الفهرس 
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Abstract
With the increased needs for esthetics the use of full ceramic restoration is becoming more common because of its superior esthetics compared to metal ceramic restorations. Dental porcelain, with its properties of wear resistance, and aesthetics is considered the preferred material to replace natural tooth tissue in prosthetic dentistry. Unfortunately all ceramic restorations with superior esthetics were limited to three or four units for fixed partial denture restorations because of strength limitation. Zirconia frameworks showed an excellent success rate, but clinical trials have shown that despite a low frequency of core fracture in zirconia supported partial fixed dental prostheses (FDPs), they have a higher rate of porcelain veneer fracture than metal ceramic FDPs. To overcome the chipping problem, zirconia is now used in direct occlusal contact, either covered with a thin glaze layer or simply polished.
The aim of this study was to evaluate the wear of human enamel when opposed to zirconia and lithium disilicate, and to compare the roughness effect of glazed and polished zirconia and lithium disilicate on human enamel wear.
The ceramics samples in this study were divided into four groups:
Polished lithium disilicate (PL), glazed Lithium disilicate glass (GL), polished translucent zirconia (PZ), and glazed translucent zirconia (GZ). Extracted human premolars from patients in the age range from 15-20 years as part of orthodontic treatment were collected.
All ceramic and enamel samples were weighted using a sensitive balance with an accuracy of 0.0001 grams. The mean surface roughness (Ra) of all specimens –ceramics and enamel samples- was measured. Ceramic and enamel samples were mounted opposing each other in a wear-simulating device designed at the department of mechanical engineering Faculty of Engineering, Ain Shams University. The wear simulation was done for 5000 cycles at a frequency of 250 cycle/min for a horizontal traveling distance of 0.5 mm under a load of 2kg. After the wear test, all ceramics and enamel samples were weighted and the differences in weight before and after the test were calculated. The surface roughness (Ra) value of all ceramics and enamel samples was also measured and the difference in Ra was calculated. One ceramic sample from each group and its opposing enamel sample were scanned after wear test using an electron microscope. One sample from each ceramic group and one enamel sample that were not subjected to the test were also scanned.
The results of this study revealed that polished zirconia and polished lithium disilicate caused the least wear to opposing enamel with no significant difference between them, while enamel samples opposed by glazed lithium disilicate showed the highest significant weight loss. There was no significant difference between the wear of polished zirconia and polished lithium disilicate, while glazed lithium disilicate showed the highest significant weight loss.
Regarding the surface roughness, polished Zirconia showed the least difference in mean surface roughness followed by polished lithium disilicate. Whereas glazed lithium disilicate showed the highest difference in mean surface roughness. Difference of mean surface
roughness for enamel antagonist was the highest for that opposing glazed lithium disilicate, followed by that opposed to glazed Zirconia. The least difference in surface roughness was that of enamel opposing polished Zirconia. The results of Pearson’s coefficient of correlation between enamel wear and Ceramic roughness showed a very strong positive correlation.
SEM images showed rough surface for all ceramics groups, zirconia surfaces (polished and glazed) showed a worn surface free of cracks while lithium disilicate surfaces (polished and glazed) showed multiple cracks and grooves.
The enamel surfaces opposed by zirconia showed a smoother surface than that opposed by lithium disilicate, and it showed multiple cracks and delamination. Enamel opposed by lithium disilicate didn’t show cracks but it showed accumulation of chipped of enamel and glass ceramic particles.