Cranial Cruciate Ligament Disease

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 deficiency (CCLD) is the most common cause of hind limb lameness in dogs. Deficiency in the ligament is frequently progressive, starting with the initial trauma and progressing through partial rupture to complete rupture, is the most common cause of hind limb lameness in dogs.

The underlying cause of CCLD in the majority of dogs is different than ACL injuries in most people. While trauma is a common cause of ACL tears in people and dogs do suffer from traumatic ruptures, CCLD 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 angle (TPA), immune-mediated disease, neoplasia (cancer), 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 CCLD.

Though the underlying cause of the disease may be different in each dog, the anatomy of the joint and the forces applied through it during weight bearing movement play a role in the continued breakdown of the ligament.

The dog stifle (knee) is anatomically very similar to a human knee. There are two long bones, the femur (thigh bone) and the tibia (shin bone), and a small bone, the patella (knee cap), which articulate together. There are also four ligaments primarily responsible for the stability of the stifle – the cranial cruciate, caudal cruciate, medial collateral, and lateral collateral. Finally, there are two menisci, medial and lateral, which help act as a cushion between the femur and the tibia and help to provide congruency and further stability to the joint. An important difference between a dog stifle and the human knee is that the top of a dog tibia, the tibial plateau, has a more significant slope than that of a human knee.

Due to the slope of the top of the tibia (referred to as the tibial plateau angle), 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, this is referred to as tibial thrust. This instability causes discomfort, frequently damages the medial meniscal ligament, and prevents normal ambulation. There is some evidence that the steeper the tibial plateau slope, the greater the likelihood of a dog developing a CCLD. As the cruciate ligament tears, changes are also taking place in the joint leading to a loss of healthy cartilage 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 progress to a complete tear over time.

Clinical signs of early (partial) CCLD may include an acute onset of hind limb lameness following activity (can be weight bearing or non-weight bearing) that improves with time and is followed by intermittent stiffness after rising or mild to moderate lameness following heavy activity. As the disease advances and the ligament progressively tears, the lameness may become more consistent. Acute complete tears may initially result in a non-weight bearing lameness on the limb, but as time goes on the dog may intermittently use the limb. Instability in the joint associated with CCLD can also lead to injury of the meniscus. Injury of the meniscus can be extremely painful for pets and may, for a period of time, lead to a non-weight bearing lameness.

There are multiple tests veterinarians can perform to help diagnose a cranial cruciate ligament deficiency. 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 fibres 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 CCLD by detection of changes that occur in the joint following CCL injury. These changes include effusion (excess fluid in the stifle), arthritis, and forward movement of the tibia relative to the femur. Radiographs are also used to rule out other concurrent injuries that may affect treatment options and recovery from CCLD.