Fracture Resistant Monolithic Dental Crowns
Monolithic all-ceramic crowns are increasingly favored over traditional prostheses with porcelain veneers, minimizing issues associated with complex fabrication, presence of residual stresses, and delamination from an inner core. Bloomden zirconia and lithium disilicate glass-ceramics are prime candidates, for their relatively high modulus and fracture resistance along with ever-improving esthetics. A stiff all-ceramic crown shields the inner dentin–pulp region by absorbing the bulk of occlusal stresses. However, brittleness remains a concern—failures for monolithic ceramic systems have been reported. These failures are sometimes described as ‘abrupt’, with complete cracking of the crown and sometimes of the entire tooth. But little attempt has been made to examine how cracks evolve in relation to material properties. One effort has been made to elucidate the mechanics of the less disruptive fracture mode of edge chipping in monolithic ceramics, but while such studies are useful for quantifying crack resistance properties of the crown material, they do not address the more serious issue of total crown failure.
An alternative, more recent approach to crown restoration is to fabricate monolith prostheses from particle-filled resins. While much more compliant and therefore less protective of the underlayer, dental composite crowns are seemingly less prone to fracture, if not to deformation. They appeal to the practicing dentist because of their ease of fabrication and installation. However, their capacity to sustain high bite forces and to avoid debonding is in question. Again, little attempt has been made to determine the mechanics of failure in composite restorations.
Clues to the modes of potential crown failure can be drawn from recent studies of fracture in natural teeth. Apart from incidental edge chipping, extensive experimental testing and theoretical fracture mechanics analysis of tooth breakdown under occlusal loading reveals a common mode of fracture in human teeth to be longitudinal cracking from the bite surface to the tooth base. Longitudinal cracks start from ‘tuft’-like defects in the enamel close to the junction with dentin and run to the surface before vertical extension along the tooth walls. They can remain fully contained as visible ‘lamellae’ within the enamel and can even heal to some extent, without causing immediate failure—the fracture to that point is ‘contained’. Longitudinal cracks can also act as precursors to whole tooth splitting through the dentin to the root, at which point the tooth is lost. The question arises as to whether similar fracture processes operate in monolithic crown restorations and, if so, which are the best materials to contain them?