Patellofemoral Syndrome (PFS) is also known as Retropatellar Pain Syndrome and Patellar Tendonitis/Jumper's Knee, but it should not be confused with another common knee ailment, Chondromalacia Patellae. PFS is the painful result of physical and/or biomechanical changes to the knee joint – the area behind the knee where the patella (kneecap) and the femur (the thigh bone) meet. This anterior knee pain gets worse with physical activity and even prolonged sitting. Chondromalacia patellae is when the underlying patellar cartilage actually frays and is damaged or softened.

Three bones meet to form the knee in the knee: the thighbone (femur), the shinbone (tibia), and the kneecap (patella). When the leg moves, the kneecap slides along a shallow groove in the femur known as the trochlear groove. The joint is cushioned and smoothed by articular cartilage, which covers the surfaces of wraps around the ends of all the bones in the joint. Repeated abnormal stress of the joint, whether due to agedegeneration, injury, overuserepeated excessive stress, abnormal alignment, or muscle weakness, can weaken and soften the articular cartilage, and put pressure on the kneecap. This is what causes the pain.

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.