MCL Anatomy

For Medical Students

What It Shows

The medial side is layered: a superficial ligament, a deep ligament continuous with the capsule, and the posterior oblique ligament behind them. 2.9.1 covers the superficial MCL.

Coronal images separate the layers, with a bursa between the superficial and deep ligaments. 5.5.12 covers the bursa.

It matters because the layers have different attachments and different jobs, so naming the layer is the first content of any medial report. 5.20.4 covers reporting MCL injuries.

How It Is Acquired

Assess the layers on coronal images through the medial joint line. 5.5.9 covers the ligaments.

Identify the superficial ligament from the femoral epicondyle to the tibia below the joint line. 2.9.1 covers the superficial MCL.

Identify the deep ligament attaching to the meniscus above and the tibia just below the joint. 2.9.2 covers the deep MCL.

Identify the posterior oblique ligament on posterior coronal images. 2.9.3 covers the posterior oblique ligament.

Identify the bursa between the superficial and deep layers, which is a normal space. 5.5.12 covers the bursa.

What Normal Looks Like

A long continuous superficial ligament with a broad tibial attachment. 2.9.1 covers the superficial MCL.

A short deep ligament in two parts, above and below the meniscus. 2.9.2 covers the deep MCL.

A posterior oblique ligament blending with the capsule and the semimembranosus. 2.9.5 covers the semimembranosus.

A thin bursa with no more than a trace of fluid. 5.5.12 covers the bursa.

What Abnormal Looks Like

Fluid in the bursa, which is not by itself a ligament injury. 5.5.12 covers the bursa.

Thickening of the superficial ligament, which follows an old injury. 2.9.1 covers the superficial MCL.

Separation of the deep ligament from the meniscus. 5.6.10 covers ramp lesions.

Loss of the posterior oblique ligament's definition. 2.9.3 covers the posterior oblique ligament.

Oedema crossing all the layers, which obscures the boundaries between them. 5.5.9 covers the ligaments.

MRCS Level

How It Is Acquired

In a viva the expected answer names the layers before the injury.

The expected points are the coronal images, the superficial ligament, the deep ligament and its meniscal attachment, the posterior oblique ligament, and the bursa.

Say that the layers have different attachments and different functions. 3.9.3 covers the superficial MCL function.

Say that the bursa is a normal structure. 5.5.12 covers the bursa.

Say that the deep layer links the medial side to the meniscus. 3.9.4 covers the deep MCL function.

What Normal Looks Like

The reasoning expected is that the superficial ligament spans the joint as a long restraint while the deep one is short and attaches to the meniscus, so each resists a different thing and each fails under different conditions. 3.9.1 covers the valgus stability.

The posterior oblique ligament tightens in extension and carries rotation, which is why it sits in a different functional group from the other two. 3.9.5 covers the POL function.

The contributions change through flexion, so a structure's importance depends on the angle at which the knee was loaded. 3.9.9 covers the contributions at different flexion angles.

The semimembranosus blends into the posteromedial structures and gives them a dynamic component. 2.9.6 covers the dynamic stabilisation.

How Good It Is

Magnetic resonance imaging resolves these layers well, which makes the medial side the best seen and most loosely reported region in the knee.

Its best contribution is separating the layers and locating an injury within them. 2.9.2 covers the deep MCL.

It is limited by acute oedema crossing the layers, by the posterior oblique ligament blending with the capsule, and by valgus laxity being clinical. 4.12.1 covers the valgus stress testing.

No figures appear for accuracy on this page, under decision 028, and no ligament lengths, thicknesses or fluid thresholds appear either: those are published quantities measured differently between studies and this environment cannot check them.

FRCS Level

How Good It Is

At decision level the layer and the site decide whether an operation is discussed. 3.9.6 covers the posteromedial corner function.

A superficial injury alone heals and is managed without surgery. 5.9.2 covers MCL sprains.

A deep injury raises the meniscus as a separate question. 5.9.3 covers deep MCL injuries.

A posterior oblique injury moves the knee towards an operation. 5.9.4 covers POL injuries.

A cruciate injury alongside changes the plan for all of them. 3.9.10 covers the combined ACL and MCL injury.

Where It Misleads

The commonest error is reporting the medial side as one structure. 5.20.4 covers reporting MCL injuries.

Bursal fluid is reported as a ligament injury. 5.5.12 covers the bursa.

A chronic thickened ligament is reported as normal, or as acute. 2.9.1 covers the superficial MCL.

The deep layer is omitted because the superficial one dominates the image. 2.9.2 covers the deep MCL.

The posterior oblique ligament is omitted because it blends with the capsule. 2.9.3 covers the posterior oblique ligament.

Fellowship Level

Where It Misleads

At fellowship level the medial side arrives described as a sprain and the referral is about rotation. 3.9.2 covers the rotational stability.

The layers were visible on the original study, so re-reading it is usually more productive than repeating it. 5.20.1 covers structured reporting.

Chronic injury thickens and shortens the ligament, so the appearance records the history rather than the current competence. 5.9.2 covers MCL sprains.

A reconstruction alters the layers it reconstructs, so the anatomy afterwards is technique specific. 5.9.7 covers the post-operative medial reconstructions.

Ultrasound assesses the superficial layer dynamically and cannot reach the posterior oblique ligament. 5.4.9 covers the MCL.

The useful report names each layer, its site of injury, the meniscal attachment and the posteromedial structures. 5.20.4 covers reporting MCL injuries.