Step 2: About ACL/CCL

The cranial cruciate ligament (CCL), equivalent to the anterior cruciate ligament, or ACL in people, is responsible for limiting over-extension of the knee, limiting internal rotation of the knee, and to prevent sliding/drawer motion of the tibia.  Cranial cruciate ligament rupture (CCLR) is the most common cause of hind limb lameness in dogs.

The underlying cause of CCLR in the majority of dogs is different from ACL injuries in most people. Whereas trauma is a common cause of ACL tears in people, CCLR in dogs is typically degenerative in nature. 

Though the underlying cause of the disease may be different in each dog, the anatomy of the joint plays a role in the continued breakdown of the ligament. Due to the slope at the top of the tibia the cranial cruciate ligament of the dog is under stress during weight bearing as it prevents sliding of the knee. When the integrity of the ligament is compromised, the tibia begins to slide forward during weight bearing. The instability that develops is partly responsible for the pain present in dogs with this injury. As the ligament progressively fails, swelling in the knee occurs and cartilage is progressively damaged – arthritis develops. In most patients, once the degenerative process of the ligament begins (partial tearing), the ligament will go on to a complete tear.