ACL Deficient Gait

For Medical Students

The Principle

When a restraint is missing, the body often protects the joint by not using the muscle that would stress it.

That protection is automatic. Nobody decides to do it and most people cannot describe it.

The cost is that the muscle weakens and the knee works differently from then on.

At the Knee

The anterior cruciate stops the shin sliding forward. 6.1 covers the ligament.

The quadriceps pull the shin forward when the knee is nearly straight, which is exactly what the missing ligament resisted.

Some people with a deficient ligament reduce that quadriceps pull during walking, which is called quadriceps avoidance. 4.4.3 covers the gait.

Others keep the knee slightly bent throughout, which keeps the ligament out of its most vulnerable position.

Both adaptations are automatic and both reduce how much the knee is used normally.

What Changes It

How long ago the injury happened.

Whether the knee gives way, and how often.

Quadriceps strength, which falls after injury regardless.

What the person does, since walking rarely provokes instability and sport does.

Clinical Relevance

A deficient knee can walk almost normally and still give way when turning. 3.14.6 covers cutting.

That is why a normal walk tells you very little about whether a ligament is intact.

The quadriceps weakness that follows is measurable and treatable. Section 8 covers the rehabilitation.

Section 4 covers how the ligament is actually tested.

MRCS Level

The Principle

An adaptation can be a strategy or a consequence, and the two need separating. Quadriceps avoidance was described as a strategy; quadriceps weakness is a consequence.

They produce a similar gait and have different treatments, which is the clinically useful distinction.

A finding that is present in a minority should not be taught as the characteristic pattern, and this one routinely is.

At the Knee

Quadriceps avoidance is a reduced or absent external knee flexion moment in early stance, so the quadriceps are not required to resist it.

It was described in a minority of deficient knees in the original work and subsequent series have found it in a minority too.

The commoner findings are reduced knee excursion, a flattened stance flexion wave and altered tibial rotation during gait. 3.14.2 covers the wave.

Dynamic anterior tibial translation and rotational instability appear in demanding tasks rather than in walking. 3.14.6 covers cutting.

Gait abnormality correlates poorly with laxity measured on examination, which is a recurring finding in this ligament. 3.7.10 covers strain during activity.

What Changes It

Time since injury and whether episodes of giving way have occurred.

Quadriceps strength and the degree of arthrogenic inhibition.

Meniscal and chondral status, which change both symptoms and gait. 6.4 covers associated injuries.

Activity demand, since walking rarely challenges the deficient knee.

Reconstruction, after which gait abnormalities persist in many patients. 3.7.20 covers the reconstruction.

How It Is Measured

Three dimensional gait analysis with force plates, which is the only way to identify a moment pattern. 3.16.7 covers it.

Dynamic radiostereometric analysis, which measures tibial translation directly in a few centres.

Instrumented laxity testing, which measures a passive property and correlates poorly with gait.

Functional hop testing, which measures performance rather than mechanism. 3.14.7 covers jumping.

The classical description came from a small cohort, and its persistence in teaching outruns its prevalence in the literature.

Clinical Relevance

Separate quadriceps avoidance from quadriceps weakness, because the second is treatable and far commoner.

Avoid teaching quadriceps avoidance as the characteristic pattern, since it is present in a minority.

Assess the knee in a demanding task rather than in walking, because walking rarely provokes the deficit.

Expect gait abnormality and examination laxity to disagree, and say which you are describing.

FRCS Level

At the Knee

This page exists partly to correct a teaching point. Quadriceps avoidance is the best known gait finding in knee surgery and it is not the commonest one.

The durable findings are reduced sagittal excursion, altered rotational kinematics and persistent quadriceps deficit, and all three survive reconstruction in a substantial proportion of patients. 3.7.20 covers reconstruction.

Persistent altered kinematics after reconstruction is the mechanical candidate for post traumatic degeneration, which a restored static laxity does not explain. 6.77 covers post traumatic disease.

That matters clinically because it reframes the aim of reconstruction from restoring laxity to restoring kinematics, and the second is not achieved by the first.

Conventional teaching reports reconstruction outcomes as laxity and return to sport; the gait literature suggests those are not the variables that predict the joint's future.

What Changes It

Graft choice and tunnel position, which affect rotational kinematics more than they affect anteroposterior laxity. 3.7.20 covers it.

Lateral augmentation, which addresses the rotational component specifically. 3.11.8 covers it.

Time from injury to surgery and the meniscal status at surgery. 6.4 covers associated injuries.

Quadriceps recovery, which is incomplete in many patients years later.

Fear of reinjury, which alters loading symmetry measurably and persists after discharge.

How It Is Measured

Three dimensional gait analysis, which shows the persistent abnormality and is unavailable to most of the surgeons it would inform.

Dynamic radiostereometric analysis and biplanar fluoroscopy, which measure true bone motion in a handful of centres.

Instrumented laxity, which is what is actually recorded in follow up and measures a different thing.

Patient reported outcome measures, which are the primary endpoints and correlate weakly with kinematics.

The gap between what is measured in follow up and what the gait literature says matters is the central problem here.

Clinical Relevance

Describe reconstruction as restoring laxity and state that kinematics are often not restored.

Expect quadriceps deficit to persist for years and measure it rather than assume recovery. Section 8 covers the rehabilitation.

Treat persistent altered kinematics as the mechanical candidate for later degeneration when counselling. 6.77 covers it.

Avoid teaching quadriceps avoidance as characteristic.

Weigh the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations, because fear of reinjury changes gait and is part of the clinical picture rather than separate from it.