Clinic for Small Animal Surgery, Department of Small Animals, Vetsuisse-Faculty, University of Zurich, Zurich, Switzerland
Cranial cruciate ligament (CrCL) rupture is one of the most common causes of hind lameness in dogs. The CrCL maintains stability of the stifle joint, thus injury to the CrCL will result in joint instability and predispose the joint to degenerative changes. In dogs, the majority of CrCL ruptures occur under normal activity, likely due to structural deterioration of the ligament and not because of a traumatic injury. Rupture of the CrCL due to degeneration can present acutely even in young dogs and eventually becomes bilateral. Osteoarthritis, meniscal injury, and persistent lameness commonly occur with CrCL rupture. Therefore, the pathological condition related to CrCL rupture is often referred to as “cruciate disease”. Cranial cruciate ligament rupture is particularly common in large and giant breed dogs; however, any breed, size, or age of dog may be affected. Although clinical features and treatment options have been well discussed in the veterinary literature, the disease mechanisms for CrCL rupture are poorly understood.
The stifle is a complex, diarthrodial, synovial joint that allows motion in three planes. Stifle motion occurs through a combination of rolling and gliding of the femur on the tibia. Rollback is asymmetric: femorotibial contact translates more caudally on the lateral than on the medial plateau, resulting in internal tibial rotation during stifle flexion. Due to the tibial translation coupled with flexion and extension, it is clear that the stifle does not function as a pure hinge joint. Although the rotational motion about the medial-lateral axis far exceeds the motion about the other two axes, approximately 20° of varus valgus and internal-external rotation occurs over an entire walking-gait cycle in normal dogs. Re-establishing this complex motion should be the goal of CrCL reconstruction techniques. However, normal kinematics is difficult to achieve. Both lack of neutralization (e.g., tibial osteotomies) and absolute constraint (e.g., extracapsular stabilization) of internal-external rotation may lead to abnormal mechanical stresses on the articular surfaces and progression of osteoarthritis.
Traditional surgical techniques attempt to impart stability by utilizing an autogenous, allogenic, or synthetic structure placed within or about the stifle that mimics the function of the normal CrCL. Extra-articular stabilization techniques are predicated on transiently restraining abnormal stifle motion until sufficient joint adaptation occurs to provide functional stability and improved limb function. A recent development in extra-articular prosthetic stabilization techniques was the use of suture anchors. Suture anchors are used to provide secure, precise fixation of the prosthesis’ origin, insertion, or both. A recent development of suture anchors is the knotless design that allows to secure the suture with an interference screw. The absence of a knot increase the stiffness of the fixation and decrease the creep, typically caused by the knot. Different anchor sizes are available, allowing to select the anchor based on size of the dog (or cat). Another extra-articular stabilization technique was developed by Cook, termed the TightRope CCL®. The technique utilizes bone tunnels drilled in the femur and tibia to place a braided polyester coated polyethylene suture on the lateral aspect of the stifle. The suture is passed through the tunnels and anchored to the medial aspect of the femur and tibia where the bone tunnels emerge using toggle buttons.
Another approach to stifle stabilization uses the concept of creating dynamic stability in the CrCL-deficient stifle by altering bone geometry. Among several tibial osteotomies, the tibial plateau leveling osteotomy (TPLO) attempts to provide stability by decreasing the tibial plateau angle, while the tibial tuberosity advancement (TTA) procedure attempts to dynamically neutralize cranio-caudal instability by altering the relative alignment of the patellar tendon to the tibial plateau.
The clinical results of both TPLO and TTA are favorable, although complications such late meniscal injury suggest that instability may persist in some cases. The meniscus acts as a secondary stabilizer in the stable stifle, while become a primary stabilizer in the CrCL-deficient stifle. Thus, a meniscal injury occurring after an extra-articular technique, TPLO or TTA suggest that these techniques do not fully re-establish normal joint stability. The concern about persistent instability after TPLO and TTA has been raised in two studies evaluating joint stability with weight-bearing radiographs. Based on these studies (n=15 TPLO; n=30 TTA) a group of dogs (30% TPLO; 70% TTA) may have a degree of tibial subluxation postoperatively, despite having a good clinical outcome. Postoperative instability after TPLO and TTA has been confirmed in in vivo studies using fluoroscopy to perform 3-dimensional kinematics.
The cause of postoperative instability after TPLO or TTA is unclear. Possible causes include 1) lack of secondary stabilization provided by the meniscus (meniscectomy); rotational instability due to acute CrCL ruptures (no periarticular fibrosis) or mild joint malalignment; failure of the dynamic stabilization mechanism due to other causes. Ideally, these cases at risk of persistent postoperative instability should be recognized preoperatively and treated differently. A possible strategy to prevent postoperative instability is to combine dynamic (osteotomy) and static (extra-articular technique) stabilization techniques. The preliminary results of a recent in vivo fluoroscopic study evaluating stifle kinematics in dogs before and after TPLO, demonstrated that dogs without instability may have a degree of external tibial rotation, in contrast to the dogs with tibial subluxation which had severe internal tibial rotation. It is possible that dogs with mild tibial internal torsion or varus may. This study would support the hypothesis that rotational instability may have an important role in postoperative sub luxation after TPLO and that a lateral anti-rotational suture may improve stability. The combination of the TPLO with an anti-rotational lateral suture in dogs at higher risk of rotational instability may improve outcome and decrease the incidence of post surgical meniscal injuries. This hypothesis needs to be confirmed with future biomechanical and clinical studies.
A novel strategy to control cranio-caudal as well as rotational instability is the new TPLO Internal Brace technique. This technique is possible using a locking plate that allows anchoring the extra-articular suture directly to the plate. This plate is designed with several new features, which allow the surgeon easier and more consistent plate placement and offer the option of a knotless anti-rotational lateral stabilization technique (Internal Brace) in dogs with severe stifle instability.
Although TPLO is widely considered a successful surgery, several reports suggest that persistent instability can be found in about 30% of dogs. In addition the occurrence of meniscal injuries after TPLO suggests that some dogs may have continuous subluxation. A recent in vivo fluoroscopic study evaluating stifle kinematics in dogs before and after TPLO2, demonstrated that dogs without instability had about 5–10° of external tibial rotation, in contrast to the dogs with tibial subluxation which had severe internal tibial rotation. This study would support the hypothesis that rotational instability may be the inciting cause of subluxation after TPLO and that a lateral anti-rotational suture may improve stability.
The combination of the TPLO with an anti-rotational lateral suture in dogs at higher risk of rotational instability may improve outcome and decrease the incidence of post-surgical meniscal injuries.
The TPLO is performed using standard technique. The anti-rotational lateral suture (Internal Brace) consists of Fibertape looped around the suture hole of the plate and secured with a knotless Swivelock anchor to the lateral femoral condyle. A bone tunnel is drilled from lateral-to-medial using an aiming device to exit at the level of the plate suture hole. The lateral anatomical landmark for the tibial tunnel has been described for Swivelock CrCL technique. A single strand of Fibertape is passed into the suture hole of the plate and shuttled through the tunnel. The Fibertape is secured to the lateral femoral condyle using a 3.5 mm or 4.7 mm Swivelock.
The suture is tensioned to eliminate excessive internal rotation. Excessive tension should be avoided, because the Internal Brace is only acting as rotational stabilizer, while the TPLO is providing dynamic cranio-caudal stabilization.