Anterior Cruciate Ligament (ACL) Injury In Dogs - Explaining the Joint and the Ligament

When a dog has been diagnosed with a torn anterior cruciate ligament (ACL) the attending veterinarian will probably suggest surgery to correct the problem.

Unfortunately, veterinarians sometimes use language that is difficult for the average dog owner to understand. This, along with the stress that the dog owner is under, makes it difficult to make appropriate decisions for the dog.

Here is what is involved in an ACL (anterior cruciate ligament) injury and its repair, explained in plain English:

In canine patients, the ACL is properly called a CCL, or cranial cruciate ligament. The term ACL refers to the same structure, but in humans. I'll be using the proper term, cranial cruciate ligament, or CCL, from here on out.

The dog's knee joint (the joint where the cranial cruciate ligament resides) is called the stifle joint. The stifle joint is composed of three major bones; the femur (thighbone in humans), the tibia (shin bone in humans), and the patella (knee cap in humans). The ends of these bones are surrounded by cartilage, the slippery stuff that allows for movement. They sit together in a fluid called joint fluid, and there is a seal around the joint called a joint capsule.

The CCL sits inside the joint, with another ligament called the caudal cruciate ligament. These ligaments cross each other, which is where the name cruciate (meaning cross) comes from. This is important later when explaining joint function.

Also in the joint are two shock absorbers called menisci (the plural of meniscus). When the stifle joint is viewed in an X-Ray, the femur and tibia appear to be separated by space - but the femur actually sits atop the meniscus - it's just that ligaments are not visible by X-Ray - so the CCL doesn't show up on an X-Ray.

The function of the stifle joint:

The stifle joint is a complex joint; but here's an analogy that might help: Think of the stifle as a hinge that can only swing two ways, forward and backward; not laterally. The center of the hinge is inside of the stifle joint. When it swings forward, it bears weight. When it swings backwards (in the swing phase of gait motion) it is non-weight bearing.

As you know, when you walk, only one foot is on the ground at any one time. The cranial and caudal cruciate ligaments are there to keep the femur in alignment with the tibia during motion. These two ligaments, which are in the shape of an X, maintain appropriate contact between the two bones.

The CCL attaches in the back of the femur and comes forward to attach to the front of the tibia. The caudal cruciate ligament attaches on the front of the femur, and goes backwards to attach to the back of the tibia. Where they cross each other is the "hinge" point.

Once you understand this, you can will have the basis to understand what happens when the cranial cruciate ligament ruptures.