Gait Following PCL Injury

For Medical Students

The Principle

When a passive restraint is lost, a muscle that pulls in the opposite direction can substitute for it.

Using a muscle to replace a ligament costs energy and loads the joint the muscle crosses, which is a transfer rather than a solution.

So an adaptation can succeed functionally and have a mechanical price somewhere else.

At the Knee

The quadriceps pulls the tibia forwards through the patellar tendon at low flexion angles, which is the opposite direction from a posterior sag. 3.4.4 covers lever arms.

A posterior cruciate deficient knee therefore uses the quadriceps to hold the tibia forward, which is the reverse of an anterior cruciate deficient knee's strategy.

The cost is patellofemoral load. More quadriceps force means more compression across the patellofemoral joint. 3.4.3 covers joint reaction forces.

Patients report symptoms going downhill and down stairs, which are the tasks demanding most quadriceps work in flexion. 3.3.12 covers stairs.

What Changes It

Quadriceps strength, which is what the adaptation depends on.

The activity, since descending demands most.

The grade of laxity, which sets how much substituting is needed.

Alignment and corner integrity, which change the mechanical problem. 2.10.4 covers the corner.

Clinical Relevance

Quadriceps strengthening is the mainstay of non operative management, and it works by a mechanism rather than by generic conditioning. Section 6 covers management.

Anterior knee pain is a recognised consequence of posterior cruciate deficiency, through the patellofemoral loading. 3.12 covers the joint.

Hamstring strengthening is unhelpful here and is helpful in anterior cruciate deficiency, which is a mechanically opposite prescription.

Gait pathology in detail belongs to 3.14.

MRCS Level

The Principle

The adaptation is a substitution of an internal moment for a passive restraint, and it has to be timed as well as produced. 3.4.7 covers internal moments.

Substitution is angle dependent, because the quadriceps' anterior shear on the tibia reverses with flexion. Near extension it pulls the tibia forward and in deep flexion it does not. 3.4.4 covers lever arms.

So the strategy works in the range where the shear is anterior, and in deeper flexion the knee has less available.

At the Knee

Conventional teaching describes increased quadriceps activity in stance in posterior cruciate deficient knees, which is the opposite of the quadriceps avoidance described in anterior cruciate deficiency.

Reduced knee flexion in stance is also described, which reduces the posterior shear and the quadriceps demand together. 3.3.9 covers walking.

The patellofemoral consequence is a recognised association, and conventional teaching reports patellofemoral and medial compartment degeneration in chronic deficiency. 6.10 covers chronic deficiency.

Descending stairs and slopes combine flexion under load with high quadriceps demand, which is why those are the reported difficulties. 3.3.12 covers stairs.

What Changes It

Grade of laxity and whether the corner is involved.

Quadriceps capacity and its endurance rather than its peak alone.

Activity and gradient, which set the demand.

Alignment, which interacts with the compartment loading.

How It Is Measured

Instrumented gait analysis with electromyography describes the muscle pattern and the kinematic adaptation.

Isokinetic testing measures the quadriceps capacity the adaptation depends on.

Imaging measures the compartment consequences over years. 5.8 covers imaging.

Separating the adaptation from pain avoidance is difficult in any individual.

Clinical Relevance

Prescribe quadriceps strengthening as the mechanical substitution it is.

Expect the strategy to work near extension and less in deeper flexion.

Anticipate patellofemoral and medial compartment consequences in chronic deficiency.

Ask about descending tasks specifically, since they demand most.

FRCS Level

At the Knee

Treat the adaptation as the reason non operative management succeeds here more often than anteriorly, and as the reason the natural history is still unfavourable in the long term.

The knee can function on a strong quadriceps and a partly healed ligament, and it pays in compartment loading over decades. Both halves have to be said to a patient. 3.5.8 covers contact mechanics.

Conventional teaching reports that reconstruction improves laxity and has not clearly altered the degenerative natural history, which is an uncomfortable position and the current one.

What Changes It

Quadriceps capacity, which is modifiable and is the adaptation's foundation.

Grade and corner involvement, which decide whether non operative management is viable.

Alignment, particularly varus with posterolateral laxity. 3.13 covers alignment.

Activity and occupation, which set the cumulative loading.

How It Is Measured

Gait analysis with electromyography describes the adaptation, in research settings.

Stress radiography measures the laxity that treatment changes.

Long term cohorts measure the degenerative outcome, over decades.

No measurement predicts which deficient knee will degenerate.

Clinical Relevance

Build quadriceps capacity as the foundation of non operative management.

Address alignment in a varus limb with posterolateral laxity.

Counsel that reconstruction improves laxity without a clear effect on the natural history.

State that no measurement predicts which knee will degenerate.