Osteoma cutis is a condition of true bone formation within the skin. It may be primary, with no preceding lesions; secondary (metastatic), when associated with parathyroid metabolism abnormalities; or dystrophic, when ossification occurs in a site of an inflammatory process.

There are four genetic disorders associated with primary osteoma cutis. These are fibrodysplasia ossificans progressiva, progressive osseous heteroplasia, Albright hereditary osteodystrophy, and widespread or single platelike osteoma cutis. In addition, primary osteoma cutis may occur as an idiopathic event, usually later in life, as a solitary osteoma. 

Multiple miliary osteomas may occur later in life as well, usually in females. These tend to occur in people with a history of acne, and lesions tend to occur within scars. Pigmented cutaneous osteomas may develop in patients treated with tetracycline or minocycline for acne. There is no correlation between severity of acne scarring and the occurrence of osteomas.

Cutaneous bone formation may occur within basal cell carcinomas, cutaneous mixed tumors, intradermal nevi, scars, venous stasis, folliculitis ulerythematosa reticulata, scleroderma, and dermatomyositis. In pilomatricomas (pilomatrixomas), ossification rarely occurs although calcification is common.

For disorders in which dystrophic calcification occurs (such as the CREST form of scleroderma and juvenile dermatomyositis), see calcinosis cutis.