The impact of a rectangular or trapezoidal flexion gap on the femoral component rotation in TKA

Purpose: The influence of soft tissue balancing on femoral component rotation in the gap technique remains unclear. The present study therefore compared the reliability of femoral component rotation determined by rectangular and trapezoidal gaps in total knee arthroplasty (TKA) using a navigation-assisted gap-balancing technique. The study also determined the correlation between femoral component rotation and gap measurement. Methods: This prospective study included 99 patients (108 knees) who underwent postoperative CT after TKA with the navigation-assisted gap-balancing technique. A trapezoidal flexion or extension gap was defined as a >3 mm difference between the medial and lateral sides in 90° flexion or extension. An outlier of femoral component rotation was defined as >3° deviation from the transepicondylar axis. Results: Postoperative CT showed that the femoral component had a tendency to rotate externally, with a mean 1.88 ± 2.49° from the surgical transepicondylar axis. Outliers and mean values of femoral component rotation were similar in the trapezoidal and rectangular flexion/extension gap groups. The medial flexion gap (r = -0.49, P = 0.007) and flexion gap differences (r = -0.59, P = 0.027) showed statistically significant negative correlations with the femoral component rotation. Multiple linear regression analysis also showed that the femoral component rotation was independently related to the flexion gap difference (β = -0.27, P = 0.031). Conclusion: In the gap technique, the rotation of the femoral component is affected more by the flexion gap than by the extension gap. However, neither the trapezoidal nor rectangular flexion gap influenced femoral component rotation. Level of evidence: Prospective study, Level I.