Popliteus Assessment

For Medical Students

What It Tests

The popliteus complex is the deep part of the posterolateral corner. It runs from the lateral femoral condyle through the joint to the tibia, with a branch to the fibula, and it resists external rotation of the tibia. 3.10.4 covers its function.

No clinical test isolates it. Assessing it means reading external rotation, because that is the plane it works in, and varus stress is close to blind to it. 4.13.1 covers that limitation.

The structure matters out of proportion to its size, because a corner reconstruction that leaves it out leaves the rotational laxity that stretches whatever else was done. 3.10.8 covers reconstruction biomechanics.

How It Is Performed

Perform the dial test. With the patient supine or prone, flex both knees to thirty degrees, rotate both feet externally together and compare the thigh foot angles. 4.13.5 covers the test in full.

Repeat at ninety degrees of flexion, because the difference between the two angles is what separates the corner from the posterior cruciate.

Perform varus stress at thirty degrees and in extension, expecting it to be close to normal in an isolated popliteus injury. 4.13.1 covers it.

Palpate the lateral collateral ligament in the figure of four position to establish whether the superficial part of the corner is intact. 4.13.2 covers that palpation.

Examine the common peroneal nerve, because the corner and the nerve are injured together often enough that the examination is not optional. 4.11.7 covers the sequence.

What a Positive Means

An increase in external rotation at thirty degrees that does not increase further at ninety degrees indicates an isolated posterolateral corner injury, and the popliteus complex is the structure that produces it.

An increase at both thirty and ninety degrees indicates the corner and the posterior cruciate together. 4.11.2 covers the posterior drawer.

A near normal varus test with an abnormal dial test is the classic popliteus picture, and it is the combination most often read as a normal lateral side. 4.13.1 covers that trap.

Because no test isolates the structure, the honest clinical conclusion is a posterolateral corner injury rather than a named component, and imaging names it where that is needed. 4.1.6 covers that relationship.

MRCS Level

How It Is Performed

In a viva the popliteus is the structure to name when asked why the dial test is performed at two angles, and that is almost always the question behind it.

The expected points are the dial test at both angles, the near normal varus test, the collateral palpation, and the nerve.

Say that the complex includes the popliteus tendon and the popliteofibular ligament, and that the second contributes substantially to external rotation restraint. 3.10.5 covers its function.

Say that the structure is intra-articular in part, which is why a corner injury can produce an effusion that an isolated collateral injury does not.

Say that the examination cannot separate the components and that saying so is the correct answer rather than a gap. 4.13.4 covers the assessment as a unit.

What a Positive Means

The reasoning expected is that external rotation of the tibia is resisted by the corner at thirty degrees and by the posterior cruciate at ninety, so the pattern across the two angles localises the injury. 3.10.7 covers the dial test biomechanics.

The popliteus also acts as a dynamic restraint through its muscle belly, which is part of why a corner injury affects gait more than a collateral injury does. 3.10.4 covers its function.

Its loss allows the lateral plateau to rotate posteriorly, which is the movement the reverse pivot shift detects and the reason that test needs the corner to be deficient. 4.11.4 covers it.

A knee with an increased dial test and a varus thrust in gait has a corner injury with a mechanical consequence, and that combination behaves differently from laxity alone. 4.11.5 covers dynamic assessment.

How Good It Is

The clinical assessment of this structure is indirect and better than its reputation suggests, because the dial test at two angles is reproducible.

What the assessment cannot do is name the component, and nothing clinical can. Magnetic resonance imaging identifies the popliteus tendon and the popliteofibular ligament where the examination identifies the pattern.

The examination remains the better guide to whether the laxity matters, which is the division this whole section keeps describing. 4.1.6 covers it.

The danger is the varus test, which reads as normal and is mistaken for a normal lateral side. That is the single most consequential false reassurance in Section 4. 4.13.1 covers it.

No figures appear for any of these comparisons, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level this structure is why a corner reconstruction is performed rather than a collateral reconstruction, and the dial test is what establishes it.

A knee with a posterior cruciate injury and an increased dial test at both angles needs both reconstructed, and reconstructing the cruciate alone is the recognised route to a stretched graft. 3.10.9 covers the combination.

A knee with an isolated corner injury and an increased dial test at thirty degrees only is reconstructed on the corner, and the cruciate is left alone.

The reconstruction that addresses the popliteofibular contribution behaves differently from one that addresses the collateral alone, which is why the examination's two angle pattern changes the operation rather than only the diagnosis. 3.10.8 covers the biomechanics.

Alignment comes first in a varus limb, for the reason it always does on this side of the knee.

Where It Misleads

The commonest error is performing the dial test at one angle, which collapses the localisation the two angles provide.

The second is relying on the varus test, which is close to blind to this structure.

The third is reconstructing a collateral and calling it a corner reconstruction, which is a different operation with a different result.

The fourth is examining a guarding patient, where both angles read as normal and the knee is recorded as intact.

The fifth is omitting the nerve examination in a knee with corner signs.

Fellowship Level

Where It Misleads

In referred practice the popliteus is the structure that was left out, and the consequence is the stretched graft sitting in front of you.

The classic case is a posterior cruciate reconstruction with an unaddressed corner, where the dial test was never performed at two angles or never performed at all. 4.11.2 covers the drawer that was.

A reconstructed corner with a persistent dial abnormality at thirty degrees means the popliteofibular contribution was not restored, and that is a different revision from a failed graft. 3.10.5 covers the structure.

A knee with a reconstructed corner that is tight in rotation presents as lateral pain and loss of internal rotation rather than instability, and over constraint on this side is under recognised.

In a knee with a previous proximal tibial osteotomy the corner's attachments and the thigh foot angle are both altered, so the dial test has to be compared with a known post-operative baseline rather than with the other leg.

A partially recovered common peroneal nerve palsy mimics a corner injury in gait, and the dial test is what separates the two: a nerve palsy does not increase external rotation laxity. 4.11.5 covers the dynamic examination.