Late dislocations in patients with Charnley total hip arthroplasty.

I analyzed the cases of thirty-two patients in whom a Charnley total hip arthroplasty had dislocated for the first time between five and ten years postoperatively. I evaluated the possible factors that caused the late dislocations. Most of the factors were similar to those that were also present in a control group of patients who had had an arthroplasty that had not dislocated and in a group in which dislocation had occurred at variable times postoperatively. Two significant factors did emerge. First, the patients with late dislocation had a greater range of motion, especially in flexion, than those in the two control groups. Second, the acetabular component showed radiographic evidence of loosening in more of the patients in the group with late dislocation than in either of the control groups. I postulated, but did not prove, that stretching of the pseudocapsule of the hip over time and extremes of motion may lessen soft-tissue constraints and allow for late dislocation.