Iso-Height Axis Provides More Physiologic Description of Condylar Kinematics and Frontal Plane Stability During Functional Activities After Unicompartmental Knee Arthroplasty

Accurate characterization of femoral condylar kinematics and its association with collateral ligament behavior are essential for maintaining overall knee stability, particularly frontal plane stability after medial unicompartmental knee arthroplasty (UKA). Conventional reference flexion axes, such as the transepicondylar axis (TEA) and geometric center axis (GCA), provide only static representations of motion. The recently proposed iso-height axis (IHA) offers a dynamic reference, yet its clinical relevance under weight-bearing conditions after UKA remains unclear. 24 patients, 13 fixed-bearing (FB) and 11 mobile-bearing (MB) UKA, underwent dual fluoroscopic imaging (DFIS) during weight-bearing (single-leg lunge, sit-to-stand) and non-weight-bearing (open-chain leg raising) maneuvers. Femoral condyle kinematics were analyzed using TEA, GCA, and IHA, with statistical analysis comparing native and UKA knees. In FB UKA, A-P condylar translation was consistently the smallest with IHA (UKA: 0.78-1.69 mm across tasks), compared with GCA (2.98-3.62 mm) and TEA (2.99-6.03 mm; mean across medial/lateral condyles). FB UKA demonstrated robust correlations between condylar kinematics and ligament changes across all three axes, with IHA uniquely capturing bilateral lateral collateral ligament (LCL) correlations during lunge (lateral: β = 0.031,β* = 0.43; medial: β = 0.028, β* = 0.39). MB UKA had reduced correlations, primarily limited to LCL-TEA during open-chain leg raising. Flexion-axis choice substantially affects the interpretation of femoral condylar kinematics after UKA. Weight-bearing assessment better reveals clinically relevant abnormalities, and IHA may provide a more physiologically meaningful reference than TEA or GCA.