Lachman Test

For Medical Students

What It Tests

The Lachman test measures anterior translation of the tibia on the femur with the knee in slight flexion, and it tests the anterior cruciate directly.

It is the most reliable single examination manoeuvre in the knee, and the reason is its position rather than anything about the hands.

At around twenty to thirty degrees of flexion the hamstrings are shortened and less able to resist, and the ligament is the structure taking the load. 3.7.8 covers anterior stability.

How It Is Performed

Position the knee in slight flexion, around twenty to thirty degrees, with the patient supine and relaxed.

Stabilise the femur with one hand and grasp the proximal tibia with the other, with your thumb on the tibial tubercle.

Apply a firm anterior force to the tibia and feel both the amount of translation and the quality of the endpoint.

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

Record the translation relative to the other side and the endpoint character, which is what makes the finding comparable later. 4.1.8 covers the record.

What a Positive Means

Increased anterior translation compared with the other knee, with a soft or absent endpoint, is a positive and indicates a ruptured ligament.

The endpoint is as important as the translation. Increased translation with a firm endpoint suggests a partial injury or physiological laxity; a soft endpoint suggests a complete rupture.

A hard endpoint with normal translation is a normal test.

A positive test does not say whether the patient is functionally unstable, which is the history's job. 4.10.1 covers it.

A negative test in an acutely guarding knee is weak and should not be recorded as excluding the injury. 4.10.2 covers the acute knee.

MRCS Level

How It Is Performed

In a viva the expected points are the flexion angle, the hand positions, the stabilised femur, the endpoint, and the comparison.

The candidate should say why the angle matters, because that is the test's whole advantage over the anterior drawer and is the most likely follow-up question. 4.10.5 covers the drawer.

Say that a large thigh or a large examiner-to-patient size mismatch makes the standard grip difficult, and name an alternative such as stabilising the femur on your own thigh or using the prone or seated variant.

Say that you would record the grade with an endpoint rather than a grade alone.

What a Positive Means

The reasoning expected is that the ligament is the primary restraint to anterior translation and supplies most of that restraint itself, with the secondary restraints contributing in a flexion-dependent way. 3.7.2 covers primary restraints.

At slight flexion the secondary restraints contribute least, which is why the test isolates the ligament better than a test at ninety degrees does.

The endpoint reflects whether any fibres remain under tension, which is the anatomical basis for distinguishing partial from complete.

Physiological laxity gives symmetrical increased translation with firm endpoints bilaterally, which is why the comparison is diagnostic rather than merely helpful. 4.23.3 covers hypermobile patients.

How Good It Is

The Lachman test outperforms the anterior drawer substantially, and that is the single most reproduced comparison in the knee examination literature. 3.17.1 covers the studies.

It is more sensitive than the pivot shift and less specific, so the two are complementary rather than competing. 4.10.6 covers the pivot shift.

It performs well in the chronic knee and less well acutely, because guarding resists translation. 4.10.2 covers the acute knee.

Examiner experience affects it, mostly through the endpoint judgement rather than the translation.

No figures appear, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level the Lachman test establishes the diagnosis and contributes little to the indication, which is the recurring structure of this chapter.

Its grade does not determine whether to reconstruct. A patient with a grade three Lachman and no instability does not need surgery, and one with a grade two and instability at work may.

Where the grade matters is in the comparison over time and in the comparison with the examination under anaesthesia. 4.10.10 covers the latter.

The endpoint matters more than the grade for partial injuries, which are the cases where the decision is genuinely difficult.

Instrumented laxity measurement adds precision and has not displaced the manual test in practice, which is worth knowing when a reported figure appears in a letter.

Where It Misleads

The commonest error is recording a grade without an endpoint, which makes the finding almost uninterpretable by the next clinician.

The second is failing to compare sides, which converts a diagnostic test into a guess, particularly in a hypermobile patient.

The third is performing it with an unstabilised femur, where the whole leg moves and the translation is apparent rather than real.

And treating a positive Lachman as the indication, which conflates diagnosis with decision and is the error this chapter returns to.

Fellowship Level

Where It Misleads

After reconstruction the Lachman test is the main clinical measure of graft integrity, and its interpretation changes.

A graft is expected to give a firmer endpoint than a deficient knee and a slightly softer one than a normal knee, so a small increase in translation with a firm endpoint is a satisfactory result rather than a failure. 4.10.11 covers post-reconstruction assessment.

A soft endpoint after reconstruction indicates graft failure and is the finding that matters.

Progressive increase in translation across visits indicates a stretching graft, which is a trajectory rather than a single finding and requires the earlier examinations to have been recorded properly. 4.1.8 covers the record.

A normal Lachman in a patient complaining of instability points at rotation rather than translation, which is this chapter's central gap. 4.10.9 covers it.

No figure here supports these claims, under decision 023.