Epidermal (Epidermoid) Inclusion Cyst of the Hand

Mechanism. The fingertip and palm are frequently subject to penetrating injuries. If a fragment of surface epidermis is driven into the deeper tissue, or buried at surgery, the trapped keratinising epithelium continues to grow and shed keratin into an enclosing cyst.

View larger

Presentation. The result is a firm, well-circumscribed, slowly enlarging subcutaneous nodule, usually on the volar fingertip/pulp or palm. It is typically painless unless it is large, ruptures or becomes infected. A careful history often reveals a previous puncture, laceration or operation at the site.

The intraosseous variant. An intraosseous epidermoid cyst of the distal phalanx presents instead as a localised lytic lesion, with swelling, nail changes or a pathological fracture.

Terminology. Two naming points get asked. The lesion is not a "sebaceous cyst": that term is a long-standing misnomer, since the cyst contains keratin, not sebum, and arises from keratinising epidermis rather than from sebaceous glands. "Epidermoid", "epidermal inclusion" and "implantation" cyst all describe the same thing in the hand, and the last is the most honest name, because in the hand the mechanism is nearly always implantation rather than a spontaneously occluded follicle.

How common. Be careful with the usual one-liner. Across hand and wrist masses overall the ganglion dominates and the epidermal inclusion cyst is one of the next commonest benign lesions, but the ranking shifts with the site studied. In series restricted to the fingers, giant cell tumour of the tendon sheath (GCTTS) often comes first, and excision-only series under-count ganglia because so many are aspirated or simply observed rather than sent to pathology.

The safe exam answer is that the ganglion is the commonest hand mass overall, with GCTTS and the epidermal inclusion cyst the other two names expected in the same breath, while noting that in the digit specifically GCTTS is at least as common.

Who gets it. Patients are typically middle-aged adults, more often men, reflecting the occupational penetrating injuries that cause it. The latency between the injury and a noticeable lump is long, months to many years, so a patient who denies trauma has usually just forgotten a splinter, a needlestick or a minor laceration.

Natural history. The course is slow and benign. Left alone the cyst enlarges very gradually and does not regress, and the events that bring it to attention are usually rupture or secondary infection rather than size alone.

Ultrasound. It shows a well-defined, often hypoechoic cyst and helps distinguish it from a ganglion or a solid tumour. The two features that earn their keep are posterior acoustic enhancement, confirming the lesion is fluid- or debris-filled rather than solid, and the absence of internal vascularity on Doppler. A cyst densely packed with keratin can look deceptively solid on greyscale alone, and GCTTS, the main solid mimic, shows internal flow, so it is the Doppler that separates them.

Radiographs and MRI. Radiographs are essential if an intraosseous lesion is suspected. They show a well-defined lytic lesion with a sclerotic margin in the distal phalanx, and exclude enchondroma and osteomyelitis. MRI can characterise deeper or recurrent lesions.

Histology decides. The definitive diagnosis is histological, and the features that make it are set out in the next section. Always send excised tissue for histology, particularly to exclude an unexpected diagnosis.

The differential. Among the soft-tissue masses of the hand it includes:

• Ganglion
• Giant cell tumour of the tendon sheath
• Epidermal inclusion cyst
• Foreign-body granuloma
• Glomus tumour - subungual, painful, cold-sensitive

An intraosseous lytic lesion is imaged and biopsied, and its differential includes:

• Enchondroma - the commonest bone tumour of the hand
• Intraosseous epidermoid cyst
• Osteomyelitis - especially with a chronic digital wound; it can coexist with an epidermoid cyst

The epidermoid cyst is lined by stratified squamous epithelium with a granular layer producing laminated (lamellated) keratin, and it has no skin adnexal structures (hair follicles or sebaceous glands), which distinguishes it from a true dermoid cyst. The whole family of keratinous cysts is sorted on histology: whether the lining has a granular layer, the type of keratin, whether skin adnexa are present, and whether there is an epithelial lining at all.

Complete excision cures it. Treatment depends on the form and on whether infection is present:

• Soft-tissue cyst - complete surgical excision including the entire cyst lining/capsule. Incomplete removal, simple rupture or aspiration leaves epithelium behind and leads to recurrence, and marsupialisation is inadequate.
• Intraosseous cyst of the distal phalanx - curettage with removal of the lining, filling the cavity with bone graft or substitute as needed. Outcomes are generally good, with a low recurrence after thorough curettage.
• Coexisting osteomyelitis (chronic digital wound) - debridement and curettage plus antibiotics treats both entities; a bone substitute carrying antibiotic is one option.
• Infected cyst - a genuinely infected cyst is drained and the infection treated first, then the cyst is excised electively once it has settled. Excision through infected tissue both fails to achieve a clean plane and risks a worse wound.

Does every cyst need an operation? No, and it is worth being able to say so. A small, asymptomatic, confidently diagnosed cyst in a site that does not interfere with function may reasonably be observed, with the patient told what would change the plan: enlargement, pain, interference with grip or pinch, repeated inflammation, or any uncertainty about the diagnosis.

Two qualifications keep that from becoming a soft option. Observation is only defensible when the diagnosis is genuinely secure, because a firm nodule that is assumed to be an inclusion cyst is the way a solid tumour gets watched. And a cyst near the pulp or a pressure point tends to declare itself eventually through rupture or infection; an elective excision on a quiet finger is a considerably better operation than an urgent one on an inflamed one, so waiting is not risk-free.

The goal. Because incomplete removal recurs, the examinable point is how a complete removal is achieved: the whole epithelial lining comes out, en bloc without rupture for the soft-tissue cyst and by thorough curettage to clean bone for the intraosseous form. The cyst sits among the digital neurovascular structures, and its contents provoke a reaction if spilled.

Soft-tissue cyst.

1. Operate under tourniquet and loupe magnification; these are not optional refinements.
2. Plan the incision to give access without crossing a flexion crease, and off the pinch surface.
3. Dissect the cyst out with its entire lining/capsule intact. Rupture spills keratin, which both incites a foreign-body inflammatory reaction and risks leaving lining behind.
4. Protect the digital neurovascular bundles in the pulp.
5. If the cyst ruptures, meticulously excise all visible lining and irrigate the wound.

Intraosseous cyst.

1. Make a cortical window.
2. Curette the lesion thoroughly, removing the entire lining to a clean, bleeding cavity; a high-speed burr of the walls helps.
3. Fill the cavity with bone graft or substitute as needed.
4. Send material for histology and culture.

The cyst itself is benign and the operation small, so the complications divide into what the untreated cyst does and what the excision costs.

• Rupture and foreign-body reaction. Extruded keratin is intensely irritant in the soft tissues, producing a painful, red, swollen digit that looks infected but is initially a sterile inflammatory reaction. It is the commonest reason an indolent lump suddenly becomes symptomatic, and the inflamed, ruptured cyst has lost the clean plane that makes complete excision easy.
• Secondary infection and abscess. These may follow rupture or arise independently, and are managed as set out under Management.
• Recurrence. It comes essentially always from retained lining, after rupture during dissection, aspiration or marsupialisation, or after incomplete curettage of the intraosseous form.
• Digital nerve or artery injury. A real risk, given how often these cysts sit in the volar pulp among the neurovascular bundles.
• Painful or adherent scar and pulp tenderness, particularly after a volar fingertip incision.
• Bone involvement in the intraosseous form. Progressive expansion, cortical thinning and pathological fracture of the distal phalanx, with nail deformity where the lesion abuts the germinal matrix.
• Malignant transformation, chiefly to squamous cell carcinoma, is genuinely rare but real, and it is the substantive reason every excised cyst goes to histology. Suspect it in a long-standing cyst that grows rapidly, ulcerates, becomes fixed or recurs atypically.