PCL Deficient Gait

For Medical Students

The Principle

If a restraint on one side of a joint is missing, a useful adaptation is to use the muscle that pulls the other way.

That muscle then works harder than it was designed to, all day.

A protective adaptation that overworks something else is how one problem becomes two.

At the Knee

The posterior cruciate stops the shin sliding backwards. Section 6 covers the injury.

The quadriceps pull the shin forwards, which is the direction the missing ligament was protecting.

So a knee without a posterior cruciate tends to use the quadriceps more during walking, which is the opposite of the anterior cruciate pattern. 3.14.9 covers that one.

The cost is more load through the kneecap joint, because the quadriceps press it against the femur. 3.12.3 covers those pressures.

3.8.7 covers what this does to the ligament and the joint; this page covers the walking pattern.

What Changes It

How much backward sag there is.

Quadriceps strength, which this adaptation depends on.

Whether other structures were injured at the same time.

Walking surface and slope, since downhill walking demands more.

Clinical Relevance

Kneecap pain years after a posterior cruciate injury is a common and mechanical story.

Strengthening the quadriceps is the main treatment and it is also what the adaptation relies on. Section 8 covers it.

Many of these knees function well, which is why they are often managed without surgery.

4.4.4 covers the gait examination.

MRCS Level

The Principle

Where the anterior cruciate deficient knee reduces quadriceps demand, the posterior cruciate deficient knee increases it. The two adaptations are mirror images with asymmetrical costs.

The anterior cruciate adaptation costs disuse. The posterior cruciate adaptation costs compartment load, which is cumulative.

That is why the long term consequences of the two injuries differ in location as well as in degree.

At the Knee

Posterior sag places the tibia posteriorly at rest, which shifts the extensor mechanism's working position. 2.4.1 covers the patella.

Increased quadriceps activity through stance raises patellofemoral contact force, and medial compartment load rises too.

The reported degenerative pattern after posterior cruciate deficiency is medial and patellofemoral, which matches that mechanical account. 3.12.10 covers the wear.

Walking on level ground is often well tolerated; descending stairs and slopes provoke the symptoms, because quadriceps demand rises. 3.3.12 covers stair kinematics.

Chapter 3.8 covers the ligament's own mechanics and 3.8.7 the gait from that perspective.

What Changes It

Grade of posterior laxity and whether it is isolated or combined.

Posterolateral corner competence, since a combined injury behaves differently. 3.10.6 covers the corner.

Quadriceps strength, which the adaptation depends on and which therefore sets function.

Activity profile, particularly stairs, slopes and squatting.

Tibial slope, which alters the sagittal equilibrium. 3.13.9 covers osteotomy.

How It Is Measured

Three dimensional gait analysis, which shows the raised quadriceps demand and altered sagittal kinematics. 3.16.7 covers it.

Stress radiography for posterior translation, which measures the passive deficit. 5.3.2 covers alignment imaging.

Isokinetic quadriceps testing, which measures the capacity the adaptation relies on.

Patellofemoral loading, which is modelled rather than measured. 3.16.4 covers modelling.

This gait has a smaller literature than the anterior cruciate equivalent, in proportion to the injury's frequency rather than its consequences.

Clinical Relevance

Ask about stairs and slopes specifically, since level walking is often well tolerated.

Measure quadriceps strength, because the adaptation depends on it and function follows it.

Expect the degenerative pattern to be medial and patellofemoral rather than diffuse. 3.12.10 covers the wear.

Assess the posterolateral corner, since a combined injury changes the picture. 3.10.6 covers it.

FRCS Level

At the Knee

The posterior cruciate deficient knee is the clearest example in this chapter of an adaptation that works and bills later.

Quadriceps substitution restores function well enough that many of these knees are managed without surgery, and the degenerative pattern that follows is consistent with the loading that substitution produces.

That gives a mechanical argument for quadriceps rehabilitation as a treatment rather than as an adjunct, and simultaneously an argument that the treatment drives the long term cost.

Nobody has resolved that tension with evidence. There is no trial comparing long term joint outcomes between reconstructed and rehabilitated posterior cruciate deficiency.

Conventional teaching manages isolated grade one and two injuries non operatively, and the degenerative consequence is accepted rather than demonstrated to be avoidable.

What Changes It

Whether the injury is isolated, which is the dividing line in practice. 3.10.6 covers the corner.

Tibial slope, which can be corrected and which changes the sagittal equilibrium. 3.13.9 covers it.

Quadriceps capacity, which is both the treatment and the mechanism of the cost.

Patient demand, which decides whether the adaptation is sufficient.

Time, since the degenerative pattern emerges over years rather than months.

How It Is Measured

Gait analysis for the quadriceps demand and sagittal kinematics, which few of these patients receive.

Serial radiography for the degenerative pattern, which is what the long term series report.

Stress radiography for the passive deficit, which is the measurement most used in decisions.

Modelled compartment loading, which connects the gait to the degeneration and is inference rather than measurement. 3.16.4 covers it.

The causal chain from adaptation to degeneration is plausible, consistent and not demonstrated prospectively.

Clinical Relevance

Counsel that non operative management works through an adaptation that itself loads the joint, rather than presenting it as cost free.

Measure and treat quadriceps capacity, which is the mechanism of function here.

Consider slope in the patient with a poor result, since it changes the sagittal equilibrium. 3.13.9 covers correction.

State that the link from adaptation to degeneration is plausible and not prospectively demonstrated.

Weigh the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations, since the choice here is between a known operation and an accepted long term cost.