Constitutional Varus

For Medical Students

The Principle

An average is not a target. Most measurements in the body have a spread, and being away from the middle of it is not the same as being abnormal.

If a body has grown and worked in one shape for decades, that shape is normal for it.

Changing someone to the population average can therefore move them away from their own normal.

At the Knee

A substantial minority of healthy adults have knees that are mildly varus and have always been. 3.13.3 covers varus.

Their soft tissue, their gait and their cartilage have developed around that shape.

This is called constitutional varus, meaning varus that is part of the person rather than the result of disease.

It is commoner in men and it is present from the end of growth rather than appearing later. 2.4.2 covers ossification.

What Changes It

Nothing, because it is the shape the limb grew into.

Later disease can add to it, which is why a varus knee needs its history taken as well as its angle measured. 3.13.8 covers that.

Growth, in a child, which is the only time the shape is still moving.

Surgery, which can move the alignment away from it. 3.13.10 covers the targets.

Clinical Relevance

It means a mildly varus leg is not automatically a problem to correct.

It is the main reason the idea of a single correct alignment for everyone has been questioned. 7.81 covers kinematic alignment.

A patient's old photographs or their account of always having been bow legged is useful information.

4.5.1 covers how alignment is assessed.

MRCS Level

The Principle

Alignment is a distribution rather than a value, and the clinically useful question is where a given patient sits in it and whether they have moved.

A current angle without a baseline cannot distinguish constitutional varus from acquired varus, and the two have different implications.

The baseline is almost never available, which is a real limitation rather than a theoretical one.

At the Knee

Population studies report that roughly a third of men and a smaller proportion of women have a natively varus mechanical alignment at skeletal maturity.

Those knees are asymptomatic, with normal cartilage and normal soft tissue tension at that alignment.

The soft tissue envelope has developed at that alignment, so correcting to neutral changes the tension the ligaments have adapted to. 3.9.1 and 3.10.1 cover the restraints.

Constitutional varus is distinguished from acquired varus by the absence of joint space loss and by history, not by the angle itself. 3.13.8 covers acquired varus.

It is the empirical basis for the kinematic alignment argument in arthroplasty, which restores a patient's own alignment rather than a population neutral. 7.81 covers it.

What Changes It

Nothing in the adult, because it is developmental.

Superimposed degenerative change, which adds an acquired component to a constitutional one.

Sex and ancestry, which affect the proportion in a population.

Skeletal maturity, since the distribution is established at the end of growth.

Surgical correction, which is the only thing that moves it and is the point at issue.

How It Is Measured

Standing long leg radiography, which gives the current angle and not its origin. 5.3.2 covers the technique.

Joint space assessment, which identifies an acquired intra-articular component.

The contralateral limb, which is the most practical proxy for a baseline when it is unaffected.

Patient history and old imaging, which are the only direct evidence of a long standing shape.

There is no measurement that returns a patient's constitutional alignment once disease has been superimposed, which is the central difficulty.

Clinical Relevance

Ask whether the patient has always been bow legged, because the answer changes the interpretation of the angle.

Use the unaffected contralateral limb as a baseline proxy where one is available.

Avoid describing a mildly varus asymptomatic knee as malaligned, since it may be that patient's normal.

Separate the constitutional and the acquired components when discussing a correction target. 3.13.10 covers the targets.

FRCS Level

At the Knee

Constitutional varus is the finding that unsettled a target. If mechanical neutral is an average rather than a norm, then correcting everyone to it overcorrects a third of men.

The mechanical consequence is soft tissue: a knee aligned away from the alignment its ligaments grew at requires release or accepts imbalance. 3.9.1 covers the medial side.

That is the argument behind kinematic and restricted kinematic alignment in arthroplasty, and the debate is about how far from neutral it is safe to leave a knee. 7.81 and 7.83 cover them.

In osteotomy the same argument cuts differently, because the operation exists to move a knee away from its own alignment deliberately. 3.13.10 covers the targets.

Conventional teaching has moved from one target to a range, and the boundaries of the range are not settled. 6.155 covers the arthroplasty controversies.

What Changes It

The patient's own constitutional alignment, which is usually unknown and proxied by the other leg.

The amount of intra-articular loss, which decides how much of the current angle is acquired.

Soft tissue adaptation, which is what makes an overcorrection felt rather than only measured.

The operation intended, since arthroplasty and osteotomy pull the argument in opposite directions.

The reference range used, which is population dependent and therefore partly a question about the cohort rather than the patient.

How It Is Measured

The contralateral limb, which is the best available proxy and is unavailable in bilateral disease.

Published population distributions, which give a prior rather than a patient value.

Joint space and bone loss measurement, which apportions the acquired component.

Intraoperative soft tissue tension assessment, which measures the consequence rather than the alignment, and is subjective.

The absence of a per patient measurement is the reason the debate is about a range rather than being settled by data.

Clinical Relevance

Estimate the constitutional alignment from the contralateral limb and from history before choosing a target. 3.13.10 covers the targets.

Expect soft tissue consequences from correcting a knee far from the alignment it developed at.

State the reference range alongside any claim that a patient is malaligned.

Distinguish the arthroplasty argument from the osteotomy argument, because the same finding supports different conclusions in each.

Take the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations into the target decision, since what counts as an acceptable alignment depends on what the knee is for.