Anteromedial instability of the knee

AIMS AND OBJECTIVES: Anteromedial rotatory instability (AMRI) may result from combined lesions to the medial capsuloligamentous structures and anterior cruciate ligament (ACL). However, the contribution made by individual structures to controlling AMRI remains controversial. The purpose of the present study was to determine how the medial structures and ACL contribute to restraining simulated clinical laxity. MATERIALS AND METHODS: Twenty-eight paired, fresh-frozen human cadaveric knees were tested in a six-degree of freedom robotic setup. After sequentially cutting the dMCL, sMCL, POL, and ACL in 4 different cutting orders, the following simulated laxity tests were applied at 0°, 30°, 60°, and 90° of knee flexion: 4 Nm external tibial rotation (ER), 4 Nm internal tibial rotation (IR), 8 Nm valgus rotation (VR) and anteromedial translation (AMT) - combined 89 N anterior tibial translation and 4 Nm ER. Knee kinematics (AMT, ER, IR, VR) were recorded in the intact state and after each cut using an optical tracking system. Differences in medial compartment translation and rotation from the intact state were then analyzed using Visual 3D. The kinematic data for each tested state, load, and flexion angle were analyzed using a 2-factor repeated-measures analysis of variance (ANOVA) and post-hoc Bonferroni corrections for multiple comparisons. STUDY DESIGN: Controlled laboratory study. RESULTS: The sMCL was the most important restraint to AMT (1.3 mm, 3.9 mm, 6.4 mm, 6.1 mm in 0°, 30°, 60°, 90°, respectively; P < 0.05), ER (1.8°, 4.4°, 6.3°, 5.3° in 0°, 30°, 60°, 90°, respectively; P < 0.01), and VR (4.1°, 4.8°, 5.8° in 30°, 60°, 90°, respectively; P < 0.05) at all flexion angles. Cutting the proximal tibial attachment of the sMCL caused no significant increase in laxity if the distal sMCL attachment remained intact. The dMCL was a minor restraint to AMT (1.3 mm, 1.8 mm, 1.6 mm in 0°, 30°, 60°, respectively; P < 0.05) and ER (1.5°, 1.2°, 1.1°, 1.0° in 0°, 30°, 60°, 90°; P < 0.05). The POL controlled IR near extension (3.3° in 0°; P < 0.05) and was a secondary restraint to AMT and ER in the ACL + MCL deficient knee. The ACL contributed in restraining AMT (4.0 mm, 6.2 mm, 4.2 mm, 3.0 mm in 0°, 30°, 60°, 90°, respectively; P < 0.01) and was a secondary restraint to ER and VR in the MCL deficient knee. CONCLUSION: The sMCL was the primary restraint to anterior subluxation of the medial tibial plateau, tibial ER and valgus rotation, which constitutes pathological laxity in AMRI. The dMCL and POL play a more minor role in restraining AMRI. Clinical Relevance: The simulated clinical tests (anteromedial drawer, external/internal rotation dial test and valgus stress test) allow determination of potential injury patterns causing AMRI. Based on the present data we propose a classification of anteromedial instability and suggest that for surgical treatment of AMRI it is important to address the sMCL.