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Updated: Apr 6, 2026
In Vitro Application of a Wireless Sensor in Flexion-Extension Gap Balance of Unicompartmental Knee Arthroplasty
Published on: May 5, 2023
Balanced Medial Collateral Ligament (MCL) Length, Not Intrinsic Laxity, Governs Load Sharing and Kinematics in
Junseo Kim1, In Jun Koh2,3, Taejin Shin4
1Department of Mechanical Engineering, Sejong University, 209, Neungdong-ro, Gwangjin-gu, Seoul, 05006, Republic of Korea.
Purpose:
As surgical practice has shifted from mechanical alignment toward kinematic alignment with growing interest in patient-specific soft-tissue laxity, we evaluated under dynamic, weight-bearing conditions-the relative impact of (1) soft-tissue laxity (modeled via MCL stiffness/compliance) and (2) the balanced medial collateral ligament (MCL) length achieved after soft-tissue balancing on tibiofemoral load sharing and kinematics in posterior-stabilized (PS) total knee arthroplasty (TKA).
Methods:
A validated, squat-based musculoskeletal model of mechanically aligned PS-TKA (0-120°) with anatomically segmented, nonlinear ligaments was used. Twenty-one simulations systematically varied soft-tissue laxity (± 20% change in MCL stiffness/compliance) and balanced MCL length (0-12% change in initial length). Outcomes included medial/lateral tibiofemoral contact forces, femoral internal-external and varus-valgus rotations, and compartmental rollback.
Results:
Changes in balanced MCL length dominated knee mechanics. Lengthening the balanced MCL (greater final length/lower pre-tension) reduced medial contact force by up to 48% (0.76 BW), increased lateral loading, and increased femoral external rotation (+ 1.5°) and posterior rollback (up to 3.7 mm). Shortening (higher pre-tension) produced features of overconstraint with elevated medial pressures and diminished rollback. In contrast, modifying soft-tissue laxity alone (± 20% stiffness/compliance) had minimal effect on load sharing or kinematics unless the ligament was already tensioned.
Conclusion:
Across the flexion arc, the balanced MCL length (functional post-balancing length/pre-tension) has a substantially greater influence on tibiofemoral load distribution and kinematics than inherent soft-tissue laxity. Intraoperatively, prioritizing precise control of balanced MCL length-via selective release, preservation, or retensioning-may better normalize compartmental forces, mitigate midflexion instability, and avoid overconstraint than attempting to accommodate small variations in intrinsic laxity. These weight-bearing simulation data provide a practical baseline for integrating patient-specific laxity into alignment strategies and support targeting a physiologic, balanced MCL length as the primary means to achieve stability and near-native kinematics in PS-TKA.
