Distal Phalanx Intraosseous Epidermoid Cyst

Summary:

An intraosseous epidermal cyst is a benign cystic lesion that occurs in the bones. It is assumed to be caused by congenital causes or trauma, and because the cyst forms in the soft tissue surrounding the bone, it can lead to bone loss. Intraosseous epidermal cysts have a well-defined radiolucent lesion with cortical extension on radiography. Due to clinical and radiological signs being similar, it is vital to distinguish an intraosseous epidermal cyst from other diseases that develop at the distal phalanx. A rare example of intraosseous epidermal cysts at the distal phalanx is reported. We describe the clinical, radiological, and pathologic aspect of this lesion, as well as our current therapeutic strategy.

An epidermoid bone cyst is a lytic lesion or a pseudotumor that affects the cranium or finger phalanges. It is classified as a traumatic or iatrogenic lesion that might be deceiving clinically and radiologically.¹ Epidermal cysts form in the periosteal soft tissue and infiltrate the bone, causing skeletal deterioration in a circular or discoid pattern. Curettage and bone grafting are used to treat the condition, which has an excellent prognosis and a low recurrence rate.² According to certain research, phalangeal cysts are caused by traumatic implantation of epidermal fragments into the bone as a result of any form of injury or by “a migration of a fragment of nail bed into the phalangeal bone.” Iatrogenic origin is frequently linked to past surgery, especially in the case of amputation stumps.³

Intraosseous epidermoid cysts can be mistaken for a malignant bone tumor because they both appear on X-ray as a destructive osteolytic lesion.² We present a case of this uncommon pathological condition in the index finger’s distal phalanx. Furthermore, this lesion was seen in an unusual location along the terminal phalanx’s longitudinal axis.

CASE PRESENTATION

A 30-year-old man with no significant medical history was referred to our hospital, with swelling of the left index finger that evolved gradually over a year. Patient denies any trauma to the finger. On examination, the swelling was present subungualy with pain and tenderness. X-ray images of the left hand showed an expansile lytic lesion involving the distal phalanx (Fig. 1). MRI showed a well-defined intraosseous lesion measuring 1.2 × 1.0 × 1.0 cm. This lesion showed high T2WI and low T2WI intensity.

Fig. 1.

Preoperative radiograph of the left fourth distal phalanx indicating a circular radiolucent lesion with cortical growth and thinning.

The patient underwent surgery under sedation with finger block; the incision was mid-lateral on the radial side of the finger after careful dissection so as to protect the neurovascular bundle.

The hole was made in the bone where the cortex is attenuated, and the cyst was removed in one piece. Total excision was made in one piece without rapture of the cyst (Fig. 2).

Fig. 2.

Intraoperative photograh after curettage.

The cyst was opened intraoperatively on the table and found to have keratinous material indicating epidermoid inclusion cyst. Demineralized bone matrix (Surefuse) was applied into the defect, the wound was closed by nonabsorbable suture, and mallet splint was applied

Postoperative recovery was uneventful with good range of motion and no pain or tenderness (Figs. 3 and 4).

Fig. 3.

Postoperative radiograph of the left fourth distal phalanx post bone graft.

Fig. 4.

Postoperative image showing good wound healing.

Histopathology shows a cyst lining by a stratified squamous epithelium with keratin material, which goes with epidermoid inclusion cyst. Two months postprocedure, the patient was seen in clinic, and his wound had healed well with acceptable range of motion and complete pain relief. The patient was also advised to avoid trauma. At 6 months assessment, there was no evidence of trauma or recurrence.

DISCUSSION

Intraosseous phalangeal lesions include a wide range of differential diagnoses, including inflammatory (ie, chronic infection), benign, and malignant diseases.⁴ Intraosseous epidermoid cysts appear on radiograph as expansile swellings with a radiolucent lytic look. Literature review revealed that average age of presentation ranges between 25 and 50, while one instance was documented in an 8-year-old child.⁶ Even though most individuals are pain-free, some do experience pain, as in our case. The intraosseous phalangeal inclusion epidermoid cyst is more common in men. It usually involves the left hand more, with the middle finger being the most common involved digit.⁷

The lesion has also been reported in amputation stumps, phalanges of toes, and the big toe; the ulna, femur, tibia, and sternum are some of the other sites of involvement.⁸ (See figure, Supplemental Digital Content 1, which displays the MRI showing intraosseous lesion measuring 1.2 × 1.0 cm. http://links.lww.com/PRSGO/C483.) (See figure, Supplemental Digital Content 2, which displays the cyst lined by benign keratinizing squamous epithelium with prominent granular layer in keeping with epidermal inclusion cyst (H&E stain 20×). http://links.lww.com/PRSGO/C484.)

Many theories have been proposed regarding the pathogenesis of intraosseous epidermoid cysts which include congenital, traumatic, and iatrogenic etiologies. However, a traumatic theory remains the most accepted hypothesis. Clinical differentiation between neoplastic, congenital, and inflammatory diseases is difficult; it can be based on the location of the cyst, but the final diagnosis can only be made histologically. Enchondromas (benign cartilage tumors) and intraosseous synovial cysts are the most common proximally based lesions. Those secondary to tophaceous gout or a localized giant cell tumor of the tendon sheath, giant cell reparative granuloma, osteoid osteoma, and epidermoid inclusion cyst are examples of distantly based erosive, cystic lesions.⁵ The most common and destructive primary bone tumor of the hand is enchondroma. An epidermoid cyst has a strong edge that is distinct from the poorly defined osteolytic lesions seen with osteolysis.⁸ Osteoid osteoma usually presents with a perifocal reactive sclerosis.⁹ In poorly defined lytic lesions, several pseudotumorous forms might be recognized (eg, osteomyelitis caused by Staphylococcus as is usually found in diabetic patients). The malignant lesions were all metastatic, including one from lung carcinoma and two from cutaneous squamous cell carcinoma bone invasion. In this investigation, there were no malignant primary bone cancers at the distal phalanx.¹⁰ The authors stated that based on clinical findings alone, it was simple to diagnose metastatic bone cancers and bone invasion. Our preoperative differential diagnosis was narrowed in our cases by the typical preoperative pictures and the history of prior trauma. To confirm the diagnosis, histopathology is needed. In epidermal bone cysts, they are made up of stratified epithelium and intracyst material with desquamated keratin that may be seen under a polar microscope. Most intraosseous epidermal cysts are harmless. Rarely, a benign lesion might transform into a malignant lesion.¹¹ Intra-lesional curettage with or without bone graft is the surgical treatment for epidermoid inclusion cysts. It is possible to expect a smooth recovery with no signs of recurrence during short-term follow-up. There have been no known cases of recurrence after curettage, according to the little published literature.⁴

The vast majority of intraosseous epidermal cysts are harmless. Intra-lesional curettage with or without bone graft is the surgical treatment for epidermoid inclusion cysts; however the need for bone grafting after curettage is still controversial (Table 1).

Table 1.

Literature Review of Epidermoid Inclusion Cysts and Intervention Used

Authors	Bone Graft	Material
Hamad et al1	No
Shin et al2	No
Mattiassich et al12	No
Schajowicz et al10	Yes (2 of 11 cases)	Not indicated
Fernández-Vázquez  et al13	Yes (2 of 3 cases)	Not indicated
Kurosawa et al14	Yes	Corticocancellous bone from the ilium

CONCLUSIONS

The distal phalanx intraosseous epidermoid cyst remains a very uncommon pathologic condition. It is important to distinguish it from other radiolucent phalangeal lesions. Radiographically, an intraosseous epidermoid inclusion cyst is difficult to distinguish from other radiolucent lesions, necessitating histological examination. Intralesional curettage and bone grafting is the treatment of choice.

ACKNOWLEDGMENTS

This case report conforms to the Declaration of Helsinki. Written informed consent was obtained from the patient for publication of this case report and accompanying images.