Infiltration of synovitis into the flexor tendon: a case report

Synovitis is a type of aseptic inflammation that occurs within joints or surrounding tendons. No previous reports have described a hypertrophic synovium eroding the tendon sheath and manifesting as synovitis within the flexor tendon. We herein report a case involving a 10-year-old girl who presented to our hospital with a 1-month history of a swollen mass and progressive inability to completely flex her left index finger. The active flexion angle of the proximal interphalangeal joint was limited to 85°. A longitudinal incision of the flexor digitorum profundus tendon was surgically performed. The synovium inside and outside the flexor digitorum profundus tendon was completely removed. After the surgical excision, normal tendon gliding returned without recurrence by the 1-year follow-up. The active flexion angle of the proximal interphalangeal joint improved to 100°. To the best of our knowledge, this is the first case of synovitis affecting the flexor tendon and leading to limited flexion of a finger. The manifestation of a double ring sign on magnetic resonance imaging is quite characteristic. Early diagnosis and monitoring of the hyperproliferation and invasiveness of the synovial tissue are required. Surgical excision can be a simple and effective tool when necessary.