Mechanical Axis

For Medical Students

The Principle

Weight passing down a limb follows a line, and anything the line passes beside is loaded less than anything it passes through.

The further the line sits from a joint's centre, the longer the lever arm, and the more that side of the joint is loaded.

So a single line drawn down the leg says something useful about how a joint shares its load.

At the Knee

The mechanical axis of the leg runs from the centre of the hip to the centre of the ankle. 2.1.2 covers the femur and 2.2.1 the tibia.

In a neutral leg that line passes close to the centre of the knee, so the two compartments share the load roughly evenly.

If the line passes inside the knee centre, the inner compartment carries more. That is varus. 3.13.3 covers it.

If it passes outside, the outer compartment carries more. That is valgus. 3.13.4 covers it.

What Changes It

The shape of the femur and the tibia, which is what the leg grew into.

Wear in one compartment, which lets that side collapse and moves the line further over. 3.13.8 covers that.

A fracture that healed at an angle.

Surgery intended to move the line. 3.13.9 covers osteotomy.

Clinical Relevance

It is the reason wear in the knee is usually worse on one side rather than spread evenly. 6.73 covers medial compartment disease.

It explains why a bow legged knee wears on the inside and a knock kneed knee on the outside.

It needs a radiograph of the whole leg to measure, because the hip and the ankle are both outside a knee film. 5.3.2 covers the imaging.

4.5.1 covers how alignment is assessed clinically.

MRCS Level

The Principle

The mechanical axis is a reference line, not a force. What it predicts is the moment arm of the ground reaction force about the knee centre. 3.4.6 covers external moments.

That distinction matters because a line can be measured and a moment cannot, at least not in a radiology department.

Everything in this chapter follows from the gap between the line we measure and the moment we care about.

At the Knee

The mechanical axis of the femur runs from the centre of the femoral head to the centre of the knee. The mechanical axis of the tibia runs from the knee centre to the centre of the ankle.

The hip knee ankle angle is the angle between them, and it is the standard expression of overall alignment.

Neutral is conventionally taken as a hip knee ankle angle of 180 degrees, with varus below and valgus above by the usual convention.

Where the axis crosses the tibial plateau is expressed as the mechanical axis deviation, in millimetres from the knee centre.

Both expressions describe the same geometry, and the millimetre version is the one used when planning a correction. 3.13.10 covers the targets.

What Changes It

Femoral and tibial bone morphology, which sets the baseline.

Joint space loss in one compartment, which adds an intra-articular component to a measurement taken as if it were bony. 3.13.8 covers it.

Soft tissue laxity, which lets the knee open under load and changes the standing angle. 3.9.1 and 3.10.1 cover the restraints.

Limb rotation and flexion at the moment of the radiograph, both of which alter the projected angle.

Previous osteotomy or arthroplasty. 3.13.9 covers osteotomy.

How It Is Measured

A long leg standing radiograph including hip, knee and ankle, which is the standard. 5.3.2 covers the technique.

Rotation of the limb during acquisition changes the measured angle, which is the commonest source of error.

Computed tomography and three dimensional reconstruction measure the same geometry without the projection error and without weight bearing. 3.16.5 covers the methods.

Clinical estimation from intermalleolar or intercondylar distance is crude and position dependent. 4.5.1 covers it.

No routine measurement captures alignment while the patient is walking, which is the state in which it matters. 3.13.7 covers that.

Clinical Relevance

Measure alignment on a standing long leg film rather than on a knee radiograph, since the landmarks are off the smaller film.

Control limb rotation at acquisition, because projection error can exceed the deformity being measured. 5.3.2 covers the technique.

Separate the bony deformity from the intra-articular component before planning any correction. 3.13.10 covers the principles.

Treat the mechanical axis as a predictor of load share rather than as a measurement of load.

FRCS Level

At the Knee

The mechanical axis is a static surrogate for a dynamic quantity, and the correlation between the two is moderate rather than strong. 3.13.7 covers the dynamic measure.

A standing radiograph records the limb under body weight with both feet down, which is a loading condition that occurs for a small fraction of the gait cycle.

Single limb stance during walking produces a larger and more variable load than the radiograph's condition, and the difference is not predictable from the film.

The measurement also mixes two things: the bony deformity and the intra-articular collapse. Only the first is correctable by an osteotomy outside the joint. 3.13.9 covers it.

Conventional teaching has kept the hip knee ankle angle as the planning variable, because it is the one that can be measured reproducibly and corrected precisely.

What Changes It

The bony contribution, which is what an osteotomy addresses.

The intra-articular contribution from joint space loss and from laxity, which it does not.

Projection and rotation at acquisition, which is a measurement problem rather than a patient one. 5.3.2 covers it.

Dynamic factors during loading, which the film cannot see. 3.13.7 covers them.

Constitutional morphology, which raises the question of what neutral means for a given patient. 3.13.5 covers it.

How It Is Measured

Standing long leg radiography, which remains the reference and carries a projection error of several degrees with rotation.

Three dimensional computed tomography, which removes the projection error and removes weight bearing with it. 3.16.5 covers it.

Weight bearing computed tomography, which keeps both and is not widely available.

Instrumented gait analysis, which measures the adduction moment rather than the angle and is confined to laboratories. 3.16.7 covers motion capture.

The quantity that predicts progression best is the adduction moment, and the quantity that surgery plans to is the angle, which is the chapter's recurring problem.

Clinical Relevance

Plan from a standardised standing long leg film, and state the rotation control used.

Apportion the deformity between bone and joint before correcting, because an extra-articular correction cannot address an intra-articular loss. 3.13.10 covers the principles.

Avoid inferring dynamic load from a static angle in an individual, since the correlation is moderate. 3.13.7 covers the measure.

State the convention used when quoting a hip knee ankle angle, since sign conventions differ between publications.

Take the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations into any correction decision, because the angle describes a limb and not a person.