Limb Length Assessment

For Medical Students

What It Tests

Limb length assessment establishes whether the two legs are the same length and, if not, where the difference is.

It belongs in a knee examination for two reasons. A knee with a fixed flexion deformity is functionally short, and a knee with lost joint height or a previous osteotomy is anatomically short. 4.5.4 covers flexion deformity.

A length difference also changes the loading on both knees, so it is a contributor to a knee complaint as well as a consequence of one. 4.5.1 covers the mechanical axis.

How It Is Performed

Lay the patient supine with the pelvis square and both legs in the same position.

Measure the true length from the anterior superior iliac spine to the medial malleolus on each side, using the same landmarks.

Measure the apparent length from the umbilicus to the medial malleolus, which differs from the true length when the pelvis is tilted or a hip is contracted.

Stand the patient up and look at the pelvic level from behind, and use blocks under the short leg until the pelvis is level, which is the more functional measurement.

Flex both knees to ninety degrees with the patient supine and look from the side, which localises a difference to the femur or the tibia.

What a Positive Means

A true length difference indicates a bony or joint height difference, and localising it to the femur or the tibia is the next step.

An apparent difference with equal true lengths indicates a pelvic tilt or a hip contracture rather than a limb problem. 4.4.5 covers a related pattern.

A functional difference in a knee with a flexion deformity is the deformity rather than a length difference, and correcting the deformity corrects the length. 4.7.7 covers contractures.

A new shortening in a knee with compartment collapse indicates loss of joint height, which is a late degenerative finding. 4.4.6 covers the gait it accompanies.

A shortening after an osteotomy or a fracture is acquired, and the operation notes and imaging explain it. 4.3.7 covers surgical scars.

MRCS Level

How It Is Performed

In a viva this is a standard measurement and the expected answer includes both the true and the apparent length and knows why both are taken.

The expected points are the squared pelvis, the true measurement with named landmarks, the apparent measurement, the standing block test, and the localisation to femur or tibia.

Say that you would square the pelvis before measuring, since an oblique pelvis makes both measurements meaningless.

Say that you would assess the hip and the spine, because a pelvic tilt originates in one of them rather than in the leg. 4.4.5 covers that pattern.

Say that long leg standing radiographs measure what the tape estimates, and that they are the measurement where a plan depends on one. 4.5.1 covers it.

What a Positive Means

The reasoning expected is that the apparent length depends on pelvic position and the true length does not, so comparing the two separates a limb problem from a pelvic one.

A knee held in flexion shortens the limb functionally without changing its anatomical length, which is why the two measurements can disagree. 4.5.4 covers flexion deformity.

A length difference alters the loading on both limbs, with the long leg typically loaded more at the knee, which is why an untreated difference contributes to knee complaints. 3.13.7 covers dynamic alignment.

Compartment collapse reduces joint height, so a degenerative knee shortens, and that shortening is part of the gait pattern it produces. 3.13.8 covers alignment and osteoarthritis.

How Good It Is

Tape measurement is reproducible to within a modest margin when the landmarks and the pelvic position are controlled, and unreliable when they are not.

The standing block test is more functional and more reproducible than the tape, and it is the measurement most useful for deciding about a raise.

Radiographic measurement is the reference where a surgical plan depends on the figure. 4.1.6 covers that relationship.

The assessment's main value in a knee clinic is identifying the knee that is short because of a deformity, since that is corrected by treating the knee. 4.7.7 covers contractures.

No figures appear for any accuracy comparison on this page, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level limb length contributes to two decisions and is a caveat on several others.

The first is whether a flexion deformity is corrected, since correcting it restores the length and that is often the patient's complaint rather than the knee itself. 4.5.4 covers the deformity.

The second is planning a bony procedure, where an osteotomy or an arthroplasty changes length and the pre-operative figure is needed. 4.5.1 covers the measurement.

As a caveat, a length difference changes the alignment's effect on both knees, so an alignment plan that ignores length is incomplete.

In a knee with multiple previous bony procedures the length is frequently abnormal and frequently unrecorded, which limits what a revision plan can promise. 4.10.12 covers revision assessment.

Where It Misleads

The commonest error is measuring without squaring the pelvis.

The second is recording one measurement rather than both.

The third is attributing a functional shortening to a bony difference when it is a flexion deformity.

The fourth is treating a small difference as significant, since modest differences are common and asymptomatic.

The fifth is planning a bony procedure without a radiographic measurement.

Fellowship Level

Where It Misleads

In referred practice limb length is most often relevant as something a previous operation changed and nobody recorded.

A patient with a previous osteotomy, a leg length difference and a knee complaint has a limb problem that a further knee operation will not address, and the pre-operative films are what establish it. 4.5.1 covers the measurement.

A patient whose main complaint after an arthroplasty is a length difference has a problem that is hard to revise and easier to manage with a raise, and saying so early is kinder than exploring it surgically.

A patient with a fixed flexion deformity and a functional shortening will gain length from correcting the deformity, and framing the expectation that way is more accurate than discussing length. 4.7.7 covers contractures.

In a multiply operated knee the length may be the limit on what any further surgery can achieve, and that belongs in the consent rather than in the operation note. 4.10.12 covers revision assessment.

In medicolegal work a length difference attributed to an injury needs a pre-injury measurement or a pre-injury radiograph, and neither usually exists. 4.23.7 covers that setting.