Intratendinous Ganglion of the Extensor Pollicis Longus: Case Report and Literature Review

Ganglion cysts are common benign lesions in the hand and wrist. However, intratendinous ganglion cysts are uncommon. We present a case of intratendinous ganglion cyst in the extensor pollicis longus (EPL) tendon of the right hand of a 73-year-old woman. The subcutaneous mass moved in concert with the EPL tendon in her right thumb. Magnetic resonance imaging showed a space-occupying lesion in the EPL tendon. Biochemical and hemato-immunological examinations ruled out diabetes, rheumatoid arthritis, and other connective tissue diseases. She reported motion pain during thumb extension, for which she desired surgery. An intratendinous cyst was identified intraoperatively within the tendon substance of the EPL, in which a part of the cyst was herniated into a slit in the tendon substance, just distal to the extensor retinaculum, without notable proliferative synovial tissue. The EPL tendon was opened longitudinally, and a cystic lesion was enucleated. Pathological examination showed that the cyst wall consisted of fibrous tissue with degeneration and no epithelial lining. Postoperative recovery was uneventful. Six months after surgery, the patient had no residual pain and no cyst recurrence. The rarity of intratendinous ganglion cysts makes diagnosis and treatment challenging. Because intratendinous ganglion cysts and associated synovitis frequently weaken the structure of affected tendons, prompt diagnosis and surgical excision are necessary.(J Nippon Med Sch 2021; 88: 500-505)