Locking

For Medical Students

What It Tests

True locking is a knee that physically will not straighten and has to be worked free. It is a mechanical block and something is in the way.

Almost every patient who reports locking means something else: a catch, a click, a knee that gives way, or a knee too painful to straighten.

Asking about locking therefore tests whether a mechanical block is present, and the whole value of the question lies in refusing to accept the word at face value.

How It Is Performed

Ask whether the knee has ever got stuck so that it would not straighten.

If yes, ask how they got it straight again, because the answer separates true locking from everything else. A patient who had to wiggle, manipulate or wait several minutes describes a block. A patient whose knee straightened as soon as the pain eased does not.

Ask how long it stayed stuck and how often it happens.

Ask whether they can straighten it fully now, and then check, because a block may still be present. 4.7.3 covers hyperextension and 4.7.8 extension lag.

What a Positive Means

A knee that is truly locked has something interposed, and the candidates are a displaced meniscal fragment, a loose body, or a torn cruciate stump.

A displaced meniscal tear is the commonest, and a bucket handle tear is the pattern that produces it. 4.9.8 covers mechanical locking.

A loose body gives intermittent locking at unpredictable positions rather than a fixed loss of extension. 4.17.3 covers osteochondral lesions.

A locked knee in a child or adolescent raises a discoid meniscus. 4.19.3 covers it.

A locked knee with an acute haemarthrosis after an injury is an urgent problem rather than a clinic one.

MRCS Level

What a Positive Means

The distinction a viva wants is between a true mechanical block, pseudo-locking and a catching sensation.

A true block has a fixed loss of extension with a hard endpoint and it is reproducible on examination.

Pseudo-locking is a knee that will not straighten because of pain or effusion. The endpoint is soft and the range returns when the irritation settles.

A catch is momentary and the knee moves freely either side of it. It points to a flap tear, a cartilage lesion or a plica rather than to a displaced fragment.

The examination is what confirms a block, so the history selects who needs examining for it carefully and the examination decides. 4.7.5 covers end feel.

How Good It Is

True locking is one of the few history features in the knee that is close to specific for a structural lesion when it is genuinely present.

The reason it has a poor reputation is that it is reported far more often than it occurs, so the question as usually asked has a high yield of false positives.

Asked properly, with the follow-up question about how the knee was freed, it becomes substantially more useful, and that single follow-up is the most valuable addition in this chapter.

It performs poorly as a screen, because a meniscal tear can be symptomatic without ever locking, so a negative answer excludes very little.

FRCS Level

How Good It Is

A genuine mechanical block is one of the clearest surgical indications in the knee, and it is the one place in this chapter where a history feature alone can justify an operation.

It is also the exception to the trial evidence on arthroscopic meniscectomy in degenerate knees. Those trials recruited knees with mechanical symptoms rather than knees that were locked, and treating them as a verdict on a truly locked knee misreads them. 4.9.11 covers the point.

The strength of the indication depends entirely on the quality of the history, which is why the follow-up question is not a refinement.

A knee that locked once, months ago, and has a full range now is a much weaker indication than one that locks repeatedly, and the difference is in the history rather than in the imaging.

Where It Misleads

Locking misleads in both directions and the costs are different.

Accepted uncritically, it produces arthroscopy for knees whose symptom was pain. That is the error the degenerate meniscus trials were addressing.

Dismissed uncritically, it leaves a displaced fragment in a young knee, where delay reduces the chance of a repairable tear being repaired rather than resected. That error is less common and more costly.

A third trap is the knee that locks in flexion rather than extension, which patients describe differently and which points towards a loose body or a patellar cause.

And a knee that locks after an operation may have a cyclops lesion, a retained fragment or impinging hardware rather than a new tear. 4.20.1 covers reconstruction assessment.

Fellowship Level

Where It Misleads

In referred practice the commonest version of this problem is a knee that has been arthroscoped for reported locking and is no better.

The history then has to establish whether the original symptom was ever a block, which the operation note will not say and the referral letter will not either.

If it was not, the symptom was something else and a second arthroscopy will not address it either.

If it was, and a fragment was resected, the remaining questions are whether the symptom is new, whether a further tear has occurred, and whether the resection itself has produced a mechanically different knee. 4.9.10 covers meniscal deficiency.

A locked knee after a meniscal repair is a different problem again and may represent failure of the repair rather than a new event. 4.20.2 covers it.

No figure on this page supports which features discriminate; they are standard teaching and the numbers were refused under decision 023.