The Patella or kneecap is a small bone which forms in the tendon of the large flexor muscle on the anterior surface of the thigh where it crosses the knee joint to insert on the tibia.

It functions to protect the joint, and to strengthen and improve the leverage of the tendon.

Note the twin articular surfaces (= facets) on its posterior aspect. These slide on the distal end of the femur. It does NOT articulate with the tibia or the fibula.