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.

Patellofemoral arthritis is arthritis, or joint disease, affecting the joint between the kneecap (patella) and the front of the femur bone (trochlea).  Initially it can start as simple cartilage fraying and softening, and over time, it can progress to complete bone on bone arthritis.  Over time, patellofemoral arthritis can spread to other parts of the knee, and contribute to degeneration between the tibia and femur bone (tibiofemoral joint), which is the major weight bearing surface of the knee.

There are many contributing factors to patellofemoral arthritis, such as patella (kneecap) instability, kneecap dislocations, direct trauma and inflammatory conditions such as rheumatoid arthritis.