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Extracapsular Stabilization for the Cranial Cruciate Ligament‐Deficient Stifle

Trent T. Gall

Gall Mobile Veterinary Surgery, Longmont, CO, USA

Introduction

Extracapsular stabilization (ECS) of the stifle joint encompasses many surgical techniques that all have similar principles and objectives. The fundamental strategy is mimicking the stabilizing properties of the cranial cruciate ligament (CrCL). This is accomplished by placing a synthetic material outside the joint capsule (extracapsular), spanning from the femur to the tibia in a caudal to cranial direction on the lateral aspect of the stifle joint. Ultimately, these techniques rely on the body to create periarticular fibrosis around the synthetic material for long‐term stability, while short‐term stability is provided by the synthetic material. The two main forces ECS controls are cranial tibial translation (cranial tibial drawer motion or tibial thrust) and excessive internal rotation. ECS is safe, relatively inexpensive (depending on the synthetic material of choice), and is a relatively easy procedure that has good results. This procedure can easily be performed by general practitioners with some practice and appropriate patient selection.

Anatomy

Knowing the anatomy of the stifle joint is important for surgical success. It is imperative to understand that the stifle joint is not a pure hinge joint; the stifle joint moves in three planes. The author likes to simplistically think of these three planes as the following: (1) Hinge, which is flexion and extension of the tibia in relation to the femur. (2) Glide, as there is a small amount of gliding motion of the femur on the tibia. (3) Rotation, the third plane is a small amount of normal internal rotation of the tibia. The term “screw‐home” mechanism has been used to describe the human knee joint and has been adopted in veterinary literature. In brief, this term is to describe the different constraints present through a normal range of motion. Since the stifle joint normally moves in three planes, it is important to keep that in mind during surgical stabilization. This is so the surgeon does not overtighten the ECS and eliminate one or more of the natural movement planes, which could result in excessive patient discomfort, cartilage damage leading to progression of osteoarthritis,^1,2 or premature ECS synthetic material failure.

The bones that make up the stifle joint are the femur, tibia, and patella. The patella is a sesamoid bone (the largest sesamoid in the body) of the quadriceps muscles. There are three other sesamoid bones (fabellae) in the stifle joint, but these can be variable on which ones are present in any given stifle joint (Figure 50.1). Two of these sesamoids are the lateral and medial sesamoids of the gastrocnemius muscle (lateral and medial fabellae), and the attachment of the lateral femoral fabellae is often called the fabello‐femoral ligament. This is not actually a ligament but is the tendon of origin of the gastrocnemius muscle to the femur. For descriptive purposes, this attachment will be called the fabello‐femoral ligament in this chapter and is also called this term in other textbooks. The third sesamoid of the stifle other than the patella is the sesamoid of the popliteus muscle. In the author’s experience, not all dogs have a lateral or medial fabella or a popliteal sesamoid. These differences can even be present in the same dog being different between the two hind limbs. This is a very good reminder and one of the reasons to perform pre‐operative radiographs to determine the specific anatomy of the limb being operated. If there is no lateral fabella, a traditional extracapsular suture stabilization will not work, since there is no lateral fabella to anchor the synthetic material. In these patients, a bone anchor or femoral bone tunnel will be needed. It is much better to be prepared for that scenario prior to surgery via radiographs than to discover the lack of a lateral fabella during surgery and not have the appropriate hardware or backup surgical plan.

A radiograph of the pelvis of a breed dog with missing left lateral fabella. — **Figure 50.1** Radiograph of a small breed dog that is missing the left lateral fabella. Blue arrow is pointing to the absence of a lateral fabella.

*Source:* © Trent Gall.

The popliteal sesamoid is also variable, both in its position (which can change on radiographs based on joint/limb position) and if it is present or not (Figures 50.2 and 50.3). This normal sesamoid can be mistaken for an intraarticular osseous body (joint mouse) on radiographs if the surgeon is not familiar with the normal anatomy.

The ligament stabilizers in the stifle are the medial collateral ligament (MCL), lateral collateral ligament, CrCL, and caudal cruciate ligament (CdCL). The CrCL has two bands, the craniomedial band and the caudolateral band. There are also the two menisci, the medial and lateral menisci, and the patellar tendon that also provide stability to the stifle joint.

Two radiographs of the popliteal sesamoid of a dog with the area of the popliteal sesamoid bone in the joint space. — **Figure 50.2** Radiograph of a dog with no right popliteal sesamoid (left image) and a radiograph of a dog with a popliteal sesamoid (right image). The green arrows point to the area of the popliteal sesamoid bone. On a cranial‐caudal view of the dog with a popliteal sesamoid, the sesamoid can appear to be in the joint space. This is not an abnormal osseous body (joint mouse) but is a normal, but variable, anatomy.

*Source:* © Trent Gall.

A radiograph of the lines drawn between the center of the tarsus to the center of the intercondylar eminence and in the tibial plateau. — **Figure 50.3** The TPA can be measured by drawing one vertical line from the center of the tarsus through the center of the intercondylar eminence (tibial long axis – green line). The second line is the top of the tibial plateau (tibial plateau axis – blue line), the reference line perpendicular to the tibial long axis line (white line).

*Source:* © Trent Gall.

Pathophysiology

CrCL pathology is the leading cause of canine hind limb dysfunction. CrCL pathology can simplistically be broken down into three categories: (1) juvenile traumatic injuries, (2) mature/adult traumatic injuries, and (3) degenerative CrCL disease. Juvenile CrCL injuries are somewhat rare. These typically occur in skeletally immature animals that sustain a traumatic event. The Sharpey’s fibers of the CrCL attaching to the bone are often stronger than the bone, which results in an avulsion fracture of either the insertion or origin of the CrCL with excess pressure. These injuries can be treated with re‐attachment of the avulsed piece of bone if that osseous segment is large enough for surgical re‐attachment, but since juvenile injuries also can result in a mid‐body rupture of the CrCL itself, other treatment options are available.

Other treatment options for juvenile patients with CrCL tears include epiphysiodesis, CBLO, and a staged approach with eventual ECS placement. Performing an epiphysiodesis involves placing a transphyseal screw in a particular position to utilize the remaining growth of the proximal tibial physis. This technique closes the cranial aspect of the proximal tibial growth plate with the screw, and as the animal grows, the caudal aspect of the growth plate continues to grow while the cranial aspect is stopped from growing by the transphyseal screw. With this ideally controlled variation in growth across the proximal tibial physis, the tibial plateau angle (TPA) decreases, which effectively levels the TPA and creates stability, consistent with the theory behind a tibial plateau osteotomy (TPLO). This technique requires precise placement of the transphyseal screw and a relatively narrow age window to effectively level the tibial plateau.³ Another technique is a corrective osteotomy that spares the growth plates, such as a CORA‐based leveling osteotomy (CBLO). The final treatment option is conservative management until the animal is skeletally mature, followed by performing an ECS technique. An ECS should not be performed in young growing animals, since the ECS could constrain the joint too much, resulting in abnormal bone growth (causing an angular limb malformation) and/or excessive cartilage strain, which, as stated before, can lead to early progression of osteoarthritis.

Pure traumatic injuries are less common in veterinary medicine as compared to human medicine. These are the result of an acute excessive force being applied to the stifle joint. Traumatic CrCL injuries can happen in any age of animal, and if there is an isolated CrCL tear (i.e., not a deranged stifle with multiple ligamentous injuries), an ECS is an appropriate surgical stabilization for this type of injury. When exploring the stifle joint, it is usually noted that the torn ends of the CrCL have a frayed, mop‐head appearance or even a hematoma on the CrCL. Once under general anesthesia, these animals should be closely evaluated for damage to the other ligaments and stabilizers of the stifle, such as the collateral ligaments and caudal cruciate ligament. It is recommended to assess for medial and lateral instability (i.e., valgus and varus, respectively) and the presence of caudal drawer (i.e., proximal tibial subluxation caudally in relation to the distal femur). When three or more stabilizing ligaments in the stifle are damaged, this is referred to as a deranged stifle. In the author’s experience, this is more common in cats than in dogs. The repair for a deranged stifle diagnosis is beyond the scope of this chapter.

The most common type of stifle pathology is a degenerative process of the CrCL. Despite years of study and countless publications, the exact mechanism is yet to be understood on why the CrCL can undergo this disease process. Most likely, there are both genetic and environmental factors that contribute to this disease process, which is seemingly why the contralateral CrCL can become affected in approximately 22–54% of patients.^4–9 Given the high predilection to the contralateral limb being affected, it is highly advisable to pass this information to the owners to prepare them for the possibility of the contralateral limb experiencing a CrCL injury at some point in the future. To date, there have been three studies looking at the heritability of CrCL rupture in purebred dogs in three high‐risk breeds for CrCL injuries: the Newfoundland, the Labrador Retriever, and the Boxer. Looking at these three studies together, the heritability is in the range of 0.3–0.5 (approximately 30–50% chance that the disease risk is genetic), and one can extrapolate that the general canine population with CrCL disease has a genetic component as well. For further information on the proposed cause of CrCL disease, an in depth description of the pathophysiology of CrCL disease can be found in the two following text books: Advances In The Canine Cranial Cruciate Ligament, second edition, ed. Peter Muir. Veterinary Surgery Small Animal, second edition, eds. Spencer A. Johnston and Karen M. Tobias.

Pre‐operative Considerations/Indications

Patient Selection

To optimize surgical success, it is key to carefully select patients for any surgical procedure. ECS stabilization is one of these procedures in which case selection has a direct impact on surgical success. Increased body weight and younger age have been associated with increased risk of postoperative complications.¹⁰ The authors of this study found that younger dogs have an increased risk of complications, and they speculated that this is due to increased activity of the young dogs with more challenging post‐op exercise restrictions. They also noted that heavier dogs had an increased risk of complications. While the authors didn’t give specific guidelines on what age or weight would be optimal in performing ECS surgery, many surgeons, including this author, have some “loose” guidelines for this recommendation.

In this author’s experience, dogs weighing over 40 lb are not ideal candidates for ECS surgery, and young and/or active dogs are not ideal ECS candidates, presumably due to increased activity and difficulty in successfully limiting their activity postoperatively. Middle age to older dogs and those that lead a more sedentary lifestyle seem to be better ECS surgical candidates, but, as stated above, these are “loose” guidelines. The patient’s activity, ability to be appropriately restricted postoperatively, owners’ expectations, patient co‐morbidities, and finances all play a role in selecting patients for ECS stabilization. Another factor to consider is the patient’s TPA. Many surgeons express concern that with a higher TPA, there is a higher ECS failure rate or decreased long‐term successful clinical outcome. A study that specifically looked at small breed dogs (<20 kg) with a TPA above 30° found that those who had a TPLO performed had better long‐term clinical outcomes and were less likely to require NSAID administration than those dogs that underwent a lateral fabello‐tibial suture procedure.¹¹ In this author’s experience, this general rule seems to apply. Measurement of the TPA on pre‐surgical radiographs is another reason to perform high quality radiographs prior to surgical decision‐making (Figure 50.3).

Patients with suspected partial CrCL injuries also are not considered ideal candidates for ECS stabilization, since these patients lack substantial laxity in the stifle to begin with. Knowing the limitations of a surgical technique is imperative to surgical success and how to direct clients in decision‐making.

Diagnosis

The diagnosis of a CrCL injury is based on the signalment and history of the patient, the physical examination, and radiographic signs. The history of the lameness can be variable but typically for an acute tear of the CrCL, the patient often presents for a non‐weight‐bearing to minimally weight‐bearing lameness of the hind limb. Another typical presentation is a waxing and waning lameness that is worse after exercise and worse after getting up from a lying position. The physical exam often finds a hind limb lameness at a walk and trot. Often, these patients will “kick out” the affected limb while sitting to avoid full flexion of the stifle joint; this is referred to as a positive “sit test.” Patients with a CrCL injury are typically painful on full flexion and extension of the stifle, and dogs with suspected partial tears are often painful on full extension of the stifle. Stifle effusion is often palpated by feeling over the medial and lateral aspects of the patellar tendon and appreciating the loss of the sharp edge of this tendon. Medial periarticular fibrosis can be felt on the proximal part of the tibia; this is termed medial buttress.

The cranial drawer test is a good test for determining the degree of laxity from a torn CrCL. The cranial drawer test is performed by first placing the dog in lateral recumbency with the affected leg facing up. One hand is placed on the femur with the pointer finger on the patella and the thumb on the lateral fabella, ensuring that the thumb gradually slides into position from cranial to caudal to avoid pinching the peroneal nerve when grasping the fabella. This hand is simply stabilizing the femur to prevent movement when the other hand is manipulating the tibia. The second hand is placed on the tibia with the pointer finger on the tibial tuberosity and the thumb on the fibular head. The hand on the tibia is then moved cranially while the hand on the femur is kept still and stabilizing the femur from moving. This cranial translation is the “cranial drawer sign.” Dogs with partial tears usually don’t have cranial drawer unless the knee is in flexion and only the craniomedial band of the CrCL is torn. It is advised to perform the cranial drawer test in flexion, extension, and a standing angle.

A cranial tibial compression test can be performed as well. This is performed by placing a hand on the femur with the thumb on the fabella, and the pointer finger spans the joint and places caudal pressure on the tibial tuberosity, which is somewhat mimicking the position and pressure of a fabello‐tibial suture. The limb is positioned at a standing angle, and the second hand is placed on the foot and then flexes the hock joint. A positive cranial tibial compression test is if the proximal tibia subluxates cranially relative to the femur when the hock is flexed. This maneuver takes more practice to effectively utilize, but if the patient is very tense, large, or heavily muscled, a tibial compression test is often easier to perform successfully in diagnosing stifle instability than eliciting cranial drawer motion. Sedation may be needed to help confirm the diagnosis and can be very helpful when performing radiographs to get ideal positioning.

Radiographs are highly recommended to help confirm the tentative diagnosis of a CrCL injury, rule out other differentials, such as neoplasia or infection, and to assess the patient’s anatomy for surgical planning. As mentioned above, the patient’s anatomy can play a key role in what procedure to advise (e.g., higher TPA or lack of lateral fabella may negate the ECS option). Another benefit of pre‐operative radiographs is that it gives a baseline of the presence of osteoarthritis. Well‐positioned radiographs are essential to accurately evaluating the patient’s anatomy, as well as identifying subtle changes that are consistent with a CrCL injury. The best positioning for a diagnostic stifle radiograph is a perfectly lateral view with the stifle in the center of the beam. The femoral condyles are directly, or as close as possible to directly, superimposed. The stifle and hock joint are at 90° (a “90/90 view”). The X‐ray beam is centered on the stifle joint, and the radiograph incorporates the hock in the field of view. Collimation is important to achieve as detailed radiographs as possible (Figures 50.4–50.6).

Suture Material Selection

There are many ECS materials on the market. The two broad categories of suture used for ECS are nylon leader line and braided material, and there are multiple brands of each on the market. Each material type has its own pros and cons to the use and application of ECS material. Nylon leader line has been in use in veterinary medicine for many years and is still a very tried and true material to use. The benefits of nylon leader line are the ease of application, affordable instrumentation for placement, inexpensive material, and has been studied extensively over the years. The downside of nylon leader line is that it has a lower tensile strength compared to braided material. Also, when tying knots in the leader line, there is considerable knot “bulk,” but this can be mitigated by using leader line size‐specific crimp clamps on certain sizes of leader line. Currently, the nylon leader line sizes come in 20‐, 40‐, 60‐, 80‐, and 100‐lb test weights. However, only 40‐, 80‐, and 100‐lb test weights come with crimp clamps of the same sizes.

A photograph of the hindlimb of a dog in lateral recumbency with X-ray beam centered on the stifle joint. — **Figure 50.4** The patient is in lateral recumbency with the stifle and hock positioned at 90° angles. The X‐ray beam is centered on the stifle joint, and the hock joint is in the field of view. While this image shows an unprotected hand holding the hind paw in position for demonstration purposes, there are commercially‐available blocks to assist in positioning the patient’s limb without exposing staff to unnecessary radiation.

*Source:* © Trent Gall.

The advantage of using crimp clamps is their low profile nature, which decreases soft tissue irritation compared to a bulky knot, and the crimp clamps are stronger than using knots.¹² A rough guideline for which suture size to use is to use the patient’s weight. For example, if the patient is 15 lb then the appropriate choice would be 20‐lb test weight. If the patient was 41 lb the appropriate weight would be either 60‐ or 80‐lb test weight. The author prefers to use nylon leader line with crimp clamps and avoids 60‐lb test weight, since this would be a very large suture knot and impossible to get a secure knot. The 20‐lb test weight is small enough that the suture knot is usually not an issue with smaller patients. That being said, the author has had several small patients with very little muscle mass and thin skin where the 20‐lb knot caused a persistent seroma. In one case, the nylon leader line and knot was removed and replaced with a softer braided material, which successfully resolved the underlying soft tissue irritation due to the knot and secondary seroma.

A radiograph of the superimposed femoral condyles with stifle effusion and fat pad displacement. — **Figure 50.5** The femoral condyles are directly superimposed over each other. Note in this radiograph there is stifle effusion (cranial displacement of the fat pad and caudal out‐pouching of the caudal joint pouch). The tibia is also in slight cranial drawer, as the intercondylar imminence should be more directly below the femoral condyle. These are all signs consistent with a CrCL injury.

*Source:* © Trent Gall.

A radiograph of the centered patella and the femoral cortices bisecting the fabellae. — **Figure 50.6** A straight anterior/posterior (AP), cranial/caudal view, or a caudal/cranial view. Note the patella is centered, the fabellae are symmetrically bisected by the femoral cortices, and the hock joint is straight.

*Source:* © Trent Gall.

A schematic illustration of the side and front views of the stainless steel crimp tube. — **Figure 50.7** Example of a 40‐lb stainless steel crimp tube. Some companies also add special coatings, such as titanium‐nitride, to their crimp tubes.

*Source:* © Trent Gall.

When using crimp tubes (Figure 50.7), which are placed around the leader line and then crimped to hold it in position, a crimping tool is needed. It is important to note that the surgeon should not mix and match materials and instrumentation from different manufacturers. Each crimping tool is calibrated to the specific manufacturer’s crimp clamp or tube. By mixing and matching systems, it is possible to either overcrimp, which could transect the leader line or tube, or under‐crimp the crimp tube, which could lead to suture slippage and implant failure. For example, do not use Securos leader line with an Everost crimping tool. Stay with the same manufacturer for whichever system you choose. Some manufacturer’s crimping tools have a “stopper” to know when the crimp tube is adequately compressed enough to securely hold the suture or leader line. This is useful to prevent under‐ or overcrimping of the crimp tube (Figures 50.8 and 50.9).

There are multiple options for obtaining suture to use for ECS placement. The first option is an individually‐wrapped suture with the swaged‐on needle of particular sizes (Figure 50.10), which is very convenient, since it uses a one‐time use a sharp swaged‐on needle (Figure 50.11), is of appropriate suture length for ECS placement in CrCL injuries, and is sterile in the pack. The second option is to purchase a roll of nonsterilized nylon leader line (e.g., essentially, fishing line), cut anticipated lengths of suture that are needed for the procedure, and then sterilize the strands individually. With this second option, a variety of cruciate needles would be needed to have on hand, since these will be non‐swaged‐on sutures that require threading at the time of surgery. When sterilizing the length of suture material, be sure to cut off enough suture material to easily work with. The author prefers approximately 14–18 in. (35.56–45.72 cm). Another tip is to sterilize the nylon leader line strand in a double peel pouch. This way, the surgeon can take the inner pouch when the outer pouch is opened over the surgical field since the line likes to “spring” out. According to the company, it is not recommended to steam‐sterilize nylon leader line more than once, since multiple steam sterilization cycles can weaken the nylon leader line.