An isolated osteochondroma is usually associated with trauma, especially in the distal phalanx of the hallux; although, in general, the cause is not clearly understood or known. The etiology has also been reported as congenital, although trauma appears to be a more commonly reported finding in the subungual tumor. The tumors may develop abnormally with or without a stalk. When the stalk is present, the tumor is termed pedunculated. When the stalk is absent, it is called sessile. Osteochondromas rarely become malignant and mitotic figures are rarely identified microscopically. In less than 1% of all solitary osteochondromas, malignant degeneration of the cartilage cap is usually heralded by a new onset of growth, pain, and rapid growth of the lesion.

The most commonly reported transformation is to a chondrosarcoma. The potential for malignant transformation occurs more often in the hereditary disorder called hereditary multiple cartilaginous exostoses. Malignant transformation to osteogenic sarcoma has also been reported, but not in solitary, subungual osteochondromas. As a subungual tumor, the lesion usually protrudes up through the soft nail bed and appears as firm, slightly lobulated, marble-like, or sometimes translucent cartilaginous bone that causes onycholysis of the nail plate. It can appear strikingly similar to enchondromas and glomus tumors. On gross examination, enchondromas also have a cartilaginous cap and appear lobulated, glassy, gray-blue, and translucent as the tumor erodes through cortical bone. Unlike osteochondromas that originate from the metaphyseal surface of the bone, enchondromas occur deep within the spongiosa of bone. Glomus or glomangioma is a common benign tumor of the fingers and toes that manifests under the nail. Glomangiomas are supplied with an efferent artery, AV anastomoses, and efferent veins measuring about 5mm in diameter on average. This gives the lesion nodular, firm consistency and its distinctive red-blue color.

Differential diagnoses of osteochondroma include enchondroma, pyogenic granuloma, verruca vulgaris, lipoma, nonossifying fibroma, glomangioma, simple bone cyst, unicameral bone cyst, fibrous dysplasia, eosinophilic granuloma, chondroblastoma, chondromyxoid fibroma, osteogenic sarcoma or clear cell chondrosarcoma.

Discussion

Osteochondroma appears to be a benign tumor most commonly described in the foot as a subungual lesion. The tumor has also been reported ‘extraskeletal’ within the foot, but is exceedingly rare. The recurrence rate after resection of osteochondroma is about 1.8 percent. Eliezri and Taylor reported two cases of recurrent subungual osteochondroma, with one recurring just a few weeks after excision. Recurrence of the tumor is likely caused by cells of the resected perichondrium or cartilage cap that is left behind in unresectedbone or nail bed. In our initial removal, it is possible some cells may have been left behind by simple excision without aggressive curettage of bone or adjuvant procedures to destroy the perichondrium. It could also be the result of persistent or repetitive trauma to the phalanx post resection. Our patient was very active and did continue to play volleyball and participate in track. It is interesting to note that in the report of two cases by Tuzuner et al, there was no incident of trauma associated with those cases. They reasoned that since there was no history of trauma in those two cases, it may be suggested that “ a neglected trauma to an immature bone may result in subungual osteochondroma.” Both patients were male at 30 and 15 years of age respectively. Histologically, their specimens revealed “classic histopathologic finding of osteochondroma” as a “well-defined trabecular bone covered with a hyaline cartilaginous cap.” Tuzuner et al, also introduced an important distinction between subungual exostosis and osteochondroma. However, our histologic findings revealed some similarities to subungual exostosis, but were histoligically more consistent with osteochondroma.