Infected intraosseous epidermoid cyst in the right ankle: a case report

INTRODUCTION

Epidermoid cysts, also known as keratin cysts, epidermal cysts, epidermal inclusion cysts, or epithelial cysts, are benign tumors originating from the infundibular portion of hair follicles. They can either be congenital, arising from the germinal epithelial layer, or acquired due to the inclusion of epidermal tissue following external injury or skin incision.¹ While they are more commonly found in subcutaneous soft tissues, intraosseous epidermoid cysts (IEpC), though rare, have been reported, particularly in middle-aged men following trauma or surgery.^2,3 These cysts most commonly occur within the phalanges of the fingers. While a congenital etiology involving the intraosseous inclusion of embryonic epithelial tissue has been suggested, post-traumatic and iatrogenic causes remain the most prevalent hypotheses.³ Bone involvement is most commonly seen in the skull and phalanges, with much rarer occurrences in the maxilla, mandible, temporomandibular joint, vertebrae, tibia, and femur.⁴

Diagnosing an IEpC can be challenging due to its rarity and resemblance to malignancies or infections, making early histopathological analysis crucial to avoid unnecessary treatments.³

CASE REPORT

A 61-year-old male presented to the outpatient clinic with progressively enlarging swelling on the lateral aspect of his right ankle, which had been present for a year and was initially asymptomatic. The swelling grew to 6.5×4.0×2.0 cm and, 20 days prior to the visit, became painful, tender, erythematous, and warm, suggesting an inflammatory or infected process. He also experienced a low-grade fever and purulent discharge from the cyst. After an incision and drainage at a local hospital provided temporary relief, the swelling continued to discharge fluid and the fever recurred, prompting further evaluation.

The patient's medical history included obstructive sleep apnea and a prior orthopedic procedure involving a metal rod in his right thigh after a traumatic fracture. He denied recent trauma to the ankle, though a mild injury occurred six months earlier. Physical examination revealed a firm, tender, nodular mass was noted over the lateral malleolus of the right ankle, with mild erythema but no fluctuance. The skin over the lesion appeared tense, suggesting pressure from the underlying mass (Figure 1).

Laboratory tests showed mild leukocytosis with WBC count of 13,000 cells/µl (reference range: 4,000–11,000 cells/µl), elevated C-reactive protein (CRP) at 5.2 mg/l (reference range: 0–3 mg/l), and increased erythrocyte sedimentation rate (ESR) of 38 mm/hour (reference range: 0–20 mm/hour), indicating inflammation or infection. Blood cultures were negative. A magnetic resonance imaging (MRI) of the right ankle revealed three closely placed multilobulated cystic lesions on the lateral aspect, likely subcutaneous, with moderate surrounding edema, raising suspicion for an infectious or inflammatory process. Differential diagnoses included an infected ganglion cyst or tumorous cyst.

Given persistent symptoms, the decision was made to perform a wide local excision of the cyst under local anesthesia. The cyst, measuring 7×5×2.4 cm, was excised with a safe margin (Figure 2).

Intraoperatively, the cyst appeared firm and nodular, consistent with a chronic cystic lesion with potential secondary infection. The hemostasis was achieved using Clinicel Knitted (Healthium Medtech, India) and the incised wound was closed using Trusynth plus neo antimicrobial coated polyglactin 910 suture. The postoperative recovery was uneventful, with fever resolution and no recurrence of discharge. The excised specimen consisted of non-sutured, oriented soft tissue pieces. Externally, the specimen appeared unremarkable, with a greyish-white to greyish-brown cystic area filled with pultaceous material.

Histopathological examination revealed microscopic findings of epidermal inclusion cyst lined by stratified squamous epithelium, with keratin flakes present on the luminal side (Haematoxylin-eosin, ×200). Higher magnification showed the cyst lining with stratified squamous epithelium and luminal keratin flakes (Haematoxylin-eosin, ×400) (Figure 3). The cyst also contained lamellated keratinous material, along with hemorrhage, fibrin, and cholesterol clefts. Multinucleated giant cell reactions were observed in some areas. Surrounding tissue showed fibrosis, hyalinization, inflammation, and collections of foamy macrophages. These features were consistent with an inclusion cyst, and no malignancy was noted. The wound healed well without complications.

DISCUSSION

Epidermoid cysts of the soft tissues were first reported by Masse in 1885 and are commonly considered benign soft tissue tumors located just beneath the skin. IEpC were first documented by Harris in 1930, with the majority of cases occurring in the digits.³ The exact prevalence is unknown, and the etiology remains unclear. One hypothesis suggests that trauma leads to the implantation of epithelial cells into subcutaneous tissues, where they survive, proliferate, and produce keratin.² Another theory proposes that intraosseous epidermoid cysts result from the proliferation of intraosseous inclusions of epithelial elements during embryogenesis.³ A third possibility is that these cysts may arise as a result of previous surgical procedures.⁵

Diagnosing an IEpC is challenging radiographically, and a biopsy along with histopathological evaluation is required for confirmation. Once diagnosed, this benign tumor is typically treated by curettage, and in some cases, bone grafting may be necessary. Successful treatment depends on thorough curettage and complete removal of the cyst capsule to prevent recurrence.⁴

Kim et al. reported successful excisions of IEpCs in the phalanges of the fingers, with no recurrence at a three-year follow-up.⁴ Blagova et al described an epidermoid cyst in the maxilla, while D’Andrea et al reported a case of an extradural-intraosseous epidermoid cyst successfully treated through a pterional approach.^6,7 These cases highlight the potential role of trauma in the development of IEpCs and demonstrate that, when completely excised, these cysts generally have a favorable prognosis.

CONCLUSION

This case of an infected epidermoid inclusion cyst in the right ankle emphasizes the importance of considering epidermoid cysts in the differential diagnosis of chronic swelling, particularly when complicated by infection. Surgical excision with complete removal and clear margins remains the treatment of choice, with histopathological confirmation critical for accurate diagnosis.

Funding: No funding sources

Conflict of interest: None declared

Ethical approval: Not required

REFERENCES

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