Accuracy and Reliability of Magnetic Resonance Imaging Measurement of Medial Femoral Condyle for Femoral Component Sizing in Oxford Unicompartmental Knee Arthroplasty

Background Accurate femoral component sizing is critical for Oxford unicompartmental knee arthroplasty (UKA) because inappropriate sizing can lead to early implant failure. Therefore, we developed a magnetic resonance imaging (MRI) measurement of medial femoral condyle (MRMFC) technique and compare its accuracy to existing methods. Methods This study included 54 Oxford UKAs. Five sizing methods were assessed: radiographic templating, intraoperative sizing spoon, anthropometric estimation (based on patient height and gender), Yang et al.'s MRI method, and the MRMFC. The MRMFC determined 3 reference points of medial femoral condyle on the sagittal plane of MRI. The primary outcome was an overhang or underhang of femoral component over posterior femoral condyle as determined on postoperative lateral radiographs which ±2 mm are considered as ideal size. Results The MRMFC method yielded the highest accuracy (50/54; 92.6%; 95% CI: 82.1–97.9), followed by the intraoperative sizing spoon (38/54; 70.4%, 56.4–82.0), Yang et al. MRI method (32/54; 59.3%, 45.0–72.4), anthropometric estimation (30/54; 55.6%, 41.4–69.1), and radiographic templating (25/54; 46.3%, 32.6–60.4). Pairwise testing demonstrated that MRMFC was better than the sizing spoon ( P = .008), radiographic templating ( P < .001), Yang et al. MRI method ( P < .001), and anthropometric estimation ( P < .001). The sizing spoon was superior to radiographic templating ( P = .029). No significant differences were observed among radiographic templating, Yang et al. MRI method, and anthropometric estimation. Conclusions The MRMFC demonstrated the highest accuracy and reproducibility in preoperative planning for Oxford UKA. Intraoperative spoon sizing, however, remains a simple and practical method and performed moderately well with significantly better than radiographic templating.