Anatomical Axis

For Medical Students

What It Shows

An anatomical axis is the line down the middle of a bone's shaft, drawn separately for femur and tibia. 5.3.7 covers the mechanical axis.

It differs from the mechanical axis because the femoral shaft is angled relative to the line from hip centre to knee. 3.13.2 covers the anatomical axis.

It matters because it is measurable on a film that does not include the hip, so it is the axis available when a long film is not. 5.3.6 covers long leg alignment films.

How It Is Acquired

Draw the mid shaft line of the femur and of the tibia, each from two points along the canal. 3.13.2 covers the anatomical axis.

Measure the angle between them at the knee, which is the tibiofemoral angle. 5.3.7 covers the mechanical axis.

Note that the femoral anatomical axis and the femoral mechanical axis differ by a consistent angle in a normal femur, which is how one is converted to the other. 3.13.1 covers the mechanical axis.

Use the tibial axis for slope and for component alignment, since the tibial anatomical and mechanical axes nearly coincide. 5.3.2 covers the lateral view.

Record which axis was used, because the two give different numbers for the same knee. 5.20.8 covers common reporting errors.

What Normal Looks Like

A femoral shaft line and a tibial shaft line meeting at a slight angle at the knee.

Shaft lines drawn through identifiable canal centres rather than estimated from the cortex.

A tibial axis nearly parallel to the limb's mechanical axis. 3.13.2 covers the anatomical axis.

Agreement with the mechanical axis measurement when both are available. 5.3.7 covers the mechanical axis.

What Abnormal Looks Like

An increased or reversed tibiofemoral angle. 3.13.3 covers varus alignment.

A shaft that is itself bowed, so no single straight line represents it. 5.3.10 covers the deformity analysis.

Previous surgery or a healed fracture distorting the shaft. 5.16.5 covers osteotomy planning.

A disagreement between the anatomical and mechanical measurements, which indicates femoral geometry outside the usual relationship. 5.3.7 covers the mechanical axis.

MRCS Level

How It Is Acquired

In a viva this is the measurement to distinguish from the mechanical axis, and the distinction is the answer.

The expected points are the two shaft lines, the tibiofemoral angle, the femoral offset between the axes, the tibial coincidence, and recording which was used.

Say that the two axes give different numbers for the same knee. 5.3.7 covers the mechanical axis.

Say that the offset relationship assumes normal femoral geometry. 3.13.2 covers the anatomical axis.

Say that a bowed shaft has no single anatomical axis. 5.3.10 covers the deformity analysis.

What Abnormal Looks Like

The reasoning expected is that the femoral head sits medial to the shaft, so a line down the shaft is not the line from the head, and the difference is the neck and offset geometry. 3.13.2 covers the anatomical axis.

That makes the conversion between the axes dependent on a patient having usual proximal femoral geometry, which a previous fracture or a dysplastic hip breaks. 5.3.7 covers the mechanical axis.

A bowed femur or tibia has curvature rather than a single angle, so representing it with one straight line hides where the deformity is. 5.3.10 covers the deformity analysis.

Component alignment in arthroplasty is referenced to these axes, which is why the distinction is a surgical one rather than a semantic one. 5.16.6 covers arthroplasty planning.

How Good It Is

The anatomical axis is available on more films than the mechanical axis and answers a narrower question.

Its best contribution is being measurable on a knee film, which makes it the practical axis when a long film does not exist. 5.3.1 covers the anteroposterior view.

It is limited by depending on normal proximal femoral geometry for any conversion, and by being meaningless in a bowed bone. 5.3.10 covers the deformity analysis.

No figures appear for accuracy on this page, under decision 028, and no axis angles or offsets appear either: those are published quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level the anatomical axis is the reference for implant position and the mechanical axis is the reference for limb alignment.

Component position in arthroplasty is set relative to bone axes, so these are the lines the implant is aligned to. 5.16.6 covers arthroplasty planning.

An osteotomy's correction is specified against the mechanical axis and executed relative to the bone, so both are needed. 5.16.5 covers osteotomy planning.

A healed fracture or previous osteotomy breaks the usual relationship between the axes, which makes the long film mandatory rather than preferable. 5.3.6 covers long leg alignment films.

Reporting a tibiofemoral angle as though it were a mechanical axis deviation is a specific and common confusion. 5.3.7 covers the mechanical axis.

Where It Misleads

The commonest error is quoting an anatomical measurement against a mechanical threshold. 5.3.7 covers the mechanical axis.

A bowed shaft is represented by one straight line. 5.3.10 covers the deformity analysis.

A patient with previous proximal femoral surgery is converted between axes using the usual relationship. 3.13.2 covers the anatomical axis.

The axis is drawn from the cortical outline rather than the canal. 5.2.2 covers the image formation.

Which axis was used is not stated, so the number cannot be interpreted. 5.20.8 covers common reporting errors.

Fellowship Level

Where It Misleads

At fellowship level the two axis systems are a long standing source of incompatible numbers in the same patient's notes.

Several angle conventions exist on each axis system and their normal ranges differ, so a value is uninterpretable without its convention. 5.21.5 covers the osteoarthritis imaging literature.

Alignment philosophies in arthroplasty reference different axes, so the same radiograph supports different implant positions depending on the philosophy rather than on the measurement. 3.13.10 covers the alignment correction principles.

Robotic and navigated systems compute their own axes from a registered bone model, which means the intra-operative number and the radiographic number are not the same quantity. 5.16.7 covers robotic surgery planning.

Three dimensional planning makes the distinction largely obsolete by measuring the geometry directly, which is where this is heading. 5.16.8 covers three dimensional reconstructions.

Until then the discipline is to name the axis and the convention with every angle, which is this chapter's repeated conclusion in another form. 5.20.1 covers structured reporting.