Osgood-Schlatters usually occurs in boys at a peak of their growth spurt, combined with excessive training or sporting activity. Not always but a high percentage. Usually occurs just below the patella (knee cap) over top a small bump called the tibial tuberosity. This is where the pain site usually is, and is often tender to touch, and can be red swollen, and painful to bend or straighten. It can happen in girls as well but is less common. If your teenager is suffering from knee pain in the above mentioned region with those signs and symptoms it is possible they have Osgood-Schlatters. However, p,ease dont self diagnosis and contact a physio for some professional guidance.