Dent Mater 10:172-177, May, 1994 Effect of cement film thickness on the fracture resistance of a machinable glass-ceramic Susanne S. Scherrer ~, Waldemar G. de Rijk 2, Urs C. Belser ~, Jean-Marc Meyer 1 Department of Prosthodontic¢ School of Dentistry, University of Geneva, Geneva, SWITZERLAND ~Department of Restorative Dentistry, University of Illinois at Chicago, College of Dentistry, Chicago, Illinoi~ USA ABSTRACT Objectives. The aim of this study was to determine the fracture resistance of a machinable glass-ceramic plate cemented to a resin composite block as a function of the cement film thickness for two types of cement. Methods. Ceramic plates were cemented to resin composite blocks using either zinc phosphate cement or a resin composite cement. For the zinc phosphate cement, the film thickness was 33 + 8 tim or 128 + 8 lim; for the resin composite cement, the thickness ranged from 26 + 11 ~m to 297 + 48 pm. The elastic modulus was determined for each of the cements. Fracture loads were obtained by using a spherical steel indenter in the center of the glass-ceramic plate. The Weibull distribution was used for the statistical analysis. Results. For glass-ceramic plates cemented with zinc phos- phate cement, the fracture resistance was independent of the film thickness. When the resin composite cement was used, a gradual decrease of the fracture strength was observed that became statistically significant at a cement thickness of 300 ~m or more. The characteristic fracture strength of glass-ceramic plates cemented with the resin composite cement was about 75% higher than when using the zinc phosphate cement. This difference is attributed to the bonding of the resin cement to the ceramic plate and the supporting structure. Significance. The findings of this study suggest that the resis- tance to fracture due to indentation of the glass-ceramic may not be affected by the cement film thickness as much as previ- ously thought. INTRODUCTION All-ceramic restorations are thought to provide better esthet- ics compared to the porcelain-fused-to-metal restorations. However, the brittle nature of ceramics and their poor frac- ture resistance have raised concerns about the longevity of such restorations (McLean, 1983; Moffa, 1988). The cause of failure is usually difficult to ascertain, as many factors can be implicated in producing a failure, such as porcelain thickness, crown design, tooth preparation, load orientation, localization and magnitude of mechanical stresses. Several factors have been found to improve the fracture resistance of ceramic crowns, in particular the use of adhesive resin-based cements to bend the ceramic to tooth structure (Eden and Kacicz, 1987; Grossman and Nelson, 1987) as well as substrates with a high modulus of elasticity (E) beneath all-ceramic restorations (de Rijk and Scherrer, 1992; Scherrer and de Rijk, 1993). Fur- thermore, the elastic modulus of the cement also constitutes a factor that influences the fracture resistance. In a previously reported study, it was observed that a cement that failed to reach final set reduced the fracture resistance of all-porcelain crowns over 50% compared to a fully set cement (Scherrer and de Rijk, 1991). The effect of cement thickness on the fracture resistance of all-ceramic restorations is not well established. This is espe- dally true for the resin-based cements. Crown retention has been measured as a function of cement thickness (Ilzuka et al., 1987), showing a decrease in retention with increasing cement film thickness. The effect of cement thickness on stresses under a porcelain crown during cyclic loading was reported by Derand (1975). In that report, the cement film thickness was actually kept constant while the crown size was altered. A small change in the fracture load was seen between the two crown configurations, which was attributed to the modification of the aspect ratio of the cement layer. The purpose of this study was to address the question of whether or not the cement film thickness significantly influ- enced the compressive fracture resistance of an all-ceramic crown. The complex geometry of a full molar crown makes a quantitative determination of the fracture strength under occlusal loading extremely difficult; hence, the current experi- ment used a specimen design with simple geometry: a point load onto a uniformly supported square ceramic plate. This configurationis commonlyused to assess the fracture strength of glass and ceramic plates (Johnson et al., 1973) and falls within the domain for stress analysis of thick plates as given 172 Scherrer et aL/Cement thickness and ceramic fracture resistance