This is a common knee injury and tends to happen in both large young dogs during play and older overweight dogs. A twisting of the dog’s hind leg, which causes the anterior cruciate ligament to tear or rupture resulting in a sudden lameness in a hind leg. When the ligament is torn or ruptured, the tibia and femur can move against each other. This can lead to arthritis fairly quickly. Surgery is one form of treatment if the ligament is completely torn. If the ligament is only partially torn and there are other extenuating circumstances that keep surgery from being an option, the rupture can be treated medically with special instruction on low impact exercise and if the dog is overweight, a diet.