Osteochondritis Dissecans
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Direct answer
Osteochondritis dissecans is a focal lesion of subchondral bone with the overlying articular cartilage, which may weaken and separate as a loose fragment. In the knee it most often affects the lateral aspect of the medial femoral condyle. Its outcome depends mainly on fragment stability and on whether the physis is open, since stable lesions in children often heal with protected weight bearing, while unstable fragments need fixation or removal.
What you must remember
- Commonest knee site: the lateral aspect of the medial femoral condyle, although the lateral femoral condyle is also involved.
- Pathology: a focal subchondral bone lesion whose cause is unclear, with repetitive microtrauma and local ischaemia proposed, and the overlying cartilage weakening and possibly separating.
- Wilson sign: pain on internal tibial rotation with the knee flexed to about 90 degrees, relieved as the knee extends.
- Stable lesion: intact overlying cartilage on MRI, usually managed with protected weight bearing and serial imaging.
- Unstable lesion: a displaced fragment or fluid behind it on MRI, which needs fixation or removal of the loose body.
- Open physis: a better chance of healing, so skeletal maturity is recorded before any decision.
- Presentation: knee pain, swelling, a sensation of catching or locking, and an effusion.
A pathway from a locked knee to fixation
A 14-year-old athlete has a painful, swollen knee with intermittent locking after a twist. The pathway sorts the lesion by fragment stability and skeletal maturity before any operation is planned.
- Examine for the Wilson sign: pain on internal tibial rotation with the knee near 90 degrees, relieved on extension, suggests a medial femoral condyle lesion.
- Image the knee: standing radiographs, including a notch view, show the lesion, although they can understate how stable the fragment is.
- Assess skeletal maturity: open physes raise the chance of healing, so growth status is recorded before the plan is made.
- Grade stability on MRI: MRI shows whether the overlying cartilage is intact and whether fluid lies behind the fragment, separating stable from unstable lesions.
- Treat stable lesions conservatively: protected weight bearing and activity restriction are used, with serial imaging to confirm healing.
- Fix or remove unstable fragments: screw fixation is chosen when the fragment can be secured, and removal of the loose body is considered when it cannot, with cartilage repair for larger defects.
Where the exam tries to catch you
Examiners test the lesion site, the Wilson sign and the decision between conservative and surgical care.
- Trap: operating on every osteochondritis dissecans lesion at once. Correct: stable lesions in skeletally immature patients often heal with conservative care, so the decision depends on stability and physeal status.
- Trap: dismissing the lesion because the plain radiograph looks normal. Correct: MRI is needed to judge cartilage integrity and fluid behind the fragment, which radiographs cannot show.
- Trap: calling a locked knee in an adolescent a meniscal tear without considering osteochondritis dissecans. Correct: a loose fragment locks the knee, and the Wilson sign and MRI help separate it from meniscal pathology.
Frequently asked questions
Which part of the knee is most often affected by osteochondritis dissecans?
Osteochondritis dissecans of the knee most often affects the lateral aspect of the medial femoral condyle, although the lateral femoral condyle is also involved. The site is best seen on a notch view, and MRI is used to judge the stability of the overlying cartilage.
What is the Wilson sign in osteochondritis dissecans?
The Wilson sign is pain on internal rotation of the tibia with the knee flexed to about 90 degrees, which eases as the knee is extended. It is a classic clinical sign that suggests an osteochondritis dissecans lesion of the medial femoral condyle.
How is a stable osteochondritis dissecans lesion managed?
A stable osteochondritis dissecans lesion, with intact overlying cartilage on MRI, is usually managed with protected weight bearing and activity restriction in a skeletally immature child. Serial imaging confirms healing, and surgery is considered only if symptoms persist or the lesion becomes unstable.
What decides fixation of an osteochondritis dissecans fragment?
Fixation or removal of an osteochondritis dissecans fragment depends on its stability, which MRI assesses through the overlying cartilage and the fluid behind the fragment. Unstable or displaced fragments are fixed with screws where they can be secured, and skeletal maturity influences the choice.