Unicompartmental knee arthroplasty (UKA) is a common treatment for medial compartment osteoarthritis. However, postoperative complications are often associated with abnormal knee kinematics. Ligament balance is critical for optimal joint function and stability. The Iso-height Axis (IHA) minimizes femoral condyle height variations during the flexion-extension path, potentially serving as an alternative reference axis compared to the traditional Tran-epicondylar Axis (TEA) and Geometric Center Axis (GCA). This study investigates the dynamic implications of IHA on soft tissue balance in UKA patients under different functional activities.

The hypothesis of this study is that iso-height points exist on the femoral condyles after UKA, and that these points can reflect the dynamic balance of soft tissues.

Twenty-four UKA patients, including 13 fixed bearing (FB) and 11 mobile bearing (MB) implants, were recruited 6–12 months after surgery. A 3D knee model reconstructed from pre- and post-operative CT scans, coupled with dual-fluoroscopic image tracking, analyzed three dynamic activities. IHA and ligament elongation were assessed in both UKA and contralateral natural knees, comparing their dynamic stability and adaptability during functional activities.

IHA demonstrated significantly smaller femoral condyle height variations compared to TEA and GCA (p < 0.001), correlating closely with the elongation of the medial collateral ligament (MCL) and lateral collateral ligament (LCL) during flexion (p < 0.001). FB UKA displayed IHA characteristics closer to those of the natural knee, whereas MB UKA showed significant deviations, particularly in the medial compartment (p < 0.01).

This study highlights IHA as a more dynamic and kinematically balanced reference axis for UKA, offering physiological alignment with the joint’s anatomical characteristics. FB UKA more effectively restores natural IHA properties compared to MB UKA, with the latter showing greater kinematic disruption.

III; Retrospective observational study.