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5 Signs Your Dog Has an ACL Injury

By: Dr. Michael King

Last Updated:

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Football, hockey and soccer players are not the only ones to get sidelined due to knee problems. Just like athletes, dogs can end up with a cruciate ligament injury. In fact, cruciate damage is the second most common orthopedic condition found in dogs after hip dysplasia, and is the most frequently operated orthopedic condition in our canine friends. 

Just like in people, there are two cruciate ligaments within the dog’s knee, and they form a cross (which is where the name “cruciate” comes from). In both people and dogs, the front part of the cross is the ligament that gets injured, except in our four-legged friends, it’s known as the “cranial” cruciate ligament—so technically, though frequently referred to as ACL, it is correctly abbreviated to CCL. 

The other difference is that ACL injuries in people are usually traumatic sporting injuries. In dogs this is not the case. Instead, the ligament gradually becomes weaker over time, partly due to genetics. This is why we see it so commonly in certain breeds. Normal activity can then cause the weakened ligament to tear without any unusual force or injury.

The CCL primarily prevents the shin bone (tibia) from shifting forward and the thigh bone (femur) from shifting backwards. An injury causes instability of the joint, resulting in discomfort, and the following common signs:

1. Lameness

This can come on without warning when the weakened ligament gives way as the dog is running or playing. They can suddenly be so uncomfortable that they will not put the foot down at all.

Alternatively, some dogs show a gradually worsening, on-and-off lameness over weeks or months. They might seem to get better with rest, but then become lame again as they become more active. 

2. Sitting abnormally

They may sit or lie with their leg sticking out to the side rather than tucked in like normal, as it hurts to bend the knee.

3. Lameness & stiffness in both back legs

Although it’s usually noticed as a lameness on one leg first, about 60 percent of patients that have a CCL injury will go on to get one in the other knee within two years. If the two injuries happen at the same time and both knees are affected at once, the dog may seem to be “stiff” in the hind end, or be reluctant to go for their normal walks or get up at all. 

4. Knee thickening & swelling

CCL injuries cause inflammation and swelling in the knee and over time scar tissue develops. This makes the injured side look bigger than the normal knee.

5. Clicking

Walking on the unstable knee puts more stress on other structures in the joint. The meniscus, a shock-absorbing pad of cartilage, can easily become torn or injured due to the knee moving in an abnormal way. It sometimes creates an audible “click” in the knee that can be heard when going on a walk. A meniscus injury is quite uncomfortable so there is typically a significant lameness as well as the clicking.



Since the CCL becomes injured due to it being too weak, it unfortunately will not heal with rest or medication. Though the scar tissue that forms around the knee over time does help, the only way to truly resolve the lameness is with surgery. Thankfully this is usually very effective, and with current surgical procedures such as the “TPLO” or “TTA”, even extremely large dogs can get back to being active and comfortable.


Last Updated:

By: Dr. Michael King
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