Passive Motion

For Medical Students

What It Tests

Passive motion is the range the examiner achieves by moving the knee, with the patient relaxed, so it tests the joint and the soft tissues around it without the muscles.

It is the structural measurement. Where the active range is limited and the passive range is not, the limitation is not structural. 4.7.1 covers active motion.

The passive examination also provides the end feel, which is the quality of resistance at the limit and is the most informative part of the range assessment. 4.7.5 covers it.

How It Is Performed

Lay the patient supine with the leg relaxed and nothing under the knee. 4.3.5 covers the supine assessment.

Support the limb yourself so the patient does not need to hold it, since a patient holding the leg is not relaxed.

Extend the knee gently to its limit, note the angle and the quality of the resistance, and record both. 4.7.5 covers the end feel.

Flex the knee, bringing the heel towards the buttock, and note the angle and what stops it: tissue contact, resistance or pain. 4.7.4 covers flexion.

Compare with the other side and record both, since a normal range varies widely between people. 4.1.2 covers the principle.

What a Positive Means

A passive extension deficit indicates a structural or soft tissue restriction, and the end feel distinguishes a mechanical block from a contracture. 4.5.4 covers flexion deformity.

A passive flexion deficit indicates the same thing in the other direction, with an effusion and tissue bulk as additional causes. 4.7.4 covers it.

A full passive range with a limited active range is a muscular or inhibitory problem, which is the most useful single comparison in this chapter. 4.7.8 covers extension lag.

Pain limiting the passive range before any resistance is reached indicates an irritable joint rather than a restricted one, and the cause is usually an effusion or inflammation. 4.8.2 covers it.

A passive range larger than the other side indicates hypermobility or ligament deficiency rather than a problem with the range. 4.23.3 covers hypermobile patients.

MRCS Level

How It Is Performed

In a viva passive motion is where the end feel is expected, and giving an angle without a quality is an incomplete answer.

The expected points are the relaxed supported limb, the extension to its limit with the end feel, the flexion with what stops it, the measurement of both, and the comparison.

Say that you would measure after the active range, since moving the knee first changes what the patient will attempt actively. 4.7.1 covers it.

Say that you would describe the end feel in words, because that is the part that distinguishes the diagnoses. 4.7.5 covers it.

Say that you would assess the effusion before the range, since an effusion limits the passive range mechanically as well as by pain. 4.8.3 covers the bulge sign.

What a Positive Means

The reasoning expected is that the passive range is limited by whichever structure reaches its length first, so the limit localises to a tissue and the end feel names the type. 4.7.5 covers that reasoning.

A firm abrupt limit suggests bone or an interposed fragment; a springy limit suggests a displaced meniscus; a leathery limit suggests capsular shortening. 4.9.8 covers mechanical locking.

The posterior capsule shortens when a knee is held flexed, which is why a flexion deformity becomes structural over weeks. 4.7.7 covers contractures.

An effusion limits flexion mechanically because the capsule's volume is finite, which is why a tense knee loses flexion before it loses extension. 4.8.1 covers normal fluid physiology.

How Good It Is

Passive range is more reproducible than active range, because it does not depend on the patient's effort, and it remains dependent on their relaxation.

The angle is reproducible with a stated method and the end feel is not, which is the familiar trade off between the measurable and the informative. 4.7.5 covers it.

It is the more interpretable measurement in a guarding or uncooperative patient, which is the complement of the active range's weakness. 4.7.1 covers it.

It cannot assess function, since nobody uses their knee passively, which is why both ranges are recorded. 4.7.9 covers functional range.

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 the passive range decides feasibility and the end feel decides which operation.

A knee that cannot be passively extended cannot be reconstructed to a normal ligament length, so the passive range is a precondition rather than an outcome. 4.7.7 covers contractures.

A firm block to passive extension in a reconstructed knee indicates a mechanical lesion with a clear operation, and the end feel is what identifies it. 4.7.10 covers post-operative range.

A springy block with a history of locking indicates a displaced meniscal tear, which is an urgent arthroscopy rather than an elective one. 4.9.8 covers mechanical locking.

Where the passive and active ranges differ, the passive one sets what is achievable and the active one sets what the patient has, and both are needed for a plan. 4.7.1 covers the active range.

Where It Misleads

The commonest error is recording an angle without an end feel.

The second is measuring the passive range in a patient who is holding their own leg.

The third is measuring before assessing the effusion, after which the figure reflects the fluid rather than the joint.

The fourth is reading a painful limit as a structural one.

The fifth is comparing with a textbook normal rather than with the other knee.

Fellowship Level

Where It Misleads

In referred practice the passive range and its end feel usually separate the two stiff knees that are referred, and they are distinguished nowhere else.

A knee with a firm abrupt block after a reconstruction has an impinging lesion, and the operation is straightforward and often delayed because the end feel was never recorded. 4.7.5 covers it.

A knee with a leathery global restriction has arthrofibrosis, and the treatment is different and the result less certain. 4.7.7 covers contractures.

A patient whose passive range has reduced while the active range has not is developing a structural problem, and the sequence of those two measurements is the finding. 4.7.1 covers the active range.

In a knee with a chronic effusion the passive range fluctuates with the effusion, so a single measurement is a poor basis for a plan and a series is needed. 4.8.8 covers chronic effusions.

In medicolegal work a passive range recorded without an end feel is a number whose cause cannot be determined later, which is a common and avoidable limitation. 4.23.7 covers that setting.