Acetabular erosion following bipolar hemiarthroplasty: A role for the size of femoral head?

Introduction: Femoral neck fractures are the most frequent fractures in the elderly and hemiarthroplasty is the treatment of choice. The objective of this multicenter study is to identify predictive factors of acetabular erosion after bipolar hemiarthroplasty (surgery between 1997 and 2007) in a mobile independent population during a follow-up of ten years. Materials and methods: Data were prospectively collected and retrospectively analyzed. Inclusion criteria were: age >60 and <85 years, BMI < 35, normal Abbreviated MiniMental Test score, ability to walk 0.8 km and live independently, non-pathological fracture, hip with no or minimal osteoarthritic changes, and availability of clinical and radiological follow-up. For each Patient were recorded: demographic data, comorbidities, time from fracture to surgery, characteristics of the implant, duration of surgery. Patients included underwent clinical and radiological follow-up. Results: Overall, 209 Patients met inclusion criteria. A press-fit implant was performed in 172 subjects; in contrast a cemented prosthesis was implanted in 37 patients. Nineteen patients underwent implant revision to total hip arthroplasty for acetabular erosion and pain. Classification of X-ray using Baker criteria showed a grade 0 in 54.5%, a grade 1 in 19.6%, a grade 2 in 18.1% and a grade 3 in 7.6%. Multivariate analysis revealed that the size of the femoral head (FH) was the only predictive factor of a higher risk of acetabular erosion. The Kaplan-Meier survival curve verified the risk of implant revision in Group 1 (FH sized >48 mm) and Group 2 (FH sized <48 mm). The probability of implant revision for acetabular erosion at ten years from surgery were 5.5% in Group 1 and 15.6% in Group 2. Conclusion: In bipolar hemiarthroplasty smaller head size lead to a polar wear implying a higher risk of acetabular erosion and migration; in our population this risk was consistent with the use of implant head <48 mm diameter. Considering the absolute risk of a smaller FH size, the surgeon must evaluate the accuracy of measurement of the caliber, since the size can be significantly underestimated.