Dislocated kneecap. Coloured X-ray of a side-view of the leg of a patient with a dislocated kneecap (patella) and ruptured patella tendon. The kneecap (dark brown, right) has moved upwards in front of the thigh bone. Dislocation can occur due to a hard knock or twisting movement, often during sport, or an imbalance in the muscles at the front of the thigh (quadriceps). It causes considerable pain and an inability to straighten the leg. Treatment, after relocation, is with anti- inflammatory drugs and physiotherapy, severe cases may need surgery. The patellar tendon attaches the quadriceps to the shin bone. Rupture is caused by a trauma to the knee. Treatment is with surgery.