THE ROLE OF CUP POSITION, CHANGE IN FEMORAL OFFSET AND LEG LENGTH ON THE RISK OF DISLOCATION AFTER TOTAL HIP ARTHROPLASTY

Abstract

Introduction: Dislocation is the most common complication resulting in re-operation after total hip arthroplasty. This study investigates the association between acetabular prosthesis position, changes in femoral offset and leg length and the risk of dislocation.

Patients and Methods: All total hip arthroplasties performed over the past 17 years at one institution were reviewed. The posterolateral approach was used in all cases. Only hips that included all of the following were included in the study: diagnosis of primary osteoarthritis, no previous surgery, unconstrained liner. 3682 hips met the inclusion criteria. 60 hips (1.6%) sustained a dislocation. Cup inclination and version was determined from scanned radiographs using Hip Analysis Suite software (University of Chicago) in all hips that dislocated and a control group of 60 patients matched for femoral head size, sex, age at surgery, side of hip replacement, time from surgery, BMI, type of prosthesis and bearing surface. We compared femoral offset and length against the contralateral normal hip, on standardised radiographs. Therefore dislocation cases where the contralateral hip had been replaced, where arthritic changes were present, or where previous surgery had been undertaken were not included in the analysis. 24 dislocating hips were measured and compared with 48 controls matched using the same criteria as above. Radiographs were analysed using Hip Analysis Suite.

Results: There is a statistically significant difference (p=0.025) in anteversion between dislocators and matched controls. Inclination is not significantly associated with dislocation (p=0.536).

There is a relative risk of 3.0 of dislocation in cups with ≤15 degrees of anteversion compared with > 15 degrees of anteversion. This difference in dislocation is statistically significant (p< 0.01).

Increased femoral offset compared with the normal contralateral hip is statistically significantly associated with an increased risk of dislocation (p=0.03). Change in leg length is not associated with dislocation risk.

Discussion: Decreased cup anteversion is associated with an increased risk of dislocation in hips operated on via the postero-lateral approach. Our results indicate that the surgeon should aim for a minimum of 15 degrees of anteversion to reduce the risk of dislocation.

The increase in femoral offset in the prosthetic hip compared with the normal contralateral hip and its association with dislocation may be due to intraoperative attempts to compensate for an unstable hip by increasing offset. These results indicate that a surgeon should be cautious when increasing femoral offset alone to try and compensate for a potentially unstable hip. Other factors, for example acetabular version should be addressed, with readjustment of cup position intra-operatively if required.