Intra-articular Osteoid Osteoma

1. Osteoid osteomas have been categorized by location as subperiosteal, cortical, and cancellous. 
2. The classic radiographic appearance of the more common cortical osteoid osteoma is that of a radiolucent nidus, with or without internal calcification and surrounding sclerosis or cortical thickening. 
3.  This is in contrast to intraarticular osteoid osteomas, which are are characterized by little or no reactive sclerosis; for this reason, the radiolucent nidus is often overlooked on radiographs.  Although endosteal sclerosis may occur with intraarticular osteoid osteomas, periosteal new bone is absent, because the periosteum is not present within the joint capsule. In this patient there is reactive calcification in the periarticular region possibly secondary to synovitis.