non-retentive adhesively retained all-ceramic posterior restoration

8/13/2019 Non-Retentive Adhesively Retained All-ceramic Posterior Restoration

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Non-Retentive, Adhesively Retained All-Ceramic Posterior Restoration

Gregg A. Helvey, DDSInside Dentistry , February 2011

Abstract

The same principles for crown preparation have been taught for over a century. As restorative and

cementing materials have improved, the preparation techniques have mostly remained the same. Adhesive

dentistry has changed many of our standard restorative techniques because retention is derived from other

sources. Ceramo-metal crowns, which have been the standard, are giving way to all-ceramic versions. As the

digital age of dentistry emerges, different preparation techniques can be employed with proper case

selection. The non-retentive, adhesively retained all-ceramic posterior crown (table-top! offers a more

conservative approach for single-tooth restorations.

Adhesive dentistry has brought many changes in the waya dentist provides a restoration. Although the changes

seem to demand more technical expertise, there is a vast difference from the days when drilling out decay, placinga matrix band, and plugging the hole with amalgam was the norm. With respect to posterior crowns, the trend has

gone from an all-metal crown to ceramo-metal to, now, all-ceramic. This trend could not have happened without

adhesive bonding technology. Generation after generation of new adhesive bonding systems have kept practicing

dentists constantly changing their protocol for placing restorations. anufacturers vying for market share will

continue to bring new ideas and techni!ues to the restorative practice.

"onding philosophies have changed over the years from being solely enamel-dependent to the development of

systems that rely on the dentin as an additional viable adhesive substrate. The one step in the entire restorative

process that has not received much attention is the design of the posterior all-ceramic tooth preparation. Therestorative process includes identification of the pathology, proper preparation of the tooth to receive the

restoration, and diligent insertion of that restoration. Technology has provided added tools in the caries

identification process, and newer bonding adhesives have reduced the technical expertise re!uired to place a

sensitive-free restoration, but the tooth preparation has been somewhat overlooked. ost posterior crown

preparations today are still based on anti!uated cementing techni!ues. The traditional resistance form and retention

form provided by axial wall reduction is still in the minds of dentists while they prepare a posterior tooth for a

crown. This is where the confusion lies. #emented crowns need resistance and retention form while adhesively

retained crowns need enamel.$The tooth preparation should be designed for the restorative material planned and

the retentive material that will be used for placement, as opposed to the restorative and retentive materials adapting

to the tooth preparation. %emoving only the pathology from a tooth and replacing the missing parts of the tooth

with a ceramic or industrial-composite material should be a consideration. This would result in less tooth structure

likely being removed. The non-retentive, full-coverage &table-top' preparation for posterior metal-free restorations

should be considered.

%e!uirements for an All-#eramic, Adhesively %etained %estoration


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There are a number of factors to consider when planning an all-ceramic or industrial-composite restoration. (irst,

the adhesive potential$ and the amount of enamel should be considered. The si)e of the existing restoration and the

extent of the pathology should be evaluated to determine the amount of remaining enamel that can be used for

adhesive retention. *econd, the functional environment must be assessed, examining it for any parafunctional

activity and the occlusal forces that are present must be examined.+,The location of the tooth in the arch bite

forces increase the further posterior the tooth,/the type of occlusal pattern, and the steepness of the incline planes

of the ad0acent teeth also must be considered. The generation of lateral stresses may be detrimental to the adhesion

portion of the restoration especially if the patient demonstrates significant parafunctional activity. Antagonistic

wear must be considered. An industrial #A12#A block or laboratory-processed composite or an all-metal

restoration may be more effective in the long term.3Third, determine if there will be ade!uate isolation, which is

critical to the success of adhesive bonding.4(ourth, gauge the ability of the ceramic composite material to match

or blend in with the inherent tooth shade.56astly, consider the patient7s desire and acceptance of the informed

consent presented.

8ncidence of #usp (ractures8t is not uncommon to find fractures at the axial-pulpal line angles after removal of a large 91 amalgam. 8n fact,

(ennis et al:found in the 1utch population fractured cusps occurring at a rate of +;.3 per $;;; examined. = than premolars +$=. axillary molars presented more fractures of buccal cusps

44= versus /=, while mandibular molars presented more fractures of lingual cusps 53= versus +3=. "ader et

al>found in their study of restored posterior teeth two clinical features that were strongly associated with the risk of

cusp fracture. The proportional si)e of the restoration and the presence of a fracture line were indicators for

posterior fractures. The type of direct restorative material does not seem to play a role in the increased occurrence

of previously restored posterior teeth. Wahl et al$;did not find a significant difference in the number of cusp

fractures in teeth restored with amalgam versus composite, although in the general practice it may seem more

fractures are found associated with amalgam-restored than composite-restored teeth. The fact that amalgam

restorative has been in service longer and more teeth have been restored with amalgam may account for this

observation.

According to Agar and Weller$$and Abou-%ass$+asymptomatic cracks that are identified with direct vision and

transillumination are precursors to cracked tooth syndrome and tooth fracture. %atcliff et al $proposed a

classification of types of cracks in teeth. Type 8 included cracks in posterior teeth with no restoration present, no

stain in the cracks, and were asymptomatic. Type 88 included cracks in posterior teeth with #l 8 or #l 88

restorations, no stain in the crack, and exhibit no symptoms. Type 888 included stained cracks detectable with an

explorer with either no restoration, or #l 8 or #l 88 with no symptoms other than mild sensitivity to sweetness

and2or temperature. Type 8? included cracks that produce &bite and release' pain and thermal and sweetness

sensitivity cracked tooth syndrome. This study also concluded that it is possible for these cracks to look like Type

8, 88, or 888 cracks.


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Traditional #rown @reparation ?ersus &Table-Top' @reparation

any clinicians were taught full-crown preparation principles that included resistance and retention form gained

from axial wall reduction. These principles were taught before the advent of adhesive dentistry. A paradigm

shifthas occurred in the preparation re!uirements for adhesively retained all-ceramic crowns. *ome dentists may be

reluctant to fully accept the retention strength of adhesives and feel that their crown preparations must still include

the retention and resistance form principles that were ingrained into them while in dental school. Traditional crown

preparations actually remove the tissueenamela material with which adhesion works so well. A split section of

an intact molar will show from an axial perspective that there is a wide band of enamel located in the supra-bulge

area approximately halfway down the clinical crown while the thinnest amount of enamel is located at the gingival

margin. Therefore, traditional crown preparation that includes axial reduction will actually reduce the surface area

of enamel, decreasing the bond strength of the restoration that is gained by the enamel substrate.$An alternative

approach to the traditional crown preparation is non-retentive, adhesively retained &table-top' crown preparation.

8n select cases, it allows for maximum preservation of the enamel, reducing the amount of tooth preparation and

creating more supra-gingival margins. Therefore, the &table-top' preparation actually increases the amount ofsurface area of bondable enamel, thus increasing the bond strength of the restoration to tooth substrate.

*tudies show that the adhesive substrate does have an influence on the fracture resistance of ceramic restorations.$3

There is higher fracture resistance for ceramic restorations that are bonded with resin to enamel versus those

bonded to dentin.$4-$:

Tensile stress is the predominant factor controlling the initial failure of ceramics. The critical tensile stress is

dependent on the elastic modulus mismatch of the ceramic, cement, and supporting material.$>%ekow et al used a

series of finite element models of an axisymmetric styli)ed ceramic crown-cement-tooth system in their factorial

analysis on the variables that influence the stress in all-ceramic crowns. They concluded that the crown materialand thickness were the primary importance in stress magnitude, but other variables cement modulus, load

position, and supporting tooth core also contribute to the stress magnitude.+;

&Table-Top' @reparation Techni!ue

When a tooth has been treatment planned for a full-coverage restoration, a traditional crown preparation is usually

the procedure of choice. Bois states that cases that re!uire cuspal protection but still maintain significant structural

integrity in an axial dimension may be suited for an adhesively retained restoration.$

The following steps show the progression of the &table-top' preparation for a typically encountered molar with a

large -surface amalgam and that has been diagnosed with &cracked tooth syndrome.' (or instructional purposes,

in this article the steps are performed on a dentaform model. Cnderstanding that this media represents the ideal

scenario will give the reader a better visual of the steps involved using this particular preparation se!uence.

Whether to cover the cusp or not is a clinical decision.


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8n addition to advocating the one-third2one-half rule, #hristensen+also warns the clinician to consider other

factors. These include the presence of hori)ontal cracks in the tooth structure, a lack of supporting dentin under the

cusp, the presence of a heavy occlusion, and highly discolored cusps in esthetically important areas. 6 carbide bur can be used. Beeping the shank of the bur parallel with the pulpal floor, a cut is made under

the entire mesialEbuccal cusp starting at the buccal groove to the mesial marginal ridge depth cut (igure /and

(igure 3. The bur is then moved back to the buccal groove and directed to the distal marginal depth cut, which

removes the entire distalEbuccal cusp (igure 4. The handpiece is then positioned on the lingual aspect of the

tooth and both the mesial and distalElingual cusps are removed (igure 5.

At this point a minimum of + mm clearance has been provided for the ceramic material. 8f any of the existing

restorative material still remains, it is then removed with a modified shoulder diamond bur. This removal will add

to the final thickness of the all-ceramic restoration and also create an isthmus that will provide orientation as a

positive seat insertion of the restoration (igure :. 8f there is no remaining restorative material an isthmus should

still be provided for orientation purposes.

8nterproximal Area

The interproximal areas become involved when there is an existing restoration, fractures, or caries. Csing the same

diamond bur, the restorative material is removed from the mesial and distal interproximal box areas. This step

lowers the interproximal margins in a more cervical direction and also provides further orientation guidance for

seating the restoration.

8n cases where there is no interproximal restorative material present and the contact is still intact after the +-mm

reduction is completed, a decision must be made to either break the contact or leave it intact. (rom a laboratory

standpoint or if a chairside digital scanner is being used to ac!uire the image of the preparation, breaking the

contact will enhance the ability to locate the margin. A D5:;$ $+-fluted finishing bur is used to break the

interproximal contact (igure >. The bur is placed on either the buccal or lingual side and swept through the

contact area. The thickness of this si)e bur provides sufficient room for impression material or a scanable view

with a digital ac!uisition camera. The outer contour of the tooth will be flattened as the bur is passed through the


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interproximal space as irregularities in the anatomical form are eliminated. The flat margin enhances the margin

tracing in a digital scenario as well as a traditional laboratory setting.

An end-cutting diamond bur is then used in the interproximal box area to eliminate any abrupt dimensional

vertical2hori)ontal platform-to-wall transition changes (igure $;. *harp line angles tend to accumulate stress and

should be avoided.+%ounded internal line angles minimi)e stress concentrations.+/,+3

An inverted cone diamond bur can be used to place further orientation grooves between the isthmus and the outerbuccal and lingual occlusal tables (igure $$. These added orientation grooves will help in the final seating of the

restoration and contribute to the blending of the ceramic-to-tooth esthetic transition. The change from a flat

occlusal platform to a varied platform helps create a chameleon or &contact lens' effect where the ceramic material

accommodates the shade of the tooth.+4

As previously stated, the ceramic thickness is critical to the success of the restoration. A +-mm flexible clearance

tab can be placed over the occlusal surface of the prepared tooth and the patient instructed to close into centric

occlusion. 8f sufficient reduction has been provided, the flexible tab should easily pull through. Any resistance

encountered is then identified and corrected.The last preparation step is to use a tapered finishing bur over the entire prepared tooth surface (igure $+and

(igure $. This is done for two reasons. (irst, carbide finishing burs will produce a smoother surface compared to

a diamond bur.+5A smooth, rounded prepared tooth surface reproduces better with all impression materials and die

stones.+:,+>Also, a smooth margin is easier to read on a digitally scanned virtual model (igure $/and(igure $3.

*econdly, coarse diamonds produce a thick, uneven smear layer, whereas carbide burs produce a thin, even smear

layer.;The significance in the different smear layers is pertinent when a self-etching adhesive is to be used. Fiu et

al;found higher bond strengths were achieved with a self-etching adhesive when it was applied on dentin surfaces

that had been prepared with carbide burs. There was less penetration of the milder acids contained in self-etchingadhesives through the thicker, more uneven smear layers produced by diamond burs. The thicker smear layers also

had more of a buffering or neutrali)ing effect on the milder acids. "arros et al $found in their study that carbide

burs leave a surface that is more conducive to bonding than diamond burs.

After this step, gingival retraction is initiated wherever necessary. This can be done with either a non-impregnated

retraction cord or a diaode laser. 8f using the traditional two-appointment method, the exposed dentinal tubules

should be sealed prior to the impression step or digital scanning.+A one-step, two-step, or three-step method of

applying a dentin adhesive can be used. 8t is imperative that the oxygen-inhibited layer be removed by applying a

water-soluble clear gel over the resin-coated prepared tooth and light-polymeri)ed. 8t is recommended thatpumicing of the sealed surface be completed prior to taking the impression.8f an in-office one-appointment

milling method eg, #%#, *irona 1ental *ystems, 8nc, www.sirona.comor /1, 1/1 Technologies,

www.e/dsky.com is used, then the sealing step is omitted and the prepared tooth can be digitally scanned.

@rovisional %estorations

@rovisional restorations are only necessary for the two-appointment method. 8n these cases, since the dentinal

tubules have already been sealed, postoperative sensitivity is not an issue. The purpose of the provisional at this
http://www.sirona.com/http://www.e4dsky.com/http://www.sirona.com/http://www.e4dsky.com/


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point is to maintain the positions of the ad0acent and opposing teeth. #omposite material can be placed using either

a free-hand techni!ue or a vacuum-formed stent from a preoperative model. %etention is gained from the

interproximal undercuts from the ad0acent teeth and by extending the provisional material into the undercut area

below the buccal and lingual margins.

#eramic @reparation for "onding

The bonding mechanism of a resin to a ceramic surface is a combination of the effects of micromechanical

interlocking and chemical bonding./There are numerous articles addressing the various methods of conditioning

the intaglio ceramic surface for bonding purposes. icromechanical interlocking is created by acid or sand-air

abrasion or roughening the surface with a diamond bur. ach of these methods creates microporosities and

increases the surface area.


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distal cusp (igure $>. The lingual cusps were then reduced in the similar fashion (igure +;. A flat-end diamond

bur was then used to remove the decay in the mesial area and the remaining restoration and decay in the distal

portion. A D5:;$ finishing bur was used to break the contact and flatten the mesial margin surface (igure +$. The

last step was to use a finishing bur D54:4 over the entire preparation to smooth any sharp angles and reduce the

thickness of the smear layer (igure ++. The tooth was scanned (igure + and a milled lithium-disilicate

restoration was fabricated. The pre-sintered restoration was tried in for fit and to check the occlusion. *urfacestains were applied (igure +/ and the crown was then removed. A spray-on gla)e was applied. The crown was

secured onto a crystalli)ation pin with ob0ect putty and placed in a two-cycle porcelain furnace for final

crystalli)ation. Cpon cooling, the restoration was bonded into place (igure +3. 8n a open-mouth view, the

unprepared axial walls of the tooth did not create any visual ob0ection (igure +4.

#onclusion

The non-retentive, all-ceramic posterior restoration is a viable option in specific situations depending on the

location, esthetics, and occlusal habits that may be present. 8ts advantages include elimination of the axial portion

of the traditional crown preparation, which provides for a more conservative approach to restoring posterior teeth.#ase selection is vital to the success of the techni!ue. Csing the previously described step-by-step preparation

techni!ue will ensure proper occlusal reduction, preservation of enamel, and supra-gingival margins where

possible, and improve margin definition and decrease the amount of time the preparation bur is in contact with the

tooth.

Acknowledgment

The author would like to thank %uth gli, %1


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%eferences

$. "akeman , Bois H#. @osterior, all-porcelain, adhesively retained restorations.Inside Dentistry. +;;>I33J+;-

;.

+. .

. Bramer K, Bun)elmann B5-$$;;.
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3. .

4. ono >-$;4.


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>. Bato .

About the Author

Gregg A.

non-retentive adhesively retained all-ceramic posterior restoration

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