• osteoid osteoma;
  • pain;
  • immunohistochemistry

Osteoid osteoma is a benign bone tumour characterized by pain which is relieved by non-steroidal anti-inflammatory drugs (NSAIDs), such as aspirin. To clarify the mechanism of the pain, five osteoid osteomas were studied immunohistochemically using polyclonal antibodies against prostaglandin E2 (PGE2), S-100 protein and protein gene product 9.5 (PGP 9.5). In all five cases, the pain had been relieved by NSAIDs. Nerve fibres positive for S-100 protein and PGP 9.5 were observed in the fibrous zone, especially close to the blood vessels, around the nidus in all the lesions and also within the nidus in three lesions. PGE2 immunoreactivity was variably positive in the nidus of three lesions. In one case a large number of actively proliferating osteoblasts reacted with this antibody. The other cases showed unevenly distributed PGE2 positivity which tended to be prominent in the plump osteoblasts. As control material we examined fibrous dysplasia (3 cases), osteosarcomas (3) and giant-cell tumours of bone (3). The plump osteoblastic tumour cells of three osteosarcomas and the bone-forming cells in two cases of fibrous dysplasia gave a positive reaction for PGE2. No S-100 or PGP 9.5 immunoreactive nerve fibres were seen in these lesions. It is concluded that the presence of nerve fibres alone might play a more important role in mediation of pain in osteoid osteomas than some effects of osteoblast-produced PGE2 on the nerves and proliferated blood vessels.