Arthritis of the knee is a leading cause of disability in the United States. Patellofemoral arthritis affects your kneecap (patella bone). It causes pain in the front of your knee and can make it difficult to kneel and climb stairs.
The patella is a small bone located in front of your knee joint — where the thighbone (femur) and shinbone (tibia) meet. It protects your knee and connects the muscles in the front of your thigh to your tibia.
The patella rests in a groove on top of the femur called the trochlear groove. When you bend and straighten your knee, the patella moves back and forth inside the groove.
The ends of the femur, trochlear groove, and the undersides of the patella are covered with a slippery substance called articular cartilage. This helps the bones glide smoothly along each other as you move your leg.
Patellar (kneecap) fractures often damage the articular cartilage that covers and protects the underside of the bone. Even though the broken bone heals, the joint surface may no longer be smooth. There is friction when the patella moves against the joint surface of the femur. Over time, this can lead to arthritis.
Dysplasia occurs when the patella does not fit properly in the trochlear groove of the femur. Because of this, when the knee moves, there are increased stresses on the cartilage. This begins to wear the cartilage down.