External Rotation

For Medical Students

The Principle

External rotation turns the tibia about its long axis so the foot points outwards relative to the thigh.

It is the larger of the two axial rotations in the normal knee, and like internal rotation it is available only once the knee has flexed.

It is also the one motion at the knee with a straightforward clinical test that gives a number, because the foot can be used as a pointer.

At the Knee

In full extension there is almost none, for the same reason as internal rotation: the joint is congruent and the collaterals are tight.

Conventional teaching gives external rotation of about 15 to 20 degrees at 90 degrees of flexion, somewhat more than internal rotation at the same angle.

The structures that resist it are the posterolateral corner, principally the popliteofibular ligament and the popliteus tendon, with the superficial medial collateral ligament and the posterior cruciate contributing. 3.10 carries the apportionment.

Popliteus resists it as a muscle as well as a ligament, which is why the lateral side has a dynamic component. 2.10.2 covers it.

What Changes It

Flexion angle again, with the available rotation rising from near zero in extension.

Posterolateral injury increases it, and the increase is larger at 30 degrees than at 90 when the posterior cruciate is intact.

A combined posterolateral and posterior cruciate injury increases it at both angles, which is the basis of the test described below.

Clinical Relevance

The dial test measures external rotation at 30 and at 90 degrees and compares sides. Increased at 30 alone suggests the posterolateral corner; increased at both suggests the corner and the posterior cruciate. 4.13 covers the examination and 3.10.7 the biomechanics.

That pair of findings is the clearest example on the site of mechanics producing a diagnostic test rather than explaining one.

A knee that gives way going downstairs or turning outwards is describing this motion.

MRCS Level

The Principle

The dial test works because two structures restrain this motion at different flexion angles. The corner carries more near extension and the posterior cruciate more in flexion, so the pattern of increase separates them.

A side to side difference of more than about 10 degrees is the conventional threshold for a positive test.

The test measures thigh foot angle, so ankle and foot position are part of what is being measured and have to be controlled.

At the Knee

The popliteofibular ligament carries the largest single share of external rotation restraint at 30 degrees, ahead of the popliteus tendon and the lateral collateral ligament. 3.10.5 carries it.

The posterior cruciate's contribution rises with flexion, which is why an isolated corner injury shows at 30 and not at 90.

The medial side contributes through the proximal division of the superficial collateral ligament, which is why a large medial injury can affect rotation. 2.9.1 covers it.

The anatomical axis of external rotation also sits nearer the medial side, so the lateral structures move further for a given rotation and are loaded more.

What Changes It

Flexion angle, as the basis of the test itself.

Injury pattern, with the two angle comparison designed to exploit the difference.

Limb position and examiner technique, which is why the test is done prone or supine consistently and compared with the other side rather than against a published figure.

How It Is Measured

The dial test is a clinical measurement in degrees, which makes it unusual at the knee.

Its reliability depends on controlling the hip and the foot, and on comparing sides.

Cadaveric sectioning gives the restraint apportionment, with the usual order effect.

Imaging contributes through stress views and through identifying the injured structures rather than by measuring the rotation. 5.10 covers it.

Clinical Relevance

Perform the dial test at both angles and record both, since one angle alone cannot separate the two patterns.

Control the hip and the foot, because both are inside the measurement.

Examine the posterior cruciate in any knee with increased external rotation at 90 degrees.

Compare sides rather than using an absolute threshold, since baseline rotation varies between people.

FRCS Level

At the Knee

Treat increased external rotation as a localising sign rather than a severity measure. Its value is in telling you which structures to examine further.

The corner's restraint is shared between three structures within two centimetres of each other, which is why the test localises to a region and not to a ligament. 2.10.4 covers it.

Rotation and varus are linked on the lateral side through the same structures, so the two findings usually travel together.

What Changes It

Alignment. A varus limb loads the lateral structures in stance and a corner reconstruction in a varus knee is loaded at every step, which is why the osteotomy question precedes the ligament question. 3.10.8 covers it.

Chronicity. Corner tissue that healed long gives increased rotation with continuity, so imaging showing intact structures does not exclude the finding.

Previous surgery, which can over-constrain rotation as readily as leave it lax.

How It Is Measured

The dial test is the most quantitative routine knee examination there is, and it is still a two person estimate in degrees.

Its threshold of about 10 degrees is conventional rather than derived, which is worth stating when it is quoted as though it were a cut off.

Stress radiography quantifies the associated translation better than the rotation, so the two investigations answer different parts of the question.

Clinical Relevance

Assess alignment in any knee with a posterolateral finding, before planning a soft tissue procedure.

Read continuity on imaging as insufficient to exclude a corner injury, since tissue heals long.

Record both dial angles and the side difference at every assessment, since this is the one rotation with a serial number available.

Avoid treating the 10 degree threshold as a boundary rather than a convention.