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.