ANSWER
The correct answer is implantation cyst (choice “b”). This type of cyst is typically caused by trauma (eg, a puncture wound), and its contents are in stark contrast to those of ganglion cysts (choice “a”), which are thick and clear.
Warts (choice “c”) are essentially an epidermal process, not subcutaneous. They almost always disrupt normal skin lines, which often curve around the wart—a finding that was missing in this case.
Acquired digital fibrokeratomas (choice “d”) are benign solid tumors frequently seen on fingers. However, they are more epidermal than intradermal and demonstrate a diagnostic feature called an epidermal collarette (missing in this case).
DISCUSSION
Sometimes called implantation dermoid cysts, these sacs have a well-defined white cyst wall and cheesy, often odoriferous contents. Although common on hands and fingers, they can occur in almost any location and as a result of many types of trauma.
This includes surgical trauma, which effectively implants surface adnexal tissue (eg, the sebaceous apparatus) where it can continue to produce and accumulate its cheesy contents over time. Patients often forget the trauma that caused the cyst, but it is still worth inquiring into.
Merely emptying the sac can confirm the diagnosis; however, this almost always results in recurrence. Fortunately, implantation cysts are usually easily removed with minimal risk to hand function.
As with almost any tissue removed from the body, the specimen needs to be sent for pathologic examination. In addition to the differential items already noted, a number of rare or unusual conditions can present in a similar fashion, including eccrine carcinoma and a variety of sarcomas.
This patient recovered from his surgery without complication. Pathologic examination confirmed the benign nature of the lesion.