Posteromedial Corner Function

For Medical Students

The Principle

Structures that are tight at the same joint position and resist the same motion behave as one unit, whatever their separate names.

Grouping them is useful because they are injured together and fail together.

A corner of a joint is such a group: the structures wrapping round one quadrant, taut in a particular position.

At the Knee

The posteromedial corner is the group of structures at the back and inside of the knee: the posterior oblique ligament, the posteromedial capsule, the semimembranosus and its expansions, and the attachment of the medial meniscus. 2.9.4 covers the anatomy.

They are taut near full extension and they resist valgus and external rotation together. 3.9.5 covers the posterior oblique ligament.

The semimembranosus tensions the capsule actively, which makes the corner dynamically reinforced rather than purely passive. 2.9.5 covers the muscle.

Losing the corner adds rotational laxity to a valgus injury, which is what changes the clinical picture. 3.9.7 covers that pattern.

What Changes It

Flexion angle, since the unit is taut near extension.

Which members are injured, although they usually go together.

Semimembranosus function, which is the dynamic component.

Concurrent cruciate injury, which raises the loads the corner carries.

Clinical Relevance

A medial injury with opening in full extension or with rotatory laxity is a corner injury, and that distinction drives the management. Section 6 covers management.

Corner injuries are the medial injuries that do not heal reliably, in contrast to isolated superficial ligament tears. 3.9.3 covers that ligament.

The corner appears on both sides of the knee as a concept, and the lateral one is separate. 2.10.4 covers the posterolateral corner.

Reconstruction of the medial side addresses the corner as well as the long ligament. 3.9.8 covers the construct.

MRCS Level

The Principle

A functional unit is defined by behaviour rather than dissection, which means its membership is a matter of convention and its boundaries are argued about.

That is acceptable as long as the behaviour it names is real. Here the behaviour is near extension valgus and rotational restraint, and it is well established.

The practical consequence is that apportionment between the corner's members is less settled than apportionment between the corner and the superficial ligament.

At the Knee

Conventional teaching includes the posterior oblique ligament, the posteromedial capsule, the semimembranosus and its tendinous expansions, and the meniscotibial attachment in the corner. 2.9.4 covers the anatomy.

Its contribution is greatest near full extension and falls with flexion, reciprocally with the superficial ligament. 3.9.9 covers the division.

It restrains external rotation and anteromedial translation, which together define the anteromedial rotatory instability pattern. 3.9.7 covers it.

Sectioning the corner after the superficial ligament produces a measurably larger increase in laxity than sectioning either alone, which is the quantitative form of the clinical observation.

What Changes It

Flexion angle, as the dominant variable.

Which members are injured and the sectioning order in any study quoted.

Semimembranosus function.

Cruciate integrity, which shares rotational restraint. 3.7.7 covers it.

How It Is Measured

Sequential sectioning with valgus and rotational loads apportions the corner's contribution, with the order dependence as a known limitation. 3.2.9 covers that.

Valgus stress radiography in full extension measures the functional consequence.

Magnetic resonance imaging demonstrates the corner's structures and their injury. 5.9 covers imaging.

No clinical test isolates individual corner members.

Clinical Relevance

Examine in full extension to detect corner involvement.

Expect a combined superficial and corner injury to be considerably more lax than either alone.

Treat the corner as a unit clinically, since no test separates its members.

Read apportionment figures within the corner as less settled than those between corner and superficial ligament.

FRCS Level

At the Knee

Treat the corner as the structure that decides the medial side's management. An isolated superficial ligament injury heals; a corner injury often does not, and the clinical question is which one is in front of you. 3.9.3 covers the ligament.

Conventional teaching reconstructs the corner as a separate limb or graft alongside the superficial ligament in anatomic medial reconstruction, which is a two structure operation rather than one. 3.9.8 covers the construct.

In a combined cruciate and medial injury the corner's state is what determines whether the medial side needs addressing surgically at all. 3.9.10 covers combined injury.

What Changes It

Whether the corner is injured and whether it has healed.

Concurrent cruciate injury and its reconstruction.

Chronicity, since the corner stretches under repeated load.

Alignment, particularly valgus. 3.13 covers alignment.

How It Is Measured

Examination under anaesthetic at full extension and at 30 degrees is the practical assessment.

Valgus stress radiography in extension quantifies the coronal consequence.

Magnetic resonance imaging identifies the structures injured.

No measurement confirms a reconstructed corner is functioning as a unit.

Clinical Relevance

Establish corner involvement before deciding on medial surgery.

Reconstruct the corner as a separate limb alongside the superficial ligament.

Reassess the corner in a chronic medial laxity even where the acute injury spared it.

Let the corner's state decide whether the medial side needs surgery in a combined injury.