Varus Motion

For Medical Students

The Principle

Varus motion is angulation in the coronal plane that opens the lateral side of the joint and closes the medial side.

It is a small motion in a normal knee, and most of what is called varus in a clinic is an alignment rather than a motion. The two are different things and the distinction is worth holding.

Varus alignment is where the limb sits. Varus motion is how far the joint opens when a force is applied. A limb can have one without the other.

At the Knee

In full extension the joint resists varus through the lateral collateral ligament, the capsule, the cruciates and the congruence of the surfaces together.

At about 30 degrees of flexion the capsule and cruciates contribute less, so the test is done at that angle to isolate the collateral ligament. 2.10.1 covers it.

Conventional teaching puts normal lateral opening at a few millimetres, with the difference between sides being the measurement that matters.

Varus thrust is varus motion seen in gait: the knee moves into varus during stance and returns as the limb unloads.

What Changes It

Flexion angle, which decides how much the secondary restraints contribute.

Alignment. A varus limb puts the load line medial to the knee centre, which produces a varus moment at every step and loads the lateral structures.

Injury to the lateral collateral ligament increases it at 30 degrees; injury to the corner and cruciates increases it in extension as well.

Medial compartment cartilage loss increases varus alignment, which then increases the varus moment, which accelerates the loss.

Clinical Relevance

Varus laxity at 30 degrees alone suggests the lateral collateral ligament. Laxity in full extension as well suggests the corner and a cruciate. 4.13 covers the examination.

Varus thrust seen on walking is a stronger finding than a table test, because it is the knee under its own load rather than the examiner's.

Alignment and laxity are recorded separately, since they have different causes and different treatments.

MRCS Level

The Principle

Separate three quantities that share the word varus: mechanical alignment, measured on a long leg radiograph; coronal laxity, measured by stress; and the dynamic varus thrust observed in gait.

They interact. A varus limb loads the lateral restraints, which stretch, which increases laxity, which increases thrust, which increases the medial load.

That loop is the mechanical account of why uncorrected varus with lateral laxity progresses.

At the Knee

The lateral collateral ligament is the primary restraint to varus, carrying the large majority at 30 degrees of flexion. 3.10.3 carries the apportionment.

The posterolateral corner and the cruciates are secondary, and their contribution rises towards extension.

Conventional teaching puts the increase in lateral opening after isolated collateral sectioning at a few millimetres, and considerably more when the corner is divided too.

The medial compartment is the fulcrum in a varus limb, so varus motion and medial compartment load are geometrically linked.

What Changes It

Alignment is the dominant variable and the only one with an effect at every step.

Flexion angle changes which structures are being tested.

Cartilage and meniscal loss on the medial side increase varus alignment directly, by reducing the height of the medial compartment.

Muscle activity resists thrust dynamically, which is why quadriceps strength changes the gait finding without changing the laxity.

How It Is Measured

Varus stress testing is graded by the opening felt and by the endpoint, in millimetres by convention.

Stress radiography quantifies it, and is the method that gives a number rather than a grade.

Long leg standing radiographs measure alignment, which is a different quantity measured in degrees. 5.3.2 covers alignment imaging.

Gait observation detects thrust, and no instrument is needed to see it.

Clinical Relevance

Watch the patient walk before testing on a couch, since thrust is a load bearing finding that a table test does not reproduce.

Measure alignment on a long leg film in any knee with coronal laxity, because the two together decide the treatment.

Test at 0 and 30 degrees and read the pair, since the pattern separates the collateral from the corner.

Expect laxity and alignment to drive each other, which is the argument for correcting alignment alongside any lateral reconstruction.

FRCS Level

At the Knee

Treat varus as a system rather than a sign. Alignment, laxity, thrust and medial compartment state are four measurements of one problem and each is treated differently.

A lateral soft tissue reconstruction in an uncorrected varus limb is loaded every step by the limb rather than by the injury, which 2.10.4 states and which is the commonest reason such reconstructions stretch out.

The fulcrum argument means an osteotomy changes the lever arm for every subsequent intervention.

What Changes It

Osteotomy changes alignment and therefore every other term. It is the intervention with the largest mechanical effect per unit of surgery at this joint.

Meniscal and cartilage restoration change compartment height and therefore alignment, by smaller amounts.

Soft tissue reconstruction changes laxity and leaves alignment untouched, which is why sequence matters.

How It Is Measured

Stress radiography gives the number, long leg films give the alignment, and gait analysis or simple observation gives the thrust. Three methods for one problem.

Published laxity figures from sectioning studies are measured without alignment, so they describe a knee that does not exist in the patients who have the problem.

Weight bearing measurement is the one that matters and the one least often done.

Clinical Relevance

Correct alignment before or with a lateral reconstruction in a varus limb, and say so in the plan rather than treating it as optional.

Measure on weight bearing films, since a supine measurement describes an unloaded knee.

Record thrust explicitly, because it predicts the mechanical environment a construct will live in.

Avoid reconstructing laxity in a limb whose alignment caused it.