Abstract:

Dental restorations such as crowns and bridges, as well as other biomechanical prostheses, are experiencing a rapid shift toward ceramic materials, partially for their strength and bioinertness but more so for their aesthetics. However, ceramics are brittle and susceptible to fracture which accounts for millions of dollars annually in replacement costs and can cause significant patient discomfort and loss of productive lifestyle. Although occlusal loading is nominally compressive, with bite forces supported in individual ''dome-like'' structures (crowns) or in frameworks with connectors (bridges), some tensile stresses at the contact and flexural surfaces are inevitable. Cracks tend to follow paths where these tensile stresses are greatest. Therefore, improving the resistance to contact and flexural damage of ceramics is the key to developing ceramic prostheses with greater long-term performance. Recent advances in theoretical and experimental work from our laboratory and elsewhere have demonstrated that damage resistance of ceramic prostheses can be substantially improved by controlled gradients of elastic modulus at the ceramic surface. This is because the gradient diminishes the intensity of tensile stresses and simultaneously transfers these stresses from the ceramic surface into the interior, away from the source of failure-inducing surface flaws. The materials science has been validated by sound fracture mechanics theory, finite element analysis and experimental work. It is interesting to note that biological systems (e.g., teeth, bone, shells) often employ layered and specifically oriented architectures to produce extremely damage-tolerant and fracture-resistant structures. For example, tooth enamel has strong gradients in elastic and other mechanical properties, arguably a contributing factor in the survivability of natural dentition. Although our findings are examined in the context of possible applications for next-generation ceramic prostheses, implications of our studies have broad impact on civil, structural, and an array of other engineering applications.