Presentation
The patient presented with a swelling over the dorsal aspect of the knee, that had been increasing progressively in size for the last 12 years. Range of motion, including flexing the knee, was limited, and the patient had difficulty sitting or standing for prolonged periods of time. On examination, a firm lesion was palpated with no tenderness, fluctuation, or cutaneous opening. No past history of trauma present. No similar family history. Ultrasound scan showed a homogeneously hyper-echoic lesion with well-defined margins in the subcutaneous plane. Few intra-lesional calcific foci were noted within. MRI was done for further assessment.
Patient Data
MRI:
- A multi-lobulated, well-defined, fluid signal intensity lesion is noted in the subcutaneous plane in the popliteal fossa of left knee.
- The lesion appeared heterogeneously hyperintense on T2W and PDFS images, with a T2W hypointense rim. It appeared hypointense on T1W images.
- No extension into the intermuscular plane or cutaneous breech.
- No involvement of underlying bony cortices, joint, synovial spaces, or knee joint ligament.
- It is seen abutting the muscle belies of gastrocnemius and semimembranosus.
Case Discussion
Epidermoid cysts are usually asymptomatic lesions, that slowly progress in size. They are formed by the invagination or the cystic expansion of hair follicles or epidermis. Most of the lesions are firm to fluctuant and dome-shaped 3 as seen in this patient. Very few cases (4 up till now) of the epidermal inclusion cyst have been described in the knee since it was first described in 2004 3. Within the knee, these cysts can occur in the popliteal fossa or the prepatellar region 1-2. With the history of trauma, possible differential of seroma could be considered.
Treatment includes complete excision. However, a biopsy is required to rule out malignancy and infection, before surgery. Careful dissection to ensure complete removal of the lesion with its capsule is imperative. Bone curettage may be required 4.
Differential diagnosis includes:
- Lipoma
- Abscess
- Seroma
- Soft tissue sarcomatous changes
- Baker's cyst