Snapping Tendons

For Medical Students

What It Shows

A snapping tendon moves abruptly over a bony prominence during knee movement, producing a palpable or audible snap. 5.5.10 covers the tendons.

The abnormality is in the movement rather than in the tendon, so a static study of a normal looking tendon is the expected finding. 5.4.3 covers the lateral knee.

It matters because this is the clearest case in knee imaging of a diagnosis that no static modality can make at all. 5.1.3 covers choosing the correct investigation.

How It Is Acquired

Place the probe over the symptomatic area and ask the patient to reproduce the movement. 5.2.6 covers sound waves.

Watch the tendon through the range rather than imaging it at one position. 5.4.15 covers dynamic instability.

Ask the patient to say when the snap happens, and correlate it with what the screen shows. 5.4.15 covers dynamic instability.

Identify the tendon and the prominence it crosses, which names the lesion. 2.10.5 covers the biceps femoris.

Record a video clip where the equipment allows, because a still image cannot show movement. 5.20.1 covers structured reporting.

What Normal Looks Like

A tendon gliding smoothly over bone through the range, with no abrupt displacement.

A normal fibrillar tendon structure at every position. 5.5.10 covers the tendons.

No fluid or thickening around the tendon. 5.5.12 covers the bursa.

No correlation between the patient's symptom and anything on the screen.

What Abnormal Looks Like

An abrupt displacement of the tendon over a bony prominence, coinciding with the patient's symptom. 2.10.5 covers the biceps femoris.

Thickening of the tendon or its sheath at the point of contact. 5.5.12 covers the bursa.

A bony prominence or an osteochondroma at the contact point. 5.13.7 covers bone tumours.

An abnormal or accessory tendon slip taking an unusual course. 2.10.5 covers the biceps femoris.

MRCS Level

How It Is Acquired

In a viva this is the diagnosis to name when asked what only ultrasound can do.

The expected points are the patient reproducing the movement, watching through range, the symptom correlation, naming the structures, and recording a clip.

Say that the symptom and the screen must coincide. 5.4.15 covers dynamic instability.

Say that a static magnetic resonance study will be normal. 5.1.3 covers choosing the correct investigation.

Say that a bony prominence may be the cause and is visible. 5.13.7 covers bone tumours.

What Abnormal Looks Like

The reasoning expected is that the snap is a mechanical event in time, so an instrument that samples one instant cannot capture it however good its resolution. 5.2.6 covers sound waves.

Real time imaging samples continuously, which makes the temporal correlation between the patient's symptom and the tendon's displacement the diagnostic observation. 5.4.15 covers dynamic instability.

The cause is usually a geometric one, an unusually prominent bone or an abnormally coursing tendon, so finding the cause follows from finding the event. 2.10.5 covers the biceps femoris.

A snap without a visible displacement points elsewhere, which is a useful negative because it redirects rather than reassures. 5.1.6 covers imaging pitfalls.

How Good It Is

Ultrasound is the only modality that makes this diagnosis, and its performance depends entirely on whether the symptom was reproduced.

Its best contribution is the temporal correlation, which is not available any other way. 5.4.15 covers dynamic instability.

It is limited by needing the patient to reproduce the symptom on demand, so a negative study in a patient who could not reproduce it means nothing. 5.1.6 covers imaging pitfalls.

No figures appear for accuracy on this page, under decision 028, and the comparison is the claim: this diagnosis is available to ultrasound and to no static modality, which is a comparison rather than a number.

FRCS Level

How Good It Is

At decision level a confirmed snapping tendon has a specific answer and an unconfirmed one does not.

A tendon displacing over a prominence, correlated with the symptom, identifies the structure to address. 5.4.20 covers tendon procedures.

A bony prominence as the cause may be the thing to remove, which is a Section 7 decision informed by this study. 5.13.7 covers bone tumours.

An unreproduced symptom leaves the diagnosis open, and the study should say that rather than reporting a normal tendon. 5.20.1 covers structured reporting.

The examination and the scan are best done together, because the person scanning needs to feel the snap as well as see it. 4.3.2 covers gait observation.

Where It Misleads

The commonest error is reporting a normal tendon when the symptom was not reproduced. 5.1.6 covers imaging pitfalls.

A displacement is seen without being correlated with the symptom. 5.4.15 covers dynamic instability.

A static study is requested for a dynamic question. 5.1.3 covers choosing the correct investigation.

The finding is recorded as a still image, which cannot show it. 5.20.1 covers structured reporting.

The bony cause is not sought once the event is seen. 5.13.7 covers bone tumours.

Fellowship Level

Where It Misleads

At fellowship level this is the purest demonstration of the chapter's argument, that the examination is the study.

The diagnostic observation exists only while the probe is on the knee and the patient is moving, so the entire diagnosis is held by the operator and transmitted as a sentence. 5.2.9 covers refraction.

Video capture would make it transmissible and is not standard, which means a genuinely unique diagnostic capability is recorded in the least reliable form available. 5.20.1 covers structured reporting.

The condition is uncommon enough that most operators have seen few, so the examination that makes the diagnosis is also the one with the least accumulated experience behind it. 5.4.3 covers the lateral knee.

Nothing in the imaging literature establishes performance here, because the reference standard is the same examination. 5.21.1 covers the meniscal imaging literature.

The practical answer is for the surgeon to scan with the patient present, which is the recommendation this chapter keeps arriving at from different directions. 5.4.1 covers the anterior knee.