Intraoperative balance in total knee arthroplasty : Development and internal validation of the Knee Balancing Score

Achieving an accurate mediolateral soft tissue balance is essential for stability, kinematics and long-term outcomes in total knee arthroplasty (TKA); however, existing classification systems are often complex and lack intraoperative usability. This study introduces and internally validates the Knee Balancing Score (KBS), a simplified and reproducible method based solely on mediolateral gap asymmetry measured at full extension (when achievable) and 90° flexion using a robotic-assisted platform.

A prospective series of 285 robotic-assisted TKAs performed between 2022 and 2025 was analyzed. Medial and lateral joint gaps were recorded at full extension (when achievable) and 90° flexion under standardized varus-valgus stress. The KBS was derived from the absolute gap difference (∆Gap = |L - M|, mm) and classified separately for extension (E1-E4) and flexion (F1-F4). Correlations between ∆Gap and alignment parameters, including the hip-knee-ankle angle (HKA), arithmetic HKA, medial proximal tibial angle (mPTA), lateral distal femoral angle (LDFA), and joint line orientation (JLO), were calculated (P < 0.05).

The mean ∆Gap was 3.02 ± 2.25 mm in extension and 3.47 ± 2.63 mm in flexion (P = 0.021). Symmetry (E1) was observed in 21.8% of knees, while mild lateral opening (E2) occurred in 36.1%. In flexion, physiological lateral laxity (F2) was observed in 29.5% and pathological imbalance (F3-F4) in 51.6%. Valgus morphotypes showed greater asymmetry in both planes (P < 0.01). The ∆Gap correlated inversely with the HKA angle (r = -0.28 to -0.40) and mPTA (r = -0.34 to -0.39), confirming construct validity.

The KBS offers a practical, physiological and objective system to quantify intraoperative balance, transforming subjective perception into a standardized and reproducible assessment applicable in modern TKA practice.