Low implant migration of the SIGMAA® medial unicompartmental knee arthroplasty

The purpose of this study was to evaluate implant migration of the fixed-bearing Sigma(A (R)) medial unicompartmental knee arthroplasty (UKA). UKA is a regularly used treatment for patients with medial osteoarthritis (OA) of the knee. UKA has a higher revision rate than total knee arthroplasty. Implant migration can be used as a predictor of implant loosening. A prospective radiostereometric cohort study was performed. Forty-five patients with medial OA of the knee were included and received a cemented Sigma(A (R)) UKA. The patients were followed for 24 months with radiostereometric analysis (RSA) and clinical outcome scores (Oxford knee score). Clinical precision was based on double determinations taken at 4 and 12 months. Tibial implants were classified as stable (difference in MTPM < 0.2 mm between 1 2 and 24 months) or as continuously migrating (difference in MTPM > 0.2 mm between 12 and 24 months). No significant differences in migration were found for the femoral component. For the tibial component, a difference of 0.05 mm was shown for translation on the x-axis between 4 months and 12 (p < 0.01) and between 4 months and 24 months (p < 0.01). A difference of - 0.23 to - 0.50A degrees was shown for rotation around the x-axis (p < 0.01) and a difference of - 0.11A degrees was shown for rotation around the z-axis between 4 and 12 months (p = 0.02). These differences in migration over time were small and fall within the clinical precision of the measurements. Tibial components were divided into a stable group (N = 26) and a continuously migrating group (N = 11), which showed a significant difference in maximal total point motion (MTPM) (p < 0.01). The Oxford knee score improved significantly from poor before surgery (23.2) to good at follow-up (37.5-40.9). The Sigma(A (R)) UKA showed low implant migration and good clinical outcomes, suggesting that the Sigma UKA can be used in clinical practice. However, continuous migration was found in 30% of our patients which could indicate a risk of later revision surgery in this group. II.