Aim: To investigate the possible influence of demographic and biomechanical factors on stability of the Endler cementless polyethylene acetabular cup.
Methods: This was a retrospective cohort study. Seventy-two patients (25 men, 47 women) bearing 82 Endler prostheses, all of which were implanted by the same surgeon in the period between 1985 and 1991, were invited for a control visit (final visit) in July 2003. During time between the surgery and the final visit, the patients were followed-up regularly and assessed for clinical and radiological signs of the Endler cup instability based on Krugluger and Eyb's criteria. The Kaplan-Meier product limit method and the Cox proportional hazard regression analysis were used to investigate the survival of the cup (time since implantation till the diagnosis of instability) and possible influence of the following factors: age and body mass index at the time of surgery, gender, achieved acetabular cup inclination angle, and acetabuloplasty and/or trochanter osteotomy performed during surgery.
Results: The median follow-up period was 15 years (range 5-18). Cumulative survival rates at 5, 10, 15, and 18 years were 97.6% (95% CI=94.2-100), 74.4% (95% CI=64.9-83.8), 53.7% (95% CI=42.9-64.5), and 44.5% (95% CI=29.5-59.6), respectively. The median survival time was 18 years (13-18). Unsatisfactory acetabular cup inclination angle (<41 or >49 degrees) was a negative predictor of the cup survival (P=0.026), whereas the interaction between the inclination angle and an unsatisfactory body mass index (>upper normal limit) was of borderline significance (P=0.056). The analyzed demographic and biomechanical factors apparently explained only a minor part of the survival variability (R(2)=0.173).
Conclusion: This study further documents the impact of the acetabular cup inclination angle achieved at surgery on the Endler cup survival. However, it also suggests that the prosthesis survival might be influenced by other, non-biomechanical factors.