The knee (stifle) joint of the dog is one of the weakest joints in the body. Just as human athletes frequently suffer knee injuries, the dog also has knee injuries. The knee joint is relatively unstable because there is no interlocking of bones in the joint. Instead, the two main bones, the femur and tibia, are joined with four ligaments. The most common injury of the knee is a rupture of the cranial cruciate ligament (CCL – in humans it is called an anterior cruciate ligament or ACL). When the CCL is torn, instability occurs that allows the bones to move in an abnormal fashion in relation to one another, causing damage to the joint cartilage.
In younger dogs, rupture of the CCL is usually the result of trauma to the stifle joint. In some cases, the ligament may only partially tear; however, this will eventually lead to complete tearing of the ligament. When CCL rupture occurs in older dogs, it is most frequently initiated by a progressive degenerative change in the ligament with eventual total rupture. Therefore the dog does not have to do anything extraordinary to cause the tear. Often they are running in the back yard, plant the foot for a turn, and the ligament breaks.
A special note is appropriate concerning the dog’s weight. Obesity or excessive weight can be a strong contributing factor in cruciate rupture. The ligament may become weakened due to carrying too much weight; this causes it to tear easily. Obesity will make the recovery time much longer, and it will make the other knee very susceptible to cruciate rupture. If your dog has a weight problem, there are prescription diets that can be used to assist weight reduction.
Rupture of the CCL is most common in middle aged and older dogs, particularly those that are overweight house pets.
Dogs with a ruptured CCL are painful and may refuse to bear weight on the affected leg. As the acute pain subsides, most dogs become more willing to bear weight but some degree of lameness remains.
The most reliable means of diagnosing this injury is to move the femur and tibia in a certain way to demonstrate the instability. This movement is called a “drawer sign.” It can usually be demonstrated with the dog awake. If the dog is very painful, has very strong leg muscles, or is uncooperative, it may be necessary to use sedation in order to examine the joint adequately.
Occasionally, the injury that causes a ruptured cruciate ligament will also result in tearing of one of the menisci or “cartilages.” At the time of surgery, these are examined and trimmed or removed if necessary.
Occasionally, small dogs that have a ruptured cruciate ligament will become sound (will no longer limp), even if surgery is not performed. However, in dogs over 30 pounds, progressive, degenerative arthritis will usually develop without surgery. Once these degenerative changes are established, the lameness cannot be corrected, even with surgery.
Following proper and prompt surgical correction, the joint is sound again in 85% of the cases. Most dogs walk and run without any lameness; however, some have either a mild limp or lameness associated with cold and damp weather. Drs. Godwin and Joiner each have over 30 years of experience performing these surgeries.