fatigue resistance of removable orthodontic appliance reinforced with glass fibre weave

Fatigue resistance of removable orthodontic appliance

reinforced with glass fibre weave

L. I . RANTALA*, †, T . M. LASTUMAKI*, T. PELTOMAKI‡ & P. K. VALLITTU* *Department

of Prosthetic Dentistry and Biomaterials Research, Institute of Dentistry, University of Turku, Turku, †Department of Dental Technology,

Helsinki Polytechnic, Helsinki and ‡Department of Oral Development and Orthodontics, Institute of Dentistry, University of Turku, Turku,

Finland

SUMMARY The aim of this study was to measure the

fatigue resistance of fibre-reinforced composite

(FRC) reinforced polymeric parts of a removable

orthodontic appliance beside the clasp. The effect of

quantity and position of FRC-reinforcement were

investigated. In addition, the influence of water

storage on the fatigue properties was determined.

The test specimens for eight groups (n = 6) were

manufactured from autopolymerizing acrylic resin.

Polymethylmethacrylate pre-impregnated woven

glass fibre was used as reinforcement of acrylic resin

specimens at the region of steel wire clasp. The test

specimens of the control group were not reinforced.

In the second group, the test specimens were rein-

forced with one fibre layer (thickness: 0Æ06 mm) on

the tension side, and in the third and fourth group

with two fibre layers. Fatigue resistance was meas-

ured by applying repeated bending force to the

clasp. The highest fatigue resistance values were

achieved when the test specimens were fibre-rein-

forced with two fibre layers. The lowest fatigue

resistance values resulted when the test specimens

were not reinforced (P = 0Æ046, ANOVA). Water stor-

age had a tendency to decrease the fatigue resistance

in all fibre reinforced test specimen groups. The

results suggest that use of the woven polymer pre-

impregnated glass FRC-reinforcement increases the

fracture resistance of orthodontic appliance made of

acrylic polymer.

KEYWORDS: fatigue resistance, orthodontic appli-

ance, fibre reinforcement, clasp

Introduction

Polymethylmethacrylate (PMMA)-based polymers are

the most common materials used when manufacturing

denture bases and polymeric parts of removable ortho-

dontic appliances. These polymers, are mainly two

component systems, which contain the PMMA powder

beads, methylmethacrylate (MMA) monomer liquid

and a small quantity of crosslinking agent such as

ethyleneglycol dimethacrylate. Numerous studies have

been published on reinforcing of the two component

polymers (Jennings & Wuebbenhorst, 1960; Schwicke-

rath, 1966; Bowman & Manley, 1984; Carroll & von

Fraunhofer, 1984; Deboer, Vermilyea & Brady, 1984;

Ruffino, 1985; Yazdanie & Mahood, 1985; Ekstrand,

Ruyter & Wellendorf, 1987; Vallittu, 1995, 1998, 1999;

Vallittu, Vojtkova & Lassila, 1995; Vallittu, 1996a, b).

The traditional method to reinforce the dental polymers

was to use different kind of metal strengtheners,

stainless steel wire being the most popular of all

(Vallittu, 1995). However, it has been showed that

metal strengtheners do only have a minor effect on the

strength of polymeric structures (Vallittu, 1996a).

Carbon ⁄ graphite, glass, aramid and polyethylene fibres

have been tested as reinforcement of two component

polymer. Of those fibres, glass fibres have an ability to

considerably increase the mechanical properties of

polymer (Vallittu, 1996a, 1999).

Tendency of orthodontic appliance to fracture is

caused by the occlusal biting force and the mechanical

ª 2003 Blackwell Publishing Ltd 501

Journal of Oral Rehabilitation 2003 30; 501–506

strain beside the clasps. It is unlikely that a steel wire

clasp with good surface quality suffer from fatigue

failures on contrary to brittle, two-component acrylic

polymer which are prone for fatigue failures (Vallittu,

1996b). Earlier studies also showed that unidirectional

fibre reinforcement considerably increased transverse

strength and stiffness of polymers while bidirectional

fibre weave had minor effect on these parameters.

However, fibre weave has been shown to increase

strain at fracture, i.e. toughness of polymer, which is a

desired property for polymeric parts of orthodontic

appliance (Vallittu, 1999).

Dental appliances are affected by water sorption in

the oral cavity. It is known that water sorption of two-

component PMMA is approximately 2 wt% (14). It is

also known that plasticization effect of water reduce the

mechanical properties of polymer to some extent

(Miettinen & Vallittu, 1997). It is therefore also likely

that water sorption influences the fatigue resistance of

material.

The aim of this study was to determine the fatigue

resistance of non-reinforced and glass fibre-reinforced

polymers with steel wire clasps. In addition, the effect

of water storage on the fatigue resistance was studied.

Materials and methods

The materials used in this study are listed in Table 1.

Autopolymerizing two component acrylic resin Pala-

press Vario (PPV) was used with a powder-to-liquid

ratio of 10 g:7 mL and mixed for 30 s according to

manufacturer’s recommendations. The clasps used in

the test specimens were manufactured from Remanium

stainless steel wire (diameter 1Æ0 mm) (Fig. 1). The

woven PMMA pre-impregnated glass fibre reinforce-

ment StickNet (SN) (thickness 0Æ06 mm) was further

impregnated with a low-viscosity mixture of PMMA

powder of PPV and monomer liquid of PPV (powder-

to-liquid ratio 10 g:10 mL) on a polyethylene sheet.

Laboratory putty polyvinyl siloxane was used to fabri-

cate the mould for manufacturing the test specimens

(Fig. 1). The clasp was placed into the retentive

impression of the mould and the mixture of PPV was

poured into the mould. In the cases of reinforced test

specimens, the further impregnated SN weaves were

placed into mould before pouring the PPV mixture. The

fibres of the weave were oriented �45� angle to the

long axis of the specimen. The test specimens were

polymerized in water at 55 � 1 �C for 15 min under air

pressure of 200 kPa (Ivomat-type IP2*). After polymer-

ization, the test specimens were wet-ground with 320

gritt (FEPA) silicon carbide grinding paper to the

thickness of 3Æ0 mm. The test specimens were cleaned

in distilled water in an ultrasonic cleaning device

(Quantrex 90†) for 15 min. The cleaned test specimens

were conditioned in a desiccator at room temperature

for 5 days or stored in water at 37 � 1 �C for 30 days.

The test specimens were divided into eight groups and

each group included six test specimens according to the

reinforcing type and storing conditions (Table 2).

A constant deflection fatigue test was carried out dry

at room temperature 23 � 1 �C. The cycle frequency of

testing machine (Custom-made fatigue resistance test-

ing device, University of Kuopio, Kuopio, Finland) was

500 cycles min)1 and the maximum initial load was

20 N with the magnitude of deflection of 1Æ0 mm. The

test was carried out to the limit of 100 cycles. Number

of loading cycles required to cause fracture to the

specimens was considered as fatigue resistance of the

specimen. Twenty-four test specimens were immersed

to distilled water in a thermostatically controlled water

bath at 37 � 1 �C. Water uptake level, i.e. sorption was

followed by weighing procedure repeated on days 1, 2,

4, 7, 8, 11, 14, 16, 21, 28 and 30. The mean values and

standard deviations of the water uptake were calculated

before fatigue-testing procedures.

Brand Manufacturer Lot no.

StickNet StickTech Ltd, Turku, Finland 1990906-W-0037

Palapress Heraeus Kultzer GmbH,

Wehrheim, Germany

Powder: 012100, 012109

Liquid: 010970,010984

Remanium, 1Æ0 mm

springhard

Dentaurum, Ispringen, Germany 58096

Laboratory-Putty Coltene AG, Altstatten, Switzerland JE 43

Table 1. Materials used in the study

*Ivoclar AG, Schaan, Liechtenstein.†L & R Ultrasonics, Elm, NJ, USA.

L . I . R A N T A L A et al.502

ª 2003 Blackwell Publishing Ltd, Journal of Oral Rehabilitation 30; 501–506

The values obtained from the fatigue test and water

uptake were statistically analysed with two-way ANOVA,

with a significance level of 0Æ05. Kaplan–Meier

survival function analysis was calculated for the

fatigue resistance values of the test specimen. Dry

and water-stored specimens were pooled for the

analysis.

Results

Mean value of loading cycles required to cause the

fracture of the test specimens of group PMMA (dry,

non-reinforced) was 25Æ849 cycles (Fig. 2). The highest

fatigue resistance was achieved when the dry test

specimens were fibre-reinforced with a fibre layer on

both sides of clasp (64Æ800 cycles) (Fig. 2). The lowest

fatigue resistance values resulted when the test speci-

mens were unreinforced and water stored (25Æ817

cycles). Mean values of loading cycles differed signifi-

cantly (P ¼ 0Æ046, F ¼ 91Æ249, n ¼ 6). Figure 3 shows

Kaplan–Meier survival function curves for the test

groups. Water storage had a tendency to decrease the

fatigue resistance in fibre-reinforced test groups (Fig. 2)

but no statistical significance of this variable was found

(P ¼ 0Æ236, F ¼ 1Æ446, n ¼ 6). Water uptake after

30 days water immersion varied between 1Æ12 and 1Æ2wt% (Fig. 4).

Discussion

This study demonstrated that correctly placed woven

glass fibres at the region of clasp can considerably

(a)

(b)

Fig. 1. Schematic drawings of the

test specimen and loading conditions.

(a) Dimensions of the test specimens

and orientation of the fibre weaves

and (b) direction of the repeated load

(F) and location of fibre weaves in

Group PMMA + 1·SN + 1·SN.

Table 2. Codes for the test groupsCode Explanation

PMMA Polymethylmethacrylate

PMMA + 1·SN Polymethylmethacrylate + 1 layer of StickNet

PMMA + 2·SN Polymethylmethacrylate + 2 layers of StickNet

PMMA + 1·SN + 1·SN Polymethylmethacrylate + 1 layer of StickNet on both sides of clasp

F A T I G U E O F R E I N F O R C E D A P P L I A N C E 503


enhance fatigue resistance of acrylic polymer appliance.

Earlier study showed that continuous unidirectional

glass fibre reinforcement increased fatigue resistance of

dental appliance up to 100 times compared with unre-

inforced appliance (Vallittu, 1996a). According to the

present study, the woven glass fibres had similar

influence on the fatigue behaviour of the construction.

The placing of fibres in the polymer matrix affects the

mechanical properties of fibre-reinforced composites

(FRCs). The importance of this factor is emphasized in

dental construction having only small quantity of

reinforcing fibres at weakest part of the construction.

These, so called partial fibre reinforcements have shown

to decrease considerably, the number of fractures in

removable dentures (Narva, Vallittu & Yli-Urpo, 2001).

The results of this in vitro study seems to support this

earlier clinical finding. However, in the case of remo-

vable orthodontic appliance with a clasp, some specific

aspects should be taken into consideration. A fibre

weave with �45� fibre angles reinforce the polymeric

parts equally in two directions on contrary to continuous

unidirectional fibres giving reinforcing effect only in one

direction, i.e. in the direction of fibres. The increased

strength and modulus by using unidirectional glass fibres

with the highest reinforcing capacity (Krenchell’s fac-

tor ¼ 1) (Murphy, 1998) might not be necessary in the

case of removable orthodontic appliances. In orthodon-

tic appliances, the fatigue failure is often caused by

repeated loads transferred from the clasp to the base

plate which result in tensile stress at certain areas beside

the clasp, and finally leads to fatigue fracture formation.

The bidirectional fibres of the fibre weave of the

polymeric part act as crack stoppers and hinder the crack

propagation. Simultaneously by acting as crack stoppers,

the �45� angle fibres increased the toughness of the

polymer. This has been reported previously (Vallittu,

1999). On the other hand, the placing of fibres with�45�angle did not result in highest possible static strength of

the FRC with that specific fibre quantity.

Comparison of water-stored and dry specimens

showed that the water saturation had a tendency to

lower the fatigue resistance. This can be explained by

the fact that water has a plasticizing effect on polymers

and polymeric composites and can therefore decrease

the mechanical properties of polymeric parts of the

orthodontic appliances (Ruyter & Svendsen, 1980;

Ruyter, 1995; Vallittu, Ruyter & Ekstrand, 1998). In

the fatigue test, the polymer chains of the polymer are

forced apart from each other by the applied stress and

–20 000

0

20 000

40 000

60 000

80 000

100 000

120 000

PM

MA

PM

MA

+ 1 X

SN

PM

MA

+ 2 X

SN

PM

MA

+ 1 X

SN

+ 1 X

SN

Nu

mb

ero

flo

adin

gcy

cles

DryWet

Fig. 2. Mean values of the number of loading cycles required to

cause fatigue fracture to the specimens (n ¼ 6). Vertical lines

present mean error values. For symbols, see Table 2.

Fig. 3. Kaplan–Meier survival function curves of the fatigue

resistance of specimens. For symbols, see Table 2.

Fig. 4. Water uptake of test specimens plotted to the storage time.



strain. This allows water molecules to penetrate more

efficiently to the spaces between the polymer chains.

By the end, the water molecules have increased the

distance of the polymer chains which decrease the

secondary chemical bonding forces (van der Waals

forces) between the polymer chains. As a result, the

fatigue resistance, as well as other mechanical proper-

ties of the polymer is lowered. However, in the case of

FRC, the influence of water sorption on mechanical

properties is more complicated than in the case of plain

polymer.

It is recognized that water sorption of composites is

dependant on the degree of impregnation of fibres on

the resins (Peltonen & Jarvela, 1992; Miettinen &

Vallittu, 1997). In the case of the existence of exposed

fibres and voids in the structure of FRC, they absorb

water by means of capillary forces. This fastens the

water saturation of the polymer matrix by increasing

the surface area, and at the same time, increase the

quantity of absorbed water in the FRC. In the case of

poorly impregnated fibres of FRC without exposed

fibres, the absorbing water has to penetrate to the voids

through the polymer matrix. However, by the end, the

voids between the fibres are filled with water. In

addition to the plasticization of polymers by water

molecules, the water can deteriorate the silane-promo-

ted adhesion of glass fibres to the polymer matrix. For

these reasons, the high degree of impregnation plays an

important role in long-term stability of FRCs. Polymer

pre-impregnation of fibres has been shown to help the

final impregnation of impregnation with the autopo-

lymerizing acrylic resin. In practice, the high degree of

impregnation of fibres by resin can be seen in translu-

cent light; the well-impregnated FRC parts of the device

are translucent. In this study, there was no noticeable

difference in water sorption between the unreinforced

and FRC-reinforced groups. This suggests that the

impregnation of fibres used in this study was adequate.

With higher fibre quantities the water uptake values for

reinforced specimens would obviously be lower. The

lower water uptake of acrylic polymer found in this

study compared with those measured earlier (Miettinen

& Vallittu, 1997) can partially be explained by the

existence of stainless steel wire clasp in the specimen.

From the clinical perspective, the present study

addressed an important aspect related to use of a

removable appliance in orthodontics, namely secure

retention of the appliance (Proffit & Fields, 2000). Even

the best springs and clasps of a removable appliances

are ineffective if the appliance moves away from the

underlying structures. Thus, by reducing likelihood of

fracture of polymeric parts beside clasps and springs

increase the stability of the appliance and together with

proper fitting of the clasps determine how well a

removable appliance performs.

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Correspondence: Dr Pekka Vallittu, Department of Prosthetic Dentis-

try, Institute of Dentistry, University of Turku, Lemminkaisenkatu 2,

FIN-20520 Turku, Finland.

E-mail: [email protected]



fatigue resistance of removable orthodontic appliance reinforced with glass fibre weave

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