Understanding why the TPLO is considered the optimal repair for ACL tears in dogs requires a short stifle (knee) anatomy and biomechanical discussion.
Canine stifle anatomy and biomechanics differ from humans and lack of this understanding has resulted in many misconceptions by the pet owning public.
Figure one illustrates the canine stifle joint with a corresponding radiograph (X-Ray). The bone above the joint is the femur and below is the tibia. The cranial cruciate ligament (more commonly know as the anterior cruciate ligament or ACL) attaches the femur to the tibia and prevents forward motion of the tibia and backward motion of the femur. The top of the tibia is the tibial plateau.
The tibial plateau angle (TPA) or slope differs greatly between dogs and people. Humans have about a four-degree TPA, which is nearly level, whereas dogs have average TPA's of 25 to 30 degrees and a few dogs have slopes above 35 degrees (figure 2).
This tibial plateau slope dictates joint biomechanics, mechanism of ACL tearing, severity of clinical signs, need for repair, why braces are relatively ineffective, frequency of bilateral (both sides) tears and repair option rational. All of these factors are different for dogs compared to humans, because in dogs, simple weight bearing causes the femur to slide down and back (figure 3).
This repetitive motion results in chronic mechanical wear and tear on the canine ACL. Figure 4 depicts the mechanism in action. Although most surgeons recognize this is as an oversimplification, most agree this is an underlying biomechanical feature found in dogs but not in humans.
In dogs, ACL tears are almost always gradual. The ligament appears to degenerate slowly rather than tear acutely. The “Canine ACL Disease Progression” video to the right depicts an arthroscopic view of the gradual degeneration leading to complete tearing. Again, this differs from human ACL tears, which usually are due to an acute athletic injury. Many dogs exhibit subtle clinical signs during the degenerative phase with acute lameness occurring only once the ligament tears completely. On the other hand, many dogs will also exhibit severe clinical signs prior to a complete tear.
The canine tibial slope and resulting sliding action are why most orthopedic veterinary surgeons have abandoned old style replacement procedures in favor of the TPLO. The replacement materials, just like the ACL, fail due to the chronic biomechanical stress. Again, this is in sharp contrast to human orthopedics where replacement with a biological graft is the standard.