The upper third of the front of the patella is coarse, flattened, and rough, and serves for the attachment of the tendon of the quadriceps and often has exostoses.

[1] The upper three-quarters of the patella articulates with the femur and is subdivided into a medial and a lateral facet by a vertical ledge which varies in shape.

In the adult the articular surface is about 12 cm2 (1.9 sq in) and covered by cartilage, which can reach a maximal thickness of 6 mm (0.24 in) in the centre at about 30 years of age.

Owing to the great stress on the patellofemoral joint during resisted knee flexion, the articular cartilage of the patella is among the thickest in the human body.

The upper three-quarters of the patella articulates with the femur and is subdivided into a medial and a lateral facet by a vertical ledge which varies in shape.

Four main types of articular surface can be distinguished: In the patella an ossification centre develops at the age of 3–6 years.

[6] The kneecap is prone to injury because of its particularly exposed location, and fractures of the patella commonly occur as a consequence of direct trauma onto the knee.

In other tetrapods, including living amphibians and most reptiles (except some lepidosaurs), the muscle tendons from the upper leg are attached directly to the tibia, and a patella is not present.

This raises the possibility that the kneecap arose 350 million years ago when tetrapods first appeared, but that it disappeared in some animals.

[10][11] The word patella originated in the late 17th century from the diminutive form of Latin patina or patena or paten, meaning shallow dish.