Varus Alignment

For Medical Students

The Principle

If a load passes to one side of a joint, that side is squeezed harder and the other side is relieved.

The further to one side it passes, the bigger the difference, because the lever arm is longer. 3.4.4 covers lever arms.

A small shift in the line produces a large shift in the share, which is why a few degrees matters.

At the Knee

Varus means the knee points outwards and the leg looks bow legged. The mechanical axis passes inside the knee centre. 3.13.1 covers the axis.

The medial compartment then carries a larger share of the load, and the lateral compartment a smaller one. 3.5.3 covers load distribution.

The ligaments on the outside of the knee are stretched slightly and the ones on the inside slackened, which adds to the effect. 3.10.1 covers varus stability.

Varus is common. A substantial proportion of healthy adults are mildly varus without any problem. 3.13.5 covers that.

What Changes It

The shape the femur and tibia grew into, which is the usual reason.

Loss of cartilage and bone on the medial side, which lets that side collapse and increases the varus. 3.13.8 covers it.

A fracture healed in varus.

Osteotomy, which moves the line deliberately. 3.13.9 covers it.

Clinical Relevance

It explains why medial compartment wear is the commonest pattern in the knee. 6.73 covers it.

It is self reinforcing: the wear increases the varus, and the varus increases the wear.

That loop is the mechanical argument for correcting alignment before the compartment is lost. 3.13.10 covers the principles.

4.5.2 covers how it is assessed clinically.

MRCS Level

The Principle

Varus alignment increases the external adduction moment about the knee during stance, which is the quantity that loads the medial compartment. 3.4.6 covers external moments.

The static angle is a proxy for that moment. The moment is what the cartilage experiences.

The loop between deformity and wear is therefore a loop between a moment and a structure that resists it, and it tightens as the structure fails.

At the Knee

Medial compartment load in a neutral knee is already greater than lateral, roughly sixty to seventy per cent of the total, before any deformity. 3.5.3 covers it.

Varus raises that share, and modelling suggests the rise is steep for the first few degrees and then flattens.

Lateral soft tissue tension rises, and lateral joint opening under load can add an apparent deformity to the bony one. 3.10.1 covers the restraints.

Medial bone loss and meniscal extrusion both add an intra-articular component that no extra-articular correction addresses. 3.6 covers the meniscus.

The thrust seen in some varus knees during walking is a dynamic phenomenon and not visible on a film. 3.13.7 covers it.

What Changes It

Bony morphology, which is the baseline and is often constitutional. 3.13.5 covers it.

Medial joint space loss and meniscal extrusion, which are intra-articular.

Lateral soft tissue competence, which decides how much the knee opens under load.

Body mass, which scales every moment. 3.5.7 covers obesity.

Gait pattern and walking speed, which change the moment without changing the angle. 3.14.1 covers normal gait.

How It Is Measured

Hip knee ankle angle on a standing long leg film, which is the standard. 5.3.2 covers the technique.

Mechanical axis deviation in millimetres, which is the planning expression. 3.13.10 covers the targets.

Joint orientation angles, which say whether the deformity is femoral, tibial or intra-articular. 3.13.6 covers them.

The knee adduction moment from gait analysis, which is the quantity that predicts progression and is not clinically available. 3.16.7 covers motion capture.

Stress radiographs, which separate the bony deformity from the correctable soft tissue component.

Clinical Relevance

Apportion a varus deformity between femur, tibia and joint before planning, because the correction site follows. 3.13.6 covers the localisation.

Expect the first few degrees of varus to matter more than the next few, since the relationship is not linear.

Assess lateral soft tissue competence, since lateral opening inflates the measured deformity.

Avoid treating mild varus as pathological on its own, because it is common in healthy knees. 3.13.5 covers it.

FRCS Level

At the Knee

The defensible account is that varus increases medial load, that medial load drives progression, and that the measured static angle is a weak predictor in an individual.

The dynamic adduction moment is the stronger predictor, and it correlates only moderately with the static angle. 3.13.7 covers it.

That is why two knees with the same hip knee ankle angle progress at different rates, and why correcting the angle does not guarantee changing the moment.

Varus thrust during gait is an independent predictor of progression and is a clinical observation rather than a radiographic one. 4.5.2 covers the assessment.

Conventional teaching corrects the angle because it is the variable that can be corrected, and the honest statement is that it is a surrogate. 6.154 covers the controversies.

What Changes It

The proportion of the deformity that is bony against intra-articular, which decides what a correction can achieve. 3.13.10 covers it.

Lateral laxity, which both inflates the measurement and limits the correction.

Dynamic factors including thrust, gait pattern and trunk sway, which change the moment independently of the angle. 3.13.7 covers them.

Body mass, which is the other modifiable term in the moment. 3.5.7 covers obesity.

Constitutional morphology, which raises the question of what the correction target should be for this patient. 3.13.5 covers it.

How It Is Measured

Standing long leg radiography with stated rotation control, which is reproducible and static.

Stress radiography, which apportions the correctable soft tissue component.

Gait analysis for the adduction moment and for thrust, which is the mechanically correct measurement and is a research tool. 3.16.7 covers it.

Clinical observation of thrust during walking, which is free, available and under used. 4.5.2 covers it.

Weight bearing computed tomography, which keeps load and removes projection error, and is not widely available.

Clinical Relevance

Watch the patient walk and look for thrust, because it predicts independently of the film and costs nothing. 4.5.2 covers it.

Apportion the deformity before correcting, and state the apportionment in the plan. 3.13.6 covers the localisation.

Treat the static angle as a surrogate and say so when explaining the rationale for correction.

Address body mass alongside geometry, since both terms sit in the same moment. 3.5.7 covers it.

Weigh the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations, because an osteotomy asks a great deal of a patient for a change in a surrogate.