Balanced Medial Collateral Ligament (MCL) Length, Not Intrinsic Laxity, Governs Load Sharing and Kinematics in Mechanically Aligned Posterior-Stabilized TKA.

purposeAs 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).

methodsA 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.

resultsChanges 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.

conclusionAcross 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.