Closing the flexion gap: Differences in femoral sizes, level of constraint, and joint anatomy between robotic assisted and conventional high volume orthopedic surgeons.

Background: One of the most common reasons for revision TKA is flexion instability. Using conventional methods, it is potentially difficult to fully assess flexion instability which can lead to pain and disability following TKA. The purpose of this study was to assess femoral size differences between surgeons with different techniques to determine the difference in femoral sizing or polyethylene constraint.

Methods: This retrospective study analyzed data from 1508 patients who underwent unilateral primary TKA performed by either restricted kinematic alignment (RKA) with robotic assistance, mechanical alignment (MA) with measured resection, and mechanical alignment with gap balancing (GB). Component sizes, joint alignment and Knee Injury and Osteoarthritis Outcome Score for Joint Replacement (KOOS, JR) values were compared. Statistical analysis was performed using chi-square and ANOVA tests with 95 % confidence intervals.

Results: A total of 1508 patients were consecutively analyzed, 789 in the robotic, kinematic technique, 371 in the manual mechanically aligned technique and 348 in the manual gap balancing technique. A significant increase in femoral size and a significant decrease in polyethylene size was observed using robotics with the RKA technique than either the gap balanced or mechanically aligned approach (p < 0.0001). The gap balancing surgeon used significantly smaller tibial implant sizing compared to the other two surgeons (p < 0.0001). RKA resulted in the smallest change in the medial joint line, increased posterior femoral offset, and a varus alignment consistent with native knee preservation. MA techniques showed greater changes in lateral joint line and proximal tibial angles.

Conclusion: Restrictive kinematic technique using robotic arm navigation led to larger femoral sizes, a significant decrease in semi-constrained polyethylene usage, and better restoration of posterior femoral offset. No significant difference in patient reported outcomes was observed. Future research should investigate long-term implications of these findings on joint function and patient satisfaction.