Three bones meet in the knee: the thighbone (femur), the shinbone (tibia), and the kneecap (patella). When the leg moves, the kneecap slides along a shallow groove at the end of the femur known as the trochlear groove. The joint is cushioned and made smooth by articular cartilage, which wraps around the ends of all the bones in the joint.

Repeated stress of the joint, whether due to age, injury, overuse, abnormal alignment, or muscle weakness, can weaken and soften the articular cartilage. This phenomenon goes by many names: anterior knee pain syndrome, runner's knee, patellofemoral pain syndrome, or chondromalacia patellae. Doctors estimate that about one-half of all non-injury knee pain complaints are due to this disorder.

When afflicted with the condition, the patient's knee rubs against the trochlear groove, instead of gliding smoothly across it. The damage may range from a slight abnormality of the surface of the cartilage to a surface that has been worn away completely to the bone. Anterior knee pain can present a diagnostic challenge because of the complex anatomy of the knee.