ACL Loading During Cutting

For Medical Students

The Principle

Loading a structure in several directions at once is worse than loading it in one, because each direction uses up part of its capacity.

A rope pulled straight takes its full load. The same rope pulled sideways while already taut takes less before it fails.

So a combined load is the dangerous case, and the knee's worst combination is flexion, valgus and rotation together.

At the Knee

Cutting plants a foot and turns the body over it, so the knee is loaded in all three planes during weight acceptance. 3.3.14 covers the kinematics.

The knee is in early flexion at that moment, which is the highest strain range. 3.7.10 covers strain.

The foot cannot rotate, so the rotation passes up to the knee. 3.3.3 covers the closed chain.

Conventional teaching therefore places cutting at the top of the loading hierarchy for this ligament, above landing and well above running.

What Changes It

The cut angle and approach speed, which set how much momentum has to be redirected.

Whether the movement was anticipated, since an unplanned cut produces a worse pattern.

Hip and trunk control, which position the knee before it is loaded.

Surface and footwear friction, since a foot that grips completely transmits all the rotation.

Clinical Relevance

This is the activity most non contact injuries occur in, which is why pivoting sports carry the risk they do. 3.7.14 covers mechanisms.

Returning to cutting is the last milestone after reconstruction, which follows from the loading. Section 6 covers management.

A rotationally deficient knee fails in this activity rather than in walking or running. 3.7.7 covers rotational stability.

Sport specific application belongs to 3.15 and prevention to 3.1.5.

MRCS Level

The Principle

The mechanical argument is that combined loading reduces the load to failure in each direction, which cadaveric work supports: a ligament loaded in rotation fails at a lower anterior force than one loaded anteriorly alone.

Anticipation changes the muscle pattern, and the muscle pattern is the protective element. An unanticipated cut has less protective co-contraction. 3.3.14 covers it.

Both factors push in the same direction, which is why this activity sits where it does in the hierarchy.

At the Knee

Peak loading occurs during weight acceptance on the planted limb, with the knee in early flexion and moving into valgus and internal rotation.

Anterior translation and internal rotation are coupled and restrained together by the cruciate and the anterolateral structures. 3.11 covers them.

Trunk position relative to the planted foot sets the external moments, and a laterally displaced trunk increases the valgus demand. 3.4.6 covers external moments.

Side step and crossover cuts load the knee differently, so the manoeuvre has to be named rather than described generically.

What Changes It

Cut angle and approach velocity.

Anticipation, which changes the kinematics measurably.

Hip abductor and trunk control, which set the loaded position.

Fatigue, which worsens the pattern in the same direction as a lack of anticipation.

How It Is Measured

Laboratory side step cutting with force plates and motion capture is the standard and measures a cued task.

Unanticipated protocols add a cue to approximate the sporting condition, which is closer and not the sport.

Modelling estimates ligament loading from the measurements, with its assumptions.

Cadaveric combined loading tests measure the reduced failure load directly, without muscle.

Clinical Relevance

Name the manoeuvre and the cut angle when recording or reading an assessment.

Include an unanticipated element where the aim is to represent the sport.

Train hip and trunk control as the route to a better loaded position.

Expect combined loading to reduce the failure load in each direction.

FRCS Level

At the Knee

Treat cutting as the activity that defines the reconstruction's job. A construct adequate for walking and running is not necessarily adequate here, and this is where rotational control is tested.

That is the mechanical case for lateral augmentation in pivoting athletes, and it is the activity the registry reinjury data is largely about. 3.11 covers the constructs.

Bony morphology changes the demand. Conventional teaching identifies increased posterior tibial slope as a risk factor for graft failure in pivoting athletes. 3.13 covers alignment.

What Changes It

Whether the anterolateral complex was addressed in the reconstruction.

Slope and lateral plateau geometry, which alter the coupled rotation for the same load.

Lateral meniscus posterior horn status.

Neuromuscular control and anticipation training, which are modifiable.

How It Is Measured

Laboratory cutting analysis after reconstruction measures restored symmetry, in cued tasks and selected cohorts.

Registry reinjury rates in pivoting athletes measure the outcome the mechanism predicts.

Instrumented pivot shift measures the laxity the activity depends on, without a threshold.

No assessment reproduces an unanticipated sporting cut, which is the gap between testing and demand.

Clinical Relevance

Consider lateral augmentation in a pivoting athlete, on this activity's mechanics.

Measure slope before revising a graft that failed without a new injury.

Repair the lateral posterior horn where torn.

State that no assessment reproduces an unanticipated cut when discussing return to sport.