Rotational Profiles

For Medical Students

What It Shows

A rotational profile measures how far the femur and the tibia are twisted along their length. 2.1.2 covers the distal femur.

Computed tomography measures it from axial slices at the hip, the knee and the ankle in one acquisition. 5.2.22 covers the CT acquisition.

It matters because rotation of the limb changes where the patella sits relative to the groove without either bone being abnormal at the knee itself. 3.12.1 covers patellar tracking.

How It Is Acquired

Acquire axial slices at the proximal femur, the distal femur, the proximal tibia and the ankle. 5.2.22 covers the CT acquisition.

State which reference axis was used at each level, because several are in use. 2.1.8 covers the imaging anatomy.

Position the limb consistently, since the measurement is taken from the image rather than from the patient. 5.1.6 covers imaging pitfalls.

Measure the femoral and the tibial component separately rather than reporting a combined value. 2.2.1 covers the proximal tibia.

Record the dose, which is higher for a study spanning the whole limb. 5.2.24 covers the radiation dose.

What Normal Looks Like

Reference landmarks identifiable at every level. 2.1.8 covers the imaging anatomy.

Femoral and tibial values each inside the range for the convention used. 2.1.2 covers the distal femur.

Symmetry between the two limbs where both were scanned. 5.20.1 covers structured reporting.

A patella seated in the groove on the axial slices through the knee. 5.12.4 covers patellar subluxation.

What Abnormal Looks Like

Increased internal femoral torsion, which turns the trochlea relative to the line of pull. 2.1.5 covers the trochlea.

Increased external tibial torsion, which carries the tubercle laterally. 2.2.5 covers the tibial tubercle.

A combination of the two, which is commoner than either alone. 3.12.11 covers the effects of malalignment.

A value reported without the axis it was measured from, which cannot be interpreted at all. 5.20.8 covers common reporting errors.

MRCS Level

How It Is Acquired

In a viva the expected answer names the axis before the value.

The expected points are the four levels, the reference axes, the consistent positioning, the separate components, and the dose.

Say that the measurement describes the bone rather than the function. 3.12.1 covers patellar tracking.

Say that rotation is a contributing factor rather than a diagnosis. 3.12.8 covers patellar instability.

Say that magnetic resonance imaging can measure the same angles without the dose. 5.5.1 covers the standard protocol.

What Normal Looks Like

The reasoning expected is that the extensor mechanism runs between the hip and the tubercle, so a twist anywhere along that path changes the line of pull across the knee. 3.12.6 covers the Q angle.

The published ranges come from particular cohorts measured with particular axes, so a value is normal only against the convention that produced the range. 5.21.4 covers the patellofemoral imaging literature.

Comparison between sides is more informative than an absolute value, because the convention applies the same error to both limbs. 5.20.1 covers structured reporting.

Rotation measured supine describes the skeleton rather than the position the limb adopts in stance. 5.18.5 covers weight-bearing MRI.

How Good It Is

Computed tomography measures torsion reproducibly within a convention, and the conventions are not interchangeable.

Its best contribution is identifying the patient whose problem sits above or below the knee. 3.12.11 covers the effects of malalignment.

It is limited by the competing axes, by supine positioning, and by rotation being one contributor among several. 5.21.4 covers the patellofemoral imaging literature.

No figures appear for accuracy on this page, under decision 028, and no torsion values, normal ranges or surgical thresholds appear either: those are published quantities tied to particular axes and this environment cannot check them.

FRCS Level

How Good It Is

At decision level the question is whether a rotational osteotomy enters the discussion, which is a different operation from anything at the joint. 5.16.5 covers osteotomy planning.

Large femoral torsion with recurrent patellar instability is the picture in which it is raised. 5.12.4 covers patellar subluxation.

The other measurements have to be read from the same study, since they interact. 5.16.4 covers the TT-TG measurement.

A value near a threshold decides nothing without the convention and the clinical picture. 5.20.8 covers common reporting errors.

The examination carries the functional rotation and the scan carries the skeletal rotation. 4.15.6 covers dynamic tracking.

Where It Misleads

The commonest error is comparing a value with a threshold from another convention. 5.21.4 covers the patellofemoral imaging literature.

Limb positioning rotates the measurement without rotating the bone. 5.1.6 covers imaging pitfalls.

A combined profile is reported as a femoral abnormality because the tibia was not measured. 2.2.1 covers the proximal tibia.

Rotation is read as the cause of anterior pain in a knee where it is incidental. 3.12.9 covers patellofemoral pain.

A paediatric profile is compared with adult ranges during remodelling. 5.15.1 covers the normal paediatric anatomy.

Fellowship Level

Where It Misleads

At fellowship level the profile arrives measured elsewhere, with no axis named and a threshold already applied to it. 5.20.8 covers common reporting errors.

Remeasuring from the images is the only way to know what a value means, and that needs the images rather than the report. 5.20.1 covers structured reporting.

A rotational correction changes every other measurement made on that limb, so the pre-operative set is the only comparable one. 5.12.8 covers the post-operative imaging.

Deformity in two planes is reported one plane at a time, which understates it. 5.3.10 covers the deformity analysis.

Automated measurement reproduces one convention consistently, which is reproducibility rather than validity. 5.18.8 covers artificial intelligence in imaging.

The useful report gives each level, the axis used at each, both components separately, the comparison between sides, and the dose. 5.20.1 covers structured reporting.