Posterior Sag Sign

For Medical Students

What It Tests

The posterior sag sign looks for the tibia resting behind its normal position on the femur, which is what a torn posterior cruciate allows gravity to do.

Every other ligament test applies a force and reads the response. This one applies no force at all. The weight of the leg is the force, and the sign is the resting position it produces. 3.8.2 covers posterior stability.

In a normal knee flexed to ninety degrees the front of the tibia sits about a centimetre in front of the femoral condyles. That offset is the tibial step off, and losing it is the sign.

How It Is Performed

Lay the patient supine with the hips flexed to about forty five degrees and both knees flexed to ninety degrees, feet flat on the couch.

Stand at the side of the couch and look along the line of the knees at eye level rather than from above. The sign is a profile, and from above it is invisible.

Compare the two knees in the same glance. A unilateral loss of the step off is the finding; a symmetrical appearance is not.

Feel for the step off with your thumbs along the joint line if the profile is equivocal, which it often is in a heavy limb.

The variant with both hips and knees supported at ninety degrees, holding the heels, removes the couch as a support and makes gravity the only force acting. It is worth knowing as the pure form of the same observation.

What a Positive Means

Loss or reversal of the tibial step off on one side indicates that the posterior cruciate is not holding the tibia forward.

The amount of sag carries the grade, and the grade is read against the femoral condyles rather than in millimetres. A reduced step off, a tibia flush with the condyles, and a tibia lying behind them are three different findings.

A sagged tibia makes the anterior drawer test read as positive, because the examiner pulls the tibia forward from a subluxed start and feels a large excursion. That is the commonest misreading in the whole chapter. 4.11.2 covers the drawer.

The sign says the ligament is deficient. It does not say the patient is unstable, and in this ligament those two things diverge further than anywhere else in the knee. 4.11.6 covers the chronic knee.

MRCS Level

How It Is Performed

In a viva the expected sequence is inspection before palpation, and this sign belongs to the inspection. Saying it after describing the drawer loses the point of it.

The expected points are the hip and knee angles, the eye level view, the comparison, and the reason the position matters. At ninety degrees of flexion the posterior cruciate is the structure resisting posterior translation, and gravity acts along the line it resists.

Say that you would reduce the tibia before testing anything else, because every anterior and posterior measurement on that knee is taken from wherever the tibia starts. 4.1.2 covers the principle.

Say that you would record the step off rather than a grade alone, for the same reason the Lachman endpoint is recorded. 4.1.8 covers the record.

What a Positive Means

The reasoning expected is that the posterior cruciate is the primary restraint to posterior translation at this angle and supplies most of that restraint itself, with the posterolateral structures contributing in a way that depends on rotation. 3.8.3 covers bundle function.

A sag that appears only at deeper flexion suggests the posteromedial bundle, which tightens with flexion. 3.8.5 covers that bundle.

A sag with increased external rotation of the foot suggests the posterolateral corner is involved as well, which changes the injury from isolated to combined. 4.11.7 covers the combined knee.

Hyperextension on the affected side points the same way, because the corner rather than the cruciate resists it.

How Good It Is

The sag sign is the most specific clinical finding for posterior cruciate injury and the one least affected by examiner experience, because it asks the examiner to see rather than to judge a force.

It is less sensitive than the posterior drawer in the acute knee, where hamstring spasm holds the tibia forward and hides the sag.

It outperforms the drawer in the chronic knee, where the sag is obvious and the drawer is hard to interpret from a subluxed starting point.

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 sag sign establishes that the ligament is deficient and contributes almost nothing to whether the knee needs an operation.

Posterior cruciate deficiency is tolerated far better than anterior cruciate deficiency, so a clear sag in a patient with no functional complaint is a finding to record and watch rather than an indication. 4.11.6 covers it.

Where the sign does drive a decision is at the extremes. A tibia lying frankly behind the condyles, with a corner injury alongside it, is a different knee from a reduced step off in isolation.

The sign is also the most useful single measure of a reconstruction over time, because it is reproducible and it does not depend on the examiner pulling equally hard on two visits.

Where It Misleads

The commonest error is reading the sag and then taking the anterior drawer as a second finding rather than as an artefact of the first.

The second is examining a knee with a fixed flexion deformity, where the thigh and the couch hold the tibia and the profile is uninterpretable.

The third is accepting a symmetrical appearance as normal. Bilateral injury is uncommon and physiological bilateral posterior laxity is not, so symmetry removes the comparison rather than excluding the injury.

A knee examined soon after injury with the quadriceps active can look normal, because a contracted quadriceps reduces the tibia and keeps it reduced. 4.11.3 covers the test built on that.

Fellowship Level

Where It Misleads

In referred practice the sag sign is usually being used to answer a question somebody has already answered, and the interesting cases are the disagreements.

A patient with a reconstructed posterior cruciate and a recurrent sag has either a stretched graft or a missed corner injury, and the examination separates them better than the imaging does. 4.11.7 covers the sequence.

A sag that was absent at the first assessment and present at the second is a trajectory, and it is the finding that most often changes a plan. It needs the first examination to have been recorded as a step off rather than as a grade.

A patient with a sag, a varus thrust and a hyperextended knee has a posterolateral corner problem that the cruciate reconstruction alone will not hold, and reconstructing the cruciate into that knee is the commonest cause of a stretched posterior cruciate graft in revision practice.

The sign is also the one that most often reveals a chronic injury presenting as something else. A patient referred with patellofemoral pain and a quiet history of a dashboard injury years before will have a visible sag that nobody has looked for. 4.2.21 covers the mechanism.