Normal Meniscus

For Medical Students

What It Shows

A normal meniscus is uniformly dark on every sequence, because dense circumferential collagen holds almost no mobile water and produces almost no signal. 2.5.5 covers its MRI anatomy.

It appears as a triangle on the coronal and sagittal images through the body, with the apex pointing into the joint and the base at the capsule. 2.5.1 covers its gross anatomy.

The useful discipline is knowing this appearance well enough that a departure from it is obvious, since every finding in this chapter is a departure. 5.6.2 covers signal grading.

How It Is Acquired

The meniscus is assessed on the sagittal and coronal images together, because a tear visible in one plane is often invisible in the other. 5.5.1 covers the standard protocol.

Proton density and intermediate weighted sequences show the meniscus best, since they give signal to a tear without flooding the image with fluid signal. 5.2.15 covers proton density imaging.

Fluid sensitive sequences are read alongside them, because a tear filled with fluid is conspicuous on those and a tear that is not is better seen on the proton density images. 5.2.16 covers fat suppression.

Thin slices matter more here than almost anywhere else, since the posterior horn and the roots occupy a small volume. 5.5.2 covers the sports knee protocol.

The slice thickness and the plane are the two technical facts to check before calling a meniscus normal. 5.1.6 covers imaging pitfalls.

What Normal Looks Like

A uniformly low signal triangle in cross section, with sharp margins and no internal signal reaching a surface.

The anterior and posterior horns are seen on the sagittal images, and on the peripheral sagittal images the two horns appear joined, which is the bow tie appearance of a normal body rather than a lesion. 2.5.1 covers its gross anatomy.

The posterior horn of the medial meniscus is larger than the anterior horn, which is normal and is the opposite of the lateral side. 2.6.1 covers the lateral meniscus gross anatomy.

The roots attach near the centre of the tibia and are small, oblique and easy to mistake for a tear. 2.5.7 covers the root attachments.

MRCS Level

How It Is Acquired

In a viva the expected answer names the sequences and the planes and says why each is there.

The expected points are the two planes, the proton density or intermediate weighted sequence for the meniscus itself, the fluid sensitive sequence alongside, the slice thickness, and the structures that need the thinnest slices.

Say that a meniscus is assessed in two planes and that a tear in one plane may not appear in the other. 5.5.1 covers the standard protocol.

Say that proton density weighting is the workhorse for the meniscus and give the reason. 5.2.15 covers proton density imaging.

Say that the roots and the posterior horn need thin slices. 5.6.9 covers root tears.

What Normal Looks Like

The reasoning expected is that signal on magnetic resonance imaging comes from mobile protons, and dense collagen has almost none, so a normal meniscus is dark because of what it is made of. 2.5.2 covers the histology.

Any internal signal therefore represents something other than intact collagen: fluid, degenerate tissue, vessels in a child, or an artefact. 5.6.2 covers signal grading.

The collagen is arranged circumferentially, which is why a tear that runs along those fibres behaves differently from one that cuts across them. 3.6.3 covers the circumferential fibres.

The meniscus has a peripheral blood supply and an avascular inner portion, which is why the position of a tear predicts whether it can heal. 2.5.3 covers the vascular supply.

How Good It Is

Magnetic resonance imaging assesses the meniscus better than any other modality, and better than the clinical examination.

Its best contribution is the normal study, because a confidently normal meniscus on an adequate protocol is strong evidence against a tear. 4.9.3 covers the McMurray test.

It is least reliable at the roots, in the post operative knee and where a previous partial meniscectomy has changed the shape. 5.6.12 covers the post operative meniscus.

No figures appear for any accuracy comparison on this page, under decision 028, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level the normal study is the one that changes management most often, because it removes the meniscus from the differential.

A normal meniscus with a positive examination sends the assessment elsewhere, and the cartilage and the ramp area are the two places to look next. 5.6.10 covers ramp lesions.

A normal meniscus in an older knee does not exclude the symptoms being meniscal in origin, because a degenerate meniscus can be symptomatic without a displaceable tear. 5.6.2 covers signal grading.

An inadequate protocol produces a falsely normal meniscus, so the technical check precedes the clinical conclusion. 5.1.6 covers imaging pitfalls.

Where It Misleads

The commonest error is calling a meniscus normal on a protocol that could not show a tear, which thick slices and a single plane both cause. 5.5.1 covers the standard protocol.

The transverse ligament crossing the anterior horn mimics a tear in one plane and resolves in the other. 2.6.3 covers the meniscofemoral ligaments.

The popliteal hiatus interrupts the lateral meniscus normally, and the interruption is read as a tear. 2.6.2 covers the popliteomeniscal fascicles.

A normal vascular meniscus in a child carries internal signal that would be abnormal in an adult. 5.15.1 covers the normal paediatric anatomy.

The magic angle effect raises signal in collagen oriented at an angle to the field and is a specific, nameable cause of false signal. 5.2.20 covers the magic angle effect.

Fellowship Level

Where It Misleads

At fellowship level the normal meniscus is the hardest call rather than the easiest, because the knees that reach a tertiary clinic have been operated on or are degenerate.

A meniscus after partial meniscectomy is not shaped like a normal one, so the reference appearance does not apply and intact remaining tissue reads as abnormal. 5.6.12 covers the post operative meniscus.

A degenerate meniscus in an older knee is diffusely abnormal without a displaceable tear, which makes normal and abnormal a spectrum rather than a distinction. 5.6.2 covers signal grading.

Protocol differences between centres change what counts as a confidently normal meniscus, and the report does not say which protocol it was. 5.5.1 covers the standard protocol.

The roots and the ramp area are the two sites where a normal report most often hides a lesion, and both need to be looked at deliberately. 5.6.9 covers root tears.

The appearance is read against an age and a surgical history, so a normal meniscus is a statement about a particular knee rather than a general one. 4.9.11 covers the examination in the worn knee.