The spatial location of impingement in total hip arthroplasty.

Impingement between acetabular and femoral components produces wear debris and results in abnormal loads on the edge of the implant. To characterize further the spatial location of impingement and the design and alignment factors associated with impingement, we reviewed 111 retrieved acetabular components from a single manufacturer. The location of impingement in the pelvis was determined by combining the location of impingement in the retrieved implants and the spatial orientation of the acetabular components measured from available radiographs. Evidence of impingement was identified in 39% of the retrieved implants and involved the posterior portion of the acetabulum in all cases. Posterior impingement was probably the result of femoral extension and external rotation, a motion that occurs during the toe-off phase of the gait cycle. Cups with impingement were more anteverted than those without impingement (P = .016). There was a significant inverse association between impingement and the size of the femoral head, and the mean head-to-neck diameter ratio for implants with impingement was smaller than that for implants without impingement (P < .0001). Factors that appear to be associated with impingement include i) excessive cup anteversion combined with posterior positioning of the extended rim and ii) femoral components with relatively small head-to-neck diameter ratio.