Biomechanics of Luxation

Luxations are a common occurrence in small animal veterinary medicine. Decisions on appropriate treatment strategies are based on many factors. One of the most important of these factors is an understanding of the biomechanics of joints. Armed with this knowledge, mechanisms of injury can be understood and sound decisions relative to treatment methods can be made to restore normal function. In this section, we address biomechanics, which is the science of the action of forces, internal and external, on the living body, an unfamiliar topic for many veterinarians.

Diarthrodial or synovial joints are complex structures located at the ends of long bones. Each joint is a composite made of hyaline articular cartilage, synovial lining tissue, joint capsule, and ligaments. Tendons and muscles cross joint articulations, lending substantial support to the joint. The sequence of events encountered by the joint during trauma and the sequelae of these events are discussed in this chapter. It is important to realize, however, that most forces are exerted indirectly on the joint, and the resultant deformation of anatomic structures depends on many factors, such as (1) the direction of the force, (2) the speed of the force, (3) the attitude (position) of the animal, (4) the age of the animal (e.g., open or closed physes), (5) the configuration of the bones and joints (e.g., bones of basset hounds versus those of a greyhound), and (6) preexisting joint disease (e.g., laxity, hip dysplasia).

Forces singularly or coupled (e.g., rotation and bending) directed at the appendicular skeleton of the animal are transmitted along the limb and may result in a joint subluxation or complete joint luxation. Additional injuries may combine with joint trauma; these may include: bone fracture, tendon tear, capsular tissue avulsion, or physis separation in young animals. The veterinarian should maintain a high degree of suspicion when examining animals subjected to trauma. A clearly evident fracture in a limb may be associated with a less noticeable luxation in the same limb. An example would be a fractured distal tibia with dislocation of the hip in the same limb. The shoulder, elbow, carpal, hip, stifle, and tarsal joints are discussed here, and a set of events that may lead to luxation of the particular joint is outlined. Surgical considerations that deal with the damage done by luxation are discussed when applicable.

Shoulder Joint

Anatomy

Being a ball and socket joint, the shoulder joint is well suited for movement in all directions. Although capable of movement in all directions, the shoulder primarily moves in flexion and extension. Joint stability is provided through a combination of passive and active mechanisms (Fig. 110-1). Passive mechanisms include the medial and lateral glenohumeral ligaments, surrounding joint capsule, joint conformation, and synovial fluid cohesion. The medial collateral ligament (MCL) commonly appears as "Y" shaped with the cranial arm coursing caudally from its origin at the medial surface of the supraglenoid tubercle. The caudal arm of the MCL originates from the medial surface of the scapular neck and joins the cranial arm to insert onto the humeral neck. The MCL and associated joint capsule are major factors in providing joint stability. Complete medial luxation occurs following transection of the medial glenohumeral ligament (MGHL) [1]. The lateral collateral ligament (LCL) originates from the lateral rim of the glenoid and extends ventrally to insert onto the humerus at the caudal region of the greater tubercle. The joint capsule originates from the periphery of the glenoid cavity. Medially, the joint capsule forms a synovial recess owing to its attachment several millimeters proximal to the glenoid rim. The concavity of the glenoid and the fit of the humeral head into the glenoid provide joint stability. This is particularly true when compression across the joint is enhanced by active muscle contraction. Dynamic active glenohumeral stability is provided by contraction of the surrounding cuff muscles. These include the biceps brachii, subscapularis, teres minor, supraspinatous, and infraspinatous muscles. Active contraction of all or selective cuff muscles induces compression across the shoulder joint as well as increasing tension in the joint capsule. When tested in a neutral position, the cranial, lateral, and medial translation of the humerus was significantly increased after biceps tendon transection. In the flexed position, translation of the humerus in the cranial and lateral directions was significantly increased after biceps tendon transection. In the extended position, the medial translation of the humerus was significantly increased after biceps tendon transaction [1].

Figure 110-1. Photograph of the passive and active restraints of the shoulder joint. Passive restraints are the medial collateral ligament, lateral collateral ligament, and surrounding joint capsule. Active restraints are the surrounding cuff muscles, which include the biceps, supraspinatus, infraspinatus, and subscapularis muscles.

Examination of the shoulder for stability should be done under anesthesia. Flexion, extension, abduction, adduction, and rotational stability of the shoulder joint should be assessed. Normal ranges of flexion and extension are 40° for flexion and 165° for extension. A normal abduction test is approximately 25°; abnormal abduction is considered present when abduction exceeds this degree and there is a difference in the abduction angle between the injured side and the normal side. Note that dogs with long-standing forelimb lameness often present with laxity. In the majority of these cases, approximately 45° of abduction exists on the affected side with a normal abduction on the sound limb (25°).

Biomechanics

A fall or jump from a height may produce a shoulder subluxation or luxation. An example in a toy breed would be the dog jumping from an owner’s arms. Another mechanism of injury causing subluxation/luxation of the shoulder would be if the animal is bearing weight at the time that a blunt force is applied to the proximal area of the humerus. The direction of subluxation/luxation is dependent on the direction of force application (i.e., anterior to posterior, medial to lateral). Because the limb is bearing weight, it forces the proximal humerus to accept most of the blow. If the foot is off the ground, the leg is able to swing in the direction of the force, diminishing the chance of a subluxation or luxation. In addition, when the limb is bearing weight, the humerus is down and away from the chest wall, and no medial, shock-absorbing protection is available from the chest wall. With a laterally to medially directed force, the humeral head can be driven medially, rupturing the medial restraints (MCL, subscapularis tendon, and joint capsule). The humeral head lies medial to the glenoid of the scapula, resulting in a medial luxation (Fig. 110-2).

Figure 110-2. A and B, Schematic drawings of an animal seen in an anteroposterior view. A medial luxation of the humeral head is due to a blunt blow to the proximal lateral surface of the humerus.

Medial Luxation

A medial to lateral force directed against the body may result in a medial subluxation or luxation if, when the force is applied, the foot is in contact with the ground. In this position, the leg starts to abduct as the body moves toward the ground. This action may be caused by a blow from a moving vehicle or a fall from a height. As the extended limb starts to abduct, a long lever arm is created from the foot to the shoulder. The greatest stress occurs on the medial side of the shoulder joint, rupturing the medial restraints (Fig. 110-3).

Figure 110-3. A. The animal’s body is moving ventrally while the forelimb is in extension and weight-bearing. B. As the chest continues ventrally, the limb begins to abduct, putting stress on the medial side of the shoulder joint. C. As the chest nears the ground, the medial side of the joint ruptures, allowing the humeral head to luxate medially. D. The stress on the medial side of the joint can be greatly exaggerated if at the same time the body is moving ventrally it is also rotating toward the affected limb.

Treatment Strategies

Closed Reduction: If, following reduction, the shoulder joint maintains the correct position, reduction can be maintained with a Valpeau sling. The elbow should be taped to the chest wall, producing a lateral force at the shoulder joint. A Valpeau sling should remain in place for 3 weeks. Once the splint is removed, the attending veterinarian should radiograph the shoulder and examine the joint under anesthesia. Either palpation to detect subtle subluxation with rotation or the abduction test is performed.

Open Reduction: A medial approach with careful identification of all injured tissue is required. Placement of suture anchors at the origin and insertion of the Y-shaped medial collateral ligament is achieved. Nonabsorbable suture is used to reconstruct the MCL.

Lateral Luxation

Lateral luxation occurs when the limb is in full extension and is forced into adduction during weight bearing. This action can be caused by a blow from a moving vehicle or a fall. The force is applied principally to the lateral surface of the shoulder. The force injures the lateral cuff muscles and collateral ligament, allowing the humeral head to luxate laterally to the glenoid (Fig. 110-4).

Figure 110-4. A. The animal’s body is moving ventrally. The forelimb is in extension and is weight bearing. B. As the body continues ventrally, the limb moves into abduction causing stress on the lateral side of the shoulder joint. C. As the chest nears the ground, the lateral side of the joint ruptures, allowing the humeral head to luxate laterally. D. The stress on the lateral side of the joint can be greatly exaggerated if at the same time the body is moving ventrally it is also rotating away from the side of the affected limb.

Another mechanism for injury is a blow to the proximal end of the humerus. The blow is directed from cranial to caudal. Because the joint is in midflexion, the humeral head is more easily driven out of the glenoid cavity. The tendons surrounding the joint are fairly relaxed, and the greater tubercle is below the scapular tuberosity and glenoid. If the force of the blow is sufficient, the humeral head is driven out of the glenoid cavity. Depending on which tendons rupture first (subscapularis or infraspinatus), the humeral head eventually lies on either the medial or lateral side of the glenoid.

Treatment Strategies

Closed Reduction: Measures are the same as for medial luxation, except that padding is placed between the elbow and chest wall, producing a medial force at the shoulder joint.

Open Reduction: A lateral approach with careful identification of all injured tissue is required. Placement of suture anchors at the origin and insertion of the lateral collateral ligament is achieved. Nonabsorbable suture is used to reconstruct the LCL.

Elbow

Anatomy

The elbow is a composite ginglymoid (hinge) joint formed by the humeral condyle, the head of the radius and the semilunar notch of the ulna. The entire joint consists of three separate articulations enclosed within a single joint capsule. The humeroradial joint occurs between the humeral capitulum and the radial head and is thought to transmit the majority of weight through the elbow. The humeroulnar joint occurs between the humeral trochlea and olecranon fossa of the humerus and the articulating surface of the semilunar notch and anconeal process of the ulna. The humeroulnar joint, along with the collateral ligaments, provides stability to the elbow, particularly in extension. The proximal radial head fits into the coronoid processes of the ulna and allows some supination and pronation. In the normal Labrador retriever, the elbow joint functions primarily in flexion and extension, allowing up to 36° of flexion and 166° of extension [2]. It has been reported that, with the elbow and carpus flexed to 90°, the collateral ligaments limit internal rotation to 60° and external rotation to 40° [3]. The collateral ligaments originate from the medial and lateral humeral epicondyles and split into cranial and caudal crura as they course distally to insert on the proximal radius (cranially) and the lateral ulna or the caudal radius, annular ligament, and medial interosseous space.

Acquired Elbow Joint Luxation

Elbow luxation is much less common than fractures involving the elbow, likely because of the inherent stability of the anatomy of the joint. When elbow luxation occurs in the dog, it is thought to be caused primarily by rotation of the body around a flexed, weight-bearing limb rather than by a direct blow to the elbow or by landing on an extended forelimb (Fig. 110-5). In the dog, lateral luxation occurs in over 90% of cases, although medial and caudal luxation have been reported [4]. The larger shape and orientation of the medial condyle make medial luxation much more unlikely. The one reported case in a cat was a medial elbow luxation [5]. The diagnosis of elbow luxation is almost always associated with significant impact, such as being hit by a car, although luxation has been reported after fighting or rough play. The animal will be severely lame, usually unable to bear weight, will have a swollen elbow with limited range of motion, and pain on attempted manipulation of the joint. Although luxation of both the radial head and the anconeal process laterally are most commonly seen, in about 20% of cases the ulna is not completely luxated. In addition, the clinician should be aware that a caudal blow to the ulna may result in radial head luxation and concurrent ulnar fracture (Monteggia fracture). Monteggia fractures, discussed elsewhere in this text, can be challenging to repair and are handled differently from a straightforward traumatic elbow luxation.

Figure 110-5. Schematic drawings show the animal from two views. A. A top view shows the left foreleg circled in solid black because that will be the weight-bearing limb. B. A lateral view of the forelimb. C. The forces acting on the animal cause the rotational forces to be in a clockwise direction as viewed from above. D. The force acting on the animal’s body causes the rotational forces to be in a counterclockwise direction as viewed from above.

Treatment Strategies

Acute Luxation: A complete physical examination should focus on other major orthopedic or thoracic injuries, which can be present in up to 30% of cases. The affected forelimb should be assessed carefully to assure intact innervation and vascular supply. Radiographs of the antebrachium should be carefully examined for ulnar fracture (which may be very distal) and fractures of the anconeal process, fragmented coronoid process, and avulsions of the epicondyles where the collateral ligaments attach. Once the animal is stable, closed reduction should be performed under general anesthesia. Reduction should be radiographically confirmed, and the elbow should be fairly stable once reduced. Post-reduction coaptation is controversial; a variety of recommendations can be found in the literature, from no bandaging at all to 4 weeks in a Spica splint, incorporating the shoulder joint. At a minimum, bandaging in flexion (for example, a Valpeau sling) should be avoided as the elbow is more likely to reluxate in that position. The animal should be confined to leash walks and crate rest for a minimum of 2 weeks. If malalignment or instability are present after reduction, or if reluxation occurs, open reduction is recommended. Some authors also recommend surgical repair of collateral ligaments even after closed reduction if it is radiographically apparent that one or both epicondyles have avulsed. Although the majority of reductions performed early after luxation give good to excellent results, the owners should be warned that degenerative joint disease and lameness can be the end result.

Chronic Luxation: Luxations of over 1 to 2 weeks’ duration are generally considered chronic. All chronic luxations require open reduction. The amount of cartilage damage, muscle contracture, and fibrosis present will dictate whether the luxation can be reduced and also correlate with decreasing prognosis for normal use of the limb. In all cases of chronic elbow luxation, the surgeon should warn the owner that arthrodesis or amputation may be an option.

Carpus

Anatomy

The carpus consists of seven bones arranged in two rows. The radial carpal and ulnar carpal bones make up the proximal row, while the first, second, third, and fourth carpal bones make up the distal row. The accessory carpal bone lies caudally and articulates with the ulnar carpal bone. The radial carpal bone and ulnar carpal bone articulate with the radius and styloid process of the ulna to form the radiocarpal joint. This joint has the greatest amount of movement. The middle carpal joint, formed by the articulation of the proximal and distal rows of carpal bones, accounts for 10% to 15% of carpal motion. Little motion occurs in the carpometacarpal and intercarpal joints. Collateral ligamentous support arises from the short radial collateral ligament medially and from the short ulnar collateral ligament laterally. Additionally, sleeves of collagenous tissue that house tendons provide medial and lateral collateral support. Palmar support is from the flexor retinaculum proximally and palmar fibrocartilage distally. Multiple small ligaments cross the intercarpal articulations between carpal bones to provide additional collateral and palmar support. Two of these, the palmar radiocarpal ligament and the palmar ulnar carpal ligament are important structures in providing palmar support for the radiocarpal joint (Fig. 110-6). Two accessory ligaments originate from the free end of the accessory carpal bone and insert onto the palmar surface of the fourth and fifth metacarpal bones. The caudal position of the free end of the accessory carpal bone, in conjunction with the accessory carpal ligaments, serves to act as a moment arm to balance the vertical force produced when the paw strikes the ground.

Figure 110-6. Photograph showing the palmar radiocarpal ligament and the palmar ulnar carpal ligament. These ligaments are important in providing craniocaudal and rotational support for the radiocarpal joint.

Carpal hyperextension injuries are divided into the following three categories. Type I injury is a subluxation or luxation of the radiocarpal joint (may include damage to the middle carpal and carpometacarpal joints). A Type II injury is a disruption of the accessory carpal ligaments, palmar fibrocartilage and palmar ligaments of the middle carpal and carpometacarpal joints. The result is a subluxation of the middle carpal and/or carpometacarpal joints with dorsal displacement of the free end of the accessory carpal bone and ulnar carpal bone. A Type III injury is a disruption of the accessory carpal ligaments, carpometacarpal ligaments, and the palmar fibrocartilage. In these injuries, a subluxation of the carpometacarpal joint occurs without disruption and displacement of the accessory carpal and ulnar carpal bones.

With an acute injury, ligamentous disruption is complete and the patient presents with a nonweight-bearing lameness. Swelling, pain, and instability are evident on examination. In Type I injuries, the patient generally remains nonweight bearing until definitive treatment is achieved; in Type II or Type III injuries, the patient may begin to bear minimal weight with the limb after the injury. However, as the patient increases the amount of weight placed on the limb, collapse and hyperextension of the carpus are evident. Standard craniocaudal and medial-to-lateral radiographs are indicated to determine the presence of bone fractures and/or joint malalignment. To accurately assess the integrity of the carpus, however, stress radiographs should be taken. The purpose of stress radiography is to assess the point of injury and to determine if the integrity of the radiocarpal joint is intact. If the integrity of the radiocarpal joint is intact, then a partial arthrodesis is indicated; if the integrity is lost, a pancarpal arthrodesis is preferred. Although stress radiographic evaluation is a valuable tool, false positives (i.e., radiocarpal joint not intact) are possible. Another valuable tool to assess the integrity of the radiocarpal posterior ligamentous support is arthroscopic examination prior to deciding on a partial carpal or pancarpal arthrodesis.

Biomechanics

The carpus is a complex structure. Type I luxations of the carpus are uncommon, and when they occur they usually involve the antebrachiocarpal joint. Luxations can be found with severe shearing injuries involving the carpus. Subluxation associated with hyperextension is the most common form of luxation in the carpus. The energy needed to cause luxation often comes from the animal’s falling or jumping from a height or jumping out of a fast-moving vehicle and landing on a fully extended limb. As the foot makes contact with the ground, the carpus hyperextends, allowing the volar aspect of the proximal metacarpal bones to strike the ground. This transmits a violent force proximally through the carpal bones toward the radius. At the same time, the force of the weight of the animal travels ventrally through the radius toward the carpus. When sufficient opposing forces travel through the column of bones held together by ligaments, luxation of one or more the carpal joints is possible. On some occasions, when an animal is falling to the ground as in the previous situation, the foot knuckles over into flexion. As the radius continues to drive ventrally, the foot is forced into extreme flexion, causing severe stress on the anterior side of the carpus. In this case, any or all of the joints of the carpus may open anteriorly, rupturing the joint capsule and interosseous ligaments. When an animal is struck by a vehicle, the carpus may be dragged along the road, causing severe shearing injury to the medial or lateral side. Generally, all the collateral support structures (skin, ligaments, tendons, joint capsule, and sometimes a considerable amount of bone) are lost. Again, the antebrachiocarpal joint is the most susceptible to luxation.

Treatment Strategies

Closed Reduction: If the damage is not too severe, it is possible to obtain healing and stability with the use of splints or casts. External coaptation must be rigid for the initial 4 weeks and then gradually stabilized over the next 4 weeks. The carpus is originally placed in mild flexion and gradually extended to a normal standing position. External coaptation is more successful with injuries to the anterior (dorsal) restraints as compared with injuries of the caudal restraints.

Open Reduction: Primary ligamentous repair, i.e., suturing the ligament ends is not successful. Likewise, reconstruction with autogenous or autologous tissue has not been rewarding. The common method for surgical repair is fusion of the injured joint(s). The method of surgical intervention depends on the classification of injury. Type I carpal injury requires a pancarpal arthrodesis; Type II injury can be repaired with a pancarpal or partial carpal fusion, whereas Type III injury requires a partial fusion. Good to excellent results can be expected when this procedure is done correctly.

Hip Joint

The hip is by far the most common site of traumatic luxation in dogs and cats. No collateral ligaments exist, and the muscles that attach to the proximal end of the femur allow a great deal of motion in the joint. The major stabilizing feature of this joint is the ball-and-socket configuration itself. Contraction of the surrounding muscles is of primary importance in lending stability to the hip joint as is the anticavitational effect of synovial fluid. The round ligament and the joint capsule are the major soft-tissue structures providing passive stability and are not of primary importance in preventing subluxation of the hip joint. These structures may become stretched, as with hip dysplasia, allowing subluxation and predisposing the hip to complete luxation. The different classifications of hip luxations are named by the location of the femoral head relative to the acetabulum. For example, with a craniodorsal luxation, the femoral head lies cranial and dorsal to the acetabulum, whereas with a ventral luxation, the femoral head lies beneath the acetabulum (often lodged in the obturator foramen).

Craniodorsal Luxation

One of the most common causes of luxation of the hip is a strong blow delivered from the rear or side of the dog or cat. As the animal starts to fall toward the hip to be luxated, the center of gravity moves lateral to the hip joint. Simultaneously, with the foot on the ground, the rear leg moves into adduction. As the hip moves ventral and laterally toward the ground, the lever action of the adducted femoral shaft subluxates the femoral head out of the acetabulum (Fig. 110-7). The center of gravity of the animal moves lateral to the hip joint. This position is exaggerated in dysplastic animals, where the hip joint conformation is poor and the passive restraints (round ligament and joint capsule) are already stretched. The dorsal rim is remodeled, further contributing to instability. When the greater trochanter strikes the ground, the energy of the blow is transmitted through the femoral neck to the femoral head. The femoral head is driven over the dorsal rim, shearing the joint capsule or teres ligament. Sometimes the teres ligament avulses from the femoral head, causing a small fragment to pull from the femoral head. Rarely, a piece of bone is chipped off the dorsal rim of the acetabulum. The femoral head comes to rest in its most common position of luxation craniodorsal to the acetabulum. The strong pull of the gluteal muscles also helps to pull the femoral head to this position. Muscular force and passive restraints (capsule and round ligament) are unable to maintain the reduced state of the joint.

Figure 110-7. A. The hip joint and hind limb viewed from the front. The leg is in extension and is weight bearing. B. A blow is struck to the rump causing the limb to go into adduction. C. Just as the greater trochanter is about to hit the ground, the femoral head is subluxated against the dorsal rim of the acetabulum. D. As the hip hits the ground, the teres ligament ruptures and the femoral head luxates to a dorsoanterior position.

Another way in which hip luxation may occur is when the body is driven ventrally toward the ground. The hind leg is extended and the foot is bearing weight. As the pelvis is forced ventrally, the knee and hip begin to flex. At some point before the pelvis strikes the ground, the knee makes contact with the ground. As the pelvis continues to move ventrally, the hip begins to rotate externally. If the force is sufficient, the teres ligament and joint capsule rupture, allowing the femoral head to luxate. The strong pull of the gluteals causes the femoral head to luxate in a craniodorsal position.

Treatment Strategies

Closed Reduction: Several techniques are used to reduce hip luxations. The longer the hip is dislocated, the more difficult it is to reduce, owing to muscle contraction and local fibrosis. Generally, the leg must be put into a sling to prevent reluxation. Reported success rates vary from 30% to 85%. Animals with hip dysplasia, severe degenerative joint disease, or avulsion fractures in the hip joint are poor candidates for closed reduction.

Open Reduction: If the luxation cannot be reduced or if the joint repeatedly reluxates, surgery is indicated. Many techniques are available to the surgeon. Procedures include capsulorrhaphy, iliofemoral suture, trochanteric transposition, transarticular pinning, teres ligament replacement (including a toggle), and triple pelvic osteotomy. Alternatives to preserving the in situ hip joint are excision arthroplasty and total hip replacement.

Ventral Luxation

A distinction may be made between cranial and caudal ventral luxation. The biomechanics are the same for both, except for the position of the femoral head as it is being forced ventrally under the acetabulum. If the leg rotates inward as the hip is luxating ventrally, the femoral head ends up in the obturator foramen. If the leg rotates outward, the femoral head ends up in front of the pubis. It has been reported that a ventral luxation can be created by overzealous reduction of a craniodorsal luxation. Those that occur naturally are usually associated with the trauma of jumping or falling and landing with the leg abducted. As the leg continues to abduct during the fall, eventually the femoral neck and greater trochanter strike the dorsal aspect of the acetabulum. This forces the femoral head out of the acetabulum, rupturing the teres ligament and ventral joint capsule. An animal with a ventral luxation carries the leg abducted and inwardly rotated and is unable to put the foot on the ground.

Treatment Strategies

Closed Reduction: Not many cases of ventral luxation have been reported, but they do appear to respond well to closed reduction. If reduction alone leaves the joint unstable, hobbles or an adduction sling may be used to prevent abduction.

Open Reduction: Any of the techniques mentioned for craniodorsal luxation that prevent luxation in any direction (e.g., transarticular pinning) could be considered. The ventral restraints may be reconstructed by advancement of surrounding soft tissue. If none of these methods works, femoral head and neck excision arthroplasty or total hip replacement should be considered.

Stifle Joint

Anatomy

The stifle joint is a complex diarthrodial (synovial) joint that allows movement in three planes. The stifle consists of three intimately associated joint spaces: the femorotibial (between the femoral and tibial condyles), the femoropatellar (between the patella and the femoral trochlea), and the proximal tibiofibular joints.

The primary motion of the joint is flexion and extension; however, the articulation of the femoral condyles with the surrounding supportive structures, menisci, and proximal tibial surface allows cranial and caudal translation, internal and external rotation, varus and valgus angulation, and medial and lateral movement of the femur in relation to the tibia. The medial and lateral menisci are C-shaped, fibrocartilaginous structures interposed between the articular surfaces of the femur and tibia (Fig. 110-8). They perform a load-transmitting function, improve joint stability by providing congruity between the femur and tibia, assist in lubrication of the joint, and are thought to provide a sensory function so they may aid in joint proprioception. The menisci are firmly attached to the femur, tibia, and joint capsule by six ligaments.

Figure 110-8. Photograph of the medial and lateral meniscal ligament. Both are important intra-articular structures providing joint stability and load transmission.

Primary ligamentous support of the stifle is provided by medial and lateral collateral ligaments, as well as cranial and caudal cruciate ligaments. The lateral collateral ligament originates proximal to the origin of the popliteal muscle on the lateral femoral epicondyle and inserts on the fibular head. The medial collateral ligament originates from an oval area on the medial femoral epicondyle and forms a distal attachment to the medial meniscus and surrounding joint capsule. Collateral ligaments are responsible for preventing varus (lateral collateral) and valgus (medial collateral) motion of the tibia and as a secondary stabilizer against rotational forces of the stifle. The cranial cruciate ligament originates on the caudomedial portion of the lateral condyle of the femur and courses in a spiral orientation cranially, medially, and distally through the intercondylar fossa to the cranial intercondyloid area of the tibia. It functions primarily to prevent cranial and caudal translation of the tibia in relation to the femur, to provide rotational stability by preventing internal rotation of the stifle, and also to assist in preventing hyperextension of the stifle. The caudal cruciate ligament begins at the lateral aspect of the medial femoral condyle and extends caudally and distally to the lateral edge of the popliteal notch of the tibia. The caudal cruciate ligament prevents excessive cranial and caudal translation of the tibia in relation to the femur and provides assistance in prevention of hyperextension of the stifle.

Other soft tissues that lend themselves to the stability of the stifle joint include the popliteal muscle, the semitendinosus and semimembranosus muscles, quadriceps femoris, straight patellar ligament, and joint capsule.

Biomechanics

Because of the dynamic variety of muscular and ligamentous structures and the articulation of the femur, tibia, and menisci, movement of the stifle is complex and does not limit itself to a single plane. During flexion of the stifle, the tibia internally rotates slightly because of the relaxation of the lateral collateral ligament and subsequent caudal displacement of the lateral femoral condyle. During extension, the tibia externally rotates because of the cranial displacement of the lateral femoral condyle as the lateral collateral ligament tightens. Cranial and caudal movement of the femur in relation to the tibia is seen in a sagittal plane during flexion and extension; a slight varus and valgus movement of the tibia is also seen but is limited by the ligamentous structures of the stifle.

The most common cause of stifle subluxation is associated with the rupture of the cranial cruciate ligament. Stifle luxation or total derangement of the knee is an uncommon injury in dogs and cats, but has been documented to have a higher incidence in cats [6]. Significant damage to multiple ligaments of the stifle is typically associated with high-energy injury, such as with a fall or vehicular trauma. Rupturing of the articular structures is most likely associated with a direct blow to a weight-bearing limb, resulting in a cranial and lateral luxation of the stifle and subsequent rupture of the cranial cruciate, caudal cruciate, and medial collateral ligaments [7]. Also, it is common to see damage to the secondary structures of the joint, such as the joint capsule, menisci, and patellar ligament. Ipsilateral fractures of the femur and pelvis were also cited as having been seen in 8 of 27 cases (30%) [7].

Damage to the cruciate ligaments and medial collateral ligament is theorized to occur because of the vulnerability of the lateral portion of the limb to blunt trauma or perhaps owing to the resistance of medial and caudal luxation by the quadriceps, hamstring, gastrocnemius, and popliteal muscles [8]. Because of the magnitude of the blow that is necessary to luxate the stifle joint, vascular and nerve complications are a consideration, especially with cranial luxations.

Treatment Strategies

Closed Reduction: Nonsurgical reduction of a stifle luxation is not recommended. The prognosis for return to function is poor.

Open Reduction: Return to good function can be expected if primary and secondary joint restraints can be restored. Multiple techniques for open reduction and repair of articular and periarticular structures have been described [9]. Even under general anesthesia, it is difficult to assess the extent of damage without direct visualization of the ligamentous and soft-tissue structures. Consequently, complete exposure and exploration of the joint is recommended [10]. Thorough examination of articular structures is important. Repair or debridement of damaged menisci and repair of the joint capsule should be the first steps in the surgical therapy.

Repair of the stifle depends on immediate repair and stabilization and long-term periarticular fibrosis. Reconstruction of collateral ligaments or placement of prosthetic ligaments using screws and spiked washers is suggested to maintain stability so that stabilizing sutures can be placed. Both intra- and extra-articular repair for the cranial and caudal cruciate ligaments have been used to provide immediate stifle stability. Reconstruction of the caudal cruciate ligament was not found to be necessary in a review of 12 cases of multiple ligamentous injuries of the feline stifle of working dogs, but methods of stabilization must be addressed on a case by case basis [11]. With extensive soft-tissue damage, it is proposed that a strong inflammatory reaction incites significant collagen-repair stages of healing, leading to long-term stability of the joint.

External skeletal fixation has been used as an adjunct therapy to support extra-articular techniques used for the repair of collateral and cruciate ligament damage. The use of a transarticular external fixator provided joint stability and return to function during the period of periarticular fibrous tissue formation. Complications seen with the external fixator are associated with pin loosening, fracture through pin sites, and severe breakdown of repair as was seen with animals that escaped from confinement. Lateral splints may also be used to provide support to surgical procedures. Use of a temporary transarticular pin without reconstruction of the damaged soft tissues has also been successfully used in small dogs and cats to provide support and allow periarticular fibrosis to form and stabilize the stifle.

Regardless of the surgical method used, mild to moderate stifle osteoarthritis is expected and can be attributed to the initial injury, as well as resultant altered biomechanical forces. Lastly, arthrodesis or amputation can be used as an option for acute severe trauma, chronic instability, or a painful joint.

Luxations involving the Tarsus and Metatarsus

The tarsal joints are composite articulations, in which more than two joint surfaces are enclosed within the same joint capsule. The tarsus and metatarsus are frequently involved in trauma and subluxations, and complete luxations are common. The regional anatomy is complex, and most surgeons consider instability at four different horizontal levels from proximal to distal: the talocrural joint, the proximal intertarsal joint, the distal intertarsal joint, and the tarsometatarsal joint. In addition, a multitude of vertically oriented intratarsal joints are considered to be more rigid than the horizontally oriented joints, although instability can occur in these joints as well [12].

Talocrural Joint Subluxation and Luxation

This joint permits the largest degree of movement (maximum flexion in normal Labrador retrievers is 38°, with maximal extension measured at 165°) [2]. The trochlea of the talus (or tibial tarsal bone), with two distinct articular ridges, fits into reciprocal grooves found in the cochlea of the tibia, providing some stability; however, the rest of the stability relies on the medial and lateral collateral ligaments and the plantar ligaments.

In dogs and cats, luxations of the talocrural joint are most commonly associated with trauma, and injuries can be classified as either closed (without soft-tissue injury) and open or as shearing injuries. Most of these injuries are presumably caused by trauma, although in many cases the actual event is not observed. The type of trauma is thought to be forced rotation around the long axis of the joint or a severe blow directly to the joint, most often to the medial aspect. If luxation of the talocrural joint results, either the medial or lateral malleoli (or both) may fracture. In our experience, the lateral malleolus is more likely to fracture and the medial collateral ligament complex is more likely to tear without fracture. Although most injuries to the collateral ligaments involve both the short and long components, injuries to the short collateral (fibulocalcanear and fibulotalar or distal and proximal short lateral collateral ligaments, respectively) can occur and may be difficult to diagnose without careful palpation under anesthesia. Radiographic changes associated with these injuries may be confused with osteochondrosis dissecans [13,14]. Damage to the lateral collateral ligaments may be more likely when the animal rotates at speed around the hindlimb in a flexed, weight-bearing position [13]. In adult cats, a torn medial collateral ligament complex is frequently associated with lateral instability caused by fracture of the fibula at the origin of the lateral collateral ligament [15]; whereas a Salter-Harris 2 fracture of the distal tibia will result in the appearance of a complete luxation of the tibiotarsal joint in the immature cat.

Treatment Strategies

Closed Injury: Using a combination of traditional orthogonal and stressed-view radiography, in addition to careful palpation, the surgeon should identify whether the medial collateral, lateral collateral, or palmar ligamentous support to the talocrural joint has been damaged. For complex injuries, computed tomography should be considered as well [16]. Although closed reduction and coaptation can be attempted, the forces applied to the joint usually exceed the ability of the scar tissue to stabilize the joint, and the joint will not become stable enough for the animal to bear weight.

Surgical repair usually involves reconstruction of the collateral ligaments. It has been shown that reconstruction of both the long and short parts (double prosthetic repair) will give a much better clinical outcome than single prosthetic repair [17]. Although the traditional method of screws and washers using nonabsorbable suture gives good results, alternative methods using bone tunnels [18] and bone anchors can also be used. Regardless of the method used, it is important to support primary repair of the collateral ligaments with either a transarticular external fixator or cast/splint for up to 6 weeks. If excessive damage to the articular surface of the tibiotarsal joint is found during surgical exploration, pantarsal arthrodesis should be done, typically by application of a dorsal plate combined with an intramedullary pin [19].

SHearing Injury: Shearing injury of the hock involves the medial aspect in two thirds or more of cases [20]. Treatment is somewhat controversial and can include immediate or delayed prosthetic reconstruction of the damaged ligaments, immediate or delayed pantarsal arthrodesis, or primary management of the wound with support from either an external fixator or cast alone [20]. If immediate arthrodesis is performed, an external fixator may be preferable to a plate, which may have to be removed owing to chronic infection that cannot be avoided when a large open wound is present [21].

Proximal Intertarsal Subluxation and Luxation

The proximal intertarsal joint is composed of the talocentral and calcaneoquartal joints, which do not allow noticeable motion in the normal dog and cat. This joint can fail in either hyperextension, which may result in damage to the dorsal ligaments, or more commonly, from failure of the plantar ligaments with or without fractures and luxations of the associated tarsal and metatarsal bones. The Shetland sheepdog appears to be predisposed to proximal intertarsal joint luxation, presumably caused by chronic degeneration of the plantar ligaments of unknown cause [22]. The condition may be bilateral.

Treatment Strategies

Hyperextension: Because during normal weight-bearing hyperextension injuries tend to be held in reduction, and because most of the support to the joint comes from the plantar ligaments, these injuries can generally be treated with a cast.

Hyperflexion: Calcaneoquartal arthrodesis is recommended for hyperflexion injuries, as external coaptation has been shown to be ineffective. Arthrodesis may be effective in a variety of ways (e.g., lateral plate, Steinmann pin, and tension band), use of a tension band in combination with the primary repair and external coaptation until radiographic evidence of arthrodesis is achieved is recommended [22].

Tarsometatarsal Luxation and Subluxation

The four distal tarsal bones, numbered from medial to lateral first through fourth, articulate with the metatarsals I through V, forming the tarsometatarsal joints. These joints are considered to be low-motion joints in the normal dog and cat. Subluxation and luxation of the tarsometatarsal joints usually occur as a result of trauma and may be associated with concurrent fractures of the proximal metatarsal bones.

Treatment Strategies

External coaptation may be appropriate for minor injuries involving only one or two metatarsal bones with minimal displacement. However, if the entire tarsometatarsal articulation is luxated, or if significant displacement is present even if only a lateral or medial injury is noted, arthrodesis is recommended. Although many forms of arthrodesis have been reported, plate fixation is likely to be the most successful. Usually the plate is applied laterally [23], and a hybrid (either 2.0/2.7 or 2.7/3.5 mm) dynamic compression plate may be advantageous. [4].

Talocalcaneal Luxation

The talocalcaneal joint is maintained by two ligaments crossing the tarsal sinus between the two bones, and is considered to be a low-motion joint. Luxation has been reported in the dog after trauma, and may be difficult to see on standard radiographic views.

Treatment Strategies

The reports of this injury are limited, but success has been observed both with reduction and external coaptation and lag screw fixation [25].