Biomechanical failure scenarios for cemented total hip replacement

Retrieval studies have indicated that debonding of the stemcement interface in Total Hip Arthroplasty precedes clinical failure of femoral cemented components. This study addressed the mechanisms that play a role in the debonding process by analyzing how debonding is likely to proceed in the course of t ime. It was investigated whether debonding is an immediate process or if it is likely to develop slowly over t ime, which interface stress components contribute particularly to its progression, if full or partial coating patches or roughness patterns could potentially arrest its progress, and whether the mechanical integrity of the cement mantle is likely to be compromised by the debonding process. To answer these questions, a threedimensional Finite Element (FE) model of the femoral THA reconstructions was developed and used to simulate the debonding process. The results showed that debonding started in the tip region of the stem, but was directly followed by debonding in the proximal, medial-anterior region. These debonded areas expanded particularly due to high shear stresses at the interface directly around the debonded areas. The probability of debonding, as measured by an interface failure index remained constant as debonding progressed. This indicates that, for this particular design, much less surface area is required for load transfer than provided by the stem, and that the debonding process does not necessarily accelerate quickly once debonding is initiated. Tensile stresses in the cement mantle remained virtually unaffected until more than fifty percent of the stem-cement interface had debonded. When more interface area debonded, the maximal cement stresses slowly increased to a level twice as high as the initial ones. Locally, cement stresses were sometimes elevated at the edges of debonded area. These local stress peaks were released again when the debonded zone expanded further along the interface. Submi t ted to the Journal of Arthroplasty