Extra-articular factors of the femur and tibia affecting knee balance in mechanically aligned total knee arthroplasty

Purpose: Although the joint gap distance and compartment loading of the knee are different parameters for obtaining balanced knees in patients with total knee arthroplasty (TKA), the extent to which a balanced knee is achieved in patients with mechanically aligned TKA (MA-TKA) is unclear. This study aimed to investigate the anatomical factors that affect the ligament balancing in MA-TKA according to a gap balancing technique by measured resection technique. Methods: The relationship between compartment loading, gap distance, and lower limb alignment was analysed in forty-three patients who underwent navigation-assisted MA-TKA with the minimum soft-tissue release. Radiographic parameters included hip-knee-ankle (HKA) angle, femorotibial angle, mechanical lateral distal femoral angle (mLDFA), medial proximal tibial angle, posterior tibial slope, varus and valgus joint line convergence angle, femoral valgus angle, femoral shaft curve, and tibial shaft curve (TSC). Results: The postoperative HKA angle was 0.4 +/- 1.5 degrees, with one coronal alignment outlier (2.3%). The gap distance showed four unbalanced knees for extension (9.3%) and flexion (9.3%), whereas the compartment loading identified 17 unbalanced knees (39.5%) at extension, 8 (18.6%) at 45 degrees flexion, 7 (16.2%) at 90 degrees flexion, and 10 (23.2%) at full flexion in MA-TKA. Mediolateral differentials of compartment loading correlated with femoral valgus angle (0 degrees, 45 degrees, and 90 degrees flexion), mLDFA (45 degrees and 90 degrees flexion), and TSC (45 degrees and 90 degrees flexion). Femoral valgus angle and TSC were risk factors of the mediolateral differentials of compartment loading. Conclusion: Extra-articular factors of femoral valgus angle, mLDFA, and TSC were associated with the mediolateral imbalance of compartmental loading, suggesting there is a limit to obtaining balanced knees in MA-TKA due to these unadjustable extra-articular factors. Level of evidence: IV, therapeutic case series.