Cranial Cruciate Ligament Disease in Dogs: A Primer

The cranial cruciate ligament (CCL), found inside the knee, is a small ligament with a big job in our pet dogs. Humans have a similar ligament called the anterior cruciate ligament (ACL), but the forces that act on the CCL in dogs and the ACL in people are not identical.  This means that while injuries to the ligament are common in both humans and dogs, the causes and considerations for treatment can be quite different and making decisions about treatment options can be complicated and confusing.

Understanding how the knee joint works can help make understanding cruciate injuries easier. The simplest way to think about the knee joint is as a hinge. The knee is flexed (bent) during what is called the swing phase of walking – when the leg is being moved forward to take the next step. During the swing phase, the knee is carried through the air and is not bearing any weight. The knee, along with the hip, is then extended (straightened) during the thrust phase of walking. This is when the knee and hip are bearing weight and providing propulsion for movement.

The ACL and CCL maintain a normal, stable alignment of the upper (femur) and lower (tibia) leg bones throughout range of motion: There is always some force during the thrust phase of movement, called tibial thrust, that causes the tibia to “want” to move forward and the femur to “want” to move backwards out of alignment when bearing weight; if they do, they rub abnormally against each other and against the cartilage cushions within the knee which causes pain and inflammation. It is the CCL’s job to keep this from happening by anchoring the front of the tibia to the back of the femur.

What is very different in dogs and people is the alignment and weight-bearing load on the cruciate ligament when simply standing. When a person stands, the knee is completely straight and there is little or no tibial thrust and stress on the ACL. In humans ACL tears are almost always due to sudden, extreme stress or impact; it is commonly a sports injury.

A dog’s knee, however, is always at an angle; this varies between breeds and individuals but the average is around 130 degrees. This means that the CCL of a dog is working against tibial thrust even when the dog is just standing around. While CCL injuries in dogs can be related to sudden injury as in humans, it is far more commonly a degenerative disease that happens slowly over time. The accumulated daily wear and tear of this constant workload combines with genetics and other factors to cause micro-tears in the CCL over time much like a rope fraying. Eventually, the CCL becomes weak enough to tear partially or completely, which is when dogs typically show symptoms.

Dogs with a CCL tear, most correctly called CCLD or cranial cruciate ligament disease, will limp on the affected rear leg. The severity varies from a subtle limp or toe touching to completely carrying the leg. Although this often seems like a sudden injury, in retrospect many dogs have had a history of minor episodes of lameness on the leg that resolved without needing intervention. These may have represented small partial tears or micro-tears leading up to a more definitive tear. Once the CCL has torn, the knee joint becomes unstable and painful because the femur and tibia cannot stay in alignment. This is painful and also damages the cartilage inside the joint, causing further pain and inflammation that may lead to the development of arthritis in the knee. Unfortunately, due to the degenerative nature of CCLD in dogs, a dog who has torn the cruciate ligament in one knee is at significant risk to tear the cruciate ligament in the other knee at some point in time.

Diagnosing CCLD involves a history, physical exam, taking x-rays of the knee, and manipulating the knee for abnormal movement of the femur and tibia in relation to one another. The ligament itself is not visible on an x-ray, but there are changes in the distribution of fat and fluid within the joint that are characteristic of CCLD. Because the knee is painful and the thigh muscles are very strong, manipulating the knee usually has to be done under sedation.

Once CCLD has been diagnosed, there are a number of options for treatment. There is no one treatment option that is best for every dog and opinions vary even among professionals about the relative benefits of various options. All treatment options have their limitations and risks, and treatment decisions should be made based on a number of factors including age, size, activity level, severity of symptoms, goals, and budget.  No treatment option can return the knee to its pre-CCLD state; any arthritis that is already present, for example, cannot be reversed. Goals of treatment are to reduce pain, restore as much normal function as possible, and prevent the formation of additional arthritis.

Generally, options fall under either conservative medical management or surgical management, although there is some overlap. For example, surgical patients still receive pain management and benefit from physical therapy post-operatively.

Conservative or medical management involves varying combinations of exercise restriction, pain management, anti-inflammatories, and physical therapy. Over time, the body will do its best to stabilize the knee joint by developing fibrosis and scarring around the joint capsule.

There are two general categories of options for surgical management. The first category includes surgeries that try to replace or mimic the function of the CCL. This is commonly performed in humans using a technique called intracapsular repair; however, intracapsular repair is not a good choice in dogs. In dogs, a technique called extracapsular repair is used instead. There are multiple specific surgeries that fall under the umbrella of extracapsular repair, but all involve using some type of synthetic material placed outside of the knee joint itself (in the joint capsule or thick tissue surrounding the joint) to recreate the CCL’s function.

The second category includes surgeries that change the geometry of the knee joint, which changes the forces acting on the knee in such a way that the CCL is no longer required for a stable joint. The two most common surgeries in this category are the Tibial Plateau Leveling Osteotomy (TPLO) and the Tibial Tuberosity Advancement (TTA). A veterinary surgeon will make the recommendation for a specific surgery based on the angle of an individual dog’s knee as well as age, size, and other factors.

There is a lot of information and many strong opinions about causes and treatment of CCLD in dogs, almost too much information. A quick internet search will bring up numerous sources that directly contradict one another regarding almost every aspect of CCLD from causes to the best treatment plan. The bottom line is that there is no single cause and no single “best” treatment option, and should you and your dog face CCLD we will help you develop a treatment plan that fits you and your dog as individuals.