Anatomical Axis

For Medical Students

The Principle

A bone's shaft and the line that load follows through it are two different lines.

They differ because the ends of a bone are not centred on its shaft. The top of the femur sits well to one side of it.

If two lines differ by a known amount, either can be used, as long as everybody knows which one is on the page.

At the Knee

The anatomical axis of the femur is the line down the middle of its shaft. 2.1.2 covers the femur.

The mechanical axis of the femur runs from the centre of the hip to the centre of the knee, and the hip sits medial to the shaft. 3.13.1 covers it.

The two therefore cross at an angle of around 5 to 7 degrees at the knee.

In the tibia the two axes are nearly the same, because the ankle sits almost under the shaft. 2.2.1 covers the tibia.

What Changes It

The length and the angle of the femoral neck, which sets how far the hip sits from the shaft.

Femoral shaft bowing, which is commoner with age.

A fracture healed at an angle, which moves one axis and not always the other.

Nothing about soft tissue, since both lines are drawn on bone.

Clinical Relevance

A knee radiograph shows the shafts, so an angle measured on it is an anatomical one. 5.3.2 covers alignment imaging.

A whole leg radiograph shows the hip and the ankle, so it can give the mechanical one.

Quoting a number from one and treating it as the other produces an error of several degrees, in the direction that matters.

4.5.1 covers how alignment is assessed in clinic.

MRCS Level

The Principle

Alignment can be described in a mechanical frame, an anatomical frame or a joint based frame, and each has its own conventions and normal ranges.

The frames are convertible, but the conversion depends on the femoral neck and on shaft bowing, which vary between patients.

So the conversion is approximate in an individual even though it is reliable across a population.

At the Knee

The anatomical axis of the femur is drawn down the mid diaphysis. The mechanical axis runs from the femoral head centre to the knee centre.

The angle between them is the femoral mechanical anatomical angle, quoted at around 5 to 7 degrees with individual variation either side.

The anatomical tibiofemoral angle, measured between the two shaft axes, therefore reads several degrees of valgus in a mechanically neutral leg.

The distal femoral and proximal tibial joint angles describe where the deformity sits rather than how large it is overall. 3.13.6 covers joint line orientation.

A correction planned in one frame and executed with landmarks from another is the standard route to an unintended result. 3.13.10 covers the planning.

What Changes It

Femoral neck length and neck shaft angle, which set the mechanical anatomical angle.

Femoral bowing, which increases with age and with some metabolic bone disease.

Prior femoral fracture or osteotomy, which may change one frame more than the other.

Patient height and segment proportions, which have a modest effect.

Measurement technique, since the shaft axis depends on where along the shaft it is drawn.

How It Is Measured

Long leg standing radiography for the mechanical frame, which needs hip and ankle on the film. 5.3.2 covers it.

Standard knee radiography for the anatomical frame, which is what most films give.

Joint orientation angles measured at the distal femur and proximal tibia, which localise the deformity. 3.13.6 covers them.

The mechanical anatomical angle measured per patient rather than assumed, which matters where the femur is bowed.

No single film gives all three frames, so the frame must be stated with the number.

Clinical Relevance

State which axis a quoted angle refers to, because the two differ by several degrees in the clinically relevant direction.

Measure the mechanical anatomical angle rather than assuming 5 to 7 degrees in a bowed or previously fractured femur.

Localise a deformity with joint orientation angles before correcting it, since overall alignment does not say where the problem is. 3.13.6 covers them.

Avoid planning a correction from a knee radiograph, since the mechanical landmarks are not on it.

FRCS Level

At the Knee

The practical importance of the anatomical axis is historical and surgical rather than mechanical. Load follows the mechanical axis; instruments reference the shaft.

Intramedullary guides, whether for osteotomy or arthroplasty, align to the canal, so they work in the anatomical frame and are corrected by a fixed offset.

That fixed offset is the assumption. It is reliable in a population and wrong in the bowed femur, where the error is a few degrees in the direction of residual deformity.

Extramedullary and navigated techniques reference the mechanical frame directly, which removes the assumption and adds its own registration error. 3.16.3 covers navigation.

Conventional teaching has moved towards measuring each patient's own mechanical anatomical angle rather than assuming a standard figure, particularly in revision and in deformity.

What Changes It

Femoral bowing, which is the main reason the standard offset fails.

Neck shaft geometry, which sets the offset in the first place.

Prior hardware, which may make a canal based reference unavailable.

The level along the shaft at which the axis is drawn, which is a measurement convention rather than an anatomical fact.

Rotational position at imaging, which distorts both frames and not equally. 5.3.2 covers the technique.

How It Is Measured

Long leg radiography with a stated rotation control, which gives both frames on one film.

Per patient measurement of the mechanical anatomical angle, which is the step most often skipped.

Three dimensional reconstruction, which measures both frames without projection error and without load. 3.16.5 covers it.

Intraoperative navigation registration, which measures the mechanical frame directly and introduces registration error of its own. 3.16.3 covers it.

Published normal ranges for the offset come from populations whose femoral morphology may not match the patient in front of you.

Clinical Relevance

Measure the mechanical anatomical angle in any femur that is bowed, short or previously fractured, rather than applying the standard offset.

Record which frame a planned correction is expressed in, and which frame the instruments will reference. 3.13.10 covers the planning.

Expect a canal referenced technique to inherit the offset assumption, and account for it.

Avoid converting between frames in an individual using a population figure, and say when that is what has been done.

State the convention and the frame alongside any alignment number in a letter or an operation note, since the reader cannot infer either.