Posterolateral Corner Assessment

For Medical Students

What It Tests

The posterolateral corner is a group of structures rather than one: the lateral collateral ligament, the popliteus tendon, the popliteofibular ligament and the posterolateral capsule. 3.10.6 covers them as a unit.

Assessing the corner means establishing whether external rotation is increased, whether varus opening is increased, and whether the two findings sit together, because the combination localises the injury better than any single test.

The corner matters because it does not heal and because an uncontrolled corner stretches whatever else is reconstructed in that knee. 3.10.8 covers reconstruction biomechanics.

How It Is Performed

Examine the common peroneal nerve first in any knee with a suspected corner injury, testing ankle dorsiflexion, great toe extension and sensation on the dorsum of the foot.

Perform the dial test at thirty and at ninety degrees, with both legs together. 4.13.5 covers it.

Perform varus stress at thirty degrees and in full extension, with your fingers on the lateral joint line. 4.13.1 covers it.

Palpate the lateral collateral ligament in the figure of four position and follow it to the fibular head. 4.13.2 covers that palpation.

Perform the posterior drawer and look for a posterior sag, because the corner and the posterior cruciate are injured together often and the examination has to separate them. 4.11.1 covers the sag.

Look at the standing alignment and the gait for a varus thrust, since the corner's mechanical consequence is read standing rather than lying. 4.11.5 covers dynamic assessment.

What a Positive Means

Increased external rotation at thirty degrees with no further increase at ninety indicates an isolated corner injury.

Increased external rotation at both angles indicates the corner with the posterior cruciate. 4.11.2 covers the drawer.

Increased varus opening at thirty degrees with a firm endpoint in extension indicates the collateral ligament; opening in extension as well indicates a larger corner injury or a cruciate.

Hyperextension and an external rotation recurvatum deformity indicate the corner rather than either cruciate. 4.13.7 covers that test.

A near normal varus test with an abnormal dial test is the commonest corner presentation, and reading only the varus test is how it is missed. 4.13.3 covers the structure responsible.

MRCS Level

How It Is Performed

In a viva the corner is a sequence station, and the order is the answer: nerve, dial at two angles, varus at two angles, palpation, posterior drawer, standing alignment.

Say the nerve first without being prompted.

Say that you would compare every test with the other knee, because every corner finding is a comparison rather than an absolute. 4.1.2 covers the principle.

Say that the examination cannot name the component and that the clinical conclusion is a corner injury with the findings listed. 4.1.8 covers the record.

Say that stress radiographs and magnetic resonance imaging are both used, the first for the grade and the second for the structures. 4.1.6 covers that relationship.

Say that examination under anaesthesia is part of the assessment in an acute high energy knee rather than an extra. 4.14.6 covers it.

What a Positive Means

The reasoning expected is that each structure has a plane and an angle where it dominates, so the pattern across planes and angles is the diagnosis. 3.10.7 covers the dial test biomechanics.

The popliteofibular ligament and the popliteus dominate external rotation restraint at thirty degrees, the collateral dominates varus at thirty degrees, and the posterior cruciate contributes to external rotation at ninety. 3.10.5 covers the popliteofibular ligament.

The corner also resists hyperextension, which is why recurvatum appears in corner injury and not in isolated cruciate injury. 3.10.6 covers the corner.

A corner injury with an anterior cruciate rupture produces a larger pivot shift than the cruciate injury alone, because the corner contributes to rotational control. 4.14.1 covers the pivot shift.

How Good It Is

The corner assessment is the most consequential examination in Section 4 and the one most often performed incompletely.

The dial test carries it. Varus stress, palpation and recurvatum add localisation and would not, on their own, make the diagnosis. 4.13.5 covers the dial test.

It is unreliable in the acute high energy knee, where guarding defeats the dial test and the result is a false negative, which is the worst way for this particular assessment to fail.

Examination under anaesthesia is therefore part of the assessment rather than a refinement, and in a multiligament knee it is usually where the corner is established. 4.11.7 covers that sequence.

No figures appear for any accuracy comparison on this page, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level this assessment decides whether a corner reconstruction is performed and whether it is performed at the same time as a cruciate reconstruction, and those are the two decisions that determine the result.

An unrecognised corner injury in a knee having a posterior cruciate reconstruction is the commonest avoidable cause of a stretched graft, and this assessment is the only thing that prevents it. 3.10.9 covers the combination.

An isolated corner injury in a knee with a varus thrust is an alignment problem as well, and an osteotomy with or instead of a reconstruction is the decision the standing examination contributes.

Acute repair and delayed reconstruction have different windows, so the timing of this assessment matters as much as its content, and that argues for the examination under anaesthesia early.

The assessment also decides what gets reconstructed. A collateral alone is not a corner reconstruction, and the dial test at two angles is what says so. 4.13.3 covers it.

Where It Misleads

The commonest error is accepting a normal varus test as a normal lateral side.

The second is performing the dial test at one angle.

The third is examining a guarding patient and recording the result rather than recording that the examination was incomplete.

The fourth is diagnosing the corner and not examining the nerve.

The fifth is planning a cruciate reconstruction on imaging without this examination, which is how the stretched grafts in revision practice were produced.

Fellowship Level

Where It Misleads

In referred practice the corner is the structure that explains most failed posterior cruciate surgery, and this assessment is usually the first time it has been performed properly.

A patient with a stretched graft, an increased dial difference at both angles and a varus thrust has three findings that belong to one diagnosis, and treating them separately is the recurring error. 4.11.2 covers the drawer.

A reconstructed corner with a residual dial difference at thirty degrees has an incomplete reconstruction rather than a failure, and the distinction changes the revision. 4.13.3 covers the component usually missing.

A knee with corner signs and a previous proximal tibial osteotomy needs the alignment correction assessed before any ligament revision, because the reconstruction will stretch again otherwise.

In the chronic corner injury the examination findings can be stable while the medial compartment deteriorates, and the deterioration rather than the laxity becomes the reason to operate. 4.11.6 covers a parallel problem.

A common peroneal nerve palsy that has not recovered changes what any reconstruction can achieve, and that has to be part of the consent rather than discovered afterwards. 4.23.7 covers the medicolegal consequence.