Bones generally grow from the ends, versus in the middle. New bone forms from a cartilage growth plate (epiphysis), located at either end of the bone. Cartilage isn't as strong as bone, and stress on the growth plate can cause it to become swollen and painful. This may be especially so if your child is very active during his or her growth spurt.

The cause of Osgood-Schlatter disease is activities that place repeated stress on the top of the tibia, the big bone in the lower leg, where the tendon of the kneecap inserts. During activities that involve a lot of running, jumping and bending — such as football, soccer, basketball, volleyball, gymnastics and ballet — the pull of the quadriceps can place tension on the band of tissue that connects the knee to the tibia (patellar tendon).

The patellar tendon may begin to pull away from the raised area on the tibia where it attaches (tibial tuberosity), resulting in pain and swelling. In severe cases, the tendon stretches to the point where it comes away from the tibia, and may take a bone fragment with it.