Insufficiency Fractures

For Medical Students

What It Shows

An insufficiency fracture is a fracture through bone too weak for ordinary load, with no unusual activity behind it. 5.13.3 covers stress fractures.

In the knee it is typically subchondral, in a femoral condyle or a tibial plateau, with marrow change above and below the line. 2.1.3 covers the medial femoral condyle.

It matters because it was named a spontaneous osteonecrosis for decades, and the mechanism is a fracture, which changes what gets treated. 5.13.5 covers osteonecrosis.

How It Is Acquired

Image with fluid sensitive and T1 weighted sequences through the affected condyle or plateau. 5.2.16 covers the fat suppression.

Find the subchondral line and its relation to the plate. 5.11.1 covers the normal cartilage.

Assess the overlying cartilage and whether the plate has collapsed. 5.12.6 covers cartilage injury.

Assess the meniscus, the root especially, because a root tear often accompanies this. 5.6.9 covers root tears.

Obtain a weight bearing alignment film, since the fracture sits in the overloaded compartment. 5.3.6 covers the long leg alignment films.

What Normal Looks Like

A continuous subchondral plate with no line beneath it. 5.3.19 covers subchondral sclerosis.

Cartilage of even thickness over the condyle. 5.11.1 covers the normal cartilage.

An intact meniscal root with the meniscus in position. 5.6.11 covers meniscal extrusion.

Alignment that loads both compartments. 3.5.3 covers the load distribution.

What Abnormal Looks Like

A subchondral low signal line parallel to the plate. 5.13.1 covers bone marrow oedema.

Depression or collapse of the plate above the line. 5.11.5 covers osteochondral lesions.

Extensive marrow change around a small line, which is the usual proportion. 5.11.7 covers bone marrow lesions.

A meniscal root tear with extrusion in the same compartment. 5.6.9 covers root tears.

Varus alignment loading the affected condyle. 3.13.3 covers the varus alignment.

MRCS Level

How It Is Acquired

In a viva the expected answer names the fracture rather than the old diagnosis.

The expected points are the subchondral line, the plate, the cartilage, the meniscal root, and the alignment.

Say that the condition was called a spontaneous osteonecrosis and is a fracture. 5.13.5 covers osteonecrosis.

Say that the root tear and the fracture are one mechanical problem. 3.6.11 covers the effects of root tears.

Say that bone quality belongs in the assessment. 5.1.3 covers choosing the correct investigation.

What Abnormal Looks Like

The reasoning expected is that losing the meniscus transfers load onto a small area of subchondral bone, and weak bone under concentrated load fractures, so the meniscus, the alignment and the bone are one problem rather than three findings. 3.6.11 covers the effects of root tears.

That explains the usual combination: a medial compartment, a varus knee, a root tear and a subchondral fracture. 3.13.3 covers the varus alignment.

It also explains why treating the meniscus alone or the bone alone fails. 5.17.4 covers the osteotomy.

A lesion deep to the plate with a serpentine margin is an infarction instead, and the distinction is geometric rather than quantitative. 5.13.5 covers osteonecrosis.

How Good It Is

Magnetic resonance imaging is the best available test for this fracture and reports a small line inside a large signal change, which invites the wrong emphasis.

Its best contribution is the line, the plate and the meniscal root in one study. 5.6.9 covers root tears.

It is limited by the extensive marrow change dominating the appearance, by overlap with infarction, and by saying nothing about bone quality. 5.13.5 covers osteonecrosis.

No figures appear for accuracy on this page, under decision 028, and no lesion dimensions or collapse thresholds appear either: those are published quantities from particular cohorts and this environment cannot check them.

FRCS Level

How Good It Is

At decision level the imaging has to answer whether the plate is intact, because that decides whether the joint can be preserved. 5.11.5 covers osteochondral lesions.

The root tear decides whether a repair is part of the plan. 5.6.9 covers root tears.

The alignment decides whether an osteotomy is part of it. 5.16.5 covers osteotomy planning.

The bone quality decides whether any of it will hold, and no part of the scan reports it. 5.1.3 covers choosing the correct investigation.

An arthroplasty discussion follows collapse rather than signal. 5.20.6 covers reporting osteoarthritis.

Where It Misleads

The commonest error is reporting the lesion as an osteonecrosis, which points the treatment at the wrong mechanism. 5.13.5 covers osteonecrosis.

The marrow change is measured and the line is missed. 5.13.1 covers bone marrow oedema.

The root tear is reported as incidental degeneration. 5.6.9 covers root tears.

Collapse is read as cartilage loss. 5.12.6 covers cartilage injury.

A knee is offered an arthroscopy for the meniscal tear while the fracture goes unaddressed. 5.1.2 covers clinical examination versus imaging.

Fellowship Level

Where It Misleads

At fellowship level the knee arrives after an arthroscopy that treated the meniscal tear and left the fracture, often worse. 5.6.9 covers root tears.

Imaging after that cannot separate the natural course from the operation's effect. 5.20.7 covers reporting the post-operative knee.

The old name persists in reports and in patient searches, so the consultation starts by correcting a diagnosis. 5.13.5 covers osteonecrosis.

Bone quality, vitamin status and medication are part of the problem and none of them is a radiological finding. 5.1.7 covers evidence-based imaging.

Serial imaging of the plate is what follows these knees, and it has to be the same projection every time. 5.3.5 covers the Rosenberg view.

The useful report gives the line, the plate's integrity, the cartilage, the root, the extrusion and the alignment, in that order. 5.20.1 covers structured reporting.