University of Groningen

Predictability of clinical wear by laboratory wear methods for the evaluation of dentalrestorative materialsHeintze, Siegward Dietmar

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155

Chapter 6

Wear of ten dental restorative materials in five wear

simulators – Results of a round robin test

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Abstract 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, SureFil, Targis cured at 95°C and 130°C, 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 centre received samples, which Ivoclar Vivadent had made using the same batch. The test centres 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 centre, 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 centres. 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. Material-wise, the best global performance was achieved by Empress, which was clearly ahead of belleGlass, SureFil and Estenia. In contrast, Heliomolar, 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 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.

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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 behaviour 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 material 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 [8]. This test included four composite materials, which were

evaluated by means of four wear methods. The test revealed important variations

with regard to the ranking of the materials relative to the test method. However, the

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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 dental restorative materials in five

wear generating and wear analysing 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.

Material Batch Composition

Amalcap Plus

C25527 Ag (70.1%), Sn (18%), Cu (11.9%). Powder:Hg = 1:0.97

Empress TC1 C35146 SiO2 (59-63%), Al2O3 (17-21%), K2O (10-14%), Na2O (3.5-65%), pigments (<5%): B2O3, BaO, CaO, CeO2,

TiO2 Matrix (weigth%) Filler (weigth%)

belleGlass enamel 911422 Bis-GMA TEGDMA

Borosilicate 0.6µm (77%)

Chromasit S4 C15082 UDMA (23.3%), Decandiol-

DMA (10%)

Copolymer (56%), SiO2 40nm (10%)

Estenia Enamel E2 202CA BisGMA, UDMA Triethylenglycol DMA

Glass ceramic 1.5µm (76%) Al2O3 20nm (16%)

Heliomolar 210 B 29157 BisGMA (14.3%), UDMA (4.4%),

Decandioldi-methyacrylate (3.3%)

Copolymer (47%), SiO2 40nm (20.2%), Ytterbium trifluoride

(10.6%)

SureFil Shade 990615 BisGMA, urethane modified

Ba-Al-F-B-silicate 0.8-10µm (82%), SiO2 (8%)

Targis Incisal S1 C05051 UDMA (9%), BisGMA (8.7%), Decandiol-

DMA (4.6%)

Ba-Al-Si-glass 1µm (72%), SiO2 40nm (5%)

Tetric Ceram 210 C16761 BisGMA (8.3%), UDMA (7.6%),

Triethylenglycol DMA (4.3%)

Barium glass 1µm (50.6%), Ba-Al-F-B-silicate 1µm (5%),

SiO2 40nm (5%), spherical mixed oxide 0.2µm (5%),

Ytterbium trifluoride (17%) Table 1: List of materials, batch-number and composition

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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°C/130°C). The same batch of each material has been used to produce flat

samples for all participating institutes. To avoid discrepancies in the production of the

samples, all samples were produced by the same employee at Ivoclar Vivadent. For

the composite materials similar colours 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,

95°C and 130°C. 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 centres, whereas the time interval until

testing occurred was taken into account. This interval was different for each test

method.

Ivoclar Vivadent method (IVOCLAR)

After processing and before testing, the samples (n=8) were kept dry at a

temperature of 37°C for 24 hours. The samples 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°C. The diameter of the 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°C/55°C). After completing the wear generating procedure, impressions of the

material were taken by using a low viscosity silicone material (President light,

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Coltène, Switzerland). After the impression material was allowed to set for four hours,

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

analysed by means of a commercially available laser device (Laserscan 3D, Willytec,

Germany) and the appropriate match-3-D-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 (n=8) were kept in water at a temperature of 36.5°C for

2 weeks. For generating wear palatal cusps, which were cut out of similar upper

molars, were pushed against the surface of the specimens (n=6) with a load of 49 N

and a frequency of 1.7 Hz. The specimens were mounted on a rubber socket at a 45°

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 (3,000x 5°C/55°C). After 120,000, 240,000, 640,000, and

1,200,000 loading cycles, the samples were subjected to toothbrushing with a slurry

of toothpaste for 30 min, 30 min, 100 min, and 140 min respectively [11]. Additionally,

at the end of the first phase (120,000 cycles) the samples were put into a solution of

75% ethanol for 20 hours 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 (CFA) were calculated by using a computerized 3D-scanner [12]. The

scanner is driven by step motors which scan the object in 1µm steps in the z-

direction and in 100 µm 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.

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

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human upper molars of similar shape, giving them a spherical shape of a diameter of

10mm. The enamel stylus was first ground with 600 grit and 1000 grit silicon carbide

slurry, polished with 5µm aluminium oxide paste and then ultrasonically cleaned for 1

min. The specimens (n=10 ) were stored in water for 24 hours at 37°C before being

mounted in the chewing simulator. The cusp was first forced onto the specimen

surface with a load of 50N, sliding across a linear path of 8mm to produce abrasive

wear. At the end of each path, a static load of 80N was applied to produce localized

attrition wear. For an entire test sequence, 100,000 cycles at 1Hz 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

to 6 correspond to the abrasion wear (OHSUABR) and the tracks 8 to 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 samples (n=8) were kept in physiological sodium chloride

solution at room temperature for 7 days. During wear testing the test specimens

(n=8) were kept under permanent contact to the spherical antagonist (Degusit

aluminium oxide, 5 mm diameter) with a linear sliding distance of 8 mm (back-and-

forth-movement) and a vertical load of 50N. During chewing simulation the samples

were rinsed with distilled water at 37°C. At 10,000, 30,000, and 50,000 double cycles

(bidirectional forth-and-back-movement), replicas 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)

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

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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 occlusal contact area (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 fulfil 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 (see Figure 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 the percentage of the total variance

in the data, caused by the variation between materials.

Agreement between methods

The sample 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 log-value, denoted by m) and the

relative rank 10 to the worst material (the material with the highest mean log-value,

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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 1+9(x-m)/(M-m).

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

MADR (V1,V2) = ∑

= 1

10

i | Rank (i:V1) – Rank (i:V2) | / 10

Rank (i:V) denoting the rank of the i th 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.

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.

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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. Figure 1

illustrates a plot of the logarithms of the raw data and the “within-material" variance,

which was stabilized by the data transformation. 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 (n=7), affecting different materials. Only in the IVOCLAR and OHSU wear

tests all specimens of each material could be analysed 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 Figure 1 and Tables 3 and 5 the materials were arranged according to their

global performance given in Table 3.

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: belleGlass, 3: SureFil, 4: Estenia, 5: Targis 130°C, 6: Amalcap, 7: Targis 95°C, 8: Heliomolar, 9: Tetric Ceram, 10: Chromasit. Specimens are represented by their identification numbers within each material.

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ANOVA and power of discrimination

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

Figure 1, where the materials do not differ much from each other for this method.

ACTA 97.2% IVVOL 95.8% IVVERT 94.3% MUNICH 90.8% OHSUABR 89.7% OHSUATT 89.4% ZURICH 52.2% Table 2: Discrimination power of the different variables

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 (MADR=0.7 in both cases). It is also

noteworthy that ACTA was the method which agreed the most with the other

variables, in particular with ZURICH, OHSUABR and OHSUATT.

Material IVVOL IVVERT ZURICH OHSUABR OHSUATT MUNICH ACTA Global Perfor-mance Gi

Consist-ency Ci

Empress 5.1 4.0 1.0 1.6 1.9 6.8 1.0 1.6 1.8 belleGlass 4.4 4.2 2.6 6.2 4.6 3.1 7.2 4.2 1.5 SureFil 5.6 4.9 6.3 5.0 6.4 2.6 7.2 5.0 1.2 Estenia 7.4 6.3 5.1 3.1 1.1 10.0 4.6 5.1 1.7 Targis 130 7.1 6.0 5.2 8.8 9.3 2.4 8.3 6.0 1.9 Amalcap 1.4 2.5 8.0 1.0 1.0 7.7 6.2 6.2 2.5 Targis 95 6.6 5.7 7.4 8.5 8.8 2.5 8.5 7.4 1.8 Heliomolar 1.0 1.0 9.7 8.2 8.1 1.0 8.8 8.2 3.3 Tetric Ceram

7.7 7.2 9.2 10.0 10.0 3.7 8.6 8.6 1.7

Chromasit 10.0 10.0 10.0 10.0 9.2 1.5 10.0 10.0 1.7 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

166

IVVOL IVVERT ZURICH OHSUABR OHSUATT MUNICH ACTA IVVOL 0 0.7 2.8 2.3 2.6 3.6 2.8 IVVERT 0.7 0 2.6 2.5 2.8 3.4 2.8 ZURICH 2.8 2.6 0 2.2 2.3 4.6 1.3 OHSUABR 2.3 2.5 2.2 0 0.7 5.9 1.3 OHSUATT 2.6 2.8 2.3 0.7 0 6.0 1.7 MUNICH 3.6 3.4 4.6 5.9 6.0 0 5.4 ACTA 2.8 2.8 1.3 1.3 1.7 5.4 0 Table 4: Agreement between methods measured by MADR (rounded up or down to one decimal place). Comparison of materials

The best global performance was achieved by Empress (G=1.6) which was clearly

ahead of belleGlass (G=4.2), SureFil (G=5.0) and Estenia (G=5.1) (Table 3). In

contrast to these materials, Heliomolar (G=8.2), Tetric Ceram (G=8.6) and especially

Chromasit showed a poor global performance (G=10.0). The best consistency was

achieved by belleGlass (C=1.5) and SureFil (C=1.2), whereas Amalcap and

Heliomolar demonstrated a poor consistency (C=2.5 and C=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).

Material IVVOL IVVERT ZURICH OHSUABR OHSUATT MUNICH ACTA

Empress 42.4 19.5 34.8 15.5 26.3 25.8 51.1 belleGlass 12.3 12.5 53.6 16.7 27.7 10.9 13.0 SureFil 17.9 11.4 47.2 21.1 18.6 24.2 10.7 Estenia 26.9 14.3 50.3 19.2 45.4 46.0 14.6 Targis 130 15.4 16.0 29.1 26.7 25.2 13.4 14.0 Amalcap 10.6 9.0 28.9 46.1 45.5 30.7 13.5 Targis 95 20.6 15.8 55.0 36.6 27.6 45.7 12.6 Heliomolar 17.8 9.2 29.9 16.8 17.7 18.8 8.0 Tetric Ceram

21.1 8.8 27.1 32.8 27.7 37.7 7.3

Chromasit 17.5 8.7 44.3 18.2 21.9 34.4 8.3 Mean 20.2 12.5 40.0 25.0 28.4 28.8 15.3

Median 17.8 12.0 39.6 20.2 27.0 28.2 12.8

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.

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Discussion

In the present study 10 restorative materials, eight composites, one amalgam and

one ceramic have been evaluated with regard to wear with five wear methods, every

centre receiving specimens made at Ivoclar Vivadent by the same employee with the

same batch. The test centres 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 4 composite materials (Ful-fil, Heliomolar, 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 centres

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 centres. When taking all materials into

account and 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

168

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 centres 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 five hours of simulated toothbrushing between the phases as

well as storage of the samples in ethanol. Wear produced by toothbrushing devices

on restorative materials in vitro is in the range of 2-5 µm 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 µm 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 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 ten 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,

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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 samples are mounted. The

rubber socket should simulate the periodontal ligament and should produce a sliding

movement of the sample, 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 sample was mounted in

the chamber [8]; a deviation of 1 mm from the centre 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 [8,22,23] while denied by others [20,24]. The MUNICH

method uses Degusit material as stylus – a material that consists mainly of highly

condensed aluminium oxide. Degusit was chosen because tests revealed a similar

material wear rate for spherical 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, 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

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normally uses mod-fillings in extracted teeth for wear testing, while, by contrast, the

present study utilized flat samples. As far as reproducibility is concerned, a similar

finding was noted for the ACTA method, as in the present study Heliomolar produced

a lower vertical loss after 200,000 cycles than the loss after 100,000 cycles published

in another study (mean vertical loss 68 µm versus 84 µm) [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, 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 movement = 100,000 cycles back and forth

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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 centres 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

influence 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 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.

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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 showed low wear at IVOCLAR and MUNICH but high wear at the other

test centres. Heliomolar 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 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 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, using EDX. In

chewing simulators like ACTA and OHSU, which use an abrasive medium, wear of

Heliomolar 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 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 3,000

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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 post-heat-curing procedure and the special glass fillers, which are well

bonded to the matrix. Consequently, the material is characterized by a general

favourable 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 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 µm vs. 0.6 µm of belleGlass)

being the main differences between the two materials. The susceptibility of Ba-

containing 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°C/95°C) 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 aluminium oxide particles,

which render the material very hard, stiff and highly fracture resistant [45].

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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 aluminium oxide antagonists are used. These antagonists

come into contact with the glass-ceramic 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 light-cured, 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. 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 aluminium

oxide than with leucite-containing ceramic (correspondingly the Degusit material is

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also harder than the antagonist used in the IVOCLAR method). It may be argued

whether ceramic is a suitable stylus material to test ceramic samples 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 are possible confounders. Furthermore, wear at

the margins is not correlated with wear of the occlusal contact area. More

sophisticated methods employ special microscopic devices [54], mechanical

computer-aided scanners [2,12,55], 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.

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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 sample 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 Göhring (University of Zurich), Karl-Heinz

Kunzelmann (University of Munich), Martin Rosentritt (University of Regensburg), and

John Sorensen (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. Längle for the preparation of the

specimens and Mrs. G. Zellweger for conducting the wear analysis for the Ivoclar

method.

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