Low implant migration of the SIGMA medial unicompartmental knee arthroplasty.

Purpose: The purpose of this study was to evaluate implant migration of the fixed-bearing Sigma 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.

Methods: A prospective radiostereometric cohort study was performed. Forty-five patients with medial OA of the knee were included and received a cemented Sigma 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).

Results: 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 - 0.50° was shown for rotation around the x-axis (p < 0.01) and a difference of - 0.11° 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).

Conclusions: The Sigma 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.

Level Of Evidence: II.