Osteoid osteoma.

Osteoid osteoma presents in the 2nd and 3rd decades. It is slightly more common in males. 90% of cases present between 5 and 25 years. The clinical history is of pain, often at night, which is relieved by Aspirin. 50% of cases occur in the proximal femur, but any bone may be affected, including the skull. If found in the spine, the most likely site is the pedicle. Osteoid osteoma is a benign tumour of bone matrix. Histologically its low density portion comprises osteoblasts and some fibrous tissue. The high density portion in the typical plain film image is bone reaction. Near the cortex, the lesion stimulates periosteal newbone, unless reduced by its intra-articular location or nearby muscle or large ligament insertion. Given the capacity of osteoid to ossify, a central nidus of higher density may be seen.

The lesion is often found with isotope bone scanning in those cases where the medullary location or neighbouring joints may not result in enough density to be diagnosed easily on the plain film.