OCD

For Medical Students

What It Shows

Osteochondritis dissecans is a subchondral lesion with overlying cartilage that may separate as a fragment, classically on the lateral aspect of the medial femoral condyle. 5.11.5 covers osteochondral lesions.

The imaging shows the lesion's extent, the state of the overlying cartilage and whether the fragment is stable. 5.11.1 covers normal cartilage.

It matters because stability decides management, and skeletal maturity changes both the prognosis and the thresholds. 5.15.3 covers paediatric OCD.

How It Is Acquired

Identify the lesion at its classic site and assess its size in two planes. 5.3.4 covers the tunnel view.

Assess the overlying cartilage for continuity, since an intact surface over a lesion is the favourable pattern. 5.11.1 covers normal cartilage.

Look for fluid signal at the interface between fragment and bed, which is the instability sign. 5.11.5 covers osteochondral lesions.

Assess the physes and record the skeletal maturity, because it changes the interpretation. 5.15.2 covers the growth plates.

Look for a displaced fragment elsewhere in the joint. 5.14.6 covers synovial chondromatosis.

What Abnormal Looks Like

A subchondral lesion at the classic site with a surrounding rim. 5.11.5 covers osteochondral lesions.

Intact overlying cartilage, which is the stable appearance. 5.11.1 covers normal cartilage.

Fluid signal fully encircling the fragment interface, which is the instability sign. 5.11.5 covers osteochondral lesions.

A displaced fragment with an empty donor bed. 5.3.4 covers the tunnel view.

MRCS Level

How It Is Acquired

In a viva the expected answer makes stability and skeletal maturity the two questions.

The expected points are the classic site and size, the overlying cartilage, the interface fluid, the skeletal maturity, and the displaced fragment.

Say that stability decides management. 4.17.4 covers osteochondritis dissecans.

Say that a skeletally immature patient has a better prognosis. 5.15.3 covers paediatric OCD.

Say that the interface fluid sign is not fully specific. 5.11.5 covers osteochondral lesions.

What Abnormal Looks Like

The reasoning expected is that the lesion is a failure at the subchondral bone rather than in the cartilage, so the cartilage above it can remain intact while the bone beneath separates. 5.11.5 covers osteochondral lesions.

Fluid completely surrounding the interface means synovial fluid has tracked all the way round, which is incompatible with bony continuity. 5.11.4 covers delamination.

An open physis means a vascular supply and a remodelling capacity that a mature skeleton lacks, which is the mechanism behind the better prognosis. 5.15.2 covers the growth plates.

The classic site is where it is because of the loading pattern there, so a lesion at an atypical site should prompt a different differential. 5.13.5 covers osteonecrosis.

How Good It Is

Magnetic resonance imaging stages this lesion and assesses its stability better than anything else, and its stability sign is imperfect.

Its best contribution is the combination of extent, cartilage continuity and the interface, which together form the staging. 5.11.5 covers osteochondral lesions.

It is limited because the interface fluid sign also appears with healing tissue, so stability is inferred rather than established. 5.21.3 covers the cartilage imaging literature.

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

FRCS Level

How Good It Is

At decision level this is a staging study whose output is a management category.

A stable lesion with intact cartilage in a skeletally immature patient is managed non-operatively, and the imaging establishes all three of those. 5.15.3 covers paediatric OCD.

An unstable lesion needs fixation or removal, and the fragment's bone quality decides which. 4.17.4 covers osteochondritis dissecans.

A displaced fragment with a locking knee is urgent. 4.17.2 covers mechanical symptoms.

A large defect after fragment removal is an osteochondral reconstruction problem rather than a cartilage one. 5.11.11 covers osteochondral grafts.

Where It Misleads

The commonest error is calling a lesion unstable on partial interface fluid. 5.11.5 covers osteochondral lesions.

Skeletal maturity is not recorded, so the prognosis cannot be given. 5.15.2 covers the growth plates.

A normal variant of ossification in a child is read as a lesion. 5.15.7 covers developmental variants.

The overlying cartilage is not assessed. 5.11.1 covers normal cartilage.

A lesion at an atypical site is called this rather than investigated differently. 5.13.5 covers osteonecrosis.

Fellowship Level

Where It Misleads

At fellowship level this lesion is staged by imaging criteria whose specificity does not match the weight the staging carries.

The interface fluid sign is the pivot of every staging system in use, and it appears both where a fragment has separated and where granulation tissue is healing one, so the same appearance supports opposite conclusions. 5.21.3 covers the cartilage imaging literature.

Staging systems were derived on particular sequences and are applied across protocols, so a stage without its protocol is not comparable. 5.20.5 covers reporting cartilage lesions.

Skeletal maturity changes both the natural history and the thresholds, and a knee protocol records it only incidentally through the physes. 5.15.2 covers the growth plates.

Contrast enhancement has been proposed to separate a healing interface from a separated one and has not become routine, which leaves the central ambiguity unresolved. 5.2.21 covers advanced magnetic resonance techniques.

The disciplined report gives the extent, the cartilage continuity, the interface appearance and the skeletal maturity as four separate observations, and lets the stage follow from them rather than replacing them. 5.20.5 covers reporting cartilage lesions.