Impact of femoral component flexion on gap balancing in varus knees using robotic technology: a comparative study of single-radius and multi-radius designs.

Accurate flexion-extension gap balancing is critical in total knee arthroplasty (TKA), influencing postoperative stability and longevity. Femoral component flexion in the sagittal plane has been suggested as a means to adjust gap balance; however, its impact across different implant designs remains unclear. This study compares the influence of femoral component flexion on gap balancing between single-radius and multi-radius femoral components using robotic-assisted TKA. A prospective observational cohort study was conducted on 200 patients (100 in each cohort) undergoing primary robotic-assisted TKA for varus deformity with end-stage osteoarthritis. Patients were randomized to receive either single-radius (Stryker MAKO with Stryker Triathlon implants) or multi-radius (Meril CUVIS with Maxx-Freedom implants) femoral components. Intraoperative medial and lateral flexion-extension gaps were assessed at femoral flexion increments from 0° to 6° using robotic measurement software, and statistical analyses were done. Femoral component flexion had distinct impacts on gap balancing depending on implant design. Multi-radius implants showed significant variability with extension medial gaps progressively loosened (mean Change 0.19 mm/degree of flexion, p < 0.001), and flexion medial and lateral gaps progressively tightened (mean Changes 0.21 mm/degree, p < 0.001 and 0.24 mm/degree, p < 0.05, respectively). Conversely, single-radius implants demonstrated minimal variability in all gap measurements across degrees of femoral flexion (p > 0.05). Femoral component flexion significantly affects gap balancing during robotic-assisted TKA, particularly in multi-radius femoral components. Multi-radius designs require careful sagittal alignment to prevent undesirable gap variations, whereas single-radius implants exhibit more predictable gap balancing profiles, potentially simplifying intraoperative adjustments. Future research should evaluate long-term clinical outcomes related to these findings.