Flexion Space Balancing Through Component Positioning and its Relationship to Traditional Anatomic Rotational Landmarks in Robotic Total Knee Arthroplasty

Background The use of the femoral component position to balance the flexion space and its relationship to the transepicondylar axis (TEA) and posterior condylar axis (PCA) has not been thoroughly evaluated. Methods 233 patients undergoing robotic arm assisted (RAA) TKA were evaluated. Native TEA and PCA were established on preoperative CT scans. Femoral component rotation was set in the axial plane to match the native trochlea and native medial femoral condyle to set the flexion gap. Knee flexion space gaps and component position were recorded. The relationship of the femoral component to the native TEA, PCA, and preoperative radiographic landmarks were evaluated. Results The intraoperative measured medial flexion space gap did not significantly correlate with the relationship of the femoral component to the PCA or TEA in varus or valgus knees. In varus knees, the preoperative mechanical axis (MA) alignment had a positive relationship to femoral component position when compared to the PCA (p=0.04) and TEA (p=0.002). In valgus knees, there was a positive correlation between the preoperative lateral distal femoral angle (LDFA) and component position when compared to the PCA (p=0.04) only. Conclusion Intraoperative measured flexion space balance through femoral component positioning did not correlate with its relationship to the native TEA or PCA. In varus knees, the preoperative MA alignment correlated with an increase in femoral component external rotation to the TEA and PCA. In valgus knees, the severity of preoperative LDFA correlated with the rotational relationship of the femoral component to the PCA only.