Understanding tibial plateau leveling osteotomies in dogs
Cranial cruciate ligament pathology is a leading cause of lameness in dogs. Many surgical treatments have been described that aim to restore stifle joint stability and minimize the progression of subsequent osteoarthritis. Most surgical treatments seek to replace the function of the cranial cruciate ligament by substituting autologous tissues or synthetic materials. More recently, the tibial plateau leveling osteotomy (TPLO) procedure has been described, which alters the mechanical forces acting on the stifle, rendering the cranial cruciate ligament unnecessary.1
The purpose of this article is to describe the clinical application of TPLO so that practitioners can better understand the surgical technique and explain the procedure and aftercare to clients. An exhaustive description of the complexities and nuances of the biomechanical basis, preoperative planning, and performance of a TPLO are beyond the scope of this article.
CHOOSING TPLOOver time, TPLO has increased in popularity among veterinary surgeons. This increased popularity is based largely on subjective clinical experience as little hard data is available upon which to base such decisions and preferences.2-5 Some studies have suggested better limb function and less progression of osteoarthritis in patients treated with TPLO as compared with extracapsular stabilization.3,5 Other studies have failed to detect a difference between TPLO and extracapsular stabilization when a walking gait across a force plate was used as the outcome measure.4 Prospective, randomized, case-controlled studies using outcome measures meaningful to pet owners would be tremendously helpful but have not been conducted because they are challenging and expensive.2,5
Among those surgeons who regularly perform TPLO, patient selection varies widely. Some surgeons prefer the technique in all dogs regardless of age, size, or activity level. Others prefer the technique primarily in younger, large-breed, particularly active dogs. Veterinarians are encouraged to discuss these preferences with the surgeons to whom they refer their orthopedic patients.
The TPLO technique was developed from an understanding of active mechanisms that induce instability in a cranial cruciate ligament-deficient stifle. During weightbearing, ground reaction forces are resisted by the contraction of the extensor, or anti-gravity, muscles (quadriceps and gastrocnemius). These combined forces across the stifle compress the femur against the caudal- and distal-sloped proximal tibial plateau. The slope of the tibial plateau converts this femorotibial compression into a cranially directed shear force called cranial tibial thrust.6 This shear force does not normally induce cranial tibial translation in healthy canine stifles because it is constrained by the intact cranial cruciate ligament with probable contribution by the pull of the hamstring muscles on the proximal tibia. The magnitude of the cranial tibial thrust is a function of external ground reaction forces, internal muscular forces, and the slope of the tibial plateau. When cranial tibial thrust exceeds the tensile strength of a healthy cranial cruciate ligament or a weakened, degenerative cranial cruciate ligament, the ligament partially or completely ruptures.7
Several in vitro studies have evaluated the mechanical effects of the TPLO procedure.8,9 The tibial plateau angle in cranial cruciate ligament-deficient dogs within my practice varies from as little as 10 degrees to greater than 45 degrees, with most patients having a tibial plateau angle between 20 and 33 degrees. Leveling the tibial plateau to an angle of about 6.5 degrees reliably eliminates cranial tibial subluxation by converting cranial tibial thrust into caudal tibial thrust that must be constrained by the intact caudal cruciate ligament.8 Progressive leveling from 6.5 degrees toward 0 degrees further increases demand on the caudal cruciate ligament.8 Thus, overcorrection may increase the risk of subsequent caudal cruciate ligament failure, and undercorrection risks persistent stifle instability. A recent recommendation for the optimal postoperative tibial plateau angle is 5 degrees.7
Cranial cruciate ligament-deficient stifles also have increased internal rotation.10 In vitro studies have shown that TPLOs on cranial cruciate ligament-deficient stifles reduced internal rotation to near a physiologic level.8,10 Clinically observing some dogs after TPLO, however, reveals internal rotatory instability called a pivot shift.7,11 The pivot shift is an abrupt cranial shift of the lateral tibial condyle relative to the femoral condyle (internal stifle rotation). When you observe the gait of a dog with a pivot shift from behind, the stifle appears to suddenly buckle laterally. Torsional moments (twisting forces) acting on the stifle causing the pivot shift are likely the result of limb malalignment, some components of which can easily be corrected at the time of the TPLO if they are recognized and deemed significant by the surgeon. When a pivot shift is noted postoperatively in patients with good limb alignment, it often resolves clinically as the hamstring muscles develop in the postoperative rehabilitation period.