Implantation Dermoid of the Distal Phalanx: A Missed Diagnosis

Epidermoid cysts of bone are rare and mainly involve the skull and terminal phalanges. Phalangeal involvement usually occurs in the distal phalanges of the fingers [1–4] although three cases of this lesion have been reported in the distal phalanx of great toe [5–7]. Epidermoid phalangeal cysts gradually enlarge and may or may not be accompanied by tenderness. These cysts are commonly regarded to be congenital, traumatic or iatrogenic in origin [1, 6, 8, 9]. A meticulous clinical and radiological examination is required but a final correct diagnosis is rarely made before corroborative operative findings and histopathological examination.

A 30-year-old man, worker at railway yard, presented to our hospital outpatient department with swollen and tender left middle finger at the level of distal phalanx (Figure 1.1 and 1.2). He was being treated as fracture of distal phalanx, after suffering from an injury due to hammer, for two months with no relief. The patient had previous injury to the same digit with avulsion of the nail 6 months ago. The nail had re-grown normally. The digit had gradually enlarged since the first injury but without any pain and tenderness. When he injured the same digit 4 months after the first injury he developed pain and tenderness. He also observed rapid increase in swelling after the second injury. The patient was being treated by a physician as fracture of the distal phalanx. The patient did not get relief in the pain for two months so he presented to us. General physical examination was normal and no other swelling or lymphadenopathy was found anywhere in the body. Radiographs showed an expansile lytic lesion at the tip of distal phalanx with associated cortical erosions and minimal soft tissue component (Figure 2.1 and 2.2). CT scan showed a well–defined lytic lesion with broken cortex at one place in the distal phalanx of the affected finger probably due to second injury (Figure 3.1 and 3.2).Hematological investigations were normal. From clinical and radiological examination, a giant cell Tumor possibly aggravated by his injury was suspected. The differential diagnosis included an inclusion cyst or chronic infection.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 culture and sensitivity. After aspiration of the creamy material, a thick, yellow rind within the bone was completely removed, which was sent for histopathology. Bone graft was taken from the ipsilateral proximal ulna and suitably shaped to fill the cavity left after thorough curettage. Histopathology demonstrated that the wall of the cyst was composed of keratinizing stratified squamous epithelium with abundant keratin debris. There were no other skin appendages seen. Culture was negative after 48 hours of incubation. The patient recovered uneventfully, with normal functional digit and satisfactory radiographs (Figure 4.1 and 4.2).

True epidermoid cysts result from the implantation of epidermal elements in the dermis and can result from trauma or a surgical procedure. Any skin puncture, laceration or incision has the potential to implant epidermal elements into the dermis and subcutaneous tissue, thereby producing a thick-walled cyst [12]. Epidermoid cysts are frequently seen in farmers, factory workers, carpenters and tailors who are subject to repeated minor traumas to the hand [13]. Human papilloma virus infection, ultraviolet exposure and eccrine duct obstruction are also suggested to be the additional risk factors

in the development of palmoplantar epidermoid cysts [14-16]. The epidermoid inclusion cysts of the hands usually present as asymptomatic, slowly enlarging, painless, superficial and localized swellings over the volar aspect of the hand, especially the distal portion of the digits. Occasional cases of epidermoid inclusion cyst arising from deeper structures like tendon and phalangeal bones have also been reported [3, 17, 18]. The epidermoid cysts are usually benign lesions; however, rare cases of malignancies have also been reported [19-21]. Epidermoid cysts are often site-specific. The history of associated trauma and basic clinical examination may be helpful in the diagnosis. Radiological investigations usually do not lead to definitive diagnosis. Computed tomography and magnetic resonance imaging [22] are helpful in delineating the anatomy of the soft tissue tumors, although seldom necessary to guide the treatment. Differentiation of epidermoid cysts from other radiolucent digital lesions remains challenging [24]. Epidermoid inclusion cysts of the hand need to be differentiated from ganglions, giant cell tumors of  bones and tendon sheath, lipomas and bony and arthropathic swellings [12, 25]. Fine needle aspiration cytology (FNAC) may have a role in making cytological diagnosis if enough keratin or sebaceous material is obtained. Histological examination helps in the definitive diagnosis as it is important to differentiate this benign lesion from other benign and malignant lesions of the bone [23].

Because most of the hand epidermoid cysts are benign and asymptomatic, surgical intervention is not warranted. The surgical treatment is needed for symptomatic cysts causing interference with hand function. The treatment requires careful complete excision, which can easily be accomplished with regional anesthesia under pneumatic tourniquet [26]. Removal of the entire cyst wall is necessary to avoid the recurrence [27]. Although it has been reported that an unduly large number of phalangeal cysts have been treated by amputation, the judicious planning of treatment can prevent this scenario. In our case, the lesion was likely caused by initial trauma and was asymptomatic initially. It became symptomatic due to second trauma.

Epidermoid cysts of the bone are rare and mainly involve the skull and phalanges. Diagnosis of such lesions can be easily missed if proper clinical and diagnostic skills are not applied. It is important to perform thorough investigations and include this lesion in the differential diagnosis to avoid unnecessary treatment.

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