Wear of ten dental restorative materials in five wear simulators—Results of a round robin test

Dental Materials (2005) 21, 304–317

www.intl.elsevierhealth.com/journals/dema

Wear of ten dental restorative materials in five wear simulators—Results of a round robin test S.D. Heintzea,*, G. Zappinia, V. Roussonb a

R&D, Ivoclar Vivadent AG, Bendererstrasse 2, FL-9494 Schaan, Liechtenstein Department of Biostatistics, Institute for Social and Preventive Medicine, University of Zurich, Switzerland

b

Received 14 October 2003; received in revised form 28 April 2004; accepted 6 May 2004

KEYWORDS Wear; Round robin test; In vitro; Composite; Ceramic; Amalgam; Wear simulator

Summary Objective: The purpose of the present study was to prove the hypothesis that different wear measurement methods generate different material rankings. Methods: Ten restorative materials, eight composites (BelleGlass, Chromasit, Estenia, Heliomolar RO, SureFil, Targis cured at 95 and 130 8C, Tetric Ceram) an amalgam (Amalcap) and a ceramic (Empress) have been evaluated with regard to the wear with five different wear methods (IVOCLAR, ZURICH, MUNICH, OHSU, ACTA). Every center received specimens, which Ivoclar Vivadent had made using the same batch. The test centers did not know which brand they were testing. After completion of the wear test, the raw data were sent to IVOCLAR for further analysis. The statistical analysis of the data included logarithmic transformation of the data, the calculation of relative ranks of each material within each test center, measures of agreement between methods, the discrimination power and coefficient of variation of each method as well as measures of the consistency and global performance for each material. Results: Relative ranks of the materials varied tremendously between the test centers. When all materials were taken into account and the test methods compared with each other, only ACTA agreed reasonably well with two other methods, i.e. OHSU and ZURICH. On the other hand, MUNICH did not agree with the other methods at all. The ZURICH method showed the lowest discrimination power, ACTA and IVOCLAR the highest. Materialwise, the best global performance was achieved by Empress, which was clearly ahead of BelleGlass, SureFil and Estenia. In contrast, Heliomolar RO, Tetric Ceram and especially Chromasit demonstrated a poor global performance. The best consistency was achieved by BelleGlass and SureFil, whereas the consistency of Amalcap and Heliomolar RO was poor. Significance: As the different wear simulator settings measure different wear mechanisms, it seems reasonable to combine at least two different wear settings to assess the wear resistance of a new material. Q 2004 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: C423-235-3570; fax: C423-233-1279. E-mail address: [email protected] (S.D. Heintze). 0109-5641/$ - see front matter Q 2004 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Different wear measurement methods: a round robin test

Introduction Wear is a phenomenon that occurs whenever a surface is exposed to another surface or to chemically active substances. Material loss or wear occurs through microploughing, microcutting, microcracking and microfatigue [1]. The various dental materials may be grouped into five different categories: metal alloys, ceramics, amalgams, composites and unfilled polymers. Of all these materials the composite resins have a particular behavior as many variables that derive from their composition directly influence their wear resistance. Composites consist of filler particles dispersed in a brittle polymer. Optimally, the loading force is completely transferred from the matrix to the filler particles. The size, shape and hardness of the fillers, the quality of the bonding between fillers and polymer matrix, the polymerization dynamics of the polymer all have an effect on the wear characteristics of a dental material. The various components of the composition, on the other hand, influence physical parameters, such as flexural strength, fracture toughness, Vickers hardness, modulus of elasticity, curing depth, etc., which may influence the wear [2]. Ceramic, on the other hand, is a brittle material. Due to its crystalline matrix, it is less sensitive to attrition wear but more sensitive to fatigue resulting from flaws in the material and the materials composition [3]. Different approaches have been taken to relate physical properties such as fracture toughness to wear [4–6]. Although some factors such as fracture toughness and the modulus of elasticity seem to be predictive for wear, both universities and companies rely more on devices that simulate wear in vitro than on physical properties alone. Those devices are based on different approaches for both wear simulation and wear analysis, like chewing simulators, the ACTA-machine, pin-on-disc-machine, etc. In 2001 the International Standard Organization ISO published a technical specification on ‘Guidance on testing of wear’ describing eight different test methods of two- and/or three-body contact [7]. However, no assessment of the different wear methods has been made. Almost no efforts have been taken so far to analyze different restorative materials of the same batch in different wear generating environments in a blind test approach and compare the results with regard to agreement. Although only published in the postdoctoral thesis by Kunzelmannn, an attempt to conduct such a test was made by the ASC MD 156 Task Force on Posterior Composites. This test included four composite materials, which were evaluated by means of four wear

305

methods [8]. The test revealed important variations with regard to the ranking of the materials relative to the test method. However, the tests were carried out only with a limited number of materials and did not include a comparative statistical analysis. The aim of the present study was to compare 10 different dental restorative materials in five wear generating and wear analyzing settings, including those of the latest generation. Special attention was put to the statistical analysis focusing on the ability to differentiate between materials, the agreement between test methods, and the consistency of test results.

Materials and methods The rationale for the material selection of composite materials was based on criteria like material composition (microfiller, fine particle hybrid), market share, clinical success record, and indication (composites for direct and indirect restorations) (Table 1). Besides these products, a ceramic and an amalgam material were chosen for comparison. Furthermore, it was the special interest of Ivoclar Vivadent to find out whether different curing procedures influence the wear rate of a specific composite material (Targis 95/130 8C). The same batch of each material has been used to produce flat specimens for all participating institutes. To avoid discrepancies in the production of the specimens, all specimens were produced by the same employee at Ivoclar Vivadent. For the composite materials similar colors were chosen. Table 1 enlists the materials with their batch number and composition. The composite materials for direct restorations were directly applied and polymerized in the material mould specific to each test method (3 min in Spectramat, Ivoclar Vivadent). The amalgam material was mixed for 20 sec in the Silamat 5 triturator and also directly applied and condensed in the specific mould. The Empress and composite resins for indirect restorations (BelleGlass, Chromasit, Estenia, Targis) were fabricated according to the manufacturer’s instructions, however curing Targis at two different temperatures, 958C and 1308C. These specimens were luted into the mould by means of Variolink II luting resin (Ivoclar Vivadent). The ceramic material had been additionally conditioned with ceramic etching gel and silanized with Monobond S (Ivoclar Vivadent). All specimens were polished with silicon carbide paper and a polishing machine until 2500 grit. The materials were coded and sent to the test centers, whereas the time interval until testing occurred was taken into account. This interval was different for each test method.

306 Table 1

S.D. Heintze et al. List of materials, batch-number and composition.

Material

Batch

Composition

Amalcap Plus Empress TC1

C25527 C35146

Ag (70.1%), Sn (18%), Cu (11.9%). Powder:HgZ1:0.97 SiO2 (59–63%), Al2O3 (17–21%), K2O (10–14%), Na2O (3.5–65%), pigments (!5%): B2O3, BaO, CaO, CeO2, TiO2 Matrix (wt%)

Filler (wt%)

BelleGlass enamel light Chromasit S4

911422

Bis-GMA TEGDMA

Borosilicate: 0.6 mm (77%)

C15082

UDMA (23.3%), Decandiol-DMA (10%)

Estenia Enamel E2

202CA

Heliomolar RO 210 B

29157

SureFil Shade

990615

BisGMA, UDMA triethylenglycol DMA BisGMA (14.3%), UDMA (4.4%), Decandioldimethyacrylate (3.3%) BisGMA, urethane modified

Targis Incisal S1

C05051

Tetric Ceram 210

C16761

Copolymer (56%), SiO2: 40 nm (10%) Glass ceramic: 1.5 mm (76%), Al2O3: 20 nm (16%) Copolymer (47%), SiO2: 40 nm (20.2%), ytterbium trifluoride (10.6%) Ba–Al–F–B-silicate: 0.8–10 mm (82%), SiO2 (8%) Ba–Al–Si-glass: 1 mm (72%), SiO2: 40 nm (5%) Barium glass: 1 mm (50.6%), Ba–Al–F–B-silicate: 1 mm (5%), SiO2: 40 nm (5%), spherical mixed oxide: 0.2 mm (5%), ytterbiumtrifluoride (17%)

UDMA (9%), BisGMA (8.7%), Decandiol-DMA (4.6%) BisGMA (8.3%), UDMA (7.6%), triethylenglyco-DMA (4.3%)

Ivoclar Vivadent method (IVOCLAR) After processing and before testing, the specimens (nZ8) were kept dry at a temperature of 37 8C for 24 h. The specimens were mounted in a chewing simulator, which is commercially available from Willytec (Germany). Antagonists were made of pressed IPS Empress ceramic (Ivoclar Vivadent) and were glazed two times at a temperature of 870 8C. The diameter of the rounded conical shaped antagonist was 2.36 mm at a height of 0.6 mm from the cuspal tip to the base. This value has been validated by analyzing the curvatures of the palatal cusp of upper first molars of young adults (unpublished data). The load was set at 50 N, the sliding movement at 0.7 mm. A total of 120,000 cycles of unidirectional antagonist movements with a frequency of 1.6 Hz were carried out. Furthermore, the specimens underwent 320 cycles of thermocycling (5/55 8C). After completing the wear generating procedure, impressions of the material were taken by using a low viscosity silicone material (President light, Colte `ne, Switzerland). After the impression material was allowed to set for 4 h, replicas were made with white super hard plaster (Fuji Superhard rock, GC, Japan) by means of a vacuum, vibrator and pressure device. The plaster replicas were analyzed by means of a commercially available laser device (Laserscan 3D, Willytec,

Germany) and the appropriate match-3D-software using the procedures ‘fit plane’ and ‘subtract plane’; the method is described elsewhere [9]. The software calculated the volumetric (IVVOL) as well as the maximal vertical loss (IVVERT) (1% percentile). The reason for using the 1% percentile was to eliminate extreme values produced by fine dust particles as well as other discrepancies.

Zurich method (ZURICH) This method has been described in detail elsewhere [10]. After processing and before testing, the specimens (nZ8) were kept in water at a temperature of 36.5 8C for 2 weeks. For generating wear palatal cusps, which were cut out of similar upper molars, were pushed against the surface of the specimens (nZ6) with a load of 49 N and a frequency of 1.7 Hz. The specimens were mounted on a rubber socket at a 458 angle, allowing the antagonist to glide over the surface of the test specimen. The test specimens were kept in water with changing temperatures according to a thermocycling protocol (3000! 5/55 8C). After 120,000, 240,000, 640,000, and 1,200,000 loading cycles, the specimens were subjected to toothbrushing with a slurry of toothpaste for 30, 30, 100, and 140 min, respectively [11]. Additionally, at the end of the first phase (120,000 cycles), the specimens were put into a solution of 75% ethanol

Different wear measurement methods: a round robin test for 20 h to simulate chemical degradation. After each thermo-mechanical sequence, the maximal vertical loss of both the specimens (OCA, occlusal contact area) and the antagonists as well as the vertical loss in the contact free area were calculated by using a computerized 3D-scanner [12]. The scanner is driven by step motors which scan the object in 1 mm steps in the z-direction and in 100 mm steps in the xy-direction [2]. In each test sequence, six different materials, which were randomly allocated to the six test chambers, were subjected to wear.

307

were taken with a polyvinylsiloxane impression material (Permadyne Garant, 3MESPE) and poured out with white plaster (Fuji Superhard rock, GC, Japan). The volumetric loss of the wear facet was determined on the plaster models with the 3D laser device (Laserscan 3D, Willytec, Germany), as briefly described above. In the course of each test sequence, eight different materials, which were randomly allocated to the eight test chambers, were subjected to wear.

ACTA method (ACTA) OHSU method (OHSU) This method has been described in detail elsewhere [13]. In principle, enamel cusps were forced into contact with the specimens through a layer of food like slurry (mixture of poppy seeds and PMMA beads). The enamel cusps were drilled out of human upper molars of similar shape, giving them a spherical shape of a diameter of 10 mm. The enamel stylus was first ground with 600 grit and 1000 grit silicon carbide slurry, polished with 5 mm aluminum oxide paste and then ultrasonically cleaned for 1 min. The specimens (nZ10) were stored in water for 24 h at 37 8C before being mounted in the chewing simulator. The cusp was first forced onto the specimen surface with a load of 50 N, sliding across a linear path of 8 mm to produce abrasive wear. At the end of each path, a static load of 80 N was applied to produce localized attrition wear. For an entire test sequence, 100,000 cycles at 1 Hz with unidirectional movements were run. The mean vertical loss of the abrasion and attrition wear facets were measured with a profilometry device at 10 defined tracks. The values of tracks 4–6 correspond to the abrasion wear (OHSUABR) and the tracks 8–9 to the attrition wear (OHSUATT).

Munich method (MUNICH) For this method, a prototype of the wear simulator used for the Ivoclar Vivadent method was utilized, though with a different machine configuration. After processing and before testing, the specimens (nZ8) were kept in physiological sodium chloride solution at room temperature for 7 days. During wear testing the test specimens (nZ8) were kept under permanent contact to the spherical antagonist (Degusit aluminum oxide, 5 mm diameter) with a linear sliding distance of 8 mm (back-and-forth-movement) and a vertical load of 50 N. During chewing simulation, the specimens were rinsed with distilled water at 37 8C. At 10,000, 30,000, and 50,000 double cycles (bidirectional forth-and-back-movement), replicas

Two metal wheels rotate in different directions with about 15% difference in the circumferential speed while having close contact [14]. The test specimens (between 24 and 28) were placed on the circumference of one wheel, while the other wheel serves as antagonist. The force with which the two wheels are put together was adjusted to 15 N. The wheels were placed in a slurry of white millet seeds in a buffer solution. After 50,000, 100,000 and 200,000 cycles, the maximal vertical loss of the test specimens was measured with a profilometry device.

Statistical methods For the sake of clarity, only the wear data after completing all cycles are presented. As the wear analysis of the antagonist is not part of all wear methods, only the wear of the material specimens are shown. For the ZURICH method, only the material loss results of the OCA are presented. Thus, we will compare seven variables related to five methods. The seven variables will be denoted by IVVOL, IVVERT, ZURICH, OHSUABR, OHSUATT, MUNICH and ACTA. As each variable has a different scale it is impossible to compare the variables with each other. In order to apply an analysis of variance for a given variable, the ‘within-material’ variance should be similar for each material. This was obviously not the case for the raw data (data not shown). Thus, a transformation of the data was needed to fulfill this requirement. Several transformations were tried, including a square-root, a cubic-root and a logarithm transformation. It turns out that the logarithm transformation was adequate to stabilize the variance for each variable (Fig. 1).

ANOVA and power of discrimination ANOVA was applied for each of the seven variables using the log-transformed data. The power of discrimination for each variable could then be measured by the R2-value provided by ANOVA. This value represents

308

S.D. Heintze et al.

Figure 1 Logarithmic transformation of raw data for each test method. The 10 materials are represented on the horizontal axis and coded as: 1: Empress, 2: Belle Glass, 3: SureFil, 4: Estenia, 5: Targis 130, 6: Amalcap, 7: Targis 95, 8: Heliomolar RO, 9: Tetric Ceram, 10: Chromasit. Samples are represented by their identification numbers within each material.

the percentage of the total variance in the data, caused by the variation between materials.

Agreement between methods The specimen means of the log-transformed data were used for ranking the 10 materials with respect

to each variable. Instead of ‘absolute ranks’, we have defined ‘relative ranks’, which should take into account the fact that some materials performed nearly equally well for some variables, whereas other materials were clearly better or worse. For each variable, the relative rank was assigned 1 to the best material (the material with the lowest mean

Different wear measurement methods: a round robin test log-value, denoted by m) and the relative rank 10 to the worst material (the material with the highest mean log-value, denoted by M). Then, relative ranks were defined in order to respect the relative differences between materials. Thus, the relative rank of a material with mean log-value x was set to 1C 9ðxK mÞ=ðMK mÞ: While these rankings are useful to compare the materials (see below), they can also be used to measure the agreement between two variables. A possible measure of agreement between two variables V1 and V2 is provided by MADRðV1 ; V2 Þ Z

10 X

jRankði : V1 Þ K Rankði : V2 Þj=10

iZ1

Rank(i:V) denoting the rank of the ith material with respect to the variable V. Thus MADR is the Mean of the Absolute Deviations of the Ranks obtained from two variables. The smaller MADR(V1,V2) the stronger V1 and V2 agree.

309

Results As the focus of this study lays on the comparison between the methods, the presentation of single results for each test method has been dispensed. Some clear outliers were identified in the data (one for the ZURICH method and four for the ACTA method) and were discarded for the rest of the analysis. In the MUNICH method, problems with the wear simulation affecting different materials method led to the exclusion of several specimens (nZ7), affecting different materials. Only in the IVOCLAR and OHSU wear tests, all specimens of each material could be analyzed for wear. Fig. 1 shows a plot of the logarithms of the raw data and the ‘within-material’ variance, which was stabilized by the data transformation. For Fig. 1 and Tables 3 and 5, the materials were arranged according to their global performance given in Table 3.

ANOVA and power of discrimination Comparison of materials Ranks can also be used to compare the materials with each other with respect to ‘global performance’ and ‘consistency of performance’ (across the methods). In order to give the same weight to each method, only the variable IVVERT for the method IVOCLAR and the variable OHSUABR for the method OHSU were considered for this procedure. The global performance Gi of a material can be measured by the median of its relative ranks with respect to the five methods. The consistency of performance Ci was calculated by the mean of absolute deviations with respect to global performance.

Reliability of methods The reliability of the different wear measurement methods is related to its variabilities with respect to the measurement of the same material. In order to assess this parameter the standard deviation could not be used, since the scale differed from variable to variable. Instead, the coefficient of variation, which is the standard deviation divided by the mean, was employed. Thus, lower coefficients of variation indicate a lower relative variability, which in turn may indicate a better reliability. For this procedure the original data have been used since logarithms may happen to be negative and the coefficient of variation is not defined for variables with negative values.

ANOVA led to clear-cut significant results (p! 0.0001) for each variable, meaning that at least some of the materials differed from the others. Each method achieved a good discrimination power (about 90% or more), except for the ZURICH method (R2 only about 50%) (Table 2). This could already be seen in Fig. 1, where the materials do not differ much from each other for this method.

Agreement between methods The relative ranks of the materials in relation to each variable are given in Table 3. One can see at first glance that the MUNICH method was very different from all the other methods. This was confirmed by the MADR values provided in Table 4. By contrast, the values IVVERT and IVVOL, as well as the variables OHSUABR and OHSUATT, strongly agreed with each other (MADRZ0.7 in both cases). It is also noteworthy that ACTA was the method which agreed the most with the other Table 2 Discrimination power of the different variables. ACTA IVVOL IVVERT MUNICH OHSUABR OHSUATT ZURICH

97.2% 95.8% 94.3% 90.8% 89.7% 89.4% 52.2%

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S.D. Heintze et al.

Table 3 Relative ranks of materials with respect to each variable (rounded up or down to one decimal place) as well as global performance and consistency for each material. Material

IVVOL

IVVERT

ZURICH

OHSUABR

OHSUATT

MUNICH

ACTA

Global performance Gi

Consistency Ci

Empress BelleGlass SureFil Estenia Targis 130 Amalcap Targis 95 Heliomolar RO Tetric Ceram Chromasit

5.1 4.4 5.6 7.4 7.1 1.4 6.6 1.0 7.7 10.0

4.0 4.2 4.9 6.3 6.0 2.5 5.7 1.0 7.2 10.0

1.0 2.6 6.3 5.1 5.2 8.0 7.4 9.7 9.2 10.0

1.6 6.2 5.0 3.1 8.8 1.0 8.5 8.2 10.0 10.0

1.9 4.6 6.4 1.1 9.3 1.0 8.8 8.1 10.0 9.2

6.8 3.1 2.6 10.0 2.4 7.7 2.5 1.0 3.7 1.5

1.0 7.2 7.2 4.6 8.3 6.2 8.5 8.8 8.6 10.0

1.6 4.2 5.0 5.1 6.0 6.2 7.4 8.2 8.6 10.0

1.8 1.5 1.2 1.7 1.9 2.5 1.8 3.3 1.7 1.7

variables, in particular with ZURICH, OHSUABR and OHSUATT.

Comparison of materials The best global performance was achieved by Empress (GZ1.6) which was clearly ahead of BelleGlass (GZ4.2), SureFil (GZ5.0) and Estenia (GZ5.1) (Table 3). In contrast to these materials, Heliomolar RO (GZ8.2), Tetric Ceram (GZ8.6) and especially Chromasit showed a poor global performance (GZ10.0). The best consistency was achieved by BelleGlass (CZ1.5) and SureFil (CZ 1.2), whereas Amalcap and Heliomolar RO demonstrated a poor consistency (CZ2.5 and 3.3). The consistency of all the other materials was between 1.7 and 1.9.

Reliability of method When the coefficients of variation are compared with each other (Table 5), it can be noted that the variables IVVERT and ACTA were those which vary the least, followed by IVVOL. In this respect the variables OHSUABR, OHSUATT, MUNICH and above all ZURICH did not perform that well (Table 5). Table 4

IVVOL IVVERT ZURICH OHSUABR OHSUATT MUNICH ACTA

Discussion In the present study, 10 restorative materials, eight composites, one amalgam and one ceramic have been evaluated with regard to wear with five different wear methods, every center receiving specimens made at Ivoclar Vivadent by the same employee with the same batch. The test centers did not know which brands they were testing. A similar attempt of round robin testing was carried out by the ASC MD 156 Task Force on Posterior Composites, including four composite materials (Ful-fil, Heliomolar RO, Silux and Herculite XR) evaluated by four wear simulators (ACTA, Alabama, Minnesota, Zurich) and two ‘pin-on-disc’ sliding wear machines (NIST, University of Indiana). There has been no official publication, but the results were discussed in the postdoctoral thesis of Kunzelmann [8]. Although the statistical analysis was not as comprehensive as in the present study, considerable variations could be seen between the test centers with regard to the ranking of the materials. A similar conclusion can be drawn from the present study. When the relative ranks of the materials were calculated, the results varied tremendously between the test centers. When taking all materials into account and

Agreement between methods measured by MADR (rounded up or down to one decimal place). IVVOL

IVVERT

ZURICH

OHSUABR

OHSUATT

MUNICH

ACTA

0 0.7 2.8 2.3 2.6 3.6 2.8

0.7 0 2.6 2.5 2.8 3.4 2.8

2.8 2.6 0 2.2 2.3 4.6 1.3

2.3 2.5 2.2 0 0.7 5.9 1.3

2.6 2.8 2.3 0.7 0 6.0 1.7

3.6 3.4 4.6 5.9 6.0 0 5.4

2.8 2.8 1.3 1.3 1.7 5.4 0

Different wear measurement methods: a round robin test

311

Table 5 Coefficient of variation (multiplied by 100) for each material and each variable, as well as mean and median of 10 coefficients of variation for each variable. Material

IVVOL

IVVERT

ZURICH

OHSUABR

OHSUATT

MUNICH

ACTA

Empress BelleGlass SureFil Estenia Targis 130 Amalcap Targis 95 Heliomolar RO Tetric Ceram Chromasit Mean Median

42.4 12.3 17.9 26.9 15.4 10.6 20.6 17.8 21.1 17.5 20.2 17.8

19.5 12.5 11.4 14.3 16.0 9.0 15.8 9.2 8.8 8.7 12.5 12.0

34.8 53.6 47.2 50.3 29.1 28.9 55.0 29.9 27.1 44.3 40.0 39.6

15.5 16.7 21.1 19.2 26.7 46.1 36.6 16.8 32.8 18.2 25.0 20.2

26.3 27.7 18.6 45.4 25.2 45.5 27.6 17.7 27.7 21.9 28.4 27.0

25.8 10.9 24.2 46.0 13.4 30.7 45.7 18.8 37.7 34.4 28.8 28.2

51.1 13.0 10.7 14.6 14.0 13.5 12.6 8.0 7.3 8.3 15.3 12.8

comparing the test methods, only ACTA agreed reasonably well with two other methods, ZURICH and OHSU, achieving mean values of absolute deviations of the ranks below 2 (Table 4). On the other hand, MUNICH did not agree with all the other methods at all. Two test methods produced two result values for the same material. IVOCLAR calculated both the volumetric and the maximal vertical loss and OHSU calculated the mean vertical loss at two different sites within the wear facet related to different forces. However, both variables within the same test method were strongly associated (mean values of absolute deviations of the ranks equal to 0.7, Table 4). Considering the single materials and relative ranks, a difference of more than 1.5 units between the variables, OHSUABR and OHSUATT was only found for the materials Estenia and BelleGlass, whereas for IVOCLAR the differences between IVVOL and IVVERT were 1.1 ranks or lower. Although different forces and number of cycles were used, similar findings were observed in other studies which employed the OHSU method [13,15]. For this method no clear tendency between abrasion and attrition could be seen (Table 5). For IVOCLAR the volumetric loss had higher coefficients of variation than the vertical loss, whereas both OHSU variables were similar in this respect. However, the abrasion variables varied less than the attrition variable. For IVOCLAR, vertical loss varied less than volumetric loss. This may indicate a better reliability of the vertical wear. This may be explained by the fact that with IVOCLAR a transversal movement is included in the test set, which generates material loss related to both attrition and microfatigue. Volumetric loss may thus vary more than vertical loss as material breakdown at the margins of

the wear facet contributes to a higher scatter of the values. ACTA achieved the highest discrimination power among the various methods, followed by IVOCLAR (Table 2). The test centers used different wear simulators, different forces, different antagonist materials, different number of cycles, with or without thermocycling, etc. Some used abrasive mediums and different methods to evaluate the material loss. ZURICH additionally included 5 h of simulated toothbrushing between the phases as well as storage of the specimens in ethanol. Wear produced by toothbrushing devices on restorative materials in vitro is in the range of 2–5 mm when using a toothpaste of medium abrasivity [16] and seems to be negligible when compared to the wear of the contact area which is in the range of 50 and 180 mm for the ZURICH device. A force of 50 N is used with ZURICH, MUNICH, IVOCLAR and OHSU (for tests on abrasion). This value is regarded to be a mean value of the physiological biting forces of non-bruxist patients [17]. Higher forces during in vitro simulation lead to higher wear rates [18]. Although the simulators use the same loading force, the actual force created on the material depends on the contact area, which not only varied from simulator to simulator due to differences in the configuration of the antagonist but also changes during the simulation due to flattening of the antagonist. Enamel as antagonist material is used with ZURICH and OHSU. However, the configuration is totally different. While in the ZURICH method, palatal cusps are cut out of first upper molars, in the OHSU test, a spherical stylus with a diameter of 10 mm is fabricated out of extracted molars. The sharper the antagonist, the higher will the wear

312 rate be [8,19]. In a study on 20 extracted first upper molars, a ball with a radius of 0.6 mm was assumed to be the most suitable for the anatomical variations in human molar cusps [20]. No other measurements of this kind were found in the literature. However, analyses of plaster models of 10 20–25-year-old patients carried out with 3D laser device revealed that the mean radius was 1.04 mm in the frontal segment of the cusp and 1.79 mm in the sagital segment (unpublished data by IVOCLAR). The latter finding was the basis for choosing a radius of 1.18 mm for the IVOCLAR method. One explanation for the scattering of the results of the ZURICH method, which was also expressed by its low discrimination power and high coefficient of variation, is the use of enamel as antagonist material. This, however, could not be seen with the OHSU method, which also uses enamel as stylus. While in the OHSU method, the enamel of the antagonist is prepared to a standardized shape and polished, in the ZURICH method the enamel cusps are only standardized by subjective molar selection and cleaned with a rotating nylon brush. Thus, the cusps may differ widely with regard to anatomical form, fluoride content in the outer surface and the amount of aprismatic enamel; the latter two parameters have an effect on the hardness of the enamel stylus. Another factor that may contribute to the scatter of the results is the rubber socket on which the specimens are mounted. The rubber socket should simulate the periodontal ligament and should produce a sliding movement of the specimen, thus leading to a dampening effect during wear simulation. Using finite element analysis Kunzelmann reported in his postdoctoral thesis that the sliding movement was very much dependent on where the specimen was mounted in the chamber [8]; a deviation of 1 mm from the center resulted in a tilting movement rather than a sliding movement. Together with changes in the elastic modulus of the rubber over time, this uncontrollable movement, which is accelerated by thermocycling, may be the cause for the high coefficient of variation. High standard deviation of material loss was also reported in other publications using the same simulator setting [18,19,21]. A substitute for enamel as antagonist may reduce both the variability and the time required to fabricate the antagonists. Steatite, a synthetic material mainly composed of magnesium silicate, has been regarded as a suitable substitute by some authors [22,23,8] while denied by others [20,24]. The MUNICH method uses Degusit material as stylus—a material that consists mainly of highly condensed aluminum oxide. Degusit was chosen because tests revealed a similar material wear rate for spherical

S.D. Heintze et al. Degusit antagonists as that of palatal cusps of upper molars. The coefficient of variation after 50,000 cycles of mastication of the Degusit spheres was lower than that of steatite and enamel. The IVOCLAR method uses IPS Empress (Ivoclar Vivadent) ceramic material for wear testing. Recently, it was reported that Empress ceramic material as antagonist produced a similar wear rate on different composites as enamel antagonists [25,26]. Concerns about the reproducibility of the test results arise as the present wear data do not necessarily match the wear data on the same material using the same wear method. While in the case of Heliomolar RO, for instance, the Zurich method provided a similar result as that published in other studies elsewhere [21], the result for Tetric Ceram was 25% lower in the present study than that of the study by Kersten et al. One confounder in this particular case may be the fact that the ZURICH simulation normally uses mod-fillings in extracted teeth for wear testing, while, by contrast, the present study utilized flat specimens. As far as reproducibility is concerned, a similar finding was noted for the ACTA method, as in the present study Heliomolar RO produced a lower vertical loss after 200,000 cycles than the loss after 100,000 cycles published in another study (mean vertical loss 68 versus 84 mm) [27]. Furthermore, the reported standard deviations were much higher than in the present study. For the MUNICH method, SureFil and Tetric Ceram were in the same statistical subgroup, while another study, which employed the same approach, revealed a significant difference between both materials [6]. For the ACTA method, these differences may be explained by the quality of the abrasive medium used (millet seeds) [28]. For the other methods, however, which do not use abrasive media, these differences are not readily explainable. For the OHSU method, it is difficult to compare data of the present study with those in the literature, as different forces and number of cycles were applied [13]. However, a critical issue for the OHSU wear simulator is the need for calibration before running tests as some mechanical features of the machine may alter decisive wear parameters of the machine. A striking fact was that the MUNICH method did not correspond at all with either of the other methods. The specific test parameters may account for this result. A sliding movement of 8 mm was chosen as an early publication indicated that the sliding distance should be at least twice the diameter of the stylus to ensure that the abraded material particles are easily washed away and the creep effects are reduced [29]. In the other two-body wear approaches, IVOCLAR and ZURICH,

Different wear measurement methods: a round robin test the sliding movements simulate the sliding of teeth that come into contact with each other [30]. In those cases, the antagonist are not in permanent contact with the specimen’s surface, Therefore, the worn particles can be easily washed away by the water flow during the simulation. It can be argued whether the MUNICH approach does reflect wear mechanisms that occur clinically. As for the OHSU method a spherical stylus with a diameter well above the diameter of molar cusps was chosen as pre-tests showed less scattering of results and lower wear [8]. It can be argued whether both effects may result in an additional lower degree of differentiation between the materials. Another influence that has not been systematically evaluated may be the material of the antagonist. The rationale to use 50,000 cycles for the MUNICH method was that experiments with different composite resins showed that during the first 50,000 cycles (bidirectional movementZ100,000 cycles back and forth movements) the increase in wear resulted in a non-linear curve, whereas after 50,000 cycles the wear increased on a linear basis. From the comparison of the results provided by five test centers and the seven wear parameters, respectively, it is clear that at least three different types of wear can be distinguished [3,31]. Type 1, produced by IVVERT and IVVOL. The wear is a consequence of a direct contact between material and antagonist and can be described as a mixed wear (adhesion, attrition and fatigue). For composite materials the discriminating characteristics which control the wear rate are above all the friction coefficient and the surface roughness, which are influenced by the composite structure, the elastic modulus, and the shear strength. The dimension and the volume of filler particles influences the wear resistance as well. A low elastic modulus, for example, leads to a higher contact area, thus to lower pressure. Large filler particles can cause a high friction coefficient, leading to high internal shear stresses in the polymer matrix. A high filler volume content can be beneficial by decreasing the tendency of the material to creep. Type 2, produced by ACTA, which agreed quite well with OHSUABR. This three-body-wear test is mainly an abrasive process with low pressure loads. The wear rate of this test is principally influenced by the hardness and fracture toughness of the material. The OHSU simulator is very different from the ACTA machine. Nevertheless, it seems that the wear processes are similar and only Amalcap and Tetric Ceram represent exceptions in the correlation. Although claimed as ‘attrition wear’, the parameter OHSUATTR is affected by the foregoing abrasion process, and differs only in the load

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amplitude (80 N instead of 50 N). It should actually approach the Type 1 of wear, yet as previously seen, its results agree more with OHSUABR and ACTA than with the IVOCLAR method. Type 3, produced by MUNICH. The simulator is the same as the one employed at IVOCLAR, yet the different configuration parameters yield different results. Mainly because of the much harder antagonist and the permanent back-and-forth 8 mm long sliding, the wear mechanism is mostly due to fatigue. Important material parameters are the elastic modulus, the strain-to-break and the type of filler. A low elastic modulus leads to a large contact area and low contact stresses, material with high strain-to-break are usually more resistant to fatigue, and once abraded, the filler particles can cause a three-body abrasion, enhancing the wear rate: since the contact is permanent, debris are entrapped between antagonist and material surface. Thus, the larger and the harder the composite filler particles, the higher could this ‘self-abrading’ effect be. The ZURICH method is difficult to interpret because of the large scattering of the results. If the type of configuration parameters is taken into account, the wear mechanism should theoretically lie between the wear types 1 and 2. If these differences and the specific mechanical and structural characteristics are taken into consideration, it is now possible to explain the reasons why the same material behaved differently in different wear simulators. Heliomolar RO showed low wear at IVOCLAR and MUNICH but high wear at the other test centers. Heliomolar RO is a composite with microfillers (40 nm) and can be polished to very low roughness values. In two-body attrition wear simulations like IVOCLAR and MUNICH the friction generated during the simulation is very low, resulting in low shear forces and low incidence of microfatigue, which ultimately causes wear [32]. By means of a pin-on-disc wear simulator with on-line wear measurement, Kunzelmann was able to determine the friction force generated by the stylus on different materials [8]. Heliomolar RO showed a slow increase of friction force with a maximum of 7 N compared to Tetric, which had a rapid increase with a maximum friction force of 16 N. Additionally, abraded Heliomolar RO particles are transferred to the antagonist, filling the voids of the abraded antagonist and thereby reducing the friction and wear [32]. This effect was confirmed in the present study when the Empress antagonists were examined for ytterbium trifluoride, the typical ingredient of Heliomolar RO, using EDX. In chewing simulators like ACTA

314 and OHSU, which use an abrasive medium, wear of Heliomolar RO was higher, indicating that the abrasives together with the shear forces of the moving stylus weaken the matrix-filler bond. Materials with a low modulus of elasticity and low filler content are more prone to wear in the ACTA machine compared to materials with high modulus of elasticity and high filler content [33]. However, it cannot be explained with certainty why Heliomolar RO showed high wear rates in the ZURICH two-body wear simulator. Possible explanations are the above-mentioned uncontrollable tilting movements of the rubber socket. The hydrolysis of the matrix described in the literature [34] may have played only a negligible role, even when the long storage time of two weeks and the long simulation procedure are taken into account. The effect of thermocycling on wear (IVOCLAR, ZURICH) has not been systematically evaluated, in spite of the fact that some studies revealed a wear-increasing effect with some materials [35–37]. As the ZURICH method includes 3000 cycles, as compared to IVOCLAR, which uses only 320 cycles, the thermocycling effect may be more pronounced in the ZURICH simulator. The microfilled composite Chromasit showed the highest wear of all materials in all wear simulators except for the MUNICH method. This material has a low filler content of only about 30% weight, resulting in a low modulus of elasticity, flexural strength, fracture toughness and hardness. These properties lead to early wear due to microploughing or plastic flow. The very low elastic modulus (low contact stresses), the low filler percentage (low friction) and the nano-size of the particles (minor self-abrading effect) could explain the low wear in the MUNICH simulator. One study using another approach of wear simulation showed equally high wear rates for Chromasit [38]. BelleGlass, a highly filled hybrid composite, produced low to medium wear rates in all wear simulators. This material possesses general good mechanical properties, partly due to the postheat-curing procedure and the special glass fillers, which are well bonded to the matrix. Consequently, the material is characterized by a general favorable resistance to adhesion, abrasion and fatigue wear. This finding corresponds with the findings of other in vitro studies using the ZURICH simulator [39], the Alabama wear simulator [40] and a pin-on-disc apparatus [41]. The performance of BelleGlass was superior to that of Targis in the IVOCLAR, OHSU and ZURICH simulator and equal to Targis in the ACTA and MUNICH simulator. Like BelleGlass Targis is a fine-particle hybrid composite with a high filler content and it

S.D. Heintze et al. is also post-cured at a high temperature. The mechanical properties are also similar, with the type of filler (the glass particles of Targis contain barium oxide) and size (Targis incorporates slightly larger particles, about 1 vs. 0.6 mm of BelleGlass) being the main differences between the two materials. The susceptibility of Bacontaining glasses to leaching as reported in the literature [42] or a weak bond between filler and matrix could be the reason for the differences between the wear rates. This result is in line with other publications on wear [40,41], but differs from a publication which used the ZURICH simulator [39]. Again, the MUNICH simulator provided different results for that material. The different curing procedures (130/95 8C) had no influence on the wear of Targis, although studies have shown an improvement of physical properties of heat-treated materials compared to light-cured ones in vitro [43] and in situ [44]. Estenia contains a hard glass-ceramic filler and nano-size aluminum oxide particles, which render the material very hard, stiff and highly fracture resistant [45]. Consequently, Estenia exhibited low wear in the OHSU and ACTA simulators, medium wear in the IVOCLAR and ZURICH simulator (high contact stresses due to the high elastic modulus and high friction due to the large hard particles), but excessive wear in the MUNICH simulator. This may be explained by the fact that in the MUNICH simulator aluminum oxide antagonists are used. These antagonists come into contact with the glassceramic fillers of the composite, resulting in a high roughness of the antagonist, which further damages the composite material. The ‘self-abrading’ effect through worn debris is particularly significant for this composite as well. In the literature a low wear rate was found in conjunction with the Alabama wear simulator [40]. SureFil, a packable composite whose physical properties are similar to other hybrid composites [46] showed wear rates similar to BelleGlass in all simulators except for ZURICH. Although only lightcured, SureFil possesses stiffness, hardness, fracture toughness and particle size which are similar to BelleGlass. For the OHSU this result is in line with another study [47]. Tetric Ceram is a direct composite. Its structure is similar to that of Targis, with a similar type and size of Ba-containing glass fillers. The mechanical properties are in general lower than those of Targis; consequently, it showed higher wear rates. Like Targis it can suffer from the possible leaching of barium, resulting in an additional drawback. Contrary to composite resins, the wear mechanisms of amalgam and ceramic materials are

Different wear measurement methods: a round robin test different. Amalgam materials exhibit good wear resistance in many wear simulators, which can be explained by the fact that surface tension is partly compensated by plastic deformation. During the simulation process an oxide layer is continuously formed and removed. Furthermore, in the two-body wear simulators, amalgam is transferred to the antagonist, thus reducing its roughness. It was surprising that the amalgam showed high wear in the ZURICH, MUNICH and ACTA wear simulators. The reasons for that are not easily explainable, especially since studies using the ACTA machine reported low wear rates for amalgam [48]. The Empress ceramic material showed low wear in the OHSU, ZURICH and ACTA simulators but high wear in the MUNICH method and medium wear at IVOCLAR. This may be explained by the type and hardness of antagonist material. In the two latter simulator settings, a ceramic material is used as antagonist, accelerating the wear processes on the ceramic specimen; this is more pronounced with aluminum oxide than with leucite-containing ceramic (correspondingly the Degusit material is also harder than the antagonist used in the IVOCLAR method). It may be argued whether ceramic is a suitable stylus material to test ceramic specimens for wear or whether enamel should rather be used in this case. Other studies in vitro, however, confirm the low wear rate of Empress [49]. All of the wear simulators lack the scientific evidence that the in vitro simulation corresponds to the in vivo situation, in spite of the fact that publications related to three of the simulators tried to establish clinical correlations. The ZURICH method even claims that 1,200,000 cycles in the simulator corresponds to 5 years in vivo. However, this assumption has not been systematically verified in longitudinal clinical studies with different materials and is only based on the extrapolation of 4-year-clinical wear data on 14 mod Dispersalloy amalgam fillings and 6-month data on inlays made of an experimental composite [50,51]. With ACTA and OHSU, which also published reasonable correlation coefficients between in vitro and in vivo data [13,14], these correlations, however, were made using semi-quantitative methods for assessing wear in vivo on pooled data from clinical trials. Methods which use scale models, like the Vivadent Scale or ML Scale, with defined steps at the restoration margin assume wear at the margin to be predictive for general wear and occlusal contact wear [52]. However, they systematically underestimate wear [53], as marginal breakdown, and under-/overfilling

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are possible confounders. Furthermore, wear at the margins is not correlated with wear of the OCA. More sophisticated methods employ special microscopic devices [54], mechanical computer-aided scanners [2,55,56], or, more recently, laser equipment [9]. The problems with all these methods are related to the quality of the impression for the replica production and the need to determine reference points that are assumed to be unchanged. However, randomized prospective clinical trials with an adequate number of subjects, standardized clinical protocols and reliable, valid wear assessment methods are necessary to evaluate the significance of wear methods in vitro.

Conclusions † IVOCLAR (vertical loss) and ACTA were the best methods with respect to the coefficient of variation. The discrimination power of the ZURICH method was clearly inferior to that of the other methods. † The variables related to the same method largely agreed with each other (volumetric and vertical wear for IVOCLAR method; abrasion and attrition for OHSU method). † The MUNICH method strongly disagreed with the other methods. † As the different wear simulator settings measure different wear mechanisms, it seems reasonable to combine at least two different wear settings to assess the wear resistance of a new material. † Well-designed clinical studies, which use an adequate specimen size and quantitative methods to measure the wear, are needed to better assess the clinical significance of the different wear simulator settings.

Acknowledgements The authors would like to thank Till Go ¨hring (University of Zurich), Karl-Heinz Kunzelmann (University of Munich), Martin Rosentritt (University of Regensburg), and John So ¨rensen (University of Portland) for conducting the wear tests in the respective wear simulator settings and sending the raw data to IVOCLAR for further analysis. Furthermore, we would like to thank W. La ¨ngle for the preparation of the specimens and Mrs G. Zellweger for conducting the wear analysis for the Ivoclar method.

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