An elasto-plastic finite element model for polyethylene wear in total hip arthroplasty

Abstract

A new finite element model (FEM) based on an elasto-plastic behavior of ultra high molecular weight polyethylene (UHMWPE) was used to study the wear behavior of UHMWPE acetabular cup, which has a 32 mm diameter femoral head. The model imposed a plastic yield stress of 8 MPa on the UHMWPE so that any stresses beyond this would automatically be redistributed to its neighbor. The FEM model adopted a unique mesh design based on an open cube concept which eliminated the problems of singularities. Wear prediction combined the influences of contact stress, sliding distance and a surface wear coefficient. The new model predicted significantly higher volumetric wear rate (57 mm 3/yr) well within the average reported clinical values. The model was also used to study the effect of friction and clearance between the acetabular cup and the femoral head. Increase in friction increased the volumetric wear rate but did not appear to affect the linear wear rate, which remained at 0.12±0.02 mm/yr. The predicted wear was sensitive to clearance. It was found that when the clearance was close to 0 and >0.5 mm, severe wear occurred. The best clearance range was between 0.1 and 0.15 mm where the average linear wear rate was 0.1 mm/yr and the volumetric wear was 55 mm 3/yr. The present work indicates the importance of avoiding too tight or too loose a diametrical clearance.