Answer BT001

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 Birmingham Radiology Training Scheme

Intraosseous epidermoid cyst of the phalanx

Case Report

The 50-year-old male patient was presented with a soft tissue mass located over the distal part of his left little finger for about two years. No obvious traumatic history could be traced. Initially, the tumor mass was small and tenderless. As time went by, the tumor enlarged progressively and occasional soreness in the finger was noted. Aspirations were performed and some topical agent was given in other clinics, but in vain. Fusiform swelling without tenderness on distal phalanx of left small finger was presented when he visited our hospital. Roentgenographic examination revealed an expansile osteolytic lesion over the distal phalanx of the small finger. There was no trabecular pattern in the lesion nor periosteal reaction around the lesion. The distal cortex of the phalanx had been broken. But the proximal margin of lesion was sharp and mildly sclerotic (Figures 1A, 1B). The first impression of the lesion was benign bone tumor or osteomyelitis, but the possibility of metastatic lesion also could not be ruled out. Incisional biopsy was planed first. However, during the operation, a well-capsulated pale-yellowish cystic mass, measured about 2x1x1 cm in size, with yellow to white cheesy content, was found in the distal phalanx of the digit. The tumor mass was easily peeled off from the distal phalanx (Figure 2). At this point, benign nature of the bone tumor was sure, and the wound was irrigated and closed. The pathologic examination of the tumor demonstrated a cystic wall composed of keratinized squamous epithelium with granular layer, and horny material arranged in laminated layers is noted in the cyst (Figure 3). Epidermal cyst was diagnosed. The postoperative course had been uneventful. There was neither sign of tumor recurrent nor remolding or regrowing sign of the bone on plain x-ray one year after the surgery.

Discussion

Intraosseous epidermoid cyst of the bone is a relatively uncommon lesion which occurred almost solely in the distal phalanxes, mainly in men [2,3]. These cysts usually appear in patients between the ages of 25 and 50. In previous reports, a history of blunt or penetrating injury is usually present in almost all instances, suggesting that such an injury may lead to intraosseous implantation of ectodermal tissue and the subsequent development of an epidermoid cyst [4].

The tibia, ulna, femur, and sternum are the other rare locations of intraosseous epidermoid cyst [3,5,6]. Most reports suggested that the cysts are of traumatic origin, resulting from displacement of epidermis into the dermis by a mild to severe crushing injury, frequently involving a nail bed [4,7]. However, trauma history was denied by the case presented.

According to plain film of the lesion (Figures 1A, 1B), the differential diagnosis may include two groups of bone tumors: well-defined osteolytic lesions and poorly-defined osteolytic lesions.

In the group of well-defined osteolytic lesions, glomous tumor should be the first consideration because this tumor most frequently occurs in subungual region of the distal phalanx. The bone beneath the glomous tumor can be absorbed under the pressure of the tumor when it growing up. However, pain, tenderness, and cold intolerance are the clinical triad of the tumor, which did not exist in the presented case. Enchondroma which is the most frequent bone tumor in the hand is another lesion to be considered. These tumors occur more commonly in the middle and proximal phalanxes than in the distal ones and roentgenographically often show spotty calcification which was not found in the case reported here. Chronic osteomyelitis or Brodie's abscess can have a swollen, severely tender fingertip. New bone formation and periosteal reaction may be seen near the affected phalanx. In the case with intraosseous epidermoid cyst, the lesion is usually tenderless and there is no new bone formation arounded.

In the group of poorly-defined osteolytic lesions, metastasis, Ewing's sarcoma and aneurysmal bone cyst were considered. In old-aged patients with radiological patterns of expansile osteolytic lesion and active destruction of cortex were the candidate of metastases which commonly from the lung and breast. The diagnosis of metastasis may be made when a chest roentgenogram is obtained. Honeycomb destructive pattern or mixed permeative destructive and sclerotic change with significant periosteal reaction and soft tissue mass are the usual radiological findings of Ewing's sarcoma. In aneurysmal bone cyst, extensive osteolytic changes with cortical destruction mimicking a malignant tumor is the typical finding presented in the distal phalanx [8].

As described previously, epidermoid cyst has been thought to originate from a traumatic incident that derives a fragment of keratinizing epithelium into subcutaneous tissue or bone. Some congenital conditions, however, such as acrosyndactyly, were reported with development of epidermal cyst [9]. If a trauma incident must be the necessary factor, can only a minor trauma implant a epithelium tissue deeply into bony tissue and cause an intraosseous lesion? Or it is purely an embryogenic lesion that initially small and cause no symptoms but enlarged progressively under some unknown stimulations? Further study may be essential for elucidating these questions.

References

1. Jones SN, Stoker DJ. Radiology at your fingertips; lesions of the terminal phalanx. Clin Radiol 1988;39:478-85.

2. Roth CSI. Squamous cysts involving the skull and distal phalanxes. J Bone Join Surg 1964;46A:1442-50.

3. Trias A, Beauregard G. Epidermoid cyst of bone. Can J Surg 1974;17:35-6.

4. Fisher ER, Gruhn J, Sperrett P. Epidermal cyst in bone. Cancer 1958;11:643-8.

5. Mollan RAB, Wray AR, Hayes D. Traumatic epidermoid cyst of the ulna. J Bone Join Surg 1982;64B:456-7.

6. Carroll RF. Epidermoid (epithelial) cyst of the hand skeleton. Am J Surg 1953;85:327-34.

7. Knickerbocker TW, Reilly R. Traumatic epidermoid cyst of terminal phalanx. Radiology 1954;63:550-4.

8. Kozlowski K, Azouz EM. Primary bone tumors of the hand (Report of 21 cases). Pediatr Radiol 1988;18:140-8.

9. Christopher TL, Walter BG. Acrosyndactyly with epidermoid inclusion cysts: evidence for the extrinsic theory. J Hand Surg 1993;18A:842-6.

                    
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