The most common cause of rearlimb lameness in the dog is rupture of the anterior cruciate ligament. This injury allows degenerative changes to occur in the stifle (knee) joint, which must be limited before permanent cartilage and bony changes occur and result in irreversible arthritis. The stifle is a hinge joint, which allows a wide range of motion of the tibia on the femur. To maintain stability through this range of motion there are two cruciate ligaments, one anterior (toward the front of the knee). These ligaments cross each other (hence the name cruciate) to provide a major role in stifle stability. The anterior cruciate ligament also prevents forward displacement of the tibia on the femur (anterior drawer motion).

The biomechanics of injury to the anterior cruciate ligament coincide with the functions of the ligament, which acts as a constraint against the bone (the tibia) moving forward on the other (the femur). See Fig.2. The most common mechanism for anterior cruciate rupture comes with sudden rotation of the stifle when the joint is in flexion (i.e.: a sudden right turn on the weight-bearing right rear pivot limb). The ligament also ruptures if the knee is hit from the front (as when a football player is hit from the front or when a dog steps in a hole and catches its foot or when jumping). Degenerative changes in the stifle joint from obesity, conformational deformities, such as knocked knees, or bowed legs and patellar luxations, or from repeated minor stresses can result in progressive deterioration of the cruciate ligament. When the anterior cruciate ligament ruptures, the compensatory stress placed upon the opposite rear limb may predispose it to ligament rupture. This places critical importance on weight reduction and immediate repair of the damaged leg to minimize the risk to the unaffected limb.
                                                         SYMPTOMS

The clinical signs of anterior cruciate rupture can vary depending on the extent and chronicity of the injury. Animals with an acute rupture present with a nonweight-bearing lameness, joint effusion, palpable pain in the stifle, and joint instability. Those with more chronic injuries generally exhibit an intermittent weight-bearing lameness, muscle atrophy, thickening of the joint capsule with palpable bone spurs present, and joint instability with a frequent "meniscal click" associated with a torn medial meniscus.

Although the exact purpose of the meniscii is still unclear, they have been described as elastic, movable washers which aid in the lubrication of the joint and also act as shock absorbers. The most common meniscal injury occurs in the medial meniscus and is associated with rupture of the anterior cruciate ligament. In some cases, the meniscus is crushed between the femur and the tibia. In others, the meniscus may