Define Osteoid Osteoma. Describe its clinical features and diagnosis.

Definition

Osteoid osteoma is a small, benign, bone-forming tumour consisting of a central vascular nidus (< 1.5 cm) of osteoid surrounded by dense reactive sclerosis. It typically affects adolescents and young adults and commonly arises in the diaphysis/metaphysis of long bones (femur, tibia) and the spine.

Clinical Features

The hallmark is persistent boring pain, characteristically worse at night, that is dramatically relieved by aspirin/NSAIDs — a very useful clinical clue explained by prostaglandin production within the nidus. A spinal lesion may cause painful scoliosis, and a periarticular one a joint effusion.

Investigations & Management

Radiographs show a small radiolucent nidus with surrounding sclerosis; CT best demonstrates the nidus, and a bone scan is intensely hot. Many lesions burn out spontaneously over years and can be managed with NSAIDs. Definitive treatment for persistent pain is removal/ablation of the nidus, now usually by CT-guided percutaneous radiofrequency ablation (or surgical excision).

Pathology

The lesion consists of a small central nidus of vascular osteoid and woven bone with active osteoblasts, surrounded by a zone of dense reactive sclerosis. The nidus produces prostaglandins, which are responsible for both the characteristic pain and its relief by prostaglandin-inhibiting NSAIDs. By definition the nidus is less than about 1.5 cm; a larger, otherwise similar lesion is termed an osteoblastoma, which is more expansile, commoner in the spine, and less reliably relieved by aspirin.

Differential Diagnosis

The main differentials are a Brodie’s abscess (subacute osteomyelitis, which can also cause night pain and reactive sclerosis) and a stress fracture. CT demonstration of the nidus, together with the classic aspirin-sensitive night pain, usually clinches the diagnosis and separates it from these mimics.

Aspirin relief of night pain is the classic clue.

• Small benign bone-forming tumour: vascular nidus (<1.5 cm) + reactive sclerosis.
• Night pain relieved by aspirin/NSAIDs; young patients; long bones and spine.
• CT shows the nidus; bone scan intensely hot.
• NSAIDs (may burn out) or radiofrequency ablation/excision of the nidus.