Relative contribution of the ACL, MCL, and bony contact to the anterior stability of the knee

Ligaments and other soft tissues, as well as bony contact, all contribute to anterior stability of the knee joint. This study was designed to measure the in situ force in the medial collateral ligament (MCL), anterior cruciate ligament (ACL), posterolateral structures (PLS), and posterior cruciate ligament (PCL) in response to 110 N anterior tibial loading. The changes in knee kinematics associated with ACL deficiency and combined MCL+ACL deficiency were also evaluated. Utilizing a robotic/universal force-moment sensor system, ten human cadaveric knee joints were tested between 0 degrees and 90 degrees of knee flexion. This unique testing system is designed to determine the in situ forces in structures of interest without making mechanical contact with the tissue. More importantly, data for individual structures can be obtained from the same knee specimen since the robotic manipulator can reproduce the motion of the intact knee. The in situ forces in the ACL under anterior tibial loading to 110 N were highest at 15 degrees flexion, 103 +/- 14 N (mean +/- SD), decreasing to 59.2 +/- 30 N at 90 degrees flexion. For the MCL, these forces were 8.0 +/- 3.5 N and 38.1 +/- 25 N, respectively. Forces due to bony contact were as high as 34.1 +/- 23 N at 30 degrees flexion, while those in the PLS were relatively small at all flexion angles. Combined MCL+ACL deficiency was found to significantly increase anterior tibial translation relative to the ACL-deficient knee only above 60 degrees of knee flexion. These findings confirm the hypothesis that there is significant load sharing between various ligaments and bony contact during anterior tibial loading of the knee. For this reason, the MCL and osteochondral surfaces may also be at significant risk during ACL injury.