ACL Strain During Activity

For Medical Students

The Principle

Strain is how much something has stretched compared with its original length, expressed as a percentage. It is not the same as force.

Two ligaments carrying the same force can be at different strains, because a longer or thicker one stretches less.

Strain is the quantity that matters for whether a structure fails, because failure happens at a strain rather than at a force.

At the Knee

The anterior cruciate's strain changes with flexion angle, with muscle activity and with the load applied.

Conventional teaching puts the ligament at its highest strain near full extension and in early flexion, falling as the knee bends further.

Quadriceps contraction raises the strain, because the patellar tendon pulls the tibia forward when the knee is near extension. 3.4.4 covers lever arms.

Hamstring contraction lowers it, by pulling the tibia backwards. That is the mechanical basis of hamstring co-contraction being protective.

What Changes It

Flexion angle, which is the largest factor.

Quadriceps and hamstring activity, in opposite directions.

Weight bearing, which raises strain in early flexion.

The activity, since different activities combine these differently. 3.7.11 to 3.7.13 cover three.

Clinical Relevance

The early flexion range with quadriceps activity is the loading condition to be careful about after reconstruction, which is why rehabilitation is staged by angle. Section 6 covers management.

Open chain extension exercises load the ligament near extension, which is why their timing is considered. 3.3.4 covers open chain.

Hamstring strength is protective by a mechanism rather than by association.

Injuries happen in positions of high strain, which is what makes the strain data useful.

MRCS Level

The Principle

The in vivo strain literature is small and unusual. A handful of studies implanted a transducer onto the ligament in awake volunteers, which is why the data exists at all and why there is so little of it.

Strain is reported as a percentage change from a reference length, and the choice of reference affects the numbers, so comparisons between studies need care.

The pattern is more reliable than the magnitudes, which is the usual position for a small measurement literature.

At the Knee

Conventional teaching ranks activities by peak strain, with squatting and active extension near full extension producing higher strain than passive motion.

Peak strain occurs in the first 30 degrees of flexion, which matches where the posterolateral bundle is tight and where the patellar tendon's anterior shear is greatest. 3.7.6 covers the bundle.

Weight bearing raises strain in early flexion compared with the same angle unloaded, which is relevant to rehabilitation prescription.

Hamstring co-contraction reduces strain measurably, and the effect is largest in the range where strain is highest.

What Changes It

Flexion angle, as the dominant variable.

Muscle activation pattern, particularly the quadriceps to hamstring balance.

Weight bearing, which raises strain in the vulnerable range.

Applied external load, including an anterior drawer or a valgus moment.

How It Is Measured

Implanted differential variable reluctance transducers in volunteers gave the in vivo data, in a small number of studies.

Cadaveric strain measurement allows more conditions and lacks muscle activity.

Modelling estimates strain across activities that cannot be measured directly, with its assumptions.

Graft strain after reconstruction is measured in cadavers and in animals rather than in patients.

Clinical Relevance

Stage rehabilitation by flexion angle, since strain is angle dependent.

Prescribe weight bearing exercise with the early flexion range in mind.

Train hamstring co-contraction, which reduces strain where it is highest.

Trust the pattern in the strain literature more than the magnitudes.

FRCS Level

At the Knee

Treat the strain data as the mechanical basis of rehabilitation protocols, and treat the protocols as having outrun the data.

The clinically important application is graft protection in the first months, when the graft is weaker than the native ligament and remodelling. 3.7.20 covers the construct.

Conventional teaching has moved towards earlier weight bearing and earlier open chain work than the original strain based protocols allowed, on outcome evidence rather than on new strain data.

What Changes It

Graft type and its initial properties. 3.7.16 covers grafts.

Time since surgery, through the remodelling curve.

Fixation method, which determines what the construct tolerates early.

Patient activity and compliance, which set the actual loading.

How It Is Measured

In vivo native ligament strain studies provide the reference pattern and did not study grafts.

Animal and cadaveric graft strain measurement describes the graft and not the patient.

Clinical trials of rehabilitation protocols measure outcome rather than strain.

No measurement of graft strain in a patient exists, which is the gap every protocol argument crosses.

Clinical Relevance

Protect the graft in the first months by angle and by loading mode.

Follow outcome based protocols rather than strain based ones where they differ.

State that graft strain in a patient is not measured when discussing protocol choices.

Account for graft type and fixation when setting early loading.