Osteochondritis  Dissecans

Primarily affecting males from 10-20 years of age, it typically involves the lateral aspect of the medial femoral condyle (LAME) as seen here.

Sagittal T2*, coronal STIR and coronal T1GE with magnified view:

The staging system is based on arthroscopic findings as follows:
Stage 1: articular cartilage intact, lesion 1-3cm.
Stage 2: articular cartilage defect without loose body.
Stage 3: partially detached osteochondral fragment.
Stage 4: loose body with crater filled with fibrous tissue.

Although the cartilage appears intact over the majority of the surface, the presence of high T2* and STIR signal surrounding the fragment and adjacent marrow oedema indicates imbibed subchondral fluid, implying fissuring of the overlying cartilage, which has a high correlation with lesion instability. Healed lesions do not show this high signal lesion/bone interface.