About ACL/CCL
The cranial cruciate ligament (CCL), equivalent to the anterior cruciate ligament, or ACL in people, is responsible for limiting hyperextension of the stifle, limiting internal rotation of the tibia in relation to the femur, and to prevent forward sliding/drawer motion of the tibia in relation to the femur. 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 than ACL injuries in most people. Whereas trauma is a common cause of ACL tears in people, CCLR in dogs is typically degenerative in nature. Some proposed predisposing factors for cruciate injuries in dogs include genetics, obesity and poor fitness level, early neutering, excessive tibial plateau slope (TPS), immune-mediated disease, and bacterial presence within the joint. Young to middle-aged, female, large breed dogs are at greatest risk for tearing their CCL, though any dog can develop a CCLR. However, as with ACL tears in people, acute traumatic ruptures can also occur.
Though the underlying cause of the disease may be different in each dog, the anatomy of the joint may play a role in the continued breakdown of the ligament. Due to the slope of the top of the tibia, or the tibial plateau, the cranial cruciate ligament of the dog is under stress during weight bearing as it attempts to keep the femur and the tibia in appropriate alignment. Once the integrity of the ligament is compromised, the tibia begins to move forward in relation to the femur during weight bearing. There is some evidence that the steeper the tibial plateau slope, the greater the likelihood of a dog developing a CCLR. The instability that develops is partly responsible for the pain present in dogs with this injury. As the cruciate ligament tears, changes are also taking place in the joint leading to a loss of cartilage health early on and a complete loss of cartilage in end-stage arthritis. In most patients, once the degenerative process of the CCL begins, the ligament will go on to a complete tear.
There are multiple tests your veterinarian can perform to help diagnose a cranial cruciate ligament tear. One of the first signs present prior to instability may be pain with full extension (hyperextension) of the knee. This pain is likely due to stretching of the intact or strained fibers of the cruciate ligament. Once the ligament tears to a certain degree the tibia can be manually manipulated to show instability in what is called the “cranial drawer test” in which the tibia can be moved forward in relation to the femur. Another sign referred to as tibial thrust, may be elicited as well. With this test, weight bearing is mimicked and the front of the tibia can be noted to be pushing forward in relation to the femur. It is important to keep in mind that many patients with clinical signs of pain and lameness may have a partial tear of the CCL. In these cases, there may not be any obvious instability (cranial drawer or tibial thrust) on the exam, however, the patient has a torn CCL that will likely progress to a complete tear.
Other signs that may be noted on the physical exam include loss of muscle mass (atrophy), detection of effusion (swelling) within the joint, and scar tissue formation around the knee (buttress). This scar tissue is the body’s natural response to try and stabilize an unstable joint. Long-term this scar tissue leads to a decreased range of motion in the knee. Finally, a “clicking” sound may be noted in a small percentage of patients with meniscal tears.
Though the cranial cruciate ligament is not visible on an x-ray, radiographs can help confirm a diagnosis of a CCLR by detection of changes that occur in the joint following CCL injury. These changes may include effusion (excess fluid in the stifle), arthritis, and forward movement of the tibia relative to the femur. Radiographs can also help rule out other concurrent injuries.