A hyperextended knee occurs when the knee is bent backward, often as a result of landing wrong after a jump. A hyperextended knee can damage ligaments, cartilage and other stabilizing structures in the knee.
Young children have softer bones because they're still growing, so a hyperextended knee can result in a chip of bone being pulled away from the main bone when the ligaments stretch too far. In older children and adults, forceful hyperextension may tear one of the knee ligaments, particularly the anterior cruciate ligament (ACL).
If the knee injury is severe enough to cause swelling, pain or instability, see a doctor immediately. Even if the injury doesn't need surgical repair, physical therapy may be needed to help restore leg strength and stability.