Osteoid osteoma (OO) is a benign bone neoplasm that occurs in children and young adults, primarily between the ages of 5 and 25 years. It is not uncommon, comprising 10% of benign bone tumors. There is a male-to-female predominance of 2-4:1. 

OO is a solitary, sporadic bone-forming tumour. This tumour has a hypervascular nidus containing osteoblastic cells (usually less than 1.5 to 2 cm in diameter) that produce prostaglandins and is associated with peripheral sclerosis. 

It typically occurs during the diaphysis and metaphysis of the long bones, especially the proximal femur. Other locations include the tibia, remaining femur, and the spine.  In terms of subtypes, it can be categorized as cortical (++), medullary, or subperiosteal.  

OO typically presents with a classic clinical picture. It causes progressive pain, which tends to worsen at night and may or may not be related to activity. The pain is generally relieved by NSAIDs within 20-25 minutes. If there is no relief, alternative diagnoses should be considered. 

In the lower limb, symptoms may include claudication, edema, muscle atrophy, limb length discrepancy, muscle contractures, and localized pain on palpation. In the hip (acetabulum and proximal femur), there may be decreased range of motion, sometimes mimicking femoroacetabular impingement. Spinal involvement can cause claudication, localized pain, paravertebral muscle contractures, and even scoliosis.

Diagnosis is confirmed via radiological imaging, particularly CT, where the characteristic nidus and surrounding sclerosis are visible.