The effect of orientation of the acetabular component on outcome following total hip arthroplasty with small diameter hard-on-soft bearings

We assessed the orientation of the acetabular component in 1070 primary total hip arthroplasties with hard-on-soft, small diameter bearings, aiming to determine the size and site of the target zone that optimises outcome. Outcome measures included complications, dislocations, revisions and Delta OHS (the difference between the Oxford Hip Scores pre-operatively and five years post-operatively). A wide scatter of orientation was observed (2SD 15 degrees). Placing the component within Lewinnek's zone was not associated withimproved outcome. Of the different zone sizes tested (+/- 5 degrees, +/- 10 degrees and +/- 15 degrees), only +/- 15 degrees was associated with a decreased rate of dislocation. The dislocation rate with acetabular components inside an inclination/anteversion zone of 40 degrees/15 degrees +/- 15 degrees was four times lower than those outside. The only zone size associated with statistically significant and clinically important improvement in OHS was +/- 5 degrees. The best outcomes (Delta OHS > 26) were achieved with a 45 degrees/25 degrees +/- 5 degrees zone. This study demonstrated that with traditional technology surgeons can only reliably achieve a target zone of +/- 15 degrees. As the optimal zone to diminish the risk of dislocation is also +/- 15 degrees, surgeons should be able to achieve this. This is the first study to demonstrate that optimal orientation of the acetabular component improves the functional outcome. However, the target zone is small (+/- 5 degrees) and cannot, with current technology, be consistently achieved.