Ceramics in Dentistry. J. Robert Kelly

Fig 2-5 Tensile stress acting on a surface flaw creates quite different responses in metals and ceramics. Crack tips in metals are blunted due to the ductile flow of atoms, which reduces the stress intensity. In ceramics, however, ductile flow is not energetically feasible, and there is no mechanism to reduce stress intensity at the crack tip other than by the creation of two new surfaces—ping!

      While strength measures provide valuable screening information, they do not directly predict relative clinical behavior for a number of important reasons. First, the failure mechanism operating clinically will generally be different from that generated in the laboratory. Clinical restorations generally withstand millions of low loads in a wet environment instead of one load in laboratory air. All ceramics are susceptible to the slow growth of cracks under these clinical conditions, and this susceptibility is not measured during simple bend strength testing. Thus, ceramics weaken at different rates during intraoral service. Second, the clinical stress state is more complicated than that during bending. As illustrated in Fig 2-6a, crowns are fully supported by dentin or core materials and often bonded by cement. As will be discussed more fully in chapter 4, failure strengths of such a structure are dependent on the stiffness of the support (elastic modulus), the square of the ceramic thickness, the thickness of the cement, the loaded contact area (ie, the wear facet), and the quality of the cement-ceramic bond.⁵ None of these variables play any role in laboratory bend testing. Unlike crowns, multiple-unit fixed dental prostheses (FDPs) are complex beams often failing from the gingival portion of their connectors (Fig 2-6b). Failure strengths function linearly with connector width, with the square of the connector height, in a complex fashion with their radii of curvature (the smaller the radii, the weaker the connector), and with the mobility of the abutment teeth (mobile teeth increase connector stresses). For connectors veneered with weaker porcelain, failures can initiate in this weaker material or from defects at the core-veneer interface. Chapter 3 discusses the clinical implications of this potential failure in the case of veneered lithium disilicate FDPs, which are not highly successful, whereas full-thickness (nonveneered) prostheses are. Again, these variables (ie, weakness of connectors) are not evaluated during standardized tensile strength testing. But prostheses are complex, multimaterial structures that do not behave as simple beams do in bending!