Mechanical Symptoms

For Medical Students

What It Tests

A mechanical symptom is something physically interfering with the joint's movement: catching, a sense of something moving, a true block to extension, or a sudden giving way without the knee being unstable.

The history chapter covers these as symptoms. This page covers them as the finding that changes a cartilage lesion from an observation into a surgical problem. 4.2.7 covers the history.

The word locking is the problem. Patients use it for stiffness, for pain and for a true block, and only the last is mechanical. 4.9.8 covers that distinction.

How It Is Performed

Ask the patient to describe exactly what happens, in their own words, rather than offering the word locking. 4.2.5 covers locking in the history.

Establish whether the knee has ever been stuck and unable to straighten, and whether they or somebody else had to do something to free it.

Establish whether anything catches intermittently, and in what position the catch occurs.

Measure the passive extension and note the end feel, since a springy block is the examination finding that corresponds to a displaced fragment. 4.7.5 covers the end feel.

Assess the effusion, since a loose fragment irritates the joint and produces one. 4.8.3 covers the bulge sign.

Ask about a loose body being felt moving, which is unusual and specific when described clearly.

What a Positive Means

A true episode of the knee being stuck, with somebody having to free it, indicates a mechanical block and that is the finding that matters.

A springy block to passive extension indicates something displaced in the joint now, and that is urgent. 4.7.5 covers the end feel.

Intermittent catching without a block suggests an unstable flap or a loose fragment that is not currently interposed. 4.17.3 covers osteochondral lesions.

A description of locking that turns out to mean stiffness is not a mechanical symptom, and recording it as one sends the patient towards an operation they do not need. 4.2.5 covers that trap.

Giving way with a mechanical description, in a knee with normal ligaments, can be a fragment rather than instability. 4.2.6 covers giving way.

MRCS Level

How It Is Performed

In a viva this is a history station and the expected answer refuses the word locking until the patient has described what happens.

The expected points are the patient's own description, the history of a true block, the position of any catch, the passive extension with its end feel, and the effusion.

Say that you would not use the word locking in the question, and say why. 4.2.5 covers it.

Say that a springy end feel with a current block is an urgent arthroscopy, which is the clinically consequential finding. 4.9.8 covers mechanical locking.

Say that a mechanical symptom in a knee with a cartilage lesion changes the decision, where pain alone frequently does not. 4.17.5 covers that argument.

What a Positive Means

The reasoning expected is that a displaced fragment physically obstructs the joint's movement, so the symptom is a mechanical event rather than a pain response. 4.7.5 covers the end feel it produces.

A fragment that is unstable but not displaced catches intermittently, which is why the symptom is episodic and the examination between episodes is normal.

An osteochondral fragment is stiffer than a meniscal one, which is why its block can feel firmer and why the end feel is read rather than assumed. 4.7.5 covers that reading.

Giving way from a fragment is a reflex response to a sudden mechanical event rather than a ligamentous failure, which is why the ligament examination is normal. 4.2.6 covers giving way.

How Good It Is

The history is the diagnostic instrument here and the examination confirms it only when a fragment is currently displaced.

A clear description of a true block is specific and is the most useful information available.

A normal examination between episodes is expected and excludes nothing, which is why the history carries the weight. 4.2.5 covers it.

Imaging identifies the fragment, and this is a case where the history, the examination and the imaging each contribute a different part. 4.1.6 covers that relationship.

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 a mechanical symptom is what converts a cartilage lesion into an operation, and pain alone frequently does not.

A current springy block is an urgent arthroscopy, and the examination makes that decision. 4.9.8 covers mechanical locking.

A clear history of true locking episodes with an unstable lesion on imaging is a surgical indication, and the examination's contribution is establishing that the locking is real. 4.17.3 covers the lesion.

Pain without mechanical symptoms in a patient with a lesion is a much weaker indication, and the alignment and the functional assessment carry that decision instead. 4.17.6 covers the functional side.

A reported mechanical symptom that turns out to be stiffness has sent patients to arthroscopy, which is the specific error this page exists to prevent. 4.2.5 covers it.

Where It Misleads

The commonest error is accepting the word locking without a description.

The second is reading an extension deficit as a block without the end feel. 4.7.5 covers it.

The third is attributing giving way to a fragment without examining the ligaments.

The fourth is expecting the examination to be abnormal between episodes.

The fifth is operating on an imaging finding in a patient whose mechanical history does not hold up.

Fellowship Level

Where It Misleads

In referred practice the mechanical symptom is the claim that has to be re-examined, because it is the claim an operation was built on.

A patient with a normal arthroscopy after reported locking had a symptom that was not mechanical, and the description rather than the imaging was where that should have been caught. 4.2.5 covers it.

A patient with recurrent true locking after a cartilage procedure has an unstable graft or an unaddressed fragment, and that is a different problem from pain. 4.20.5 covers that assessment.

A patient with locking and a previous meniscectomy may have a meniscal rather than a cartilage cause, and the two coexist in the same compartment. 4.9.10 covers meniscal deficiency.

A patient whose locking episodes stopped after an operation and whose pain did not has had the mechanical problem treated and the pain problem left, which is a result worth framing before the operation. 4.17.5 covers the defect.

In medicolegal work a recorded description of a true block is useful evidence, and a recorded word locking without a description is not. 4.23.7 covers that setting.