Osteoma Cutis (Bone Growth in the Skin)

What is osteoma cutis?

Osteoma cutis refers to the presence of bone within the skin. It is considered to be a type of cutaneous ossification, which means formation of bone tissue in the skin. However, osteoma cutis can easily be differentiated from other types of cutaneous ossification. Sometimes calcium deposits within the skin may also be mistaken for osteoma cutis. However, the lesions of osteoma cutis represent abnormal bone formation within the skin, whereas calcium salt deposition in the skin (calcinosis cutis) does not include formation of bone.

How common is osteoma cutis?

Osteoma cutis may occur at any age and does not show a predisposition to any particular race or gender. Yet, Albright hereditary osteodystrophy, which is a cause of osteoma cutis in some patients, affects women twice more likely than men.

Osteoma cutis is considered to be a rare condition. However, the true incidence of this anomaly is not well known. The reason is that many conditions and syndromes may be found linked with osteoma cutis. These conditions change the frequency of occurrence of osteoma cutis accordingly, making assessment of its incidence difficult.

In 20% of all cases of osteoma cutis, no underlying causes can be found.

Types

Osteoma cutis can be divided into the following categories:

Primary osteoma cutis

Albright’s hereditary osteodystrophy
Not associated with Albright’s hereditary osteodystrophy :
– Isolated osteoma
– Multiple miliary (millet seeds-like) osteomas of the face
– Congenital plaque-like osteoma
– Widespread osteoma

Secondary osteoma cutis

Inflammatory skin disease
– Progressive systemic sclerosis and CREST syndrome/scleroderma
– Dermatomyositis (inflammation of the muscles and the skin)
– Morphea (hardened skin)
Tumors
Trauma and scars

Location

Osteoma cutis most commonly affects the face, legs, arms, scalp, fingers, and the skin under the nails. It can occur at other locations in the body.

Abnormal Bone Formation in the Skin

Usually bone formation happens in the body in one of the following two ways: transformation of cartilage to bone (intramembranous ossification), or in absence of a cartilage (enchondral ossification). Formation of bone in skin and soft tissues in osteoma cutis progresses through the first type, intramembranous ossification.

In osteoma cutis, bone formation happens in the absence of an associated or pre-existing lesion. It arises spontaneously. On the contrary, almost all incidences of secondary cutaneous ossification occur by metaplastic reactions to various stimuli. Metaplastic changes are transformation of one type of tissue to another. Inflammation, trauma, and cancerous transformation in cells can result in metaplastic changes. In ostemoa cutis, however, bone formation in the skin happens in the absence of any of these causes.

Signs and Symptoms

The main symptoms of osteoma cutis reported by patients is feeling hard areas in the skin. This can be verified by the doctor during physical examination. However, the presentation of this condition can vary significantly in different individuals. The form, the number, and the location of the lesions may change from patient to patient.

Osteomas may present as:

Single or multiple nodules
Extremely hard nodules
Plaques
Miliary tumors (of the size and appearance of millet seeds)

Complications

Although osteoma cutis can cause considerable local discomfort and significant degree of disfigurement, it is not a life threatening condition.

Causes and Risk Factors

More than a disease on its own, osteoma cutis is a feature. The following conditions may cause osteoma cutis:

A disease called Albright hereditary osteodystrophy is commonly linked with osteoma cutis. In patients suffering from Albright hereditary osteodystrophy, skeletal system abnormalities and lesions of osteoma cutis are frequently seen.
Bony fragments undergoing transepidermal (through the skin) elimination may cause single, small osteomas. It is more common in later stages of life.
Complication of acne vulgaris
A disease called neurotic excoriation, which is characterized by repetitive scratching of the skin
Surgical procedures like dermabrasion, which involves removal of the upper layers of the skin
Congenital plaque-like (present from birth); more common in the skin of the scalp, legs and arms
Transformation of muscles into the bone (for example, in stone man syndrome or fibrodysplasia ossificans progressiva)

Tests and Diagnosis

There are various possible causes of hard nodules in the skin and therefore the clinical presentation alone is often insufficient to diagnose osteoma cutis. Several tests can be performed to diagnose osteoma cutis. Such tests may include the following:

Lab tests: Blood samples are checked for levels of serum calcium and parathyroid hormone (PTH). A change in these levels may be indicative of Albright hereditary osteodystrophy.
Plain x-rays can be used to identify lesions.
A small biopsy sample may be taken to confirm the diagnosis.
Histological test: When a small part of osteoma tissue is taken on a slide, stained with dyes, and observed under the microscope, the cells that form different types of blood cells (like RBCs, WBCs, platelets) are absent. In normal tissues, these elements can easily be seen in the medullary cavities, which are enclosed areas of fat.

Treatment and Removal

Excision

Osteoma cutis may be surgically removed from the skin. In these procedures, the overlying skin is removed by excision or laser resurfacing.

In the laser resurfacing technique, a laser beam is used to break the molecular bonds of the skin. Osteoma cutis may be treated with any of the two options available for laser surfacing – Er:YAG laser or carbon dioxide laser.

Side effects of laser surgery may include the risk of hypopigmentation, in which the affected area of the skin becomes lighter in shade. Scarring may also arise after a laser surgery, despite taking all possible precautions.

Er:YAG laser treatment for osteoma cutis is seen to cause less hypopigmentation and tissue scarring. Carbon dioxide laser treatment might also result in longerrecovery time, apart from higher risks of hypopigmentation and scarring.

Medication

Tretinoin cream may be applied topically to promote transepidermal elimination of the bony tissue; however, the efficacy of this method has not been proven. Similarly, etidronate disodium can be taken orally. Though etidronate sodium is used to prevent or treat bone problems, its efficacy on treating osteoma cutis is not proved.

Needle microincision-extirpation technique

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This technique may be used in osteoma cutis patients, who are unresponsive to non-invasive treatment options. In the procedure, the skin overlying the lesions is incised with a needle and then the bony papules are extirpated (eliminated from the base) by using a small curettage device. The resulting lesions are left to secondary healing, which means the wound is left open and allowed to close by growth and contraction of skin on the affected site. The results of this procedure may be cosmetically acceptable in many cases.

References :

http://dermnetnz.org/pathology/osteoma-cutis-path.html

http://emedicine.medscape.com/article/1103575-overview

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1294330/