patella

Patella is the diminutive of Latin patina (a shallow dish or pan). The kneecap was named for its shape: a small, round, slightly concave bone that looks like a tiny dish. The word is precise. The patella is roughly circular, about five centimeters across, and its posterior surface is smooth and curved — dish-shaped. The anatomist who named it picked up a kneecap, looked at it, and saw a saucer.

The patella is a sesamoid bone — a bone embedded within a tendon. The word sesamoid comes from Greek sēsamoeidēs (resembling a sesame seed). Most sesamoid bones are tiny, grain-sized nodules in the tendons of the hands and feet. The patella is the exception: a large sesamoid bone embedded in the quadriceps tendon, protecting the knee joint and improving the mechanical leverage of the quadriceps muscle. It increases the muscle's force by about 33%.

Patella injuries are common in sports. Patellar dislocation, patellar tendinitis (jumper's knee), and chondromalacia patellae (runner's knee) fill orthopedic waiting rooms. The patella is the bone that takes the impact when you kneel, the bone that slides when you run, and the bone that cracks when you fall. Its position — exposed at the front of the knee — makes it the body's most vulnerable bony structure.

The word patella entered English from Latin in the early 17th century. The everyday English term 'kneecap' is a straightforward compound: the cap (covering) of the knee. The Latin diminutive is more elegant. A small dish. A tiny saucer balanced at the front of the leg's central joint.