ACL injuries are the most common orthopedic injuries seen in the dog. You may also here them referred to as a cranial cruciate ligament rupture (this is the anatomically correct description, but we will call it the ACL since most are familiar with this). When I talk to owners, I try to reference it to what people experience to make it a little easier to understand.
The ACL, while small, is a very major player in stability of the knee. If this is ruptured (partial or complete) this will cause instability to a varying degree which will cause inflammation, arthritis and cartilage wear in the knee.
There are three big forces that the ACL counteracts: knee hyperextension, tibial(shin bone) internal rotation (twisting inward) and cranial (anterior) shear force. When you think about how we rupture our ACL it is usually by hyperextending the knee and a large force (like a linebacker) impacting the knee or by planting your foot and turning abruptly (internal rotation). Most of the time we rupture our ACL by strenuous activity. The third force (cranial shear force) is greater in the dog than you or I. With us, the top of our tibial (tibial plateau) is relatively flat with only a very small angle of inclination. When the top of our tibial meets the rounded femur (thigh bone) there is very little push forward. In the dog, due to the way the dog stands (4 legs) and develops (much higher angle of inclination of the tibia (normally between 20-40 degrees) this push forward is much greater as the tibia meets the femur. Think of a wheel on a hill model – the steeper the hill the more the wheel will roll down it (the hill is the tibial and the wheel is the femur). The ACL runs from the front part of the tibia to the back of the femur and counteracts the above force. When it is torn there is no holding the tibial back as the dog steps and walks on the injured dog.
So this is a little about what a ruptured ACL is, next we will talk about the signs. Have a great Monday and be sure to check back!!!
Please look like at the following video: arthroscopic evaluation of the canine stifle joint.