Superficial MCL

For Medical Students

Structure and Location

The superficial medial collateral ligament is a long flat band running down the inner side of the knee. Conventional teaching gives its length as about 10 cm, which makes it the longest ligament at the knee.

Its femoral attachment is in a shallow depression close to the medial epicondyle, slightly above it and slightly behind it. Conventional teaching places the centre about 3 mm proximal and 5 mm posterior to the epicondyle itself.

It has two tibial attachments rather than one. The proximal attachment is to soft tissue about 12 mm below the joint line, over the anterior arm of semimembranosus. The distal attachment is to bone on the tibia about 6 cm below the joint line, under the pes anserinus tendons.

The medial side is described in three layers. Layer one is the fascia over sartorius, layer two is this ligament, and layer three is the capsule. 2.9.2 and 2.9.4 use the same scheme.

It is extra-articular. Nothing about it sits inside the joint, which is the single fact that explains most of its behaviour after injury.

Relations

Superficial to it: the fascia of layer one, then the pes anserinus tendons of sartorius, gracilis and semitendinosus running across to their insertion in front of it.

Deep to it: a bursa, then the deep medial collateral ligament and the capsule. The bursa is what lets the two ligaments move on each other.

Behind it: the posterior oblique ligament, which 2.9.3 covers, and the posteromedial corner.

Crossing the field: the saphenous nerve and its infrapatellar branch, and the medial inferior genicular artery running under the ligament close to the joint line.

Function

The ligament is the primary restraint to valgus force. That is its main job and it is what a valgus stress test examines.

Its two tibial attachments do different work. The proximal division resists external rotation and the distal division resists internal rotation, so the ligament restrains rotation in both directions as well as valgus.

It is tightest in slight flexion rather than in full extension, which is why the stress test is done at about 20 degrees. 3.9.3 covers its contribution through range.

Clinical Relevance

Valgus laxity at about 20 degrees of flexion with a firm endpoint is the finding that localises an injury to this ligament. Laxity in full extension points further back, to the posteromedial corner.

It usually heals. Three things favour it: the ligament sits outside the joint, it has a good blood supply along its length, and its torn ends stay in contact. The anterior cruciate has none of the three, which is why one injury is treated in a brace and the other in theatre.

The two tibial attachments mean an injury can involve one and spare the other, and the pattern of rotation on examination says which.

The saphenous nerve crosses the field, so numbness over the inner shin after a medial injury is a nerve finding rather than a ligament one.

MRCS Level

Structure and Location

Describe the femoral attachment by the depression rather than by the epicondyle. The epicondyle is a prominence and the attachment sits in the hollow behind and above it, which is why an attachment described as on the epicondyle is usually drawn too far forward.

Conventional teaching gives the femoral footprint an area of the order of 100 square millimetres and the distal tibial footprint rather more, spread along the bone.

The proximal tibial attachment is not to bone. It attaches to the soft tissue over the anterior arm of semimembranosus, which couples the ligament to a muscle and gives the medial side a dynamic component. 2.9.5 and 2.9.6 carry that.

The ligament is broad proximally and narrows as it descends, then widens again at the distal attachment.

Relations

The three layer description is the one to use in a viva. Layer one is the deep crural fascia continuous with the sartorius fascia; layer two is this ligament with the patellofemoral ligaments anteriorly; layer three is the capsule, thickened centrally as the deep medial collateral ligament.

Anteriorly the layers are distinct. Posteriorly layers two and three merge, and the posterior oblique ligament is that merged tissue.

The pes anserinus tendons lie in layer one over the distal attachment, with the pes bursa between them and the ligament.

The infrapatellar branch of the saphenous nerve crosses the field transversely between sartorius and gracilis. It is the structure most often injured in a medial approach.

Blood Supply and Innervation

The supply is from the superior and inferior medial genicular arteries, with contributions from the descending genicular artery proximally.

The vessels reach the ligament along its whole length rather than at one point, which is the vascular difference from the cruciates and the reason this ligament repairs itself.

Nerve supply is from the medial articular branches and from branches of the saphenous nerve. The ligament carries mechanoreceptors near both attachments.

Function

It provides the large majority of the restraint to valgus through the mid range. The capsule and the cruciates take a larger share near full extension.

The proximal and distal divisions behave differently through flexion: the proximal division tightens in flexion and the distal division in extension. 3.9.3 and 3.9.9 carry the detail.

Through its proximal attachment to semimembranosus it is tensioned by muscle as well as by position, so it is partly a dynamic restraint.

Clinical Relevance

Examine at 20 to 30 degrees for this ligament and at 0 degrees for the posteromedial corner. Reading the two together localises the injury, and 4.12 covers the examination.

Grade by the opening and the endpoint rather than by the pain, because an acute injury is painful whatever its grade.

Expect healing in most isolated injuries. The question is the position in which it heals, which is why the knee is held rather than simply rested.

Protect the infrapatellar branch of the saphenous nerve in any medial approach. The nerve crosses where the incision runs, and 2.9.7 covers the landmarks.

FRCS Level

Relations

The anatomical boundary that matters is the point where layers two and three merge. In front of it the ligament can be isolated; behind it the posterior oblique ligament and the capsule are one sheet, and an injury there is a corner injury rather than a ligament one.

The proximal tibial attachment couples the ligament to semimembranosus. An injury that strips that attachment removes a dynamic tensioner as well as a static restraint.

The distal tibial attachment lies under the pes. Reaching it means working through or around three tendons, which is the practical reason the distal end is approached less often than the proximal.

Function

Treat valgus restraint as shared rather than solitary. This ligament carries the large majority through mid flexion; near extension the posteromedial corner and the cruciates carry more, and a knee lax in extension has lost more than this ligament.

Rotation restraint divides between the two tibial attachments, so an injury pattern can produce rotational laxity with little valgus laxity.

Anteromedial rotatory instability is the combination of this ligament with the posteromedial corner and, usually, the anterior cruciate. 3.9.7 carries the mechanics and 2.9.4 the anatomy.

Clinical Relevance

Assess rotation as well as valgus before calling a medial injury isolated. The anatomy divides the work between two attachments and the examination has to follow it.

Consider the healing position rather than only the healing. A ligament that heals long leaves a knee that opens, and the anatomy of the two attachments explains why position matters more here than in most ligaments.

Where a reconstruction is planned, the femoral attachment is found from the depression behind the epicondyle and the distal tibial attachment from the measurement below the joint line, not from the appearance of the remaining tissue. 2.9.7 covers the landmarks.

Protect the dynamic coupling at the proximal tibial attachment where the tissue allows it. Restoring the static restraint without it restores part of the anatomy.