PREDICTING THE PRESENCE OF ZONE 1 RADIOLUCENCY IN THE EARLY POST OPERATIVE PERIOD FOLLOWING CEMENTED TOTAL HIP REPLACEMENT

Abstract

Introduction: Radiolucency in the DeLee and Charnley zone 1 of the acetabulum in the early post operative period is a strong predictor of long-term failure of the cemented acetabulum. There is a wide variety in the acetabular anatomy of patients presenting for total hip replacement. Zone 1 radiolucency is an indicator of the failure of penetration of cement into the relatively hard cortical bone encountered in zone 1. Cement penetration is achieved by adequate preparation, achieving containment and effective pressurisation.

Aim: To use pre operative radiological measurements to predict the risk of radiolucency around the cemented acetabular component post operation.

Hypotheses:

1. Dysplastic acetabuli are associated with a higher incidence of zone 1 radiolucency.

2. Retroverted acetabuli are associated with a higher risk of zone 1 radiolucency.

3. Radiolucencies progress in the early post operative period.

Materials and Methods: A cohort of 300 patients undergoing cemented THR in our institution was identified. Radiographs performed on the patients pre operatively, post operatively, at first follow up (6 weeks to 3months) and follow up at 1 year were analysed. The following measurements of the native acetabulum were performed: Tonnis grade of osteoarthritis, Crowe grade of dysplasia, acetabular index of depth to width, ACM angle, peak to edge distance, acetabular index of weight bearing zone, centre-edge angle of Widberg, acetabular angle of Sharp, cross over sign and posterior wall sign to assess retroversion, acetabular inclination and anteversion angle. Post operative films were then assessed for the presence of zone 1 keyholes, incidence and degree of radiolucency, cup inclination and anteversion.

Results: Patients with an acetabulum outside the normal range were more likely to have a post operative radiolucency. Radiolucency tended to progress with time. Zone 1 keyholes appeared to terminate this progression. Retroverted and steeply inclined acetabuli demonstrated a higher incidence of radiolucency. A large change in version from the native to prosthetic acetabulum was associated with an increased risk of radiolucency.

Conclusion: Thorough pre operative radiological assessment of the acetabular anatomy allows us to predict patients at high risk of post operative radiolucency. Patients with unsuitable anatomy may be more appropriate for an alternative method of fixation or require different techniques of acetabular preparation or augmentation in order to reduce their risk of loosening of the acetabular component in the long term.