Do Ultracongruent Inserts Reproduce the Intraoperative Sagittal Plane Kinematics of Posterior Stabilized TKR? Assessment Using a Modern Robotic System.

Background: Ultracongruent (UC) total knee replacement (TKR) designs, serving as alternatives to posterior stabilized (PS) and cruciate retaining (CR) designs, lack conclusive evidence regarding posterior femoral rollback. This study aimed to compare intraoperative posterior femoral rollback and maximal knee flexion between UC and PS inserts, addressing the paucity of literature on femoral rollback achieved with UC designs in total knee replacement.

Methods: A consecutive cohort of 20 patients undergoing robotic-assisted primary total knee replacement, posterior femoral rollback and maximal intraoperative knee flexion were assessed. Robotic imaging at varying flexion angles (0°, 45°, 90°, and 120°) was conducted after implanting femoral and tibial components with Ultracongruent and Posterior-Stabilized trial inserts. Femoral contact on the tibia was estimated as a percentage of the sagittal dimension of the tibial component with 0 representing the anterior edge and 100 representing the posterior edge of the tibial component.

Results: In extension, UC inserts exhibited a statistically significant posterior contact point versus PS inserts (56.3 + 4.3 vs 53.5 + 5.3, p = 0.003). Between 0 and 45° flexion, 13 of 20 UC inserts showed a paradoxical anterior translation exceeding 5%, unlike the PS group. At 90° flexion, both displayed consistent posterior femoral rollback, with PS inserts having a more posterior contact point (63.5 + 6.2 vs 67.2 + 5.1, p = .008). At 120° flexion, rollback was similar (70.1 + 8.4 vs 71.3 + 8.4, p = 0.128). Mean maximal flexion was 130° (SD = 6.87) and 133° (SD = 6.72) for UC and PS inserts, respectively (p = 0.0001).

Conclusions: The study indicates UC inserts achieve comparable posterior femoral rollback in deep flexion, supporting their alternative use, despite minor intraoperative flexion differences. However, paradoxical anterior translation in mid-flexion with UC inserts warrants further investigation into wear and clinical outcomes.