Moments About the Knee

For Medical Students

The Principle

A moment is the turning effect of a force about an axis, and it equals the force multiplied by its lever arm. 3.4.4 covers lever arms.

Several forces can act at once, each with its own arm. The net moment is the sum, with direction taken into account.

A net moment of zero does not mean nothing is happening. Two strong opposing pulls sum to zero and compress the joint hard.

At the Knee

Moments are named by plane. In the sagittal plane they flex or extend, in the coronal plane they angulate into varus or valgus, and in the transverse plane they rotate.

The sagittal moment is the largest and the one most activities are described by. The coronal moment is smaller and matters most for compartment load.

During walking the knee's sagittal moment changes sign several times in one cycle, which is why muscle activity switches between the extensors and the flexors. 3.3.9 covers walking.

The joint reaction force follows from the moment. A larger moment needs a larger muscle force, and a larger muscle force compresses the joint more. 3.4.3 covers it.

What Changes It

The size of the external force, usually the ground reaction force.

Where the force line passes relative to the knee, which can change without the force changing.

Flexion angle, through the moment arms.

Muscle activity, which produces the internal moment that balances it. 3.4.7 covers internal moments.

Clinical Relevance

Moments explain why the same body weight hurts a knee in one activity and not another.

Reducing a moment can be done by reducing the force or by moving its line, and the second is often easier.

A net moment of zero with high muscle activity is a loaded knee, which is why guarding is not protective.

Describing an intervention by the moment it changes makes two treatments comparable.

MRCS Level

The Principle

Keep the external and the internal moment distinct. The external moment is produced by gravity and the ground; the internal moment is produced by muscle, ligament and capsule to balance it.

The convention in the gait literature is to name the external moment and describe the muscle demand from it, which is why a knee adduction moment is an external quantity.

The net internal moment computed by inverse dynamics is a single number standing in for many muscles, so it cannot distinguish a strong agonist from two muscles pulling against each other.

At the Knee

The external knee flexion moment in early stance is resisted by the quadriceps and its size sets the eccentric demand of the loading response.

The external knee adduction moment is the coronal quantity, and conventional teaching associates a larger one with medial compartment load and with progression of medial osteoarthritis.

Transverse plane moments are small and are the hardest to measure reliably, so they are reported with caution. 3.3.14 covers cutting, where they are largest.

Moment and angle together define the task. Plotting one against the other through a cycle is the standard way the literature describes an activity.

What Changes It

Alignment, which sets the coronal moment for a given force.

Speed, which scales the sagittal moments.

Trunk and foot position, which move the force line in both planes.

A brace or a wedge, which applies or redirects an external moment.

How It Is Measured

Inverse dynamics from force plate and motion capture data is the method behind every published moment, and it reports net quantities.

Normalising to body mass and height is conventional, which makes the figures comparable and hides absolute load.

Electromyography is added when the question is which muscle, and it reports timing rather than force.

Co-contraction and the muscle redundancy problem are the standing limitations of the whole approach.

Clinical Relevance

Read a quoted moment as external unless the paper says otherwise, since that is the convention.

Treat a net internal moment as unable to separate co-contraction from a single strong agonist.

Expect normalised figures to hide absolute load, which matters in a heavy patient.

Use the moment against angle curve rather than a peak value when comparing activities.

FRCS Level

At the Knee

Treat the knee adduction moment as the kinetic variable with the clearest clinical application, since it is associated with progression and is modifiable by three different routes: osteotomy, gait modification and bracing.

The sagittal moment is what an extensor mechanism has to meet, so it sets the rehabilitation demand and explains why a quadriceps deficit presents on stairs rather than on the flat.

A moment is the quantity in which two unlike treatments can be compared. An osteotomy and a lateral wedge both reduce the adduction moment, by different amounts and with different costs.

What Changes It

Correction angle in osteotomy, which is chosen to produce a target moment change rather than an appearance.

Gait modification, trunk lean and toe out among them, with small effect sizes and a sound mechanism.

Body mass, which scales every moment and is the largest single modifiable term in many patients.

Muscle capacity, which determines whether the required internal moment is produced efficiently or by co-contraction.

How It Is Measured

Gait analysis gives the adduction moment and is a research measurement rather than a clinical one.

Alignment on a long leg film is the clinical proxy and correlates imperfectly with the dynamic moment.

Prospective cohort studies link the moment to progression, which is association rather than a demonstrated causal threshold.

No agreed moment threshold defines who should be offered an osteotomy, so the decision remains clinical.

Clinical Relevance

Compare unlike treatments by the moment change each produces.

Plan osteotomy to a target that changes the moment rather than to an angle alone.

Address body mass alongside any mechanical correction, since it scales every moment.

State that no moment threshold defines the indication, and decide clinically.