What is the cranial cruciate ligament?

The cranial cruciate ligament is a ligament located inside the stifle (knee) joint. The stifle is composed of the bottom of the femur (thigh bone), which is shaped into two rounded condyles with a deep furrow running between them (the intercondylar fossa), the top of the tibia (shin bone), which provides a relatively flat surface with a mild downward slope from front to back (called the tibial plateau), and the patella (kneecap). As the knee flexes and extends, the femoral condyles rock back and forth atop the tibial plateau, and the patella slides up and down within the trochlear groove running along the front of the bottom of the femur. A number of ligaments are present within and around the knee joint to maintain stability as the limb is used. Two of these, the cranial and caudal cruciate ligaments, run between the intercondylar fossa of the femur and the middle of the tibial plateau within the joint. The cranial cruciate ligament (analogous to the anterior cruciate ligament, or ACL, in humans) starts far back on the femur and runs toward the front of the tibia, while the caudal cruciate does the opposite, starting toward the front of the femur and running to the back of the tibia. The cranial cruciate ligament performs the important job of preventing backward sliding of the femur on the sloped tibial plateau when the animal bears weight on the limb. It also helps to limit internal rotation of the stifle. 

What kinds of problems can occur with the cranial cruciate ligament? 

Degeneration of the cranial cruciate ligament is the most common cause of hind limb lameness in dogs. This degeneration, which ultimately results in weakening and tearing of the ligament, is caused by a combination of biological and biomechanical factors, most of which are thought to have a genetic basis. Lifestyle-related variables, including activity level and body weight, likely also contribute. These genetic and lifestyle-based factors cause gradual weakening, progressive inflammation, and micro-tears to the ligament that ultimately result in a complete tear. Once the ligament tears, the femoral condyles slide backward down the tibial plateau every time weight is placed on the leg, resulting in an unstable stifle joint. This instability causes pain and inflammation within the joint, resulting in progressive arthritis, and predisposes the patient to meniscal tears. 

While dogs of any breed can develop cruciate ligament degeneration and tearing, the Labrador Retriever, Boxer, Rottweiler, Newfoundland, Malamute, St. Bernard, American Staffordshire Terrier, Chow Chow, Akita, Airedale Terrier, and Mastiff breeds all appear to be at increased risk. 

While cats can also tear their cranial cruciate ligament, it is much less common than in dogs. In cats, cranial cruciate tearing is usually caused by a sudden trauma, like in humans, and generally does not have a genetically-based chronic degenerative component. 

What are the clinical signs of a damaged cranial cruciate ligament?

The most common sign of cranial cruciate ligament damage is a limp in the affected limb. This often begins as a mild on-and-off lameness that after a variable period of time abruptly progresses to non-weight bearing as the degenerating ligament either partially or fully tears. Within a few days, most affected dogs start to bear a little bit of weight on the injured leg. Because the body begins to lay down scar tissue around the joint in an attempt to stabilize it, many affected individuals go on to bear progressively more weight over time. Without prompt intervention, however, the lameness will not resolve, as ongoing instability and inflammation cause progressive arthritis and pain. 

How is injury to the cranial cruciate ligament diagnosed?

The diagnosis of a damaged cranial cruciate ligament begins with an orthopedic exam. Common exam findings include joint effusion (an increased volume of joint fluid secondary to inflammation), thickening of the joint capsule, decreased muscle mass, reduced range of motion, and discomfort on palpation of the stifle joint. A specific manipulation called the cranial drawer test, in which the veterinarian grasps the top of the tibia and bottom of the femur and attempts to move the tibia forward with respect to the femur will reveal if the ligament is fully torn. A similar test, called the cranial tibial thrust test, involves palpating the patella (kneecap) and front of the tibia while flexing the hock; if the cranial cruciate ligament is fully torn, the front of the tibia will shift forward relative to the position of the patella. Due to the stabilizing effect of tense leg muscles, sedation may be needed in order to perform these manipulations. 

Once a physical exam has been performed, sedated x-rays are taken of the stifle joint. While the ligament itself is not visible on x-rays, characteristic changes to the soft tissues and bones are present in the cranial cruciate-deficient stifle. A variable degree of distension of the joint capsule is seen as a result of inflammation-induced joint effusion. The top of the tibia (tibial plateau) may be positioned too far forward (subluxated) relative to the femoral condyles. Depending on the duration of the ligament damage, arthritic changes may also be present. Finally, in a small fraction of cases, abnormal mineralization may be present within the torn ligament. 

How is injury to the cranial cruciate ligament treated?

Complete or partial tearing of the cranial cruciate ligament typically requires surgical treatment for a functional outcome. The only exception is in cats and small dogs, who may recover near-normal function with several months of activity restriction, anti-inflammatory medications, and weight management. Even these patients, however, will achieve a better outcome with surgery. 

Several different surgical procedures exist to address degeneration and tearing of the cranial cruciate ligament. The three most commonly performed surgeries are the extracapsular repair, the tibial plateau levelling osteotomy (TPLO), and the tibial tuberosity advancement (TTA). Which surgery is best for a particular animal will depend on a number of factors, including body size, age, activity level, and the specific anatomy of their stifle joint (typically determined through measurements on stifle x-rays.)

The extracapsular repair, also known as the lateral suture technique, aims to stabilize the knee by installing a synthetic substitute for the torn ligament outside of the joint against its lateral (outer) surface. First, the surgeon enters the joint and removes any damaged portions of the cranial cruciate ligament and medial meniscus. After closing the joint, the surgeon passes a thick, heavy suture—most commonly composed of nylon, around the lateral fabella, a small round bone located behind the lateral femoral condyle. The suture is then brought forward across the lateral aspect of the stifle joint, and one end is passed under the patellar ligament to the inner (medial) side of the stifle. It is then passed back to the lateral side through a bone tunnel drilled in the top of the front portion of the tibia. The two ends of the suture are brought together and adjusted to the appropriate tension, then a tiny metal clamp is used to secure the two strands together. This nylon loop now runs in the same direction from the front to the back of the knee as the cranial cruciate ligament, effectively assuming the job of preventing the femur from backsliding on the tibial plateau. Over the months following surgery, scar tissue is laid down along and around the suture, providing essential reinforcement, as the suture will inevitably weaken and loosen with time and use. This surgery is most appropriate for animals weighing less than 30 pounds who live a relatively inactive lifestyle and who do not have an abnormally steep tibial plateau slope.

The tibial plateau levelling osteotomy (TPLO) aims to stabilize the knee by flattening the slope of the tibial plateau, thereby eliminating the driving factor behind the backward slippage of the femur. First, the surgeon enters the stifle joint and removes the damaged remnants of the cranial cruciate ligament. They also inspect the medial meniscus, which can become damaged as a result of joint instability. If damage is found, the affected portion of the medial meniscus is removed, leaving only healthy and functional meniscal tissue to cushion the knee. The levelling of the tibial plateau is achieved by making an arc-shaped cut (called an osteotomy) through the top of the tibia, starting at the front of the joint surface of the tibia and ending at the back of the tibia a small distance below the joint. The bone segment created, which encompasses the joint surface of the tibia, is then rotated slightly so as to nearly eliminate the slope of the tibial plateau. A special surgical plate is used to secure the bone segment in this position. The cut in the bone takes several months to heal, and during this time, it is essential to limit physical activity to avoid loosening and breakage of the implants and allow healing to occur. This surgery is an appropriate option for virtually any animal with a torn cranial cruciate ligament. 

The tibial tuberosity advancement (TTA) surgery aims to stabilize the knee by moving the tibial tuberosity, with its patellar tendon attachment, forward, so that the patellar tendon becomes perpendicular to the tibial plateau. The increased forward pull of the patella tendon when weight is placed on the affected limb helps to counteract the back-sliding force on the end of the femur that occurs when the cranial cruciate ligament is ruptured. First, the surgeon enters the stifle joint and removes the damaged remnants of the cranial cruciate ligament. They also inspect the medial meniscus, which can become damaged as a result of joint instability. If damage is found, the affected portion of the medial meniscus is removed, leaving only healthy and functional meniscal tissue to cushion the knee. Next, the surgeon makes a straight vertical cut through the front of the tibia from the inner (medial) to the outer (lateral) surface, separating the tibial tuberosity from the rest of the tibia, leaving only the periosteum (thin flexible outer lining of the bone) at the bottom of the cut attached. This attachment serves as a hinge, and a small metal cage is inserted higher up in the cut to maintain a precisely-sized wedge-shaped gap between the two cut surfaces of the tibia. A special surgical plate is then secured across the gap to both bone segments, fixing them in their new configuration. Finally, a bone marrow graft or synthetic bone graft is placed into the gap to help facilitate bone ingrowth. The cut in the bone takes several months to heal, and during this time, it is essential to limit physical activity to avoid loosening and breakage of the implants and allow healing to occur. This surgery is an appropriate option for virtually any animal with a torn cranial cruciate ligament. 

What post-operative care is required following surgery?

Regardless of which surgery is selected for a specific patient, proper care in the months after surgery is essential to a successful outcome. Most patients are able to go home the day after surgery with oral medications to treat pain and inflammation. Instructions are provided on how to perform some initial physical rehabilitation techniques such as cryotherapy (ice packing), stretches, and range of motion exercises. 

The most critical component of a patient’s post-operative care is strict activity restriction until healing is complete. Following an extracapsular repair, this takes about two months, while after a TPLO or TTA, this typically takes three to four months. During this period, patients should only be permitted to engage in low-impact activity, such as slow walks on a short leash. These must initially be kept to only a few minutes a day, but instructions will be provided on gradually increasing the duration of daily activity. A formal physical rehabilitation program during the months following surgery, consisting of weekly or twice-weekly appointments with a physical rehabilitation technician as well as at-home exercises, is provided to aid in healing, comfort, and reconditioning.

Recheck appointments with the surgeon are scheduled at two, four, eight, and twelve weeks following surgery to assess comfort and healing. At the two week recheck, the skin sutures or staples are removed. For patients who have had a TPLO or TTA, sedated x-rays are taken at each monthly recheck to monitor bone healing and evaluate for any signs of complications. Based on the exam and x-rays, an assessment is made on how much a patient’s physical activity can be increased until the next recheck appointment. 

What are the possible risks and complications of surgery?

The complication rate following surgery for a torn cranial cruciate ligament is about 15%. Potential complications with the extracapsular repair include stretching or breakage of the suture and infection of the surgery site. Failure of the suture is usually a consequence of too much activity in the two months following surgery, and necessitates a second suture to replace the suture. Dogs over 30 pounds are at a higher risk of this complication than are smaller dogs and cats. Most surgical site infections are superficial and respond to antibiotics, but in some cases, removal of the suture may be necessary to clear the infection. 

Potential complications of the TPLO surgery include surgical site infection, patellar tendonitis, loosening or breakage of the surgical implants, and fracture of the fibula (the very slender bone running parallel to the tibia). Complications following the TTA surgery may include surgical site infection, patellar luxation, loosening or breakage of the surgical implants, and tibial fracture. Most complications can be successfully resolved with medications and physical rehabilitation therapy, but in the case of damaged surgical implants or deep infection, a second surgery may become necessary.

What is the prognosis for cranial cruciate ligament injuries?

The prognosis for return to normal or near-normal function following surgery is very good to excellent; for all three of these surgeries, 85-90% of patients return to normal limb use once fully healed. 

It is important to understand that degenerative changes that have already developed in the joint cannot be reversed, and some degree of arthritis will develop even with surgical stabilization. This, however, is much less severe than it would be without surgery, and is typically successfully managed with weight management, joint supplements, nutraceuticals, and anti-inflammatory medication as needed. 

While a patient who has had surgery for a torn cranial cruciate ligament has a lower risk of going on to develop a meniscal tear than one who has not had surgical stabilization, there is still about a 10% chance of medial meniscal injury in the future. This usually presents as a sudden-onset lameness developing up to a year after surgery, and may necessitate another procedure to remove the damaged portion of the meniscus.  This surgery is much simpler than the extracapsular repair, TLPO, or TTA, and has a much more rapid recovery.