Mechanical Locking

For Medical Students

What It Tests

Mechanical locking is a physical block to extension caused by something interposed in the joint. Confirming it is the most important thing this chapter does.

It matters because it is the one meniscal finding that is a surgical indication on its own, and because the thing causing it is often repairable now and not later.

4.2.5 covers the history. This page covers confirming it on examination.

How It Is Performed

Ask the patient to straighten the knee fully themselves, and compare with the other side.

Then attempt passive extension gently, supporting the heel and letting gravity assist, and feel for the endpoint.

Characterise the endpoint. A hard, abrupt endpoint with a springy resistance is a block. A soft endpoint that creeps further with gentle sustained pressure is not. 4.7.5 covers end feel.

Measure and record the loss of extension in degrees, and record the endpoint character beside it. 4.1.8 covers the record.

Do not force extension. A block that is forced can be made worse, and the examination is complete once the endpoint is characterised.

What a Positive Means

A fixed loss of extension with a hard springy endpoint, in a knee with a matching history, is a mechanical block and the commonest cause is a displaced meniscal fragment.

A bucket handle tear is the classic pattern, and in a young knee it is frequently repairable if addressed promptly.

A loose body gives a block at variable positions rather than a fixed loss of extension. 4.17.3 covers osteochondral lesions.

A torn cruciate stump can block extension after an acute injury, which is a recognised and less common cause.

A soft endpoint with a loss of extension is pain, effusion or a developing contracture rather than a block. 4.7.7 covers contractures.

MRCS Level

How It Is Performed

In a viva the expected sequence is active extension, comparison, gentle passive extension, endpoint characterisation, measurement, and a statement that you would not force it.

The endpoint is the mark. A candidate who records a loss of extension without characterising the endpoint has not distinguished a block from a contracture.

Say that you would examine the other knee, because a patient with physiological hyperextension has a loss of extension relative to themselves that is easy to miss. 4.7.3 covers hyperextension.

Say what you would do next, which for a confirmed block in a young knee is urgent rather than routine.

What a Positive Means

The reasoning is mechanical: a fragment displaced into the intercondylar region physically prevents the femur completing its roll and glide into extension. 3.3.2 covers roll and glide.

That produces a hard endpoint, because bone is meeting an interposed structure rather than a tightening capsule.

A contracture produces a soft endpoint, because the limiting structure is elastic.

Pain inhibition produces a variable endpoint that changes with reassurance and with analgesia, and re-examining after analgesia is a legitimate step.

An effusion produces a mild loss of extension that resolves as the effusion does.

How Good It Is

A confirmed mechanical block is the most specific finding in this chapter by a wide margin, and it is the only one that acts alone.

Its sensitivity for a meniscal tear is low, because most tears do not lock, so it is a finding to confirm rather than a test to screen with.

Examiner agreement on the endpoint is better than agreement on the provocative tests, because a hard endpoint is a more discrete observation than a click.

The acute knee is the exception: guarding and effusion make the endpoint hard to characterise, and re-examination after a few days is more reliable than persisting.

No figures appear, under decision 023, and the claim is the ranking against the other findings in this chapter.

FRCS Level

How Good It Is

At decision level a confirmed block changes the pathway from elective to urgent, and it is the one place in this chapter where the examination alone justifies an operation.

It is also the exception to the arthroscopic meniscectomy trial evidence. Those trials recruited knees with mechanical symptoms rather than knees that were locked, and applying them to a locked knee misreads them. 3.17.3 covers the point.

The urgency is about repairability rather than about the block. A displaced peripheral tear in a young knee may be repairable now, and a delay converts a repair into a resection. 4.2.9 covers onset.

In an older degenerate knee a block is still an indication and the operation is more often a resection, so the urgency is lower and the symptom relief is the aim.

A knee with a soft endpoint and a loss of extension does not need urgent surgery and frequently needs rehabilitation.

Where It Misleads

The commonest error is accepting a reported locking without confirming a block, which sends a knee to arthroscopy on a history alone. 4.2.5 covers the history.

The second is the reverse: dismissing a genuine block in a young knee because the imaging was reported as a small tear.

The third is failing to compare with the other side in a hyperextending patient, where a ten degree loss relative to their own normal is a real block that reads as full extension on its own. 4.7.3 covers hyperextension.

And forcing extension to prove the point, which can displace a reducible fragment further and is never necessary.

Fellowship Level

Where It Misleads

A block after knee surgery has its own differential and the meniscus is not at the top of it.

After a cruciate reconstruction, a cyclops lesion produces a block near extension and is frequently recorded as stiffness instead, which delays its treatment. 4.20.1 covers the assessment.

After any arthroscopy, a retained fragment or a loose body can block, and the history of whether the knee was ever straight after the operation separates the two.

After an arthroplasty, a block raises impingement, a retained osteophyte, cement or a component issue rather than a meniscal cause. 4.20.8 covers the assessment.

In a multiply operated knee the distinction between a block and arthrofibrosis is the one that matters most, because the treatments differ completely, and the endpoint character is the main clinical discriminator available.

No figure here supports these claims, under decision 023.