MRI Anatomy

For Medical Students

Structure and Location

On magnetic resonance imaging a normal meniscus is uniformly dark on every sequence. It contains little free water, so it returns almost no signal.

On a sagittal image through the body of the meniscus the anterior and posterior horns appear as two dark triangles, one at each end of the plateau, with their apices pointing towards the centre of the joint.

On a sagittal image through the periphery the meniscus appears instead as a continuous dark band, the bow tie, because the slice passes through the body rather than the horns.

On a coronal image the meniscus appears as a dark wedge at the margin of the joint on each side, thick peripherally and tapering centrally.

Relations

The dark meniscus sits between the bright cartilage of the femoral condyle above and the plateau below, which is what makes it easy to identify.

Peripherally it is continuous with the capsule, and the deep fibres of the medial collateral ligament attach to it. On a coronal image the superficial collateral is seen as a separate dark band further out.

The posterior horn of the medial meniscus is larger than the anterior horn, which is a normal asymmetry and the first thing to check when deciding which meniscus a slice shows.

Function

Imaging exists here to answer whether the meniscus is intact, where a tear is and whether it reaches the surface. Knowing the normal appearance is what makes any of that possible.

The sagittal and coronal planes answer different questions. Sagittal shows the horns and the anteroposterior extent; coronal shows the radial width, the peripheral attachment and whether the meniscus has extruded.

Clinical Relevance

A uniformly dark meniscus of normal shape with a normal peripheral attachment is normal, and that is most of what this page is for.

Tear signal grading and the criteria for calling a tear belong to 5.6.2. This page stops at normal.

MRCS Level

Structure and Location

Count the bow ties. Because the meniscus is a continuous ring at its periphery, consecutive sagittal slices through the outer part of the joint show a continuous band, and the number of such slices reflects the radial width of the meniscus at that point.

Moving centrally, the band becomes two triangles as the slice passes inside the free edge of the body and catches only the horns.

On the coronal images, follow the meniscus from front to back: the anterior horn, the body at the mid coronal slice, then the posterior horn, with the peripheral attachment visible at each level.

The horn attachments are seen on the sagittal images closest to the midline and on the coronal images through the intercondylar area. 2.5.7 covers the roots.

Relations

The deep medial collateral fibres blend with the meniscus on coronal images, so there is no clear line between them. The superficial collateral is a discrete band further peripherally with a bursa between the two layers.

The transverse intermeniscal ligament crosses in front of the joint and is seen on sagittal images as a small dark structure just anterior to the anterior horns. Where it inserts, it can mimic a tear of the anterior horn, which is a recognised pitfall.

The meniscal periphery and the tibial margin should be close. A gap between them on a mid coronal image is meniscal extrusion.

Posteriorly the semimembranosus and the posterior oblique fibres lie outside the capsule next to the posterior horn.

Blood Supply and Innervation

Vascularity is not directly visible in the meniscus on standard sequences. The perimeniscal plexus lies in tissue that appears as capsule rather than as vessels.

Contrast enhancement at the periphery after injury reflects the vascular zone and is one of the few ways the zones can be inferred radiologically, but the zone is still assessed at arthroscopy.

Signal change within the meniscus reflects water content rather than perfusion, which is why intrasubstance signal in a middle aged meniscus is a tissue change rather than a vascular one.

Function

The two planes together answer the anatomical questions. Sagittal defines which part of the ring is abnormal; coronal defines how far across the width and whether the peripheral attachment is intact.

Thin slices and good positioning matter more here than field strength, because the structures are small and the pitfalls are partial volume effects.

Normal variants have to be known before abnormality can be called, and the three that matter on the medial side are the intermeniscal ligament insertion, the oblique meniscomeniscal ligament where present, and a prominent peripheral vascular cleft.

Clinical Relevance

Meniscal extrusion is assessed on the mid coronal image and is a functional finding: it implies the hoop has failed, usually at the root.

Partial volume effects at the free edge and at the horns produce apparent signal that is not a tear, which is why a finding on one slice in one plane is treated with suspicion.

The intermeniscal ligament pitfall is worth knowing by name, because it produces an apparent anterior horn tear in a normal knee.

Tear criteria, signal grading and the accuracy of each sign are owned by 5.6 and 5.6.2. This page is the normal appearance only, which is what the architecture assigns it.

FRCS Level

Relations

The relation worth checking before operating is the peripheral one. A tear whose peripheral attachment is intact and which sits close to the margin is probably repairable, and the coronal images answer that better than the arthroscopy report of the previous surgeon.

In a previously operated meniscus, imaging cannot reliably distinguish a healed repair from a recurrent tear, because both show signal reaching a surface. That limit is anatomical rather than technical.

Function

Imaging sets the plan and arthroscopy sets the operation. Planning repair from imaging and finding unrepairable tissue is a common and manageable sequence; planning resection and finding repairable tissue without the instruments is not.

That asymmetry is why the imaging question before a meniscal operation is where and how peripheral rather than whether torn.

Clinical Relevance

Assess extrusion, root integrity, rim width and tear location before arthroscopy, and plan for repair wherever those four favour it.

In the degenerate knee, imaging findings in the meniscus are common and frequently incidental. The anatomical point is that intrasubstance change is a property of an ageing meniscus rather than a lesion.

Where imaging suggests root pathology, 2.5.7 is the anatomy and 7.14 the procedure.

No measurement appears on this page. Rim width, extrusion distance and every signal criterion are measured values owned by Section 5, and this page describes appearances rather than thresholds.