Dial Test

For Medical Students

What It Tests

The dial test measures external rotation of the tibia on the femur and compares the two knees at two flexion angles.

At thirty degrees the posterolateral corner is the principal restraint to external rotation. At ninety degrees the posterior cruciate takes a larger share. 3.10.7 covers the biomechanics.

That difference is the whole test. The pattern across the two angles separates a corner injury from a corner and cruciate injury, which no other clinical test does. 3.10.2 covers external rotation stability.

How It Is Performed

Lay the patient prone, which is the easier position, with both knees together and both feet free.

Flex both knees to thirty degrees and hold both feet.

Rotate both feet externally at the same time with the same force, and compare the angle the feet reach relative to the thigh on each side.

Note the difference between the two sides rather than the absolute angle, since the absolute angle varies between people.

Repeat the whole thing with both knees flexed to ninety degrees, and record both results.

What a Positive Means

An increase in external rotation on the injured side of about ten degrees or more compared with the other knee is a positive test.

An increase at thirty degrees that does not increase further at ninety indicates an isolated posterolateral corner injury. 4.13.4 covers that assessment.

An increase at both thirty and ninety degrees indicates the corner and the posterior cruciate together, which is the combination that changes the operation. 4.11.2 covers the posterior drawer.

An increase at ninety degrees alone is unusual and should prompt a careful posterior cruciate examination rather than a corner diagnosis.

A symmetrical increase on both sides is normal variation in a hypermobile patient and is not a finding. 4.23.3 covers those patients.

MRCS Level

How It Is Performed

In a viva the dial test is the answer to how you would assess the posterolateral corner, and performing it at two angles with both legs together is the mark.

The expected points are the prone position, both knees together, both feet rotated at once, the thigh foot angle, and both flexion angles recorded.

Say that you would rotate both feet simultaneously rather than one after the other, because a sequential test loses the comparison that makes it work.

Say that the supine version is acceptable and harder, because holding both knees at a matched flexion angle supine takes an assistant.

Say that you would also test the posterior drawer and the varus stress, so the pattern across the lateral examination is complete. 4.13.1 covers varus stress.

Say that stress radiographs and the examination under anaesthesia are the two things that settle an equivocal result. 4.14.6 covers the latter.

What a Positive Means

The reasoning expected is that external rotation is resisted by the popliteofibular ligament and the popliteus at thirty degrees, and that at ninety degrees the posterior cruciate contributes more. 3.10.5 covers the popliteofibular ligament.

The lateral collateral ligament contributes to external rotation restraint and contributes less than the deep structures, which is why the dial test and the varus test can disagree. 3.10.3 covers its function.

An isolated corner injury therefore gives a difference at thirty degrees that does not grow at ninety, because the structure that would have added to it at ninety is intact. 3.10.7 covers the mechanics.

A corner and cruciate injury gives a difference that grows, because both contributors are gone. 3.10.9 covers the combination.

How Good It Is

The dial test is the most useful lateral examination finding and the most reproducible rotational test in the knee.

It is more reproducible than the reverse pivot shift, which it has largely replaced in practice for this question. 4.11.4 covers that test.

Its reliability depends on rotating both legs together and on the patient being relaxed, and it fails in a guarding patient in the direction of a false negative.

Stress radiographs and instrumented measurement give numbers where this gives a compared angle, and they matter where a plan turns on a borderline difference.

No figures appear for any accuracy comparison on this page, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level this test decides the content of the operation on the lateral side, which is more than any other clinical test in Section 4 does.

An increase at thirty degrees only means the corner is reconstructed and the posterior cruciate is left. An increase at both means both, and the cruciate alone is not an option. 3.10.9 covers the combination.

That is the decision a stretched posterior cruciate graft usually comes from getting wrong, and it is the reason this test appears in every chapter on this side of the knee. 4.11.7 covers the combined assessment.

The test also follows a reconstruction. A dial difference that reappears after a corner reconstruction is the clearest clinical evidence that it has failed. 4.13.4 covers that assessment.

Where the difference is borderline and the decision is real, examination under anaesthesia with both legs is how it is settled, since guarding is the test's main enemy. 4.14.6 covers that setting.

Where It Misleads

The commonest error is performing it at one angle, which discards the whole point of the test.

The second is rotating the legs one after the other, which removes the comparison.

The third is reading the absolute thigh foot angle rather than the difference, which produces findings in people who simply rotate more.

The fourth is a guarding patient, where a normal result is an absent test rather than a normal lateral side.

The fifth is performing it in a knee that cannot reach ninety degrees, where the second angle is not the angle described and the pattern cannot be read.

Fellowship Level

Where It Misleads

In referred practice the dial test is the test that was not done, and doing it answers most of the questions the referral asks.

A patient with a stretched posterior cruciate graft and an increased dial difference at both angles has the explanation for the stretch, and the revision has to address the corner or it repeats. 4.11.2 covers the drawer.

A patient with a reconstructed corner and a dial difference at thirty degrees only has a failed or incomplete corner reconstruction rather than a new injury. 4.13.3 covers the structure usually left out.

A knee with no dial difference and a persisting rotational complaint is pointing at the anterolateral structures rather than the corner, and that is a different examination. 4.14.3 covers it.

After a proximal tibial osteotomy the thigh foot angle is altered on the operated side, so a post-operative baseline is needed and the other leg is no longer a valid comparison.

In an over constrained corner reconstruction the dial difference is negative, meaning the operated knee rotates less, and that presents as lateral pain and stiffness rather than instability. 4.2.4 covers stiffness.