A cranial cruciate ligament tear or rupture is the most common rear leg injury in dogs, according to Matthew Mahn, D.V.M., an American College of Veterinary Surgeons diplomate and surgeon at the Midwest Veterinary Referral Center in St. Louis., which treats about 200 to 300 such injuries each year.

You may have heard the term ACL, for anterior cruciate ligament, in reference to this tiny ligament inside the knee joint. ACL is the human term for this part of the anatomy. Nonetheless, a tear or rupture is exactly the same injury in dogs as in people, Mahn says. “The primary function of that ligament is to keep the knee nice and stable,” he says. “When it tears, the knee becomes unstable.” It works in conjunction with the caudal – or posterior – cruciate ligament which crosses it.

Despite its size – only about three-fourths of an inch long in the average Labrador – it’s made up of hundreds of thousands of bundles that wrap around each other. “Mid-body tears are in the middle of the ligament,” Mahn explains, while in an avulsion, the ligament is pulled off at the attachment to the femur – the thigh bone – or the tibia – the shin bone. “Several of those bundles can rupture, or all of them can,” he says. The ligament runs from the back of the thigh bone diagonally to the front of the shin bone.

Because dogs’ back legs are like ours and their front legs are more like our arms, dogs only have knees, thus cranial cruciate ligaments, in their back legs.

Doing the Damage

Mahn and his surgical colleague at Midwest Veterinary Referral see two major kinds of CCL injuries: acute and chronic.

In the first, for example, “Dogs will be at a dog park running with other dogs, and all of a sudden develop an acute lameness,” Mahn says. They may even “yelp out” in pain.These are “very acute” injuries, he says. “CCL injuries can also be very, very chronic. The owners don’t really even know what happened.They literally will say, ‘We went to work this morning, and he was fine. We came home this evening, and he’s limping.” The ligament can tear from such simple activities as walking to the feed bowl or getting off the couch, Mahn says. Chronic CCL is more typically seen in an older dog, he adds, one at least 4 years old. Acute cases are often found in younger dogs, though almost all are adults.

Large breed dogs seem to be more predisposed, he says, and Labrador Retrievers seem to be over-represented as a breed. He’s quick to point out, however, that the breed’s standing as the most popular breed in the U.S. could explain that. Mahn also sees a lot of Rottweilers, Newfoundlands, Boxers, Golden Retrievers and Cocker Spaniels. While most of these are larger breeds, “any size dog can have this injury,” he says.

Why CCL tears and ruptures are so common remains a mystery. “We don’t have the answer to that – not one single anything that we can put our finger on. It’s frustrating for us as doctors and frustrating for clients who have dogs with this injury.”

It’s easy to speculate that canine athletes would be most susceptible, but Mahn says he “very, very rarely” sees a racing greyhound, for example, with a CCL rupture. Nonetheless some evidence suggests that overweight dogs and those that compete in field trials and similar events may experience the injury more often than dogs that have appropriate body conditioning scores and have a less demanding physical regimen. Current research is looking at whether an autoimmune component may be involved.

In the meantime, owners spend $1.6 billion a year in the U.S. and Europe on treatment for their dogs’ CCL injuries, according to Mahn.

The primary issue in a CCL injury is “instability within the knee because the ligament isn’t serving its primary function,” Mahn says. From an owner’s standpoint, you can see anything from limping while walking to holding up the leg to prevent pain. “Some owners can recognize the fluid-filled knee,” as it is swollen from the inflammation.

Any lameness lasting more than 12 to 24 hours means a veterinary checkup is in order.

How Bad Is It?

A ligament tear is determined through a physical exam. “You can feel for the instability,” Mahn says. “Sometimes you need X-rays to help support your diagnosis.”

Too much fluid in the joint is also an indicator, as are the results of two tests: the tibial thrust test and the cranial drawer test. In the first test, your vet will hold the thigh bone steady, then flex the ankle. If the tibia moves forward, the ligament is injured. In the second, the vet again holds the thigh bone steady and tries to move the shin bone forward. If it does – kind of like a drawer opening – there’s likely a CCL injury.

The best way to figure out the extent of the injury is through a “surgical look at the joint.” But most tears, whether partial or complete, can be diagnosed without surgery.

Options for Treatment

Treatment for CCL injuries falls into two major categories: medical management and surgical.

For minor injuries, such as mid-body, partial tears, medical management can be sufficient. The veterinarian or veterinary surgeon manages the dog’s pain with medication and the owner manages the dog’s activity “in order to allow the body to heal the knee,” Mahn says. “Dogs that weigh 15 kg – about 33 pounds – or less “have better responses to medical management than larger dogs, but still may not be successful. Usually with medical management, I get them on a nonsteroidal anti-inflammatory drug (NSAID) with a cocktail of tramadol and gabapentin to help with the acute inflammation and pain of the knee.”

This continues for at least six weeks, he says, during which time the dog cannot run, jump or “roughhouse.” It involves “a lot of crate confinement” whenever the dog cannot be directly surprised. The dog can only go outside to relieve itself – and, even then, on a leash. “They’re pretty strict restrictions,” he says.

In addition, Mahn “highly recommends” underwater treadmill therapy at a canine rehabilitation center starting as soon as treatment begins.

Medical management is not typically an option for dogs involved in competitive sporting events, such as field trials or agility, he adds. To continue in such knee-stressing activity, the CCL must be supplemented with an artificial tendon.

Partial tears can advance to complete tears in the future, he says. Either way, in the long-term, arthritis management may be required. Any CCL injury will, at some point, lead to arthritis of one degree or another.

The Less Invasive Surgeries

“No surgeries are effective at directly replacing or repairing the ligament,” Mahn says.

In the history of veterinary medicine, “tens upon tens” of surgeries were developed to repair ruptured CCLs, he says. Today four surgical procedures are the most common with two being less invasive and two being more so. Which type of surgery a dog will have is based on the severity of the injury, the owner’s comfort level with the various methods and the desired activity level for the dog post-surgery.

The less invasive procedures– extracapuscular stabilizations – use prosthetic, or fake, ligament to replace the natural ligament. “We essentially place an implant that mimics the function of the ligament,” Mahn says.

Lateral suture stabilization commonly uses heavy gauge, nylon, single filament suture material in a 20-, 40-, 80- or 100-pound strength, depending on the dog’s size. It’s anchored on the lateral fabella, the small round bone on the outside of the knee, Mahn explains. It’s secured around the knee with crimps.

The TightRope accomplishes the same thing, with a combination of FiberTape and FiberWire. A braided material, it’s “pretty darn strong,” Mahn says. “It looks like a flat piece of woven cloth.” The attachment requires the drilling of holes in the femur and the tibia, then the TightRope is anchored with little “buttons” to these bones.

Generally these procedures are best used in dogs that are older, Mahn says, whose slope at the back of the knee joint is less than 30 degrees and who have an average active lifestyle. “For the most part, they’re an indoor dog. They go outside to take care of business, chase a squirrel here or there, and take a mile or two walk a day.”

Most dogs recover fully between 11 weeks and four months after the procedure. As with medical management, dogs must have severely restricted activity for some of this time.

The Serious Stuff

The more invasive surgeries involve osteotomy, or the cutting of bones.

“In general, my candidates would be younger dogs, highly active dogs and very large to giant breed dogs,” Mahn says.

During an osteotomy, the veterinary surgeon alters the upper portion of the tibia to the point “where it alters the biomechanics of the joint itself and converts that shearing force that’s occurring into a compression force,” he says. To explain this to clients, he compares it to rubbing sandpaper on a wall versus touching sandpaper to a wall.

The most common osteotomy is TPLO or tibial plateau leveling osteotomy. “We make a semicircular cut, then rotate the bone backward to level the tibial plateau. Instead of that slope being 25 or 26 degrees, it’s five to seven degrees. Once we do that rotation, we fix it into place with a specialized plate and screws. They hold the fracture together, and it heals up.”

A tibial tuberosity advance, or TTA, is based on a human model of knee biomechanics. “Instead of making semicircular cut, we make a very straight cut just behind tibial tuberosity. We pull that attachment forward, then we put in special plates, and a 3-, 6-, 9-, 12- or 15 mm titanium cage, plus a “fork,” to create a gap where the bone was. Eventually that gap fills with solid bone. The end result is the tibial tuberosity and patellar ligament are at 90 degrees to each other.

Despite the more invasive nature of osteotomies, they require the same recovery time as adding lateral suture or TightRope to stabilize the joint.

Mahn points out, however, that every surgeon has his or her own preferred rehabilitation protocol, based on experience.

Common complications to the surgeries – in 10 to 22 percent of cases – can include infection, a risk with any surgery; further tears to the fibrocartilaginous structure in the knee – the meniscus; ongoing lameness or instability; and what Mahn calls “catastrophic failures,” such as implant failures that cause other fractures and necessitate a revision surgery.

However, he says that no matter what a veterinary surgeon does, the knee will never be restored to the condition that nature intended, “but we get pretty close.” In addition, “there will always be a progression of arthritis. It typically is minimal to no issue at all for the patient. The knee is a pretty forgiving joint when it comes to arthritis.” Also the meniscus may be damaged, and there may be other injuries in the knee joint along with the CCL rupture.

“In the grand scheme of things, as far as prognosis is concerned, regardless of the type of surgery, you have an 85- to 95-percent chance of a good to excellent return to function,” Mahn says. “It’s essentially being able to return to the former level of activity with occasional lameness or no lameness whatsoever.

While current research continues to better identify predisposing causes of CCL injury, Mahn says, “Someone is always trying to develop a new technique or procedure. They’re still looking into ways to replace ligament, though he doesn’t expect the introduction of anything new in the short term.

“The best advice that I can give owners, and it’s not 100 percent, is to keep their animals active and to keep them at an ideal body condition.” Because the factors that cause CCL injuries are not known, no preventive effort can be 100 percent, he adds. But even then it’s not 100 percent preventive.

However, for a dog that’s treated appropriately based on the extent of the injury, the dog’s age and size, and its future activity level, “for the most part, they do very, very well,” Mahn says.

To watch a video that shows the parts of the knee and manipulation of a ruptured CCL for diagnosis, click here. Please note that a related video shows a procedure that has fallen out of favor, the “over the top method” or intracapsular repair.