IN VITRO WEAR OF 4 DIFFERENT UNIVERSAL COMPOSITES
Figure 1a. Wear facets were mostly symmetrical and round. By tracing the perimeter in the Keyence Microscope, the radius of the circle determining the“ wear dome” could be determined
By using geometric software( Geomagic Control 2014, Geomagic, Cary, NC, USA), the scanned data was used to measure the wear of the samples after each round. The flat surface of the sample was used as a reference plain. All wear facets at 120,000 cycles were examined with a digital microscope and digital images recorded( Keyence VHX 1000, Keyence Corporation of America, Elmwood Park, NJ, USA). The wear of the Steatite antagonists was not measured with the laser scanner due to difficulty of establishing reference plane. They were determined indirectly by the geometric relationship( Fig. 1a). The radius( b) of the wear facet was measured using the Keyence digital microscope. Knowing the radius( r) of the sphere, we calculated the height of the abraded dome( h) using the following formula( Fig 1 b),
The volume of the wear dome( V) was calculated using the following spherical cap formula from standard mathematical tables,
Samples C1, C3, C4, F2 and F7, experienced delamination at the interface between increments before conclusion of the experiment. Therefore they were excluded from the analyses. Due to imbalanced numbers of specimen per group of composites, GLM( SAS, 9.4; SAS Institute Inc., Cary, NC, USA) was used to analyze the variance of wear volume of the composites and antagonists. After the initial wear-in period, linear relationship between the wear volume and number of cycles from 2,000 to 120,000 cycles was apparent for all samples investigated. Linear regression was performed using SAS to determine the slope
Figure 1b. With the known radius( r) of the sphere and the radius of the wear facet( b), the volume of the“ wear dome can be calculated
of the curve. The values represent the wear in μm 3 / cycle of the samples and were called wear rate in this paper. GLM was used to determine statistical differences of wear rates among the four composite groups. The correlation coefficients( r 2) between wear of antagonists and volumetric wear of composites, and between wear of antagonist and wear rate of composites were calculated by linear regressions.
3. Results
GLM analyses showed that after 120,000 chewing cycles there were no statistical differences in total volumetric wear among the four composites( p = 0.1183) and wear of antagonist( p = 0.3027) with its respective composite. Linear regressions of the composite wear volume vs. number of cycles showed that the degree of fit( r 2) was > 0.99 for each of the specimen investigated. GLM analysis of the values of wear rate determined for each specimen shows there was statistically significant difference among composites groups( p = 0.0488). It is important to note that the p-value was almost at the point of no significant difference( p = 0.05). The mean values and standard deviation of the total wear volume at 120,000 chewing cycle, wear of respective antagonist and the wear rates are shown in Table 3. The mean cumulative wear volumes as a function of the number of cycles, along with the best fit straight line of the mean values for each group of composite are displayed in Fig. 2. Analysis of the correlation showed that both wear volume and wear rate increased slightly as the wear of antagonist increased but with low correlation coefficient( r 2 = 0.0027 and r 2 = 0.2081, respectively). Some illustrative pictures of wear facets of the composites are shown in Fig. 3.
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