This animal has a congenital abnormality, which is present in small breed dogs, that causes patellar luxation. This luxation can be due to altered positioning of the quadriceps femoris, a shallow trochlear groove, and various dysplasias of the femur and tibia. This luxation of the patella caused a laxity within the stifle joint. The laxity within the stifle predisposed this dog to a rupture, or partial rupture of the cranial cruciate ligament. This rupture of the right cruciate ligament occurred when the animal fell off the bed. The joint was able to compensate and the clinical signs disappeared within one month. This caused the medial buttressing palpated on physical exam. However, at the time of the right cruciate rupture, the left cruciate was less severely torn, again, due to the congenital patellar luxation. Since the right was worse than the left, the dog did not appear to be lame in the left leg. Approximately six months later, the dog became lame again in the right leg probably due to a re-injury to the stifle joint causing a joint laxity. During this re-injury, the dog may have also more severely torn the left cruciate giving the clinical signs now present.