IM0410

I research _ dental surfaces 22 I implants4_2010 _Discussion Wear of dental hard tissues is a naturally occur- ing and inevitable phenomenon. However, when human enamel is opposed by ceramic (or other restorative material) the enamel experiences accel- erated wear. Developing a material of equivalent hardnesstohumanenamelwasseenasthesolution to the enamel wear problem.1, 2 However, it has now been shown that microstructural differences and changes in surface topography are much more im- portant than hardness.3 To account for these vari- ables the chemical make up of the veneering porce- lain was (and still is being) modified to produce the conventional aluminous porcelains e.g. VITADUR ALPHA, machinable ceramics e.g. VITA Mark II and the hydrothermal porcelains e.g. Duceram-LFC. Studies on the wear characteristics of these porce- lains produced varying results13, 14, 15 with no signif- icant reduction in abrasiveness to human enamel. The pattern of enamel wear varies according to the ceramicsystemusedanditssurfacecharacteristics.4, 6,7,8,9,13,16 Theamountofwear that will occur on both the restored surface and opposing enamel is an im- portant consideration in dentistry as this will affect tooth movement and the vertical dimension of oc- clusion. Restorative dentistry must therefore pro- vide restored occlusal surfaces that are wear resist- ant themselves and more importantly, do not pro- mote excessive wear of the opposing occlusion.4, 5 The major driving force in dentistry today is aes- thetics. Several all ceramic systems have been de- veloped.Mostofthesesystemsrelyonacoremate- rial onto which a porcelain veneer is applied. Some of the most studied systems are Dicor™ (a castable glass ceramic), IPS Empress (leucite core, lithium disilicate core in Empress II and E-max) and In-Ce- ram(aluminiumoxidecore).Theadventofimproved zirconia systems such as Procera™ and Etkon™ shows a wide availability of systems on the market today. However, all these systems rely on veneering porcelains and several studies have demonstrated thattheseveneeringporcelainsaremoreabrasiveto human enamel than the core material itself.4, 6, 7, 8, 9, 13, 16 Zirconiabasedcorematerialshaverecentlybeen introduced. Several companies have introduced CAD/CAM systems to produce cores for natural toothrestorationsaswellasabutmentsandbridges for implant based single and multiple unit restora- tions. This study examined the in vitro wear of hu- man enamel against a zirconia (YTZP) core material with two surface finishes and various dental porce- lains designed specifically for veneering zirconia and alumina cores to produce all ceramic restora- tions.ApolishedtypeIVgoldsurfaceactedasacon- trol surface. The porcelain surface was compared to the zirconia surface to determine if the application of the veneering porcelain was beneficial or detri- mental to human enamel i.e. is it less harmful to enamel to have the zirconia core exposed or ve- neered? Studies have shown that a polished dental porcelainsurfaceistheleastabrasiveporcelainsur- face finish to human enamel.10, 11, 12 The entire study did evaluate the autoglaze and powder glaze sur- facefinishesaswellbutonlythepolishedfinishwas thesameforbothzirconiaandthetestporcelainsso it was the only data presented here. Weardoesnotoccurinisolation.Itisnotjustthe enamel that is abrading but also the opposing restorative surface. The ideal situation is for the enamel and the opposing restorative material to have the same physical and mechanical properties. This way the restorative: enamel interface should wear at the same rate as an enamel: enamel inter- face. To date, the least abrasive surface to human enamelisahighlypolishedtypeIVgoldsurface.5 This is seen as the standard to which all other materials are compared. Most, if not all, studies show that dental porcelains (regardless of surface finish) and all the ceramic core materials are far more abrasive to human enamel than a polished type IV gold sur- face.4, 6, 7, 9 Zirconiahasgreaterstrengthandflexibilitycom- paredtoaluminiumoxide,allowingthinnercoresof equivalentstrengthtobefabricated.Thisallowsthe use of zirconia in situations where inadequate oc- clusal clearance exists for an aluminium oxide core. The only alternative would then be a metal-ceramic restoration with exposed metal occlusal contacts. Zirconiahasnowlargelysupersededaluminiumox- ide as the core material of choice for most all ce- ramic restorations. Initially the zirconia cores had a bluish white colour; however, shaded versions are now available to improve aesthetics. ThePowervaluefortheone-wayANOVAdataset forthepolishedzirconiaandporcelainsurfacesver- sus the type IV gold control is 1.00. The Power value for the one-way ANOVA data set for zirconia versus type IV gold is 0.774 for zirconia mm wear data set makingtheresultsstatisticallysignificantalthough the sample size is small. Considering the polished zirconia vs type IV gold results (Tab. 1) samples 1, 3 and 4 produced virtually identical levels of enamel wear(mean0.035mm)whereassample2produced significantly more wear (0.055 mm). Thus sample 2 had a significant negative impact on the data set. The significance level for this study is P = 0.05. The results of this experiment show that the zirco- nia core material in its as-manufactured state and