The healing medial collateral ligament following a combined anterior cruciate and medial collateral ligament injury--a biomechanical study in a goat model.

The ideal treatment of a combined anterior cruciate ligament (ACL) and medial collateral ligament (MCL) injury to the knee is still debated. In particular, the question of whether reconstruction of the ACL can provide the knee with sufficient multidirectional stability to allow for effective MCL healing needs to be better elucidated. Therefore, the first objective of this study was to quantify the changes in the function of goat knees between time-zero and 6 weeks following a combined ACL/MCL injury treated with ACL reconstruction. Using a robotic/universal force-moment sensor testing system, the kinematics of the knee and in situ forces in the ACL/ACL graft as well as in the sham-operated and healing MCL were evaluated in response to (1) a 67 N anterior-posterior (A-P) tibial load and (2) a 5 Nm varus-valgus (V-V) moment. The second objective was to evaluate the structural properties of the healing femur-MCL-tibia complex (FMTC) and the mechanical properties of the healing MCL at 6 weeks under uniaxial tension. In response to the 67 N A-P tibial load, the A-P translations for the experimental knee increased by as much as 4.5 times from time-zero to 6 weeks (p<0.05). Correspondingly, the in situ forces in the ACL graft decreased by as much as 45% (p<0.05). There was no measurable changes of the in situ force in the healing MCL. In response to a 5 Nm V-V moment, V-V rotations were twice as much as controls, but similar for both time periods. From time-zero to 6 weeks, the in situ forces in the ACL graft dropped by over 71% (p<0.05), while the in situ force in the healing MCL was as much as 35+/-19 N. In terms of the structural properties of the healing FMTC, the stiffness and ultimate load values at 6 weeks reached 53% and 29% of sham-operated contralateral controls, respectively (p<0.05). For the mechanical properties of the healing MCL substance, the values for tangent modulus and tensile strength were only 13% and 10% of sham-operated controls, respectively (p<0.05). These results suggest that the ACL graft stabilized the knee initially, but became loose over time. As a result, the healing MCL may have been required to take on excessive loads and was unable to heal sufficiently as compared to an isolated MCL injury.