Polyethylene wear vector in vivo: A three-dimensional analysis using retrieved acetabular components and radiographs

Polyethylene wear of the acetabular component can bt described as one or mo re vectors. To help clarify the mechanisms of wear advancement ill vivo, we used a combination of retrieved implants and radiographs to describe the t hree-dimensional wear vectors in total hip arthroplasty. The wear vectors i n 41 retrieved implants from a single manufacturer were measured with use o f the shadowgraph technique, and the spatial orientation of each implant wa s calculated from serial anteroposterior pelvic radiographs, On the basis o f the combination of the wear vector in the implant and implant orientation in the pelvis, the wear vectors in vivo were determined. The mean wear vec tor was directed 8.1 degrees lateral in the coronal plane and 4.1 degrees p osterior in the sagittal plane. The wear vectors in vivo showed a relativel y wide range of directions, not necessarily coinciding with the commonly pr esumed resultant force in the hip. The wear vectors were not associated wit h the spatial orientation of the acetabular components. but cups with impin gement demonstrated more anterior wear than did those without impingement. Our results suggest that the process of polyethylene wear is not as simple as previously described and that several factors influence advancement of w ear in vivo.