2.6. survival rate and fracture strength of porcelain veneers

$: 2.6. Survival rate and fracture strength of porcelain veneers$
Aus der Universitätsklinik für Zahn-, Mund-, und Kieferheilkunde der

Albert-Ludwigs-Universität Freiburg

Abteilung für Zahnärztliche Prothetik

(Ärztl. Direktor: Prof. Dr. J. R. Strub)

Survival rate and fracture strength of maxillary incisors,

restored with different kinds of full veneers.

An in-vitro study

INAUGURAL-DISSERTATION

zur Erlangung des

Zahnmedizinischen Doktorgrades

Der Medizinischen Fakultät der Albert-Ludwigs-Universität

Freiburg

Vorgelegt 2003

Von

Nektaria Stathopoulou

Geboren in Athen, Griechenland

Dekan: Prof. Dr. med. J. Zentner

1. Gutachter: Prof. Dr. J. R. Strub

2. Gutachter: PD. Dr. P. Hahn

Jahr der Promotion: 2003

To my parents and my brother

Table of contents:

1. Introduction...................................................................................................... 1

2. Literature review ............................................................................................. 2

2.1 Historic perspective .................................................................................... 2

2.2 Indications and contraindications of porcelain veneers........................ 3

2.3 Classification of currently available all-ceramic systems for porcelain

veneers................................................................................................................ 5

2.3.1. Sintered ceramics .................................................................................................... 5

2.3.2. Infiltrated ceramics................................................................................................. 7

2.3.3. Castable ceramics.................................................................................................... 8

2.3.4. Pressable ceramics ................................................................................................ 10

2.3.5. Machining techniques .......................................................................................... 11

2.3.5.1. Copy-milling technique.............................................................. 11

2.3.5.2. CAD/CAM Systems ................................................................... 12

2.4. Preparation design................................................................................... 14

2.5. Luting procedure ..................................................................................... 18

2.5.1. Conditioning of the ceramic surface .................................................................. 18

2.5.2. Conditioning of the tooth surface....................................................................... 19

2.5.3. Luting composite .................................................................................................. 21

2.5.4. The adhesion complex: tooth/luting composite/porcelain ........................... 22

2.6. Survival rate and fracture strength of porcelain veneers................... 22

2.6.1. Biting forces and range of temperature on anterior dentition ....................... 22

2.6.2. Survival rate of porcelain veneers...................................................................... 23

2.6.3. Fracture strength of porcelain veneers .............................................................. 24

3. Aim of the study ............................................................................................ 27

4. Materials and Methods ................................................................................ 28

4.1. Materials .................................................................................................... 28

4.1.1. Abutment teeth ..................................................................................................... 28

4.1.2. Ceramic system ..................................................................................................... 28

4.1.3. Luting agent........................................................................................................... 29

4.1.4. Impression materials ............................................................................................ 31

4.1.5. Die materials.......................................................................................................... 31

4.1.6. Additional materials used ................................................................................... 32

4.2. Methods..................................................................................................... 32

4.2.1. Teeth ....................................................................................................................... 32

4.2.1.1. Selection of the abutment teeth ................................................. 32

4.2.1.2. Diagnostic Wax-up...................................................................... 33

4.2.1.3. Tooth preparation........................................................................ 33

4.2.2. Fabrication of the master models ....................................................................... 36

4.2.2.1. Impression .................................................................................... 36

4.2.2.2. Fabrication of the master dies.................................................... 36

4.2.3. Fabrication of the veneers.................................................................................... 36

4.2.3.1. Wax-up.......................................................................................... 36

4.2.3.2. Investment .................................................................................... 37

4.2.3.3. Preheat and Pressing................................................................... 37

4.2.3.4. Divestment ................................................................................... 38

4.2.3.5. Glazing.......................................................................................... 38

4.2.4. Cementation of the veneers................................................................................. 39

4.2.4.1. Pre-treatment of the veneers (Fig. 4.5) ..................................... 39

4.2.4.2. Pre-treatment of the abutments (Fig. 4.6) ................................ 39

4.2.4.3. Bonding procedure (Fig. 4.7) ..................................................... 40

4.2.5. Preparing the test specimens for the artificial-mouth ..................................... 41

4.2.5.1. Artificial periodontal membrane .............................................. 41

4.2.5.2. Fabrication of the master models for the artificial mouth..... 41

4.2.6. Dynamic loading in the artificial mouth ........................................................... 42

4.7. Examination of post-bonding cracks..................................................... 46

4.8. Fracture strength test (Fig. 4.12)............................................................. 46

4.9. Statistic analysis of data .......................................................................... 47

5. Results ............................................................................................................. 47

5.1. Dynamic loading in the artificial oral environment............................ 47

5.1.1. Survival rate .......................................................................................................... 47

5.1.2. Crack development .............................................................................................. 50

5.1.2.1. Crack formation rate (Fig. 5.3)................................................... 50

5.1.2.2. Crack pattern of porcelain veneers (Table 5.1)........................ 52

5.2. Fracture strength test............................................................................... 54

5.2.1. Fracture strength of individual samples ........................................................... 54

5.2.2. Statistical evaluation of data ............................................................................... 57

5.2.3.1. Component of the restoration that failed................................. 60

5.2.3.2. Position of the restoration where fracture occurred .............. 62

6. Discussion....................................................................................................... 64

6.1. Discussion of the materials..................................................................... 64

6.1.1. Extracted human teeth as abutments................................................................. 64

6.1.2. The storage solution ............................................................................................. 65

6.1.3. EPC as a material for the fabrication of full veneers ....................................... 65

6.2. Discussion of the methods...................................................................... 66

6.2.1. Preparation of the specimens for the artificial mouth..................................... 66

6.2.1.1. Preparation ................................................................................... 66

6.2.1.2. Cementation ................................................................................. 68

6.2.1.3. Artificial membrane .................................................................... 69


6.2.2.1. Angulation in the artificial mouth ............................................ 70

6.2.2.2. Contact point of the applied force in the artificial mouth ..... 71

6.2.2.3. Artificial ageing and its clinical relevance on the survival rate

of ceramic veneers .................................................................................... 71

6.2.3. Load-to-fracture testing ....................................................................................... 73

6.2.3.1. Angulation in the Zwick testing machine................................ 73

6.2.3.2. Contact point of the load-to-fracture applied force................ 73

6.2.3.3. Clinical relevance of the fracture strength tests of ceramic

veneers ....................................................................................................... 74

6.3. Discussion of the results ......................................................................... 75


6.3.1.1. Survival rate ................................................................................. 75

6.3.1.2. Crack formation........................................................................... 76

6.3.2. Load-to-fracture testing ....................................................................................... 78

6.3.2.1. Fracture strength of the specimens in the Zwick universal

machine...................................................................................................... 78

6.3.2.2. Fracture pattern of the specimens in the Zwick universal

machine...................................................................................................... 79

7. Conclusions .................................................................................................... 81

8. Summary ......................................................................................................... 82

9. Zusammenfassung ........................................................................................ 83

10. References..................................................................................................... 84

11. Curriculum vitae.......................................................................................... 97

12. Acknowledgements .................................................................................... 98

Introduction 1

1. INTRODUCTION

The publics demand for the treatment of unaesthetic anterior teeth is

steadily growing. Accordingly, several treatment options have been

proposed to restore the aesthetic appearance of the dentition. For many

years, the most predictable and durable aesthetic correction of anterior

teeth has been achieved with full crowns. However, this approach is

undoubtedly the most invasive, requiring removal of large amounts of

sound tooth structure and resulting in possible adverse effects on the pulp

and adjacent periodontal tissues.

The introduction of multi-step total-etch adhesive systems, along with the

development of high-performance hybrid resin composites has led to a

more conservative restorative technique. Resin composite veneers can be

used to mask tooth discoloration and/or to correct unaesthetic tooth form

and/or position. However, such restorations still suffer from a limited

longevity, since resin composites remain suspect to discoloration, wear

and marginal fractures.

In search for more durable aesthetics, porcelain veneers have been

introduced. Several studies have proven their long time prognosis in-vivo

and high level of patient acceptance (Rucker et al. 1990; Calamia 1993;

Fradeani 1998; Friedman 1998; Kihn and Barnes 1998; Peumans et al. 1998b;

Dumfahrt and Schaffer 2000). The failure rate ranged in these studies ranged

between 0% (Rucker et al. 1990) and 33% (Shaini et al. 1997). Additionally,

in-vitro studies demonstrated long term retention of porcelain veneers

(Magne and Douglas 1999c; Castelnuovo et al. 2000; Hahn et al. 2000).

Despite the promising results, there are certainly limits to the possibilities

that veneers can offer. Key factors for prognosis are the material and the

Literature review 2

kind of preparation used for such restorations. There are limited studies

concerning the design and thickness of the preparation. Regarding the

design of the preparation, four basic types have been described: the

window, the feather, the bevel and the overlapped incisal edge

preparation. As far as the thickness of preparation is concerned, the early

concepts suggested minimal or no tooth preparation. Nevertheless,

current beliefs support removal of varying amount of tooth structure.

Further studies are required to evaluate the influence of ceramic materials

and different types of preparation on the survival rate and fracture

strength of laminate porcelain veneers.

2. LITERATURE REVIEW

2.1 Historic perspective

Dr Charles Pincus (1938) was the first to describe the use of veneers to

enhance the appearance of actors for close-ups in the movie industry.

They were temporarily held in place with adhesive denture powder while

the actors were before a camera. The fragile restorations then had to be

removed, because no adhesive system existed at that time to provide a

permanent attachment of veneers to tooth structure.

Buonocore´s (1955) research on acid-etch techniques, combined with

Bowen´s (1978) later use of filled resin, enabled the mechanical bonding

between etched tooth and filled resins. The use of direct resin veneers was

now possible. The limitation of the self-curing technique was the limited

working time for the dentist to recreate a labial surface before the

composite resin chemically cured itself. The introduction of light-cured

composite resins allowed the dentist greater flexibility, due to a greater

working time and improved chemistry.

Literature review 3

Faunce and Meyers (1976) described a one-piece acrylic resin prefabricated

veneer as an improved alternative to a direct acid-etched bonded veneer.

By using a chemical primer applied to the veneer and a composite resin to

lute the veneer onto an etched tooth, both chemical and mechanical

bonding contributed to the attachment.

The concept of acid-etching porcelain was cited in the dental literature,

when Rochette described the restoration of a fractured incisor with an

“etched silanted porcelain block” (1975). Essential to the attachment of

porcelain veneers is the ability of porcelain to be etched and bonded to

composite resin as reported by Horn (1983) and Simonsen and Calamia

(1983). Continued research by Calamia (1985) also showed that the

treatment of the etched porcelain veneer with a silane coupling agent

produced a chemical bond that enhanced the porcelain-composite resin

mechanical bond.

From the moment porcelain veneers could be adhesively luted, the clinical

and laboratory techniques have continued to be refined. Today we have at

our disposal long term in-vivo and in-vitro studies focusing on porcelain

veneers, performed during the last 10 to 15 years (Karlsson et al. 1992;

Nordbo et al. 1994; Jäger et al. 1995; Fradeani 1998; Friedman 1998; Peumans et

al. 1998b). Furthermore, new preparation designs and extensions of

porcelain veneers are tested (Magne and Douglas 1999c).

2.2 Indications and contraindications of porcelain

veneers

According to the last classification of Belser et al. (1997), the three principal

indications for porcelain veneers are:

Literature review 4

Type I: Tooth discoloration resistant to vital bleaching procedures

Examples of type I indications include heavily discoloured teeth due to

tetracycline therapy [degrees III and IV according to Jordan and Boksman

(1984)], fluorosis and amelogenesis imperfecta (Calamia 1988). Anterior

teeth that present severely worn down edges, that subsequently lead to an

infiltration of dentine also belong to the same category.

Type II: Major morphological modifications

Type II a: Conical teeth

Type II b: Diastema and open embrasures to be closed

Type II c: Augmentation of incisal length and prominence

In the above cases, a single isolated problem can easily be handled with

the free-hand application of composite resins. However, in the case of

multiple modifications, the use of composites does not permit the control

of form, shade or colour and also presents signs of early fatigue when

used to restore incisal edges. Porcelain laminate veneers offer a more

predictable result.

Type III: Extensive restorations

Type III a: Extensive coronal fracture

Type III b: Extensive loss of enamel by erosion and wear

Type III c: Generalized congenital and acquired malformations

Literature review 5

In these situations, the tooth damage sometimes involves the major part of

the coronal volume or of the tooth surface. The primary advantage of

porcelain laminate veneers is that the vitality of the tooth is maintained.

Nevertheless, there are cases where porcelain veneers are contraindicated.

An extensive reduction of natural tooth structure, in extended caries and

multiple fillings, often requires a full crown because the enamel surface is

not enough to support a resin bonding procedure. Also, in patients with

bad oral hygiene and inadequate compliance veneers should be avoided

(Castelnuovo et al. 2000). Being an aesthetic treatment option, it should be

carried out under the complete collaboration of the patients, who demand

a refined appearance of their dentition.

Furthermore, patients with deep bites, open bites or bruxism are also

contraindicated, because pathological forces are placed on the teeth, that

can lead to fracture of the ceramic veneers (Sheets and Taniguchi 1990;

Fradeani and Barducci 1996).

2.3 Classification of currently available all-ceramic

systems for porcelain veneers

According to the fabrication procedure, the all-ceramic systems used for

porcelain veneers are:

2.3.1. Sintered ceramics

These products are available as powders to which the technician adds

water to produce a slurry. The ceramic mass is built up in layers on a

platinum foil or a refractory die material to form the restoration. The

Literature review 6

powders are available in various shades and translucency, and are

supplied with characterizing stains and glazes.

• Feldspathic porcelain

Vitadur N (Vita, Bad Säckingen, D) was one of the first feldspar ceramic

systems used for the fabrication of veneers (McLean and Sced 1987; Gilde et

al. 1989; Rucker et al. 1990; Hui et al. 1991). As per the technique described

by McLean and Scen (1987), a platinum foil of 0.02 mm is placed and

burnished to adapt to the die. The ceramic mass is then layered on the foil.

During sintering, microporosities are formed on the surface of the veneer,

which decrease the strength of the restoration. Thus, the clinical use of this

material is nowadays strongly limited.

• Optec HSP (Jeneric/Pentron, Wellingford, Connecticut, USA)

Optec is a leucite reinforced feldspar ceramic that condenses like an

alumina ceramic and is sintered like a traditional feldspar ceramic. The

leucite concentration was reported to be 50.6 wt% (Kelly et al. 1996). Due to

the higher amount of leucite crystals, it has greater strength than

conventional feldspathic porcelain. Because of its increased strength, it

does not require a core when used to fabricate all-ceramic restorations.

The leucite and glassy components are fused together during the baking

process at 1020°-1035°C. The build-up and contouring of the crown is

accomplished using the powder-slurry technique on a special semi-

permeable refractive die (Kühn 1992; Rosenblum and Schulman 1997). The

body and incisal porcelains are pigmented to provide the desired shade

and translucency.

Optec is recommended for the fabrication of inlays, onlays and veneers

(Schäffer 1990; Anusavice 1993). A flexural strength of 146 MPa has been

Literature review 7

reported for this material, but no survival rate data is available in the

literature.

• Duceram LFC (Ducera Dental, Rosbach, D)

Duceram is a low-fusing hydrothermal ceramic material composed of an

amorphous glass containing hydroxyl ions. The restoration is made in two

layers. The base layer is Duceram Metal Ceramic (a leucite-containing

porcelain) that is placed on a refractory die using standard powder-slurry

techniques and then baked at 930°C. Duceram LFC is applied over the

base layer using the powder-slurry technique and baked at a relatively

low temperature (660°-680°C). The material is supplied in various shades

and can be surface-characterized with compatible stains and modifiers.

Duceram is recommended for inlays, full-contour crowns and veneers

(Rosenblum and Schulman 1997). A flexural strength of 110 MPa has been

reported for this material. No survival rate data for veneers is available in

the literature.

2.3.2. Infiltrated ceramics

These products consist of two components: a powder, which is fabricated

into a porous substrate, and a glass, which is infiltrated at high

temperature into the porous substrate. The infiltrated ceramic is then

veneered, using the conventional feldspathic porcelain technique.

• In-Ceram Spinel (Vita, Bad Säckingen, D)

This glass-infiltrated ceramic is based on spinel, a composition containing

aluminum oxide and magnesium oxide. The powder is mixed with

distilled water to form a thick slip. The slip mass is placed on a refractory

Literature review 8

die and heat-treated at 1120°C for 10 hours to produce the opaque, porous

core. Then an appropriate shade of glass powder is applied to the core

which is baked again at 110°C for four hours. During this process the

molten glass infiltrates the porous alumina core by capillary action. This

procedure increases the strength of the core to about 20 times to its

original strength. The spinel crystals limit crack propagation and the glass

infiltration reduces porosity. Vitadur Alpha (Vita, Bad Säckingen, D)

aluminous veneering porcelain is then applied using conventional

powder-slurry techniques to create the proper shade and contour. The

core of spinel is not etchable. Sandblasting and tribochemical silica coating

is recommended for this material before its adhesive luting.

In-Ceram Spinel is the most translucent material among the glass

infiltrated sintered alumina ceramics and is therefore indicated for

veneers. Nevertheless, some strength has been sacrificed for the

translucency. The flexural strength of In-Ceram is reported to be 450 MPa

and that of In-Ceram Spinell 350 MPa. No survival rate data is available in

the literature.

2.3.3. Castable ceramics

These products are supplied as solid ceramic ingots, which are used for

the fabrication of cores or full-contour restorations using a lost-wax and

centrifugal casting technique. Generally, a monotone shade is available,

which is then veneered with conventional feldspathic porcelain or is

stained to obtain the proper shading and characterization of the final

restoration.

• Dicor (Dentsply, York, Pennsylvania, USA)

Literature review 9

Dicor ceramic is a polycrystalline glass-ceramic (45% glass phase and 55%

crystal phase). Its main component is silica that contains tetrasilicic

fluormica crystals. It is initially moulded as a glass (vitreous phase) and

subsequently heat treated under controlled crystallisation conditions to

produce a glass-ceramic material (mica phase) (Kelly et al. 1996; Rosenblum

and Schulman 1997). The fabrication method uses the lost-wax and

centrifugal casting techniques similar to those used to fabricate alloy

castings. A full contour transparent glass restoration is cast at 1350°C, then

is heat treated at 1075°C for 10 hours. To achieve the appropriate shade,

the extrinsic stains are baked on the surface of the glass-ceramic material

(Rosenblum and Schulman 1997).

The flexural strength of this ceramic material has been reported to be 152

MPa. Barnes et al. (1992) investigated in a short-term clinical study the

efficacy of the Dicor system for the fabrication of veneers. Although

limited survival rate data is available for inlays and crowns, no data is

available for veneers. This ceramic system has not further evolved,

especially for porcelain veneers and its clinical use is rather limited.

• Cera-Pearl (Kyocera Bioceram, Kyoto, Japan)

Cera-Pearl is a castable ceramic similar to the Dicor. It is an apatite ceramic

and its crystal phase is a hydroxyapatite phase in contrast to Dicor´s mica

phase (Hobo and Iwata 1985). The fabrication is also based on the lost wax

and centrifugal casting procedures. It is moulded at 1460°C and heat

treated at 870°C, similar to the Dicor technique. No data is available about

this ceramic system and its clinical use is also limited.

Literature review 10

2.3.4. Pressable ceramics

These products are also supplied as ceramic ingots. They are melted at

high temperatures and pressed into a mould, using the lost-wax

technique. The pressed form can be made to full contour, or can be used as

a substrate for conventional feldspathic porcelain build-up.

• IPS Empress®

IPS Empress® is a leucite reinforced glass ceramic designed for the

fabrication of single crowns, inlays and veneers (Dalloca and Demolli 1994).

The glass-ceramic ingot is partially precerammed by the manufacturer and

then processed in the laboratory and completed, using either a surface-

coloration or layering technique.

A full contour restoration is waxed-up, invested and placed in a special

mould that has an alumina plunger. After wax elimination, the glass-

ceramic is pressed into the preheated muffle. A temperature of 1050°C is

required for pressing the complete form, and a temperature of 1180°C is

required if the layering technique is to be used. Both techniques require 5

bar pressure for 30 to 40 minutes. When the layering technique is used, the

substructure is covered with dentine and enamel porcelain followed by a

final glazing cycle. For the surface-coloration technique, the restoration

form is obtained directly from the wax-pattern (Dong et al. 1992).

This glass ceramic material presents high optical and aesthetic qualities

(Fradeani and Barducci 1996). It shows also an abrasion behaviour close to

that of enamel (Wohlwend and Schärer 1990a). The marginal fit of IPS

Empress veneers has been reported to be relatively good at 95 µm (Schmalz

et al. 1994). Furthermore, Castelnuovo et al. (2000) reported mean fracture

loads between 164 N and 274 N for maxillary incisors restored with IPS

Empress® veneers. Hahn et al. (2000) reported fracture strength values


between 466 N and 693 N for mandibular incisors restored with IPS

Empress® veneers. However, no survival rate data was mentioned in the

literature.

2.3.5. Machining techniques

In order to eliminate the necessary time and effort to manufacture a

restoration, machining techniques have become an option in the field of

veneer fabrication. Both precision copy-milling machines and CAD/CAM

system are commercially available.

2.3.5.1. Copy-milling technique

• Celay (Mikrona Technologies AG, Spreitenbach, CH)

In this technique the model (pro-veneer) is formed using a light-cured

resin on the master model. Based on the size of the piece to be copied, an

appropriate prefabricated ceramic block is pre-selected and fixed in the

milling unit. The model is fixed in a special attachment unit and a three-

dimensional construction is copied, by means of mechanical censoring and

synchronous milling under water cooling (Grüninger 1996).

For veneers, Alumina Celay® Blanks (Vita, Bad Säckingen, D) are

recommended. Alumina Celay blanks are ingots of a fine-grained

feldspathic porcelain. The milled copings are veneered with aluminous

porcelain (Vitadur Alpha, Vita, Bad Säckingen, D) (Eidenbenz et al. 1994).

Unfortunately, no data is mentioned in the literature concerning veneers.


2.3.5.2. CAD/CAM Systems

• Cerec (Siemens, Bensheim, D)

Mörmann and Brandestini (1987) introduced the first computer-assisted

design/ computer-assisted manufacturing (CAD/CAM) system, Cerec® 1,

for the fabrication of inlays, onlays and veneers. This compact chair-side

system consists of an optical data acquisition camera and CAD/CAM

software. The prepared tooth is scanned using the intra-oral video camera

(“optical impression”). The preparation is then shown on the screen,

where the restoration is designed with the help of a trackball. Afterwards,

the electronic information is transferred to the 3-axis milling device. The

milling unit generates the restoration from a standard ceramic block

(Mörmann and Krecji 1992; Otto and De Nisco 2002).

The first generation equipment produced a marginal fit that was barely

below the 100 µm criterion under the best conditions. One of the design

goals of the second-generation equipment, Cerec® 2, was to produce

restorations that fit closer than 100 µm at the margins. The evolution from

Cerec® 2 to Cerec® 3 led to a further development in the fabrication of

inlays, onlays, veneers and crowns. A more accurate morphology and a

finer ceramic surface is possible with the new systems. Unfortunately, no

survival rate data is available for veneers.

For veneers, the recommended blocks are:

Cerec Vitablocs Mark® II (Vita, Bad Säckingen, D)

These are ingots of feldspathic porcelain of increased strength (152 MPa)

and finer grains size than the Mark® I compositions. These products are

supplied as ceramic ingots in various shades. The machined restoration

can be stained and glazed to obtain the desired appearance.


Alumina Celay® Blanks (Vita, Bad Säckingen, D)

The Celay blanks are also compatible with the Cerec system.

• Procera

The Procera system utilizes the concept of computer-assisted design and

computer-assisted machining to fabricate all-ceramic veneers. The

restoration is composed of a densely sintered, high purity aluminium

oxide coping combined with a compatible low-fusing AllCeram®

veneering porcelain (Andersson 1998). The Procera system consists of a

computer-controlled scanning and design station, located in the dental

laboratory. This station is connected via a modem to Procera Sandvik in

Stockholm, Sweden. The fabrication of the coping takes into account the

sintering shrinkage of 20% by enlarging the model of preparation used in

the manufacturing process. A high pure aluminium oxide powder is

compacted against the enlarged preparation model, milled and sintered to

fully density. By the addition of the veneering porcelain, the veneer is

completed (Andersson 1998).

The flexural strength of the Procera AllCeram® coping material ranges at

650-687 MPa (Wagner 1996; Razzoog 1997), and its precision of fit has been

reported to be less than 100 µm (May et al. 1998; Boening et al. 2000).

Especially for veneers, no survival rate data is available in the literature.

For veneers, the recommended blocks are (as in the Cerec system):

Cerec Vitablocs Mark® II (Vita, Bad Säckingen, D)


Alumina Celay® Blanks (Vita, Bad Säckingen, D)

Both ceramic blocks are compatible for the Procera system.

2.4. Preparation design

A key element for the success of porcelain veneers is the carefully

controlled but appropriate tooth reduction. The goals of tooth preparation

are:

• Generate sufficient space onto which the dental technician can build

the porcelain without over-contouring the tooth.

• Maintain as much tooth structure as possible.

• Provide a finished preparation that is smooth and has no sharp

internal-line-angles which would result in areas of high stress

concentration.

• Hide the preparation margins.

• Provide a well-defined insertion direction.

Aspects of tooth preparation that are of concern and have to be

systematically reviewed are the depth of preparation, the incisal edge

reduction, the interproximal extension and the marginal finish line.

Depth of preparation

Concepts regarding the amount of tissue removal have been changed over

the past few years. When the technique for porcelain veneers was

introduced, it was considered preferable to undertake minimal tooth


preparation or non at all (Calamia 1983; Horn 1983; Garber et al. 1989).

Nevertheless it is believed that this approach might be a factor

contributing to a higher failure rate (Shaini et al. 1997). Tooth preparation

is necessary, because it creates some space for the technician to fabricate a

veneer without over-contouring the tooth, resulting in a better periodontal

response (Walls et al. 2002). It facilitates the correct positioning of the

restoration during cementation (Schneider 1981; Gilmour 1993). It also

creates the necessary space for the composite resin cement to mask the

underlying discoloration and most significantly to increase the bond

strength between tooth and ceramic. This is achieved by removing the

aprismatic and hyper-mineralized enamel layers which can be resistant to

acid etching (Schneider 1981).

In tooth reduction, the standard preparation remains within the enamel.

Troedson and Derand (1998), Hahn et al. (2000), Peumans et al. (2000) suggest

an enamel reduction of 0.5 mm, which allows the preservation of enamel

and enhances strong bonds, and at the same time allows a sufficient

thickness of porcelain. Christensen and Christensen (1991) state that 0.75 mm

is the optimum amount of enamel reduction.

According to Ferrari et al. (1992), however, the extent and thickness of

enamel in the gingival area of anterior teeth does not permit a reduction of

0.5mm without extending into the dentine. In addition, Natress et al. (1995)

showed that in case of freehand preparation, the proximal and cervical

enamel was reduced more than 0.5 mm in the vast majority of cases. This

means that very often dentine can be exposed during the tooth

preparation.

Incisal edge reduction


In the literature, four preparation designs have been described with regard

to the incisal edge (Walls et al. 2002):

• Window, in which the veneer is taken close but not to the incisal edge.

This has the advantage of retaining the natural enamel over the incisal

edge, but has the disadvantage of weakening the enamel at the incisal

edge. In addition, it is difficult to hide the margins of the veneers.

• Feather, in which the veneer is taken up to the height of the incisal

edge but the edge is not reduced. This has the advantage of

maintaining anterior guidance, but the veneer is liable to be fragile at

the incisal edge and may be subject to peel/shear forces during

protrusion.

• Bevel, in which a bucco-palatal bevel is prepared across the full width

of the preparation and there is some reduction of the incisal length of

the tooth. This permits better aesthetic control at this area and

facilitates the seat of the veneer during try-in and cementation. The

margin is not in a position which will be subjected to direct shear

forces during protrusion. However, this type of preparation involves

more extensive reduction of tooth tissue.

• Incisal overlap, in which the incisal edge is reduced and the veneer

preparation extends onto the palatal tooth surface. This provides a

positive seat of the veneer during luting. However, it involves a more

extensive tooth preparation. Care is also to be taken concerning the

path of insertion, since the veneer has to be seated from the buccal-

incisal direction rather than from the buccal alone.


Limited studies are available in order to come to a decision on incisal edge

preparation. Highton et al. (1987) claimed that an incisal reduction provides

suitable stress distribution and increases the longevity of laminate

veneers.

Hui et al. (1991) reported fracture strength values of 1190 N for the

window preparation of maxillary incisors, 788 N for the feather

preparation and 688 N for the incisal overlap preparation.

Castelnuovo et al. (2000) reported mean fracture loads of 310 N for

unrestored maxillary incisors, 274 N for the bevel preparation, 237 N for

the feather preparation, 192 N for the full veneer preparation and 164 N

for the incisal overlap preparation.

Hahn et al. (2000) reported fracture strength values of 653 N for unrestored

mandibular incisors, 693 N for the feather preparation and 466 N for the

incisal overlap preparation.

Wall et al. (1992b) and Meijering et al. (1997), furthermore, were unable to

distinguish any difference in failure rate between incisal preparation

designs.

Interproximal extension

In the case of minimum preparations, interproximal contacts should be

maintained. However, if major changes in form or the closure of diastemas

are planned, a preparation through the contact area is recommended

(Belser et al. 1997). This extension permits the dental ceramist to build a

veneer that matches to the form and the emergence profile of the tooth. It

also facilitates numerous treatment phases and, most of all, final

cementation with better stabilization of the porcelain veneer and easy

access to all of the margins during the bonding procedure (Belser et al.


1997; Magne and Belser 2002). Unfortunately, the systematic creation of an

interdental lapping has not been sufficiently studied.

Finish line

The preparation line should allow an optimum adaptation of the final

restoration. Troedson and Derand (1999) suggested that a featheredge

preparation in the cervical and interproximal region is contraindicated.

The creation of a chamfer without internal line angles is recommended.

Magne and Douglas (1999a) also showed in their study that palatal mini-

chamfers or butt-joint margins are generally recommended.

2.5. Luting procedure

The porcelain veneer technique includes the bonding of the fabricated thin

porcelain laminate to the prepared tooth surface using a luting composite.

The success of the porcelain veneer restoration is greatly determined by

the strength and durability of the bond between these three components;

the veneer, luting agent and tooth, which form the adhesion complex.

2.5.1. Conditioning of the ceramic surface

A combination of micromechanical interlocking (hydrofluoric acid) and

chemical coupling (silanization) is necessary to obtain the most effective

bonding of porcelain (Jardel et al. 1999).


Etching the inner side of the porcelain veneer with hydrofluoric acid

creates a retentive etch pattern. SEM of the etched porcelain surface

reveals an amorphous micro-structure with numerous porosities (Peumans

et al. 1999). These micro-porosities increase the surface area for bonding

and lead to a micro-mechanical interlocking of the resin composite.

Several factors determine the micro-morphology of the etch pattern and

consequently the bond strength of the resin composite to the etched

porcelain. Such factors include the etching time, the concentration of the

etching liquid, the method of fabrication of the porcelain restoration

(Simonsen and Calamia 1983; Stangel et al. 1987) and the type of porcelain

(Calamia 1985; Roulet et al. 1995). Ultrasonic cleaning of etched porcelain

with 95% alcohol, acetone or distilled water is essential to remove all

residual acid and dissolved debris from the porcelain surface (Peumans et

al. 2000).

Silanization of etched porcelain with a bi-functional coupling agent

provides a chemical link between the luting resin composite and

porcelain. A silane group at one end bonds chemically to the hydrolyzed

silicon dioxide at the ceramic surface, and a methacrylate group at the

other end copolymerises with the adhesive resin (Peumans et al. 2000).

Contamination of the pre-tested surface with die stone (Swift et al. 1995),

latex gloves (Holtan 1995), saliva (Nicholls 1988), silicone-based fit checker

paste (Sheth et al. 1988) and try-in paste (Della Bona 1994) will negatively

influence the bond strength of resin composite to the ceramic restoration.

2.5.2. Conditioning of the tooth surface

The procedure followed differs according to the clinical situation:

• Preparation maintained into the enamel


Enamel reduction is required to improve the bond strength of the resin

composite to the tooth structure (Stacey 1993). Thus, the aprismatic top

surface of enamel, which is known to offer only a minor retention

capacity, is removed. When the enamel is cut, an organic smear layer is

formed on the remaining surface. Etching with a 30-40% phosphoric acid

dissolves the inorganic components of smear layer and changes the

smooth surface of the enamel to an irregular one in order to achieve a

micro-mechanical interlock with the composite resin. The enamel prisms

form a rough surface that makes it more receptive to adhesion. After

polymerisation, resin tags are formed in the microporosities of the etched

enamel, creating a resistant and lasting bond (Lopes 2002).

• Preparation with significant dentine exposure

If a considerable area of dentine has been exposed during tooth

preparation, dentine adhesion must take place. During cutting, a smear

layer is formed, consisting of burnished components and hydroxylapatite

fragments. The smear layer blocks the dentine tubules, stops the tubule

fluid from escaping and prevents the formation of a chemical and/or

micromechanical retention of the dentine bonding agent. Thus, pre-

treating the dentine surface becomes essential to obtaining effective

adhesion. Traditional dentine protocols include:

1. Etching/conditioning with a primer

2. Application of an adhesive

3. Application of a bonding resin

Etching dissolves the smear layer and demineralizes the dentine, resulting

in a wide opening of the dentine tubules (Gwinnett 1993). Primer

penetrates the collagen fibres and promotes the flow of bonding agent.

The applied bonding agent penetrates the collagen net and forms a mixed


zone of demineralized dentine and unfilled composite (Gordan et al. 1998).

This zone is known as the "resin-dentine interdiffusion zone" and

represents a key factor for the success of dentine bonding.

It is, however, not necessary that these steps (conditioning, priming,

bonding) are sequentially performed. Some of these functions have been

combined in the form of self-conditioning primers and primer adhesives

in order to reduce the number of applications (Watanabe et al. 1994).

Recently, a new type of bonding system has been introduced which

combines all three functions, the so-called self conditioning primer

adhesives (Yoshiyama 1998).

2.5.3. Luting composite

For the cementation of porcelain veneers, light-curing composites have

been proposed as luting agents. A major advantage is that they allow a

longer working time compared with dual cure or chemically curing

materials. This allows the dentist to improve on position of the restoration,

and removal of excess composite prior to curing. This may reduce the

finishing time required for these restorations. In addition, their colour

stability is superior compared with the dual-cured composites, that are

characterised as colour unstable due to amine degradation (Peumans et al.

2000).

Nevertheless, it is important that there is enough light transmittance

throughout the porcelain veneer to polymerise the light-curing luting

composite. In cases of ceramic veneers with extreme facial thickness, or in

situations where the veneers have been rendered highly opaque (as in the

case of severe residual discoloration), light-cured resin composites do not

reach their maximum hardness (Linden et al. 1991). A dual-cured luting

composite, which contains the initiation system for both chemical and

light-cured composites, is then recommended.


2.5.4. The adhesion complex: tooth/luting composite/porcelain

Stacey (1993) reported that a very strong complex was obtained in-vitro by

luting the porcelain veneer. The strength of the combined porcelain/luting

composite/enamel complex (63 MPa) was significantly higher than the

separate composite/etched enamel (31 MPa) and the separate luting

composite/porcelain (33MPa) bond strengths. Andreasen et al. (1991) and

Stokes and Hood (1993) noted that extracted incisors restored with porcelain

veneers were recovered to their original strength.

Magne and Douglas (1999a) also demonstrated that porcelain veneers

restore the mechanical behaviour and microstructure of the tooth in-vitro,

when they are bonded to an extensive dentine surface using an optimized

application of dentine adhesives. A sufficient and even thickness of

ceramic combined with a minimal thickness of luting agent provides the

restoration with a favourable configuration.

2.6. Survival rate and fracture strength of porcelain

veneers

2.6.1. Biting forces and range of temperature on anterior

dentition

Exposing dental materials to various loads and temperature ranges is a

common part of in-vitro testing. Both force and temperature should reflect

the situation that exists intraorally. Extreme values could over-stress the

material and probably indicate that it is unsatisfactory for clinical use. In

contrast, limited values may not adequately stress the material and

possibly promote the use of clinically deficient materials.


In terms of loading, several investigations have evaluated the biting forces

in the anterior area. Parameters such as sex, age, state of the dentition,

training, muscular strength and bruxismus have proved to be correlated

with biting forces.

Helkimo et al. (1977) reported average values of 147 N for the canines and

137 N for the incisors. The mean values were higher for males than for

females. In males the maximal bite forces measured were 176 N in the

incisor region and in 108 N female.

Sonnenburg et al. (1978) found that the average load values for anterior

teeth are between 215 N and 360 N for men and 115 N and 269 N for

women.

Kalipcilar and Kedici (1993) reported maximal biting forces of 121.6 N for

the canines and 94.1 N for the incisors.

Kelly (1999) reported that the physiologic forces in the anterior region

during chewing and swallowing range between 5 N and 364 N, and the

maximum forces during clenching between 216 N and 890 N.

In terms of temperature ranges, there is limited data on the maximum or

minimum temperatures that can actually be reached at the tooth surface

in-vivo. Palmer at al. (1992) reported an average high temperature of

58.5°C in the maxillary anterior region and an average low temperature of

1.0°C for the mandibular posterior region. Thus, they suggested that a

range of 0°C to 67°C might be appropriate for thermocycling dental

materials.

2.6.2. Survival rate of porcelain veneers

The longevity of dental restorations is one of the most important factors

pertinent to the success and predictability of a proposed technique. Only

long-term studies are appropriate to draw to conclusions on the longevity

of restorations. It is important that 5-year clinical results should be


available before new all-ceramic restorations can be recommended for use

in daily private practice (Strub 1992). According to Pröbster (1996), a

prosthetic restorative system can be considered successful if it

demonstrates a survival rate of 95% after 5 years and 85% after 10 years.

Aspects to determine the survival rate of porcelain veneers are fracture,

debonding, chipping, cracking and marginal integrity including caries,

staining and overcontouring. Several clinical studies have reported a very

low failure rate (0-5%) due to loss of bonding and fracture (Clyde and

Gilmour 1988; Strassler and Nathanson 1989; Rucker et al. 1990; Nordbo et al.

1994; Kihn and Barnes 1998; Peumans et al. 1998b; Dumfahrt and Schaffer

2000). Somewhat higher failure rates were noted by Christensen and

Christensen (1991) (13% after 3 years) and Strassler and Weiner (1995) (7%

after 7-10 years).

In addition, several in-vivo studies report a high number of restorations

with acceptable marginal adaptation (65-98%) and periodontal response

(Strassler and Nathanson 1989; Rucker et al. 1990; Christensen and Christensen

1991; Strassler and Weiner 1995; Kihn and Barnes 1998; Meijering et al. 1998a;

Dumfahrt and Schaffer 2000). Only a few clinical studies report small

marginal defects along the entire outline of porcelain veneers (Jäger et al.

1995; Peumans et al. 1998b).

2.6.3. Fracture strength of porcelain veneers

A natural tooth's ability to withstand masticatory and thermal loads

during a lifetime is the result of the structural and physical relationship

between an extremely hard tissue (enamel) and a more pliable tissue

(dentine). The moment some tooth reduction takes place to provide space

for a porcelain veneer, the biomechanical response of the tooth to the

restorative procedure is to be taken into consideration. The question is

whether a recovery of crown rigidity can be expected.


It was demonstrated that crown rigidity could be recovered completely

when feldspathic porcelain is used as an enamel substitute, as with

porcelain veneer restorations (Magne and Douglas 1999a). Teeth restored

with dentin-bonded porcelain veneers demonstrated increased stiffness

when subjected to fracture tests (Magne and Douglas 1999c).

Susceptibility to fracture of ceramic veneers is the result of several factors.

Fracture strength is most dependent upon the number and severity of

cracks in the ceramic restoration. These cracks may occur before or after

the cementation procedures. Ceramics always contain a large number of

pre-existing cracks. They are caused by the condensation, melting, and

sintering process; by differences in the coefficient of thermal expansion

between cores and veneers; and by grinding and abrasion (Hondrum 1992).

Postoperative cracks are also reported as a possible consequence of

loading, polymerization shrinkage and thermocycling (Magne et al. 1999a).

Repetitive loading may be the most common mechanism of failure of

dental ceramics, causing a combination of bending and tensile forces.

These low energy flexural forces provoke surface flaws under tension

(Hondrum 1992). Slow crack growth of subcritical flaws occur when local

stresses are relieved by the growth of existing cracks until the crack

reaches a critical size for a catastrophic, fast fracture (Ritter 1995).

During polymerization, shrinkage of the luting agent also generates

compressive forces on the ceramic, either at the restoration surface or at

the interface. The ratio of the thickness of the luting agent appears to have

a relevant influence on stress distribution in porcelain veneers.

Restorations that are too thin combined with poor internal fit resulted in

higher stresses and crack propensity at the ceramic (Magne et al. 1999a).

Thermocycling is also to be taken under consideration. Temperature

ranges in the oral environment may vary between 0°C and 67°C and it is

assumed that the thermal expansion mismatch between tooth and

restorative materials can create significant stress in the porcelain.


Furthermore, failures can result from a stress-dependent chemical reaction

between water vapour and micro-cracks on the surface of the ceramic

material. Absorbed moisture lowers the energy required for crack

propagation at the crack tip. There is a 20% to 30% reduction in strength in

a moist environment (Hondrum 1992).

Aim of the study 27

3. AIM OF THE STUDY

The aim of this comparative in-vitro study was to evaluate the survival

rate and fracture strength of four groups of anterior full veneers after

thermo-mechanical fatigue in a dual-axis chewing simulator. The

influence of the depth and the palatal extension of the veneer preparations

on the survival rate and fracture strength were of high interest.

In detail the following parameters were examined:

a) The influence of ceramic thickness on the stability of the full-veneers.

b) The influence of dentin exposure on the adhesion complex and the

fracture strength of the restorations.

c) The influence of the contact point on the fracture strength and crack

formation rate of the restorations.

Materials and methods 28

4. MATERIALS AND METHODS

4.1. Materials

4.1.1. Abutment teeth

Eighty caries-free, human maxillary central incisors were used as

abutments. The teeth were obtained directly after extraction and stored in

0.1% thymol solution at room temperature (Sparrius and Grossman 1989).

4.1.2. Ceramic system

An experimental press ceramic (EPC) from Ivoclar-Vivadent AG (Schaan,

FL) was used. It concerns a glass-ceramic material which similar to IPS

Empress® 2, is based on a SiO2-Li2O-system. The tables 4.1 and 4.2 show

the physical properties and the composition of the EPC:

Properties Experimental press ceramic (EPC)

Mechanical:

• Flexural strength

• Fracture toughness

525 ± 75 Mpa

3.0 ± 0.5 MPa • m0.5

Optical:

• Translucency

Very high translucency, similar to

the natural tooth

Thermical:

• Coefficient of thermal expansion

10.6 ± 0.5 • 10-6 K-1 m/m

Chemical:

• Solubility

< 100 µg/cm2


Technical:

• Press temperature

• Firing temperature of sinter

ceramic resp. sinter glass ceramic

(Dentin and Incisal)

910 – 920 °C

800 °C

(New Type of material:

740 °C)

Table 4.1: Physical properties of the EPC

Components ma.-%

SiO2 57 – 80

Al2O3 0 – 5

La2O3 0.1 – 6

MgO 0 – 5

ZnO 0 – 8

K2O 0 – 13

Li2O 11 – 19

P2O5 0 – 11

Additional ingredients 0 – 8

Table 4.2: Composition of the EPC

4.1.3. Luting agent

Variolink® II -Professional-Set (Ivoclar-Vivadent AG, Schaan, FL)

Variolink® II is a micro-filled, dual-curing (light- and chemical-curing)

luting composite, indicated for adhesive cementation of all-ceramic,


Ceromer and composite restorations. This material is available in 5 shades,

3 degrees of translucency and 2 degrees of viscosity (low viscosity and

high viscosity). Its standard composition is the following (Table 4.3):

Standard composition (% in weight) Base Catalyst

high

Catalyst

low

Bis-GMA 13.1 11 13.9

Urethane dimethacrylate (UDMA) 6.6 5.5 7

Triethylenglycoldimethacrylate

(TEGDMA)

6.6 5.5 7

Barium glass filler, silanised 38.4 42.2 36.2

Ytterbiumtrifluoride (YbF3) 25 25 25

Mixed oxide, silanised 5 5 5

Ba-Al-Fluoro-Silicate glass 5 5 5

Catalyst and Stabilisers 0.3 0.8 0.9

Pigments < 0.1 < 0.1 < 0.1

Table 4.3: Standard composition of Variolink® II

The Variolink® II professional set consists of:

Total Etch®

Gel for enamel etching and dentine conditioning (phosphoric acid 37%

wt., silicon dioxide and pigments).

Syntac® primer

Polyethylene glycol dimethacrylate, maleic acid and ketone in aqueous

solution designed to make the superficial dentine collagen suitable for the

following bonding procedure.

Syntac® adhesive


Polyethylene glycol dimethacrylate and glutaraldehyde in an aqueous

solution.

Heliobond®

Light-curing single component bonding agent (Bis-GMA 60% wt. and

triethylene glycol dimethacrylate 40% wt.).

IPS Ceramic etching gel®

4.9% hydruofluoric acid, recommended for the EPC material.

Monobond-S®

Silane (99% wt. water/ethanol solution, acetic acid, 1% 3-

methacryloxypropyl-trimethoxysilane).

Liquid strip

Glycerine gel for the prevention of contact with oxygen.

4.1.4. Impression materials

Dimension® Garant L (3M-Espe, Seefeld, D): hydrophilic low consistency

addition polymerisation silicone material (polyvinyl-siloxane) with 99.9%

elastic return after deformation, -0.20% linear dimensional change after 24

hours and setting time of 5.30 minutes.

Permagum® Putty Soft (3M-Espe, Seefeld, D): high viscosity addition

silicone impression material with -0.05% linear dimensional change after

24 hours and setting time of 5 minutes.

4.1.5. Die materials

GC Fujirock® (GC Belgium): type 4 dental stone (water/powder ratio

20ml/100gr, setting expansion 0.08% and compressive strength 53 MPa).


4.1.6. Additional materials used

The additional materials used are the following (Table 4.4):

Materials Company

Anti-Rutsch Lack Wenko-Wenselaar, GmbH Hilden,D

Technovit 4000 Heraus-Kulzer,Wehrheim, D

Diamonds No 386.023, No 8368.023,

No 7KR.012, No 8837KR.012

Gebr. Brasseler, Lemgo, D

Steatite ceramic balls Hoechst Ceram Tec, Wunsiedel, D

Thermocycling system Gebrüder Haake GmbH,Karlsruhe,D

Artificial oral environment Willytech, Munich, D

Table 4.4: Additional materials used in the study

4.2. Methods

4.2.1. Teeth

4.2.1.1. Selection of the abutment teeth

For this study 80 maxillary central incisors of comparable size were

selected and used as abutments. After the extraction they were stored in

0.1% thymol solution at room temperature (Sparrius and Grossman 1989).

Calculus deposits and soft tissue were removed using a hand scaler and

an ultra-scaler device where necessary.

Only teeth that had been visually examined and found to be sound and

free from caries, hypoplastic defects and cracks were included in this


study. The teeth were randomly divided into five equal groups,

designated A, B, C, D and E. Each group contained 16 teeth.

4.2.1.2. Diagnostic Wax-up

An average-sized, completely healthy maxillary incisor, with a height of

11mm and width of 8.5 mm (Stambaugh and Wittrock 1977), was selected in

order to provide a reference for the preparation and the wax-up procedure

in the laboratory. The wax-up was modified to give the tooth an ideal

form for the final veneer restoration.

This model tooth was duplicated several times, using a silicone putty

matrix (Formasil®, Heraus-Kulzer, Wehrheim, D). A sagittal cut through

the impression offered the dentist and the dental technician the earlier

mentioned reference (Magne and Douglas 1999b). In this way a similarity of

design among the test groups was achieved.

4.2.1.3. Tooth preparation

In this study four different types of full veneer preparations were

compared. For this reason, the teeth were divided into the following five

groups:

• Group A = control group: natural teeth without preparation

• Group B = 1. test group: full veneer preparation maintained in the

enamel/ contact point on the natural tooth

• Group C = 2. test group: full veneer preparation maintained in the

enamel/ contact point on the veneer

• Group D = 3. test group: full veneer preparation extended into dentine/

contact point on the natural tooth

• Group E = 4. test group: full veneer preparation extended into dentine/

contact point on the veneer


The standard preparation was carried out as follows:

Group A: No preparation as control group

Group B (Fig. 4.1): Shortening of the incisal edge 3 mm

Labial reduction 0.5 mm

Cervical finish 0.5 mm deep chamfer

Proximal extension with a clear chamfer palatinally

Palatal extension 2 mm

Contact point on the natural tooth.

Fig 4.1: Full veneer preparation for Group B

Group C (Fig. 4.2): Shortening of the incisal edge 3 mm

Labial reduction 0.5 mm




Contact point on the veneer


Fig 4.2: Full veneer preparation for Group C

Group D (Fig. 4.3):

Shortening of the incisal edge 3 mm

Labial reduction. 1 mm




Contact point on the natural tooth

Fig 4.3: Full veneer preparation for Group D

Group E (Fig. 4.4):

Shortening of the incisal edge 3 mm

Labial reduction 1 mm




Contact point on the veneer

Fig 4.4: Full veneer preparation for Group E


4.2.2. Fabrication of the master models

4.2.2.1. Impression

After preparation, the abutment teeth were dried with air and cotton

pellets. Then, impressions of them were taken with a polyvinyl-siloxane

impression material (Dimension Garant L + Putty Soft, 3M-Espe, Seefeld,

D). For the impression procedure, perforated custom plastic trays were

used.

4.2.2.2. Fabrication of the master dies

One hour later, the impressions were sprayed with a surfactant and dried.

Distilled water and Fujirock® type 4 dental stone (GC, Tokyo, J) were

vacuum-mixed in a ratio of 20ml/100gr for 60 sec, and a cast of each

impression was poured. After the stone had set, the models were taken

out of the impressions and the prepared abutments were covered with a

thin flowing silicone (Prävegum; Pratzner, Böbingen, D) to protect the

margins from the trimming procedure. The models were trimmed, the

silicon films were removed and the preparation margins of the abutment

teeth were set free.

4.2.3. Fabrication of the veneers

All 64 veneers in groups B through E were fabricated out of EPC

VP2117/TC2 ceramic of the Fa. Ivoclar-Vivadent AG, Schaan, FL. The

following procedures were performed as recommended by the

manufacturer:

4.2.3.1. Wax-up

A sealer (Margidur, Benzer Dental AG, Zürich, CH) was applied on the

dies to harden the surface and to protect them without changing the


dimensions. A layer of spacer (Purargent, Benzer Dental AG, Zürich, CH)

was applied up to 0.5 mm above the preparation margin.

A full wax-up was fabricated and a silicone index prepared. Organic

model dental wax that burns out without leaving residue was used for the

wax-up (Pro Art Sculpturing Wax, Williams, Ivoclar-Vivadent AG,

Schaan, FL).

4.2.3.2. Investment

A wax sprue with a round profile, 3-8 mm of length and 2-3 mm/8 gauge,

was attached directly to the wax-ups. The investment was mixed with its

liquid for 1 minute under vacuum, according to the manufacturer’s

instructions. After setting (1hour), the ring gauge and ring base were

removed. The paper ring and rough spots at the bottom of the investment

cylinder were also removed.

4.2.3.3. Preheat and Pressing

The investment cylinder was preheated in a conventional KaVo

preheating furnace, Type 5636. The following parameters were set:

-Temperature increase 5° C per minute

-Holding time 30 minutes at 250° C

-Final temperature 60 minutes at 850° C

For the pressing procedure of the ceramic veneers a special furnace (IPS

Empress® EP 500 hot press furnace, Ivoclar-Vivadent AG, Schaan, FL) was

used. The investment cylinder along with the ingot was placed at the

centre of the EP 500 press furnace and the pressing procedure followed.

The pressing temperature was 915°C. The exact pressing parameters were

set:

-Start temperature 700°C



-Final temperature at 915° C

-Holding time 20 minutes

-Vacuum start 500°C

-Vacuum stop 915°C

4.2.3.4. Divestment

After approximately 1 hour, the investment cylinder was removed from

the press furnace. Occasionally, the cylinders presented cracks due to

differences in the coefficients of thermal expansion of the various

materials. However, these cracks do not have to be considered as failures

of the pressing procedure.

The investment cylinder was separated, using a separating disc to create a

predetermined breaking point. The rough divestment was removed with

glazing blasting pearls at 2 bar pressure. The pressed veneers were cut

from the sprues and the attachment points were removed with

appropriate burs. Subsequently they were cleaned in Invex liquid (Ivoclar-

Vivadent AG, Schaan, FL) in an ultrasonic unit for 10 minutes, rinsed with

water and dried. Finally, they were carefully blasted with Al2O3 at 1 bar

pressure. The veneer fit was verified on the master models.

4.2.3.5. Glazing

The application of glazing and staining materials followed. Two glazing

procedures took place in Programat P100 with Empress® Universal Glasur

D64847. The firing parameters of these materials are reported as follows:

-Start temperature 403°C


-Final temperature at 770° C

-Holding time 20 minutes

-Vacuum start 450°C

-Vacuum stop 769°C


The fitting surfaces of the crowns were sandblasted with Al2O3 Type 100

at 1 bar pressure and cleaned with jet steam.

4.2.4. Cementation of the veneers

4.2.4.1. Pre-treatment of the veneers (Fig. 4.5)

The inner surfaces of the veneers were initially etched with hydrofluoric

acid 4,9% (Ceramic etchant, Ivoclar-Vivadent AG, Schaan, FL) for 20

seconds. Then, they were rinsed-off with water, dried with air and

silanized with Monobond-S® (Ivoclar-Vivadent AG, Schaan, FL). 60

seconds after the silanization the veneers were dried with air. A thin layer

of Heliobond® was applied with a brush on the etched and silanized

ceramic surface. To prevent premature setting of Heliobond®, the ceramic

restorations were stored in the Vivapad® (Ivoclar-Vivadent AG, Schaan,

FL) and thus protected from light.

Fig 4.5: Pretreatment of the full veneers

4.2.4.2. Pre-treatment of the abutments (Fig. 4.6)

The abutments were initially etched with phosphoric acid 37% (Total

Etch®, Ivoclar-Vivadent AG, Schaan, FL) for 30 seconds in enamel and 15

seconds in dentine. The phosphoric acid was rinsed-off with water for

approximately 15 seconds, and the tooth surface was air-dried. Syntac®

primer (Ivoclar-Vivadent AG, Schaan, FL) was applied with a brush and

allowed to react for 15 seconds; then the primer was dried with water- and


oil-free air. Syntac® adhesive was applied with a brush on the dentine,

allowed to react for 10 seconds and then dried. Application of Heliobond®

(Ivoclar-Vivadent AG, Schaan, FL) bonding agent followed.

Fig 4.6: Pretreatment of the abutment teeth

4.2.4.3. Bonding procedure (Fig. 4.7)

All veneers were cemented with Variolink® II (Ivoclar-Vivadent AG,

Schaan, FL), a dual-curing resin cement. The base paste was mixed with

the catalyst paste in a 1:1 ratio on a mixing pad for 10 seconds. Mixed

Variolink® II was applied with a spatula on the restorations, which were

placed in situ with slight finger pressure initially, and then the finger

pressure was increased slowly and maintained for 15 seconds. Excess

material was removed with foam pellets. An easily accessible, small

marginal area was polymerised for 40 seconds for initial fixation; the

pressure was taken off the restoration and additional marginal excess was

carefully removed with an explorer so that the cement was not brushed

out of the margins. Then it was polymerized step by step for 40 seconds

per segment. Polymerized excess was removed with polishing discs.

Fig 4.7: Cementation of the veneer on the abutment teeth


4.2.5. Preparing the test specimens for the artificial-mouth

4.2.5.1. Artificial periodontal membrane

In order to represent physiological tooth mobility, all roots of the

abutment teeth were covered with an artificial periodontal membrane (0,5

mm) made out of gum resin (Anti-Rutsch Lack, Wenko-Wenselaar, Hilden

D).

Each tooth was coronally waxed until 2 mm short of the cemento-enamel

junction and then dipped once in the gum resin. After the gum resin had

dried, the excess resin on the root tip was removed using a scalpel so that

a uniform coating remained on the root surface.

4.2.5.2. Fabrication of the master models for the artificial mouth

A sample holder of the artificial mouth was filled with Putty Soft (3M-

Espe, Seefeld, D). One of the teeth was embedded in it at a buccal-lingual

inclination angle of 135° between the long axis of the tooth and the

horizontal plane of the sample holder, replicating the clinical situation.

After the material has set, an impression of this first master model was

taken several times with a polyvinyl-siloxane impression material (Putty

Soft, 3M-Espe, Seefeld, D). This impression was used as a guide for the

fabrication of all master models used in the study. The natural teeth were

fixed into the impression with the use of wax, that was extended 2 mm

apically. Then, the abutment teeth were embedded in a self-curing

polyester resin (Technovit 4000®, Hereaus-Kulzer, Wehrheim, D)

throughout the sample holder of the artificial oral environment. In this

way the clinical situation was simulated with the bone level positioned

approximately 2 mm below the cementoenamel junction.


4.2.6. Dynamic loading in the artificial mouth

All specimens were artificially aged in a computer-controlled dual axis

chewing simulator (Willytec, Munich, D). The chewing simulator had

eight identical sample chambers and two stepper motors that allowed

computer-controlled vertical and horizontal movements between two

antagonistic specimens in each sample chamber (Fig. 4.8 and 4.9). The

masticatory load curve was programmed by combining this horizontal

(0.5 mm) and vertical (6 mm) motion. The computer unit controlled the

mechanical motion and the water flow of the cold and warm water baths

for the thermal cycling of the samples.

Each of the eight sample chambers had a plastic sample holder which was

adjusted and fixed with a butterfly nut to the base of the sample chamber

and the underlying lower crossbeam (Fig. 4.10). The samples were

embedded in resin into the lower sample holder. The lower crossbeam

was moved by one stepper motor and allowed a horizontal, sliding motion

of the samples.

Vertical guide rails were freely mounted within the bearings of the upper

crossbeam. The vertical height of the antagonistic, 6 mm in diameter,

steatite balls was adjusted by the adjustment screw on top of the upper

crossbeam (Fig. 4.10). Weights of 5 kg were mounted on top of the guide

rails and established a chewing force of 49 N. The upper crossbeam was

moved by the second stepper motor and moved the antagonistic steatite

balls vertically. Because the guide rails were freely mounted within

bearings in the crossbeam, their individual weight was fully transferred to

each lower sample. The effective impact force is dependent on the

antagonist’s total weight and on its velocity, and both could be precisely

controlled. The chewing machine’s computer unit calculates and displays

the effective impact as kinetic energy.


Each sample chamber was equipped with a water nozzle through which

the sample was sprayed alternately with cold and warm water (Fig. 4.10).

To prevent the mixing of cold and warm water, the preceding water was

fully suctioned out before water of the other temperature was applied.

The cyclic dynamic occlusal load of 49 N was applied to the veneers for 1.2

million chewing cycles and 3.000 thermocyclings (5°/55°C) (Fig. 4.11). The

cycle frequency was set at 1.6 Hz. The following parameters were

assessed: rising speed: 55 mm/s; descending speed: 60 mm/s; forward

speed: 60 mm/s; backward speed: 55 mm/s (Table 4.5).

Chewing cycle 1.200.000

Cycle frequency 1.6 Hz

Vertical movement 7 mm

Horizontal movement 0.5 mm

Descending speed 60 mm/s

Rising speed 55 mm/s

Forward speed 60 mm/s

Backward speed 55 mm/s

0 point subincisal 2 mm

Applied weight pro sample 49N

Hot dwell time 60s

Hot bath temperature 55°C

Cold dwell time 60s

Cold bath temperature 5°C

Intermediate pause 12s

Table 4.5: Overview of the assessed parameters in the artificial mouth


Fig 4.8: The dual-axis chewing simulator

1. upper crossbeam 6. pump for cold water removal

2. lower crossbeam 7. pump for warm water removal

3a. water reservoir in 8. pump for cold water application

3b. water reservoir out 9. pump for cold water application

4. filter for cold water 10. motor block

5. filter for warm water 11. table

Fig 4.9: Schematic drawing of the dual-axis chewing simulator with

eight sample chamber according to Kern et al. 1999


vertical guide rail

adjustable weight

shock absorber ring

sample chamber base

adjustment screw

fixation ring

upper crossbeam

upper sample holder antagonistic sample sample lower sample holder

lower crossbeam

butterfly nut

water supply

water nozzle

sample chamber

water outlet

Fig 4.10: Schematic drawing of one chewing chamber according to Kern

et al. 1999

Fig 4.11: Test sample during dynamic loading


4.7. Examination of post-bonding cracks

After dynamic loading in the artificial mouth, the samples that survived

were examined under the stereomicroscope (Zeiss, Jena, D) at a

magnification of 200x, in order to observe whether post-bonding crack

formation had occurred. The samples that failed were not examined

further.

4.8. Fracture strength test (Fig. 4.12)

All samples that survived the exposure to the artificial mouth were

statically loaded in the Zwick Z010/TN2S universal testing machine

(Zwick, Ulm, D), until fracture of the veneer or the tooth occurred.

Each sample was mounted in the test machine. A double layer of 1 mm tin

foil (DT Dental Tradint, Bad Kissingen, D) was placed in the incisal edge

of each veneer to ensure homogeneous stress distribution. The test started

via a computer connected to the testing machine. The 135° axially inclined

samples were subjected to an increasing compressive load with a speed of

1.5 mm/min until fracture occurred. The fracture strength and the

displacement were automatically recorded by the Text X-pert (Zwick,

Ulm, D) controlling the software of the machine. The recorded data was

automatically analysed, and a graph was drawn for each sample.

Fig. 4.12: Test sample loaded to fracture

Results 47

4.9. Statistic analysis of data

The fracture strength values were summarized in tables according to the

groups. Then, they were graphically displayed using Box-plots. The

multiple pair wise comparison of the fracture strength was evaluated with

the Wilcoxon-Rank Test.

5. RESULTS

5.1. Dynamic loading in the artificial oral

environment

5.1.1. Survival rate

During the dynamic loading of the samples in the artificial mouth,

different results were observed, starting from cracking of the veneers and

ending with fractures.

The cracking of the veneers was not considered to be a failure of the

sample. As far as the fractures are concerned, 6 out of the 80 samples

failed in the chewing simulator. All fractures occurred at the root level,

while the veneered crowns remained intact (Fig. 5.1).

Results 48

Fig 5.1: Fracture of the test sample at the root level

The survival rate of the 5 groups was as follows (Fig. 5.2):

• Group A (no preparation)

In the control group, 3 out of the 16 samples fractured.

Sample no.6 fractured at 530.000 chewing cycles, sample no.8

fractured at 430.000 chewing cycles and sample no.14 fractured at

210.000 chewing cycles.

• Group B (preparation maintained in enamel/ contact point on the

tooth)

In test group B, all 16 samples survived the exposure to the artificial

environment.

• Group C (preparation maintained in enamel/ contact point on the

veneer)

In test group C, 2 out of the 16 samples fractured.

Sample no.5 fractured at 170.000 chewing cycles and sample no.10

fractured at 470.000 chewing cycles

• Group D (preparation extended into dentine / contact point on

the tooth)

In test group D, all 16 samples survived the exposure to the

artificial environment. • Group E (preparation extended into dentine / contact point on the

veneer)

In test group E, 1 out of the 16 samples fractured.

Sample no.7 fractured at 520.000 chewing cycles

Results 49

81%

100%

88%

100%94%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Control group A Test group B Test group C Test group D Test group E

Control group A: no preparation

Test group B: preparation maintained in enamel / contact point on the tooth

Test group C: preparation maintained in enamel / contact point on the veneer

Test group D: preparation extended into dentin / contact point on the tooth

Test group E: preparation extended into dentin / contact point on the veneer

Fig. 5.2: The survival rate of the different groups

Results 50

5.1.2. Crack development

After dynamic loading in the artificial mouth, the samples that survived

were examined under the stereomicroscope (Zeiss, Jena, D) at a

magnification of 200x. The samples that failed were not examined further.

The rate and the position of the cracks were the topics of interest.

5.1.2.1. Crack formation rate (Fig. 5.3)

Among the different test groups, cracks in the porcelain veneers were

developed in 28 out of the 64 samples (44%). The crack formation rate was

as follows:

• Group B (preparation maintained in enamel/ contact point on the

tooth)

In test group B, cracks were observed in 3 out of the 16 samples.

• Group C (preparation maintained in enamel/ contact point on the

veneer)

In test group C, cracks were observed in 10 out of the 14 samples.

• Group D (preparation extended into dentin/ contact point on the

tooth)

In test group D, cracks were observed in 6 out of the 16 samples.

• Group E (preparation extended into dentin/ contact point on the

veneer)

In test group E, cracks were observed in 9 out of 15 samples.

Results 51

19%

71%

38%

60%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Test group B Test group C Test group D Test group E

Test group B: preparation maintained in enamel / contact point on the tooth

Test group C: preparation maintained in enamel / contact point on the veneer

Test group D: preparation extended into dentin / contact point on the tooth

Test group E: preparation extended into dentin / contact point on the veneer

Fig. 5.3: The crack formation rate of porcelain veneers in the different test groups

Results 52

5.1.2.2. Crack pattern of porcelain veneers (Table 5.1)

Among the different test groups, differences were observed in the position

where the crack had developed. The crack pattern was as follows:

a. no crack

b. palatal (Fig. 5.4)

c. proximal (Fig. 5.5)

d. palatal and proximal (Fig. 5.6)

e. vestibular (Fig. 5.7)

f. circular, extended around the tooth

Fig 5.4: Cracks on the palatal Fig 5.5: Cracks on the prox.

surface of the veneer surface of the veneer

Fig 5.6: Cracks on the palatal and Fig 5.7: Cracks circumferentially

the prox. surface of the veneer on the veneer

Results 53

N no crack palat. prox. pal.+prox. vestib. circular

Group B 16 13 0 3 0 0 0

Group C 14 4 3 0 3 0 4

Group D 16 10 3 3 0 0 0

Group E 15 6 8 0 1 0 0

Table 5.1: Position on the porcelain veneers where the cracks were observed A statistical analysis of the above information is presented in Table 5.2. It

was of interest to compare if a higher crack formation rate occurred when

the contact point was on the veneer (groups C and E) instead of on the

tooth (groups B and D).

Group B and D Group C and E

No crack 23 10

Crack 9 19

Total 32 29*

*3 sampes fractured in the oral environment Table 5.2: Crack formation on the full veneers according to the contact point The pair-wise comparison was performed with the Fisher exact test. The

p-value was 0.005, which showed that there was a significant difference

between the groups.

Results 54

5.2. Fracture strength test

5.2.1. Fracture strength of individual samples

The values at which fracture occurred for each veneered abutment are

mentioned on the following tables, according to group. It must be taken

into consideration that the specimens that failed in the artificial mouth

were considered to have fractured at 50 N, from a statistical standpoint.

That was the cyclic load that they withstood in the artificial environment,

under which they fractured.

• Group A: no preparation

Sample# 1 2 3 4 5 6 7 8

F(N) 689.34 886.36 769.59 558.58 763.62 50.00 737.18 50.00

Sample# 9 10 11 12 13 14 15 16

F(N) 686.08 985.71 848.29 424.02 745.12 50.00 345.67 799.76

No.6 fractured at 530.000 chewing cycles



Results 55

• Group B: preparation maintained in enamel/ contact point on the

tooth

Sample# 1 2 3 4 5 6 7 8

F(N) 848.78 643.78 848.48 649.55 661.73 669.87 816.69 589.26

Sample# 9 10 11 12 13 14 15 16

F(N) 489.77 414.14 644.57 463.51 861.17 698.07 268.72 221.98

• Group C: preparation maintained in enamel/ contact point on the

veneer

Sample# 1 2 3 4 5 6 7 8

F(N) 680.02 580.52 446.54 911.11 50.00 609.00 575.48 454.43

Sample# 9 10 11 12 13 14 15 16

F(N) 804.38 50.00 551.58 721.45 503.19 640.97 827.42 715.53


No.10 fractured at 470.000 chewing cycle

Results 56

• Group D: preparation extended into dentin/ contact point on the

tooth

Sample# 1 2 3 4 5 6 7 8

F(N) 670.99 450.73 363.95 375.16 478.16 483.83 483.76 660.89

Sample# 9 10 11 12 13 14 15 16

F(N) 794.20 400.82 464.56 520.52 597.24 599.87 237.89 792.46

• Group E: preparation extended into dentin/ contact point on the

veneer

Sample# 1 2 3 4 5 6 7 8

F(N) 422.89 300.46 612.74 598.02 655.31 515.35 50.00 547.63

Sample# 9 10 11 12 13 14 15 16

F(N) 462.94 694.18 336.83 226.56 360.24 489.92 537.31 799.85


Results 57

5.2.2. Statistical evaluation of data

The statistical analysis of the recorded fracture strength test results was

performed with a Boxplot representation of the median, quartiles,

minimum and maximum values (Table 5.3; Fig. 5.8).

Min. 1st

Quartile

Median Mean 3rd

Quartile

Max.

Control

group A

50.0 404.4 713.3 586.8 777.1 985.7

Test

group B

222.0 483.2 647.1 611.9 727.7 861.2

Test

group C

50.0 491.0 594.8 570.1 717.0 911.1

Test

group D

237.9 438.3 483.8 523.4 615.1 794.2

Test

group E

50.0 354.4 502.6 475.6 601.7 799.9

Table 5.3: Statistical analysis of the fracture strength test results in N

Results 58

a= Control group A: no preparation

b= Test group B: preparation maintained in enamel / contact point on the tooth

c= Test group C: preparation maintained in enamel / contact point on the veneer

d= Test group D: preparation extended into dentin / contact point on the tooth

e= Test group E: preparation extended into dentin / contact point on the veneer

Fig. 5.8: Box plot of the fracture strength test results in N

Results 59

The result of the pair-wise comparison via Wilcoxon test showed that

there was no significant difference between the groups (Table 5.4).

Group p-value Significance

Group A versus Group B 0.7486 not significant

Group A versus Group C 0.5088 not significant

Group A versus Group D 0.1933 not significant

Group A versus Group E 0.0896 not significant

Group B versus Group C 0.6109 not significant

Group B versus Group D 0.1269 not significant

Group B versus Group E 0.0570 not significant

Group C versus Group D 0.2067 not significant

Group C versus Group E 0.1091 not significant

Group D versus Group E 0.6376 not significant

The difference is only significant when p-value ≤ 0.05

Table 5.4: Multiple pair-wise comparison of the different groups via

Wilcoxon test

Results 60

5.2.3. Fracture patterns

Among the control and test groups, differences were observed in the

manner that the veneered abutments fractured. Several aspects were

examined. First of all, it was of interest to see which component failed

first, i.e the ceramic, the tooth, or both of them. Furthermore, it was

observed in which part of the restoration the fracture occurred.

5.2.3.1. Component of the restoration that failed

When the samples fractured, differences were observed concerning the

component that failed. Three possibilities were reported:

a. fracture of the ceramic (Fig. 5.9)

b. fracture of the tooth (Fig. 5.10)

c. fracture of the ceramic and the tooth (Fig. 5.11)

Fig 5.9: Fracture of the Fig 5.10: Fracture of the Fig 5.11: Fracture of the

ceramic tooth ceramic and tooth

Results 61

The results, according to group, were the following (Table 5.5):

N tooth tooth+ceramic ceramic

Group A 13 13 0 0

Group B 16 11 5 0

Group C 14 7 4 3

Group D 16 4 3 9

Group E 15 4 3 8

Table 5.5: Component of the restoration that failed

Based on the above table, a statistical analysis followed. It was of interest

to see if the thickness of the ceramic or the extension of dentine bonding

influenced the fracture of the full veneers. The thinner veneers in group B

and C were compared to the thicker ones in group D and E. It was

examined in which of the above cases the ceramic has resisted better to

fracture (Table 5.6).

intact ceramic fractured ceramic total

Group B and C 18 12 30*

Group D and E 8 23 31*

*3 samples fractured in the oral environment Table 5.6: Resistance of the ceramic to fracture according to thickness and extension of dentine bonding

The pair-wise comparison was performed with the Fisher exact test. The

p-value was 0.01, which showed that there was a significant difference

between the groups.

Results 62

5.2.3.2. Position of the restoration where fracture occurred

The possible positions, where the restoration fractured, were:

a. the root level (Fig. 5.12)

b. incisally (Fig. 5.13)

c. palatally (Fig. 5.14)

d. circumferentially (Fig. 5.15)

e. the tooth internally (no fracture was clinically observed)

Fig 5.12: Fracture at the root Fig 5.13: Fracture of the

level of the restoration restoration incisally

Fig 5.14: Fracture of the restoration Fig 5.15: Fracture of the

palatally restoration circumferentially

Results 63

The fracture patterns are shown in table 5.7:

N root palatal incisal circum. internal

Group A 13 5 0 0 8 0

Group B 16 4 0 8 0 4

Group C 14 3 6 2 0 3

Group D 16 3 10 3 0 0

Group E 15 1 2 0 11 1

Table 5.7: Position of the restoration where the fracture occurred

Furthermore, every specimen was investigated in order to see if there was

a correlation between the position of the pre-existing cracks and the

position where the fracture took place. No correlation was found.

Discussion 64

6. DISCUSSION

6.1. Discussion of the materials

6.1.1. Extracted human teeth as abutments

In the present study, extracted human maxillary incisors were used as

abutments. This replicates other in vitro studies that were dealt with

porcelain veneers (Simonsen and Calamia 1983; Highton et al. 1987; Clyde and

Gilmour 1988; Hui et al. 1991; Karlsson et al. 1992; Troedson and Derand 1998;

Troedson and Derand 1999; Magne and Douglas 1999d; Magne et al. 1999a;

Castelnuovo et al. 2000; Hahn et al. 2000).

Human teeth were preferred to acrylic (Edelhoff and Sorensen 2002) or

bovine teeth (Andreasen et al. 1991; Munksgaard et al. 1991; Andreasen 1992),

that have been used in other studies. As far as acrylic teeth are concerned,

they present identical properties and dimensions among each other.

However, their properties cannot be compared to those of natural teeth,

giving neither real information about the microstructure of the hard tissue

of teeth after preparation, nor about the micro- and chemomechanical

adaptation of the luting material to the dentine.

The properties of bovine teeth, on the other hand, especially the modulus

of elasticity, the tensile strength and the bonding characteristics, are

similar to those of human teeth (Sano et al. 1994). Therefore, they have

been used to test the fracture strength of ceramic restorations (Andreasen et

al. 1991; Andreasen 1992), the bonding strength of luting materials (Rada

and Jankowski 1991; Phrukkanon et al. 1998) and the fracture resistance of

post systems (Isidor et al. 1999). Nevertheless, they present a great size

Discussion 65

discrepancy in comparison to human teeth, and consequently a difficulty

in the fabrication of the ceramic restoration.

In the case of human extracted teeth, the advantage is that they possess

similar modulus of elasticity, hardness, strength and bonding

characteristics as the teeth in the oral environment. A disadvantage is that

there is a great variation in age, size, shape and quality, making it difficult

to standardize the samples (Morgano and Milot 1993; Isidor et al. 1999). In

order to minimize this danger, an attempt was made to select abutments

with comparable sizes. For that purpose a silicone control key of an

average-sized incisor was used. The extracted human teeth that were too

big or too small, as well as teeth with caries were excluded.

6.1.2. The storage solution

In the present study the extracted teeth were stored in thymol solution.

Other authors have reported the use of distilled water (Andreasen et al.

1991; Andreasen 1992; Reeh and Ross 1994; Castelnuovo et al. 2000), NaCl

(Stacey 1993; Magne et al. 1999a), or 10% formol solution (Stokes and Hood

1993).

A thymol solution of 0.1 % concentration was used in this study to prevent

the specimens from drying out and thereby becoming brittle. The thymol

solution also functioned as an antibacterial means of storage (Sparrius and

Grossman 1989).

6.1.3. EPC as a material for the fabrication of full veneers

In this study an EPC from Ivoclar-Vivadent AG (Schaan, FL) was used for

the fabrication of the full veneers. According to manufacturer it is

considered to have a higher flexural strength (525 Mpa) in comparison to

sintered ceramics. It seems to combinate a higher fracture than Empress I®

(Ivoclar-Vivadent AG, Schaan, FL) and a higher translucency than

Discussion 66

Empress II® (Ivoclar-Vivadent AG, Schaan, FL), similar to the natural

tooth. According to Isenberg et al. (1992) veneers fabricated with such a

pressable ceramic have also a better marginal adaptation compared to the

CAD/CAM or copy milling system.

6.2. Discussion of the methods

6.2.1. Preparation of the specimens for the artificial mouth

6.2.1.1. Preparation

It remains controversial whether different tooth preparation designs can

affect the strength of ceramic veneers, or whether one configuration of

tooth is superior to another. Highton et al. (1987) claimed that preparation

with incisal overlap contributed to a better distribution of load forces and

was consequently less prone to fracture.

Gilde et al. (1989), Hui et al. (1991), Hahn et al. (2000), however, reported

that porcelain veneers without overlap demonstrated more resistance to

load, when compared to overlapping veneers.

Wall et al. (1992b) and Meijering et al. (1997), furthermore, were unable to

distinguish any difference in failure rate between incisal preparation

designs.

However an overlapping veneer preparation is necessary when aesthetics

on the incisal edge third or occlusal corrections demand it (Dumfahrt 2000).

Castelnuovo et al. (2000) showed that if the reduction of incisal edge is

considered necessary then a bevel preparation is preferred than an incisal

overlap.

In this study, the preparation forms that were examined can be used as

restorative options in more severe defects. When a tooth fracture is located

Discussion 67

at the middle third of the crown, or when extended proximal caries and

old fillings are present, a full veneer preparation can be a restorative

option. This “¾ crown preparation” has proven to be a more conservative

solution, thus avoiding the substantial removal of tooth substance,

necessary for a full crown.

Two different aspects were under study, the depth and the palatal

extension of such a preparation. As far as the depth is concerned, in two

out of four test groups the preparation was retained in the enamel.

Although the results of the newest generation dentine adhesive systems

are very promising, the bond strength of porcelain bonded to enamel is

still superior when compared to the bond strength of porcelain bonded to

dentine (Van Meerbeek et al. 1996; Van Meerbeek et al. 1998). This concurs

with the standard preparation design, recommended by the majority of

authors (Calamia 1985; Garber et al. 1989; Troedson and Derand 1998; Hahn et

al. 2000; Peumans et al. 2000). They propose an enamel reduction of 0.5 mm,

since such a preparation offers a stronger bond with the ceramic and

reduces microleakage (Peumans et al. 2000).

In the other two groups, the preparation was extended into the dentine.

Ferrari et al. (1992) suggested that enamel in the gingival area of anterior

teeth is less than 0.5mm. Natress et al. (1995) also showed that in case of

freehand preparation, the proximal and cervical enamel was reduced

more than 0.5 mm in most of the cases. This means that very often the

preparation can be extended into dentine. So, it was desirable to examine

how a thicker ceramic restoration behaves in comparison to a thinner one,

since Magne and Douglas (1999c) reported that with the optimized

treatment of the dentine surfaces, veneers bonded to dentin can be a

successful treatment option.

Furthermore, the extension of the full veneer palatinally was also a subject

of discussion. In the literature, the available studies compared until

recently overlapping and not overlapping patterns. Data about different

Discussion 68

preparation designs palatinally is limited. Magne and Douglas (1999a)

reported in a finite-element study that extensive chamfering into the

palatal concavity must be avoided, due to the accumulation of functional

stresses in this area, and recommended a short chamfer or bevel.

Castelnuovo et al. (2000) also reported the palatal chamfer appeared to be

the weak feature that displayed cracks extending to the facial surface of

the veneer. In our study, instead of a short chamfer, a short bevel was

created to examine if a thicker preparation and consequent ceramic

restoration at the palatinal margin represented a favourable feature in the

prevention of palatinal cracks and provided a better stress distribution.

6.2.1.2. Cementation

Since ceramics have a lower flexural strength, it is the bond to the

underlying tooth structure that provides the full veneers the strength

necessary to resist the incisal shear loads and survive intraorally.

In the present study, the ceramic surface was etched (IPS Ceramic etching

gel®, Ivoclar-Vivadent AG, Schaan, FL) and silanated (Monobond-S®,

Ivoclar-Vivadent, Schaan, FL) as the majority of authors reported

(Simonsen and Calamia 1983; Calamia 1985; Friedman 1998; Hahn et al. 2000).

Malament and Socransky (1999) reported the importance of acid etching for

the survival of Dicor glass-ceramic restorations. Nonacid-etched

restorations had a 2.2 times greater risk of failure compared to the acid-

etched ones. Calamia and Simonsen (1983) also showed that silane coupling

agents improved the bond strength to a resin composite-based luting

material by about 20%.

For the conditioning of the tooth surface, a total etch system (Total Etch®,

Vivadent-Ivoclar AG, Schaan, FL) for both enamel and dentin was

preferred. As mentioned, in half of the specimens the preparation was

extended into dentine. In the other half, where the preparation was

maintained in the enamel, there was always a suspicion of dentine

Discussion 69

exposure at the cervical and interproximal margin. For the above reasons

and since all specimens should be identically treated, the total-etch

technique was chosen.

As far as the luting agent itself is concerned, a dual-cured resin (Variolink®

II, Vivadent-Ivoclar AG, Schaan, FL) was used. The use of light cured

resins is also possible, as long as the right polymerization is guaranteed by

light-curing. Shade, thickness, and transmission coefficient of the bonded

ceramic restoration and the composite itself influence the conversion rate

of the light-activated material and could limit its application.

Polymerization can be considered safe up to 3mm distance from the tip of

a standard curing light. For security reasons and since half of the veneers

were thicker than 1mm, Variolink® II, a dual-activated composite, was

preferred. It offered extended working times and ensured a high degree of

polymerization.

As the manufacturer recommended, a high viscosity luting paste was

chosen. A high filler ensures a higher bond strength, reduces the thermal

expansion coefficient and the amount of polymerisation shrinkage (Sim et

al. 1994). Thus, microleakage at the dentine/composite interface can be

minimised (Peumans et al. 2000). Furthermore, the removal of the excess

cement is facilitated and an overreduction of the material at the luting gap

during cleaning is avoided.

6.2.1.3. Artificial membrane

Tooth mobility is a decisive factor in the evaluation of fracture strength

(Kelly 1999) and its effect is contoversial. Kelly et al. (1996) showed that

when a small amount of abutment rotation is allowed, failure is more

likely.

Kern et al. (1993) showed that the use of an artificial periodontal membrane

did not significantly affect the fracture strength of porcelain resin bonded

Discussion 70

bridges in comparison to abutments which were directly embedded in

acrylic material.

Kappert and Knode (1990) tested the fracture strength of 3-unit bridges on

metal dies. They used elastic rings and silicone material around the teeth

in half of the samples to simulate physiological tooth mobility. They

reported a mean fracture strength of 703 N for the group with

physiological tooth mobility in comparison to 225 N for the group with no

abutment mobility.

Furthermore, the materials used in order to achieve the tooth mobility

vary in the literature. In some studies, samples were embedded directly

into the resin (Beschnidt and Strub 1999), when the periodontal movement

was already foreseen in the chewing simulator. Other authors have

embedded their samples in thiokol resin (Stokes and Hood 1993), in nylon

rings and orthodontic resin (Reeh and Ross 1994), or in phenol rings and

epoxy resin (Castelnuovo et al. 2000). In the present study, the simulation of

the physiological tooth movement was desirable, in an attempt to

resemble the clinical situation. For this reason, a thin 0,5 mm layer of gum

resin (Anti-Rutsch-Lack®, Wenko-Wenselaar, GmbH Hilden, D) was

applied on the root of the incisors, up to 2 mm below the cementoenamel

junction. This simulated the position of the alveolar bone with regard to

the biological width (Gargiulo 1961; Kois 1994). Kern et al. (1993) showed

that this method could imitate the tooth mobility during dynamic loading

and fracture strength testing.


6.2.2.1. Angulation in the artificial mouth

Load angulation can represent a crucial factor when testing the strength of

the teeth. In some studies that compared fracture strength of different

Discussion 71

designs for ceramic veneers, the specimens were tested by loading the

veneer-tooth system directly at the incisal edge and in a direction parallel

to the long axis of the tooth (Highton et al. 1987; Hui et al. 1991).

Nevertheless, the orthognathic interincisal angle in case of natural

dentition is 135° (Reitz 1973). Stresses that affect maxillary ceramic veneers

during mastication and protrusive mandibular movements are not usually

directed parallel to the long axis of a tooth. Elevated loads generated

during parafunction are not applied in this direction either. Such loads are

usually directed toward the palatal surface of maxillary incisors and are

not parallel to the long axis of the tooth (Castelnuovo et al. 2000).

For the above reasons, in the present study the veneered abutments were

fixed in the specimen holder at a 135° angle to the saggital plane and 2° to

the vertical plane. This concurs with other studies (Castelnuovo et al. 2000;

Hahn et al. 2000), that also tried to simulate the clinical situation.

6.2.2.2. Contact point of the applied force in the artificial mouth

In the present study, the force was applied palatinally on the veneered

abutment, 6 mm apical to the incisal edge. Thus, an attempt was made to

reproduce the clinical situation, since the points of application of the forces

during occlusion on maxillary anterior teeth are predominantly within a

range of 2-6 mm from the incisal edge (Coca and Schwickerath 1987).

6.2.2.3. Artificial ageing and its clinical relevance on the survival rate of

ceramic veneers

In order to consider a dental material or design acceptable for clinical use,

5-year clinical results should be available (Strub 1992). However, clinical

studies are time consuming, expensive and, additionally, it is often

difficult to standardize the test parameters (Kern et al. 1999). Therefore,

various in vitro tests have been developed to simulate clinical conditions

and to save evaluation time (Krejci et al. 1994; Kern et al. 1999).

Discussion 72

Intraorally, occlusal forces create dynamic repetitive loading. Therefore,

instead of monotonic static loading, it is more clinically relevant to test the

specimens under physiologic fatigue load in a chewing simulator (Krejci et

al. 1994; Kelly 1999; Kern et al. 1999). Thus, the test sample and mode of

loading chosen simulate more faithfully the actual component in function.

Krejci et al. (1994) suggested that the chewing machine fullfils the

parameters concerning chewing motion and thermal changes reported in

the literature. For the above reasons, in the present study a dual-axis

chewing simulator was used for the artificial ageing.

All parameters assessed for the chewing simulation were based on values

already used by investigators in previous studies. Fatigue produced by

240.000 to 250.000 masticatory cycles in a chewing simulator corresponds

to the results obtained after 1 year of clinical service (De Long 1985;

Sakaguchi 1986). Therefore, to simulate a service time of 5 years, a total

number of 1.200.000 masticatory cycles were set to be performed in the

chewing simulator (Kern et al. 1999).

A force of 49 N was applied during the dynamic loading. This reflects

mean values of chewing forces in the anterior dentition according to the

literature. Kalipcilar and Kedici (1993) reported maximal biting forces of 94.1

N for the incisors, and Kelly (1999) reported that the physiologic forces in

the anterior region during chewing and swallowing range between 5N

and 364 N.

Thermocycling with temperature changing from 5°C to 55°C was also

performed. This is often reported by other authors in order to evaluate the

long-term success of restorations (Kern et al. 1993; Krejci et al. 1994;

Beschnidt and Strub 1999). This is in accordance to Palmer et al. (1992) who

reported minimum and maximum temperatures intraorally between 1.0°C

and 58.5°C and suggested a range of 0°C to 67°C for the thermocycling

tests of dental materials.

Discussion 73

6.2.3. Load-to-fracture testing

6.2.3.1. Angulation in the Zwick testing machine

In the present study, the specimens were tested in the Zwick universal

testing machine, until fracture occurred. They were set at an angle of 135°

to their long axis, like in the artificial mouth. The angulation is in

agreement with the intraoral orthognathic interincisal angle of 135°, as

mentioned before (Reitz 1973).

In the literature, however, other angulations have also been used. Highton

et al. (1987) loaded specimens at an angle of 0° to the tooth long axis.

Castelnuovo et al. (2000) loaded specimens at an angle of 90°. Troedson and

Derand (1999) loaded specimens at an angle of 0°, 30°, 60° to the tooth long

axis and reported higher values in the case of 0°.

Nevertheless, Hahn et al. (2000) tested an angle of 135°, that similar to our

study was considered to be the one closer to the clinical situation.

6.2.3.2. Contact point of the load-to-fracture applied force

In our study, the veneers were loaded until fracture on their incisal edge

(Hölsch 1992). This is comparable with other studies, like Highton et al.

(1987), Gilde et al. (1989), Hui et al. (1991), and Troedson and Derand (1999),

who loaded specimens to fracture directly on the incisal edge.

However, Castelnuovo et al. (2000) loaded 2,5 mm apical to the incisal edge.

Andreasen et al. (1992) and Stokes and Hood (1993), on the other hand,

loaded specimens vestibularly.

In the clinical situation, the occlusal forces are applied approximately 3

mm apical to the incisal edge (Coca and Schwickerath 1987). However, our

study required the total force to be applied on the veneer, which is the

weakest feature in our restoration design. If a palatinal loading pattern

was to be chosen, then the tooth surface would have been loaded in half of

Discussion 74

our specimens (in the groups were the contact point was on the natural

tooth). This could have lead to a failure of the tooth first and not of the

veneer. This was not the purpose of our test.

Furthermore, a double layer of 1mm tin foil was placed on the incisal edge

of each veneer. The goal was to avoid a loading on one point only and to

ensure a homogeneous stress distribution (Hölsch 1992).

6.2.3.3. Clinical relevance of the fracture strength tests of ceramic

veneers

Fracture strength tests of ceramic restorations are of substantial

importance in deciding the efficiency, longevity and subsequent success of

the reconstructions. Ideally, in vitro load-to-failure tests should simulate

clinical failure, in order to evaluate new materials or designs and

recommend them for clinical use.

Kelly (1999) questioned the clinical validity of the such tests. He suggested

that significant differences were found between the failure behaviour

created during traditional load to failure tests and that observed to have

occurred during clinical failure of all ceramic restorations. Examination of

clinically filed glass-ceramic restorations revealed that failures initiated

from flaws and stresses existing at the cementation surface. In contrast, in

vitro tests induce failure from damage at the occlusion surface.

Another problem of laboratory testing is that extremely high loads are

required compared with those during mastication, swallowing or

clenching movements. He concluded that traditional laboratory tests fail

to (1) create appropriate stress states, (2) cause failure from clinically

relevant flaws, or (3) create crack systems modelled on clinical failure.

For the above reasons, several sections found in the dental literature

regarding the strength and function of ceramic veneers could be

misleading. More realistic conditions must be produced, so that the

clinical situation can be reproduced more closely.

Discussion 75

6.3. Discussion of the results


6.3.1.1. Survival rate

In group B and D, where the load was applied directly on the natural

tooth, a survival rate of 100% was reported. In Group C and E, where the

load was applied on the full veneers, a lower survival rate of 88% and 94%

respectively, was reported.

Although it seems that group B and D had a higher survival rate, there

was no statistical difference between the groups. Furthermore, in all cases

of failure, the fracture occurred on the root and not at the palatal or any

other surface of the tooth. The veneered crown remained intact. The same

root failure pattern was observed in the control group of the unprepared

teeth. Thus, it is concluded that the tooth structure was the one that failed,

and specifically in the control group in a higher level than in the test

groups.

Based on the above, it was shown that all kinds of full veneer preparation

in our study provided a safe treatment option that did not influence the

survival rate of the teeth. The restorations with full veneers did not seem

to reduce the rigidity and original stiffeness of the teeth, compared with

the unrestored ones in the control group.

This coincides the results of Magne and Douglas (1999c), who also

concluded that veneers, even when bonded to dentine, can restore both

the mechanical behaviour and microstructure of the intact tooth. In their

study they showed that the crown compliance increased by a factor of 2.16

after facial enamel removal and returned to 96% of its original value after

Discussion 76

the placement of the veneer. Similar results were found in the study of

Magne and Douglas (2000a), who reported no significant differences in

crown flexure between natural and veneered incisors. Even when

composite build-ups were used to replace the missing dentine in cases of

fracture defects, the original tooth compliance was almost restored (Magne

and Douglas 1999d).

6.3.1.2. Crack formation

After the dynamic loading in the artificial mouth, the full veneers were

examined under the stereomicroscope. Crack formation was observed at

64% of the samples.

The rate of the existing cracks was higher in groups C (69%) and E (56%),

where the antagonist was coming in contact with the full veneer. In

groups B and D, on the other hand, where the antagonist was coming in

contact with the natural tooth, the reported rate was lower (19% and 38%

respectively). The statistical analysis showed a significant difference

between the two categories.

Based on the above results, it can be concluded that the extension of full

veneers palatally plays a preliminary role in crack formation. And since

this crack formation is a possible consequence of polymerization

shrinkage, function and thermocycling (Magne et al. 1999a), these are the

parameters that have to be relatively examined.

Initially, thermocycling have the same influence in all groups and can be

thus excluded as a reason that caused the above results. On the other

hand, function seems to be decisive. When the antagonist is coming in

contact with porcelain, numerous cracks are developed. According to the

literature, the level of the fossa happens to be the area of maximum tensile

stress (Magne et al. 1999a). Because of the ceramic brittleness, these tensile

forces are the most detrimental for the restoration during mechanical

Discussion 77

fatigue (Magne and Douglas 2000b). Thus, the increased stress accumulation

in that area leads to the development of several cracks.

Another point in this study was the fact that the increase of the porcelain

thickness between groups C and E did not seem to prevent the high crack

formation (from 69% to 56%). Between the two groups there was no

statistical significance in crack formation. This is in accordance with Magne

et al. (1999a) who demonstrated that the ceramic thickness itself does not

seem to influence the development of flaws. They reported no significant

differences in the ceramic thickness in relation to crack propensity

between cracked and uncracked specimens.

The critical factor was instead the ratio of cement to porcelain veneer

thickness that distinguishes cracked laminates from intact laminates.

Magne et al. (1999b) supported that a thicker ceramic with a minimal

thickness of luting composite provides the restoration with a favourable

configuration with regard to crack propensity. The same results were

reported by Magne et al. (1999a), who suggested a ceramic to luting

composite thickness ratio above 3 as a favourable one.

Furthermore, another topic of this study was the position of the crack

formation. In groups C and E, the specimens displayed cracks that started

from the contact point palatally and extended to the rest of the palatal

surface or further to the proximal and facial areas. On the other hand, in

group B, cracks were observed only proximally, and in group D 50%

proximally and 50% palatally.

This can be explained for group C and E, by the particular geometry of the

area. Concavities, like the palatal one, demonstrate higher stress levels,

than smooth and convex surfaces. When the veneers are palatally directly

loaded, it is rational that a crack formation starts from that point and

extends to the rest of the tooth.

On the other hand, in groups B and D, where there is no direct loading on

the full veneer, a tendency of crack formation is observed in the proximal

Discussion 78

surface rather than in the palatal. In the literature, there is no available

data explaining the above phenomenon. A speculation could be that

ceramic is slightly thinner in the proximal aspect than in the neighbouring

palatal or incisal aspect, making thus the veneer there prone to cracking.

6.3.2. Load-to-fracture testing

6.3.2.1. Fracture strength of the specimens in the Zwick universal

machine

The specimens that fractured in the artificial mouth could not be tested in

the universal testing machine. From a statistical standpoint they were

considered to have fractured at 50 N, since that was the cyclic load that

they withstood in the artificial environment, under which they fractured.

The full veneers in Group B and C, that were bonded to enamel, presented

mean fracture strength values of 611.88 N and 570.1 N consequently. The

full veneers in Group D and E, that were bonded to dentin, presented

mean fracture strength values of 523.41 N and 475.6 N consequently.

All the above mean fracture strength values were within the limits of

clinical acceptance when compared with the physiologic forces developed

in-vitro. According to the literature, Helkimo et al. (1977) reported average

values of 137 N for the incisors. Sonnenburg et al. (1978) found that the

average load values for anterior teeth are between 215 N and 360 N for

men and 115 N and 269 N for women and Kalipcilar and Kedici (1993)

reported maximal biting forces of 94.1 N for the incisors. Kelly (1999)

reported that the physiologic forces in the anterior region during chewing

and swallowing range between 5 N and 364 N, and the maximum forces

during clenching between 216 N and 890 N.

The question whether the bonding of veneers to enamel, in comparison to

dentine, would provided higher values of fracture strength was negatively

Discussion 79

answered by these results. Among the groups, the differences were not

found to be statistically significant.

In-vitro, there is limited data comparing the fracture strength of veneers

bonded to these two different surfaces. The studies of Castelnuovo et al.

(2000) and Hahn et al. (2000) compared different preparation designs

within the enamel. They report values between 164-310 N for maxillary

incisor and 466-653 N for mandibular incisors respectively. The great

difference between the values is the support provided by the jaw and

varying angle of loading. Castelnuovo et al. (2000) loaded speciments at an

angle of 90° to the tooth long axis, whereas Hahn et al. (2000) loaded

speciments incisally as in this study. That is why the last results can be

comparable to the ones in our study, concerning only groups B and D,

where the preparation remained in the enamel.

On the other hand, Magne and Douglas (1999c) and Magne and Douglas

(2000a) experimented with dentine-bonded porcelain veneers in relation to

crown flexure and stiffness, but they did not study their fracture resistance

compared to strength. In-vivo Dumfahrt (2000) reported that in 6 out of 7

cases, the failures were located at the restoration-dentine interface,

supporting the suggestion of Christensen and Christensen (1991) that the

connection of resin cement to dentine may be the weakest link in bonding

porcelain veneers. To this weak link are also attributed the lower fracture

values in groups D and E.

6.3.2.2. Fracture pattern of the specimens in the Zwick universal

machine

After the load-to-fracture testing, three failure patterns were observed. In

some cases it was the tooth that fractured, in some others the ceramic, or a

third possibility was that both the tooth and the ceramic broke.

Discussion 80

It was of interest to see whether the thickness of the ceramic influenced the

resistance of the material to fracture. In groups B and C, where thin

ceramic veneers were bonded to enamel, 18 out of the 30 ceramic samples

remained intact and it was the tooth that broke.

Another scenario was often recorded in group D and E, where thicker

ceramic veneers were bonded to dentine. In 23 out of the 31 specimens, a

fracture was observed on the ceramic. In some of the above cases (6) the

tooth broke at the same time, but in most of them (17) only the veneers

fractured and not the tooth.

The above results were statistically analyzed and a significant difference

was found between the two groups. Thinner veneers remained intact more

often after fracture in comparison to thicker ones. In concurrence with

extensive literature on the subject, it is proven that the resistance of a

ceramic restoration depends not on the thickness of the ceramic, but on the

strength of the adhesion complex. Characteristically, Stacey (1993) reported

that the strength of the complex porcelain/luting composite/enamel (63

MPa) was almost double than that of the separate composite/etched

enamel (31 MPa) and the separate luting composite/porcelain (33MPa)

bond strengths. Andreasen et al. (1991) and Stokes and Hood (1993) proved

also that extracted incisors restored with porcelain veneers recovered to

their original strength.

In groups B and C, where the thinner ceramic was luted on enamel, the

composite/etched enamel bonding was so strong that it resisted the

transmitted stresses and the tooth structure failed first. On the contrary,

although thicker, the veneers that were bonded to dentine fractured in

most of the cases first, leaving the tooth intact. This can be also explained

by the fact that the link in bonding porcelain veneers is the weakest at the

restoration-dentine interface (Christensen and Christensen 1991).

Discussion 81

Initially, it seems that thinner veneers behaved better in our in-vitro study.

But it has to be taken into consideration that clinically a full veneer

restoration is a treatment option in cases of more extended defects, where

less enamel is left. Thus, it is clinically important that in case of the dentin

bonded veneers, if a failure is to happen, it is more likely that the ceramic

will fail, protecting the tooth structure.

7. CONCLUSIONS

The following conclusions can be drawn from this study:

(a) Full veneers are indicated in clinical practice, since all mean fracture

strength values obtained were within the limits of clinical acceptance.

Thus, the removal of large amounts of sound tooth substance, necessary

for a full crown preparation, can be avoided.

(b) According to the fracture strength values detected, restorations with

thin labial ceramic surfaces bonded to enamel were as successful as

restorations with thick labial ceramic surfaces bonded to dentine.

(c) It is preferable that the contact point remains on the tooth and not on

the ceramic. Thus, the danger of crack formation is minimised.

(d) The EPC (48 samples) in combination with an adhesive bonding

system (Variolink® II, Ivoclar-Vivadent AG, Schaan, FL) seems to be a

promising material for the restoration of anterior teeth with full veneers.

Clinical data is necessary before the EPC is indicated for everyday clinical

use.

Summary 82

8. SUMMARY

In this study, the survival rate and fracture strength of different kinds of ceramic

full veneers, fabricated with a new experimental press ceramic (EPC), were

evaluated after exposure to the artificial mouth.

Eighty, caries-free human maxillary central molars were used as abutments for

the full veneers. The teeth were randomly divided into one control group and

four test-groups of 16 samples each. In control group A, unprepared teeth served

as a control. In the test groups, four different types of full veneer preparations

were performed. In test group B, the preparation was maintained in enamel and

the contact point was on the tooth. In test group C, the preparation was

maintained in enamel and the contact point was on the veneer. In test group D,

the preparation was extended into dentin and the contact point was on the tooth.

In test group E, the preparation was extended into dentine and the contact point

was on the veneer. In all test groups, heat-pressed glass ceramic was used for the

fabrication of the porcelain veneers. They were luted using Variolink® II (Ivoclar-

Vivadent AG, Schaan, FL). All samples were exposed to the artificial mouth.

After exposure in the artificial mouth, a survival rate of the veneers of 81-100%

was reported among the different groups, but no failures of the ceramic materials

could be recognised. The mean fracture strength in group A was 586.34 N, for

group B 611.9 N, for group C 570.1 N, for group D 523.4 N and for group E 475.6

N.

Among the different groups no statistically significant differences were found.

All mean values obtained were within the limits of clinical acceptance, indicating

that the use of full veneers and the EPC is acceptable. Furthermore, it seems to be

preferable to maintain the contact point of the adjacent tooth on the tooth

structure. Thus, it may result in reduced crack formation.

Zusammenfassung 83

9. ZUSAMMENFASSUNG

Ziel dieser Studie war die Bestimmung der Überlebensrate und Frakturresistenz

unterschiedlicher Arten von Vollveneers, nach Exposition im künstlichen Mund,

hergestellt mittels einer neuen experimentellen Presskeramik (EPC).

Als Ankerzähne für die Vollveneers dienten 80 kariesfreie menschliche

Oberkiefer-Zähne. Die Zähne wurden mittels des Zufallsprinzips in eine

Kontrollgruppe und 4 Testgruppen zu jeweils 16 Zähnen aufgeteilt. In Gruppe A

dienten unpräparierte Zähne als Kontrollgruppe. In den Testgruppen wurden 4

unterschiedliche Arten von Vollveneer- Präparationen vollzogen. In Testgruppe

B waren die Präparation vorwiegend im Schmelz und der Kontaktpunkt auf der

natürlichen Zahnhartsubstanz. In Testgruppe C war die Präparation vorwiegend

im Schmelz und der Kontaktpunkt war auf der Zahnhartsubstanz. In der

Testgruppe E reichte die Präparation bis in das Dentin und der Kontaktpunkt

war auf dem Veneer. In allen Testgruppen wurde für die Anfertigung der

Keramischen Veneers eine Presskeramik verwendet. Die adhäsive Befestigung

fand mittels Variolink® statt. Danach wurden alle Proben dem künstlichen Mund

ausgesetzt.

Nach der Exposition im künstlichen Mund wurde eine Überlebensrate von 81-

100% in den verschiedenen Testgruppen und keine Misserfolge des keramischen

Materials beobachtet. Die durchschnittliche Frakturresistenz in Gruppe A war

586.34 N, für Gruppe B 611.9 N, für Gruppe C 570.1 N, für Gruppe D 523.4 N und

für Gruppe E 475.6 N.

Innerhalb der verschiedenen Gruppen war kein statistisch signifikanter

Unterschied festzustellen. Alle so erhaltenen Durchschnitteswerte waren

innerhalb der klinischen Akzeptanz und zeigten somit, dass die Anwendung von

Vollveneers und von EPC vertretbar ist. Weiterhin scheint vorzuziehend den

Kontaktpunkt auf dem Zahn zu belassen, da so Rissbildungen reduziert werden

können.

References 84

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Curriculum vitae 97

11. CURRICULUM VITAE

Date of birth: October 11th, 1975

Place of birth: Athens, Greece

Parents: Stathopoulos Georgios

Stathopoulou Irini

Marital status: Single

Nationality: Greek

Education: 1987-1993 College “Saint-Paul”,

Athens, Greece (High School Certificate)

1994-2000 Faculty of Dentistry, University of

Athens, Greece

2000-2003 Postgraduate Student, Department of

Prosthodontics, Albert Ludwigs

University, Freiburg, Germany.

Program director: Dr. R.J. Kohal,

Associate Professor

Acknowledgements 98

12. ACKNOWLEDGEMENTS

I would like to express my most sincere gratitude to Prof. Dr. J. R. Strub,

Chairman, Department of Prosthodontics, Albert Ludwigs University,

Freiburg, Germany for offering me the possibility to conduct research

under his supervision, as well as for his revision of this manuscript.

I would also like to express my gratitude to the Bakala Foundation,

Athens, Greece for supporting me financially through a two-year

scholarship.

I would also like to thank:

Dr. C. F. J. Stappert, Senior Lecturer, Department of Prosthodontics, Albert

Ludwig University, Freiburg, Germany, for the idea for the present

research and his continued help, advice and support during the entire

period of the present study.

Dr. P. Hahn, Associate Professor, Department of Operative Dentistry and

Periodontology, Albert Ludwigs University, Freiburg, Germany for the

revision of this manuscript.

Mr. H. P. Foser, Chief, Dental Laboratory, Ivoclar-Vivadent AG, Schaan,

FL for the fabrication of the porcelain laminate veneers.

Dr. M. Krah, Division of Dental Materials, Albert Ludwigs University,

Freiburg, Germany for his help in carrying out the fracture strength tests.

Dr. W. Att for his guidance and Dental technician W. Stathopoulos for his

support in operating the artificial oral environment.

Mr. T. Gerds, Institute of Medical Biometry and Medical Informatics,

Albert Ludwigs University, Freiburg, Germany, for the statistical analysis

of the data.

Table of contents

2.6. survival rate and fracture strength of porcelain veneers

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