Analysis of medial flexion gap after medial release for varus deformity by navigation-guided TKA

The goal of this study was to analyze medial flexion gaps after medial release for varus deformity by navigation-guided total knee arthroplasty (TKA). In each patient, a preoperative standing anteroposterior (AP) radiograph of the lower extremity and an AP valgus stress radiograph of the knee were used to measure preoperative mechanical axis angle and valgus stress angle, respectively. The correlation between preoperative varus deformities and medial flexion gap increases as measured by navigation was examined. Patients were assigned to 2 groups: group A (25 knees), in which the difference between the lateral flexion gap (LFG) and the medial flexion gap (MFG) (LFG-MFG) was ≤1 mm; and group B (73 knees), with an LFG-MFG of >1 mm. Mean preoperative mechanical axis angles in groups A and B were 13.21°±5.01° varus (range, 3.7°-23.6°) and 10.05°±3.70° varus (range, 1.9°-23.7°), respectively. Mean preoperative valgus stress angles in groups A and B were 1.72°±0.89° valgus (range, 0.1°-4.0°) and 4.84°±2.61° valgus (range, 0.1°-11.7°), respectively. A significant difference was observed between the groups in terms of mechanical axis angle (P=.002) and valgus stress angle (P<.001). Furthermore, valgus stress angle was found to be more strongly correlated with medial flexion gap increase than mechanical axis angle. The cutoff values of mechanical axis angle and valgus stress angle in group A were 13.4° and 2.45°, respectively. This study shows that preoperative valgus stress angle measurements can be used to predict the extent of medial release for varus deformity.