Insufficiency of the cranial cruciate ligament (CrCL) is a common cause of lameness in dogs that leads to degenerative joint disease of the stifle. CrCL insufficiency is most often observed in adult, large breed dogs with an overrepresentation of breeds including Rottweilers, Newfoundlands, and American Staffordshire terriers. Dogs with CrCL insufficiency can be managed conservatively or with surgery. Conservative management may result in improvement of lameness for dogs < 15 kg, however most dogs (especially those greater than 15 kg) benefit from surgical management of CrCL insufficiency. An array of surgical techniques has been described including tibial osteotomies, intracapsular stabilization, and extracapsular stabilization techniques. Tibial osteotomies provide dynamic stability by modifying the geometry of the stifle to eliminate the cranial shear force responsible for the tibial subluxation. 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 techniques utilize peri-articular heavy gauge suture or wires, or the transposition of soft tissues to reduce stifle laxity, whereas intra-articular techniques attempt to anatomically reconstruct the CrCL using autogenous tissues, allografts or synthetic materials. Most authors cite good to excellent limb function in the majority of dogs that have undergone extra- or intra-articular procedures.

Lateral suture stabilization (LS) is one of the most commonly used extracapsular techniques. LS is a relatively simple surgical technique that consists of placing a prosthetic suture around the lateral fabella and through a bone tunnel drilled across the tibial tuberosity. After tightening the LS, tibial cranial translation and internal rotation are neutralized by the suture placed across the femur and the tibia. While it has been demonstrated that the outcome of LS is similar to TPLO 6 months after surgery, only 40% of the dogs treated with LS improved in a prospective clinical study. The causes of incomplete return to normal function may depend on multiple factors, including the severity of OA and meniscal pathology at initial presentation, the body condition score, the level of activity of the dog and the occurrence of complications. Among others, premature failure of the LS can be a cause of lameness and discomfort.

In the attempt of addressing some of the shortcomings of LS, new extracapsular techniques have been developed. These techniques share some common features, which will be discussed in the next section. The techniques that will be described are the Tightrope CCL and the LS with bone anchors.

Isometry

For a suture that spans a joint to be isometric and provide support without limiting range of motion, its attachment points on either side of the joint must remain at the same distance from each other from full extension to full flexion. Changes in distance between these points can increase or decrease the suture tension at different joint flexion angles. It is clear that suture placement is critical as isometric suture placement can potentially greatly improve the magnitude of forces placed on the stifle after extracapsular stabilization. In a radiographic study, Roe, et al. recently defined sagittal plane isometric suture placement points in the femur and tibia. It is also important to understand that the only true isometric points are the origin and insertion of the CrCL. However, the points reported by Roe may be the closest approximation to isometric points of extracapsular suture fixation. Both Tightrope CCL and anchor technique utilize similar isometric points in the femur and in the tibia. It is imperative to choose the appropriate points of fixation for the success of these techniques. Small errors in placement (1-3 mm) can cause laxity of the suture because of the decrease in distance between the points. It may also cause premature failure due to increase in distance between the points loading the suture past the yield point. The femoral point is 2 mm cranial and distal to the lateral fabella-femoral condyle junction and within the caudal portion of the condyle (anchors and Tightrope CCL). The tibial point is 2 mm caudal and distal to the Gerdy tubercle (within the long digital extensor groove for the Tightrope CCL; caudal to the groove for the anchor technique).

Bone-to-Bone Fixation

Anchor placement is important, not only for isometric suture placement, but also to reduce anchor pull-out as this is the most noted mode of failure observed with in vitro and clinical studies of anchors. In the femur, it has been demonstrated that higher acute load to failure of the anchor occurs when the anchor is placed in the caudolateral aspect of the lateral femoral condyle. This point has also been shown to be an isometric location for anchor placement in extracapsular suture stabilization. Tibial suture anchors are less commonly used for extracapsular suture stabilization, although an isometric point has been described for the tibia as well. Tightrope CCL also attempts to improve the stiffness of the anchor points by using bone tunnels in the femur and tibia. The suture are passed through the tunnels and fixed on the opposite side (medial side) using 2 or 4 holes buttons that act as toggle pins. By fixing the suture directly to bone, a stiffer construct than the fabellar suture may be achieved. The potential mechanical advantages of bone-to-bone fixation compared to fabellar fixation remains to be demonstrated.

Suture Material

As extracapsular suture stabilization techniques have progressed, suture materials, suture sterilization methods and suture securing methods are all topics that have been evaluated. Numerous suture materials have been proposed for use in extracapsular suture techniques. Polyester, coated caprolactam, and braided polyesters were some of the early suture materials that were initially chosen due to the availability of large diameter size (up to No.5 in the polyesters) and thus perceived increased strength. Monofilament suture material is another solution that may reduce the risk of draining tracts observed with braided material such as caprolactam, however the largest size available in nylon and polypropylene is No. 2. For this reason, larger diameter monofilament materials such as monofilament nylon fishing line and monofilament nylon leader line have been used in extracapsular suture stabilization. One of the disadvantages of nylon leader line is its poor resistance to cyclic displacement and poor stiffness, which clinically may be recognized as early failure due to suture loosening. New suture material used in Tightrope CCL and anchor technique such as Fiber tape suture and Fiber wire (Polyblend polyethylene sutures) are stiffer sutures and more resistant to cyclic displacement than leader line.

Several questions remain to be answered about extracapsular techniques. Abnormal external tibial rotation, valgus angulation and lateral compartment pressure may occur following excessive tensioning of the extracapsular suture. It is not known what is the appropriate range of tension to provide stability without causing detrimental effects on stifle compartment pressure and joint range of motion. Furthermore, subsequent meniscal tears have been reported after LS and TR (as well as TPLO and TTA), suggesting that extracapsular techniques fail to protect the medial meniscus from tear. Subsequent tears may be due to misdiagnosis at the time of the original surgery, or persistent instability causing meniscal tear. Further studies will be needed to confirm if extracapsular techniques can be improved. Potential advantages and unique features of Tightrope CCL and anchor techniques for stabilization of the CrCL deficient stifle.