Tibial plateau leveling osteotomy--part I.

527 N o subject in veterinary orthopedics has been written about more than rupture of the canine cranial cruciate ligament (CCL). It’s fair to say that the problem continues to be written about because the definitive answer has yet to be found. For some, the closest thing to a definitive answer is the tibial plateau leveling osteotomy (TPLO). For the past 5 decades, during which the diagnosis and treatment of CCL rupture has been recognized and debated, the dozens of surgical techniques described have had one thing in common; namely, the attempted elimination of stifle instability as detected by the cranial drawer sign. The late Dr. Barclay Slocum examined the work of Henderson and Milton (1), the first to describe the tibial compression mechanism. In simple terms, the biomechanics of the stifle approximate the actions of a pulley with the quadriceps muscles proximal and cranial to the stifle counteracted by the gastrocnemius muscle mechanism caudal and distal to the joint. During weightbearing and locomotion, the gastrocnemius muscle mechanism pulls distally from its origin on the caudal aspect of the femoral condyles. In the absence of an intact CCL, the downward force of the femoral condyles against the tibial plateau results in a cranial movement of the proximal tibial extremity that Slocum called cranial tibial thrust (2). Whereas Henderson and Milton recognized the diagnostic utility of the tibial compression mechanism, Slocum proposed the cranial tibial thrust as an integral part of the pathogenesis of degenerative joint disease in the CCL-deficient stifle. While the cranial drawer sign was a passive test for stifle instability, cranial tibial thrust represented the dynamic stifle instability that occurred with weight-bearing. Slocum theorized that unless the forces acting on the CCL-deficient stifle could be altered, such that cranial tibial thrust could be neutralized, surgical treatment was doomed to failure and degenerative joint disease was inevitable. In an effort to neutralize cranial tibial thrust, Slocum described the role of the tibial plateau angle. The tibial plateau in the dog slopes distad from cranial to caudal. The degree of the tibial plateau slope varies dramatically from dog to dog from a low of 15° to 18°, up to 40°, or more in rare instances. Most surveys place the mean slope in the low to mid 20°s (2,3). Slocum theorized that dogs with greater tibial plateau slopes would be more prone to CCL rupture due to greater stress on the CCL (2). He proposed that only by “leveling” the tibial plateau could the damaging cranial tibial thrust be neutralized and the dog returned to function (4). Initially, Slocum performed a closing wedge osteotomy of the cranial surface of the proximal tibial extremity. The wedge osteotomy would tip the proximal tibial extremity cranially, thus reducing the tibial plateau slope. A standard bone plate was used to stabilize the osteotomy (4). Supplementing an intraarticular or extracapsular repair technique with the closing wedge osteotomy was meant to increase the chances of success of the repair by removing the stress of cranial tibial thrust. Over the next decade, Slocum came to believe that a stabilization procedure in addition to tibial plateau leveling was redundant. While he recognized that the procedure did not eliminate cranial drawer motion, he was convinced that if cranial tibial thrust could be eliminated, the joint would be functionally stable. It really didn’t matter much that drawer motion could be elicited in a laterally recumbent patient! In addition, Slocum came to recognize the limitations of the closing wedge osteotomy technique. The procedure lacked versatility in dealing with different sized patients with varying tibial plateau slopes. In addition, he found that the procedure produced varus or valgus tibial deformities in some dogs. His solution was the development of what is known today as the TPLO. If a radial osteotomy of the proximal tibial extremity was utilized, the procedure became more versatile; saw blades of varying radius could be adapted to different sized bones. It became a relatively simple geometric equation to determine how much caudal rotation of the proximal tibial fragment was required along the circumference of the cut to level the measured tibial plateau slope (5). A special TPLO plate was developed to stabilize the osteotomy after rotation (Figure 1). These weren’t the only refinements Slocum produced. The TPLO became registered under U.S. patent #4677973. Slocum taught the procedure out of his practice, sold licenses and the necessary surgical equipment to his students, and was decidedly short on specifics in his publications and lectures on the subject. To say that this approach generated controversy would be an understatement! While debates over the merits of bypassing the conventional route of publication and peer review have raged on this issue, one fact is irrefutable: Slocum’s approach delayed research, research publication, and eventual acceptance of the technique by almost 2 decades. Animal Clinic of Regina, 1800 Garnet Street, Regina, Saskatchewan S4T 2Z2. ORTHOPEDICS ORTHOPÉDIE