Osteochondritis Dissecans

For Medical Students

What It Tests

Osteochondritis dissecans is a lesion of the subchondral bone and its overlying cartilage that develops without a single injury, most often in an adolescent and most often on a femoral condyle. 4.19.1 covers growth plate assessment.

Examining for it means establishing an insidious onset, a compartment specific pain and an effusion, and then asking whether the fragment is stable. 4.17.2 covers the mechanical half of that question.

Stability is the question because a stable lesion in a growing knee can heal and an unstable one cannot, and the examination's contribution is the mechanical symptom. 4.17.3 covers the acute equivalent.

How It Is Performed

Establish the age and the skeletal maturity, since the prognosis depends on growth remaining. 4.19.1 covers that assessment.

Establish the onset: insidious, activity related, with no single injury, which is the characteristic history. 4.2.9 covers onset.

Assess the effusion, which is usually present and activity related. 4.8.3 covers the bulge sign.

Palpate the compartments with the knee flexed, since the lateral aspect of the medial femoral condyle is the commonest site and is reachable in flexion. 4.6.4 covers joint line tenderness.

Ask about mechanical symptoms specifically, since a true block or catching indicates an unstable or loose fragment. 4.17.2 covers them.

Assess the range and look for a block, with the end feel recorded. 4.7.5 covers it.

What a Positive Means

An adolescent with insidious activity related knee pain, an effusion and compartment specific tenderness has a presentation consistent with this diagnosis, and the imaging confirms it.

A mechanical symptom or a block indicates an unstable or detached fragment, which changes the management from observation to intervention. 4.17.2 covers it.

A skeletally immature patient with a stable lesion has the best prognosis, because remaining growth supports healing. 4.19.1 covers that assessment.

Bilateral lesions occur and are worth looking for, which means examining the asymptomatic knee too. 4.19.6 covers developmental variants.

A normal examination in a patient with a lesion on imaging is common, and the mechanical history is what adds the stability information. 4.17.1 covers the presentation.

MRCS Level

How It Is Performed

In a viva this is an adolescent knee station and the expected answer gets to skeletal maturity and stability quickly.

The expected points are the age and maturity, the insidious onset, the effusion, the flexed knee palpation of the condyle, the mechanical history, and the range with its end feel.

Say that the commonest site is the lateral aspect of the medial femoral condyle and that palpating it requires flexion. 4.6.4 covers the technique.

Say that the examination cannot grade stability and that the imaging does, and that the mechanical history is the clinical proxy. 4.1.6 covers that relationship.

Say that you would examine both knees, since the condition can be bilateral. 4.19.6 covers developmental variants.

What a Positive Means

The reasoning expected is that the lesion is a failure of the subchondral bone, so the cartilage above it loses its support and may separate with it. 3.5.1 covers contact areas.

A lesion that remains attached can revascularise and heal while growth continues, which is the basis for observing a stable lesion in an immature knee. 4.19.1 covers that assessment.

A lesion that separates becomes a loose body and produces mechanical symptoms, which is why the history is the clinical marker of stability. 4.17.2 covers them.

The commonest site is loaded in flexion rather than in extension, which is why the pain is activity related and why the arc localises it. 3.5.4 covers the effects of flexion.

How Good It Is

The examination raises the diagnosis in an adolescent with the characteristic history and cannot grade the lesion, which the imaging does.

Its most useful contribution is the mechanical history, because stability is the decision and the history is the clinical route to it. 4.17.2 covers it.

It is insensitive for a stable lesion, which may produce only an effusion and vague pain.

Magnetic resonance imaging grades stability and radiographs show the lesion, so both have a role and the examination's is the referral. 4.1.6 covers that relationship.

No figures appear for any accuracy comparison on this page, under decision 023, and the comparison is the claim being made.

FRCS Level

How Good It Is

At decision level the stability and the skeletal maturity decide the treatment, and the examination contributes to the first and establishes the second.

A stable lesion in a skeletally immature knee is observed and protected, and the examination's job is confirming there are no mechanical symptoms. 4.17.2 covers them.

An unstable or detached lesion is treated surgically, and the mechanical history with the imaging makes that decision.

A skeletally mature patient with a lesion has a worse prognosis for healing, and the decision moves towards intervention earlier. 4.19.1 covers that assessment.

In every case the alignment is assessed, because a malaligned limb loads the affected compartment and changes what any procedure achieves. 4.5.1 covers the measurement.

Where It Misleads

The commonest error is not asking about mechanical symptoms, which are the clinical marker of stability.

The second is palpating the condyle with the knee extended, where the commonest site cannot be reached.

The third is not establishing skeletal maturity, which changes the prognosis more than any examination finding.

The fourth is examining one knee.

The fifth is attributing an adolescent's knee pain to an apophysitis when the effusion and the compartment tenderness point elsewhere. 4.16.4 covers that alternative.

Fellowship Level

Where It Misleads

In referred practice this lesion arrives either as an adolescent needing a prognosis or as an adult with the consequences of one.

An adolescent with a stable lesion and remaining growth has a good prognosis, and the conversation is about protection and time rather than about surgery. 4.19.1 covers that assessment.

An adult with a healed lesion and compartment pain has an early degenerative problem in that compartment, and the alignment is the treatable part. 4.5.2 covers varus alignment.

An adult with an unhealed lesion and a loose body has a cartilage defect problem, and the decision is a reconstruction one rather than a healing one. 4.17.5 covers it.

A patient with bilateral lesions and a family history has a developmental pattern, and the other knee's prognosis belongs in the conversation. 4.19.6 covers developmental variants.

In medicolegal work this condition is developmental and is not attributable to a single injury, which is a clear statement the examination supports. 4.23.7 covers that setting.