Summary
Using cadaveric knees and robotic testing, we show that the sMCL is the major stabilizer to external rotation torques and combined anterior and external loading conditions related to anteromedial rotatory instability across the arc of knee flexion, while the dMCL, POL, and ACL play a less prominent role, with the exception of the ACL and dMCL near full extension.
Abstract
Background
Injury to the medial knee ligaments (sMCL, dMCL, POL) and ACL can cause anteromedial rotatory instability (AMRI), which involves excess tibial external rotation, anterior tibial translation, or a combination of both motions. AMRI can cause instability and ACL graft failure; however, it is unclear how the sMCL, dMCL, POL, and ACL resist AMRI. There is little data regarding the forces in these ligaments during AMRI loading. Thus, we investigated the forces in sMCL, dMCL, POL, and ACL during clinically-relevant AMRI loading scenarios.
Aim
Characterize the in-situ forces of the sMCL, dMCL, POL, and ACL under 1) isolated external tibial rotation torque (ER), 2) isolated anterior tibial force (Ant), and 3) combined ER+Ant loading.
Methods
Twenty-eight human cadaveric knees (18 male; mean age, 48±13; 21-65 years) were tested on a robotic manipulator with force sensing. Tibiofemoral kinematics were recorded under isolated ER (4Nm, 0-90°), isolated Ant (134N at 0-90°), and combined ER+Ant (4Nm+100N at 15, 30, 90°). The sMCL, dMCL, POL, and ACL were dissected in random order. The in-situ force (N) in the sMCL, dMCL, POL, and ACL at the peak applied load for each loading condition was calculated using superposition and compared with Kruskal-Wallis tests with post-hoc pairwise testing using a Bonferroni-Holm correction for multiple comparisons (α=0.05).
Results
Under isolated ER, the force in the sMCL (32–52N) from 0°-90° exceeded that of the ACL, dMCL, and POL at each flexion angle (p<0.001), except for at 0°, where force in the ACL and sMCL were similar (p>0.05). Force in the ACL was the second highest (26–6N from 0°-90°). Force in the dMCL and POL was low (≤12N). Under isolated Ant, the ACL carried the highest force at all flexion angles (≥113N) (p<0.001). Force in the sMCL was the second highest (22–31N from 0°-90°). Force in the dMCL and POL was low (≤11N). Under combined ER+Ant, forces in the sMCL and ACL were comparable in early knee flexion (15°, 30°) (p>0.05), but at 90° the sMCL carried the highest force of all ligaments (p<0.01). Combined ER+Ant produced elevated force in the dMCL that was comparable to the sMCL and ACL at 15° (p>0.05). At 90°, force in the dMCL diminished (<3N).
Conclusions
We show that the sMCL is the major stabilizer to isolated ER, while force in the dMCL, POL, and ACL is low, except for at 0°, where the ACL and sMCL carry similar force. This contrasts with prior work, which emphasized the dMCL in resisting isolated ER (Ball, et al, Amis, 2020). Also, under combined ER+Ant, sMCL and ACL force was high, indicating that if AMRI is detected, clinicians should consider sMCL and ACL injury. Force in the dMCL was high under combined ER+Ant only near full extension, thus if AMRI is detected near extension, dMCL injury may also be present. Overall, the dMCL may bear some, but comparatively less, force near full extension, but clinical suspicion for sMCL and ACL injury should be high if AMRI is detected clinically.