Osteonecrosis

For Medical Students

What It Shows

Osteonecrosis is death of bone and marrow following loss of blood supply. 5.5.6 covers the bones.

Magnetic resonance imaging shows a geographic lesion with a low signal margin separating dead bone from living. 5.2.13 covers the T1 imaging.

It matters because the cause is usually outside the knee, so finding it redirects the assessment away from the joint. 5.1.3 covers choosing the correct investigation.

How It Is Acquired

Image with fluid sensitive and T1 weighted sequences through the whole affected bone end. 5.2.16 covers the fat suppression.

Define the lesion's margin, since the margin rather than the signal inside it carries the diagnosis. 5.2.13 covers the T1 imaging.

Define the lesion's position relative to the subchondral plate. 5.11.1 covers the normal cartilage.

Assess whether the plate has collapsed. 5.11.5 covers osteochondral lesions.

Take the systemic history, since steroids, alcohol, haemoglobinopathy and chemotherapy are the usual causes. 4.2.9 covers the onset.

What Normal Looks Like

Marrow following fat throughout the bone end. 5.5.6 covers the bones.

No geographic margin anywhere in the marrow. 5.2.13 covers the T1 imaging.

A continuous subchondral plate with normal cartilage above it. 5.11.1 covers the normal cartilage.

No lesion in the other knee or in the hips where those were imaged too. 5.20.1 covers structured reporting.

What Abnormal Looks Like

A geographic lesion with a serpentine low signal margin. 5.2.13 covers the T1 imaging.

A lesion sitting deep to the plate rather than against it. 5.13.4 covers insufficiency fractures.

Preserved fat signal inside the lesion early, which is the dead bone before it is resorbed. 5.5.6 covers the bones.

Collapse of the plate, which is the step that changes the operation. 5.11.5 covers osteochondral lesions.

Lesions in more than one bone or more than one joint, which confirms a systemic cause. 5.13.7 covers bone tumours.

MRCS Level

How It Is Acquired

In a viva the expected answer separates infarction from subchondral fracture.

The expected points are the whole bone end, the margin, the position relative to the plate, the collapse, and the systemic history.

Say that a lesion against the plate with a line is a fracture, and one deep to it with a margin is an infarction. 5.13.4 covers insufficiency fractures.

Say that the staging systems were written for the femoral head. 5.1.7 covers evidence-based imaging.

Say that both knees and both hips are worth imaging where a systemic cause is suspected. 5.1.3 covers choosing the correct investigation.

What Abnormal Looks Like

The reasoning expected is that an infarct is demarcated by the body's reaction at its boundary, so the margin is the lesion's signature and the signal inside it changes with time rather than with severity. 5.5.6 covers the bones.

The subchondral plate's fate decides everything, because an intact plate over a lesion behaves unlike a collapsed one. 5.11.5 covers osteochondral lesions.

Staging systems transferred from the hip assume a spherical head and a different load pattern, so they fit the knee poorly. 5.1.7 covers evidence-based imaging.

In a child the same appearance in a condyle is more likely a lesion of a different mechanism. 5.11.6 covers OCD.

How Good It Is

Magnetic resonance imaging is the best available test for osteonecrosis and separates it from a subchondral fracture by geometry rather than confidently.

Its best contribution is the margin, the extent and the state of the plate. 5.11.5 covers osteochondral lesions.

It is limited by overlap with insufficiency fracture, by staging systems borrowed from another joint, and by appearances that change with time rather than with severity. 5.13.4 covers insufficiency fractures.

No figures appear for accuracy on this page, under decision 028, and no lesion volumes or staging thresholds appear either: those are published quantities defined for the femoral head and this environment cannot check them.

FRCS Level

How Good It Is

At decision level the questions are the extent, the plate and the cause. 5.11.5 covers osteochondral lesions.

A small lesion with an intact plate is watched while the cause is addressed. 5.1.3 covers choosing the correct investigation.

Collapse moves the discussion to joint preservation or replacement. 5.20.6 covers reporting osteoarthritis.

A systemic cause makes the other joints part of the plan rather than part of the history. 5.13.6 covers transient osteoporosis.

Alignment matters here as it does in every subchondral problem of the knee. 5.3.6 covers the long leg alignment films.

Where It Misleads

The commonest error is naming an insufficiency fracture an osteonecrosis, which the literature did for decades. 5.13.4 covers insufficiency fractures.

A resolving marrow change is reported as an early infarct. 5.13.1 covers bone marrow oedema.

Preserved fat inside a lesion is read as viability. 5.5.6 covers the bones.

Secondary degeneration above a collapsed lesion is reported as primary arthritis. 5.20.6 covers reporting osteoarthritis.

A single knee lesion is reported with no comment on the systemic possibilities. 5.20.8 covers common reporting errors.

Fellowship Level

Where It Misleads

At fellowship level these knees arrive with the lesion named and the cause unexamined. 5.1.3 covers choosing the correct investigation.

Steroid exposure is often historical and absent from the referral, so the history has to be retaken. 4.2.9 covers the onset.

Serial imaging is used to watch the plate, and the plate is best watched radiographically in a consistent projection. 5.3.5 covers the Rosenberg view.

After a joint preserving procedure no agreed expected appearance exists for the lesion. 5.20.7 covers reporting the post-operative knee.

Metabolic imaging can report whether the lesion is active, which is exactly the question, and is rarely obtainable. 5.18.7 covers the PET-MRI.

The useful report gives the margin, the extent, the plate, the cartilage, the other compartments, and the systemic question stated as open. 5.20.1 covers structured reporting.