Mechanical Symptoms

For Medical Students

What It Tests

Mechanical symptoms are the group of complaints suggesting something physical is interfering with movement: clicking, catching, grinding, a sensation of something moving, or giving way during a specific position.

Asking about them tests whether there is a structural interference, and the main job is sorting the significant from the harmless.

A painless click in a knee with full range and no swelling is usually normal. The same click with pain, swelling or a loss of range is not.

How It Is Performed

Ask whether anything in the knee clicks, catches or grinds, and then ask whether it hurts when it does.

Pain accompanying the symptom is what makes it worth pursuing.

Ask whether it happens in a particular position, because reproducibility at a specific angle suggests a structural cause.

Ask whether anything ever stops the knee moving, which is the locking question. 4.2.5 covers it.

Ask whether they can make it happen on request, since a patient who can demonstrate it gives you an examination finding rather than a history.

What a Positive Means

A painful catch reproducible at one angle suggests a flap tear, a chondral lesion or a plica. 4.17.2 covers the cartilage version.

Grinding through a range, worse on stairs and on rising, suggests patellofemoral surface change. 4.15.10 covers patellofemoral arthritis.

A sensation of something loose, intermittent and unpredictable, suggests a loose body. 4.17.3 covers osteochondral lesions.

A painless click in a young knee with no other findings usually needs reassurance rather than investigation.

MRCS Level

What a Positive Means

The expected reasoning separates mechanical symptoms by what they imply about tissue.

A catch implies something that moves into and out of the way, which is a flap or an unstable fragment.

Grinding implies two rough surfaces in continuous contact, which is cartilage loss rather than a fragment.

A block implies something interposed and not moving out of the way, which is the locked knee.

A sensation without any of these, in a knee that examines normally, is most often describing crepitus, which is common and usually unimportant.

How Good It Is

Mechanical symptoms as a group perform modestly, because the harmless versions are so common that the question has a low yield unless pain is attached.

Pain plus reproducibility at a defined angle raises the value substantially, and that combination is the useful question rather than the symptom alone.

They perform better in a young knee after an injury than in an older degenerate knee, where crepitus and clicking are near universal and tell you little.

This is the group of symptoms the arthroscopic meniscectomy trials recruited, which is worth knowing before quoting those trials in either direction. 4.9.11 covers it.

FRCS Level

How Good It Is

At decision level mechanical symptoms are the group most likely to lead to an operation that does not help, which makes careful history taking here disproportionately valuable.

The strong indication is a reproducible painful mechanical event at a defined position in a knee whose imaging shows a matching lesion.

The weak indication is diffuse grinding and clicking in a degenerate knee, where the symptoms reflect surface change that arthroscopy does not reverse.

Separating the two is a history task, because the imaging in the second case will show findings and those findings will be real and will not be the problem.

Where It Misleads

The commonest error in this chapter is treating any mechanical symptom as a surgical indication.

The second is the reverse: dismissing a genuine reproducible catch in a young knee because the imaging was reported as normal. A small flap tear or a chondral flap can be missed on imaging and found at arthroscopy.

The third is attributing mechanical symptoms to the compartment with the most impressive scan finding rather than the one the patient points at.

And symptoms described as mechanical after an operation may be hardware, a retained fragment or scar tissue rather than a new lesion. 4.20.5 covers cartilage surgery assessment.

Fellowship Level

Where It Misleads

In referred practice mechanical symptoms after surgery need a differential that includes the surgery itself.

After meniscal resection, the symptom may be the altered mechanics of a deficient meniscus rather than a new tear. 4.9.10 covers meniscal deficiency.

After cartilage surgery, a graft or a scaffold that has not integrated can produce a mechanical symptom that imaging describes poorly.

After reconstruction, a cyclops lesion produces a reproducible block near extension and is a recognised cause that is frequently attributed to stiffness instead. 4.20.1 covers the assessment.

And in a knee that has been arthroscoped more than once for mechanical symptoms without benefit, the useful conclusion is usually that the symptom was never mechanical, and the history is where that is established rather than at a further arthroscopy.

No figure here supports which features discriminate, under decision 023.