Summary

This study demonstrates that even for partial medial injuries of the knee, force on the ACL graft is relatively increased with the largest change between grade 2 and 3 injuries.

Abstract

Objective

Anteromedial laxity can be present during anterior cruciate ligament (ACL) reconstruction due to concomitant injury of the medial collateral ligament complex. The most common injury includes a complete rupture of the deep medial collateral ligament (dMCL) with a combined partial injury to the superficial medial collateral ligament (sMCL). Although ACL graft forces are expected to increase with anteromedial deficiency, the severity may depend on the grade of sMCL injury as well as the type of loading activity. The objective of the study was to determine how different grades of sMCL injury effect the restraining force in an ACL graft during external rotation and anteromedial loading.

Methods

A subject-specific knee model of ACL reconstruction was derived from a computed tomography scan of a single cadaveric knee specimen. Ligaments were modeled as non-linear springs and were calibrated to match experimentally measured subject-specific knee laxity data. The dMCL was completely resected from the model. Four levels of sMCL injury were then simulated by reducing its stiffness by 33% increments, ranging from 100% stiffness to complete rupture. The models simulated two functional activities: 1) 4 N·m tibial external rotation (ER) and 2) a combined 89-N anterior tibial translation and 4-N·m ER for anteromedial rotation (AMR) and anteromedial translation (AMT) at 30° of knee flexion. Medial gap was measured for each grade of injury with a 5 N·m applied valgus stress. ACL graft forces were recorded for the MCL intact state, dMCL deficient state, and four different levels of sMCL deficiency.

Results

Simulated grade I, II, and III injuries resulted in 5.3 mm, 6.2 mm, and 10.3 mm of gap. Transecting the dMCL alone did not increase the ACL graft force, while decreased sMCL stiffness resulted in higher forces in the ACL graft. Compared to the intact state, ACL graft forces increased by 76.2% during ER and 38.5% during AMR+AMT at 30° of flexion for grade III injuries.

Conclusions

This study demonstrates that even for partial medial injuries of the knee, force on the ACL graft is relatively increased with the largest change between grade 2 and 3 injuries.