IM0410

_Introduction Porcelain fused to metal restorations are the most widely used restorations in dentistry today. However, in someclinicalsituationsalack of inter-occlusal space does not allow for the appropriate thicknessofopaqueandden- tine porcelains to be applied to the metal substructure. Consequently, the opaque layer may still be visible and imparts a matt, lifeless appearance to the final restoration.Thedentistshouldalsoconsidertheef- fect the restorative surface will have on opposing enamel.Thehardnessofaceramicmaterialhasbeen viewed as a predictor of its potential to abrade hu- man enamel, and thus manufacturers have been pushed to develop ceramics with equivalent hard- ness to enamel to try and solve the wear issue.1, 2 However,themicrostructuralelementsofaceramic aswellasfracturetoughnessandhardnessallinflu- ence the wear characteristics of the material.3, 4, 5 In an attempt to replicate the colour, texture, translu- cencyandshapeofthenaturaldentitionavarietyof all ceramic systems have been developed. Many of these systems have a dense core material, replacing themetalsubstructure,ontowhichdentalporcelain is veneered to achieve the desired aesthetics. Zirco- niahasrapidlybecomethematerialofchoiceforuse as the core of all ceramic (implant and tooth borne) restorations. The translucency and colour of these cores allow the ceramist to produce a natural look- ing, aesthetic restoration. Yttrium-Stabilized-Zir- conia (YTZP) is one such material and several com- panies have recently introduced CAD/CAM based systems for milling such units. The physical proper- tiesofzirconiahavebeenwidelydocumented:how- ever, the effect of these materials on the natural dentitionandonotherrestorativematerialshasnot been fully investigated. In normal masticatory function if the veneering porcelainislostduetomodificationoftheocclusion (chairsideadjustmentbythedentistorattrition)the zirconia core may come into direct contact with the opposing dentition. Another situation where this might occur is when there is insufficient interarch space for the veneering porcelain resulting in oc- clusal stops directly on the zirconia core material. One must then consider if it is possible to place the I research _ dental surfaces In vitro wear of human enamel opposingYTZP zirconia And various polished dental porcelain surfaces Authors_T. R. Tambra, M. E. Razzoog, B. R. Lang, R. F. Wang, B. E. Lang, UK 14 I implants4_2010 Fig. 3 Fig. 1a Fig. 2 Fig. 1b