Popliteus Function

For Medical Students

The Principle

A muscle with a tendon that crosses a joint inside the capsule can act as a ligament when it is not contracting and as a motor when it is.

A tendon with attachments to a third structure on its way past also guides that structure.

So one unit can be an active rotator, a passive restraint and a guide, depending on which question is being asked.

At the Knee

The popliteus runs from the popliteal sulcus of the lateral femoral condyle, inside the capsule, to the posteromedial tibia. 2.10.2 covers the anatomy.

As a muscle it internally rotates the tibia and unlocks the knee from full extension. 3.3.6 covers the reverse screw home mechanism.

As a passive restraint it resists external rotation and contributes to varus restraint, through the popliteofibular ligament. 3.10.5 covers that ligament.

Through the popliteomeniscal fascicles it retracts the lateral meniscus during flexion, keeping the posterior horn out of the closing joint. 2.6.2 covers the fascicles.

What Changes It

Whether the question is about active or passive function.

Flexion angle, which changes both.

Whether the tendon, the fascicles or the popliteofibular ligament is injured.

Lateral meniscal integrity, since the fascicles attach to it.

Clinical Relevance

A popliteus injury is almost always part of a posterolateral corner injury rather than isolated. 3.10.6 covers the corner.

Its three roles mean an injury affects unlocking, rotational stability and meniscal behaviour together.

A hypermobile lateral meniscus without a tear can be a fascicular problem. 3.6.10 covers extrusion.

No clinical test isolates popliteus function, so it is assessed through the corner's findings. Section 4 covers the tests.

MRCS Level

The Principle

Treating it as a complex rather than a muscle is the useful move. The popliteus complex is the muscle, its tendon, the popliteofibular ligament and the popliteomeniscal fascicles.

Each component has a different mechanical role, and sectioning studies that divide the tendon are measuring something different from those that divide the popliteofibular ligament.

That is one reason corner apportionment figures vary between studies: the complex is not divided the same way twice. 3.2.9 covers the sectioning order limitation.

At the Knee

Conventional teaching gives the popliteus complex a major role in restraining external rotation, with the popliteofibular ligament carrying much of the passive share. 3.10.5 covers it.

Its intra-articular, extrasynovial course means it is visible at arthroscopy, which is how a corner injury can be inferred intraoperatively.

The tendon's femoral origin sits in the popliteal sulcus, anterior and distal to the lateral collateral's origin, and the two are distinguishable in reconstruction. 2.10.7 covers the surgical anatomy.

As an active internal rotator it opposes the external rotation that a corner injury allows, so its loss removes an active and a passive restraint in the same direction.

What Changes It

Which component of the complex is injured.

Flexion angle, which changes the passive tension and the active lever arm.

Lateral meniscal status, through the fascicles.

Fibular styloid integrity, which the popliteofibular ligament depends on.

How It Is Measured

Selective sectioning of the complex's components apportions the restraint, with the components divided differently between studies.

Electromyography describes the muscle's timing and is technically difficult because the muscle is deep.

Magnetic resonance imaging demonstrates the tendon and the fascicles. 5.10 covers imaging.

Arthroscopic assessment of the popliteal hiatus identifies tendon and fascicular injury.

Clinical Relevance

Treat it as a complex and name the component when discussing injury or reconstruction.

Inspect the popliteal hiatus arthroscopically where a corner injury is suspected.

Expect its loss to remove an active and a passive restraint in the same direction.

Read apportionment variation as reflecting different sectioning of the complex.

FRCS Level

At the Knee

Treat the complex as the structure that makes anatomic corner reconstruction a three component operation: the lateral collateral, the popliteus tendon and the popliteofibular ligament. 3.10.8 covers the construct.

Conventional teaching holds that reconstructing all three restores rotational and coronal restraint better than reconstructing a subset, which is the mechanical argument behind anatomic over non anatomic techniques.

The active component cannot be reconstructed. A graft replaces the passive restraint and the muscle's rotational and meniscal roles are not restored, which is worth stating rather than implying. 3.3.6 covers the unlocking function.

What Changes It

Which components are reconstructed and where they are attached.

Fibular head and styloid bone quality for fixation.

Alignment, which loads the construct. 3.13 covers alignment.

Graft availability for three limbs.

How It Is Measured

Robotic testing compares anatomic and non anatomic corner reconstructions against the intact knee.

Varus stress radiography and the dial test measure the clinical result, the second without an instrument.

Postoperative imaging confirms tunnel positions against footprint maps.

No measurement confirms the active functions have been restored, because they have not.

Clinical Relevance

Reconstruct all three components where the injury and the graft allow.

State that the muscle's active roles are not restored by a graft.

Confirm tunnel positions on imaging, since the femoral origins are close together.

Correct alignment, since the construct cannot carry a varus limb's load.