Sometimes the kneecap is sitting in an unbalanced way, usually too far toward the outside of the knee. This can cause the kneecap to slip out of the joint or put more pressure on the thighbone and the cartilage in the knee. All of these problems can cause pain.
Tight thigh muscles can also lead to knee and kneecap pain. If the muscles on the front of the thigh (quadriceps) are tight, the kneecap gets pushed hard against the thighbone. Tight muscles on the back of the thighs (hamstrings) force the quadriceps to work harder, which may cause kneecap pain.
Patellar tendonitis (PDF), or jumper’s knee, happens when the patellar tendon becomes inflamed. It is common in children who do repetitive jumping activities, like jump rope or basketball.
Patellofemoral syndrome (PDF) is pain at the front of the knee caused by irritation of the kneecap or end of the thigh. Irritation can be caused by overuse of the knee from a repetitive activity, doing weight-bearing work that puts extra pressure on the joint or from a kneecap that is out of alignment.
In children, spots on the bones around the knee that contribute to growth are weaker than other surrounding bone. If the growth centers (growth plates) become inflamed or irritated, it can cause pain.
When this happens at the top of the shinbone, the problem is called Osgood-Schlatter syndrome (PDF). When it happens at the bottom of the kneecap, it is called Sinding-Larsen-Johansson syndrome (PDF).