Reverse Screw Home Mechanism

For Medical Students

The Principle

A locking mechanism needs an unlocking step. If extension and external rotation are one movement, flexion from full extension has to begin with internal rotation.

The unlocking is active where the locking was passive. Shape and ligament tension lock the knee and a muscle has to start it moving again.

This is the reverse of 3.3.5 rather than a separate mechanism, which is why the conventional name is the reverse screw home.

At the Knee

The popliteus internally rotates the tibia on the femur in open chain, and externally rotates the femur on the tibia in closed chain. Either way it unlocks the joint.

It is positioned for the job. It runs from the lateral femoral condyle, passes inside the joint capsule and attaches to the posterior tibia, so its line of pull produces rotation rather than flexion.

Once the first few degrees of rotation and flexion have occurred, the larger flexors take over and the knee bends normally.

2.10.2 covers the tendon's anatomy and 3.10 the lateral side's biomechanics.

What Changes It

Popliteus injury, which is usually part of a posterolateral corner injury rather than isolated. 2.10.4 covers the corner.

Whether the knee was locked at all, since a knee held in flexion needs no unlocking.

Load, because an unloaded knee is easier to rotate and a loaded one resists.

Pain and guarding, which can prevent the small initiating movement.

Clinical Relevance

The popliteus has a role out of proportion to its size, which is why it is named in examinations more often than its bulk suggests.

A patient who struggles to start bending the knee from standing is describing a problem with the first few degrees rather than with the range.

Posterolateral corner injury affects rotation and therefore this step, which is part of why those injuries are functionally significant. Section 6 covers management.

Describing the popliteus as a flexor understates it. Its first job is rotation.

MRCS Level

The Principle

Treat the popliteus as a rotator with a flexion component rather than as a flexor, and its clinical significance follows from the first description.

The unlocking step is small and obligatory. A mechanism that must occur first is a single point of failure, which is why a small muscle matters.

Its second role is to retract the lateral meniscus during flexion, through the popliteomeniscal fascicles, which makes it a guide as well as a motor.

At the Knee

The popliteus originates from the popliteal sulcus of the lateral femoral condyle, runs intra-articular and extrasynovial, and inserts on the posteromedial proximal tibia.

The popliteofibular ligament connects the tendon to the fibular styloid and is part of the static posterolateral restraint. 2.10.3 covers it.

Popliteomeniscal fascicles attach the tendon to the lateral meniscus, so popliteus activity retracts the posterior horn out of the closing joint space during flexion.

The muscle therefore appears in three accounts: the unlocking motor, a posterolateral static restraint through its attachments, and a meniscal guide.

What Changes It

Posterolateral corner injury, which usually involves the popliteus complex and alters external rotation at 30 degrees.

Lateral meniscal injury or meniscectomy, which removes part of what the fascicles were guiding.

Fibular styloid avulsion, which disconnects the popliteofibular contribution.

Previous lateral surgery, which can scar the tendon and its fascicles.

How It Is Measured

Electromyographic studies describe the timing of popliteus activity and place it at the start of flexion, in a small number of invasive studies because the muscle is deep.

Cadaveric sectioning quantifies the static contribution of the popliteus complex to external rotation restraint, with the usual sectioning order limitation.

Magnetic resonance imaging demonstrates the tendon and the fascicles, which is anatomy rather than function. 5.10 covers imaging.

No clinical test isolates popliteus function, so the muscle is assessed through the rotational findings of the corner as a whole.

Clinical Relevance

Assess rotation at 30 degrees when a posterolateral injury is suspected, since that is where the complex is the restraint.

Expect a corner injury to affect the initiation of flexion as well as rotational stability.

Consider the popliteomeniscal fascicles when a lateral meniscus is hypermobile without a tear.

Avoid attributing a rotational finding to the popliteus alone, since no clinical test isolates it.

FRCS Level

At the Knee

Treat the popliteus complex as a reconstruction target rather than as a muscle to be noted. Conventional teaching holds that anatomic posterolateral reconstruction should address the popliteus tendon, the popliteofibular ligament and the lateral collateral ligament rather than a subset.

The functional loss in a deficient corner is rotational and it is positional, which is why these patients describe difficulty with the limb in extension rather than with pivoting alone.

A corner injury alongside a cruciate injury changes the cruciate's prognosis, and missing it is a recognised cause of graft failure. 3.8 and 3.10 cover the biomechanics.

What Changes It

Varus alignment, which loads a reconstructed corner and is addressable by osteotomy. 3.13 covers alignment.

Which components of the complex are injured, since the pattern decides what has to be reconstructed.

Chronicity, because a chronic corner injury stretches the remaining restraints and changes the operation.

Lateral meniscal status, through the fascicular attachments and through load sharing.

How It Is Measured

Robotic sectioning studies apportion the corner's restraint between the popliteus tendon, the popliteofibular ligament and the lateral collateral ligament, in specimens and in a sectioning order that changes the figures.

Varus stress radiography and the dial test quantify the clinical deficit, and neither separates the components.

Examination under anaesthetic adds information by removing guarding, which matters on the lateral side where the findings are subtle.

No measurement distinguishes a reconstructed popliteus that is functioning from one that is merely in place.

Clinical Relevance

Examine the posterolateral corner in every cruciate injury, since missing it fails the graft.

Address varus alignment before or with a corner reconstruction in a varus limb.

Reconstruct the components the injury pattern names rather than a standard subset.

Interpret corner apportionment figures with the sectioning order in mind.