Posterior Translation

For Medical Students

The Principle

Posterior translation is the tibia moving backwards relative to the femur.

It differs from anterior translation in one practical way: gravity acts in the same direction, so a deficient knee sits subluxed before anyone touches it.

That is why posterior instability is missed. The examiner pushes from an already abnormal starting point and finds a normal looking excursion.

At the Knee

The posterior cruciate ligament is the primary restraint. Conventional teaching gives its share as about 95 per cent of the resistance to posterior translation at 90 degrees of flexion.

The posterolateral corner is the main secondary restraint, and its contribution rises towards extension. 2.10.4 covers it.

Normal posterior translation is small, and the clinical measure is the step off between the medial tibial plateau and the femoral condyle rather than a distance.

Conventional teaching puts the normal step off at about 10 mm of tibia in front of the condyle at 90 degrees of flexion.

What Changes It

Flexion angle. The posterior cruciate carries most of the restraint at 90 degrees and less near extension, which is why the drawer is tested in flexion.

Gravity, which produces the sag that masks the finding.

Quadriceps activity, which pulls the tibia forwards and can reduce the sag during an active test.

Corner injury, which increases posterior translation near extension as well as rotation.

Clinical Relevance

Read the resting step off before pushing. A reduced step off is the finding; the excursion from that position can look normal.

The grading is anatomical: plateau still in front of the condyle is partial, level with it is complete, behind it means the corner has gone too. 4.11 covers the examination.

A posterior cruciate deficient knee troubles people going downhill and down stairs rather than on turning, which is the history that distinguishes it.

MRCS Level

The Principle

Posterior instability is a position problem before it is an excursion problem, which is the opposite of the anterior side.

Every test and every imaging measurement therefore has to establish the starting position before measuring movement from it.

That single difference explains most of why the two cruciates are assessed so differently despite being geometrically symmetrical.

At the Knee

The anterolateral bundle of the posterior cruciate carries most of the restraint in flexion and the posteromedial bundle more near extension, which 2.8.2 and 2.8.3 cover.

The meniscofemoral ligaments contribute a measurable share, so a deficient knee retains some restraint from structures that are not the cruciate. 2.8.1 covers them.

Corner injury adds rotational and varus laxity to the posterior translation, and the combination is commoner than an isolated posterior cruciate injury in high energy trauma.

Conventional teaching grades by step off: 0 to 5 mm grade one, 5 to 10 mm grade two, more than 10 mm grade three.

What Changes It

Flexion angle, gravity, muscle activity and the state of the corner.

Chronicity. A posterior cruciate injury can heal in continuity at a greater length, so imaging can show an intact ligament in a lax knee. 2.8.4 covers why.

Quadriceps strength, which compensates functionally and is why some deficient knees cope well.

How It Is Measured

The posterior drawer with the step off read first is the clinical method, graded in millimetres of step off rather than millimetres of excursion.

Stress radiography quantifies it and is the method used where the grade decides treatment.

The quadriceps active test uses muscle contraction to reduce the sag, which is a mechanical confirmation rather than a separate measurement.

Imaging shows continuity and not length, which is the limitation that matters most here. 5.8 covers it.

Clinical Relevance

Establish the resting step off before any posterior drawer, and record it.

Examine the posterolateral corner in every posterior cruciate injury, since the combination changes the treatment.

Read an intact posterior cruciate on imaging as compatible with laxity, because healing in continuity at length is common.

Expect downhill and stair symptoms rather than rotational giving way.

FRCS Level

At the Knee

Treat the posterior cruciate as the restraint and the corner as the modifier. Isolated injuries behave differently from combined ones and the examination separates them by flexion angle.

The partial healing that this ligament achieves is the reason its natural history differs from the anterior cruciate's, and it is an anatomical difference rather than a clinical preference. 2.8.4 covers it.

Posterior sag changes the resting contact position, which loads the patellofemoral and medial compartments differently over time.

What Changes It

Corner integrity, which decides whether the knee is a posterior problem or a posterolateral one.

Alignment, because a varus limb with posterolateral laxity is a different mechanical problem from the same ligament injury in a neutral limb.

Time, through partial healing and through the secondary compartment changes that a chronically sagged knee develops.

How It Is Measured

Stress radiography is the method that settles grade and is underused relative to its value here, because the clinical finding is the one most affected by starting position.

Examination under anaesthetic removes guarding, which matters more on this side because quadriceps activity directly opposes the motion being measured.

Sectioning studies describe an acutely divided ligament, and most patients present with one that healed long.

Clinical Relevance

Measure with stress radiography where the grade decides the treatment, rather than relying on a clinical estimate from an uncertain starting position.

Assess alignment in any chronic posterolateral injury before planning reconstruction.

Expect partial healing and reassess rather than treating the early finding as final.

Avoid reconstructing a posterior cruciate in a varus limb without addressing the alignment.