Pain in left knee.
A 8 years old boy presented with a pain in the left knee while practicing basketball.he had no history of knee injury.1week h/o his left knee pain and hydrathrosis had become too severe to continue playing basketball well.On physical examination, full range of motion,tenderness at the level of lateral joint line,pain and catching on deep flexion and a postive mcmurray test on the lateral side was observed. His gait was normal and there was no sign of giving way of the affected knee.valgus test and varus stress test,anterior drawer test,pivot shift test and postive drawer test had negative results,indicating an absence of ligamentous abnormalities. the neurovascular examination shows normal findings. x rays and MRI are normal.Arthroscopic images are shown. Dx??