Patella
Sesamoid bone; part of the limb (appendicular) skeleton.
Also known as: Kneecap
Classification
Sesamoid bone; part of the limb (appendicular) skeleton.
Dimensions
Generally heart-shaped or teardrop-shaped. In humans, it is about 2 inches in diameter, while in larger mammals like horses, it can be much thicker and more block-like.
Key Features
Look for a thick, chunky bone with one side that is very smooth and slightly waxy — this is the back that glides against the leg. The front side is usually rough and pitted where tendons attach. It typically has a rounded top and a pointier bottom, making it look a bit like a rounded triangle or a thick peach pit.
Similar Bones
From a distance, it can be confused with a large pebble or a rounded wrist bone (like the pisiform). However, the kneecap is much larger than wrist bones and lacks the intricate interlocking facets found on the bones of the hand or foot.
Across Species
The kneecap is a marvel of animal engineering. In many mammals, it acts like a pulley to increase the power of the thigh muscles. Birds have a similar structure, but it is often much smaller or even remains as cartilage. Interestingly, some animals like whales lack a kneecap altogether because they do not have the same mechanical need for a powerful walking stride.
Evolutionary History
The kneecap first emerged in early reptiles and birds as a way to protect the knee joint and provide better leverage for movement. Over millions of years, as mammals transitioned to more upright or specialized gaits, the bone became larger and more robust to handle the increased Forces of land-based locomotion.
Photography Tips
Take photos from at least three angles: the smooth back surface, the rough front surface, and a side profile to show the thickness. Place a coin or a ruler next to it for a size reference, and use soft natural light to highlight the texture of the pitted front surface.