Understanding the ACL in Dogs; Structure, Function & Injury
The ACL (anterior cruciate ligament) in Dogs is also commonly referred to as the cranial cruciate ligament, so don't get confused if your veterinarian uses this terminology. They are one and the same, just one is used for people and the other for animals. Pet owners though most often use the term ACL as apposed to CCL.
Structure of the ACL Ligament
There are a number of different types of ligaments in the body, but the ACL in dogs, is known more specifically as an articular ligament. All this means is that it connects bone to other bones, therefore forming a joint. It is composed of a very tough band of fibrous connective tissue. There are hundreds if not thousands of these bands that make up the entire ACL in dogs. This is why often dogs will only partially tear the ligament as apposed to fully tearing it. Another interesting thing about ligaments is that they are elastic, whereas a tendon, which attaches muscle to bone is inelastic or not elastic. Therefore the ACL ligament in dogs has the ability to stretch or lengthen to some degree.
Within the dogs knee, there are actually two cruciate ligaments. The anterior (cranial) cruciate ligament and the posterior (caudal) cruciate ligament. 99% of the time dogs injure only the ACL.
Function of the ACL in Dogs
As mentioned earlier this ligament holds bone to bone. In this case it is connecting the dogs femur to the tibia to make up the dogs knee joint or stifle, as referred to in veterinary medicine. Its main functions are to prevent the forward movement of the tibia, prevent any internal rotation of the tibia and lastly to prevent any excessive hyperextension of the knee joint.
If you were to look at your dog from the side, you will notice that different from humans, your dogs knees are always bent to some degree while standing. This is a very important point to realize. It is because of this normal bending of the knee or flexion that the acl in dogs always has some degree of tension on it. We also refer to this as the ligament is "loaded", meaning it is carrying some weight.
Injury To the ACL in dogs
Of all the orthopedic problems is dogs, from hip dysplasia to broken bones, injury to the acl in dogs is hands down the most common orthopedic problem seen in veterinary hospitals. Why you ask? Well that is a great question, yet unfortunately there is no one simple answer because there are so many factors involved.
Some of the most important factors to consider are as follows. It is estimated that 50% of the pet population in the USA alone are considered overweight. This excess weight puts increased stress on the ACL, considering as we mentioned previously that the ligament is always "loaded". In addition to weight we have to consider the lifestyle of many domestic dogs today. Because most families have a two family income the majority of dogs are left home during the day and are not exercising. In general, most dogs are natural athletes and we all know that if you don't use it you lose it. Muscles, ligaments and tendons need to be conditioned to stay in their optimal health and prevent injury. Therefore, when our dogs are out exercising they are potentially prone to more injuries due to their lack of conditioning.
Lastly it is important to understand the difference between partial and full acl tears in dogs. Many times dog will only partially injure the ligament. Though this partial tear still creates an inflammatory process in the knee, pain and resulting limping often times with rest and some anti-inflammatory medications they can recover and return back to normal function. With that said, the vast majority of partial tears will eventually turn into full tears and therefore many veterinarians feel that dogs even with partial tears are surgical candidates. It is best to discuss this with your veterinarian and have them perform a complete physical and orthopedic examination.