Intraosseous epidermoid cyst in the toe of a dog

By Emma Scurrell

The fourth digit of the left foreleg was amputated in a 3-year-old, female Labrador Retriever in which radiographs indicated a focal expansile lytic lesion affecting the third phalanx (P3).

Fig 1. Radiograph of the left foot reveals a discrete cystic mass within P3 of the fourth digit. Radiograph courtesy of Gilmore Vets Ltd of Durham.
Fig 2. Histopathology. The bone of P3 is focally replaced by a discrete cyst. The cyst contains abundant keratin and is lined by a layer of well-differentiated, benign squamous epithelium (arrows).

Final Diagnosis

Intraosseous epidermoid cyst of the third phalanx

Discussion

Intraosseous epidermoid cysts are not common but account for lytic lesions occuring most commonly in the distal phalanges of dogs. Other sites occasionally affected include the vertebrae and skull. The cause of these benign cysts is not known but the main thoughts include nests of heterotopic ectoderm sequestered within bone during embryonic development or a penetrating wound resulting in traumatic implantation of epidermis into the underlying bone.