Bone Marrow Lesions

For Medical Students

What It Shows

A bone marrow lesion is an area of increased signal in subchondral marrow on a fat suppressed fluid sensitive sequence. 5.2.16 covers fat suppression.

It sits beneath a cartilage defect or in the loaded part of a worn compartment. 5.11.2 covers chondral defects.

It matters because it is detected easily, accompanies almost everything, and is reported as though it named something. 5.13.1 covers bone marrow oedema.

How It Is Acquired

Assess on a fat suppressed fluid sensitive sequence, which is what makes it visible. 5.2.16 covers fat suppression.

Assess its location relative to the cartilage and the loaded compartment, since location is most of its meaning. 5.11.1 covers normal cartilage.

Assess the overlying cartilage, because the pair is the finding rather than the marrow alone. 5.11.2 covers chondral defects.

Assess for a pattern that names a mechanism, such as the pair of impaction sites in a cruciate injury. 5.7.4 covers the acute cruciate injury.

Exclude failed fat suppression, which produces patchy brightness that imitates it. 5.2.16 covers fat suppression.

What Abnormal Looks Like

Ill defined increased signal in subchondral marrow on the suppressed sequence. 5.2.16 covers fat suppression.

Reduced signal in the same area on the T1 weighted sequence. 5.2.13 covers T1 imaging.

Location beneath a cartilage defect or in the loaded compartment. 5.11.2 covers chondral defects.

Subchondral cysts within the area in advanced change. 5.3.19 covers subchondral sclerosis.

MRCS Level

How It Is Acquired

In a viva this is the finding to describe as a sign rather than a diagnosis.

The expected points are the suppressed sequence, the location, the overlying cartilage, the mechanistic pattern, and the suppression failure.

Say that it is sensitive and non-specific. 5.13.1 covers bone marrow oedema.

Say that a pattern can name a mechanism where a single area cannot. 5.7.4 covers the acute cruciate injury.

Say that it occurs in asymptomatic knees. 5.1.6 covers imaging pitfalls.

What Abnormal Looks Like

The reasoning expected is that subchondral bone responds to the load transmitted through the cartilage above it, so when that cartilage fails the bone beneath remodels and the marrow signal changes. 3.5.2 covers the contact pressures.

That makes the lesion a consequence of a mechanical change rather than a disease, which is why it appears beneath defects and in loaded compartments. 5.11.2 covers chondral defects.

The same signal appears from trauma, from stress, from infection and from infiltration, which is why its location and pattern carry the interpretation. 5.13.3 covers stress fractures.

In a cruciate injury the pattern of two impaction sites names the mechanism, which is the clearest case of pattern over presence in this section. 5.7.4 covers the acute cruciate injury.

How Good It Is

This is the most sensitive and least specific finding in knee magnetic resonance imaging.

Its best contribution is the patterned case, where two or more sites together name a mechanism. 5.7.4 covers the acute cruciate injury.

It is limited because an isolated area is compatible with almost anything, including a normal asymptomatic knee. 5.13.1 covers bone marrow oedema.

No figures appear for accuracy on this page, under decision 028, and no lesion volume or signal intensity values appear either: those are published sequence dependent quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level this finding describes a compartment's state and does not generate an indication.

Subchondral marrow change beneath a cartilage defect describes the compartment and bears on whether a repair is worth doing. 5.11.8 covers cartilage repair procedures.

The impaction pattern in a cruciate injury names the mechanism and supports the diagnosis when the ligament is equivocal. 5.7.4 covers the acute cruciate injury.

Marrow change in a compartment with preserved cartilage and no malalignment is discordant and should prompt a different explanation. 5.13.5 covers osteonecrosis.

Marrow change alone in a symptomatic knee is a prompt for a clinical question rather than an answer. 5.1.2 covers clinical examination versus imaging.

Where It Misleads

The commonest error is reporting the lesion as a diagnosis. 5.13.1 covers bone marrow oedema.

Failed fat suppression is read as marrow oedema. 5.2.16 covers fat suppression.

Normal red marrow is read as a lesion. 5.2.13 covers T1 imaging.

Population level prognostic associations are applied to the individual patient. 5.21.3 covers the cartilage imaging literature.

The overlying cartilage is not described, so the pair is lost. 5.11.2 covers chondral defects.

Fellowship Level

Where It Misleads

At fellowship level this is the finding whose citation frequency in this section is most out of proportion to what it establishes.

Its prognostic associations in osteoarthritis are real, population level and measured over years, and they are routinely quoted to an individual patient as though they predicted their course. 5.21.3 covers the cartilage imaging literature.

Its prevalence in asymptomatic knees means its presence shifts a diagnostic probability very little while reading as a significant abnormality. 5.1.6 covers imaging pitfalls.

Quantitative volume measurement would make it a measure rather than an impression, and it is a research technique, so a qualitative finding is used quantitatively. 5.18.1 covers quantitative magnetic resonance imaging.

The one setting where it is genuinely decisive is the patterned acute injury, where two sites together name a mechanism, and that use is both the most reliable and the least discussed. 5.7.4 covers the acute cruciate injury.

The disciplined report gives the location and the pattern, pairs it with the cartilage, and says when it names nothing. 5.20.5 covers reporting cartilage lesions.