MRI Anatomy

For Medical Students

Structure and Location

A normal lateral meniscus is uniformly dark on every sequence, like the medial. The appearance conventions on 2.5.5 apply to both.

Two differences are visible. Its anterior and posterior horns are of similar size, whereas the medial posterior horn is clearly larger than its anterior horn. And it is more nearly circular, so it appears as a continuous band on more sagittal slices.

On a coronal image through the mid joint, the lateral meniscus appears as a dark wedge at the lateral margin, slightly thicker than its medial counterpart at the same level.

Relations

The popliteus tendon appears as a dark round structure immediately posterolateral to the posterior horn, in its sheath. The gap between the two is the popliteus hiatus and it is a normal finding.

The meniscofemoral ligaments are seen as small dark bands running from the posterior horn towards the medial femoral condyle, in front of and behind the posterior cruciate.

The transverse intermeniscal ligament crosses in front of both anterior horns.

The lateral collateral ligament is seen peripherally and does not attach to the meniscus.

Function

The imaging questions are the same as for the medial meniscus: intact or torn, where, and whether the tear reaches a surface.

One extra question applies here. A lateral meniscus that is too large or covers too much of the plateau is discoid, and that is assessed on the coronal and sagittal images together.

Clinical Relevance

Four normal structures around the lateral posterior horn can mimic a tear: the popliteus tendon and hiatus, the two meniscofemoral ligaments, and the transverse ligament anteriorly. Knowing them by name is most of what prevents overcalling.

Criteria for calling a tear, for grading signal and for diagnosing a discoid meniscus belong to 5.6 and 5.6.2.

MRCS Level

Structure and Location

Count the bow ties here too, but expect more of them than on the medial side, because the meniscus is wider relative to its plateau and more nearly circular.

On sagittal images the posterior horn is seen with the popliteus tendon behind it and the hiatus between. Moving medially, the meniscofemoral ligaments appear alongside the posterior cruciate.

On coronal images follow the meniscus front to back as on the medial side, and assess the ratio of meniscal width to plateau width, which is how discoid shape is judged. The criteria belong to 5.6.

The anterior horn is seen close to the anterior cruciate footprint, and the two can appear continuous on the slices where the root attaches.

Relations

The popliteus hiatus is the structure most often reported as a tear. It is a gap in the capsular attachment, it contains tendon and fluid, and it is normal. Its position posterolateral to the posterior horn is the identifying feature.

The meniscofemoral ligaments produce an oblique line adjacent to the posterior horn, the pseudotear. Tracing the structure to the medial femoral condyle identifies it.

The transverse intermeniscal ligament inserting on the anterior horn produces the same pitfall anteriorly as it does medially.

The popliteomeniscal fascicles are thin and inconsistently seen. Their absence on imaging is not a reliable sign of a fascicle tear, which is why hypermobility is diagnosed arthroscopically.

Blood Supply and Innervation

As on the medial side, vascularity is not directly assessable. The watershed at the hiatus is anatomical rather than visible.

Peripheral enhancement after injury may suggest a vascular response, and the zone is still assessed at arthroscopy.

Intrasubstance signal reflects water content and tissue change rather than perfusion, and it increases with age in both menisci.

Function

Sagittal images define which part of the ring is involved; coronal images define radial extent, peripheral attachment and discoid shape.

The region that most needs good imaging is the posterolateral periphery, because it is the region the arthroscope sees worst and the region with the most normal structures to confuse.

Thin slices matter more here than on the medial side, for the same reason.

Clinical Relevance

Learn the four pitfalls and trace each structure to its other end before calling a tear. The hiatus to the popliteus muscle, the meniscofemoral ligaments to the medial condyle, the transverse ligament to the opposite anterior horn.

A discoid meniscus is a shape diagnosis made across both planes and is commonly associated with abnormal peripheral attachment, which imaging assesses poorly.

A lateral posterior root tear is harder to detect than a medial one, because extrusion is held in check by the other tethers. Suspicion based on the cruciate injury matters more than the imaging sign.

Signal grading, tear criteria and discoid classification all belong to 5.6 and 5.6.2. This page stops at the normal appearance, which is what the architecture assigns it.

FRCS Level

Relations

The posterolateral periphery is where imaging and arthroscopy disagree most often, because imaging sees it better than the arthroscope does while also containing the structures most likely to be misread.

That combination means the imaging report and the arthroscopic finding should each be read in the light of the other rather than one overruling the other.

Function

Imaging decides whether to operate and informs the plan; it does not decide the operation on this side, because peripheral attachment and mobility are not reliably assessable.

A plan for a lateral meniscal procedure should therefore include the possibility of finding an attachment problem rather than a tear.

Clinical Relevance

Before lateral meniscal surgery, assess shape, root integrity and the posterolateral periphery, and plan to test mobility whatever the imaging says.

In a young patient with lateral symptoms and normal imaging, the differential includes a hypermobile meniscus, and imaging will not settle it.

In discoid meniscus, expect the peripheral attachment to be abnormal more often than imaging suggests, and plan stabilisation as a possibility rather than a surprise.

No measurement appears on this page. Every discoid criterion and every signal threshold is a measured value owned by Section 5.