De novo intra-osseous keratinous cyst – A rare cause of lytic bone lesion

De novo intra-osseous keratinous cysts are esoteric bone lesions. A 61 yr old maleꬷ presented to the Orthopaedic outpatient department of Base Hospital, Lucknow, Uttar Pradesh, in March 2020 with a swelling of the right thumb for 20 yr with recent onset of pain. The patient did not recall any history of trauma. The phalanx was erythematous and tender with associated painful, restricted range of movements. X-ray showed a solitary, well-circumscribed, lytic lesion involving the distal phalanx with the destruction of the dorsal cortex (Fig. 1A and B). Per-operatively, a cystic lesion filled with cheesy material was seen. Complete excision of the cyst wall was done followed by autologous bone grafting. Histopathology showed a cyst wall lined by stratified squamous epithelium with preserved granular layer enclosing lamellar keratin, indicating it to be an epidermoid cyst (Fig. 1C and D). Post-operative radiograph showed the lesion being filled by a bone graft (Fig. 2A). At the end of three months, the scar was seen to be healing well (Fig 2B); at nine months, he was free of pain, having clinical and radiological evidence of healed lesion (Fig. 2C and D); at 16 months, he had full functional recovery of the thumb.

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Fig. 1 Radiographs showing (A) anteroposterior; (B) lateral view of a radiolucent lesion (white arrow) involving the distal phalanx of the right thumb with the destruction of the dorsal cortex; (C) haematoxylin and eosin-stained section (×40) showing fragmented cyst wall lined by stratified squamous epithelium enclosing lamellated keratin; and (D) (×400) with preserved granular layer (blue arrow).

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Fig. 2 (A) Post-operative lateral view radiograph showing the lytic lesion in the distal phalanx of right thumb (white arrow) being filled up by the bone graft; (B) clinical appearance at three months, post-surgery shows a linear scar (black arrow) with overlying scab in the dorsal aspect of the right thumb; (C) lateral view radiograph at six month post-surgery showing the bone graft to have been well incorporated into the lesion (white arrow) in the distal phalanx of the right thumb; and (D) clinical appearance at nine month post-surgery showing a well healed scar (white arrow) in the right thumb.

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Intra-osseous keratinous cysts have a male preponderance and are present in the fourth and fifth decades. These present as deformed nails in the distal phalanx with pseudo-clubbing and rarely with pain due to underlying pathological fracture. The postulated pathogenesis includes secondary to traumatic implantation of epithelial cells or proliferation of intra-osseous rests of epithelial elements, as seen in the index case. These are rare expansile radiolucent entities commonly affecting the phalanges. Definitive diagnosis is by histopathology and curettage with bone grafting is curative.