Biochemics of injury and TPLO repairs:

The Tibial Plateau Leveling Osteonomy reduces the slope of the tibial plateau as measured in the parallel plane of the tibia, by rotation of the plateau about an axis perpendicular top the sagittal plane. The forces of weight bearing applied at the foot are transmitted through the knee where the slope of the tibial plateau causes a forward tibial thrust to project the proximal tibia forward. In addition, the muscular forces of the quadriceps mechanism and the gastrocnemius muscles enhance the effect of the tibial plateau slop by increasing the compressive forces across the stifle. The cranial ligament is the primary structure that prevents the cranial translation (forward tibial thrust) of the tibia. The causal horn of the medial meniscus is the next structure to stabilize the stifle. The cranial tibial thrust is opposed buy hamstring and biceps femoris muscles that are protective to the cranial (forward) cruciate ligament. By decreasing the slope of the tibial plateau, the cranial (forward) movement of the proximal tibia is easily overcome by the hamstring muscles. In addition, the damage to the medial meniscus is addressed at the time of surgery.

Medical Meniscal Injruy and Treatment:

Meniscal injury must be addressed with complete or advanced partial ruptures of the Cranial Cruciate Ligament. Cranial translation of the tibia unopposed by the cranial cruciate ligament brings the caudal horn of the medial meniscus under compression by the medial femoral condyle. The reason the caudal horn of the meniscus is damaged lies in the fact that the caudal horn is held firmly to the tibial and the caudal tibial plateau by the short portion of the medial collateral ligament and the caudal tibial ligament of the medial meniscus. To avoid damage to the caudal horn of the medical meniscus, the meniscus caudal to the short portion of the medial collateral ligament or the caudal tibial ligament of the medial meniscus must be incised. This prevents the forced trapping and crushing of the caudal horn between the medial tibial plateau and the medial femoral condyle. This enables the meniscus to avoid being further damaged. Once a meniscal release is performed, the ability of the caudal horn of the medial meniscus to prevent the cranial translation of the tibia with respect to the femur is lost.

Excerpt from the developer of the TPLO - Dr. Barclay Slocum

I love working with working dogs because they represent the most noble spirit I can imagine, that is, they perform beyond human capabilities with grace and ease, they are in service to man with loyalty and charm. Even though they test me and push me to my limits, come back for more because the experience enriches me. When a patient comes to me with a ruptured anterior cruciate ligament, I eagerly wait for the patient to return to the field and training. This sense of satisfaction comes from the understanding of how the stifle works. In 1978, a new test for identifying rupture of the anterior cruciate ligament was described. After using the test for several years, I began to understand how the stifle really works.
In 1982, I described the cranial tibial thrust which is a natural force created in the dog's stifle. The cranial tibial thrust is responsible for rupture of the anterior cruciate ligament. Understanding how this force is created, led to the design of a surgical procedure for controlling this force and its destructive side effect. Although that surgery neutralized the eftects of the cranial tibial thrust, a new version of the procedure, called Tibial Plateau Leveling Osteonomy, has proven effective in returning dogs to full function.
The key to understanding lameness is understanding the forces generated in the stifle. Since the tarsal tendon within the Achilles tendon of the dog is a fixed length, all the forces of the foot are transmitted through the tibia. The proximal portion of the tibia, the tibial plateau, is sloped. This slope causes the tibia to slide forward unless it is restrained from doing so by the anterior cruciate ligament. The single force between the tibia and the femur can be thought of as having a component for compression and a component for slippage. The compressive force is composed of weight bearing and muscular forces for propulsion, On the other hand, the cranial tibial thrust is opposed only by the anterior cruciate ligament. When the cranial tibial thrust is too great, it ruptures the anterior cruciate ligament. Ruptures come in several varieties. There are singular incidents which cause a sudden complete rupture with a lot of pain and a non-weight bearing lameness. Other ruptures occur in small increments or a little bit at a time, These are known as partial ruptures of the anterior cruciate ligament which cause a small amount of pain and a mild lameness with poor performance. When partial ruptures proceed to complete ruptures, the transition is often gradual.
The diagnosis of at rupture of an anterior cruciate ligament is made by eliciting forward motion of the tibia (anterior drawer sign). This is easy in an acute complete rupture, but subtle in partial ruptures or chronic ruptures ofthe anterior cruciate ligament. Secondary to rupture of the cruciate ligament, the medial meniscus (the football player's cartilage) is injured. This is constantly painful and will soon be followed by the inability to sit with the stifle in a fully flexed position. A crooked sit may represent a mild to sever form of stifle injury.
For years I used traditional surgical techniques which were designed to stabilize the stifle by replacing the anterior cruciate ligament with a synthetic or natural ligament. The material was supposed to be as strong or stronger than the original ligament and be able to heal without stretching in the fluid environment of the stifle joint. The ligament would stretch, the cranial, the cranial tibial thrust would be unopposed, and arthritis would follow.