Cruciate Ligament Rupture- A Common Cause of Rear Limb Lameness

By Mark Hale ,DVM

Bolivar, MO

            A friend of mine recently ruptured his cruciate ligament and is having surgery on it this week. This is the second time for him; the other knee had surgery for this a few years ago. This scenario reminds me a lot of the situation we see in dogs concerning their knees.

            Cruciate ligament surgery is THE most common orthopedic surgery for dogs. If a large breed dog comes in for an examination of sudden onset of lameness in a rear leg, it is very likely due to some type of cruciate ligament injury. Not all of these will be surgical. Oftentimes, only a partial tear may occur which usually does not require surgery. These injuries may respond favorably to exercise restriction, pain medication, and anti-inflammatories. But if surgery is needed, it should be done before degenerative changes in the knee start happening.

            The canine knee joint is very similar to a human knee joint. Both are made up of three main bones; the femur (thigh), tibia (shin), and the patella (kneecap). These bones are held together by four main ligaments, which allow the joint to flex and bear weight, without bending the wrong way. Two of these are actually "cruciate" ligaments but it is usually the cranial (called the anterior in humans) one that causes the problems. These two ligaments form an "X" in the joint to prevent front to back movement of the femur and tibia. There are also two pads called menisci that give cushioning to the ends of the bones. These can also be injured and become another source of pain when the ligament is torn.

            To diagnose a cruciate injury, your veterinarian will perform what is called a "drawer test". This involves holding the femur and the tibia in certain areas and trying to move the tibia forward, much like opening a drawer. If the ligament is intact, this movement cannot occur and so we know the ligament is not ruptured. However, if the ligament is partially torn, or strained, pain may be elicited with the drawer test instead. Other manipulations are also done to confirm the findings. One problem is that if the dog's muscles are very tense it can be difficult to perform a good test on the tender knee. For this reason, it may be necessary to sedate your pet for a more thorough examination. If sedation is necessary, this is an excellent time for radiographs to check for degenerative changes in the knees or hips. Findings on x-rays may help your veterinarian decide if surgery is the best option for your pet. 

            We see two main groups of dogs with this injury, which is much more common and severe in large breed dogs. One group is the young athletic dog who takes a bad step and injures the knee while playing. The other group, which is more common, is the older large overweight dog. Many of this last group probably have weakened ligaments that may have partially torn earlier, and now suddenly rupture completely. This group may have some arthritis already present in the knee. This fact, combined with their age and being overweight, do not make for as good of a response to surgery. Weight loss and controlled exercise, especially swimming, are keys to their postoperative rehabilitation. Sometimes this group also has a second knee rupture within a few months after the first one. This is definitely one time that pet health insurance would be a great thing to have.

            Surgical repair of the ruptured ligament is now mainly done in one of two ways. Most dogs have what is called an extracapsular repair. In this surgery, after the torn ligament and the meniscus are addressed, a heavy suture is placed in a certain fashion to limit the undesirable movement that is present because of the rupture. This surgery usually works great in dogs that are not too large and don't have preexisting arthritis. The other surgical repair must be done by surgical specialists. It involves cutting the tibia and plating the pieces back together at a particular angle. Due to the high cost of this procedure (several thousand dollars versus a few hundred dollars), not many pet owners choose to go this route. One thing to keep in mind is that either surgery is not a complete cure for life. Arthritis and degeneration of the joint may still occur to some degree, just like it can in humans after surgery.