Dial Test Biomechanics

For Medical Students

The Principle

If two structures resist the same motion at different joint angles, measuring at both angles separates them.

The comparison does the work. A finding at one angle and not the other implicates the structure tight at that angle.

That is a subtraction rather than a direct measurement, which is why the test is read as a difference.

At the Knee

External rotation of the tibia is resisted mainly by the posterolateral corner at 30 degrees of flexion and increasingly by the posterior cruciate as flexion increases. 3.10.2 covers external rotation.

Measuring external rotation at 30 and at 90 degrees therefore separates a corner injury from a combined corner and posterior cruciate injury.

Increased rotation at 30 degrees alone indicates an isolated corner injury. Increased rotation at both, worse at 90, indicates the posterior cruciate as well. 3.8.1 covers that ligament.

The test is performed on both legs and read as a side to side difference, because individual variation is large. Section 4 covers the technique.

What Changes It

Which structures are injured.

The flexion angles used, since the comparison depends on them.

Muscle guarding, which reduces the measured rotation.

Whether both sides were compared.

Clinical Relevance

The test's value is in the comparison, so performing it at one angle wastes it. 4.13 covers the lateral examination.

A positive test at 30 degrees changes the management, because an isolated corner injury is still an operative one. Section 6 covers management.

It is the clinical test that most directly reflects the corner's function, which is why it appears in every account of the corner. 3.10.6 covers the corner.

Under anaesthetic the findings are clearer, because guarding is removed.

MRCS Level

The Principle

The test is an applied example of angle dependent restraint, which is the organising idea of the whole section. 3.2.10 covers stability through range.

Its logic assumes the two structures' contributions change in opposite directions with flexion, which they do: the corner slackens and the posterior cruciate tightens. 3.8.3 covers the cruciate's bundles.

Where that assumption fails, the test fails. A knee with both injured and a large laxity at both angles is harder to interpret than the clean two case model suggests.

At the Knee

Conventional teaching reads an increase of more than 10 degrees of external rotation compared with the uninjured side as significant.

An increase at 30 degrees that does not worsen at 90 is read as an isolated posterolateral corner injury.

An increase at both angles that is greater at 90 is read as a combined corner and posterior cruciate injury. 3.10.9 covers combined injury.

The test can be performed prone or supine, and the position affects reproducibility more than the mechanics. 4.13 covers the technique.

What Changes It

Which structures are injured and to what degree.

Guarding, which reduces the measurement.

Examiner technique and patient position.

Whether the comparison with the other side was made.

How It Is Measured

Clinical measurement of the thigh foot angle or the tibial tubercle position gives the rotation, with modest reproducibility.

Computed tomography and radiographic measurement under applied rotation quantify it in research settings.

Sectioning studies establish which structures the test is sampling at each angle.

No instrument in routine clinical use quantifies rotatory laxity, which is the standing limitation. 3.10.2 covers that point.

Clinical Relevance

Perform it at both angles and record both, since the comparison is the finding.

Compare with the uninjured side rather than against an absolute figure.

Repeat under anaesthetic where the finding will change the operation.

Expect interpretation to be harder when both structures are badly injured.

FRCS Level

At the Knee

Treat the dial test as the intraoperative check as well as the clinical one. Performed under anaesthetic before and after fixation, it is the measurement that says whether the corner reconstruction worked. 3.10.8 covers the construct.

Its limitation is that it has no instrument, so a comparison between two examinations by different people is weak. That limits its use as an outcome measure and not its use as an intraoperative check.

Conventional teaching holds that a residual positive dial test after corner reconstruction predicts a poorer result, which makes the intraoperative check the one worth doing.

What Changes It

Which components were reconstructed and their orientation.

Graft tension and the flexion angle at fixation.

Alignment, which loads the construct. 3.13 covers alignment.

Posterior cruciate status, where it is also injured.

How It Is Measured

Examination under anaesthetic before and after fixation, at both angles, is the practical check.

Varus stress radiography measures the coronal component reproducibly and not the rotational one.

Research instrumentation quantifies rotation and is not available clinically.

Inter observer comparison is weak, which limits its use as an outcome measure.

Clinical Relevance

Perform the dial test under anaesthetic before and after corner fixation.

Use it as an intraoperative check rather than as a comparative outcome measure.

Record both angles and the side to side difference.

Investigate a residual positive test before leaving theatre.