Posted on 09-12-2012
Today we're going to discuss cruciate ligament tears, how they happen, why they are important, and what can be done to repair them.
Cruciate ligament tears account for about 80% of hind limb lameness in dogs. If your dog is limping on a back leg, chances are it has to do with damage to a cruciate ligament. The cranial cruciate ligament (CrCL, also called the anterior cruciate ligament or ACL) runs inside the knee joint from the back of the femur (upper leg bone) to the tibia (lower leg bone). It acts like a cable that keeps the tibia from riding forward during walking or standing. If it breaks or tears, the knee becomes unstable and pain, limping, joint inflammation, and joint damage can occur.
In dogs there are two main ways the cruciate ligament gets damaged. The first and likely the most common is as a result of a degenerative disorder. The body weakens the ligament gradually over time until one day the dog jumps for a ball, slides on the kitchen floor, or hops off the sofa and the ligament breaks. It should be noted here that many dogs with a degenerative tear WILL go on to tear the cruciate ligament in the other knee sometime in the future. The second way to tear a CrCL is by trauma. Either hyperextending the knee or by an impact on the knee while weight bearing can cause the ligament to tear and make the joint unstable.
CrCL tears don't heal. The ligament is under constant tension and, being a thick fibrous tissue, has a very limited blood supply. Trying to get a CrCL to heal with conservative therapy is like trying to glue a rope together while it is supporting a weight. It just doesn't work. The body will make an attempt to stabilize the joint by thickening the joint capsule and the surrounding tissues. While the joint will eventually become stable (months) it will also have a decreased range of motion and decreased load bearing capacity. It's almost like stabilizing the knee by wrapping it in layer after layer of duct tape (don't do that, by the way). By the time you get enough around the leg to stabilize it, you have significantly compromised the knee's normal function.
Lastly, what can we do to repair cruciate ligaments? Two main approaches are commonly used to repair CrCL tears. The first, usually restricted to less active dogs under 35 pounds body weight, is called the Extracapsular Lateral Suture or ELS technique. This is where two pieces of heavy nylon suture are placed on the outside of the knee joint in parallel to the (failed) CrCL. The sutures act to take on the forces that the CrCL used to handle and stabilize the joint. Very active or heavy dogs can break these sutures or the points where they are anchored, so they aren't good candidates for the ELS procedure.
The second method involves changing the geometry of the knee joint so the dog no longer needs a CrCL. Remember, we said that the CrCL handles the forces that want to move the tibia forward during standing or walking. By changing the angle between those forces and the weight bearing portion of the knee joint we can eliminate the need for the CrCL without trying to replace it with something that can break. Tibial Tuberosity Advancement (TTA) is a procedure that moves the front of the tibia bone forward and eliminates the tibial thrust experienced during weight bearing. Dogs enjoy a dynamically stable joint immediately after surgery and often begin to bear weight on the leg within the first few days. Exercise still must be restricted while the bone heals, but early return to weight bearing results in less degenerative joint disease in the future and a happier pet in the short and long term. Tibial Plateau Leveling Osteotomy (TPLO) is a similar procedure that changes the geometry of the knee to eliminate the need for the CrCL. Instead of moving the front of the tibia forward, the TPLO changes the angle of the weight bearing surface to eliminate the forward forces. The dynamic stability is the same as the TTA, but the bone cut goes through the weight bearing portion of the tibia and this can lead to a longer time before return to weight bearing.
That's it for today! Next time we will talk about pain control before, during, and after cruciate surgery!
I hope your pet never needs surgery, but if they do, we're here to help. Dr. Wight
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