Common in adolescence (15-20 years). It affects males more often than women. This condition may be entirely asymptomatic or patient may present with pain on movement, limited range of motion and/or locking. Osteochondritis dissecans of the knee may be familial - autosomal dominant. Associations: short stature, endocrine dysfunction, Scheuermann\'s disease, Osgood-Schlatter disease, tibia vara, carpal tunnel syndrome 

Repeated microtrauma may lead to vascular compromise and arrest of bone growth with subchondral band of sclerosis. Surface load is converted to shear at the shoulder of the defect so that separation occurs first on the deep surface. Diagnostic features include a plate of laminar calcification in the detached fragment or a small focus of ossification forming a bridge to the main portion of the epiphysis.

Knee - commonest. Unilateral : bilateral = 3 :1. Classic and extended classic forms affect the lateral aspect of medial femoral condyle, It may occur in the inferocentral medial or lateral condyles or on the anterior aspect of lateral condyle.                                                               Patella (medial facet - most common site.)

Ankle - Talus - mid third of lateral border, posterior third of medial border.

Elbow - capitulum.

Purely chondral lesions require CT arthrography, MRI or arthroscopy for accurate diagnosis. 

MRI: demonstration of  a rim of high signal intensity fluid surrounding the fragment on T2WI is suggestive of an active unstable fragment.