ACL Injury Mechanisms

For Medical Students

The Principle

Working out how an injury happens means combining what can be seen with what can be measured. Neither on its own is enough.

Video of real injuries shows the positions and the timings. Laboratory work shows what loads the structure and at what point it fails.

Where the two agree, the mechanism is reasonably established. Where they disagree, something is being missed.

At the Knee

Most anterior cruciate injuries are non contact, meaning nothing struck the knee. The patient's own movement produced the load.

Video analysis consistently shows a knee near extension, collapsing into valgus, with the tibia rotating, during a landing or a sudden deceleration. 3.3.13 covers landing.

The timing is very short. Conventional teaching places the injury within the first 50 milliseconds of ground contact, which is too fast for a correction.

Laboratory work agrees that this combination loads the ligament most, which is the point where the two sources meet. 3.7.13 covers cutting.

What Changes It

The activity, with pivoting and landing sports carrying the risk.

Technique, which is modifiable and is what prevention programmes train. 3.1.5 covers prevention.

Fatigue and anticipation, both of which worsen the pattern.

Bony and anatomical factors, which are fixed and set the baseline.

Clinical Relevance

The history usually fits the mechanism, which is why the account of the injury is diagnostic before any examination. Section 4 covers the tests.

A pop, immediate swelling and an inability to continue is the classic account and it matches the mechanics.

Prevention programmes target the modifiable part of the mechanism and reduce injury rates. Section 6 covers management.

Knowing the mechanism also predicts the associated injuries, because the same movement loads the menisci and the collaterals. 3.7.3 covers secondary restraints.

MRCS Level

The Principle

Three evidence streams contribute and each has a weakness. Video analysis shows real injuries without measuring loads. Laboratory loading measures loads without producing real injuries. Prospective screening measures risk factors without establishing mechanism.

The mechanism is accepted because the three converge rather than because any one is decisive.

Anterior tibial shear with the knee near extension is the loading that cadaveric work fails the ligament with, and valgus and internal rotation reduce the shear needed.

At the Knee

Video analysis describes a loaded, nearly extended knee with dynamic valgus and tibial internal or external rotation, in a deceleration, a landing or a change of direction.

Quadriceps contraction at low flexion angles produces anterior shear, which is the internal contributor to the loading. 3.4.4 covers lever arms.

Bone bruise patterns on imaging are consistent with the described mechanism, which is a fourth and independent line of evidence. 5.7 covers imaging.

Associated injuries follow from the mechanism: lateral meniscal tears, ramp lesions, Segond fractures and medial collateral injury. 6.4 covers associated injuries.

What Changes It

Anticipation and fatigue, which change the muscle pattern.

Trunk and hip control, which set the knee's position at loading.

Posterior tibial slope and lateral plateau geometry, which alter the shear for a given load.

Footwear and surface friction, which determine whether the foot releases.

How It Is Measured

Video analysis of injuries captured incidentally is the direct observation and depends on camera angle and frame rate.

Cadaveric loading to failure establishes the loading combinations that rupture the ligament, without muscle.

Prospective cohorts measure candidate risk factors against subsequent injury and report modest predictive value.

Bone bruise mapping is a retrospective reconstruction of the position at injury.

Clinical Relevance

Take the mechanism from the history, since it is usually diagnostic.

Expect the associated injuries the mechanism predicts and look for them.

Read bone bruise patterns as evidence about the position at injury.

Treat screening as measuring risk factors rather than establishing mechanism.

FRCS Level

At the Knee

Treat the mechanism as settled in outline and unsettled in detail. Whether the primary driver is sagittal shear, valgus collapse or rotation remains argued, and the clinical consequences of the argument are small.

Where it matters is prevention and return to sport, because a programme targets a mechanism. Conventional teaching supports multi component neuromuscular programmes, which hedge across the candidate mechanisms. 3.1.5 covers prevention.

The second place it matters is bony risk. Slope and lateral plateau morphology act through the sagittal and rotational components, and they are now partly modifiable. 3.13 covers alignment.

What Changes It

Neuromuscular training, which is the modifiable element.

Bony morphology, now partly addressable by osteotomy.

Sport, position and exposure, which set the number of at risk events.

Previous injury, which is the strongest single risk factor for another.

How It Is Measured

Randomised prevention trials measure injury rate reduction and are the strongest evidence for acting on mechanism.

Registry data measures reinjury and contralateral injury rates after reconstruction.

Imaging measures the bony risk factors, which are the ones that can be acted on surgically.

No measurement predicts an individual injury, which is the limit of the whole literature.

Clinical Relevance

Prescribe multi component neuromuscular programmes, which hedge across mechanisms.

Measure slope in a young pivoting athlete, particularly before revision.

Treat previous injury as the strongest risk factor when counselling.

State that no measurement predicts an individual injury.