A bone inside a tendon
Your kneecap is a small, flat, roundish bone at the front of your knee. Its science name, patella, means little dish in Latin.
It's an unusual bone. It grows inside a tendon, the strong cord that joins your big thigh muscle to your shin bone.
Bones that grow inside tendons are called sesamoid bones, because small ones look like sesame seeds. Your kneecap is the biggest one of all.
Babies' kneecaps are soft
Myth: Babies have no kneecaps. Truth: They do! But at first, a baby's kneecap is made of soft cartilage, so it doesn't show up on an X-ray.
Somewhere around age 2 to 6, the cartilage starts turning into bone. It keeps hardening for years, all through childhood.
Soft kneecaps are handy for babies and toddlers, who crawl and tumble a lot.
Sliding in a groove
The back of your kneecap is smooth and covered with slippery cartilage. It sits in a groove at the bottom of your thigh bone.
When you bend your knee, the kneecap slides down the groove. When you straighten it, the kneecap slides back up, like a train on a track.
Try it: sit with your leg straight and relaxed. Gently wiggle your kneecap side to side with your fingers. Now squeeze your thigh muscle. The kneecap gets pulled tight and stops wiggling!
A shield and a lever
Your kneecap protects the front of your knee when you kneel or bump it.
It also helps your thigh muscle work. It lifts the tendon away from the joint, a bit like a pulley. That gives the muscle more pulling power to straighten your leg, so you can kick, jump and climb stairs.
When a doctor taps just below your kneecap with a little rubber hammer, your leg kicks all by itself! That's called a reflex. It shows your nerves and muscles are working well.