Discoid Meniscus

For Medical Students

What It Tests

A discoid meniscus is almost always lateral, and it presents in a child as a snapping knee, a block to full extension, or lateral joint line pain. 4.9.10 covers meniscal deficiency.

The examination is deciding whether there is a mechanical abnormality on the lateral side and whether the knee reaches full extension. 4.5.4 covers the flexion deformity.

Many discoid menisci never cause symptoms, so the examination is looking for the consequences rather than for the shape. 2.6.1 covers the lateral meniscus anatomy.

How It Is Performed

Take the history: the age, whether the knee snaps audibly, whether it has ever locked, and whether the child can straighten it fully. 4.2.7 covers mechanical symptoms.

Ask the parent as well as the child, since a snapping knee is often noticed by an observer first.

Watch the child walk and squat, because the snap frequently appears during movement rather than on testing. 4.4.10 covers the squat.

Measure the extension carefully, comparing with the other side, since a small deficit is the common finding. 4.7.2 covers passive motion.

Take the knee through the range with a hand on the lateral joint line, feeling for a palpable clunk. 4.6.1 covers systematic palpation.

Palpate the lateral joint line for tenderness. 4.6.4 covers joint line tenderness.

Perform the meniscal provocation tests, accepting that they are harder to interpret in children. 4.9.3 covers the McMurray test.

What a Positive Means

A palpable or audible clunk on the lateral side through the range, with or without an extension deficit, is the characteristic finding.

An extension deficit in a child with a snapping knee suggests an unstable discoid meniscus rather than a simple variant. 4.5.4 covers the flexion deformity.

Lateral joint line tenderness with a snap suggests a tear within the discoid meniscus, which changes the management. 4.9.10 covers meniscal deficiency.

A snap without pain, without a deficit and without an effusion can be a symptomless variant, and the examination's conclusion is that no intervention follows from it. 2.6.1 covers the lateral meniscus anatomy.

Bilateral snapping raises a bilateral variant, which is common. 4.1.4 covers the structured approach.

MRCS Level

How It Is Performed

In a viva this is the snapping knee in a child and the expected answer names the lateral side and the extension deficit early.

The expected points are the age, the parental history, the observed gait and squat, the extension measurement, the palpated range, the lateral joint line, the meniscal tests and the other knee.

Say that you would examine both knees, since the variant is often bilateral. 4.1.4 covers the structured approach.

Say that you would measure extension against the other side rather than against a textbook value. 4.7.2 covers passive motion.

Say that the examination cannot confirm the diagnosis and that imaging does. 4.1.6 covers that relationship.

What a Positive Means

The reasoning expected is that a discoid meniscus covers more of the lateral plateau than a normal meniscus, so it is caught between the condyle and the plateau during movement, which produces the snap. 3.5.3 covers load distribution.

An unstable discoid meniscus displaces, which is why it blocks extension and why the deficit matters more than the snap.

The abnormal shape also makes it thicker and more prone to tearing within its substance, which is why pain accompanies the mechanical findings. 4.9.10 covers meniscal deficiency.

A symptomless variant transmits load differently and is still a variant rather than a lesion, which is why the examination's negative findings matter. 2.6.1 covers the lateral meniscus anatomy.

How Good It Is

The examination detects the mechanical consequences well when they are present and says nothing about the shape.

Its best contribution is the extension deficit and the palpable clunk, both of which are specific in a child of this age. 4.5.4 covers the flexion deformity.

It cannot grade the variant or detect a tear within it.

Magnetic resonance imaging shows the shape and the tear, so the examination raises the question and the imaging answers it. 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 the question is whether the discoid meniscus is stable, unstable or torn, and the examination contributes to the first two.

A stable symptomless variant needs no treatment, and the examination is what establishes that there are no consequences. 2.6.1 covers the lateral meniscus anatomy.

An unstable variant with an extension deficit is a different decision, and the deficit is clinical. 4.5.4 covers the flexion deformity.

A torn discoid meniscus with lateral pain and an effusion moves towards intervention, and the examination raises it. 4.9.10 covers meniscal deficiency.

Lateral compartment status in a knee with a long standing discoid meniscus is examined, because the altered load transmission has consequences over time. 4.18.4 covers the valgus worn knee.

Where It Misleads

The commonest error is attributing a child's knee snap to a discoid meniscus when a patellar or a tendon snap produces the same sound. 4.15.10 covers patellofemoral arthritis.

A small extension deficit is missed when the knee is not compared with the other side. 4.7.2 covers passive motion.

Meniscal provocation tests are less interpretable in children, and a positive result is given more weight than it deserves. 4.9.3 covers the McMurray test.

A symptomless variant found on imaging is treated as the cause of unrelated pain, and the examination is the check against that. 2.6.1 covers the lateral meniscus anatomy.

A bilateral finding is read as normal because it is symmetrical. 4.1.4 covers the structured approach.

Fellowship Level

Where It Misleads

At fellowship level the difficulty is deciding whether a known discoid meniscus is the source of the symptoms, and the examination is the main instrument.

The variant is common enough and often symptomless, so its presence on imaging does not establish causation and the clinical findings have to do that work. 2.6.1 covers the lateral meniscus anatomy.

An extension deficit, a clunk and lateral tenderness together make a convincing case, and any one of them alone does not. 4.5.4 covers the flexion deformity.

A previously treated discoid meniscus examines differently, and the baseline for the examination is the post operative knee rather than normal. 4.20.2 covers the repaired meniscus.

Long term lateral compartment change after a discoid meniscus produces degenerative findings that overlie the mechanical ones. 4.18.4 covers the valgus worn knee.

The examination cannot assess the peripheral attachment, which is the finding that determines whether the meniscus is stable. 4.9.10 covers meniscal deficiency.