Teaching Case

35-year-old female patient presents to our emergency department with swelling of the big toe with redness and pain. The patient has undergone multiple surgeries on this toe because of an ingrown toenail; the last surgery took place 1 year ago.

Plain films show an expansive, eccentric, lucent lesion involving the distal phalanx of hallux with sclerotic well-defined margins and a cortical defect. There is scalloping of the dorsal aspect of the phalanx on the lateral view, suggesting possible remodeling from adjacent soft tissues. On MRI, there is a well-defined round lesion of intermediate signal intensity on T1-weighted images and intermediate to high signal intensity on T2-weighted images. Contrast-enhanced MR shows peripheral enhancement and DWI-ADC sequences show restriction of diffusion in its centre probably due to its high content of keratin which was confirmed by histology.

Epidermal cysts are benign cystic lesions caused by ectopic proliferation of epidermal cells within the dermis. Intraosseous epidermal cyst are rare, compared to their intradermal or subcutaneous counterparts. They are usually subungually located [1, 2]. The exact pathogenesis of epidermal cysts is unknown, but –as in our case- entrapment of an epidermal cyst inside bone tissue due to repetitive surgeries at this area was suggested. An intraosseus epidermal cyst typically presents as expansive lytic lesions, which can mimic a cyst on ultrasound and x rays due to its relative hypoechoic and radiolucent aspect. On MRI, they usually present as low or intermediate T1 signal intensity lesions because of its high keratin and cholesterol content respectively and high or intermediate signal intensity on T2-WI depending on the total amount of water they may have. Diffusion restriction (DWI-ADC) may be seen, probably due to its high keratin content. An intraosseous epidermal cyst characteristically presents with peripheral enhancement after gadolinium intravenous administration because of the absence of intralesional vessels and the presence of a peripheral vascular capsule [1]. Osteomyelitis is the main differential diagnosis in our case but the absence of a high white blood cell count and low PCR on laboratory tests argued against this diagnosis. Aneurysmal Bone Cyst (ABC) and simple bone cyst may be quite similar to our lesion on X Ray images, but there is no diffusion restriction. In ABC, intralesional fluid-fluid levels are seen. Giant cell tumour and enchondroma may also have a similar appearance on plain films but these lesions have a different enhancement pattern on MRI (septal pattern in enchondroma and homogeneous central in giant cell tumour). The lesion was resected and histologically confirmed after surgery.