HSS What’s the Diagnosis Case 162

What’s the Diagnosis – Case 162

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What’s the Diagnosis – Case 162

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What’s the Diagnosis – Case 162

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What’s the Diagnosis – Case 162

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What’s the Diagnosis – Case 162

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What’s the Diagnosis – Case 162

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What’s the Diagnosis – Case 162

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Findings The radiographs demonstrate deformity of the distal phalanx of the index finger and a soft tissue mass causing indolent erosion about the dorsum of the distal phalanx. The MRI shows a high T2 weighted/near fluid signal soft tissue mass causing indolent erosion of the bone. Within the mass are areas of persistent low signal that bloom/become more conspicuous on the MPGR (gradient based sequence). Noted is that on the radiograph there is no calcification or metal identified about the soft tissue mass.

What’s the Diagnosis – Case 162

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What’s the Diagnosis – Case 162

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What’s the Diagnosis – Case 162

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What’s the Diagnosis – Case 162

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What’s the Diagnosis – Case 162

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Diagnosis: Epidermoid Inclusion Cyst These benign masses are related to prior trauma where the epithelium of the skin gets displaced deeper into the subcutaneous tissue and forms a reactive mass which can cause indolent erosion of the adjacent bone as in this case.

Typicaly and as in this case, the mass is not calcified and there is no periosteal reaction of the bone. In this case prior pins had been removed and although it can not be detected on the radiographs, minimal metal fragments are present. These minimal metal fragments produce the low signal with the somewhat odd appearing surrounding high signal on the PD and IR pulse sequences. Related to the physics of a gradient sequence, it is more susceptible to local field inhomogeneity and as such, metal becomes more conspicuous or blooms. The same principle is utilized for diagnosis of PVNS utilizing a gradient based sequence.

The differential diagnosis for terminal finger lesions includes a glomus tumor which is normally much smaller because they are so painful that patients seek attention before they grow. They also do not frequently cause this degree of indolent erosion and have no association with prior trauma. Multiple other lesions are possible including enchondroma but as the physis of the distal phalanx is at the base, this is very uncommon but possible. Other more aggressive processes are possible such as infection and even the extremely rare case of distal metastases. These lesions would of course look markedly different on imaging with areas of destruction of the bone, periosteal reaction, and soft tissue abnormality.

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References MRI Findings of Subcutaneous Epidermal Cyst: Emphasis on the Presence of Rupture. Sung Hwan Hong, Hye Won Chung, Ja-Young Choi, Young Hwan Koh, Jung-Ah Choi, Heung Sik Kang. American Journal of Roentgenology.

April 2006; Vol186, Issue4, 961-966. 10.2214/AJR.05.0044

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