ACL Injury and Repair, part 1

"Every cloud has a silver lining." "When life gives you lemons, make lemonade." If Grace had to have knee surgery, at least I can get a few blog posts out of it, no? This way, she and I can help educate others about life after an ACL.

First, some anatomy:

I don't know who created the knee, but it is the worst designed joint in the body. Instead of a ball and socket joint, like the hip or shoulder, the knee joint is simply one bone sitting on top of another, held together by several ligaments.

In this first drawing of the knee (stifle) joint, we have the bottom of the thigh bone (femur) sitting on top of the shin bone (tibia.) In between the bones are the menisci (singular meniscus) - cushions that act as shock absorbers of the knee. The kneecap (patella) is at the very top of the drawing. On either side of the stifle joint are the collateral ligaments, those on the sides of the knee. In the middle are the two cruciate ligaments.


Cruciate comes from the Latin crux, meaning cross. You can see the crossed ligaments forming an "X" in the middle of the joint. The ligament in the front is the Anterior Cruciate Ligament, which vets call the Cranial Cruciate Ligament. I'll use the abbreviation ACL for short, since most people are familiar with that term. It is the one most prone to injury or failure.

These cut-away drawings show the ACL, highlighted in pink, in action. The first drawing shows the knee in extension (straight)


while the second drawing shows the joint flexed. In this second drawing, you can see the tension in the ACL as the knee bends. It is this action which puts the ACL at risk for trauma or failure.


If you watch any sports at all, you have surely heard the phrase "blew out his knee," or "ruptured his ACL." Traumatic ACL rupture occurs when the leg is suddenly rotated while the knee is bent at a 20-50 degree angle. This might happen if the dog made a sudden right turn while bearing weight on the right rear leg. The internal stresses cause the ligament itself to rupture, or cause it to tear loose from its attachment to the bone.

Ruptured ACL

How does your veterinarian diagnose a ruptured ACL? Or, better yet, how do I diagnose an ACL in every other dog that comes through the door except mine?

History is very important. "He was running in the back yard and suddenly, I heard him cry out. He came back carrying a back leg, and he won't put it down at all." Most people guess "broken leg" with these symptoms, but my first guess is always a knee injury. ACL is the most common cause of acute rear leg lameness in dogs.

Breed, age, and body type are important clues in making the diagnosis. Large breed dogs, like Labradors and Rottweilers, are predisposed to ACL rupture, although I have seen it in small breed dogs like Beagles and even in one cat. There are two basic types of dogs who present with ACL: middle aged, overweight, sedentary "couch potatoes," and young, athletic, active dogs. While the exact mechanics vary between the two types, the end result is the same.

On physical exam, there is usually some degree of pain on manipulation of the stifle joint, laxity (looseness) in the joint, called a "drawer sign," and occasionally effusion (fluid) in the knee joint.

demonstrating drawer movement

Radiographs (X-rays) are usually negative for any abnormalities, but are often taken to rule out other injuries.

Grace's right rear leg

and her (normal) X-ray
Sorry, I couldn't flip the picture to keep the orientation the same.

So, how did I miss the diagnosis of ACL in Grace's case? Well, first, I told you I wasn't always rational when it comes to my own pets. Secondly, Grace's history didn't fit the typical ACL presentation. Yes, she is an older Rott, but she is neither overweight and sedentary nor young and athletic. Also, she didn't present as an acute, non-weightbearing lameness, but as a slowly progressive condition. Finally, I was unable to demonatrate any laxity (lack of drawer sign) whenever I palpated her knee joint.

Dr. Shrader explained to me that Grace most likely had a degenerative failure of her ACL, rather than an acute trauma, which fit her clinical signs and the slow progression of her lameness. Also, he showed me how she was painful on hyperextension of the joint, something which he said was classic for ACL. When I asked about the lack of a drawer sign, he told me, "I think we rely too much on drawer sign for diagnosis," and gently reminded me that "Every stifle lameness is ACL until proven otherwise," something I knew, but blanked out.

Next post: Two surgical options for ACL repair, and I why chose the one I did.