Knee Varus and Ligamentous Laxity
In knee varus deformity, laxity appears in the medial collateral ligament (MCL), not the lateral collateral ligament (LCL).
Biomechanical Explanation
The varus deformity creates a characteristic loading pattern that directly affects the medial structures of the knee:
Varus alignment increases the external knee adduction moment (EKAM), which represents the torque pulling the knee into varus and concentrates 70-80% of joint load through the medial compartment during gait 1
The MCL becomes the primary structure subjected to chronic stretch and overload in varus knees, as the medial compartment experiences focal overload while the lateral compartment is relatively unloaded 1
Ligamentous laxity develops as a consequence of chronic varus malalignment, where erosion of articular cartilage results in malalignment that interferes with the ability of ligaments to restrict joint motion and preserve physiological load distribution 1
Clinical Evidence
The relationship between varus deformity and MCL laxity is well-established:
Biomechanical studies demonstrate that the MCL is the main contributor to valgus stiffness and the primary restraint preventing valgus angulation 2
In varus knees, the MCL undergoes chronic stretching due to the abnormal loading pattern, leading to progressive ligamentous laxity over time 1
The LCL remains relatively protected in varus deformity because the lateral compartment is unloaded, and the LCL primarily contributes to varus stiffness rather than being stressed by varus alignment 2
Treatment Implications
Understanding this pattern is critical for surgical planning:
Valgus bracing is used to treat medial compartment osteoarthritis with varus deformity because it shifts load from the medial to lateral compartment and reduces stress on the stretched MCL 1
In total knee arthroplasty for varus knees, MCL release (not LCL release) is required to achieve proper ligament balancing, with complete MCL release sometimes necessary in severe varus deformity 3
MCL reconstruction may be needed in chronic cases where the ligament has healed with persistent medial laxity, particularly when combined with cruciate ligament injuries 4, 5
Common Pitfall
Do not confuse the direction of deformity with the location of laxity. Varus deformity means the knee bows inward (medially), which paradoxically creates tension and eventual laxity in the MCL on the medial (concave) side, not the LCL on the lateral (convex) side 1.