Post-operative Meniscus

For Medical Students

What It Shows

A meniscus that has been operated on no longer looks like a normal meniscus, and most of the change is expected rather than abnormal. 5.6.1 covers the normal meniscus.

The commonest operation is a partial meniscectomy, which removes the torn fragment and leaves a shorter meniscus with a cut surface. 15.24.1 covers partial meniscectomy.

It matters because the usual rule for calling a tear was written for a meniscus nobody had cut, and applying that rule to a cut meniscus over-reports failure. 5.6.2 covers meniscal signal grading.

How It Is Acquired

Find out what was done before reading the study, because the appearance cannot be interpreted without the operation note. 5.20.7 covers reporting the post-operative knee.

Compare with the pre-operative study where one exists, which converts a question about appearance into a question about change.

Use the post-operative protocol rather than the standard one, since metal artefact from fixation and the altered shape both need accommodating. 5.5.3 covers the post-operative protocol.

Assess the remaining meniscus for its size and its position as well as for signal within it. 5.6.11 covers meniscal extrusion.

Assess the compartment, because what happens to the cartilage after meniscal surgery is the outcome the patient notices. 5.11.2 covers chondral defects.

What Normal Looks Like

A shorter meniscus with a smooth, often blunted free edge, which is the cut surface rather than a tear.

Some persistent internal signal, which is expected in a meniscus that was degenerate enough to tear in the first place. 5.6.2 covers meniscal signal grading.

A meniscus that occupies less of the compartment than the other side, with no fluid signal reaching its surface.

An unchanged position within the joint, with the body still within the tibial margin. 5.6.11 covers meniscal extrusion.

What Abnormal Looks Like

Fluid signal reaching the surface of the remaining meniscus on more than one image, which is the finding that carries weight after surgery.

A displaced fragment within the joint, which is unambiguous where a signal line is not. 5.6.6 covers bucket handle tears.

New extrusion compared with the pre-operative study, which says the remaining meniscus has lost its anchor. 5.6.9 covers root tears.

New cartilage loss and subchondral signal change in the operated compartment. 5.11.7 covers bone marrow lesions.

MRCS Level

How It Is Acquired

In a viva this is the study where the first answer is a question about what was done, and saying so is the expected opening.

The expected points are the operation note, the comparison study, the post-operative protocol, the remaining meniscus, the position, and the compartment.

Say that fluid reaching the surface on more than one image is the finding you would act on. 5.6.3 covers horizontal tears.

Say that persistent internal signal in a resected meniscus is not evidence of a new tear. 5.6.2 covers meniscal signal grading.

Say that a displaced fragment settles the question where signal does not. 5.6.7 covers flap tears.

What Abnormal Looks Like

The reasoning expected is that the standard surface rule works because an intact meniscus has an intact surface, and a resected meniscus has a cut one, so the rule loses the thing it depended on. 5.6.1 covers the normal meniscus.

A cut surface heals as fibrous tissue that holds fluid, which is why signal persists at the margin without any new tear. 2.5.2 covers the histology.

Fluid tracking through the substance on consecutive images is a different observation from signal at a cut margin, and the distinction is the whole reading.

Loss of meniscal volume matters mechanically whether or not anything is torn, because the circumferential tension depends on the tissue being there. 3.6.2 covers hoop stress.

How Good It Is

Magnetic resonance imaging after meniscal surgery is better at describing the remaining meniscus than at calling a re-tear.

Its best contribution is the comparison: a change between two studies is far more informative than a single appearance. 5.20.7 covers reporting the post-operative knee.

It is limited by the altered surface, by metal artefact, and by the absence of a reliable rule for what signal means in resected tissue. 5.2.19 covers artefacts.

No figures appear for accuracy on this page, under decision 028. The claim being made is a comparison between the post-operative and the intact meniscus, and that comparison does not need a number.

FRCS Level

How Good It Is

At decision level the question is almost never whether the meniscus looks abnormal, since it does. The question is whether anything has changed.

A symptomatic operated knee with a displaced fragment has a mechanical answer. 5.6.6 covers bucket handle tears.

A symptomatic operated knee with persistent signal, no displacement and no new cartilage loss has no imaging answer, and this is the common case. 5.1.6 covers imaging pitfalls.

New extrusion and new cartilage loss in the operated compartment describe a compartment problem rather than a meniscal one, and that changes who the patient is. 5.3.16 covers Kellgren-Lawrence grading.

A failed repair is a different question from a failed meniscectomy, and the imaging approaches differ accordingly. 5.6.13 covers meniscal repair imaging.

Where It Misleads

The commonest error is reading the operated meniscus against the rule for an intact one and reporting a re-tear. 5.6.2 covers meniscal signal grading.

A short meniscus is reported as a radial tear when it is the resection margin. 5.6.5 covers radial tears.

Signal at the cut edge is called a horizontal tear. 5.6.3 covers horizontal tears.

The study is read without the operation note, so the baseline is unknown. 5.20.7 covers reporting the post-operative knee.

Metal artefact from fixation is read as the lesion. 5.2.19 covers artefacts.

Fellowship Level

Where It Misleads

At fellowship level the post-operative meniscus is the clearest example in this chapter of an appearance that cannot be interpreted from the images alone.

The same study reported with and without the operation note yields different reports, which makes the clinical information part of the investigation rather than context for it. 5.20.1 covers structured reporting.

There is no agreed definition of a re-tear on imaging, so published comparisons of re-tear detection are comparing differently defined events. 5.21.1 covers the meniscal imaging literature.

Intra-articular contrast improves the reading of a resected meniscus and is rarely available in the pathway where the question arises, so most reports are made on the less informative study. 5.2.21 covers advanced MRI techniques.

The compartment's state is the information that predicts what happens next, and it is routinely subordinated to a search for a tear. 3.5.9 covers the contact mechanics after meniscectomy.

Automated reading is being applied to post-operative studies that were excluded from the training sets, which is the setting where it should be trusted least. 5.18.8 covers artificial intelligence in imaging.