Analysis of model variables and fixation post length effects on stresses around a prosthesis in the proximal tibia.

A two-dimensional, finite element model of a single-posted, metal, tibial prosthetic component implanted with PMMA in the proximal tibia is developed. The effects upon the stresses at the PMMA-bone interface and in the cancellous bone around the component due to the inclusion in the model of the cortical shell and the heterogeneity and anisotropy of the cancellous bone, are demonstrated. Various lengths of the fixation post are considered. The cortical shell adds support and stiffness to the bone structure, and allows generation of significant stresses proximally. The heterogeneity of the cancellous bone, as assumed here, dictates the form of the stress distributions and the magnitudes and locations of the peak stress values around the post of the component. Peak stress values can occur at locations proximal to the post tip. The anisotrophy of the cancellous bone significantly alters the magnitudes of the stresses. Lower stresses along the post interface and higher stresses beneath the plate of the prosthesis are seen in the anisotropic models. Lengthening of the fixation post reduces stresses proximally, but the effects of post lengthening upon the maximum stress values around the post are highly dependent upon the assumed properties of the cancellous bone.