In a series of 300 total hip replacements, nine (3 per cent) dislocated.
Precise measurements of the orientation of the acetabular cup were made and
it was found that anterior dislocations were associated with increased
acetabular-component anteversion. There was no significant correlation
between cup-orientation angle and posterior dislocation. The dislocation
rate for cup orientation with anteversion of 15 +/- 10 degrees and lateral
opening of 40 +/- 10 degrees was 1.5 per cent, while outside this "safe"
range the dislocation rate was 6.1 per cent. Other factors that were
documented include time after surgery (with the greatest risk in the first
thirty days) and surgical history (with a greater risk in hips that have
had prior surgery).