This report describes a congenital solitary osseous choristoma of the left lateral canthus, a rare condition that is seldom reported [4,5,6,7,8]. The term “osteoma cutis” is used when the ossification involves cutaneous layers. However, “osteoma cutis” and “osseous choristoma” have frequently been used interchangeably in the literature [4,5,6,7,8,9]. While the exact pathophysiology of osseous choristoma remains unclear, one theory suggests that it involves the migration of mesenchymal cells to abnormal sites, and their subsequent differentiation into osteoplastic cells [9].

Osseous choristoma may originate from different ocular or periocular tissues, including choroid, episclera, conjunctiva, and extraocular muscles [2]. The formation of osseous choristoma from the lateral canthus is considered extremely rare, however, and only a few cases have been reported [4,5,6,7,8]. In most published cases, the masses were present since birth, without ocular involvement, and were located in the lateral canthus of the eyelid skin. In one case, a medial canthal location was reported [10]. Gupta et al. [5] published two cases of left lateral canthal masses that were similar to the current case in terms of size, shape, and pedunculated appearance, though in those cases there was adherence of the mass to the conjunctiva.

The diagnosis of osseous choristomas can be challenging because their clinical presentation varies depending on the location and size of the lesion, and they are often mistaken for other benign or malignant skin tumors [9, 11]. Histopathologic examination is required to confirm a diagnosis of osseous choristoma, and they exhibit mature bone formation surrounded by dense fibrous stroma with concentric osteocytes arranged around central canals containing fibrovascular tissue. Osteoblastic or osteoclastic activity is rarely present [3, 11].

The treatment of osseous choristoma depends on the location and size of the lesion, as well as the patient’s symptoms and cosmetic concerns. Small asymptomatic lesions that do not cause cosmetic concerns may not require treatment, but larger lesions causing functional or cosmetic problems may require surgical excision [9, 12]. In cases where the lesion is located in a cosmetically sensitive area, such as the face or neck, careful surgical planning is required to minimize scarring and achieve satisfactory cosmetic results [13]. The prognosis of osseous choristoma is generally excellent, with low recurrence rates reported in the literature, suggesting that the benign nature of the tumor can help reduce patient anxiety and avoid unnecessary interventions [9, 11].