Patella

The patella from the side, showing its relationship to the femur bone and tibia bone.	The patella is contained within the tendon of the quads muscle and the whole muscle-bone-tendon complex is known as the 'extensor mechanism'.	A stable patella is contained within the walls of the underlying groove of the femur.

The extensor mechanism

The patella is a 'sesamoid bone', contained within a tendon, and subject to the forces that pass through that tendon and its related muscles. The part of the tendon below the patella is called the patellar tendon, while the part above is called the quads tendon, but it is all part of a single functional structure. When the knee straightens or extendes, the forces go through all parts of the extensor mechanism.