Balanced flexion gap is more strongly associated with postoperative outcomes than femoral component rotation in total knee arthroplasty.

Introduction: This study evaluated (1) the degree of femoral component rotation required to achieve a balanced flexion gap in total knee arthroplasty (TKA) using the extension-first technique, (2) the proportion of patients who achieved balanced flexion and extension gaps, and (3) whether femoral component rotation or gap balance was more closely associated with postoperative outcomes.

Materials and methods: A total of 169 patients who underwent total knee arthroplasty using the extension-first technique were retrospectively reviewed. Femoral component rotation was determined based on the posterior condylar line and adjusted to the angle required to attain a balanced flexion gap at 90°. Based on the degree of femoral component rotation (FCR) relative to anatomical landmarks, patients were classified into within-range (WR, between the clinical and surgical transepicondylar axes), internal rotation (IR, FCR less than sTEA), and external rotation (ER, FCR greater than cTEA) groups. The proportions of patients categorized as WR, IR, and ER were assessed, and within each group, the proportion of patients demonstrating balanced mediolateral laxity and symmetric flexion-extension gaps was evaluated. At 2 years postoperatively, the WOMAC score, range of motion, and patellar tilt angle were analyzed and compared between groups categorized by gap balance and femoral component rotation.

Results: The proportions of patients classified as within-range (WR), internal rotation (IR), and external rotation (ER) were 51%, 11%, and 38%, respectively. A total of 68% of patients (n = 115) achieved balanced gaps in extension, flexion, and between both, whereas 32% (n = 54) failed to achieve a balanced gap in at least one of these assessments. Among these variables, unbalanced mediolateral laxity in flexion showed a stronger association with worse 2-year postoperative WOMAC scores than femoral component IR or ER, with regression coefficients of 4.993, 2.303, and 2.682 (P = 0.014, 0.274, and 0.152), respectively.

Conclusions: Achieving a balanced flexion gap may have greater clinical significance than positioning the femoral component within the range between the clinical and surgical transepicondylar axes (cTEA and sTEA).