Intraosseous Epidermoid Inclusion Cyst in a Great Toe: A Case Report and Review of the Literature

Epidermoid cysts of bone are rare and mainly involve the skull and terminal phalanges. In the published literature, terminal phalangeal involvement usually occurs in the distal phalanges of the fingers.1–4 Three cases of this lesion have been reported in the distal great toe.5–7 Those phalangeal cysts with gradual enlargement of the soft tissues may or may not be accompanied by tenderness or pain. They are commonly regarded to be congenital, traumatic, or iatrogenic in origin.1,6,8,9 Some studies suggest that the origin of phalangeal cysts is either directly related to traumatic implantation of epidermal fragments into the bone by any type of injury1,2,9,10 or due to “a migration of a fragment of nail bed into the phalangeal bone.”11 Iatrogenic origin usually is related to previous surgery, particularly in relation to amputation stumps.6,8 Epidermoid cysts of the bone can be clinically and radiologically challenging. A correct diagnosis is rarely made before operation by radiologists or surgeons primarily because of failure to recognize this entity. It therefore is important to consider this lesion in the differential diagnosis to avoid unnecessary digital amputation.

A 54-year-old man was involved in an accident at work 3 years before this admission at which time he injured his back, left shoulder, and right great toe. He required surgical rotator cuff repair of his left shoulder and conservative treatment of his back. Meanwhile, his right great toe injury was ignored. He had not been able to work full time because of the injuries, particularly because of the chronic problems associated with his left shoulder. With regard to his right toe, it was initially thought to have been fractured without requiring treatment. The toe had gradually enlarged, intermittently causing discomfort, largely because it no longer fit into his shoe. The patient had a foot x-ray film taken shortly after the time of injury, which showed a radiolucent lesion within the distal phalanx having a “ground glass” appearance. Nothing was done at that time. Recently, he had another x-ray film taken, which demonstrated that the radiolucency had enlarged with destruction of most of the distal phalanx (Figure 1). Magnetic resonance imaging was described as demonstrating a signal abnormality of this area. His medical history was otherwise unremarkable. On physical examination, he was a slender man in no acute distress. His gait pattern was normal. Examination of his right lower extremity revealed no lymphadenopathy. There were no masses in the leg and no skin lesions. His right great toe was substantially enlarged when compared with his left toe (Figure 2). The nail was enlarged, tender, and thought to be infected; sensation was normal. Clinically, a giant cell tumor possibly aggravated by his injury was suspected. The differential diagnosis included an inclusion cyst or chronic infection. Planned treatment was an incisional biopsy followed by an immediate curettage if the lesion was benign; an amputation would be performed if the lesion was malignant.

During the operation, after incision of the soft tissue over the phalanx, a creamy material was obtained from within the bone, which was sent for cultures. Afterward, a thick lining shell was easily curetted from within the cavity. After aspiration of the creamy material, a thick, yellow rind within the bone was completely removed. A frozen section showed this to be consistent with an epidermal inclusion cyst. The patient recovered uneventfully, with normal toe function restored.

The specimen was received fresh for frozen section. It appeared to be a whitish tan, firm cystic structure. On the cryostat microtome, it was difficult to cut thin sections, and sections easily rinsed off the slides during the dehydration procedure. Frozen section diagnosis was “hyperkeratotic cyst, consistent with epidermoid inclusion cyst.” Permanent sections confirmed this diagnosis, which demonstrated that the wall of the cyst was composed of keratinizing stratified squamous epithelium with abundant keratin debris. There were no skin appendages seen. The cyst had a granular layer similar to that of the epidermis, which has the same morphologic features as other epidermoid cysts in the body (Figure 3).

Epidermoid inclusion cysts are commonly seen in subcutaneous soft tissues. They are more rarely found in the bones, presenting as lytic lesions or pseudotumors. Bone involvement is mainly seen in the skull and phalanges5 and much less in the maxilla, mandible, temporomandibular joint, vertebrae, tibia, and femur.1,8,12 A large number of epidermoid cysts are located in the distal phalanges of the fingers, with only 2 cases involving the great toes described in the English literature5,6 and 1 case in the non–English European literature.7 There was one more case involving the second toe.13 

The patients are usually between 25 and 50 years of age, although one case was reported in an 8-year-old child.4 Although most patients are free of pain, some experience pain and seek medical attention because of the increased size of the terminal phalanx. Men are more commonly affected than women, especially those whose work involves potential traumatic injury. Particularly frequent are the cysts located in the terminal phalanges of the fingers, especially the left middle fingers, occasionally involving the midphalangeal bone.6 Most bony epidermoid cysts have been solitary, but in rare instances the patients had several cysts involving the fingers of the same hand.5 The origin of phalangeal epidermoid inclusion cysts is traumatic and secondary to implantation of epidermal elements into the bone.1,2,9,10 The radiologic presentation is a well-defined, expanding radiolucent intraosseous lesion, often with an associated soft tissue swelling (Figure 1).

Although chronic infections are high on the differential list of diagnoses for radiolucent lesions of the phalanx, both benign and malignant tumors, including chondroma, intraosseous ganglion, osteoid osteoma, simple bone cyst, aneurysmal bone cyst, giant cell tumor, giant cell reparative lesions, and even metastatic lesions, must be considered.

Enchondroma or chondroma is the most frequently occurring lesion in the phalanges; however, it usually occurs in the proximal segments of the phalanges, because it usually is associated with the metaphyseal side of the growth plates, which are at the proximal ends of the phalanges. Enchondromas are statistically the most common space-occupying process of the short tubular bones of the hands and feet, although they are more common in the hands than in the feet. Characteristically, spotty, or stippled calcifications in a radiolucent lesion favor the diagnosis of chondroma.

Other than benign cartilage lesions, a few other tumors of the terminal digits should be considered in the differential diagnosis. Osteoid osteoma usually presents as a painful lesion with perifocal reactive sclerosis. If a clinically painful lesion is accompanied by sensitivity to cold temperatures, the differential diagnosis should include glomus tumor, which has scalloped edges and bone erosion on radiographs. The glomus tumor is frequently located at the tip of a digit or subungually; however, primary involvement of bone is rare. A lack of pain combined with perilesional sclerosis frequently permits a radiologic diagnosis of intraosseous epidermoid cyst.6 Although aneurysmal bone cyst and giant cell tumor may occur in almost any bone, their presence limited to the terminal great toe would be extremely uncommon, especially since they involve the ends of the bone that have growth plates. In addition, aneurysmal bone cyst without an associated underlying lesion is rare after the second decade of life. Myeloma rarely involves the terminal phalanx and is usually a radiolucent lesion without significant periosteal or endosteal reaction. Phalangeal involvement is usually by multiple lesions.

It is rare that metastatic tumor to the great toe presents as an initial clinical complaint. In this circumstance, the patient usually has a known primary neoplastic manifestation. Although solitary metastasis from bronchogenic carcinoma has been described in the distal phalanx, a primary neoplasm of the bone cannot be excluded by this clinical appearance.14 

Several pseudotumorous conditions can be seen in poorly defined lytic lesions (eg, osteomyelitis caused by Staphylococcus as is usually found in diabetic patients). Gout generally can be recognized with little difficulty because of its juxta-articular location on both sides of the joint accompanied by evident articular changes.6 

Regardless of the origin, histologic examination is required for the correct diagnosis. During the operation, frozen section diagnosis is reliable and can be valuable. Often, a thin shell of cyst wall is usually present, which may not be easily sectioned satisfactorily on a cryostat because of thick keratin. The epidermoid cyst is basically a keratin-filled cavity lined by stratified squamous epithelium usually with an apparent granular layer. Morphologically, it has to be distinguished from dermoid cyst. The dermoid cyst is also lined by keratinizing stratified squamous epithelium; however, it contains one or more skin appendages, such as sebaceous glands, sweat glands, or hair follicles. The lumen is filled with keratin or sebaceous material. If sufficient material is obtained, a diagnosis of epidermoid cyst in the terminal phalanx can even be suggested by fine needle aspiration.10,15 Curettage of the cystic component with or without bone graft not only is adequate treatment but also can avoid unnecessary further treatment. In summary, intraosseous epidermoid inclusion cysts of the great toe constitute a rare clinical entity often associated with previous trauma that should be differentiated from other radiolucent osseous neoplasms.