RESTORATION OF NATIVE PASSIVE KINEMATIC PATTERNS THROUGH FUNCTIONAL ALIGNMENT IN CRUCIATE-RETAINING TOTAL KNEE ARTHROPLASTY

Introduction Total knee arthroplasty (TKA) aims to restore knee function, with increasing focus on obtaining medial stability to promote medially pivoting kinematics. Abnormal translation patterns have been linked to altered kinematics and reduced patient satisfaction. Polyethylene insert design and thickness may influence pivot behaviour, but the magnitude of this effect is not well defined. This study assessed how well native pivot patterns are restored during cruciate-retaining, robotically assisted TKA using functional alignment (FA), and how incremental insert thickness changes affect tibiofemoral kinematics. Methods Intraoperative kinematic data were collected from 66 patients with intact PCLs. Before bone cuts, the native knee was passively moved from ∼110° flexion to full extension to record medial and lateral anteroposterior (AP) contact positions. During trialling, this assessment was repeated for up to three tibial inserts differing only in thickness. A proprietary script calculated medial and lateral AP motion defining passive kinematic pattern. Medial and lateral contact positions at 10° and 90° were compared across insert sizes. Results Native pivot pattern was reproduced in 73% of TKAs. Pivot distribution was similar between native knees and TKAs (medial pivot: 79% vs 73%; lateral pivot: 3% vs 0%; all p>0.05). Total AP travel was comparable to native values and consistently greater laterally. Increasing insert thickness reduced medial AP laxity at 10° (8.1 mm at −2 mm to 4.6 mm at +2 mm) and at 90° (4.7 mm to 2.7 mm). At 90°, thicker inserts shifted medial contact progressively posteriorly, while no consistent trend appeared in extension. Conclusion Not all osteoarthritic knees demonstrate a medial pivot preoperatively, yet FA with a cruciate-retaining design frequently restores one. Insert thickness influences medial compartment behaviour, suggesting a role in optimizing kinematic targets. Further research is needed to define ideal pivot characteristics to guide insert selection and improve outcomes.