Posteromedial Corner

For Medical Students

Structure and Location

The posteromedial corner is the region behind the superficial medial collateral ligament and in front of the posterior cruciate ligament, on the inner side of the knee.

Five structures make it up: the posterior oblique ligament, the semimembranosus tendon with its arms, the oblique popliteal ligament, the posteromedial capsule, and the posterior horn of the medial meniscus with its attachments.

The three layer scheme that 2.9.1 introduced is what organises them. Layers two and three merge here, and the merged sheet is the posterior oblique ligament.

The corner has no clear boundary at either end. In front it becomes the collateral ligament, behind it becomes the posterior capsule, and the structures named above are regions of a continuous sheet rather than separate objects.

Relations

In front: the superficial medial collateral ligament and the interval between it and the posterior oblique ligament.

Behind: the posterior capsule and the medial head of gastrocnemius.

Deep: the posterior horn of the medial meniscus and the joint.

Crossing it: the oblique popliteal ligament, running up and laterally from the semimembranosus tendon to the opposite side of the capsule.

Function

The corner restrains valgus near extension, when the collateral ligament is at its slackest.

It is the primary restraint to internal rotation of the tibia in the extended knee.

It resists hyperextension, mainly through the oblique popliteal ligament.

Semimembranosus tensions the whole corner when it contracts, so the corner is both static and dynamic.

Clinical Relevance

A corner injury is missed because no single test finds it. Valgus at 20 degrees tests the collateral ligament; valgus at 0 degrees and rotation in extension test this.

A corner injury rarely occurs alone. It usually accompanies a collateral ligament injury, an anterior cruciate injury, or both.

The combination of an anterior cruciate injury and a corner injury produces a particular pattern of instability on rotation, which 3.9.7 covers.

Because the corner is capsular, an injury here bleeds into the joint.

MRCS Level

Structure and Location

Describe it by layers rather than by a list. Layer one is the sartorius fascia, layer two the collateral ligament fading into the posterior oblique ligament, layer three the capsule. Posteriorly two and three are one sheet.

The posterior oblique ligament is the static centre of the corner and semimembranosus is its dynamic centre. Everything else attaches to one or the other.

The oblique popliteal ligament is the largest structure on the back of the knee. Conventional teaching gives it a width of the order of 15 mm where it crosses the midline.

The posterior horn of the medial meniscus is part of the corner rather than a neighbour, because the central arm of the posterior oblique ligament attaches to it.

Relations

The interval between the collateral ligament and the posterior oblique ligament is the anterior boundary and it is only a real plane proximally.

The medial head of gastrocnemius arises just behind the corner's femoral attachments and covers them from behind.

The semimembranosus bursa separates the tendon from gastrocnemius and communicates with the joint in a substantial minority of knees.

The posterior cruciate ligament lies medial to nothing here: it is intra-articular, and the corner is the capsule outside it. 2.8.1 covers the relationship.

Blood Supply and Innervation

Supply is from the medial superior and medial inferior genicular arteries and from branches of the popliteal artery through the capsule.

The corner is capsular tissue with a better blood supply than an intra-articular ligament, which is the anatomical basis for repairing acute injuries here.

Nerve supply is from the posterior articular branches of the tibial nerve and from the medial articular branches, which is why posteromedial pain localises poorly.

Function

Through the last 30 degrees of extension the corner takes over from the collateral ligament as the main valgus restraint.

It is the primary restraint to internal rotation near extension and a secondary restraint to anterior translation, which is how it interacts with the anterior cruciate.

Anteromedial rotatory instability is the combination of corner and anterior cruciate loss. The tibia rotates and translates anteromedially as a unit. 3.9.7 carries the mechanics.

Semimembranosus tensions the corner, so its competence depends on a working muscle as well as intact tissue.

Clinical Relevance

Test valgus at 0 and at 20 degrees, and test rotation in extension. Three findings separate corner from collateral, and 4.12 covers the examination.

Suspect a corner injury whenever valgus laxity is present in full extension, whatever the finding at 20 degrees.

Examine the anterior cruciate in any knee with a corner injury, because the combination is commoner than either alone in this pattern.

Expect an effusion. The corner is capsular, so bleeding reaches the joint.

FRCS Level

Relations

The corner's anterior boundary is a plane that exists at the femur and closes distally, so an approach that separates the collateral ligament from the corner can do so proximally and not below.

Its femoral attachments cluster around the gastrocnemius tubercle and the adductor tubercle. Three structures take their origin from a few square centimetres of bone, which constrains every fixation point. 2.9.7 carries the landmarks.

The posterior horn's attachment through the central arm means corner surgery and meniscal position are the same question asked twice.

Function

Treat the corner as the extension restraint and the collateral ligament as the flexion restraint. The pattern matches the cruciate bundles in 2.8.2 and 2.8.3, and it is the reason a single structure repair restores part of a range.

Its contribution to anterior restraint explains why an anterior cruciate graft in a knee with an unrecognised corner injury is loaded beyond what it was tensioned for.

Dynamic tensioning by semimembranosus means a reconstruction that restores static tissue restores less than the corner's whole function.

Clinical Relevance

Assess the corner before any anterior cruciate reconstruction in a knee with medial findings. An unrecognised corner injury loads the graft.

Document valgus at 0 degrees, valgus at 20 degrees and rotation in extension separately, because the three together stage the injury and no one of them does.

Consider acute repair where the tissue is capsular and recently injured, since the blood supply supports it. 7.38 carries the operative content.

Avoid treating capsular tissue that has already stretched as repairable. Capsule that has failed once is not tissue to rely on a second time.

Fellowship Level

Clinical Relevance

Staging is the whole problem. The corner is five structures and the examination gives three findings, so a stage is an inference and the operative plan has to survive being wrong. Plan the approach to expose more than the plan needs.

Repair and reconstruction answer different anatomies. Acute capsular tissue with its blood supply intact is repairable; the same tissue three months later has remodelled at the length it healed to, and advancing it moves a structure that has already failed.

In a multiligament knee the corner is usually injured and usually not the first thing addressed. That order is defensible, but the anatomy says the corner determines the rotational position in which everything else is fixed.

Reconstruction points compete for bone. The posterior oblique ligament, the collateral ligament and the medial head of gastrocnemius originate within a few square centimetres, so a construct that reproduces two structures has to place two points in one small field. 2.9.7 carries the measurements.

The dynamic component cannot be reconstructed. Semimembranosus tensions the corner and no graft does, so a static reconstruction in a limb with a poor medial hamstring restores a limit without restoring the tension that keeps the limit from being reached.

Consider the meniscus in every corner plan. The central arm carries the posterior horn, so a corner that is advanced, repaired or reconstructed moves it, and a corner left lax leaves it moving.