It is a benign bone forming tumor which is usually smaller than 1cm in greatest dimensions and had limited growth potential
Incidence-13% of all benign bone tumors
Clinical features:
Age – Usually seen in adolescents and young adults
Sex – Male predominance with male to female ratio of 2 or 3 to 1
Symptom –
Progressive pain which is typically worse at night is characteristic symptom of patient
Pain is immediately relieved by NSAIDS
Pain is thought to be due to the presence of nerve fibres in the nidus and production of prostaglandins by the nidus which can be suppressed by NSAIDS
Tenderness and swelling may present if the lesion is located superficially.
Sites:
Common sites are long bones particularly tibia and femur but any portion of skeleton may be involved
In long bone, diaphysis or metaphysis are involved by lesion
Osteoid osteoma rarely presents with multicentric involvement
Radiographic findings:
Lesions involve medulla, cortex, subcortical bone or even bone surface
In long bones-lesion is located in cortex with nidus appearing as oval lucency which is well demarcated and may have variable meneralization
Central lucency may show calcification with lucent halo
Associated sclerosis in cortical bone usually appears as fusiform thickness of cortex with thickest area on the nidus
If the lesion is in spongy bone or in the intracapscular location, there may not be any associated sclerosis
CT is sensitive for the detection of nidus in osteoid osteoma. In the sites like femoral neck and spine, nidus can be detected by CT scan
Gross findings:
When surgeon chisels thick cortical bone, red granular area is exposed which is the nidus
Entire sclerotic bone need not be removed but if the nidus is removed sclerotic bone is remodled over months or years
Gross appearance of nidus depends upon site of involvement
If the lesion is in medullary cavity-it appears as well demarcated red marble without associated sclerosis
If the lesion is in cortical bone or subcortical, reactive sclerosis is extensive with nidus appearing as well demarcated red granular area
Nidus can be separated by scalpel from the sclerotic bone
Microscopic findings:
Nidus shows interlacing network of bony trabecular and osteoid with variable mineralisation
Anastomosing trabeculae are usually thin
In the nidus central part is more mineralised than peripheral part
Bony trabeculae are rimmed by single layer of osteoblasts which are small and polygonal
Intertrabecular space contains spindle shaped fibroblasts and proliferating capillaries
Thickened bony trabeculae may show cement lines
Surrounding sclerotic bone is sharply demarcated from the nidus
If the lesion is within capsule of joint, lymphocytes and plasma cells show infiltration
Differential diagnosis:
Osteoblastoma
Difficult to distinguish as morphological features overlap
Osteoid osteoma has limited growth potemtial with size less than 1cm where as osteoblastomas are always larger than 2cms
Osterosarcoma- Distinguished by morphological features. Osteoid osteoma is well demarcated lesion whereas Osteosarcoma is infiltrative lesion with Pleomorphic cells
Stress fractures may simulated osteoid osteoma with reactive new bone formation however leaks the rounded nidus seen in osteoid osteoma
Treatment and prognosis:
Complete surgical removal of nidus
Medical management by NSAIDS is preferred if surgical treatment is contraindicated
CT guided percutaneous resection of the nidus can be perfomed