Patellar instability is a range of injuries that occur when the patella, or kneecap, is displaced from its intended resting place. The causes include a traumatic dislocation, such as occurs during a sport activity, or a displacement caused by daily activities.
Most of the patella injuries develop when your foot is placed on the ground and then you change or twist your direction. Direct blow to the knee cap also causes the injury. Risk factors include laxity or joint looseness, and girls are at higher risk than boys.