Summary
A multi-modal analysis of ACL injuries and associated lesions leading to a hypothesized mechanism
Abstract
Introduction
There continues to be uncertainty about the mechanism of injury in many cases of ACL rupture. Video of football injuries shows a valgus + external rotation posture. Investigations have included video image analysis, reconstructing the posture of the knee injury by opposing bone bruises (BB) seen in MRI, and laboratory studies in-vitro. This presentation will explore these modes of analysis and attempt to synthesize some consensus.
Methods
Clinical: This work is based on 73 Premiership-level footballers with non-contact ACL injury. They arrived with videos of their injuries occurring. The knees were scanned by MRI. In addition, the players described the mechanism of injury, and the surgeon recorded damaged structures found during ACL reconstruction.
Image analysis: The MRI scans were examined for ligament damage and BBs, and their locations recorded. Twenty scans were used for bone modelling. 3D bone and BB models were constructed from the scan slices. The BB models were assembled into the bone models. The bone models were manoeuvred into the posture assumed to be when the BB were created by the impact at injury, opposing the centres of the BB and then (given that most knees did not have 4BB) rotating the bones to minimize the gaps around the points of contact. The resulting posture was recorded. The path of motion from the initial anatomic pose to the final BB posture was calculated by interpolation. Length changes between ligament attachments were calculated across the injury process, showing the posture when ligaments failed prior to arriving at the BB posture, and compared to published failure data.
Results
Clinical: It was routinely found that the injured knees required a lateral extraarticular stabilization in addition to the ACL graft.
Image analysis: No knees had 4 articular BB. 5 had 4 BB but had a deep MCL lesion as the 4th BB. Medial compartment BBs were traction lesions at the femoral attachment of the deep MCL and/or a ramp lesion on the posterior rim of the tibial plateau, so not caused by impact during injury. The bone posture reconstructions showed the lateral femoral condylar sulcus balanced on the posterior rim of the lateral tibial plateau, at approximately 60 deg flexion. The ligament length changes showed clearly that the ACL and deep MCL had to have been ruptured in all cases. In addition, the anterolateral (ALL) and sMCL were elongated into the failure strain region, with failures in most knees. The BB postures showed that there was no posteromedial impact. The LCL always remained intact.
Discussion
These results are compatible with the injury mechanism of Slocum and Larson, in which the anteromedial soft tissues are initially ruptured by external rotation plus valgus loading, then the lateral femoral condyle slips down the posterior slope of the lateral plateau. The ACL+ALL/ITB injury would then be more likely if the lateral tibia moves anteriorly due to the coupled internal rotation caused by the lateral posterior slope of the tibial plateau. We may discuss these suggested mechanisms!