Patella

The patella, or kneecap, is a flat triangular sesamoid bone embedded in the tendon of the quadriceps, just in front of the knee. It glides in a groove on the femur as the knee bends and works as a pulley: by holding the tendon away from the joint, it increases the thigh muscle's leverage for straightening the knee.

Key facts

The patella is the largest sesamoid bone in the body.
It increases the quadriceps' leverage by roughly a third.
Its back surface has the thickest cartilage in the body.
It protects the front of the knee joint.
It starts to ossify in early childhood and is fully formed by the teens.

What can go wrong

The kneecap can dislocate sideways, most often in teenagers, when a twist pushes it out of its groove. It can fracture from a direct fall onto the knee. Pain behind the kneecap on stairs and after sitting ('movie-goer's knee') commonly comes from irritation of its joint surface and the structures around it.

Related procedures & tests

Related symptoms

Knee pain

Related muscles

Watch: related videos

▶ WatchPatella fracture repair ▶ WatchKneecap patella button loosens after total kneereplacement kneeinjury fracture

Frequently asked questions

Can you manage without a kneecap?

People do function after its removal, but the thigh muscle loses leverage, so straightening the knee against resistance is weaker — which is why surgeons preserve or repair it whenever possible.

Why does the kneecap sometimes slip out?

Its groove can be shallow, the thigh muscles unbalanced, or ligaments loose — a twisting injury then lets it jump sideways, usually to the outer side. It often pops back as the knee straightens.

Educational information only. This atlas explains normal anatomy and describes conditions in general terms; it is not medical advice and does not replace assessment by a qualified clinician. If you have symptoms that worry you, seek medical care.