A finite element stress analysis of an endodontically restored tooth.

Abstract

Previous work has indicated that teeth restored with posts and pins of non-noble metals can fail by longitudinal fracture. In particular, teeth restored by the Wiptam method have shown significant demineralization of the dentine by metallic salts due to corrosion of the Ni–Co–Cr post. This investigation concentrated on the stress distribution within a Wiptam restored tooth for a variety of mechanical and thermal loads calculated using the finite element method. It is shown that the dentine is more highly stressed in restorations with long posts than short posts for a given load. The maximum tensile stresses exist in the vicinity of the bottom of the post and the FEM calculations indicate a similar pattern of stress distribution to the observed cracking pattern. In no case were the calculated stresses above the fracture stress for dentine. However, by simulating the gradual leaching of cement from the dentine/gold-porcelain interface, it is shown that significant tensile stresses well above the fracture stress develop in the zinc phosphate cement in the pulp chamber. Subsequent breakdown of this layer, with the consequent ingress of oral fluids to the Ni–Co–Cr post, then leads to corrosion and demineralization of the dentine. The demineralized dentine is then readily fractured by normal masticatory forces. Thermal transients of the order of 25 C and above also generate high stresses in the cementing medium in the pulp chamber. Normal thermal cycling can also lead to cement breakdown and subsequent tooth fracture by the processes already described.