Ruptured patellar tendon. X-ray of a knee with a ruptured patellar tendon due to a sport's injury. The patella (kneecap, at upper right) is not held tightly against the femur (thigh-bone, upper frame). This is because of a rupture, or tear, to the patellar tendon, an extension of the thigh muscle. The patellar tendon connects to the tibia (shin-bone, larger bone in lower frame), holding the kneecap in place. The smaller bone in lower frame is the fibula. Injuries such as this are commonly caused by stress due to strenuous activity such as running and jumping. Surgical repair of the tendon is often necessary.