Summary

ACL deficiency greatly increases the ATT as well as the strain in the superficial MCL with the highest stress values were seen at the proximal sMCL attachment

Abstract

Background

Anterior cruciate ligament (ACL) injury is common and there are questions on the effect of this on other structures of the knee. It is stated that the medial collateral ligament (MCL) is a secondary restrain to anterior tibial translation (ATT). While studies have looked at the strain in the MCL, no studies have measured the strain distributions in the MCL in the ACL deficient knee under controlled knee loads. In this study the strain field was measured in the MCL under ATT loading in the intact and ACL deficient knee.

Methods

Eight (N=8) left, fresh frozen cadaveric knees were checked for abnormalities and kinematics recorded using a robotic testing system. The kinematics of the intact knees with and without the ACL were measured under a 89 N ATT load at flexion angles of 0, 15, 30, 45, 60, 75 and 90 degrees. The knees were then dissected to expose the superficial MCL, and it was marked. The kinematics of the undissected knee with and without the ACL under the ATT loading was imposed on the dissected knee while recording a video of the MCL. From the video, digital image correlation (VIC3D, Correlated Solutions Inc.) was used to calculate the strains in the ligament at the end of loading. Strains from the specimens were converted to a common file format and any missing data was interpolated and the data for the specimens was averaged at each flexion angle.

Results

SECTION: The anterior tibial translation (ATT) values across different knee flexion angles (0° to 90°) demonstrated a significant difference between the intact and deficient states. In the intact group, the ATT remained relatively stable across all flexion angles, with a mean translation ranging from 5.5 ± 0.6 mm at 0° to 4.6 ± 0.8 mm at 90°. In contrast, the deficient group exhibited substantially higher ATT values, particularly at lower flexion angles, peaking at 12.1 ± 2.9 mm at 15° and gradually decreasing to 7.2 ± 2.5 mm at 90°.
Mean longitudinal strains in the superficial MCL varied greatly between the intact and deficient states. ACL insufficiency doubled the global strain of the proximal sMCL attachment (ACL intact: 0.01 – 0.04; ACL deficient: 0.04 – 0.14) throughout all flexion angles.

Discussion

The main finding of this study is that ACL deficiency greatly increases the ATT as well as the strain in the superficial MCL. The highest stress values were seen at the proximal sMCL attachment which increased by a factor of 3-6 in the ACL deficient state comapred to the intact state.

SIGNIFICANCE/CLINICAL RELEVANCE: These data suggest that injury to the ACL leads to additional stress on the proximal superficial MCL. This additional load is particularly relevant in combined ACL - MCL injuries, as persistent anterior knee instability may reduce the healing potential of the MCL, and adequate treatment strategies need to be applied.