Definition:
Osteoma is a localized protruding mass resulting from an exaggeration of intramembranous bone formation. In essence it is normal bone that is abnormally dense. The periphery contains dense lamellae with organized Haversian canals, and the intertrabecular stroma contains osteoblasts, fibroblasts, and giant cells but not hematopoietic cells.

Three histologic types of osteoma: compact, spongiotic, and mixed have been described.

Osteomas are most commonly found in the skull and facial bones. The entire temporal bone can be involved. This includes the squama, mastoid, internal and external auditory canal, glenoid fossa, middle ear, eustachian tube, petrous apex, and styloid process.

Clinical Symptoms:
Osteomas are most often asymptomatic. However, there location can effect the clinical presentation (e.g. cosmectic vs obstruction vs compression).

Osteomas in the external auditory canal can produce meatal obstruction, conductive hearing loss and chronic suppuration.

Imaging findings:
Osteomas are usually dense, unilateral, solitary, round masses and can have pedunculated or wide base. Non-enhanced axial and coronal high resolution computed tomographic scans are most useful in localizing and characterizing these lesions.

Differential diagnosis:

In the External auditory canal (EAC) the two most commonly described are exostosis and an osteoma. Exostoses are usually multiple, bilateral, broad-based elevated lesions that protrude into the medial canal compared to the solitary, unilateral, pedunculated osteoma.

Other lesions involving the skull and facial bones that can be considered are osteosarcoma, osteoblastic metastasis, isolated eosinophilic granuloma, Paget’s disease, giant cell tumor, osteoid osteoma, calcified meningioma, and monostotic fibrous dysplasia. Imaging findings should allow differentiation of these lesions from osteomas.

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