ACL: Ruptured Anterior Cruciate Ligament



The Knee

The knee is a joint consists of three bones: The Femur bone (the long bone extending down from the hip), the  Tibia bone (the bone between the knee and ankle) and the Patella (the kneecap). These bones are joined together by a number of ligaments. Two ligaments crisscross in the joint from the femur to the tibia and are called cruciate ligaments. The one towards the front of the leg is called the anterior cruciate ligament and the one crossing behind it is the posterior cruciate ligament. These ligaments prevent the ends of the femur and tibia from moving back and forth across each other.

ACL Rupture

When the anterior cruciate ligament ruptures, the joint becomes unstable and the femur and tibia can move back and forth across each other. The anterior cruciate ligament is commonly torn when the dog twists on his hind leg. The twisting motion puts too much tension on the ligament and it tears. This often occurs if the dog slips on a slippery surface or makes a sudden turn while running. Obesity puts too much weight on the knee and overweight dogs tend to have more occurrences of ruptured cruciate ligaments. It appears that in most dogs with the problem, the ACL slowly degenerates and becomes weaker until it ruptures, without any sudden injury. Certain breeds appear to be at increased risk of ACL degeneration and include the Newfoundland, Labrador Retriever, Sint Bernard, Rottweiler and also the English Bulldog. Many dogs with a degenerating ACL will have the condition in both knees.


Dogs who have ruptured their cruciate ligament will appear suddenly lame, and usually hold the foot of the affected leg off the ground or only tip toe with the injured leg (the 2nd video in this blog shows a good example at 2 minutes and 39 seconds). In time, the dog may start to use the leg again, but often lameness returns. Dogs with a degenerating ACL may also show some pain and can have swelling on the inside aspect of the knee.


The diagnosis of a ruptured cruciate ligament is made by a veterinarian through observing movement of the joint. The vet will place one hand around the femur and one around the tibia in a precise manner. By applying pressure on the knee, the veterinarian will feel the bones move abnormally in what is called a ‘drawer sign.’ If an animal is in a lot of pain, or very nervous, the muscles near the knee may be so tense that they prevent the drawer movement from occurring. If a veterinarian suspects a ruptured cruciate ligament in a dog but cannot elicit the drawer sign, the dog may be sedated to relax the muscles and then re-examined for the drawer sign.


If the ligament is completely torn, your dog will need surgery. There are several different methods used to repair the knee joint when an anterior ligament is torn.

TPLO stands for Tibial Plateau Leveling Osteotomy. Basically the TPLO surgery changes the bones of the knee joint to make them work in a different, more “level” manner. A portion of the tibia is cut, moved, and reattached to a different portion of the tibia using plates and screws. By changing the conformation of the tibia, the joint is stabilized. This is a technically difficult surgery but it has shown to produce excellent results, often with less arthritis. The recovery period is similar to that with the other surgical procedures.

TTA stands for Tibial Tuberosity Advancement, a procedure where the dog’s knee joint is manipulated to provide stability without the use of a functional Cranial Cruciate Ligament (or CCL). This is another surgery in which a different portion of the tibia is cut, and allowed to heal at a different angle to change the mechanical stresses on the joint. As with the TPLO, this surgery is more complex.

TTO stands for Triple Tibial Osteotomy, combines different aspects of the TPLO and TTA. Similar to the TPLO and TTA, the TTO involves cutting the tibial bone to change the angle of the knee joint.


If the dog’s exercise is restricted as instructed, and overweight dogs return to normal body weight, the prognosis is good.
Depending on the amount of injury to the knee and length of time between the injury and correction of the problem, degenerative joint disease may occur as the pet ages.

Most surgical techniques require two to four months of rehabilitation.  A second surgery may be required in 10 to 15 percent of cases, because of subsequent damage to the meniscus (a crescent-shaped cartilage located between the femur and tibia in the stifle). Regardless of surgical technique, the success rate generally is over 85 percent.