Abstract Title: TKA Femoral Flexion Angle Distribution and Range of Motion

Background: Femoral implant flexion angle (FF) is an underutilized metric with wide reported ranges. Increased understanding of current FF use may establish specific sagittal alignment targets.

Methods: Demographics, knee ROM, and implant alignment parameters were collected for patients undergoing TKA with a CT-based robotic knee replacement system. The range and distribution of FF angles was determined. “High-FF” and “Low-FF” groups were divided at the FF mean. ROM measurements were taken in four windows: Preoperative, Early postoperative (7- 42 days), Middle postoperative (43-135 days), and Late postoperative (>136 days). ROM was compared between groups.

Results: 976 patients of age 66.3±8.74 years, BMI 32.9±10.9, and 56.6% female. Average FF was 4.04°±2.10° flexion 0°-9°. The Low-FF group 0.0°-4.0° had 528 patients. The High-FF group 4.5°-9.0° had 448 patients. There were no differences in preoperative knee parameters: varus/valgus alignment, flexion/extension ROM (p>0.2). High-FF patients were often female (Low-FF 53.0%; High-FF 62.5%; p=0.006) and more frequently cemented implants (Low-FF 14.4%; High-FF 20.4%; p=0.020). High-FF patients had smaller femoral implants (Size 4 or lower: Low-FF 52.2%; High-FF 68.7%; p<0.001), smaller tibial implants (Size 4 or lower: Low- FF 50.1%; High-FF 63.9%; p<0.001), and lower tibial slope (Low-FF 3.2°; High-FF 3.0°; p<0.001). There were no differences in patient size (p>0.1) or femur-tibia size mismatch frequency (p=0.539). 193 patients had measurements in all four windows. Repeated measures found no ROM differences between groups at any timepoint.

Conclusion: The observed femoral implant flexion angles assumed a normal distribution ranging from 0° to 9° flexion, centered at 4° flexion. Patients receiving a femoral flexion angle above 4° were more often female, cemented, had lower tibial slope angle, and smaller femur and tibia implant sizes. Patients with larger femoral flexion angles showed no difference in flexion or extension range of motion at any timepoint when compared to patients with smaller femoral flexion angles.