Hypothesis 1 Cranial Cruciate Ligament Rupture due to Trauma The cranial cruciate ligament is one of a pair of crossed ligaments which stretch between the femur and the tibia in the stifle joint. The function of this ligament is to restrict cranial movement and internal rotation of the tibia in relation to the femur. In an acute traumatic rupture, there are abnormal stresses placed on the joint which cause catastrophic loading of the ligament. This often happens during athletic activity when the animal takes a misstep or steps in a hole and hyperextends the stifle joint. Other possible etiologies involve forcible rotation of the tibia internally with the stifle joint flexed 20-50 degrees or rotation of the animalÅs body (femur) externally, which cause twisting and rupture of the cranial cruciate ligament. Catastrophic stress could also result, as it often does in people, from being hit with blunt force from the side or an appropriate angle which overloads the cruciate ligaments. When the cruciate ligament is ruptured, pain, intra-articular hemorrhage, and joint effusion may result. The hemorrhage will cease with the subsequent joint distention, and joint instability will also be limited by the distention. The ruptured ligament retracts and becomes frayed at the end. The instability and abnormal joint function that result from the ligament rupture can cause meniscal injury, joint effusion, and pericapsular fibrosis. Eventually, these changes in the joint result in osteophyte development, chronic instability, and degenerative joint disease. The clinical signs associated with acute traumatic rupture of the cranial cruciate ligament includes an acute hindlimb lameness occurring within the first three days after the incident which gradually improves over several weeks. The lameness may be partial or non-weight bearing. Palpation of the stifle often elicits pain, and stifle joint swelling, effusion, and crepitation are frequently present. The signalment of an acute rupture patient is often a large breed, obese dog. The diagnosis of a cranial cruciate ligament rupture is based on a test called the cranial drawer sign. The distal femur is grasped firmly in one hand and the proximal tibia in the other, and the examiner attempts to move the tibia cranially. A cranial drawer sign (cranial movement of the tibia in relation to the femur) of greater than 3- 5 mm is indicative of cranial cruciate ligament rupture. A cranial drawer sign of less than 2 mm is normal. Many of the historical and clinical pieces of data fit this case. Bonnie is a large breed (Labrador retriever), obese, and very active dog, and could have easily stressed or torn her cruciate ligament while jogging or walking with her owner. She is also let out into a fenced backyard and likes to chase other dogs or cats. She could have been chasing a critter in the yard and suffered an acute rupture of the ligament. The fact that she is obese would add to the abnormal stresses placed on her joints during activity. Bonnie became lame rather suddenly, and displays a partial weight bearing, toe touching lameness which is fairly typical for a CCL rupture. She has effusion and distention of the stifle joint capsule, and pain on manipulation of the joint. While under sedation, a cranial drawer sign of 4-5mm on the left stifle was obtained, which is indicative of a CCL rupture. The cranial drawer sign on the right stifle was normal at <2 mm. Also, the fact that her hips, collateral ligaments, and patellas all appeared normal suggest that the cause of the lameness is most likely in the stifle, where pain and abnormalities were elicited.