(1) Background: Little is known about the etiology, clinical features, diagnosis methods, treatments, and the prognosis of synovial fistula of the ankle joint. The purpose of this study is to investigate the clinical features of synovial fistula of the ankle joint. (2) Methods: Between March 2003 and December 2018, 40 cysts associated with synovial fistula of the ankle joint were treated consecutively by two surgeons. Case histories, clinical manifestations, intraoperative findings, surgical treatment methods, and treatment outcomes were evaluated to characterize fistula-associated cysts. The clinical results were assessed using the visual analog scale (VAS) and the American Orthopaedic Foot and Ankle Society (AOFAS) ankle-hindfoot functional scores, preoperatively and at the last follow-up. (3) Results: The main complaints were ankle instability and pain (15 patients), pain only (15 patients), instability (seven patients), and cosmetic problems (three patients). Eleven patients had a cyst with an open skin wound, and eight of these patients had undergone surgery under a misdiagnosis of bursitis. Cysts were located anterior to the lateral malleolus in 22 cases, next to the lateral malleolus in 13 cases, posterior to the lateral malleolus in three cases, and across the entire lateral malleolus in two cases. Mean VAS and AOFAS scores improved from 5.2 (range, from 1 to 7) and 72.3 (range, from 65 to 87) preoperatively to 1.1 (range, from 0 to 3) and 93.6 (range, from 85 to 100), respectively, at final follow-up visits. (4) Conclusions: Cyst occurrence due to a synovial fistula should be considered when treating a cyst around the lateral malleolus. Fistula repair and reinforcement with fibular periosteum provides a good treatment option for cysts attributed to synovial fistula of the ankle that fail to respond to conservative treatment.
Keywords: ankle; characteristics; prognosis; synovial fistulae.