Osteoma Cutis

Strictly defined, osteoma cutis refers to the presence of bone within the skin in the absence of a preexisting or associated lesion. This is opposed to secondary types of cutaneous ossification that can occur in reaction to inflammatory, traumatic, and neoplastic processes. ^{[1, 2]}

Bone arises in skin and soft tissues through mesenchymal (membranous) ossification without cartilage precursors or models (as in endochondral ossification of the skeletal system).

The lesions of osteoma cutis differ from calcinosis cutis in that they represent bone formation (dermal deposition of hydroxyapatite crystals) versus calcium salt deposits.

The pathogenesis of primary osteoma cutis has the following two proposed mechanisms ^[3] :

• Through mesenchymal cells differentiating into osteoblasts and then migrating to an abnormal location

• Through an osteoblastic metaplasia of mesenchymal cells already in the dermis, such as fibroblasts

Osteoma cutis can be a feature of several groups of patients. In can be secondary to inflammatory skin diseases, trauma/scars, and cutaneous tumors. It can also occur with genetic syndromes or in isolation.

Albright hereditary osteodystrophy is due to an autosomal dominant defect in the alpha subunit of intracellular G proteins. ^[4] The characteristic phenotype includes short stature, round facies, defective teeth, mental retardation, brachydactyly (fourth and fifth metacarpals/"knuckle knuckle dimple dimple" of the Archibald sign), and osteomas of the soft tissue and the skin. Classically, it presents with pseudohypoparathyroidism (elevated parathyroid hormone [PTH], hypocalcemia, hyperphosphatemia secondary to renal PTH resistance). Tetany is often the presenting sign, secondary to the low calcium level. Albright hereditary osteodystrophy can cause osteoma cutis without endocrine abnormalities in the pseudo-pseudohypoparathyroidism variant.

Progressive osseous heteroplasia is also, like Albright hereditary osteodystrophy, associated with a defect in the alpha subunit of G proteins (GNAS1 gene). It is characterized by ossification of the dermis in infancy, with progression to the subcutaneous and deeper connective tissues throughout childhood. It is not associated with endocrine changes, but it can have a severe affect on growth and joint mobility. ^[5]

Congenital platelike osteomatosis, a type of primary osteoma cutis, meets the following criteria:

• Lesion present at birth (or within first year of life)

• No evidence of abnormal calcium or phosphorous metabolism

• No evidence of trauma or infection

• Presence of at least one bony plate

Congenital platelike osteomatosis is most commonly found on the scalp. It should be monitored, as its diagnosis could be representative of a slow-evolving progressive osseous heteroplasia (something with much more severe consequences). ^[6]

Fibrodysplasia ossificans heteroplasia and fibrodysplasia ossificans progressiva (stone man syndrome) are possible causes.

Osteoma cutis can be found in patients with Gardner syndrome, which includes colonic polyposis, retinal hyperplasia, and other osseous and soft tissue growths.

Multiple miliary osteomas of the face often present in patients with a history of severe acne, sunburn, neurotic excoriation, or dermabrasion. ^[7] These small, hard papules can have a bluish hue, especially in patients who have been exposed to tetracycline treatment. ^{[8, 9]}

Osteoma cutis has been noted in the setting of lichen planopilaris. ^[10]

US frequency

Although considered rare, with no well-defined data on incidence, a plethora of conditions and syndromes may be found in association with osteoma cutis. Hence, the frequency of its occurrence varies accordingly. Primary lesions with no underlying cause are even rarer, but they account for approximately 20% of all skin ossifications. Reported in 1977, of 20,000 consecutive skin biopsies, only 35 cutaneous osteomas were found. Ten of them were primary, while 25 appeared secondary to another abnormality (although long ago, this allows some insight into its rarity). ^{[1, 2]}

Race

No particular race is predisposed to developing osteoma cutis.

Sex

Generally, no distinct sexual predominance exists. However, one cause of osteoma cutis, Albright hereditary osteodystrophy, occurs with a female-to-male ratio of 2:1.

Age

Osteoma cutis may occur at any age. Of note, multiple miliary osteoma cutis classically presents in middle-aged white women. ^[8]

Osteoma cutis is not life threatening, although local discomfort and/or disfigurement may lead the patient to seek consultation. Osteosarcoma or other malignancies have not been reported to arise within osteoma cutis.