Posterolateral Corner Function

For Medical Students

The Principle

Structures that resist the same motions and are injured together behave as one unit, whatever their separate names.

Grouping them is useful because the clinical finding is the group's failure rather than any member's.

A group containing a muscle is partly active, so its function changes with whether the muscle is working.

At the Knee

The posterolateral corner is conventionally the lateral collateral ligament, the popliteus tendon and the popliteofibular ligament, with the posterolateral capsule and the biceps femoris contributing. 2.10.4 covers the anatomy.

Together they resist varus, external rotation and posterior translation, with the share of each changing through the arc. 3.10.1 and 3.10.2 cover the first two.

The popliteus is a muscle, so part of the corner's restraint is active. 3.10.4 covers it.

The corner heals badly, which is the opposite of the medial side. 3.9.6 covers the posteromedial corner.

What Changes It

Flexion angle, which changes the members' shares.

Which members are injured, although they usually go together.

Alignment, since varus loads the corner in stance. 3.13 covers alignment.

Concurrent cruciate injury, which raises what the corner carries.

Clinical Relevance

A missed corner injury is a recognised cause of cruciate graft failure, on both cruciates, which is why it is looked for rather than noticed. 3.10.9 covers combined injury.

Patients describe giving way into hyperextension and varus, which is the corner's characteristic account.

The peroneal nerve runs through this region, so a foot drop can accompany the injury. 2.10.6 covers the nerve.

Poor healing is why corner injuries are reconstructed rather than braced. Section 6 covers management.

MRCS Level

The Principle

The corner is defined by behaviour, so its membership is conventional and its boundaries are argued about. What is not argued about is the behaviour: varus and external rotation restraint with a posterior component. 3.9.6 covers the medial equivalent.

Apportionment within the corner is less settled than apportionment between the corner and the cruciates, because the members are adjacent and the sectioning order changes the figures. 3.2.9 covers that limitation.

The active component makes the corner different from the medial one in kind as well as in healing.

At the Knee

Conventional teaching gives the lateral collateral the main varus restraint near extension and the popliteofibular ligament and popliteus complex the main external rotation restraint. 3.10.5 covers the popliteofibular ligament.

The corner contributes to posterior translation restraint, which is why a corner injury increases posterior laxity and why a posterior cruciate injury with a large laxity suggests corner involvement. 3.8.2 covers posterior stability.

Hyperextension and varus thrust are the gait findings of chronic corner deficiency, which 3.14 covers in detail.

The biceps femoris and the iliotibial band contribute dynamically and passively, and are usually included in the wider account. 2.10.5 covers the biceps.

What Changes It

Flexion angle and which members are taut.

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

Alignment, which loads the corner in stance.

Cruciate integrity, particularly posterior.

How It Is Measured

Sequential sectioning with varus, rotational and posterior loads apportions the corner's contribution, with order dependence.

Varus stress radiography and the dial test measure the clinical consequence. 3.10.7 covers the dial test.

Magnetic resonance imaging demonstrates the structures inconsistently, particularly the popliteofibular ligament. 5.10 covers imaging.

Examination under anaesthetic is where the corner's findings are most reliably obtained.

Clinical Relevance

Assess varus, external rotation and posterior laxity together when a corner injury is suspected.

Read a large posterior laxity as suggesting corner involvement.

Examine under anaesthetic, since the findings are subtle and the imaging inconsistent.

Expect apportionment within the corner to be less settled than between corner and cruciate.

FRCS Level

At the Knee

Treat the corner as the structure whose assessment belongs in every cruciate operation. Conventional teaching identifies a missed corner injury as a leading cause of graft failure for both cruciates. 3.10.9 covers combined injury.

It is also the structure whose chronic deficiency is least forgiving, because it heals badly and because varus alignment loads it. A chronic corner injury in a varus limb needs both addressed. 3.13 covers alignment.

Reconstruction is anatomic and three component where the graft allows, and the active function is not restored. 3.10.8 covers the construct.

What Changes It

Whether the corner was assessed at the index cruciate operation.

Alignment, and whether an osteotomy is included.

Chronicity, since the structures stretch rather than heal.

Peroneal nerve status, which changes the operation and the counselling. 2.10.6 covers the nerve.

How It Is Measured

Dial test and varus stress under anaesthetic at the time of cruciate surgery is the practical assessment.

Varus stress radiography quantifies the coronal component.

Long leg standing radiography measures the alignment that loads the corner.

Revision series report corner findings at reoperation, which is the evidence it was missed.

Clinical Relevance

Assess the corner under anaesthetic in every cruciate reconstruction.

Correct varus alignment where a corner reconstruction would otherwise be loaded.

Document peroneal nerve function before and after corner surgery.

Expect a chronic corner injury not to improve without surgery.