Location: More commonly known as your kneecap, the patella is a thick, triangular bone that makes up part of the knee joint. It is the largest sesamoid bone — a bone embedded within a tendon — in the body. It becomes hard somewhere between the age of 2 and 6.

Function: The patella protects the knee joint and increases the angle between the tendon and the femur, allowing for more leverage to move the knee joint.

Potential problems: Overuse or injury can lead to a degradation of the cartilage on the underside of the patella. That can cause pain or a grinding sensation with movement. People who often kneel during work or play sports in which they often fall on their knees can experience prepatellar bursitis, an inflammation of the bursa sac that causes the knee to swell. And athletes in jumping sports, such as volleyball or basketball, often experience patellar tendonitis, the inflammation of the tendon connecting the kneecap to the shin bone. In some cases, the patella can slip out of the groove in the knee. Such dislocation is generally painful and can become a recurrent problem.

Source: American Academy of Orthopaedic Surgeons, Mayo Clinic