IN VITRO WEAR OF 4 DIFFERENT UNIVERSAL COMPOSITES
initial stage of testing. There is no agreement in the literature about the material and the shape of the antagonists to be used in in vitro wear studies. The following materials have been reported 34: stainless steel, natural teeth, tooth cusps shaped to a specific shape and highly polished, leucite reinforced ceramic( Empress), Steatite( magnesium silicate ceramic), Degusit( aluminum oxide) and Zirconium oxide. 36 Average radius of natural cusps is 1.04 and 1.79 mm. 34 Artificial materials are used with diameters of 3mm, 36 4mm, 20, 33 6mm 37, 38 or 12mm. 27, 32 In the present study 6mm Steatite antagonists were chosen. For horizontal movement, both 0.3 and 0.7 mm have been used, we used 0.7 mm as it is more commonly used. As a measurement tool a laser scanner was used. Heintze et al. 39 have shown that laser scanners give the same results as the ones obtained with optical or mechanical profilometers. Due to the notable differences in in vitro wear testing methods described above it is almost impossible to directly compare the present results to other studies. Therefore, comparisons only to studies done with Willitec / Mechatronik wear testing machines are reported. As in the studies of Heintze et al. 34, 39 and Wang et al. 27 the wear development over the number of cycles was linear. We can confirm Heintze’ s data 34, 39 that the Antagonist wear is about half the wear of the composite materials. In contrast, the wear reported for a multitude of materials by Ivoclar Vivadent R & D is slightly lower than the wear found in this study. This may be due to the higher load used in the present study and the use of different antagonists( Empress vs Steatite). Lazaridou 38 found for Tetric Evoceram 0.3297 mm 3, while Tetric N Ceram in the present study showed 1.15 mm 3, which is substantially higher. Again, there are differences in the method which may explain the different findings. The load used in the present study was 20 % higher, and we used thermoclycling in contrast to Lazaridou et al. 38 Most in vitro wear test methods demonstrate a steep increase in wear initially, also called wear-in or run-in phase, and then a flattening of the curve that appears increasing in a linear fashion, thereafter. The wear profile of individual sample tested and the profile of mean of the material groups all exhibited the wear-in pattern( Fig. 2). The duration of wearin varies among material groups. In the literature,
27, 34, 39, 40 this linear relationship is often recognized but not used for calculating wear rates. Often for comparison, the final volumes of wear were used for comparison. Since the total volume of wear also depends on the number of cycles and the extent of vertical movement, it becomes necessary have both information available for comparison. Wear-in phase does not truly reflect the wear of the material but include the wear generated during the initial stage when the composite and the antagonist are adjusting to accommodate each other in forming a sliding interface. The linear portion of the curve can be used to calculate the wear rate without the influence of the wearin phase. When the load and the horizontal movement remains the same, the effect of testing duration by cycles disappears when wear rates are used. A straightforward comparison of in vitro wear will become possible. Lastly, the unit for the wear rate should also be standardized. The unit of mm 3 is commonly used in discussing of volumetric wear and µ m is used in presenting wear in depth. We used the unit µ m 3 / cycle for wear rate in this study. However, the quantity of µ m 3 / cycle is very small, a factor of 10 3 is needed( Table 3). The unit of mm 3 / cycle, on the other hand is so big that a factor of 10-6 is needed. As a compromise, we suggest that mm 3 / megacycle be used in expressing volumetric wear for comparison. As such the values of wear rates shown in Table 3 would be presented without the factor of 10 3.
5. Conclusions
The four tested composites showed a linear development of wear over the number of cycles and showed a wear which was comparable to wear in other studies. It can be expected that in the clinical reality they will behave similarly to other composites of their class.
Acknowledgments
The authors declare no conflict of interest related to this study. There are no conflicts of interest and no financial interests to be disclosed.
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