No effect of additional screw fi xation of a cementless, all-polyethylene press-fi t socket on migration, wear, and clinical outcome

Background and purpose — Additional screw fi xation of the all-polyethylene press-fi t RM cup (Mathys) has no additional value for migration, in the fi rst 2 years after surgery. However, the medium-term and long-term effects of screw fi xation remain unclear. We therefore evaluated the infl uence of screw fi xation on migration, wear, and clinical outcome at 6.5 years using radioste- reometric analysis (RSA). Patients and methods — This study involved prolonged fol- low-up from a previous randomized controlled trial (RCT). We analyzed RSA radiographs taken at baseline and at 1-, 2-, and 6.5-year follow-up. Cup migration and wear were assessed using model-based RSA software. Wear was calculated as translation of the femoral head model in relation to the cup model. Total trans- lation, rotation, and wear were calculated mathematically from results of the orthogonal components. Results — 27 patients (15 with screw fi xation and 12 without) were available for follow-up at 6.5 (5.6–7.2) years. Total transla- tion (0.50 mm vs. 0.56 mm) and rotation (1.01 degrees vs. 1.33 degrees) of the cup was low, and was not signifi cantly different between the 2 groups. Wear increased over time, and was similar between the 2 groups (0.58 mm vs. 0.53 mm). Wear rate (0.08 mm/ year vs. 0.09 mm/year) and clinical outcomes were also similar. Interpretation — Our results indicate that additional screw fi xation of all-polyethylene press-fi t RM cups has no additional value regarding medium-term migration and clinical outcome. The wear rate was low in both groups.