Biomechanics

Although the knee joints of both dogs and humans are similarly constructed, the forces applied to the surfaces of these joints during weightbearing are vastly different. This is due to differences in anatomic configuration. In humans, the hip, knee, and ankle joints are parallel to each other and perpendicular to the weightbearing surface (the feet). Humans can stand easily with little stress on any ligamentous structure. Dogs, however, stand on their toes with their ankles up in the air and their knees bent forward (see Fig.1). The upper portion of the canine tibia (the tibial plateau) is sloped. Weightbearing creates a force that pushes the femur down the sloping tibial plateau, thereby moving the tibia forward. This force is called cranial tibial thrust. It is opposed only by the anterior cruciate ligament (see Fig.2). Just as an unrestrained car on a hill would tend to roll down the hill, the anterior cruciate ligament acts like the cable in Figure 3A to restrict the downhill roll of the femur. With every step a dog takes, stress is applied to the anterior cruciate ligament. Over time, dogs with a high tibial plateau slope place enormous stress on the anterior cruciate ligament. Therefore, when the cranial tibial thrust is too great, the anterior cruciate ligament ruptures (see Fig.4).

Anterior cruciate ligament ruptures can occur in several different ways. There may be a single incident which causes a sudden complete rupture of the ligament with severe pain and nonweightbearing lameness. Anterior cruciate ligament ruptures can also occur in small increments or a little bit at a time. These are known as partial ruptures of the anterior cruciate ligament. These partial ruptures cause a small amount of pain and a mild lameness. When partial ruptures proceed to complete ruptures, the transition is often gradual.

Two other important structures in the knee are the medial and lateral menisci (cartilage pads) (see Fig.1). The menisci are also prone to injury when the stifle joint is unstable from a cruciate ligament tear.

The TPLO procedure is used mostly for large, active dogs due to the stability it provides under extreme repetitive stress. Traditional surgical techniques require prolonged confinement to allow healing of the synthetic or natural anterior cruciate ligament replacements. These surgical repairs may fail due to the difficulty in confining large, active dogs for prolonged recovery periods. Any activity may lead to stretching of the artificial cruciate ligaments.

Clinical Signs

Once the cranial cruciate ligament ruptures, the tibia can slide forward and the femur is free to ride down the slope of the tibial plateau, just as the car rolls down the hill once the cable is cut (see Fig. 3A). The meniscus is often damaged as the femur rides over the top of it. When the ligament tears, pain, swelling, and marked lameness will occur. If not stabilized, the joint will become dramatically arthritic over time. Rest and antiinflammatory medications have little effect upon the pain and lameness the dog experiences.

Diagnosis

Diagnosis is made upon eliciting forward motion of the tibia (cranial drawer sign). This is easy in acute, complete ruptures but may be more subtle in chronic or partial tears. Mild sedation to allow muscle relaxation and radiographs (x-rays) to demonstrate arthritic changes and swelling may be necessary to obtain a diagnosis.

TPLO Surgery

The tibial plateau leveling osteotomy is used to neutralize the effect of cranial tibial thrust (see Fig. 5). The procedure levels the tibial plateau, thereby eliminating the need for the cranial cruciate ligament as a restraint against cranial tibial thrust (see Fig. 3B). In other words, rather than replacing the cable which broke in the first place, this procedure will level the surface and eliminate the need for the cable. Meniscal injuries are also corrected during the surgery in order to prevent further arthritic changes in the joint.

Postoperative Care

Healing takes about two months for the bone and slightly longer for the soft tissues. Strict confinement is mandatory during the healing process. Because the plateau leveling allows the joint pain to rapidly subside, the major problem during recovery is excessive patient activity prior to the completion of bone healing. Most patients return to controlled activity in two months and full activity in three to four months. Patients can usually return to athletic competition (field trial, hunting, agility trials) by six months postoperatively.