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

Polyethylene wear of the acetabular component can be described as one or more vectors. To help clarify the mechanisms of wear advancement in vivo, we used a combination of retrieved implants and radiographs to describe the three-dimensional wear vectors in total hip arthroplasty. The wear vectors in 41 retrieved implants from a single manufacturer were measured with use of the shadowgraph technique, and the spatial orientation of each implant was calculated from serial anteroposterior pelvic radiographs. On the basis of 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 vector was directed 8.1 degree lateral in the coronal plane and 4.1 degree posterior in the sagittal plane. The wear vectors in vivo showed a relatively wide range of directions, not necessarily coinciding with the commonly presumed resultant force in the hip. The wear vectors were not associated with the spatial orientation of the acetabular components, but cups with impingement 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 wear in vivo.