applied fracture mechanics

122Applied Fracture Mechanicsrelates to the brittleness of ceramics, making them prone for chipping and fracturing [21-27].In the 1980s and 1990s, crowns were fabricated as enamel-like monoliths from micaceousglass-ceramics (Dicor, Dentsply/Caulk, Milford, DE) and high leucite porcelains (IPSEmpress, Ivoclar, Schaan, Lichtenstein), but these ceramics showed unacceptably highfailure rates and were soon replaced by improved ceramics [28, 29]. Subsequent crowndesign has focused on retention of porcelain as an aesthetic veneer fired to much strongeralumina-based ceramics, either glassinfiltrated (InCeram, Vita Zahnfabrik, Bad S.ackingen,Germany) or pure and dense (Procera, Nobel Biocare, Goteborg, Sweden) alumina, assupporting cores. Although alumina-based crowns continued to replace metal-basedcrowns, failure rates remained an issue [30]. During the past 15 years, ultra-strong coreceramics, e.g. yttria-stabilized zirconia (Y-TZP) and alumina-matrix composites (AMC)[31]have gained in popularity but have yet to be documented regarding their clinical long-termsuccess.Clinically, bulk fractures are the reported cause of all ceramic crown failure whether thecrown is a monolith or a layered structure. According to a fractographic evaluation byThompson et al.[32], in which they evaluated fractured and recovered Dicor and Cerestorecrowns, they found that failures generally did not ensue from damage at the occlusal surface.Instead, for Dicor the cracks emerged from the internal surface, while in the case ofCerestore, the initiation occurred at the porcelain/core interface inside the core materials. Inother studies it has been shown that radial cracks are initially contained within the innercore layer, but subsequently propagate to the core boundaries, ultimately causing irretrievablefailure. This failure mechanism raises an interesting question: If the ceramic core materialsare so strong, why do the cracks not originate in the weak outer porcelain? In the case ofporcelain-fused-to-metal, porcelain failures do seem to occur preferentially in the porcelain,although there is some indication that such failures may be preceded by plasticity in theductile metal [33]. That in turn raises the question: What are the important material parametersthat govern these failure modes in crown structures, and how may they be optimized?Maybe McLean’s [33] suggestion from 1983 that layered all-ceramic crowns should performwell if the core fracture strength exceeded the yield strength of base metal alloys (about 400–500MPa for gold).Before diving deeper into the fracture strength of the core, let us accept that there are severalfactors which can be associated with crack initiation and propagation in dental ceramicrestorations. These factors include: (a) shape of the restoration; (b) micro-structuralinhomogeneities; (c) size and distribution of surface flaws; (d) residual stresses and stressgradients, induced by polishing and/or thermal processing; (e) the environment in contactwith the restoration; (f) ceramic/cement interfacial features; (g) thickness and thicknessvariation of the restoration; (h) elastic module of restoration components; and (i) magnitudeand orientation of applied loads. The possible interactions among these variables complicatethe interpretation of failure analysis observations, explaining why fracture behavior of allceramiccrowns is rather tricky problem to understand.Even though McLean’s[34] suggestion that the core fracture strength exceeded the yieldstrength of base metal alloys (about 400–500MPa for gold) might be tempting to adopt to, it