The knee, known medically as the stifle, is a complicated joint prone to injury in dogs. By far, the most common injury to the hind leg is rupture of Cranial Cruciate Ligament (also called the ACL) of the stifle. It is most common in larger breed dogs especially the Labrador retriever, Golden retriever, Rottweiler, Newfoundland, Akita, St. Bernard, Chesapeake Bay retriever, and American Staffordshire Terrier. Other large breed dogs, including mixed breeds, may also suffer this injury.  The Cranial Cruciate ligament is hugely important from the perspective of function, and while quite small, it is tremendously important in stifle stability.

Without functioning Cruciate ligaments, the upper and lower bones that comprise the stifle joint (the femur above and tibia below), do not stay in proper alignment when the dog puts weight on the limb and the stifle becomes very unstable as the bones slide across each other during weight-bearing movement. 

In smaller breed dogs, the Cruciate injury occurs more often if the dogs also have a luxating or dislocated patella (kneecap).   The luxation puts extra pressure on the Cruciate ligament and increases the risk of a tear. In such situations, both problems require repair.

The ligament is not capable of healing itself, so the main changes over time are the progression from acute injury and inflammation to a more chronic state.  Once the knee becomes unstable, the inflammation begins immediately.  At a microscopic level, changes can be seen within 24 hours.  The chemistry of the joint fluid in the stifle shifts as the inflammatory process starts, leading to a destructive impact on the joint cartilage, swelling, changes in the joint fluid, and significant pain. Pain may decrease over time, but the stifle remains unstable and negative forces are continuing within the joint and arthritic changes are progressing.

There are two basic scenarios for rupture of the Cruciate ligament. In humans the common form of injury is an acute rupture due to trauma – think football player injuries or snow skiing accidents. More commonly in dogs, it is a chronic degenerative change in the ligament that leads to thinning and inflammation prior to complete rupture of the structure. In the first situation, the normal dog abruptly becomes lame and has a marked inability to bear weight normally on the hind leg associated with some injury or trauma. In the latter, the dog may have had prior incidents of lameness or limping on the hind leg that resolved over a few days, weeks, or longer time.  Then the dog suddenly becomes persistently lame, often with no apparent trauma having occurred. Such dogs likely suffered some chronic degeneration before the final ligament tear that causes the more painful and unstable scenario of a full ligament rupture.

The diagnosis of Cruciate injury is fairly straight-forward. The physical exam may reveal pain on certain movements in the knee, an increase in joint fluid, and a palpable loss of stability noted on manipulation of the joint (anterior drawer sign and cranial tibial thrust). In many dogs that are painful and tense, a light sedation may be required for a full assessment of the degree of instability, especially in strong, heavily muscled dogs. Radiographs (x-rays) will clarify if there are other related problems such as avulsion of bone at the site of trauma, bone spur formation, or from an unexpected problem such as bone tumor affecting the same area. Many vets will also take a radiograph of the hips since the large breed dogs are prone to hip dysplasia and it helps to know if the patient has both problems.  Stabilization of the affected joint typically requires surgery. 

There are three commonly used surgical techniques to provide joint stability in the absence of an intact ACL. For smaller dogs, a procedure called an extracapsular repair uses heavy nylon line or fiber wire to recreate the forces of the missing ACL to reduce the instability of the femur and tibia. The other two procedures are more invasive and require metal surgical implants, the TPLO and TTA. Both of these procedures involve the movement of bone in order to change the physics of the joint to compensate for the missing ligament. The choice between surgical techniques would be based on the specific patient and the surgeon’s experience with the procedure. A 28lb Beagle, even if overweight, is typically managed with an extracapsular repair.  Large and giant breed dogs will usually have a TPLO or TTA procedure. In all cases, the joint will be inspected to see if the fibrous cushions, called the menisci are intact. If the meniscus (usually the medial one) is damaged, it will need to be trimmed to remove the damaged portion as spontaneous healing of that tissue is poor. The damaged remnants of the Cruciate ligament are also removed as they contribute to inflammation. There is a recent movement to manage these dogs with rehabilitation (physical therapy for dogs) alone, but it is a slower process to weight bearing than surgery followed by rehab.

Controlled exercise restriction is needed following any major orthopedic surgery.  Excessive restriction leads to stiffening of the soft tissues, joint restriction and muscle loss due to lack of use. Controlled exercise through rehabilitation can have a positive impact on return to function, both in recovery time and the degree of recovery. Pain management is essential throughout the process. 

Without an intact Cranial Cruciate ligament, the stifle will never be as normal as the original and some arthritis is inevitable. But without surgical stabilization of the stifle, the arthritis and dysfunction are typically much more severe. 

There are many treatments to help reduce the arthritis and disability from ACL trauma just as there are from other causes of arthritis like hip dysplasia. Adequan® injections, appropriate joint supplements and weight management are very important.

One last word of caution, about 50% of dogs that have ruptured a Cruciate ligament will suffer a rupture of the ligament in the other stifle with 12-18 months. These cases are usually in dogs that had gradual degenerative change prior to the full rupture. The forces that lead to failure of the first ligament may lead to failure of the second one as well. The good news is that the majority of dogs who had a Cruciate ligament injury and had the stifle treated will go on to normal, happy, functional lives, and can enjoy an active lifestyle once fully recovered and rehabilitated after surgery. There are many examples of competition dogs returning to their sport after recovery from Cruciate injury, whether it be agility, lure coursing, dock diving or fly-ball, particularly with the combined application of surgical, biologic and rehabilitation therapies.

Elizabeth F. Baird, DVM, CVPP, CCRT, cVMA
Steele Pain Management & Rehabilitation Center

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