Intraosseous epidermoid cyst of the distal phalanx reconstructed with synthetic bone graft

Introduction

Intraosseous epidermoid cysts are exceedingly rare. They are known as pseudotumors as they are not true neoplasms. They usually involve the skull and the phalanges of the fingers and toes. ^{1 –3} The most common site of phalangeal involvement is the distal phalanx of the finger. ^{4 –6} Intraosseous epidermoid cysts can mimic a malignant bone tumor, as both typically present as a destructive osteolytic lesion on X-ray. ⁷ Although malignant tumors are very rare in the distal phalanges, the most common malignant bone tumor at this site is a metastatic tumor. We therefore have to keep intraosseous epidermoid cysts in our differential when evaluating bony lesions that occur at this site. Here, we present two cases of intraosseous epidermoid cysts in the distal phalanx treated with curettage and synthetic bone graft, followed by a review of the literature.

Case 1

A 25-year-old male was referred to our hospital with complaints of a painful enlargement of his right index finger over the past 6 months. On exam, the patient had painful swelling at the tip of his right index finger, accompanied by bulging of the nail bed (Figure 1(a)). He had a history of trauma at the tip of the right finger. Regular blood tests were normal. X-rays showed an osteolytic lesion of the distal phalanx with bone expansion and cortical thinning (Figure 1(b)). The cortical bone on the dorsal side of the distal phalanx was eroded. Magnetic resonance imaging (MRI) demonstrated a low signal lesion at the distal phalanx on T1-weighted imaging (WI), with high intensity signal in this area on T2-WI (Figure 1(c)). This lesion was not enhanced by gadolinium (Figure 1(c)). The initial differential diagnosis included enchondroma, glomus tumor, intraosseous epidermoid cyst, and bone metastasis. Given the lack of enhancement of the lesion on MRI with the gadolinium, a benign cystic lesion was suspected. The patient was consented for surgery with plans for intraoperative pathology. During the operation, intra-regional curettage was performed, and a whitish, brittle material was obtained (Figure 1(d)). Frozen sections showed a squamous epithelium without malignant cells. After curettage, hydroxyapatite graft was used to reconstruct the bone defect (Figure 1(e)). Histopathological examination revealed that the wall of the cyst was composed of squamous cells, and the cavity was filled with keratinized lamellar material (Figure 1(f)). Postoperative recovery was uneventful. X-rays obtained 3 months postoperatively that showed good remodeling of the artificial bone with the surrounding host bone (Figure 1(g)).

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Figure 1.

Clinical findings of case 1. (a) The right index finger is remarkably enlarged, with bulging of the nail bed. (b) Preoperative X-rays show a radiolucent lesion with cortical thinning of the distal phalanx. The dorsal cortex is eroded. (c) Preoperative MRI images. The lesion on the distal phalanx has a low signal on T1-WI and a high intensity on T2-WI. The lesion was not enhanced by gadolinium. (d) Intraoperative findings: a whitish, brittle material was obtained from the distal phalanx lesion. (e) Hydroxyapatite grafting was performed after curettage. (f) Histopathologic appearance of the lesion, showing a capsule with stratified squamous epithelial cells and keratin lamellae. (g) Three months after surgery, good remodeling of synthetic bone and union to the host bone were obtained. MRI: magnetic resonance imaging; WI: weighted imaging.

Case 2

A 24-year-old woman presented with a 1-week history of the painful enlargement of her left fifth digit. She had an injury by pinching this finger in a door 10 years ago. The tip of her left little finger was swollen, with bulging of the nail bed. Regular blood tests were normal. Preoperative radiographs demonstrated a defect of the radial cortex of the distal phalanx on the anteroposterior view, and a radiolucent lesion causing bone expansion and cortical thinning on the lateral view (Figure 2(a)). A preoperative MRI of the distal phalanx demonstrated a low signal lesion on T1-WI, with high signal on T2-WI. No gadolinium enhancement was observed. Based on the experience of Case 1, an intraosseous epidermal cyst was suspected. Intraregional curettage was performed. A white, brittle material with a thin membranous tissue was obtained from the lesion. After curettage, beta-tricalcium phosphate synthetic bone grafts were used to fill the cavity to reduce the risk of a pathologic fracture (Figure 2(b)). Histopathological analysis confirmed the diagnosis of an epidermoid cyst. There were no postoperative complications. X-rays revealed complete healing of the bone defect 18 months following surgery (Figure 2(d)).

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Figure 2.

Preoperative and postoperative X-rays of case 2. (a) Preoperative X-rays show a thinning of the cortex of the distal phalanx with erosion of the radial cortex. (b) Postoperative X-rays. After curettage, the cavity is filled with β-TCP bone substitutes to prevent a pathologic fracture. (c) X-rays show good remodeling of the β-TCP 3 months after the operation. (d) Union between the host bone and β-TCP is observed 18 months after operation. The patient had no problem using her finger. β-TCP: beta-tricalcium phosphate.

Discussion

Intraosseous epidermoid cysts are rare and usually occur in the skull and phalanges. The most common site for phalangeal involvement is the distal phalanx. Roth notes that pain is the most common presenting symptom of intraosseous epidermoid cysts, in some cases accompanied by fingertip swelling and nail deformity. ¹ In prior reviews on intraosseous epidermoid cysts, it is well established that they are predominately found in males. ^{6, 8} Although the pathogenesis of intraosseous epidermoid cysts remains undefined, various theories have been proposed, which include congenital, traumatic, and iatrogenic etiologies. ^{6, 8} A traumatic theory remains the most prevalent hypothesis, as many patients have a history of a traumatic event to the involved anatomy prior to symptom onset. ^{8, 9} The interval between the trauma and the appearance of symptoms varied from one to several years. Some studies have suggested that an intraosseous epidermoid cyst is the direct result of the traumatic implantation of epidermal or nail bed fragments into the phalanx. ^{9 –12} Radiographically, intraosseous epidermoid cysts typically present as a well-defined osteolytic lesion without any trabecular pattern. The cortex is expanded and thinned, but bony sclerosis or reaction is typically absent. The characteristic X-ray findings of intraosseous epidermoid cysts mimic those of malignant bone tumors. ² Nearly all bone tumors that occur in the distal phalanx are benign primary tumors. A malignant primary bone tumor at this site is exceedingly rare. Schajowicz et al. reviewed 37 cases of cystic lesions of the terminal phalanx of hand and feet, of which 34 were benign and 3 malignant. ⁸ The benign bone tumors included 13 chondromas, 11 intraosseous epidermoid cysts, 6 osteoid osteomas, 2 aneurysmal bone cysts, and 2 glomus tumors. The malignant lesions were all metastatic, one from lung carcinoma and two from the bone invasion by cutaneous squamous cell carcinoma. There were no malignant primary bone tumors at the distal phalanx in this study. The authors noted that it was easy to diagnose metastatic bone tumors and bone invasion based on clinical findings alone. ⁸ In our cases, the characteristic preoperative images and the history of preceding trauma helped narrow our preoperative differential diagnosis. Asking about a history of preceding trauma is key to diagnosing a lesion as an intraosseous epidermoid cyst. If the diagnosis is not established before the operation, a needle biopsy could be performed prior to curettage. The whitish-yellow cheese-like substance also supports the diagnosis of an intraosseous epidermoid cyst. In our first case, intraoperative pathologic examination of frozen sections helped keep this a single-stage surgery.

The surgical treatment of intraosseous epidermoid cysts is intraregional curettage. To prevent the recurrence of intraosseous epidermoid cysts, complete curettage is needed that includes the wall of the cyst. The need for bone grafting after curettage is still controversial (Table 1). ^{1,8,10,13 –17} In cases without cortical thinning, bone grafting may not be needed. However, because of the destructive nature of this disease, the potential for a pathologic fracture after curettage is worrisome. Although both autologous and synthetic bone grafts can facilitate healing process, synthetic bone is superior in terms of graft site morbidity. As both of our cases were relatively young and desired an immediate return to active occupations (police officer and kindergarten teacher), we chose to use synthetic bone graft substitutes. During the postoperative period, good bone remodeling was observed and both patients returned to work 2 weeks following surgery without any problem.

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Table 1.

Bone grafts reported by previous authors.

Authors	Bone graft	Material	Reference
Hamad et al.	No		13
Shin et al.	No		14
Mattiassich et al.	No		7
Schajowics et al.	Yes (2 of 11 cases)	Not indicated	8
Fernández-Vázquez et al.	Yes (2 of 3 cases)	Not indicated	15
Kurosawa et al.	Yes	Corticocancellous bone from the ilium	16

In conclusion, it is important to consider intraosseous epidermoid cysts as part of the differential diagnosis prior to the treatment of bony lesions of the terminal phalanx. For physically active patients with substantial bone defects, synthetic bone graft may be recommended.