The cranial cruciate ligament (CCL) is part of the stifle joint and acts to limit internal rotation and forward movement of the lower and upper part of a dog's hind leg. In people, this is called the anterior cruciate ligament, or the ACL, and it is part of the human knee. Damage to a dog's cranial cruciate ligament is one of the most common causes of hind limb lameness in companion dogs. Most cases will require surgical stabilization or reconstruction, along with moderate lifestyle changes and medical management, to restore stability to the stifle and reestablish a good, pain-free quality of life for affected animals.
The "stifle" is the dog's equivalent of the human knee. It is normally stabilized by a number of ligaments. The main two are the cranial and caudal cruciate ligaments, which cross over the front of the stifle joint. Two other ligaments, the medial and lateral collateral ligaments, stabilize the sides of the stifle joints in dogs. Rupture of the cranial cruciate ligament is a common, and potentially quite serious, condition in pet dogs.
Causes & Prevention
Causes of CCL Injuries
The stifle joint of the dog is especially vulnerable to injury, because it has no interlocking bones to provide structural support. Instead, a dog's stifle, or knee joint (in its hind legs), depends upon a meshwork of ligaments and muscles for stability. The cranial cruciate ligament (CCL) provides most of this support for the stifle and, when partially torn or completely ruptured, leaves only the muscles and soft tissues surrounding the stifle to hold the stifle joint in place. The cranial cruciate ligament in dogs is called the anterior cruciate ligament in people. The CCL can be damaged from acute trauma or from chronic repetitive injury. Most acute (sudden) CCL injuries in dogs happen during strenuous or exuberant athletic activities, such as romping, roughhousing, running, hunting, playing, chasing, jumping or engaging in other enthusiastic antics. Owners often report that their dog suddenly stumbled, possibly yelped and then "came up lame," or held its rear leg up at an exaggerated angle.
The affected ligament probably experienced excessive wear and tear well before observable signs of damage became apparent. Chronic damage develops slowly over time and can be caused by poor neuromuscular coordination, conformational abnormalities or poor muscle tone. Obesity certainly can contribute to chronic ligament damage as well, because it puts abnormal stress and weight on the stifle joint.
Prevention of CCL Injuries
Unfortunately, there is no fool-proof way to prevent stifle injuries in dogs, other than keeping them from playing and running around exuberantly, which is not a good option. Conditioning seems to be the best approach. Dogs that are in poor physical condition are at the greatest risk of injuring their CCL, while healthy, well-conditioned dogs are better able to avoid injuries, because their stifles are protected from outside stressors by strong surrounding musculature. Owners should keep their dogs on a high-quality diet and give them regular, moderate exercise. After a period of inactivity, a dog should not be asked to engage in rigorous physical activities all of a sudden. He should be allowed to gradually work up to a point where vigorous activities are enjoyable and not overly stressing. Finally, dogs with conformational abnormalities in their rear legs that predispose them to CCL injuries probably should be spayed or neutered and not bred. This is something best discussed with the dog's breeder and its attending veterinarian.
Dogs that develop cruciate ligament injuries in both hind legs can look like they have a neurological disorder, because they typically rise and move with an unusual, exaggerated and uncoordinated gait. The cause of bilateral hind limb lameness can be difficult to diagnose. Most veterinarians will want to perform a thorough neurological examination in addition to an orthopedic examination when evaluating a dog that comes in with a sudden onset of abnormal rear end locomotion.
Dogs with cruciate ligament injuries often have a condition called "lymphoplasmacytic synovitis." Many experts think that the dog's immune system initiates an inflammatory reaction directed against the collagen of the torn ligament after a CCL injury. Another theory is that lymphoplasmacytic synovitis occurs first and causes the stifle joint laxity, weakness and instability, which in turn predisposes the dog to cranial cruciate ligament injury. The actual relationship between CCL damage and immune-mediated lymphoplasmacytic synovitis is still the subject of much debate in medical circles.
Symptoms & Signs
Effects of CCL Injuries
Damage to the cranial cruciate ligament (CCL) is the most common cause of canine hind limb lameness and is a major cause of degenerative joint disease in the stifle (knee) joint of companion dogs. The cranial cruciate ligament is critical to stabilizing a dog's stifle, which is the equivalent of the human knee. When the CCL is torn, the upper ends of the two long lower leg bones (the tibia and fibula, like the shin bone in people) become loose within the knee joint, causing abnormal friction, wear and tear. This, in turn, leads to joint swelling (effusion), stifle joint instability, pain, lameness in the affected rear legs and, ultimately, chronic irreversible degenerative joint changes. Dogs with ruptured CCLs will become lame suddenly, because of the pain associated with this injury. They may feel better after rest, but once they rise, walk around or exercise, the pain will return, as will the lameness. They will just plainly hurt.
Symptoms of CCL Injury
The outward signs of cruciate damage depend upon the severity of the injury, which can range from a partial tear to a complete rupture of the affected ligament. Without surgical correction and appropriate post-operative management, the effects of CCL injuries can progress and become permanent. Most owners of dogs with cranial cruciate ligament injuries first notice a sudden onset of lameness or limping in one or both rear legs. Most of these injuries happen when a dog is romping, roughhousing, running, playing, jumping or engaging in other enthusiastic antics. Owners often report that their dog suddenly stumbled, possibly yelped and then "came up lame." However, the affected ligament probably experienced excessive wear and tear well before observable signs of damage became apparent.
Symptoms of this condition that owners may notice include:
Lameness in one or both hind legs; may be non-weight-bearing; may come and go; may come on very suddenly
Limping or reluctance to use one or both hind limbs, which worsens with exercise and improves with rest
Abnormal posture, especially over the back and hip areas
Reluctance to rise, run or jump
Morning stiffness that takes time to "warm out of"
Sitting at an odd angle, with a hind leg slanted off to one side
Swelling (effusion) around the stifle joint
Atrophy (withering away) of the muscles of the affected limb
All or only some of these signs may be noticed by owners. However, regardless of the severity of the injury, the signs of CCL damage usually become worse if they are not treated, because the pain felt by the dog increases as the stifle joint progressively deteriorates.
Dogs At Increased Risk
All breeds and both genders are susceptible to this injury. Rottweilers, Labrador Retrievers, Staffordshire Bull Terriers, Newfoundlands and other active, large-breed dogs seem to be at an increased risk of suffering from CCL damage. Most affected animals are in good body condition and are not systemically ill at the time of their injury. Rupture of the CCL can happen in any dog at any age, but it is more likely to occur in young, active animals. When a cruciate ligament ruptures in one leg, there is an increased chance that the CCL in the other leg will eventually become compromised, probably because of the increased weight that will be required to support the other leg as it heals.
Diagnosis & Tests
Cranial cruciate ligament injuries are quite common in domestic dogs. Fortunately, they are not particularly difficult for skilled veterinarians to diagnose. When presented with a patient limping on one or both of its hind legs, the veterinarian with initially do several things. First, she will take a thorough history from the dog's owner, paying particular attention to whether the dog had any recent trauma that may have caused an injury to the affected leg (such as jumping off the couch, running too quickly down the stairs, jumping out of the back of the truck, zooming around the yard and skidding in the mud, etc.). She will especially want to know whether the lameness came on slowly or suddenly. Next, the veterinarian will examine the dog physically, from nose to tail. She will observe the dog's posture when sitting, standing up and walking. She will palpate (feel) the muscles and bones of the legs, usually starting at the feet and working upward. The veterinarian will be assessing the stability or instability of the stifle on the affected hind leg. There are special manipulations that she will do on the rear legs that will identify whether the cranial cruciate ligament has been ruptured, torn or stretched. Most dogs accept these gentle manipulations quite well, without the need for sedation. However, depending on the severity of the injury, sedation may be appropriate for the dog's comfort.
It is important for owners to take hind limb lameness in their dogs seriously and to seek veterinary advice and assistance as soon as possible. Without treatment, the underlying reason for the lameness can progressively worsen and ultimately become permanent.
After the initial evaluation, most veterinarians will recommend routine blood work, which includes a complete blood count (CBC) and a serum chemistry panel, together with a urinalysis. The blood sample will be taken from the dog's jugular vein or from one of the large veins in a leg. The urine sample will be collected "free catch" (if the veterinary technician is lucky), or more easily by a procedure called "cystocentesis," which involves inserting a sterile needle through the abdominal wall directly into the bladder, and retrieving a sterile urine sample by pulling back on the plunger of the attached syringe. Neither of these samples requires sedation. The results of blood work and a urinalysis will provide a snapshot of the dog's overall health. If a damaged CCL is the only problem affecting the dog, the test results typically will be normal. However, it's always a good idea to check a dog thoroughly, to get a complete picture of what may be contributing to its symptoms. For example, lameness can be caused by many things other than a blown cruciate ligament. Cancer, soft tissue injuries and bacterial infections in muscles or joints can all cause lameness. Evaluation of a blood sample will show whether the dog's white blood cells are elevated, which happens when the dog's immune system is fighting an infection.
The attending veterinarian will probably also recommend that radiographs (X-rays) be taken of the stifle joints of both rear legs, even if the dog is only lame in one leg. It is important to look at both legs for comparison purposes. The veterinarian will assess the angles of the bones that come together at the stifle. In some cases, radiographs of the whole legs may be recommended, especially if physical limb deformity or some other structural or conformational defect is suspected. Sedation may be necessary to get good films.
Arthroscopy and/or magnetic resonance imaging (MRI) may be recommended to confirm the diagnosis, especially if the cruciate ligament is only stretched or partially torn. "Arthroscopy" is the examination of the inside of a joint using an arthroscope (endoscope), which is a wand-like instrument with a camera at its tip. "Magnetic resonance imaging" involves placing the dog into a large tube-like structure under general anesthesia. The MRI instrument produces a very strong magnetic field around and through the dog. Radiofrequency signals are sent to a computer and processed in cross-sectional images, which the veterinary radiologist will carefully evaluate. MRIs are best used in veterinary medicine to assess soft tissue injuries. This diagnostic test is usually only available at veterinary teaching hospitals and highly specialized veterinary clinics.
Arthrocentesis, which involves sampling the synovial fluid from inside a joint capsule, can also be done to help to rule out infectious causes of lameness. Biopsies of the affected ligament may be taken during a surgical exploration or repair procedure, especially when the injury is not thought to have been caused by acute trauma. The sample will be sent to a diagnostic laboratory for microscopic assessment, to try and figure out what caused the ligament to fail.
Owners should not blame themselves if their pet injures its cranial cruciate ligament. Normal roughhousing and romping, and even more strenuous activities, are important to a dog's overall fitness and physical and mental health. These activities should not be restricted just to prevent possible damage to the stifle. CCL injuries are fairly common in domestic dogs, and usually can be treated surgically with great success.
Goals of Treating CCL Injuries
When a dog suddenly comes up lame in one or both hind legs, its owner should take it to a veterinarian as soon as possible. Left untreated, damage to the stifle (knee) joint usually is progressively degenerative; any chance of reasonable recovery wanes without treatment. A dog that favors an injured leg for a long period of time also runs a significant risk of eventually damaging some part of its "good" leg, because it is being over-used. The goals of treating canine cruciate ligament (CCL) injuries are to relieve pain, improve stability and function of the stifle joint and minimize the progression of degenerative changes.
Dogs that injure their cranial cruciate ligament suddenly (acutely) should start medical management as soon as the injury is diagnosed. "Medical management" means treatment with medication and other sorts of supportive care. Conservative medical management of CCL injuries includes administration of non-steroidal anti-inflammatory drugs (NSAIDs), subcutaneous or intravenous fluids, rest, exercise restriction and possibly corticosteroid therapy. Steroids should not be given at the same time as NSAIDs, because severe gastrointestinal and other adverse side effects can occur when these two types of drugs are combined. Sometimes, medical management without surgery is all that is needed for small dogs, older dogs or dogs whose CCL is only stretched or partially torn. However, in most cases, surgical stabilization of the stifle joint is the only effective way to treat this injury and reduce the risk of future degenerative damage to the joint. Torn or ruptured cruciate ligaments will not re-attach or re-grow without surgical intervention.
There are a number of surgical options for stabilizing the stifle joint, and more are being developed all the time. Some of the current techniques include intra-articular grafts, extra-capsular suture stabilization, tibial plateau leveling osteotomy (TPLO), tibial tubercle advancement (TTA), arthroscopic reconstruction and medial meniscal release. A detailed description of these surgical techniques is beyond the scope of this article. A dog's veterinarian is the best one to assess its injuries and select the best surgical procedure in any given case. Frequently, a general practitioner will consult with or refer the dog to a veterinary orthopedic specialist, because selection of the "best" surgical technique remains controversial even among experts.
After surgery, the dog normally will be managed medically with non-steroidal anti-inflammatory drugs, and possibly other drugs or supplements to promote healthy cartilage repair (these are called "chondroprotective agents"). A number of chondroprotective supplements are available, both over-the-counter and by veterinary prescription, to help keep cartilage and joints well-lubricated. These include polysulfated glycosaminoglycans, glucosamine, chondroitin sulfate, hyaluronan, Vitamin C, omega 3 and 6 fatty acids and MSM, among others. These can be quite effective in reducing inflammation and relieving pain.
In many cases, good dietary management and weight loss alone can dramatically reduce the pain and other symptoms that accompany CCL damage, as overweight dogs tend to suffer more from this injury than do fit dogs. Moderate, regulated exercise and/or exercise restriction can stimulate cartilage growth and help delay joint degeneration, and long controlled walks in early or mild cases of CCL damage may help prevent loss of rear muscle mass. Physical therapy, hydrotherapy (swimming and other water exercises), passive flexion and extension of the affected limb and controlled low-impact on-leash walks often are incorporated into the recovery process. It takes a long time – often months – for a dog to heal from CCL surgery. During much of that time, the dog should be strictly confined and only walked outside on a leash to potty.
Other, less traditional techniques that may or may not benefit affected dogs, in addition to medical treatment, might include: massage therapy to stimulate blood flow to the stifle area and reduce joint stress; application of acupuncture and/or acupressure techniques; use of herbal or other non-regulated supplements or homeopathic "remedies"; and other forms of supportive care that may help to ease pain, increase circulation, speed healing and otherwise promote wellness, relaxation and comfort. Some of these adjunct approaches lack controlled studies of their effectiveness and may not have established quality control methods or ways to assess their benefit to dogs with partially or totally torn or ruptured cranial cruciate ligaments. Chiropractic adjustment or manipulation of dogs with musculoskeletal conditions is highly controversial among veterinary professionals, unless it is performed by or under the direct supervision of a veterinarian with orthopedic expertise.
Dogs that undergo surgical correction of injured cruciate ligaments have a very good prognosis for return to long-term, pain-free function. Stabilization of the stifle joint, post-operative treatment with non-steroidal anti-inflammatory medications, and judicious rehabilitation typically result in rapid resolution of the pain that caused the lameness. If joint effusion (swelling) and discomfort persist, some dogs respond well to steroid therapy at immunosuppressive levels (but not at the same time as NSAIDs are being administered). Owners should know that following damage to the CCL in one leg, the other leg is at a greatly increased risk of suffering a similar injury – even if the first injury is surgically corrected. The contralateral (or "good") limb is forced to bear an unusually large amount of additional weight, stress, wear and tear before surgery on the "bad" limb takes place, and also during the long recovery process, which typically takes months.