The kneecap, which is also called patellar, transfers the energy of the extensor muscles (quadriceps tendon) of the thigh onto the lower leg (patellar tendon). The kneecap slides like a kind of train of barges in a channel-shaped slide bearing of the thighbone. The sliding surfaces of the kneecap joint consist of joint cartilage, which reduces the load on the joint via its elasticity and which makes a frictionless function of the joint possible via its specific material. If these cartilage surfaces wear out, a cartilage damages will appear.