Functions of the PCL

For Medical Students

The Principle

A ligament resists the movement that stretches it. One running from the front of the femur to the back of the tibia resists the tibia sliding backwards.

A ligament at the centre of a joint also sets where the bones sit relative to each other at rest, which is a position rather than a limit.

Position matters because every other measurement is taken from it. Move the starting point and every reading changes.

At the Knee

The posterior cruciate runs from the lateral wall of the medial femoral condyle to the back of the tibia, below the joint surface. 2.8.1 covers the anatomy.

Its primary function is resisting posterior translation of the tibia. 3.8.2 covers it.

It is the strongest ligament in the knee. Conventional teaching gives it roughly twice the ultimate strength of the anterior cruciate.

It is often called the central pivot of the knee, because the joint rotates about it and it holds the tibia in its resting position.

What Changes It

Flexion angle, since its tension rises with flexion. 3.8.3 covers the bundles.

Quadriceps activity, which pulls the tibia forwards and unloads it.

The posterolateral corner, which shares the restraint of posterior and rotational motion. 2.10.4 covers the corner.

Gravity and limb position, which is why a sag appears when the knee is held flexed and unsupported.

Clinical Relevance

A deficient posterior cruciate lets the tibia sag backwards, which changes the resting position before it changes any excursion. 3.8.6 covers the drawer.

That is why the step off between the femoral condyle and the tibial plateau is examined before any drawer is performed. Section 4 covers the tests.

Patients describe difficulty going downhill and down stairs rather than rotational giving way, which follows from the direction it resists.

It heals in continuity more often than the anterior cruciate, which is one reason its management differs. Section 6 covers management.

MRCS Level

The Principle

Separate three functions: a limit, a position and a guide. The ligament limits posterior translation, sets the resting position, and guides the rollback of the femoral condyles in flexion. 3.3.2 covers roll and glide.

The position function is what makes posterior instability different from anterior. An anterior cruciate deficient knee sits normally and translates further; a posterior cruciate deficient knee sits wrongly.

That difference changes how it is measured, because a drawer measured from a sagged starting position underestimates the excursion.

At the Knee

Conventional teaching gives the posterior cruciate roughly 95 per cent of the restraint to posterior translation at 90 degrees of flexion, which is a larger share than the anterior cruciate holds for its direction. 3.7.2 covers that comparison.

Its tension rises with flexion, so the restraint is greatest where posterior loads are largest. 3.8.3 covers the bundles.

It is a secondary restraint to external rotation, which is why an isolated injury and a combined corner injury behave differently. 2.10.4 covers the corner.

The meniscofemoral ligaments provide some posterior restraint independently, which is why a deficient knee is not completely unrestrained. 2.6.3 covers them.

What Changes It

Flexion angle, as the dominant variable.

Quadriceps activity, which reduces posterior translation actively.

Posterolateral corner integrity, which decides whether the picture is posterior or posterolateral.

Time, since partial healing changes the measured laxity over months.

How It Is Measured

Sectioning studies with a posterior load apportion the restraint, with the usual sectioning order limitation. 3.2.9 covers that.

Stress radiography measures the posterior translation against bony landmarks and is the most reproducible clinical measurement here.

The quadriceps active test demonstrates the sag mechanically by reducing it.

Instrumented arthrometry is less reliable posteriorly than anteriorly, because the starting position is uncertain.

Clinical Relevance

Establish the resting position before measuring any posterior excursion.

Use stress radiography where the grade will decide the treatment.

Examine the posterolateral corner in every posterior cruciate injury.

Expect measured laxity to change over months through partial healing.

FRCS Level

At the Knee

Treat the position function as the one that explains the natural history. A chronically sagged knee loads the medial and patellofemoral compartments differently, which is the mechanical route to degeneration after posterior cruciate injury. 3.5.8 covers contact mechanics after cruciate injury.

Partial healing in continuity at a greater length is the finding that separates this ligament from the anterior cruciate. The imaging can look normal in a knee that is measurably lax. 5.8 covers imaging.

That healing is why non operative management succeeds more often here, and why reconstruction results are less reliable: the operation competes with a ligament that is partly working. Section 6 covers management.

What Changes It

Corner integrity, which decides the pattern and the operation.

Alignment, since a varus limb with posterolateral laxity is a different problem. 3.13 covers alignment.

Time and the degree of healing achieved.

Quadriceps capacity, which substitutes for the restraint dynamically.

How It Is Measured

Stress radiography quantifies the grade and is underused relative to its value.

Examination under anaesthetic removes the quadriceps contribution, which matters more here than anteriorly.

Sectioning studies describe an acutely divided ligament; most patients present with one that healed long.

Serial measurement over months captures the healing, which a single measurement cannot.

Clinical Relevance

Measure with stress radiography where the grade decides the treatment.

Reassess over months rather than treating an early measurement as final.

Assess alignment in any chronic posterolateral injury before planning reconstruction.

Expect reconstruction to compete with a partly working ligament.