Osteoma cutis is a rare benign tumor in which bone formation occurs within the skin. There are two types of osteoma cutis, primary and secondary. Primary osteoma cutis is idiopathic in nature, whereas secondary osteoma cutis is caused by bone formation in an area of trauma or another form of cutaneous inflammation. It can also be seen secondary to abnormalities of parathyroid hormone metabolism, and this form of osteoma cutis is called metastatic ossification. Secondary osteoma cutis is much more common than the primary idiopathic form.
Clinical Findings: Primary osteoma cutis is not associated with any defined underlying disorder and can manifest as a solitary nodule, plaque, or plate-like hard-ening of the skin. Some are quite small, whereas others are large and cause discomfort. Males and females are equally affected, and there is no race predilection. The age at onset is variable. Plate-like or plaque-like osteoma cutis is a form of primary osteoma cutis that occurs during the first few months of life and can even be present at birth. The acral regions are most commonly affected. Over time, these osteomas tend to develop ulcerations or erosions of the overlying epidermis. With this ulceration, small parts of the osteoma are extruded from the underlying dermis and expelled from the skin. This may be the cause for presentation to the clinician. Most patients present with a thickened or hardened area of skin with no preceding trauma or inflammatory condition. There is no malignant potential.
Primary osteomas of the skin may be seen in the genetically inherited disease, Albright’s hereditary osteodystrophy. This condition is characterized by a constellation of findings including short stature, osteoma cutis, mental and physical delay, and brachydactyly. Varying degrees of obesity and a round appear- ance to the face are also seen. This condition is caused by an underlying defect in the GNAS gene. This gene encodes a stimulatory G protein (Gs) that is responsible for cell signaling through the eventual production of cyclic adenosine monophosphate (cAMP). Albright’s hereditary osteodystrophy has been reported to manifest with resistance to parathyroid hormone, but other Albright’s patients have not shown this resistance. These differences are likely due to the complex inheritance pattern and whether the defective gene was inherited from the maternal or paternal side or both. Most patients have associated hypocalcemia and hyperphosphatemia.
Secondary osteoma cutis is far more common than the primary form, by a ratio of about 9 : 1. Bone formation may occur in any area of previous skin trauma, acne cysts, or epidermal inclusion cysts and are commonly seen in pilomatricomas. Pilomatricomas are benign tumors that most often manifest in childhood. Inflammatory conditions associated with osteoma cutis include dermatomyositis and scleroderma.
Fibrodysplasia ossificans progressiva is a rare genetic condition in which connective tissue is turned into bone after minor trauma, causing secondary osteomas. The skin can be involved, but so can the muscle and other underlying tissue. This disease is progressive and can result in premature death. This condition is unique in that it is caused by endochondral bone formation.
Pathogenesis: Primary forms of osteoma cutis show intramembranous ossification that is centered within the dermis. There is no preceding cartilage formation to act as a scaffolding for the bone to form. The exact cause is unknown. The G protein that is defective in Albright’s hereditary osteodystrophy has been found to be important in bone regulation. The precise reason why some areas of skin are involved while others are left intact in this genetic disease is not well understood.
Histology: Areas of bone formation are seen ectopically in the dermis or subcutaneous tissue. The bone is formed by an intramembranous mechanism without the assistance of a preceding cartilage scaffolding.
Treatment: Secondary osteoma cutis can be removed with a number of surgical techniques. Creation of a small, nick-like incision over the area of osteoma formation and removal with a small curette or laser resurfacing has produced the best results. This treatment can be very time-consuming and labor intensive in cases of multiple secondary osteoma cutis (e.g., in some cases of acne-associated osteoma cutis).
The treatment of primary plaque-like osteoma cutis is surgical removal. Albright’s heriditary osteodystrophy and fibrodysplasia ossificans progressiva are rare diseases that require a multidisciplinary approac at centers with experience treating these conditions.