Patella
Sesamoid bone; part of the limb skeleton.
Also known as: Kneecap
Classification
Sesamoid bone; part of the limb skeleton.
Dimensions
In land mammals, this is typically a thick, heart-shaped or almond-shaped bone, often roughly two inches across in an adult human, though its size varies significantly with the overall scale of the animal.
Key Features
Look for a sturdy, rounded bone that is smooth and slightly hollowed on the back face (where it glides against the thigh bone) and rough or pitted on the front face. Its general shape is often described as a rounded triangle pointing downward.
Similar Bones
It can be confused with the kneecap of a different animal or sometimes a large pebble when found in the wild. Its unique 'bread-loaf' thickness and the single smooth, glassy facet on the back side distinguish it from simple stones or the smaller bones found in the wrists and ankles.
Across Species
The patella is the largest 'sesamoid' bone, meaning it develops within a tendon. While humans and many other mammals have a prominent bony kneecap to help movement, some animals like whales lack them entirely because their legs evolved into flippers that don't need a bending knee joint. In birds, it is often very small or missing, while in some desert-dwelling lizards, it remains made of soft cartilage rather than hard bone.
Evolutionary History
The kneecap first appeared as a specialized adaptation in early reptiles and birds to provide mechanical leverage for walking and running. It gained prominence in mammals as they moved toward an upright or semi-upright posture, acting like a pulley to help the large muscles on the front of the thigh move the lower leg more efficiently.
Photography Tips
To identify a kneecap, take one photo looking directly at the front (rough side) and another looking at the back (smooth side). Adding a coin or a ruler next to the bone helps show its scale for comparison against different animal species.