http://www.orthoinfo.org/fact/thr_report.cfm?Thread_ID=252&topcategory=Children
American Academy of Orthopaedic Surgeons
A teenager who is active in sports and starts to feel a dull, aching pain behind the kneecap (patella) on either or both legs may need to adjust his or her training routine. Chronic pain in the front and center of the knee (anterior knee pain) is common among active, healthy young athletes especially girls.