Flap Tears

For Medical Students

What It Shows

A flap tear has a free leaf attached along one edge, so the leaf can lift and move while the rest of the meniscus stays put. 5.6.3 covers horizontal tears.

The leaf commonly displaces into the gutter beside the meniscus or beneath the meniscal body, and in either place it is away from where the meniscus is being looked at. 5.6.1 covers the normal meniscus.

It matters because a displaced leaf explains mechanical symptoms in a knee whose meniscus is reported as only degenerate. 4.9.8 covers mechanical locking.

How It Is Acquired

Assess the meniscus itself first, since the parent tear is usually a horizontal or oblique split. 5.6.3 covers horizontal tears.

Look in the gutter, the recess between the meniscus and the capsule, because a displaced leaf sits there and is outside the usual search area. 5.5.1 covers the standard protocol.

Look beneath the meniscal body, where a leaf can tuck under and produce a double layered appearance. 5.6.1 covers the normal meniscus.

Compare the meniscal height across the slices, since a leaf lifted away makes the parent thinner.

Assess both the coronal and sagittal images, because a leaf is often visible in only one. 5.5.2 covers the sports knee protocol.

What Abnormal Looks Like

A tongue of meniscal tissue displaced from its parent, attached along one margin and lying in the gutter or beneath the meniscus.

A parent meniscus that is thinner or shorter than its opposite number, with a split running to the displaced leaf.

A double layer where a leaf has tucked under the body, which can be mistaken for a normal variant. 5.6.1 covers the normal meniscus.

Fluid outlining the leaf on a fluid sensitive sequence, which is what makes it visible at all. 5.2.16 covers fat suppression.

MRCS Level

How It Is Acquired

In a viva this is the mechanical symptom with an unremarkable meniscal report, and the expected answer says where to look.

The expected points are the parent tear, the gutter, the area beneath the body, the meniscal height, and both planes.

Say that you would look in the gutter and beneath the meniscus specifically. 5.5.1 covers the standard protocol.

Say that a flap tear is a common cause of mechanical symptoms with a meniscus reported as degenerate. 5.6.2 covers signal grading.

Say that the leaf may have reduced by the time of scanning. 4.9.8 covers mechanical locking.

What Abnormal Looks Like

The reasoning expected is that a leaf attached along one margin behaves like a hinge, so it moves with the knee and catches between the surfaces. 3.6.3 covers the circumferential fibres.

Because it remains attached it is not a loose body, which is why it produces catching rather than true locking. 4.9.8 covers mechanical locking.

Its displacement into the gutter takes it out of the joint space, which is why the meniscus itself can look unremarkable. 5.6.1 covers the normal meniscus.

The parent split is usually degenerate, which is why this pattern appears in older knees with a mechanical history. 5.6.2 covers signal grading.

How Good It Is

This is the meniscal pattern most often missed, because the displaced tissue is outside the structure being assessed.

Its best contribution, when found, is explaining a mechanical symptom that the meniscal report did not account for. 4.9.8 covers mechanical locking.

It is missed when the search is confined to the meniscus and when the leaf has reduced. 5.6.1 covers the normal 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 a displaced flap is often the finding that justifies operating in a knee that would otherwise be managed without surgery.

A mechanical symptom with a displaced leaf is a different proposition from the same symptom with degenerate signal alone. 4.9.11 covers the examination in the worn knee.

The leaf is usually resected rather than repaired, because the parent tissue is degenerate, which makes this one of the remaining clear indications in a worn knee. 5.6.3 covers horizontal tears.

The compartment's cartilage still governs the expected benefit, so the flap is a reason to look rather than a reason to proceed. 5.11.2 covers chondral defects.

A specific request to look for a displaced flap changes what the report says, which is the clearest case in this chapter for stating the clinical question. 5.20.2 covers reporting meniscal tears.

Where It Misleads

The commonest error is confining the search to the meniscus. 5.6.1 covers the normal meniscus.

A reduced leaf gives a study that does not explain the symptoms. 4.9.8 covers mechanical locking.

A leaf tucked beneath the body is read as a normal double layer. 5.6.1 covers the normal meniscus.

The parent tear is reported and the displacement is not, which loses the part that matters. 5.20.2 covers reporting meniscal tears.

A displaced leaf in an arthritic knee is taken as the whole explanation when the compartment is also worn. 4.9.11 covers the examination in the worn knee.

Fellowship Level

Where It Misleads

At fellowship level the flap tear is the strongest argument in this chapter for asking the radiologist a question rather than reading a report.

The finding is conspicuous once looked for and invisible when not, so its detection depends on the request more than on the protocol. 5.20.1 covers structured reporting.

A leaf's position varies with knee position and time, so a scan is one sample of a mobile fragment. 4.9.8 covers mechanical locking.

The gutter and the subpopliteal recess are anatomically variable, which makes a small displaced leaf hard to distinguish from a normal fold. 2.6.2 covers the popliteomeniscal fascicles.

In a degenerate knee the flap coexists with everything else that is abnormal, and attributing the symptom to it is a judgement rather than a finding. 4.9.11 covers the examination in the worn knee.

Resecting a flap in a worn compartment relieves the catching and does not change the compartment, which is the expectation the consent conversation needs. 3.6.5 covers load sharing.