Kinematics During Cutting

For Medical Students

The Principle

Cutting is a change of direction while moving. The foot plants, the body's momentum carries on in the old direction, and the limb has to redirect it.

A planted foot cannot rotate with the body, so the rotation has to be taken by the joints above it. The knee takes a share.

This is the task that combines all three planes at once. Flexion, valgus and rotation occur together rather than in sequence.

At the Knee

The knee is loaded in relatively early flexion during the plant, which is where it depends least on bony congruence. 3.3.8 covers the zones.

Valgus and rotation are added to that loaded flexion, and the combination is larger here than in any other everyday task.

The motion is coupled rather than independent, so the three planes cannot be trained or assessed one at a time. 3.2.9 covers coupling.

Closed chain conditions apply, so the hip and the trunk position set much of what the knee has to do. 3.3.3 covers the chain.

What Changes It

The angle of the turn and the approach speed, which together set the redirection required.

Whether the movement was anticipated. An unplanned change of direction produces a different pattern from a rehearsed one.

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

Footwear and surface friction, since a foot that grips completely transmits all the rotation to the limb.

Clinical Relevance

This is the mechanical setting most non contact cruciate injuries are described in, which is why the task appears in every discussion of prevention.

A patient who gives way does so in tasks like this rather than in walking, because this is where rotation is demanded.

Return to cutting sports is a later milestone than return to running, since the task demands rotation under load.

Sport specific application belongs to 3.15 and prevention to 3.1.5.

MRCS Level

The Principle

Describe cutting by its three plane demand and by its unpredictability. Both matter, and the second is what laboratory tasks most often leave out.

Anticipated and unanticipated cutting produce measurably different kinematics, with the unanticipated pattern showing greater valgus and rotation.

The pattern is therefore partly a motor control phenomenon rather than purely a mechanical one, which is why training changes it.

At the Knee

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

Anterior tibial translation and internal rotation are coupled, and the coupled pair is what the anterior cruciate and the anterolateral structures restrain together. 3.11 covers them.

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

The side step and the crossover cut load the knee differently, so the manoeuvre has to be named rather than described generically.

What Changes It

Approach velocity and cut angle, which scale the demand.

Anticipation, which changes the kinematics measurably.

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

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

How It Is Measured

Laboratory side step cutting with optical capture and force plates is the standard, and it measures a cued task in a fixed geometry.

Unanticipated cutting protocols add a light or auditory cue to approximate the sporting condition, which is closer and still not the sport.

Field based video and inertial sensors measure the sport and measure it less precisely.

Soft tissue artefact is at its worst here, because the transverse plane quantities are small, fast and the ones of interest.

Clinical Relevance

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

Include an unanticipated element where the aim is to represent the sporting demand.

Train hip and trunk control when the knee's loaded position is the problem.

Read transverse plane cutting data with the measurement error in mind.

FRCS Level

At the Knee

Treat cutting as the task a rotationally deficient knee fails, which is why rotatory laxity and not translation decides the functional result in a pivoting athlete.

The mechanical argument for lateral augmentation rests on this task: the anterolateral structures restrain the coupled rotation that cutting demands, and an isolated intra-articular graft restrains it less well. 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, which is a loading argument about this task. 3.13 covers alignment.

What Changes It

Graft construct and whether the anterolateral complex was addressed, which decides what restrains the coupled rotation.

Posterior tibial slope and lateral plateau geometry, which alter the coupled rotation for the same applied load.

Lateral meniscus posterior horn status, through its role as a restraint to the coupled translation.

Neuromuscular control and anticipation, which are modifiable and are what training targets.

How It Is Measured

Laboratory cutting kinematics after reconstruction measures whether symmetry was restored, in cued tasks and selected cohorts.

Pivot shift grading and instrumented pivot shift measure the rotatory laxity the task depends on, without an agreed threshold.

Return to sport and reinjury rates in pivoting athletes are the outcome the kinematic measures are proxies for, and they are what construct comparisons are judged on.

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

Clinical Relevance

Assess rotatory laxity rather than translation alone when counselling a pivoting athlete.

Consider lateral augmentation in high grade rotatory laxity and in revision, on the mechanics of this task.

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

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