The Superficial Medial Collateral Ligament Is the Primary Restraint to External Tibial Rotation Among Medial Knee Structures: A Robotic Biomechanical Study.

Background: External rotational stress of the knee leads to several knee problems and persistent pain. Clarifying the role of knee structures in external rotational stability aids in optimizing nonoperative treatment and guiding surgical indications. No previous studies have simultaneously compared the biomechanical contributions of both medial and lateral soft tissue structures to external rotational stress using a robotic system. Purpose: To investigate the influence of multiple soft tissue knee structures on stability during external tibial rotation at 0° to 90° of flexion using a robotic testing system. Study Design: Descriptive laboratory study. Methods: A total of 9 fresh-frozen cadaveric knee specimens and a robotic testing system were used. First, 5 N·m of external tibial rotation was applied to the intact knee at 0°, 15°, 30°, 60°, and 90° of knee flexion. The anterior cruciate ligament, anterolateral capsule, lateral collateral ligament, popliteus tendon (PT), posterior root of the lateral meniscus, superficial medial collateral ligament (sMCL), posterior root of the medial meniscus (MMPR), and posterior cruciate ligament (PCL) were then completely transected in sequence. After each transection, intact knee motion was reproduced for each knee condition, applying 5 N·m of external tibial rotation. By employing the principle of superposition, the resultant force of each structure was determined based on the 6 degrees of freedom force/torque data of each state. Resultant forces were statistically compared using the Kruskal-Wallis test, followed by the post hoc Steel-Dwass test. Results: The sMCL exhibited the greatest resultant force across all knee flexion angles from 0° to 90°. Between 30° and 90°, the MMPR and PT generated the highest resultant forces after the sMCL, while the PCL showed the greatest force at 90° after the sMCL, MMPR, and PT. At 60° of knee flexion, the sMCL, MMPR, and PT showed significantly greater resultant forces than the other structures ( Conclusion: Our study demonstrated that the sMCL exhibited the greatest resultant force under external tibial rotation across all knee flexion angles from 0° to 90°. Clinical Relevance: This study emphasizes the need to avoid excessive external rotation during the nonoperative or postoperative management of sMCL injuries, as rotational stress may compromise healing and functional recovery of the sMCL.