Continuous periprosthetic bone loss around the TOP® cup and inferior survival rate at an 8-year follow-up: a prospective cohort study.

ABSTRACT

BACKGROUND: The trabeculae-oriented pattern (TOP®) cup was designed to minimize acetabular periprosthetic bone loss. In our previous prospective study comprising 30 patients with a two-year follow-up we found a substantial decrease in periprosthetic bone mineral density (pBMD) in the proximal and medial regions of the TOP cup. The present study aims to investigate pBMD changes in the mid-term and how this affects implant survival. METHODS: We followed the previous cohort and estimated implant survival by Kaplan-Meier analysis, evaluated pBMD with dual-energy X-ray absorptiometry (DXA) and clinical outcome using the Harris Hip Score (HHS). RESULTS: Mean follow-up was 8.6 (range 7.8-9.1) years. The eight-year implant survival rate for cup revision for all reasons was 83% (95% confidence interval {CI} 70-97) and 86% (CI 74-99) when cup revision due to aseptic loosening was the endpoint. Mean HHS at eight years was 95 (range 77-100). A further 12% (CI 5-17) loss in pBMD was detected in the proximal Digas zone 1 and 12% (CI 7-17) loss in Digas zone 2 also between two and eight years after surgery. pBMD continued to decrease up to 30% (CI 24-36) in Digas zones 1, 2 and 3 compared to pBMD immediately postoperatively. CONCLUSIONS: The TOP cup shows inferior mid-term survival rates compared to other uncemented cups, as well as a continuous decrease in pBMD. Periprosthetic bone loss cannot be prevented by this uncemented cup. CLINICAL TRIAL NUMBER Not applicable.