d5aa8617_University_of_Chicago_Medicine__Possible_cause
[Type of overuse injury] Osteochondritis dissecans [Symptoms] Knee pain and swelling [Possible cause] 
A piece of the cartilage in the knee joint that separates from the joint surface. Theories suggest that it may run in families or be caused by a metabolic problem.
Overuse Injuries - The University of Chicago Medicine
Possible cause