Osteoma Cutis

Etiology

Osteoma cutis can be either primary or secondary. Osteoma cutis is classified as primary when there is no preexisting lesion. Primary osteoma cutis can occur in isolation or in association with metabolic syndrome.[3]

Osteoma cutis is secondary when it is associated with inflammatory processes, scars, dysembryoplasia, or neoplasia.

Some authors have found a correlation between osteoma cutis and chronic acne.[4] About 85% of osteoma cutis arise as a result of prolonged acne. This is reflected in various sites where osteoma cutis develops, including the face in females and the scalp or chest in males.

Recently, some authors have demonstrated a relationship of osteoma cutis to GNAS1 gene mutations, which is a key regulatory gene in progressive osseous heteroplasia and Albright hereditary osteodystrophy.[5]

Epidemiology

Osteoma cutis accounts for 14% of all cutaneous ossifications.[6] Osteoma cutis can affect adults as well as children. There is a female predominance in osteoma cutis with a peak incidence in the second and third decades.[7] Osteoma cutis can be primary or secondary, which can be due to either inflammatory or neoplastic processes.[7]

The most common locations of osteoma cutis are the face in females and the scalp in males. Other sites of involvement include the breasts, the buttocks, and the extremities. Rarely, osteoma cutis has been seen in mucosae such as the tongue, which is also called osteoma mucosae or osseous choristoma.[2]

Pathophysiology

The mechanism of the formation of osteoma cutis has not been fully established. Several theories have been proposed ranging from hamartomas to nevoid tumors.[7] The most recognized hypothesis is fibroblast metaplasia. According to one theory, the development of osteoma cutis may result from the metaplasia of fibroblasts to osteoblasts secondary to the alterations of the genes that regulate bone formation.[8] In fact, the technique of in situ hybridizations demonstrated that dermal fibroblasts could differentiate into osteoblasts resulting in osteonectin production and increased collagen type 1.[9] Another hypothesis stipulated that primitive mesenchymal cells usually differentiate into osteoblasts but migrate to an ectopic location.[9] According to some authors, skin ossification may also result from gene mutations.

Histopathology

Histologically, osteoma cutis is characterized by the existence of dense eosinophilic deposits in the subcutaneous tissue and the dermis. There are bony spicules with prominent cement lines and calcification. Bony spicules can sometimes perforate the epidermis through a process of transepidermal elimination. There is no associated cartilage formation since the majority of bone formation arises through membranous ossification. Osteoma cutis may demonstrate osteoblasts, osteoclasts, and osteocytes. In large deposits, Haversian systems can be observed. Rarely, bone marrow elements are identified.[10]

Perforating osteoma cutis is a sporadic and peculiar form characterized by a central crater enclosed by squamous epithelium, corresponding to a transepidermal elimination channel.[11]

The immunohistochemical study, as well as special stains, are usually not contributory to the diagnosis of osteoma cutis.

History and Physical

The clinical presentation of osteoma cutis is variable and can range from asymptomatic single to multiple lesions. They range in size from 0.1 cm to 5.0 cm.[7] These lesions may present as papules, plaques, nodules, or as miliary lesions. On palpation, they are hard and can sometimes be responsible for skin discoloration that becomes white or yellowish.[12]

In rare cases, the overlying epidermis may be ulcerated with the release of bony spicules. This rare form corresponds to perforating osteoma cutis.[13]

There are four distinct clinical variants of osteoma cutis:[14]

• Solitary: Occurs anywhere on the skin as a solitary nodule
• Widespread: Characterized by the presence of multiple generalized osteomas that arise in the neonatal period 
• Plate-like: Is often present at birth or grows during the first years of life
• Multiple miliary osteomas: Characterized by the presence of multiple punctate foci of bone tissue inside the skin[15]

For the diagnosis of plate-like osteoma cutis to be made, four criteria are required, including:[16]

1. The presence of a bony plate since birth
2. The absence of metabolic disorder
3. The presence or not of osteomas elsewhere in the body
4. The absence of trauma, infection or other underlying events

Physical examination:

Physical examination of patients with osteoma cutis is crucial in order to eliminate dysmorphic features. Primary osteoma cutis can occur in association with progressive osseous heteroplasia, Albright's hereditary osteodystrophy, or fibrodysplasia of progressive ossification.

Progressive osseous heteroplasia:

Progressive osseous heteroplasia (POH) is a rare entity, which can be either sporadic or an autosomal dominant inherited disease. It is characterized by progressive ossification of the dermis in infancy. Whereas, during childhood, there is progressive ossification of the subcutaneous and deep connective tissue. This disease is not accompanied by endocrinological anomalies. Its clinical course is slower in adulthood and may result in ankyloses of the joints as well as growth retardation of the limbs.[17]

Albright's hereditary osteodystrophy:

Albright's hereditary osteodystrophy (AHO) is a disease clinically characterized by short stature, a round face, obesity, and mental retardation. It can be associated with osteoma cutis (25-50%) and with endocrinological abnormalities.[18]

Evaluation

Serum Calcium and Parathyroid Hormone Levels

Serum calcium and parathyroid hormone levels are performed to rule out Albright’s hereditary osteodystrophy.

Conventional soft tissue radiographs, computed tomography (CT) scans, or ultrasonography of the skin can help to evaluate calcification.

Conventional 2D Radiographs

The conventional 2D radiograph is an imaging study that poses diagnostic difficulties, especially regarding the interpretation and the location of osteoma cutis.[7] 

Cone-beam CT:

Cone-beam CT, with its three-dimensional (3D) capabilities, is a beneficial and accurate imaging modality that helps to detect and establish the diagnosis of osteoma cutis.[7] 

Osteoma cutis is a radiopaque regularly outlined lesion that can occasionally have a radiolucent center. The density of osteoma cutis is identical to the bone. Different shapes of osteoma cutis are described, including donut-or snowflake-like or washer-shaped.[7]

Skin Biopsy

The histopathological examination of the cutaneous biopsy confirms with certainty the diagnosis of osteoma cutis.

Treatment / Management

The treatment modalities of osteoma cutis depend on several factors, including its severity, extension, location, and etiology.[19](B3)

There are several treatment options proposed in the literature. Among the different treatment options, we can distinguish between non-invasive and invasive modalities.

Non-invasive treatment options include the application of tretinoin cream with limited results essentially in small and superficial lesions.[19][20](B3)

Invasive treatment modalities include a combination of dermabrasion and punch biopsy, erbium:yttrium-aluminum-garnet (YAG) laser, scalpel incisions, curettage, and CO2 laser.

Ablative laser procedures using CO2 and erbium:YAG lasers may induce cutaneous pigmentary changes. Recently, some authors successfully performed non-ablative Q-switched neodymium:YAG laser treatments for miliary osteoma that did not induce pigmentary changes.[2](B3)

Concerning secondary osteoma cutis, the associated metabolic abnormalities must be investigated and treated accordingly. For instance, patients with pseudohypoparathyroidism may need calcitriol and calcium replacement therapy when hypocalcemia occurs.[3](B3)

Differential Diagnosis

Histological differential diagnosis:

• Pilomatrixoma in its ossified form
• Osteochondroma 
• Ossified hair follicle (for example, in the case of a melanocytic nevus)
• Osteosarcoma in its extraskeletal form located in the dermis or in the subcutaneous tissue
• Cutaneous calcinosis

Radiological differential diagnosis :

• Calcified phleboliths
• Calcified nodules resulting from surgical clips, wires, or sutures may radiologically mimick osteoma cutis.

Prognosis

Osteoma cutis is a benign tumor that never metastasizes or infiltrates the surrounding structures. The prognosis of this lesion is excellent.

Complications

Ablative laser procedures using CO2 and erbium:YAG lasers may induce cutaneous pigmentary changes.