Factors governing the mechanical behavior of the implant-porous coating-trabecular bone interface

Abstract

Mechanical advantages can arise from the use of implants with a porous surface layer that allows bone ingrowth. This study first outlines the factors which can influence the mechanics at the interface between the porous coating and the surrounding bone. The influence of some of these factors is assessed quantitatively by finite element stress analysis. The basic assumption made prior to the stress analyses is that bone ingrowth has occurred and thus that skeletal fixation has been established. A mechanically successful fixation by ingrowth implies that a critical value for the shear stress at the porous coating-bone interface is not exceeded. The stresses have been calculated for a modelled prosthesis with a porous coating, surrounded by trabecular bone and subjected to either a compression or a bending loading mode. The results show that the value of the elastic modulus of the porous coating has no major influence on the porous coating spongy bone interfacial shear stress upon compression and bending. This finding is explained by the deformation and stress pattern of the whole structure. It is pointed out that load uptake by the cortex is essential and that the underlying trabecular bone where the stresses in the bone are the highest, will probably build up a gradient in elasticity in order to minimize the stresses at the interface and in the underlying bone.