Patella

Bones

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The patella, also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint.

The patella is found in many tetrapods, such as mice, cats, birds and dogs, but not in whales, or most reptiles. In humans, the patella is the largest sesamoid bone in the body.

Babies are born with a patella of soft cartilage which begins to ossify into bone at about four years of age.

Definition from Wikipedia, CC BY-SA 4.0.