“The patella is commonly referred to as the kneecap. It is a small, freestanding, bone that rests between the femur (thighbone) and tibia (shinbone). The femur has a dedicated groove along which the kneecap slides. As a form of protection, both bones also contain cartilage — strong, flexible tissue — in the areas near the patella.”

“The kneecap plays a vital role in how the knee bends” 

http://www.healthline.com/human-body-maps/patella-bone

Image; Google Images