RESUMEN
A 12-year-old female athlete, training for an international career in pentathlon, was referred to our clinic because of 2 years of recurrent localized swelling and activity-related pain in the medial aspect of her right knee, since falling from a horse and hitting her knees on the ground. She had been examined by a number of doctors over this period and treated with physiotherapy without a proper diagnosis and with no improvement. She could run only very limited distances before excruciating pain and swelling, and she had to abandon participation in competitions. During rest, the swelling decreased but never disappeared entirely. Clinical examination revealed minor effusion and localized tenderness on palpation around the medial joint line. Magnetic resonance imaging showed fluid between the medial capsule and medial collateral ligament, but could not identify any connection to the joint. Arthroscopy revealed a cleavage in the posterior medial joint capsule, superior and close to the medial meniscus, that was connected to the medial collateral ligament bursa. The cleavage was simply expanded, which emptied the bursa, followed by compression bandage and ice for 2 weeks. An immediate positive effect was observed and she could run pain free without swelling within 3 weeks of surgery. Six months after surgery, there has been no recurrence. To our knowledge, this is the first time this injury has been described in a child.