Introduction

In the last five years the technique described by Barclay Slocum and Theresa Devine to surgically treat dogs with cranial cruciate ligament (CCL) injures became more and more diffused and appreciated by orthopaedic surgeons all over the world. Conversely to all the other described techniques the TPLO is a technique addressing the biomechanics failure underlining the cranial cruciate ligament failure and not the injured ligament itself.

Different traditional methods have been addressed to replace intra-capsular the CCL or to substitute its function with extra-capsular joint stabilization. Results from the traditional techniques to repair the cranial cruciate ligament have been good in some cases, however in many dogs, particularly large and giant, have been poor, with failure of the fixation methods, relapsing of the instability, periarticular fibrosis, progression of the degenerative joint disease and incomplete return of limb musculature and function.

A successful outcome in the treatment of a CCL failure deficient should include the ability of the dog to sit with full flexion of the knee, the return of thigh musculature, the cessation of the degenerative joint disease, and the return to preinjury limb function.

Biomechanics of the knee joint

Slocum invented his technique developing the cranial tibial thrust concept from the observations described by Henderson and Milton in 1978. They described the tibial compression test observing that when the hock was flexed, keeping the knee in a fixed position, cranial translation of the tibia occurred when the CCL was ruptured.

The cranial tibial thrust, responsible for the positive tibial compression test when the CCL is ruptured, is a force generated in the knee by weight bearing and joint anatomy. In the weight bearing the knee and hock joints are kept extended by the quadriceps muscle and the Achilles' tendon, and the loading forces are transmitted in the knee across the cartilage surfaces of the femoral condyle and of the tibial plateau. If the tibial plateau would be perpendicular to the long axis of the tibia, the loading forces would be transmitted completely to the ground. This happens in the human knee, where the few degree of tibial slope (5°-7°) are balanced by the hamstring muscles. In the dog the tibial plateau is not perpendicular to the long axis of the tibia, being at an angle varying in different breeds and dogs from 18° to 60°. Since the tibial plateau is sloped the loading between the femoral condyle and the tibial plateau generated by weight bearing produces a force that divides itself into two perpendicular components, one distal in direction of the tibial axis and one cranial, the cranial tibial thrust. Higher is the slope and higher is the cranial tibial thrust. The cranial tibial thrust is opposed passively by the CCL and actively by the biceps muscle and by the hamstring muscle group. The cranial tibial thrust is enhanced by the magnitude of the tibial slope, by the weight of the dog and by the extension of the knee, because the extension of the knee during weight bearing produces a less favourable lever arm for the caudal muscle of the thigh. In a perfectly balanced knee joint the cranial tibial thrust is neutralized by the active muscular forces and the CCL is not stressed. Since in the dog this balance is often altered, the CCL is under intermittent or continued stress caused by the force of the cranial tibial thrust; its rupture occurs when this force exceeds the ligament's strength.

When CCL fail, the cranial tibial thrust elicited by each weight bearing on the limb causes a friction damage to the cartilage and a pressure damage to the caudal horn of the medial meniscus; the medial meniscus, being strongly connected to the medial collateral ligament, moves cranially with the tibia and its damaged by the femoral condyle pressing it or passing over it.

Neutralization of the cranial tibial thrust

As a consequence of the biomechanics of the canine knee, the problem of the CCL deficient stifle in the dog is the control of the cranial tibial thrust, more than the reconstruction of the CCL itself. Without neutralizing the cranial tibial thrust any reconstruction of the CCL could fail, reproducing the same configuration that was at the origin of the initial CCL injury. The innovative solution proposed by Slocum was to modify the anatomy of the joint with the purpose to stabilize the joint without the help of the CCL or its substitutes. To neutralize the cranial tibial thrust it is necessary to eliminate the tibial slope modifying the angle of the tibial plateau in such a way that all the loading forces are transmitted perpendicularly from the femoral condyle to the tibial plateau. The modification of the angle of inclination of the tibial plateau relative to the tibial long axis was achieved by Slocum with a proximal tibia osteotomy, called Tibial Plateau Levelling Osteotomy (TPLO).

The Tibial Plateau Leveling Osteotomy

With a semicircular osteotomy cut in the proximal tibia, caudal to the tibial tubercle, it is possible to modify the angle of inclination of the tibial plateau until a neutral position is found. Since the caudal muscles of the thigh, the biceps femoris and the hamstring muscles of the pes anserinus group pull the tibia caudally, balancing a limited cranial tibial thrust, the neutral tibial slope is achieved at 5° to 8°. Further reduction of the tibial slope could cause stress to the caudal cruciate ligament. After TPLO, no reconstruction of the CCL is necessary, as the knee become stable during weight bearing when the tibial thrust was eliminated. This causes the forces on the femoral condyle to be perpendicular to the articular surface of the tibia.

The TPLO is a patented technique, being necessary to attend a specific Course and wet-lab to learn the technique and to buy the instruments and implants for the surgical procedure. The purpose is to have the technique performed by trained surgeons and to reduce or eliminate the risk of complication associated to an invasive surgical technique like this one.

Meniscal release

In the CCL deficient knee, the cranial tibial thrust causes the femoral condyle to crush against the caudal horn of the medial meniscus, being strongly connected to the medial collateral ligament and to tibial plateau. Resulting damages include horizontal cleavage lesions, crushing of the caudal horn and bucket handle tears.

As an additional procedure associated to TPLO, when the CCL is absent or damaged, Slocum suggested to free the caudal horn of the medial meniscus (meniscal release) to avoid any impingement or crashing by the femoral condyle. Meniscal release is performed through a small medial approach to the joint, caudal to the medial collateral ligament and proximal to the tendon of semimembranosus. A number 11 scalpel blade is used to cut the medial meniscus transversely just behind the medial collateral ligament, until the caudal horn can move freely backward.

Clinical experience

The TPLO procedure has been performed on 310 knees with acute and chronic CCL injures, with good to excellent results. According to the Slocum's criteria for a successful surgical outcome, TPLO provided on most dogs the ability to return to full sit, return of full thigh circumference, cessation of the degenerative process and return to pre-injury function. Many dogs were treated for both knees, in different times. Several dogs had previous unsatisfactory traditional surgeries. Weight bearing was observed very early in the post-operative period. Clients were given strict instruction on the postoperative management, to reduce physical activity in the first two months post-operative, then gradually increasing it in the following two months, to avoid stress to the patellar tendon and to allow a gradual return to complete muscular mass and strength. Sporting dogs were returned to full activity after four months. The postoperative complications observed occurred in only three dogs: one had fracture of the tibial tubercle, one had distal tibia fracture and one had disruption of internal fixation.

The overall experience of TPLO for the surgeon, patient and client has been excellent, with early return to function of both pet and working dogs.

A: tibial plateau B: tibial long axis C: slope of tibial plateau	A: tibial plateau after TPLO B: tibial long axis C: Slocum TPLO plate