Multidirectional deformation in fully congruent acetabular components

Most clinical studies have used femoral head migration as an index of aceta bular wear, but a previous study showed multiple wear vectors in 30% of ret rieved acetabular components with noncongruent liners. The origin of multip le wear vectors is unclear, and it has been suggested that polyethylene cre ep in a noncongruent shell might influence deformation on the articular sur face. We used shadowgraph and volumetric methods to evaluate the extent and direction of surface deformation of 37 retrieved polyethylene liners that were fully congruent to a single design of metal backing. The results show that multiple deformation vectors are relatively common in retrieved acetab ular cups (27% in this study) and are independent of congruency between lin er and metal backing, rim impingement, and backside creep. Polyethylene lin ers with multiple wear vectors were significantly thinner than those of cup s with a single vector. The origin of multiple vectors is still unclear, bu r clinical and laboratory studies measuring linear wear alone without recog nizing multiple vectors underestimate total in vivo volumetric wear.