Menisci

For Medical Students

What It Shows

The menisci are two fibrocartilaginous structures between femur and tibia, appearing uniformly dark on every routine sequence. 5.6.1 covers the normal meniscus.

They are triangular in cross section at the body and thinner at the free edge. 2.5.1 covers the medial meniscus gross anatomy.

It matters because uniform darkness is the normal state, so any internal signal is the finding, and several normal appearances imitate it. 5.6.2 covers meniscal signal grading.

How It Is Acquired

Assess on the intermediate weighted sequence, which gives the internal detail. 5.2.15 covers proton density imaging.

Assess in the sagittal and coronal planes, and confirm any finding on both. 5.5.1 covers the standard protocol.

Follow each horn to its root attachment, since the roots are at the edge of attention. 5.6.9 covers root tears.

Assess the position of the body relative to the tibial margin. 5.6.11 covers meniscal extrusion.

Know the normal appearances that imitate a tear: the transverse ligament, the popliteal hiatus and the meniscofemoral ligaments. 5.6.1 covers the normal meniscus.

What Normal Looks Like

Uniformly dark triangular menisci on every sequence. 5.6.1 covers the normal meniscus.

A medial meniscus with a larger posterior horn than anterior, and a more symmetrical lateral meniscus. 2.6.1 covers the lateral meniscus gross anatomy.

The transverse ligament crossing anteriorly, which produces a line at its junction with the anterior horn. 2.5.1 covers the medial meniscus gross anatomy.

A fluid filled popliteal hiatus laterally, which separates the meniscus from the capsule normally. 2.6.1 covers the lateral meniscus gross anatomy.

MRCS Level

How It Is Acquired

In a viva this is the structure to name when asked what normal appearances imitate pathology.

The expected points are the intermediate weighting, the two planes, the roots, the position, and the normal imitators.

Say that the transverse ligament produces a pseudo-tear at the anterior horn. 5.6.1 covers the normal meniscus.

Say that the popliteal hiatus normally separates the lateral meniscus from the capsule. 2.6.1 covers the lateral meniscus gross anatomy.

Say that a finding on one plane only is treated with caution. 5.5.1 covers the standard protocol.

What Normal Looks Like

The reasoning expected is that a meniscus is dense organised collagen with little free water, so it returns almost no signal, and that is why any signal within it is interpretable at all. 2.5.2 covers the medial meniscus histology.

The normal imitators are all structures crossing or abutting the meniscus, so their junction with it produces an interface that looks like a line. 5.6.1 covers the normal meniscus.

Knowing where those junctions are is therefore a prerequisite, because the appearance is identical to a tear and only the location distinguishes them. 5.6.3 covers horizontal meniscal tears.

The roots are small, obliquely oriented and at the edge of the usual visual sweep, which is why they need to be sought deliberately. 5.6.9 covers root tears.

How Good It Is

Magnetic resonance imaging reads the normal meniscus reliably, which is what makes the abnormal one interpretable.

Its best contribution is that a uniformly dark meniscus on two planes is a strong normal. 5.6.1 covers the normal meniscus.

It is limited by the normal imitators, by the roots, by the posteromedial junction and by intermediate signal in a degenerate meniscus having no agreed meaning. 5.6.2 covers meniscal signal grading.

No figures appear for accuracy on this page, under decision 028, and no signal or extrusion threshold values appear either: those are published quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level the normal appearance is what prevents an unnecessary arthroscopy.

A uniformly dark meniscus in a painful knee moves the question elsewhere, which is a useful negative. 5.6.1 covers the normal meniscus.

A pseudo-tear at a known imitator site is a normal meniscus, and recognising it avoids an operation. 5.6.3 covers horizontal meniscal tears.

A degenerate meniscus with intermediate signal is the common ambiguity, and it sits in most knees over a certain age. 5.6.8 covers complex meniscal tears.

Extrusion is the mechanically informative measurement and belongs in the assessment of every degenerate meniscus. 5.6.11 covers meniscal extrusion.

Where It Misleads

The commonest error is reporting a normal imitator as a tear. 5.6.1 covers the normal meniscus.

The roots are not followed, so a root tear is missed. 5.6.9 covers root tears.

Intermediate signal in a degenerate meniscus is resolved towards a tear. 5.6.2 covers meniscal signal grading.

Extrusion is not measured. 5.6.11 covers meniscal extrusion.

A single plane finding is reported without confirmation. 5.5.1 covers the standard protocol.

Fellowship Level

Where It Misleads

At fellowship level the normal meniscus is read well and the degenerate one is the problem, which is a statement about the population being imaged.

A young knee's meniscus is uniformly dark and a tear in it is conspicuous, so the imaging performs best in the patients whose management is least ambiguous. 5.6.4 covers vertical meniscal tears.

An older knee's meniscus carries intermediate signal with no agreed meaning, and that is the population in whom a meniscal report most often leads to an intervention that the trial evidence does not support. 5.6.8 covers complex meniscal tears.

The grading conventions were derived on particular weightings and are applied across protocols, so the ambiguity is partly a protocol artefact. 5.2.15 covers proton density imaging.

Extrusion is the finding with the clearest mechanical meaning and the least consistent reporting, which this section's meniscal chapter argues at length. 5.6.11 covers meniscal extrusion.

The disciplined report names the imitators it excluded, reports intermediate signal as intermediate, and measures extrusion. 5.20.2 covers reporting meniscal tears.