The patella is the kneecap, the bone sitting over the front of the knee. Plural = 'patellae'.
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.
Patellar position in patients with patellofemoral syndrome as characterized by anatomo-radiographic study. Franco BAFM, Sadigursky D, Daltro GC. Rev Bras Ortop. 2018 Jul 7;53(4):410-414.
Acute traumatic patellar dislocation. Duthon VB. Orthop Traumatol Surg Res. 2015 Feb;101(1 Suppl):S59-67.
A simple explanation of the patella, and why it is important.