Posterior Drawer Test

For Medical Students

What It Tests

The posterior drawer test measures posterior translation of the tibia on the femur with the knee flexed to ninety degrees, and it tests the posterior cruciate directly.

At ninety degrees the ligament is the structure resisting that translation, and the secondary restraints contribute least. 3.8.6 covers the mechanics.

It is the mirror image of the anterior drawer in position and the opposite of it in reliability, because the posterior cruciate is tested well at this angle and the anterior cruciate is not. 4.10.5 covers the anterior drawer.

How It Is Performed

Position the patient supine with the hip flexed to about forty five degrees and the knee flexed to ninety degrees, with the foot flat on the couch.

Sit on the foot to stabilise it, or hold it with your own thigh, so the tibia cannot rotate out of the plane you are testing.

Find the tibial step off with both thumbs on the joint line and reduce the tibia forward until the step off is restored. This is the step that decides the test and the step most often skipped.

Grasp the proximal tibia with both hands, thumbs still on the joint line, and apply a firm posterior force.

Feel the amount of translation and the quality of the endpoint, and read the amount against the femoral condyles under your thumbs rather than estimating it in the air.

Compare with the other knee, which is what makes the amount interpretable. 4.1.2 covers the principle.

What a Positive Means

Increased posterior translation compared with the other knee indicates a deficient posterior cruciate.

The grade is read from where the tibial plateau ends up. A step off that is reduced but still present is a first degree injury; a plateau flush with the femoral condyles is second degree; a plateau that can be pushed behind the condyles is third degree.

A third degree result on its own suggests that more than the cruciate is torn, because the corner structures resist the last part of that translation. 4.11.7 covers the combined knee.

The endpoint matters as it does in the Lachman test. A firm endpoint with increased translation suggests a partial injury; a soft endpoint suggests a complete one. 4.10.4 covers the comparison.

A positive test says the ligament is deficient and says nothing about whether the patient has a problem. 4.11.6 covers that distinction.

MRCS Level

How It Is Performed

In a viva the single point that distinguishes a good answer is the reduction before the force. A candidate who describes grasping and pushing has described a test that cannot be graded.

The other expected points are the flexion angle, the stabilised foot, the thumbs on the joint line, the endpoint, and the comparison.

Say that you would look for the sag first, because the inspection tells you where the tibia is starting and the drawer does not. 4.11.1 covers the sag.

Say that you would test the drawer in neutral rotation and then with the tibia internally rotated, because internal rotation tightens the posterior oblique and intact posteromedial structures reduce the translation.

Say that you would record the grade with the step off and the endpoint rather than a grade alone. 4.1.8 covers the record.

What a Positive Means

The reasoning expected is that the posterior cruciate supplies most of the restraint to posterior translation at ninety degrees, and that the two bundles contribute differently across the flexion range. 3.8.3 covers bundle function.

The anterolateral bundle is the larger and tightens with flexion, which is why the test is performed at ninety degrees rather than in extension. 3.8.4 covers that bundle.

Translation that reduces when the tibia is internally rotated suggests the posteromedial structures are intact, and translation that does not suggests a combined injury.

Increased external rotation at thirty degrees of flexion in the same knee moves the diagnosis to the posterolateral corner. 4.13.5 covers the dial test.

How Good It Is

The posterior drawer is the reference clinical test for this ligament, and it is more reliable for the posterior cruciate than the anterior drawer is for the anterior cruciate.

The reason is the angle. Ninety degrees is where this ligament is tightest and where the hamstrings cannot substitute for it, so the test is working with the anatomy rather than against it.

Its weakness is the starting position rather than the manoeuvre, and that weakness is examiner dependent in a way the sag sign is not. 4.11.1 covers the alternative.

It performs less well in the acute knee, where guarding resists the force and the patient cannot tolerate ninety degrees of flexion.

No figures appear for any of these comparisons, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level the drawer grade is one of the few examination findings in the knee that genuinely changes the operation being considered.

A first or second degree injury in isolation is usually managed without reconstruction, because the ligament heals in continuity more often than the anterior cruciate does and the knee compensates well.

A third degree injury is a different problem, because the grade implies corner involvement and the corner is what determines whether a reconstruction holds. 4.11.7 covers the assessment.

The grade is also the measure that follows a patient through non operative treatment. A grade that worsens over months in a braced knee is the finding that converts a plan.

Instrumented and radiographic stress measurement gives a number where this test gives a grade, and the number is what appears in a letter from another unit. The two are not interchangeable and the examination remains the primary record.

Where It Misleads

The commonest error is testing from a subluxed start, which under reads the posterior translation and over reads the anterior drawer on the same knee.

The second is grading in millimetres rather than against the condyles, which sounds more precise and is less reproducible between examiners.

The third is reading a third degree drawer as a severe isolated injury rather than as a sign of a combined one.

The fourth is examining at ninety degrees in a knee that cannot reach ninety degrees, where the angle achieved is not the angle described and the grade is not comparable with anybody else's.

Fellowship Level

Where It Misleads

In revision and referred practice the drawer is usually being compared with somebody else's drawer, and the comparison is only as good as what they wrote.

A grade recorded without the step off or the rotation is close to useless at the second assessment, which is why this chapter insists on recording both. 4.1.8 covers the record.

After reconstruction the expected finding is a restored step off with a firm endpoint and slightly more translation than the normal side. A soft endpoint is graft failure and a progressive increase across visits is graft stretch.

The commonest cause of that stretch is an unaddressed posterolateral corner, and the drawer alone will not tell you. The dial test at thirty and ninety degrees will. 4.13.5 covers it.

A drawer that is normal in a patient who describes the knee giving way going downstairs is pointing at the extensor mechanism or the patellofemoral joint rather than at this ligament, and the history is the discriminator. 4.2.6 covers giving way.

Examination under anaesthesia removes the guarding and changes the grade in the acute knee, and it is the setting where the difference between a second and a third degree injury is actually settled. 4.23.1 covers it.