The medial ligaments and the ACL restrain anteromedial laxity of the knee

Abstract

Purpose The purpose of this study was to determine the contribution of each of the ACL and medial ligament structures in resisting anteromedial rotatory instability (AMRI) loads applied in vitro. Methods Twelve knees were tested using a robotic system. It imposed loads simulating clinical laxity tests at 0° to 90° flexion: ±90 N anterior-posterior force, ±8 Nm varus-valgus moment, and ±5 Nm internal-external rotation, and the tibial displacements were measured in the intact knee. The ACL and individual medial structures-retinaculum, superficial and deep medial collateral ligament (sMCL and dMCL), and posteromedial capsule with oblique ligament (POL + PMC)-were sectioned sequentially. The tibial displacements were reapplied after each cut and the reduced loads required allowed the contribution of each structure to be calculated. Results For anterior translation, the ACL was the primary restraint, resisting 63-77% of the drawer force across 0° to 90°, the sMCL contributing 4-7%. For posterior translation, the POL + PMC contributed 10% of the restraint in extension; other structures were not significant. For valgus load, the sMCL was the primary restraint (40-54%) across 0° to 90°, the dMCL 12%, and POL + PMC 16% in extension. For external rotation, the dMCL resisted 23-13% across 0° to 90°, the sMCL 13-22%, and the ACL 6-9%. Conclusion The dMCL is the largest medial restraint to tibial external rotation in extension. Therefore, following a combined ACL + MCL injury, AMRI may persist if there is inadequate healing of both the sMCL and dMCL, and MCL deficiency increases the risk of ACL graft failure.

Medial soft-tissue complex of the knee: Current concepts, controversies, and future directions of the forgotten unit

“…But studies have also demonstrated that they work as a complex unit in response to valgus and rotational forces, with the sMCL, dMCL, and POL slackening or tightening at different degrees flexion. 1,9,16,28,29 Valgus forces are resisted primarily by the sMCL in all range of movement (ROM) but especially at 30º, with the dMCL acting as a secondary restraint in all ROM, and the POL as a stabilizer in extension. 1,9,[28][29][30] When these structures are injured, the anterior cruciate ligament (ACL), which has been shown to resist valgus forces, may become overloaded.…”

“…1,9,16,28,29 Valgus forces are resisted primarily by the sMCL in all range of movement (ROM) but especially at 30º, with the dMCL acting as a secondary restraint in all ROM, and the POL as a stabilizer in extension. 1,9,[28][29][30] When these structures are injured, the anterior cruciate ligament (ACL), which has been shown to resist valgus forces, may become overloaded. 1,9,10 Internal rotation forces (IR) are resisted primarily by the POL, mainly close to extension, 1,9,16,28,29 although a study found it to be relevant through all ROM.…”

“…1,9,[28][29][30] When these structures are injured, the anterior cruciate ligament (ACL), which has been shown to resist valgus forces, may become overloaded. 1,9,10 Internal rotation forces (IR) are resisted primarily by the POL, mainly close to extension, 1,9,16,28,29 although a study found it to be relevant through all ROM. 28 The dMCL and sMCL also restraint IR between 0-90º, especially at 30º of flexion.…”

“…If left untreated, it may cause pain, instability, and overload other stabilizers. 8,9 Nevertheless, few well-designed studies have been published around this topic, and most overlook the other MSC structures. 2,10,11 This has led to misunderstanding and controversy around MSC injuries, with some authors considering it the neglected side of the knee.…”

The medial collateral ligament: the neglected ligament

“…It defines the femoral and tibial attachments of the components of the MCL complex [3], introduces the concept that the dMCL is slack in neutral rotation, and describes an inverted fan shape passing from a small femoral attachment to a wide tibial attachment coursing anteriorly as well as distally. This orientation is shown to resist tibial external rotation [4]. The anterior dMCL on the medial side of the knee seems analogous to the ALL on the lateral side… perhaps an 'anteromedial ligament'!…”