Abstract
INTRODUCTION
The introduction of hard-on-hard bearings and the consequences of increased wear due to edge-loading have renewed interest in the importance of acetabular component orientation for implant survival and functional outcome following hip arthroplasty. Some studies have shown increased dislocation risk when the cup is mal-oriented which has led to the identification of a safe-zone1. The aims of this prospective, multi-centered study of primary total hip arthroplasty (THA) were to: 1. Identify factors that influence cup orientation and 2. Describe the effect of cup orientation on clinical outcome.
METHODS
In a prospective study involving seven UK centers, patients undergoing primary THA between January 1999 and January 2002 were recruited. All patients underwent detailed assessment pre-operatively as well as post-op. Assessment included data on patient demographics, clinical outcome, complications and further surgery/revision. 681 primary THAs had adequate radiographs for inclusion. 590 hips received cemented cups. The primary functional outcome measure of the study was the change between pre-operative and at latest follow up OHS (OHS). Secondary outcome measures included dislocation rate and revision surgery. EBRA was used to determine acetabular inclination and version.
The influence of patient's gender, BMI, surgeon's grade and approach on cup orientation was examined. Four different zones tested as possibly ± (Lewinnek Zone, Callanan's described zone and zones ± 5 and ±10 about the study's mean inclination and anteversion) for a reduced dislocation risk and an optimal functional outcome.
RESULTS
There were 21 dislocations (3.1%) and 8 (1.2%) patients required revision at a mean follow up of 7 years. Experienced surgeons (2=0.047) and those operating with the patient in the lateral decubitus position (p=0.04) were more likely to achieve a cup orientation within any of the tested zones. Surgical approach (2=0.14) and patient's BMI (2=0.93) had no influence on whether a cup was within or outside any zone. There was no difference in dislocation rate between the posterior and anterio-lateral approaches (2=0.88). None of the zones tested had a significantly reduced dislocation risk (2=0.13), nor revision risk (2=0.55). OHS was not different for patients with cups within or outside any of the zones tested (p=0.523).
DISCUSSION
There was a wide variation in cup orientation. Despite the wide scatter in cup orientation, no safe zone could be identified that would reduce dislocation and revision rate, nor improve patient reported outcome (OHS). Hence, these data suggest that acetabular component orientation should not be considered predictive of patients' early/mid-term complication/revision rate and outcome following THA
