Intraoperative Joint Gap Affects Postoperative Knee Kinematics in Posterior-Stabilized Total Knee Arthroplasty

Introduction: Adjusting joint gaps and establishing mediolateral (ML) soft tissue balance are considered essential interventions for better outcomes in total knee arthroplasty (TKA). However, the relationship between intraoperative laxity measurements and weight-bearing knee kinematics has not been well explored. The goal of this study was to establish how intraoperative joint gaps and ML soft tissue balance affect postoperative kinematics in posterior-stabilized (PS)-TKA. Methods: We investigated 44 knees with 34 patients who underwent primary PS-TKA. Subjects averaged 71 ± 7 years at the time of surgery, included 8 male and 36 female knees with a preoperative diagnosis of osteoarthritis in 38 knees and rheumatoid arthritis in 6 knees. A single surgeon performed all the surgeries with mini-midvastus approach. After independent bone cutting, soft tissues were released on a case-by-case basis to obtain ML balance. The femoral trial and a tensor were put in place, and the patella was reduced to the original position. A joint distraction force of 40 lb was applied by the tensor, and the central joint gaps and ML tilting angles were measured at 0°, 10°, 30°, 60°, 90°, 120° and 135° flexion (Fig. 1). We defined a “gap difference” as a gap size difference between one gap and another, which represents the gap change between the two knee flexion positions. ML soft tissue balance was assessed by measuring the mean joint gap tilting angle over all flexion angles for each patient. Based on the tilting angle, the 44 knees were classified into three groups: The knees with the mean joint gap tilting of less than −1.0° (13 knees), between −1.0 and 1.0° (14 knees), and over 1.0° (17 knees). At least 1.5 year after surgery, a series of dynamic squat radiographs and 3 static lateral radiographs of straight-leg standing, lunge at maximum flexion, and kneeling at maximum flexion, were taken for each patient. The 3-dimensional position and orientation of the implant components were determined using model-based shape matching techniques (Fig. 2). Correlations between intraoperative measurements and knee kinematics were analyzed. The knee kinematics was also compared among three tilting groups. Results: Gap difference of 135° minus 0° showed positive correlations with total posterior lateral translation during squatting (r = 0.336, p = 0.042) and total femoral external rotation (r = 0.488, p = 0.002). Gap difference of 135° minus 0° also demonstrated negative correlations with lateral condyle AP position in maximal kneeling (r = −0.501, p = 0.001, Fig. 3), and showed positive correlations with maximal knee flexion during squatting (r = 0.342, p = 0.038) and kneeling (r = 0.355, p = 0.031). Gap differences of 120° minus 90° and 135° minus 90° exhibited positive correlations with femoral external rotation at maximal kneeling and lunge. No correlation was demonstrated between joint tilting angle and post-operative knee kinematics. Also, there were no significant kinematic differences among three tilting groups. Conclusion: These findings suggest adequate intraoperative joint gaps beyond 90° flexion are important to obtain greater lateral femoral translation and femoral external rotation, both factors for better postoperative flexion, while several degrees of ML imbalance is unlikely to negatively affect postoperative knee kinematics.