Screw Home Mechanism

For Medical Students

The Principle

A screw advances as it turns. The knee does something comparable through terminal extension: the rotation and the extension are one movement rather than two.

The rotation is obligatory. It happens because of the shape of the surfaces and the length of the ligaments, not because a muscle produces it.

Its purpose is to lock. A locked joint can hold a load with very little muscle activity, which is why standing still is not tiring.

At the Knee

Through roughly the last 30 degrees of extension the tibia rotates externally relative to the femur. Conventional teaching gives about 5 to 10 degrees of rotation.

In closed chain the same relationship appears as internal rotation of the femur on a fixed tibia.

Three features produce it. The medial femoral condyle has a longer articular surface than the lateral, the anterior cruciate tightens through extension, and the collateral and posterior structures tighten with it.

At the end of the movement the joint surfaces are maximally congruent, the ligaments are taut, and the knee has no useful rotation left. This is the position of maximal stability.

What Changes It

The anterior cruciate, since its tightening is part of the mechanism and a deficient knee loses part of it.

The shape of the medial femoral condyle, which varies between individuals.

Anything preventing full extension, because the mechanism occupies the last part of the range and a knee that cannot reach it never locks.

Soft tissue tension generally, which is why a swollen or contracted knee loses the position.

Clinical Relevance

A knee held in slight flexion cannot lock, so the quadriceps must work continuously to hold a load. That is one mechanical reason a flexion contracture is tiring.

Restoring full extension restores the locking position, which is why extension is protected so carefully after injury and surgery.

The locked knee is the position a limb is most stable in and least mobile in, which is the trade the mechanism makes.

Unlocking is an active step rather than a passive release. 3.3.6 covers it.

MRCS Level

The Principle

Describe it as a consequence of constraint rather than as a mechanism with a purpose. Given these surfaces and these ligament lengths, the rotation is the only way the joint can reach full extension.

The term screw home is conventional and slightly misleading, because nothing threads. The useful content is that terminal extension is coupled to external tibial rotation.

The end position is usually called the position of maximal congruence or the terminal locked position, and it is the one position in which the knee has essentially no axial rotation available.

At the Knee

Conventional teaching attributes the rotation chiefly to the greater anteroposterior length of the medial femoral condylar articular surface, which obliges the lateral side to finish its travel first.

The anterior cruciate ligament tightens through terminal extension and contributes to the coupled rotation rather than only limiting translation. 3.7 covers it.

The iliotibial band and the posterior capsule tighten in extension and add to the terminal constraint.

The patella sits at its most proximal trochlear position in extension, with the smallest contact area, which 3.12 covers.

What Changes It

Anterior cruciate deficiency, which alters terminal rotation and is one reason a deficient knee feels different in extension rather than only in pivot.

Condylar morphology, which varies and is part of why normal terminal rotation has a range rather than a value.

A fixed flexion deformity, which removes the part of the range the mechanism needs.

Previous surgery that changes extension, including a graft placed so that it limits terminal extension.

How It Is Measured

Fluoroscopic and model based tracking through terminal extension measures the coupled rotation in vivo, over a small range where measurement error is proportionally large.

Cadaveric work established the relationship and measures it without muscle, which is acceptable here because the mechanism is passive.

Clinical assessment detects loss of full extension reliably and the coupled rotation not at all, so the mechanism is inferred rather than measured in clinic.

Conventional teaching's figure of 5 to 10 degrees comes from a small range measurement and should be held loosely.

Clinical Relevance

Protect and restore full extension after any knee injury or operation, because the locking position depends on the last few degrees.

Expect a knee that cannot fully extend to fatigue the quadriceps in standing, and treat the extension rather than the muscle.

Suspect a graft or a cyclops lesion when terminal extension is lost after reconstruction.

Avoid quoting terminal rotation as a precise figure, since the published range is wide relative to the quantity.

FRCS Level

At the Knee

Treat terminal extension as a design and a technical target. An arthroplasty that cannot reach full extension has lost the locking position, and the patient pays for it in standing endurance rather than in range.

The coupled rotation constrains component rotation in replacement, because an implant that cannot rotate as it extends will either not extend or will rotate the whole tibia.

In reconstruction the same coupling explains why a graft placed too anteriorly limits terminal extension. Conventional teaching describes this as impingement, and the mechanical account is that the construct cannot accommodate the coupled motion.

What Changes It

Component geometry and insert conformity in replacement, which set whether coupled rotation is possible.

Posterior capsular release and osteophyte removal, which decide whether the last degrees of extension are available at all.

Graft tunnel position, particularly on the femoral side, which changes the angle at which the graft becomes the limit.

Tibial slope, which shifts the whole flexion extension arc and therefore where the mechanism sits in it.

How It Is Measured

Intraoperative assessment of full passive extension is the practical measurement, and it is made in an anaesthetised knee that will swell.

Fluoroscopic kinematic study of replaced knees measures whether coupled terminal rotation survived the operation, in selected series performing prescribed tasks.

Lateral radiography measures component position and sagittal alignment rather than the coupled motion itself.

Loss of extension is detected clinically with certainty and attributed to a cause only with imaging and examination together.

Clinical Relevance

Achieve full extension intraoperatively rather than accepting a few degrees, since the locking position has no partial version.

Place the femoral tunnel to allow terminal extension, and check it on the table.

Investigate lost extension after reconstruction rather than attributing it to stiffness.

Consider component rotation when a replaced knee loses terminal extension without mechanical obstruction.