Knotless Anchors for Joint Instability: Advantages and Applications
World Small Animal Veterinary Association Congress Proceedings, 2017
A. Pozzi
Clinic for Small Animal Surgery, Department of Small Animals, Vetsuisse-Faculty, University of Zurich, Zurich, Switzerland

Injuries causing joint instability are very common in dogs and cats. These injuries can be treated conservatively in some cases, but often require surgical stabilization. For example, treatment of tarsal medial collateral ligaments sprain depends on the severity of the injury. Some cases can be treated with coaptation only, but the more severe cases require surgical stabilization, which can be performed using a prosthetic reconstruction of the ligaments using anchors, screws, or bone tunnels. Other examples of applications of bone anchors for prosthetic stabilization are stifle and elbow luxation, carpal ligament reconstructions, and in selected cases medial patellar luxation and cruciate ligament rupture.

Bone anchors are a typical implant for reattachment of soft tissues to bone or anchoring of prosthetic material to bone. Soft tissue fixation to bone is a basic technique of orthopedic surgery for which many procedures and devices have been developed. Early techniques used bone tunnels, screws and washers, and bone staples. These techniques are relatively simple, but can have disadvantages including increased surgical exposure, damage to suture material, interference with joint structures and non-isometric placement. Since the suture anchors were developed in the late 1980s, several modifications and improvements have been made. Suture anchors can be used effectively for reattaching avulsed soft tissues to bone, thus reestablishing integrity to tendons and ligaments. Suture anchors are usually made of stainless steel, titanium, PEEK, or an absorbable polymer. Design features that vary with suture anchors include size, method of anchoring (toggle, thread, tine), thread design, whether they are preloaded with suture, suture type, position of the anchor after insertion and method of insertion. Suture anchors have the advantage of a lower profile than screws and washers, which help avoid interference and abrasion of articular surfaces and adjacent soft tissues during joint movement. They also can be more precisely placed to allow improved reattachment of ligamentous structures to their isometric origin or insertion.

In the knotted bone anchors the suture material is loaded through an eyelid. Examples include the Corkscrew® and Fastak® anchors (Arthrex Vet Systems, Naples, FL), IMEX, Secures, DePuy Synthes anchors. This method of suture fixation differs from the knotless anchors such as the SwiveLock® anchor (Arthrex Vet Systems, Naples, FL, USA), where the suture is secured to the bone as an interference screw. This anchor can be used for reconstruction of a variety of ligament tears, including cranial cruciate and collateral ligament tears. This anchor sits flush to the bone and functions as an interference screw. One of the benefits of the knotless anchors is the ability to apply tension on the suture as desired without the need for a knot. Additionally, the suture and anchors are flush to the bone, leaving a very low-profile repair.

One of the advantages of knotless anchors is the versatility. Because of their low profile and the various available sizes, they can be applied to multiple joints. In our experience, one of the most common locations is the tarsus. It is important to understand the anatomy of the tarsus before considering reconstruction. The tarsus is a complex joint and consists of four joint levels, the talocrural, the proximal intertarsal, the distal intertarsal, and the tarsometatarsal joint. The ligamentous support of the tarsus consists of the small dorsal ligaments, the plantar ligaments, which are reinforced by a fibrocartilage plate, and the collateral ligaments. In the dog three distinct parts form the medial collateral ligamentous support. The long part runs from the medial malleolus to the first tarsal bone, but also attaches to the central tarsal bone. During extension this long part becomes taut and during flexion it becomes loose. The short part of the ligament lies deeper and consists of two distinct parts. Both attach to the medial malleolus, but the first one runs to the talus (tibiotalar portion) and the second one to the central tarsal bone (tibiocentral portion). The medial collateral ligaments prevent varus and the lateral collateral ligaments prevent excessive valgus motion. The lateral collateral ligaments in the dog consist of three components: long lateral ligament, calcaneofibular short ligament, and the talofibular short ligament. All of these parts are attached to the lateral malleolus of the fibula. During extension of the tarsal joint, the long lateral and the calcaneofibular short ligament are taut, while the talofibular short component is tensioned when the tarsus is in a flexed position.

In contrast to dogs, cats are missing the long collateral ligaments, although a tendon running on the lateral as well as on the medial aspect of the tarsus act as ligaments. This is the tendon of the tibialis caudalis muscle on the medial side and the tendon of the fibularis brevis muscle on the lateral side. All of the medial collateral joint stabilizers originate at the medial malleolus of the tibia. The tibiocentral ligament as well as the tibialis caudalis tendon attach then to the central tarsal bone. The tibiotalar ligament consists of two parts: The craniodistal part attaches dorsodistomedial at the talus and the caudoproximal part plataroproximomedial at the talus. The caudoproximal part of the tibiotalar ligament is taut in flexion, all remaining ligaments are taut in extension of the tarsal joint. The lateral collateral ligaments originate at the distal part of the fibular malleolus and the fibularis brevis tendon attaches to the lateral aspect of this bony prominence. The fibulotalar ligament inserts proximolateral on the talus, immediately deep to the fibular malleolus. The fibulocalcaneal ligament inserts proximal and lateral on the calcaneus. The fibularis brevis tendon attaches distal and lateral on the calcaneus. The only structure on the lateral side of the tarsus taut in extension is the fibularis brevis tendon. Both ligaments, fibulocalcaneal as well as fibulotalar, are taut in flexion.

One of the advantages of these noel techniques is that they may not require immobilization of the joint with coaptation, transarticular external fixator or transarticular pin to spare the repair. Immobilizing the joint with a transarticular fixator has the potential to lead to degenerative joint disease, but the described repair techniques have not sufficient strength to withstand the repetitive loading in the postoperative period. An important consideration is that cats presenting with tarsal instability typically have open access to the outdoor and may not be confined by the owners. A novel technique that we developed combining Fiberwire and knotless anchors offer significant advantages such as decreased risk of implant failure, earlier return to joint mobility and better long-term function. Our initial clinical experience using small Bone PushLock anchors in a cat with tarsal instability has been excellent. Further testing is necessary to determine which technique offers the best biomechanical performance.

The Swivel Lock or PushLock anchor can be used to reconstruct the MCL of the stifle or collateral ligaments of other joints including the shoulder, elbow, carpus and tarsus. The PushLock anchor is introduced into the bone tunnel by impaction rather than screwing into place. One end of the FiberWire or FiberTape is attached to bone using a suture anchor or suture button. The suture is tightened to the desired tension and securely fastened into a bone tunnel using a Peek Swivel Lock or PushLock anchor, reconstructing the collateral ligament. Stabilization of the shoulder for medial instability has been recommended by ligament reconstruction, imbrication of the subscapularis tendon or imbrication of the medial glenohumeral ligaments (MGHL) and medial joint capsule. Reconstruction of the MGHL is typically performed through a craniomedial approach to the shoulder, followed by ligament reconstruction using suture anchors and Fiberwire or FiberTape. It is important to reconstruct both arms of the ligament if they are both damaged. Stabilization of the lateral glenohumeral ligament by joint capsule imbrication using a suture anchor technique was recently reported with good results in 2 dogs having tears of the LGHL. Thermal shrinkage of the joint capsule was previously recommended as a means of stabilizing the shoulder, but this technique has fallen out of favor due to inconsistent results. Following any type of surgical repair of surgical instability, it is recommended that the shoulder be protected with restricted, controlled activity, and a thoracic jacket or bandage. Immobilization of the shoulder has been recommended for 6 weeks, followed by rehabilitation exercise for an additional 6 weeks.

Speaker Information
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A. Pozzi
Clinic for Small Animal Surgery
Department of Small Animals
University of Zürich
Zürich, Switzerland

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