Nine times out of 10, you know this injury the moment you see it: A large-breed dog hobbles in on three legs, with one hind paw dangling just above the ground. It could be a fractured leg or a dislocated hip, but if I were playing the odds, I’d bet it’s a ruptured cranial cruciate ligament.
A quick comparative anatomy lesson is in order. Just like people, our pets have knee joints in their hind limbs (and only in their hind limbs). The human knee is partly held together by a pair of criss-crossing ligaments called the anterior and posterior cruciate ligaments (ACL and PCL). The same is true in dogs, except the nomenclature is different. Instead of a knee, we call it a stifle. And instead of anterior and posterior cruciate ligaments, we say cranial and caudal.
Regardless of what you call it, if a dog injures a cranial cruciate ligament it has the same problem as a human with a torn ACL: The joint simply isn’t being held together anymore. Sometimes this injury is purely the result of trauma. Just as people can rupture an ACL while skiing or turning too quickly on a basketball court, dogs can blow the ligament by just running around in the yard. But that’s not how it usually happens.
Far more often, a pre-existing joint disease contributes to a dogs’ cruciate ligament injury. While my patients may suffer sudden lameness after exercise, the groundwork for that injury was almost always laid down in advance. Degenerative joint disease is common in the stifle, and that constant inflammation weakens the cruciate ligament. With hindsight, many owners recall their dogs showing intermittent lameness in the prior weeks and months. It improved after a few days of rest, but with each round of exertion, the ligament weakened until finally giving out.
This brings good news and bad news. First, the good: Preventing degenerative disease can reduce the risk of cruciate injuries later on. The single best way to do this is to make sure dogs maintain a healthy weight and engage in routine low-impact exercise like walking and swimming.
The bad news is that degenerative joint disease results from a convergence of environmental and genetic causes, many of which are not well understood. Although weight is a factor, it is not the most important one. Some dogs just seem destined to have bad joints. So while you might be able to reduce the injury’s odds, you can’t eliminate them.
Once a dog has been diagnosed with a cruciate injury, options can be frustratingly limited. Smaller dogs can sometimes recover with time and rest, but the joint often remains unstable and prone to re-injury. Larger dogs very rarely respond well to conservative efforts, and the majority of dogs with cruciate injuries cannot recover without surgical repair. And worse, since the underlying causes affect ligaments on both sides, an injury to one is very likely to eventually injure the other one. Delaying treatment can multiply their troubles since overuse of the opposite leg steadily increases the odds of both legs being affected.
While the prospect of knee surgery can be emotionally and financially stressful, I’m at least happy to say that the results are broadly excellent. Orthopedic surgery is increasingly the province of highly-trained specialists, and nearly all of these dogs eventually return to a happy and active life. That doesn’t make the news of a cruciate injury any more welcome, but it hopefully makes it a little less daunting.
Dr. Mike Fietz is a small animal veterinarian at Georgetown Veterinary Hospital. He has lived in Charlottesville since 2003, the same year he received his veterinary degree from Cornell University.
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