Posterior Drawer Mechanics

For Medical Students

The Principle

Measuring how far something moves needs a known starting point. Move the start and the measurement changes without anything else changing.

A joint that sags under gravity has moved its own starting point before the examiner touches it.

So a test of excursion in a sagging joint measures the excursion plus or minus the sag, depending on where the examiner thinks the start was.

At the Knee

With the knee flexed and the foot supported, gravity pulls the tibia backwards in a posterior cruciate deficient knee. 3.8.2 covers posterior stability.

The examiner therefore starts by finding the step off between the front of the femoral condyle and the front of the tibial plateau. That is the starting position.

Conventional teaching gives the normal step off as around 10 millimetres at 90 degrees of flexion.

Pushing the tibia backwards from a position that has already sagged produces a small apparent excursion in a badly deficient knee, which is the trap.

What Changes It

Flexion angle, which changes both the sag and the restraint.

Whether the limb is supported, since an unsupported limb sags more.

Quadriceps activity, which pulls the tibia forward and hides the sag.

The examiner's assumption about where the start was.

Clinical Relevance

Assessing the step off before the drawer is the whole technique, and it is where the finding comes from. Section 4 covers the tests.

The quadriceps active test uses the muscle to pull the sagged tibia forward, which demonstrates the sag by reducing it.

A knee that appears to have a small posterior drawer may have a large sag, which is the opposite conclusion.

This is also why fixing a graft without reducing the sag first locks the deformity in. 3.8.8 covers the construct.

MRCS Level

The Principle

Two quantities are being confused in one test: the resting position and the excursion from it. A grade based on step off measures the first and a drawer measures the second.

Only the first is reproducible by hand, because the second depends on where the examiner believes the neutral position was.

This is why posterior laxity is better measured radiographically than instrumented, which is the reverse of the anterior situation. 3.7.8 covers anterior stability.

At the Knee

The quadriceps active test is performed at around 90 degrees with the foot fixed: active extension effort pulls the tibia anteriorly, and a visible anterior shift of 2 millimetres or more is conventionally taken as indicating posterior cruciate deficiency.

The posterior sag sign and the reverse pivot shift test other aspects, the second involving the posterolateral corner. 2.10.4 covers the corner.

Flexion angle changes the result. The restraint is greatest at 90 degrees and the corner contributes more at 30. 3.8.3 covers the bundles.

External rotation at 30 degrees separates an isolated cruciate injury from a combined corner injury, which is the distinction that changes the operation.

What Changes It

Starting position, which is the dominant source of error.

Limb support and relaxation.

Quadriceps activity, deliberate or involuntary.

Corner integrity, which changes the rotational findings.

How It Is Measured

Stress radiography with a standardised posterior load measures translation against bony landmarks and is the reference.

Clinical grading by step off is the practical method and depends on technique.

Instrumented arthrometry is less reliable posteriorly because the neutral position is uncertain.

Examination under anaesthetic removes the quadriceps contribution, which matters more here than anteriorly.

Clinical Relevance

Record the step off and the flexion angle, not a drawer figure alone.

Test external rotation at 30 degrees to separate isolated from combined injury.

Prefer stress radiography to arthrometry for posterior laxity.

Use the quadriceps active test when the sag is uncertain.

FRCS Level

At the Knee

Treat the starting position as the thing to control, in examination and in surgery. The same error that makes a drawer misleading makes a reconstruction fail: fixing from a sagged position.

Intraoperatively that means an anterior drawer applied to reduce the tibia before the graft is fixed, which is the step that distinguishes this reconstruction from an anterior cruciate one. 3.8.8 covers the construct.

Conventional teaching holds that residual posterior laxity after reconstruction is common and is attributed partly to this, partly to graft creep, and partly to unaddressed corner injury.

What Changes It

Whether the sag was reduced before fixation.

Corner integrity and whether it was reconstructed.

Graft creep over the first months. 3.7.16 covers graft properties.

Alignment, which loads the construct. 3.13 covers alignment.

How It Is Measured

Intraoperative assessment of the step off after fixation, before closure, is the practical check.

Postoperative stress radiography measures the result reproducibly.

Serial measurement captures creep over the first year.

No measurement confirms the tibia was reduced at the moment of fixation.

Clinical Relevance

Reduce the tibia with an anterior drawer before fixing the graft.

Check the step off after fixation and before closure.

Measure the result with stress radiography rather than by hand.

Expect some residual laxity and attribute it before revising.