Meniscal Extrusion

For Medical Students

What It Shows

Extrusion is the meniscal body lying beyond the edge of the tibial plateau rather than within the joint. 5.6.1 covers the normal meniscus.

It happens when the circumferential tension that holds the meniscus in place is lost, which a root tear, a complete radial tear or diffuse degeneration all cause. 3.6.10 covers meniscal extrusion.

It matters because it is the imaging finding that corresponds most directly to the meniscus not working. 3.6.2 covers hoop stress.

How It Is Acquired

Measure on the coronal image through the mid body of the meniscus, where the tibial margin is a clear landmark. 5.5.1 covers the standard protocol.

Measure from the edge of the tibial plateau to the outer edge of the meniscus, excluding osteophyte from the tibial landmark. 5.3.18 covers osteophytes.

Record which compartment, because medial extrusion is much more common and carries the clearer association with cartilage loss. 5.11.2 covers chondral defects.

Look for the cause in the same study: the roots, a complete radial tear, or diffuse degeneration. 5.6.9 covers root tears.

Note that the measurement is made on an unloaded supine scan, which understates what happens standing. 5.18.5 covers weight bearing imaging.

What Abnormal Looks Like

The meniscal body sitting outside the tibial margin on the coronal image, with a measurable distance between the two.

A meniscus that appears thinned or absent within the joint, because it is elsewhere.

Cartilage loss and subchondral signal change in the same compartment, which extrusion precedes and accompanies. 5.11.7 covers bone marrow lesions.

A root tear, a complete radial tear or a degenerate meniscus as the cause, which should be sought rather than assumed. 5.6.9 covers root tears.

MRCS Level

How It Is Acquired

In a viva this is the finding to name when asked what to measure in a degenerate meniscus.

The expected points are the coronal mid body image, the tibial landmark, the exclusion of osteophyte, the compartment, the cause, and the supine limitation.

Say that you would look for the cause of the extrusion rather than reporting it alone. 5.6.9 covers root tears.

Say that osteophyte makes the tibial landmark unreliable. 5.3.18 covers osteophytes.

Say that a supine measurement understates the loaded position. 5.18.5 covers weight bearing imaging.

What Abnormal Looks Like

The reasoning expected is that the meniscus is held in the joint by its own circumferential tension acting against the compressive load, so losing that tension lets the load push it outwards. 3.6.2 covers hoop stress.

Once extruded the meniscus is not between the surfaces, so the compartment loads a smaller area at higher pressure whatever the meniscus looks like. 3.6.5 covers load sharing.

That is why extrusion tracks the compartment's state better than the tear morphology, which is this chapter's recurring point. 5.6.8 covers complex tears.

Extrusion is reversible in principle if the anchor is restored, and in practice repair does not reliably reverse it. 5.6.13 covers meniscal repair imaging.

How Good It Is

Extrusion is measured reproducibly on a given protocol and compared poorly across protocols and centres.

Its best contribution is mechanical: it says the meniscus is not doing its job, which no tear description does as directly. 3.6.10 covers meniscal extrusion.

It is limited by the supine position, by the osteophyte landmark problem, and by the absence of an agreed threshold. 5.18.5 covers weight bearing imaging.

No figures appear for any accuracy comparison on this page, under decision 028, and no millimetre threshold appears either: the conventional cut off is a published quantity and this environment cannot check it.

FRCS Level

How Good It Is

At decision level extrusion is prognostic information that is available and usually unused.

Extrusion with a root tear and preserved cartilage identifies the knee where restoring the anchor might protect the compartment. 5.6.13 covers meniscal repair imaging.

Extrusion with established cartilage loss identifies a compartment already past that point, and the decision becomes the compartment's. 5.3.16 covers Kellgren-Lawrence grading.

Extrusion with a varus limb is a mechanical argument for addressing the alignment, because the load is what drives both. 5.3.7 covers the mechanical axis.

Extrusion without a nameable cause suggests diffuse degeneration, which is a different conversation from a focal lesion. 5.6.8 covers complex tears.

Where It Misleads

The commonest error is not measuring it, so the most mechanically informative finding goes unreported. 5.20.2 covers reporting meniscal tears.

Osteophyte is included in the tibial margin, which understates the extrusion. 5.3.18 covers osteophytes.

A supine measurement is treated as the loaded state. 5.18.5 covers weight bearing imaging.

Extrusion is reported without its cause, so a treatable root tear is missed behind it. 5.6.9 covers root tears.

A threshold is quoted from memory, which is why none appears on this page. 5.21.1 covers the meniscal imaging literature.

Fellowship Level

Where It Misleads

At fellowship level extrusion is the best mechanical marker the chapter has and the least standardised.

The measurement depends on which coronal slice is chosen, and the mid body is defined by eye, so two readers measure different distances on the same study. 5.20.8 covers common reporting errors.

The tibial landmark is corrupted by osteophyte in exactly the knees where extrusion matters most. 5.3.18 covers osteophytes.

A supine scan measures the unloaded position, and the clinically relevant quantity is what happens standing, which almost nobody images. 5.18.5 covers weight bearing imaging.

Thresholds in the literature come from specific cohorts and protocols, and this page states none for that reason. 5.21.1 covers the meniscal imaging literature.

Whether reducing extrusion changes a compartment's course is the question the whole root repair field rests on, and it is not answered. 5.6.13 covers meniscal repair imaging.