Flexural fatigue behavior of resin composite dental restoratives

Flexural fatigue behavior of resin composite dental restoratives

Dental Materials 19 (2003) 435–440 www.elsevier.com/locate/dental Flexural fatigue behavior of resin composite dental restoratives Ulrich Lohbauera,*...

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Dental Materials 19 (2003) 435–440 www.elsevier.com/locate/dental

Flexural fatigue behavior of resin composite dental restoratives Ulrich Lohbauera,*, Tina von der Horstb, Roland Frankenbergera, Norbert Kra¨mera, Anselm Petschelta a

Policlinic for Operative Dentistry and Periodontology, University of Erlangen-Nuremberg, Gluckstrasse 11, 91054 Erlangen, Germany b Department of Medical Informatics, Biometrics, and Epidemiology, University of Erlangen-Nuremberg, Erlangen, Germany Received 22 February 2002; revised 13 June 2002; accepted 31 July 2002

Abstract Objectives. The aim of this study was to evaluate the mechanical properties of resin composite dental restoratives under quasi-static and cyclic loading. Methods. Four-point-bending bars of 10 different resin composite materials were manufactured according to ISO standard and stored for two weeks in distilled water. The fracture strength (FS) was measured with the four-point-bending test in an universal testing machine. The flexural fatigue limits (FFL) for 105 cycles were determined under equivalent loading. All specimens were tested and fatigued in water at 37 8C. The data were analyzed using ANOVA, Weibull statistics of FS and the ‘staircase’ approach of FFL. Fractographic analysis was performed using SEM. Results. The initial flexural strength values for the resin composite materials varied from 55.4 MPa for Solitairew up to 105.2 MPa for Filtekw Z250. The mean flexural fatigue limit for 105 cycles ranged between 37 and 67% of the initial strength. SEM analysis of the fractured surfaces suggests two kinds of failure mechanisms for initial and fatigue fracture. Significance. The fatigue behavior of resin composite materials does not correlate with initial strength values. Materials providing high initial strengths do not obviously reveal the best fatigue resistance. Flexural fatigue measurement of resin composite materials should be viewed as a useful tool to evaluate long term mechanical properties. q 2003 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Fracture strength; Fatigue; Resin composite; In vitro; Staircase; Mechanical properties

1. Introduction Within the last few decades, modern restorative materials were developed with a focus on amalgam-like mechanical properties, excellent aesthetics and biocompatibility. Such materials were further improved for application in stress bearing areas. Therefore, mechanical properties under masticatory load and above all fatigue resistance are important. Fatigue fractures after years in clinical use were found to be a common failure reason. Damage of restorations like bulk, cusp, or marginal fractures were frequently reported [1,2]. Using resin composite materials, Burke et al. [3] reported marginal fracture (18%) and bulk fracture (7%) as the most prevalent reasons for rerestoration. * Corresponding author. Tel.: þ 49-9131-853-4236; fax: þ 49-9131-8533603. E-mail address: [email protected] (U. Lohbauer).

Fatigue in dental restoratives is influenced by corrosive water attack at a certain temperature (37 8C) and by cyclic masticatory forces. The naturally occuring loading of a filling was estimated at between 5 and 20 MPa [4]. Contemporary approaches to fatigue principles consider a fracture process in three phases: crack initiation, slow crack growth, and fast fracture. The latter phase is very short in duration and thus the time of crack initiation and of slow crack growth account for the useful fatigue resistance of a material. Crack initiation nucleates at heterogenities like surface and subsurface microcracks, porosities, filler particles, crazes, etc. within the material [5]. Cyclic loading is able to drive a crack, called slow crack growth. Additional water exposure causes a variety of weakening effects on resin composites: degradation of the filler – matrix interface, eluation, and swelling or a visco-elastic effect on the matrix which all accelerate slow crack growth [6 –8]. The purpose of this in vitro study was to determine the strength of today’s resin composite materials under fatigue conditions,

0109-5641/03/$ - see front matter q 2003 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S0109-5641(02)00088-X

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Table 1 Resin composite materials under investigation Brand name (LOT, shade)

Manufacturer

Composite material

Filler fraction (wt%/vol%)

Charismaw (020030, A2) Definitew (218, A2) Filtekw Z250 (OCY 2002-10, A3) Heliomolarw (905566, A3) Solitairew (22, A2) Solitairew II (010221, A2) Surefilw (9804205, A3) Tetricw Ceram (A07612, A3)

Heraeus Kulzer (Germany) Degussa (Germany) 3M ESPE (USA) Vivadent (Liechtenstein) Heraeus Kulzer (Germany) Heraeus Kulzer (Germany) Caulk/Dentsply (USA) Vivadent (Liechtenstein)

Fineparticle hybrid Fineparticle hybrid, ormocer Fineparticle hybrid Inhomogenously microfilled Porous silica hybrid Porous silica hybrid Fineparticle hybrid Fineparticle hybrid

78/61 78/61 82/60 76.5/64 66/46 75/58 82/65.2 78.6/60

simulating the clinical situation. The methodology was developed considering earlier findings on material degradation. 2. Materials and methods 2.1. Materials A variety of present commercially available, light-curing resin composite materials were used in this study (Table 1). They were chosen to be representative for the differences in filler configuration.

sequentially, with the maximum applied stress in each succeeding test being increased or decreased by a fixed increment, according to whether the previous test resulted in failure or not. The first specimen was tested at approximately 50% of the initial flexural strength value. As the data are concentrated around the mean stress, the number of specimens required is less than with other methods [11,12]. Fractographic examination was performed under a light microscope (SV11, Zeiss, Germany) on all specimens and under a scanning electron microscope (SEM, Leitz ISI SR 50, Akashi, Japan) on representative specimens (five for each set). 2.4. Statistical treatment

2.2. Specimen preparation Depending on the material’s density, around 0.2 g was weighed and placed in a special mold (2 £ 2 £ 25 mm3). The light polymerization was performed with a halogen light curing unit (Transluxw CL (800 mW/cm2), Heraeus Kulzer, Germany) on five overlapping points on each upper and lower side. The illumination time on a single point was 20 s for Filtekw Z250 and 40 s for the remaining materials. The procedure followed the manufacturers’ recommendation and ISO 4049 standard. The specimens’ surfaces were ground with silicon carbide paper up to 800 grit, to avoid and remove cracks at their edges. All specimens were stored for two weeks in distilled water at 37 8C. 2.3. Experimental procedure To evaluate the initial flexural strength the four-pointbending test was used (n ¼ 12). Bars of 25 mm in length were fixed between four fins (B ¼ 2 mm, distance of inner fins ¼ 10 mm, distance of outer fins ¼ 20 mm) and were subsequently loaded until fracture with a crosshead speed of 0.75 mm/min (universal testing machine Zwicki, Zwick, Ulm, Germany). The tests were carried out under distilled water at a temperature of 37 8C. The flexural fatigue limits (FFL) of the composite materials were determined for 105 cycles under equivalent test conditions at a frequency of 0.5 Hz (n ¼ 20). The ‘staircase’ approach method [9,10] was used for fatigue evaluation. For every cycle the stress alternated between 1 MPa and the maximum stress. Tests were conducted

According to the assumption of the weakest link, the fracture strength (FL) of brittle materials will be limited by the longest crack size in the loaded volume. Hence, a distribution of crack lengths results in a strength distribution which is commonly described by fracture probability P(Fsc)   m  s PFðsc Þ ¼ 1 2 exp 2 c ð1Þ s0 where s0 is the scale parameter (PF(sc) ¼ 63.2%) and m is the Weibull modulus, respectively [13,14]. The strength data was evaluated according to this two parameter cumulative Weibull distribution by plotting the fracture probability PF(sc) versus fracture strength sc ln ln

1 ¼ m ln sc 2 m ln s0 ð1 2 PFðsc Þ Þ

ð2Þ

The parameters m and s0 were determined by a maximum likelihood approach. The groups among each other were analyzed using the non-parametric Mann – Whitney U-Test ( a ¼ 0.05/8 ¼ 0.00625; SPSS 10.0 for Windows). Commonly this test analyses two groups. However, to compare eight groups a Bonferroni correction was applied. The mean FFL for 105 cycles was determined using Eq. (3) and standard deviation, respectively, using Eq. (4) P  ini ð3Þ FFL ¼ X0 þ d P ^ 0:5 ni ! P  P P2 ni i ni 2 ini 2 SD ¼ 1:62d þ 0:029 ð4Þ P 2 ni

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Table 2 Four-point-bending strength and fatigue strength data (SD) Brand name

Scale parameter s0 (MPa)

Weibull modulus m

FFL (SD) (MPa)

FFL decrease (%)

Charismaw Definitew Filtekw Z250 Heliomolarw Solitairew SolitairewII Surefilw Tetricw Ceram

97.94a,b 88.92b.d 105.16a 91.52b 55.39c 66.80c,d 88.59b,d 78.04b,d

9.2 9.1 10.8 8.1 5.6 9.6 8.4 12.3

33.3 (6.2)a 47.2 (2.3)b,c 45.9 (7.0)b,c 39.4 (9.1)a,c 17.9 (5.1) 34.6 (3.8)a 55.5 (7.2) 45.3 (11.8)b

65.9 46.9 56.3 56.9 67.6 48.2 37.3 41.9

Data with same superscript letter are not significantly different (Mann–Whitney U-Test; a , 0.00625).

where X0 is the lowest stress level considered in the analysis and d is the fixed stress increment. To determine the FFL, the analysis of the data is based on the least frequent event (failures versus non-failures). In Eq. (3) the negative sign is used when the analysis is based on failures, otherwise the positive sign is used. The lowest stress level considered is designated as i ¼ 0, the next as i ¼ 1, and so on and ni is the number of failures or non-failures at the given stress level. Analogously to the initial strength comparison, the fatigue data were analyzed using the non-parametric Mann – Whitney U-Test (a ¼ 0.00625; SPSS 10.0 for Windows).

3.1. Fracture strengths Fig. 1 displays the results listed by increasing scale parameters. Regarding Weibull statistics, the fine particle hybrid composites Filtekw Z250 and Charismaw pointed out the significantly highest strength values while most materials were medium ranged. The most homogenous behavior and the lowest scatter in strength, expressed by a high Weibull modulus m, was measured for the fine particle hybrid composites Tetricw Ceram and Filtekw Z250. Solitairew, however, exhibited the lowest results in both cases. 3.2. Flexural fatigue limits

3. Results The mean flexural strengths s0 at a fracture probability PFðsc Þ of 63.2%, the Weibull modulus m and the FFL of the different materials are presented in Table 2.

In the case of cyclic fatigue measurement the strength ranking changed. The materials with high initial strength values point out a rather low fatigue resistance. The FFL percentage points out a decrease in strength between 37 and

Fig. 1. Weibull plots of the investigated materials.

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Fig. 2. FFL according to the staircase method.

68%. The significantly best fatigue resistance was found for the fine particle hybrid composite Surefilw. Here, an initial FS of 88.59 MPa was measured and decreased within 10,000 cycles to a value of 55.5 MPa. The significantly worst fatigue

resistance was documented for the porous silica hybrid composite Solitairew. The FS of 55.4 MPa dropped to a FFL value of only 17.9 MPa indicating a decrease of 68%. Fig. 2 shows the FFL for the investigated materials.

Fig. 3. Typical fracture surface of an initial fracture. The arrow indicates the fracture origin (a), the typical mist and hackle regions (b), and lance hackle mark (c).

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Fig. 4. Typical fracture surface of a fatigue fracture.

3.3. Fractographic examination The fracture surfaces of initial strength measurements as well as those of the fatigued specimens were compared with each other in order to find specific characteristics regarding their fracture mechanism. Fig. 3 exhibits a fracture surface with typical macroscopic patterns of a fast and inert fracture. Fig. 3 is taken from the material Tetricw Ceram and shows a representative fracture surface compared with all resin composite materials. The fatigue surfaces point out a quite different macroscopic pattern, Fig. 4. The micrograph here is taken from the material Surefilw and shows a smooth fracture after 8503 cycles.

4. Discussion The results for the resin composite materials indicate a distinct variation according to their initial fracture strength. Considering four-point-test loading, the initially investigated properties behave in a similar way to those of other studies [15,16]. To range and assess the materials’ behavior their reliance on strength, expressed by Weibull modulus m, has to be considered. High strength materials with a low modulus m may be worse than lower strength materials with less scatter in strength. Filtekw Z250 points out the best values for both m-value and scale parameter s0, although following the manufacturer’s recommendation of a half light curing

period (20 s.). Solitairew on the other hand exhibited the worst results which may be due to a delayed initiator system or due to the porous silica fillers itself and its optical properties [16]. Based on worse findings for Solitairew, the material was taken from the market and replaced by Solitairew II. The strength ranking within the initial measurement has changed by determination of FFL. Materials with higher filler contents exhibited a tendency towards improved fatigue resistance. For the highly filled Surefilw, as a so called ‘packable’ material, the best results under cyclic fatigue conditions were measured. However, all materials suffer from a decrease in strength which is derived from a mechanical fatigue within 10,000 cycles and therefore described by FFL percentage. Surefilw shows a decrease of 37% from an initial fracture strength of 88.6 to 55.5 MPa. This example points out that the material with the highest initial strength value may not obviously be recommended when focused on fatigue resistance. A correlation of FFL with different filler types (Table 1) could not be computed. Htang et al. [17] described a correlation of filler content on fatigue resistance. A maximum fatigue resistance, however, was determined with a 75 wt% filler fraction. The authors summarized by fractographic analysis that crack propagation in dental resin composites is mainly determined through the matrix and its adhesion to the filler particles. Drummond [8] stated, whether crack propagation in resin composite materials is mainly around or through the second phase particles (inter- or intracrystalline) is dependent on filler content and interparticle distance, correspondingly.

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This topic has to be considered to fully understand the mechanisms of fatigue. Especially when a destructive corrosion, caused by water exposure, weakens the matrix – filler interface [6,7]. Ferracane et al. [18] discussed a significant influence of silanization agents on mechanical properties during long term water storage. To show whether there is a correlation of different filler types and fatigue resistance or not, the specific surface areas, particle shape and interparticle spacing of the fillers might be compared [8]. Hence, porous silica fillers exhibit a much larger surface area to comparable filler sizes. Obviously extended interface areas might be more sensitive to corrosive attack. Further differences between FS and FFL were determined by SEM examination of the specimens’ fracture surfaces. Figs. 3 and 4 show typical fracture surfaces for initial and fatigue loading. The specimens were tilted in the SEM to display the fracture origin at the top of the micrograph. The experimental loading was applied at the bottom (compressive zone). Fig. 3 exhibits the features typical of a brittle fracture. The source of failure may be located close to the tensile surface (Fig. 3(a)) surrounded by a smooth mirror, a mist, and a hackle region in Fig. 3(b) symmetrically around the fracture origin [19,20]. A further crack deviating lance hackle mark is observed (Fig. 3(c)). These marks are often determined on the compressive side of a flexure specimen due to the obvious presence of mixed mode conditions [21]. The failure characteristics under cyclic fatigue are different. No sign indicates brittle fracture. The fractographic analysis shows a smooth fracture surface (Fig. 4). This might be a hint for a diverging fracture mechanism. Subcritical crack growth, especially under water exposure, or maybe a moderate association to visco-elastic creep are reported as failure criteria under cyclic fatigue loading [22 –24]. Whether the fracture mechanisms under fatigue are based on subcritical crack growth, on visco-elastic creep or on a combination of both, has to be cleared up in further studies. Research also has to be done assessing the influence of different filler types and varying specific surface areas on fatigue behavior.

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