CHAPTER 13 Jim Schumacher, DVM, MS, Diplomate ACVS, MRCVS and Ted S. Stashak, DVM, MS, Diplomate ACVS Wounds of the distal aspect of the limb of horses are common and account for more than 60% of all wounds. These wounds are often more problematic than wounds located elsewhere because of their proximity to the ground, making them more likely to become contaminated and then infected, and because skin in the distal portion of the limb is poorly vascularized compared to skin located elsewhere on the horse’s body. A wound to the distal aspect of the limb is much more likely to involve a vital structure and, if allowed to heal by second intention, to develop exuberant granulation tissue (EGT). The greatest difference in the rate of healing of wounds involving the body and those involving the distal aspect of the limb can be attributed to the greater contribution of contraction to the healing of wounds on the body. Although there may be geographic differences in terms of type and incidence of wounds encountered, wounds of the distal aspect of the limb (i.e., up to and including the carpus and tarsus) of horses are quite common and account for more than 60% of all wounds.1 Geographic differences in types of wounds encountered relate to how the horse is confined (e.g., paddock or pasture – barbed wire fences versus board, pipe, or plastic fences, etc. – or a stall and run) and the manner in which the horse is used (e.g., Western performance versus hunter/jumper). Sharp objects, such as sheet metal, broken glass, exposed nails or bolts, and barbed wire, are responsible for most lacerations and avulsion injuries, but serious wounds can be caused by smooth, high‐tensile wire or by rope. Protruding objects, such as stubs of wood projecting from tree trunks or logs, or nails and bolts protruding from fences, buildings, or trailers, are often the cause of penetrating wounds. If the penetrating object becomes embedded within tissue, a foreign‐body reaction develops, usually resulting in a persistently or intermittently draining tract. The injury may be sustained by running into, brushing against, kicking at, or stepping on an object, or by becoming entangled in barbed or smooth wire or a rope (Figure 13.1). Horses that jump fences may sustain blunt trauma resulting in an abrasion or a penetrating wound, often on the dorsal surface of the pastern. Blunt trauma to the dorsal surface of the carpus can occasionally cause a substantial hematoma or hygroma (Figure 13.2). Penetrating wounds from jumping injuries often occur just proximal to the coronet on the hindlimb or at the distal end of the antebrachium, and, often, a splinter of wood becomes embedded in soft tissue (Figure 13.3). Horses that become entangled in barbed wire may sustain a serious wound, such as a degloving injury, which is particularly common in the metacarpal or metatarsal region (Figure 13.4), or a laceration that extends into a synovial cavity (Figure 13.5) or through a heel bulb (Figure 13.6). The extensor or flexor tendons may also be injured. For more information regarding injuries to tendons and their sheaths, see Chapter 17. Figure 13.1 A full‐thickness wound to the plantar surface of the pastern caused by a rope. Figure 13.2 A hygroma unresponsive to medical treatment, on the dorsal surface of the carpus. Figure 13.3 A case of persistent drainage from a wound at the coronary band. (a) Splinter of wood being removed. (b) Wood splinter after removal. Figure 13.4 A degloving wound to the left hind limb. The metatarsal bone and the flexor tendons were exposed. Courtesy of Dr. Jon Koella. Figure 13.5 Acute laceration of the palmar carpal region that extended into the carpal canal. Figure 13.6 Chronically infected heel bulb laceration. Wounds involving the distal aspect of the limb are often more problematic than wounds located elsewhere because their close proximity to the ground makes them more likely to become contaminated, and because skin in the distal portion of the limb is less vascular than skin located elsewhere. A wound to the distal aspect of the limb is much more likely to involve a vital structure because the distal aspect of the limb is populated by a high number of synovial structures, ligaments, and tendons. Wounds involving the distal aspect of the limb are often more difficult to suture than are wounds of similar size on the body or proximal aspect of a limb because skin surrounding a wound on the distal aspect of the limb is more difficult to mobilize. Sutured wounds in this area are also more likely to dehisce. Second‐intention healing of a wound on the distal aspect of the limb proceeds more slowly than it does in a wound proximal to the carpus and tarsus, because an unsutured wound in the distal aspect of the limb expands more after injury and has a longer preparatory phase of healing and a slower rate and earlier cessation of wound contraction.2 Wounds of the distal aspect of the limb increase in size during the first 11–16 days after trauma, whereas those on the body change little in size.2–4 Although one study showed that wounds of the distal aspect of the limb of horses epithelialized more slowly than did wounds on the thorax or flank,2 an unpublished study demonstrated little difference between the rate of epithelialization of experimentally created, full‐thickness, 7–9 cm2 wounds on the distal aspect of the limb and the rate of epithelialization of similar wounds on the buttocks.5 In that study, epithelialization progressed over metatarsal wounds at a rate of 0.48 mm/week and over wounds on the buttocks at a rate of 0.62 mm/week between the 3rd and the 7th weeks of healing. The greatest difference in the rate of healing of wounds involving the body and those involving the distal limb is the greater contribution of contraction to the healing of wounds on the body.6 Wounds that heal by contraction produce a more cosmetic outcome than those that heal by epithelialization because, with contraction, all layers of skin, including cutaneous adnexa (i.e., hair follicles, sebaceous and sweat glands), are carried centripetally (i.e., toward the center). The final appearance of wounds left to heal by second intention on the body or proximal aspect of the limbs is, therefore, much more cosmetic than the appearance of those of similar size on the distal aspect of a limb (Figure 13.7). Figure 13.7 Wounds that heal to a large degree by epithelialization are less cosmetic than wounds that heal primarily by contraction because the epithelial scar is devoid of adnexa. Horses have a greater propensity to develop EGT within a wound than do other domesticated animals, and a wound healing by second intention on the distal aspect of the limb is much more likely to develop EGT than is a similar wound proximal to the carpus and tarsus. Other factors, besides location, that promote exuberant production of granulation tissue in wounds of horses include chronic inflammation, motion, bandages, and casts.7,8 More information regarding treatment for EGT can be found in Chapter 15. A wound to the distal aspect of the limb should be meticulously examined after it has been properly prepared, as outlined in Chapter 4, so that damage to a vital structure is not overlooked. A wound that appears to be relatively innocuous (Figure 13.8) may be accompanied by damage to one or more vital structures deep within it, or it may contain a foreign body. Horses with an acute injury to the distal aspect of the limb involving a synovial structure often show little or no lameness, giving the owner and the attending veterinarian a false sense of security about the horse’s prognosis for uncomplicated recovery. Lameness becomes apparent when intrasynovial pressure and concentration of inflammatory products increase. Ultrasonographic examination of the wounded region may provide the first evidence of synovial sepsis and/or identify a foreign body (see Figure 12.25c). Early ultrasonographic evidence of sepsis includes the presence of excess fluid, fibrin, or cells within the synovial fluid (Figure 13.9). Ultrasound may also be useful to guide collection of synovial fluid for cytologic analyses and for bacterial culture. Air introduced into the wound from breach of tissue, however, may limit the effectiveness of ultrasonographic examination if the wound is acute. Generally, primary closure is indicated for an acute, clean wound in which a vital structure has not been penetrated. For contaminated or contused wounds, delayed primary closure or delayed secondary closure is often selected. The decision to close the wound primarily or to delay closure – primary or secondary – or to allow it to heal by second intention can be made only after considering many factors (see Chapter 8 for more information regarding approaches to wound closure and Chapter 10 for more information regarding reconstructive surgical techniques, such as skin mobilization). Figure 13.8 A harmless appearing wound over the tuber calcis that entered the calcaneal bursa, resulting in sepsis of that synovial structure. Courtesy of Dr. Blake Everett. Figure 13.9 This ultrasonographic image of a tarsocrural joint shows evidence of sepsis, including the presence of fibrin and cells within the synovial fluid. The asterisks lie on the talus, and the arrow points to fibrin within the tarsocrural joint. Heavily contaminated or infected wounds and those that are so large that they cannot be closed with sutures must heal by second intention. Without proper care, however, these wounds often develop EGT, leading to an unsightly epithelial scar that is susceptible to re‐injury (Figure 13.10) or to a fibrous granuloma (see Figure 15.12). The reader is referred to Chapter 15 for discussion of EGT. See Chapter 18 if the wound is so large that skin grafting must be considered. Figure 13.10 A wound that healed with an unsightly epithelial scar is susceptible to re‐injury. (a) A rectangular‐shaped wound over the dorsal fetlock. (b) Healing after 75 days resulted in an unsightly epithelial scar susceptible to re‐injury. For purposes of discussion, this chapter has been separated into wounds (both open and closed) located from the fetlock to the carpus or tarsus, wounds of the pastern, avulsion injuries to the hoof capsule, and penetrating wounds to the hoof. A wound to a synovial cavity and treatment of horses with an avulsion injury accompanied by exposed bone and the presence of bone sequestra are not discussed in depth in this chapter. For more information, see Chapter 16, which covers treatment of horses with wounds involving synovial structures, and Chapter 14, which covers degloving injuries and bone sequestra. Wounds may be categorized as open (i.e., full thickness) or closed (i.e., partial thickness). Open wounds are those in which the entire thickness of the skin has been separated. They include incisions, which are wounds produced by a sharp object, either intentionally with a scalpel or accidentally by glass or sheet metal, for example. The skin edge of an incision is cut cleanly, resulting in little damage to underlying tissue. The victim experiences little pain. Lacerations are the most common type of open wound and are characterized by an irregular cutaneous margin and extensive damage to underlying tissue. They are accompanied by bruising, which results in considerable pain. A laceration accompanied by loss of tissue is termed an avulsion. A puncture is another type of open wound and is produced by a sharp object that perforates tissue. The perforating object may carry dirt, manure, or other debris into the depths of the wound and may enter a synovial cavity. Puncture wounds are easily trivialized because their size belies their severity. Closed wounds are those that do not involve the entire thickness of skin; they include abrasions and contusions. An abrasion is an injury to the superficial layers of the skin caused by friction and is characterized by oozing of serum and only a small amount of hemorrhage. Exposure of many nerve endings results in considerable pain to the victim. A rope burn is an example of an abrasion. A contusion is a closed wound characterized by bleeding and destruction of tissue within and beneath undivided skin (see Figure 10.27). Wounds involving the fetlock, metacarpus or metatarsus, and carpus or tarsus, are commonly inflicted by barbed wire or other metal objects. The hindlimb appears to be more susceptible than the forelimb to wounding in these regions. The soft tissues of the dorsal surface of the carpus are particularly prone to contusions and punctures, which are often incurred when the carpus strikes a fence or stall door, and these closed injuries and punctures often involve one of the many synovial structures found in the carpal region (see Figure 16.6a). Occasionally, a wire laceration or a puncture to the palmar surface of the antebrachium or carpus enters the synovial sheath of the carpal canal (Figure 13.5). The most common type of closed injury to the distal portion of the limb is the carpal hygroma, which appears as a fluid‐filled swelling over the dorsal surface of the carpus. Its usual cause is blunt force to the dorsum of the carpus or antebrachium, such as that which might occur when the horse jumps a fence or hits the stall door with a forelimb. A carpal hygroma forms from accumulation of fluid in an adventitious bursa in the subcutaneous tissue, usually from a subcutaneous hematoma that develops into a seroma.9 A hygroma often resembles an abnormal accumulation of fluid in the tendon sheath of an extensor muscle, making the two conditions sometimes difficult to differentiate. A carpal hygroma appears as a cyst‐like, spherical, fluctuant swelling containing serous fluid (Figure 13.2). Its spherical appearance helps distinguish a hygroma from a longitudinally oriented, distended sheath of the tendon of an extensor muscle (i.e., the extensor carpi radialis muscle or the common digital extensor muscle) and from a horizontally oriented, distended carpal joint (i.e., the antebrachiocarpal joint). Carpal flexion may be restricted if the hygroma is large. Palpation of the swelling does not usually cause the horse to show signs of discomfort, unless the hygroma is infected, and usually the horse is lame only if the hygroma interferes mechanically with the gait or is infected. Although a carpal hygroma can typically be differentiated from a swollen sheath of an extensor tendon or a distended carpal joint by inspection and palpation, ultrasonographic examination or contrast radiographic examination of the hygroma provides definitive diagnosis. The affected horse can usually be treated successfully by centesis and drainage of the hygroma, application of a pressure bandage, and confinement. A corticosteroid injected into the cavity of the hygroma at the time of drainage may be helpful in preventing reformation of fluid. If the hygroma fails to resolve in this manner, it can be drained through a distally located stab incision, through which the lining of the cavity is removed, by using a large curette, with the horse standing, or by using an arthroscopically guided, motorized synovial resector, with the horse anesthetized. A Penrose drain inserted through the stab incision ensures that it remains open until production of fluid has subsided (Figure 13.11a). The limb is bandaged and held in extension with a splint for several weeks (Figure 13.11b). If the hygroma fails to resolve with these treatments, the capsule can be excised with the horse anesthetized. After surgery, the limb is held in extension with a cast or splint for several weeks, after which a pressure bandage is applied for an additional 1–2 weeks. The horse can begin walking exercise after about 1 month. Figure 13.11 Same horse as in Figure 13.2, at surgery. (a) A Penrose drain is inserted through the stab incision at the distal extent of the hygroma. The drain ensures that the incision remains open until production of fluid has subsided, usually within 7–10 days. (b) The limb is bandaged and held in extension with a splint for several weeks. If the horse is chronically affected, treatment may not restore full range of carpal motion due to the presence of fibrosis. The owner should be warned that surgical treatment may worsen the condition if healing is disturbed by motion or infection. The carpus, tarsus, metacarpus, or metatarsus may suffer a degloving injury, which is a type of injury in which an extensive segment of skin and underlying tissue is lacerated, thereby exposing bone and often tendons. A large portion of the lacerated segment of skin is often devoid of vascular supply. The injury may result from a circumferential laceration accompanied by detachment of skin from the limb (Figure 13.4) or from an avulsion that is accompanied by such an extensive loss of tissue that the wound cannot heal by second intention alone (see Figure 5.4). Contraction of large wounds over the flexor surface of a joint often results in contracture of the associated joint and distortion of the limb in people, dogs, and cats.10,11 Wound contraction in horses, however, does not cause contracture and should be considered beneficial because it accelerates wound healing and decreases the size of the epithelial scar. Wounds healed by contraction are more cosmetic than those healed by epithelialization. Wounds to the carpus, metacarpus, tarsus, or metatarsus frequently involve one or more of the many tendons, tendon sheaths, bursae, or joint capsules found in these regions (Figures 13.5, 13.8, 13.12). Breach of a synovial structure can be confirmed by injecting sterile fluid into the synovial cavity suspected of being penetrated, at a site remote from the wound, and observing it egress from the wound (see Figure 4.4). Synovial fluid aspirated from a synovial structure suspected of being penetrated can be analyzed to determine if the synovial structure is infected. Bacteria are not commonly seen during cytologic examination of synovial fluid from an infected synovial structure, and consequently, infection is usually determined by identifying an elevated concentration of neutrophils and protein in fluid aspirated from the cavity. Bacterial infection of a synovial structure can be definitively diagnosed by identifying bacteria within the cells present in synovial fluid, by bacterial culture, or by polymerase chain reaction (PCR).12 For more information, see Chapter 16, which discusses lacerations associated with synovial structures. Figure 13.12 This horse suffered a puncture wound to the tendon sheath of the extensor carpi radialis muscle. (a) Appearance of the carpus and antebrachium. (b) Penetration of this synovial structure was confirmed by instilling a radiocontrast medium into the tendon sheath. Courtesy of Dr. Albert Sole. Injury to the calcaneus and associated soft‐tissue structures is common because the calcaneus is prominent and has little soft‐tissue protection. Injury to the point of the hock, or calcaneal tuber, is frequently associated with lameness, swelling of surrounding soft tissues, periosteal new bone production and lysis of the calcaneal tuber, and infection of the calcaneal bursa (Figure 16.6b).13 Injury to the distomedial enlargement of the calcaneus, or sustentaculum tali, is frequently associated with lameness, swelling of surrounding soft tissues, periosteal new bone production and lysis of the sustentaculum tali, effusion of the tarsal sheath, and damage to the deep digital flexor tendon (sometimes referred to as the lateral digital flexor tendon, which is the principal tendon of the deep digital flexor muscle at this level).14,15 The tendon of the medial digital flexor muscle (medial head of the deep digital flexor muscle, sometime called the long digital flexor muscle) passes plantar to the medial collateral ligament joining the deep digital flexor tendon distal to the tarsometatarsal joint. A flexed, proximoplantar‐to‐distoplantar tangential (skyline) radiographic projection of the calcaneus is helpful in identifying lesions of the calcaneal tuber and sustentaculum tali. Sepsis of the calcaneal bursa or tarsal sheath is determined by analyzing synovial fluid obtained by centesis, and ultrasonographic examination is used to identify damage to the deep digital flexor tendon. The extensor tendons are frequently involved in lacerations located in the carpal, tarsal, metacarpal, or metatarsal regions because they are located superficially in these regions. When the tendon of the extensor carpi radialis muscle is lacerated, the horse is still able to extend the distal aspect of the limb owing to the action of the common and lateral digital extensor muscles, but the carpus overflexes because resistance to the action of the flexor muscles is diminished.16 Transection of the tendon of the common digital extensor muscle above or below the carpus seems to have no long‐lasting detrimental effect on gait because the actions of the extensor carpi radialis and lateral digital extensor muscles and the extensor branches of the suspensory ligament compensate for the loss of action of this muscle.17 Transection or rupture of the peroneus tertius tendon on the dorsal surface of the hock allows this joint to move independently of the stifle as a result of disruption of the reciprocal apparatus (Figure 13.13). Figure 13.13 This horse sustained an avulsion injury to the dorsal surface of the hock that was complicated by laceration of the tendon of the peroneus tertius. Note that the hock is extended while the stifle is flexed. Laceration of an extensor tendon is often associated with a large avulsion wound on the dorsal surface of the metacarpus/metatarsus (Figure 13.4, Figure 13.14).16 Extensor tendons of the hindlimb are lacerated far more frequently than are those of the forelimb,18 and the tendon of the long digital extensor muscle is lacerated more frequently than is the tendon of the lateral digital extensor muscle, although, frequently, both tendons are lacerated. Because the primary function of the extensor tendons of the distal aspect of the limb is to extend the digit during locomotion, rather than to support weight, laceration of one or more of the extensor tendons does not alter the conformation of the limb but may result instead in an inability of the horse to extend the toe, causing the fetlock to knuckle during locomotion. Figure 13.14 Large, degloving wounds on the dorsal surface of the metacarpus/metatarsus often are associated with laceration of the long and lateral digital extensor tendons. Courtesy of Dr. Michael Caruso. In contrast, the conformation of the limb is altered when a flexor tendon of the distal aspect of the limb is severed. The digit hyperextends when one or both flexor tendons have been severed. If only the superficial digital flexor tendon has been severed, the metacarpo‐/metatarsophalangeal joint hyperextends, causing the fetlock to drop when weight is borne on the limb. Severance of the deep digital flexor tendon, coupled with severance of the superficial digital flexor tendon, causes the distal interphalangeal joint to also hyperextend, causing the toe to elevate upon weight bearing. If both flexor tendons and the suspensory ligament have been severed, the palmar/plantar surface of the fetlock contacts the ground when the horse bears weight on the limb, and the solar surface of the foot fails to contact the ground. Whereas laceration of an extensor tendon is often associated with a large wound, laceration of a flexor tendon is typically associated with a small wound. The cutaneous wound may be slightly remote from the tendinous wound if the laceration occurred while the limb was flexed. Ultrasonographic examination of the flexor tendons may be necessary to detect partial disruption of a digital flexor tendon because partial disruption results in no postural changes. See Chapter 17 for more information about diagnosis and treatment of horses with lacerations associated with tendons, paratenons, and tendon sheaths. A laceration to the dorsal, lateral, or medial surface of the metacarpus/metatarsus often exposes a large area of bone, which is often devoid of periosteum (Figure 13.15). The extreme thickness of the dorsal portion of the cortex of the diaphysis of the third metacarpal/metatarsal bone may predispose the outer third of this bone to develop a sequestrum, because trauma may deprive the bone of its periosteal blood supply leaving it dependent on medullary vessels traversing the cortex. Figure 13.15 Laceration to the lateral surface of the metacarpus exposed a large area of bone. The upper layer of bone devoid of periosteum is ischemic and is likely to form a sequestrum. Occasionally, a foreign body becomes embedded within the soft tissues on the palmar or plantar surface of the metacarpal or metatarsal region. Although a foreign object may sometimes be identified by inserting a probe into the wound’s tract, ultrasonographic examination or plain or contrast radiographic examination of the affected region is usually required to identify the foreign object (Figure 4.3). Sutured wounds of the carpus, tarsus, metacarpus, metatarsus, and fetlock are prone to dehisce because wounds in these regions rapidly become contaminated due to their proximity to the ground and because their blood supply is often attenuated from the tension exerted on the skin when closing the wound. Wounds of the fetlock, carpus, and tarsus are especially prone to dehisce because of tension caused by motion upon joint flexion/extension. The risk of dehiscence caused by infection can be diminished by properly preparing the wound or by using delayed primary or delayed secondary closure. See Chapters 4 and 8 for detailed accounts of techniques used to prepare wounds for suturing. Wounds of the carpus, tarsus, metacarpus, metatarsus, and fetlock are often difficult to suture without tension because tissues in these regions are difficult to mobilize and because these regions are highly mobile or contain highly mobile structures (e.g., tendons). Dehiscence of wounds sutured under tension and excessive expansion of unsutured wounds can best be prevented by immobilizing the distal portion of the limb with a cast or with a splint applied over a bandage. Immobilizing the distal portion of a limb with a distal limb cast or a splint may be helpful in preventing dehiscence of a sutured wound in a mobile region, such as the fetlock, especially if the wound is perpendicular or oblique to the limb’s long axis. Enclosing a lacerated tarsus in a cast is difficult because of the reciprocal apparatus of the hindlimb, but application of a full‐limb cast or a bandage and splint to a hind imb may sometimes be indicated. For more information regarding techniques for casting and bandage splinting, see Chapter 7. The length of time during which the distal aspect of the limb is immobilized depends on the amount of tension exerted on the wound during closure. Wounds with a good blood supply that are sutured with minimal tension are immobilized for only 10–14 days, whereas wounds sutured under great tension or those with a blood supply marginal enough to delay healing should be immobilized for 17–21 days. A limb with a wound left to heal by second intention may require a considerably longer period of immobilization (e.g., several months), especially if an extensor or flexor tendon has been lacerated. Wounds closed under tension, especially those that have been excessively undermined, may dehisce because stretching the skin beyond its limits of maximal extensibility may obstruct blood flow through the dermal blood vessels, causing the sutured margin of the wound to necrose.19 Techniques most commonly used to avoid excessive tension on the sutured laceration include: undermining of the skin adjacent to the wound; presuturing the wound; applying tension sutures, such as walking sutures or horizontal mattress, vertical mattress, or far–near tension sutures; and creating relaxing incisions.20–23 For information regarding techniques used to relieve skin tension, see Chapter 10. If the wound cannot be closed completely, as much of the wound as possible should be sutured to prevent the margin of the wound from retracting. Preventing retraction improves the cosmetic outcome and speeds healing. Treatment of injury to the calcaneus and associated soft‐tissue structures may entail excising infected bone, endoscopically irrigating the tarsal sheath or calcaneal bursa, and administering antimicrobial therapy systemically and/or by regional limb perfusion. Excising the portion of the deep digital flexor tendon within the tarsal sheath may be necessary to resolve infection of the tarsal sheath if this tendon is infected.14 The reader is referred to Chapter 16 for more information regarding treatment of infected synovial cavities. Movement of an intact or severed or partially severed flexor or extensor tendon within a wound on a metacarpus or metatarsus often results in the formation of two separate granulation beds, one on the tendon and the other on tissue surrounding the tendon (see Figure 17.18). Granulation tissue provides myofibroblasts that normally span the wound, causing it to contract, but the presence of two separate granulation beds prevents myofibroblasts from spanning the wound. When movement of the tendon is restricted by immobilizing the limb, the granulation beds rapidly become confluent, resulting in prompt contraction of the wound. Horses with an infected wound containing a foreign body or a bone sequestrum are usually presented with an intermittently or persistently draining tract within the wound. The wound fails to heal until the foreign object or sequestrum is removed.24 A foreign object or sequestrum provides a nidus for bacterial growth, and its presence can cause infection when only a small quantity of bacteria contaminates the wound. For more information on the management of infected wounds, see Chapter 19. Treatment for a bone sequestrum is discussed in Chapter 14. A degloving wound can be managed by primary closure or by delayed primary or secondary closure when no skin has been lost, provided that the lacerated skin has sufficient blood supply to sustain its survival. Often, a degloving wound has such an extensive loss of skin that the wound can be healed only by second intention or by applying a free skin graft to the wound (Figure 13.15). The reader is referred to Chapter 18 for more information about skin grafting. A degloving injury may, rarely, be accompanied by vascular damage to the limb so severe that the hoof capsule loses its laminar attachments (Figure 13.16). See Chapter 14 for more information about the treatment of degloving injuries Figure 13.16 (a,b) This horse suffered a degloving injury to the medial and dorsal surface of the metatarsus that so severely impaired the vascular supply to the foot that the hoof capsule was lost. The horse is in dorsal recumbency on a surgery table. Courtesy of Dr. Gal Kelmer. When treated properly, a horse with a wound of the fetlock, metacarpus, metatarsus, carpus, or tarsus that does not extend into a supporting structure or synovial cavity has a good prognosis for an excellent cosmetic outcome and soundness. Treatment is difficult when a synovial cavity, tendon, or bone becomes infected, and the outcome is often disappointing. Horses that have incurred damage to a synovial structure have a guarded prognosis for return to soundness.25,26 The nature of the injury affects outcome. For example, cutaneous incisions, whether created accidentally or purposely, result in minimal trauma and negligible contamination. Lacerations caused by impact, whether clean or heavily contaminated, are more susceptible to infection than are incisional wounds, and they tend to heal poorly because of injury to the blood supply. Sutured wounds parallel to the long axis of the limb are more likely to heal without complication than are sutured wounds transverse to the long axis of the limb. Sutured wounds aligned obliquely or transversely to the long axis of the limb are best protected from distractive forces by immobilizing the injured region in a bandage splint or a cast and commonly dehisce when protected by a bandage alone. A deep wound to the dorsal surface of the metacarpal or metatarsal region is not usually career ending, even if extensor tendons are injured, because extensor function is usually restored if the distal portion of the affected limb is immobilized for 4–6 weeks with a cast or splint. Damage to the tendon of the lateral digital extensor muscle of the hindlimb, however, sometimes causes the horse to develop stringhalt in the affected limb when the tendon becomes immobile from fibrous adhesions that develop during healing. Because extensor tendons do not support weight, a decrease in their mechanical strength does not adversely affect the outcome of the horse. A wound to the palmar/plantar surface of the metacarpal/metatarsal region, however, is frequently career ending or even life threatening if one or both flexor tendons, suspensory ligament, or digital flexor tendon sheath are lacerated. In one study, 75% of horses with wounds of the hock involving only the calcaneal bursa survived, but if the calcaneal tuber of the calcaneus was also infected, only 44% of survived.27 Another study examining the treatment and outcome of 10 horses with a wound of the hock resulting in infection of the sustentaculum tali of the calcaneus, with or without infection of the tarsal sheath, found that of nine horses for which long‐term follow‐up was available, seven became sound.14 The authors of that report concluded that horses with osteomyelitis of the sustentaculum tali, with or without concomitant infection of the tarsal sheath, can have an excellent to good outcome and may return to their previous use after surgical debridement of affected tissues and lavage of the tarsal sheath. They also observed that resection of the deep (lateral) digital flexor tendon, however, should be regarded as a salvage procedure, because athletic use after this procedure is unpredictable. Infected wounds containing a foreign body or an osseous sequestrum generally heal without complication after the object or sequestrum has been removed. Antimicrobial therapy administered parenterally may temporarily decrease swelling and discharge from the wound,28 but antimicrobial therapy alone is an ineffective treatment for a horse with a foreign object or sequestrum embedded in a wound, because drugs fail to contact bacteria harbored by the foreign object or sequestrum. Organic foreign objects, such as pieces of wood, have a greater tendency to potentiate infection and provoke a greater inflammatory response, resulting in more discharge of exudate, than do non‐organic foreign objects, such as pieces of glass or metal. Bacterial culture of exudate from a draining sinus is not helpful in determining the organism associated with the foreign object because secondary pathogens rapidly colonize a draining sinus. An avulsion injury to the carpus is particularly devastating because the success of sheet grafting a wound over the dorsum of the carpus is often compromised due to the movement the graft must withstand in this region (Figure 13.17). The carpus must be completely immobilized in a full‐limb or sleeve cast for the graft to be accepted. After the graft has been accepted and after the limb is no longer immobilized, the graft and granulation tissue beneath the graft may split transversely along the dorsal surface of the antebrachiocarpal and middle carpal joints, sometimes opening the sheath of the tendon of the extensor carpi radialis muscle. Figure 13.17 This horse suffered an extensive avulsion injury to the carpus. (a) Granulation tissue fills the wound. (b) The wound was covered with a free, split‐thickness, meshed skin graft. A large portion of the graft was accepted. The pastern is particularly susceptible to trauma because of its proximity to the ground. Lacerations of the pastern are commonly inflicted by barbed wire or other metal objects (Figure 13.6; see Figure 4.2a), and occasionally, the pastern is injured when it becomes entrapped between immovable objects, such as the rails of a cattle guard or a wall and stall door. An injury caused by entrapment of the pastern is often accompanied by vascular trauma leading to ischemic necrosis of soft tissue and infection. The longer the horse is entrapped and the more it struggles to free itself, the greater the vascular injury. Occasionally, a rope encircling the pastern causes a rope burn severe enough to result in loss of a partial‐ or full‐thickness portion of skin (Figure 13.1). The coronary band of a hindlimb sometimes incurs a penetrating wound when the horse jumps a barrier, and often a splinter of wood is embedded within the wound (Figure 13.3). Lacerations of the pastern frequently involve the coronary band and variable portions of the hoof capsule, as well as structures deep to the hoof wall. Lacerations to the coronary band may result in permanent defects in the hoof wall (see the section on laceration and avulsion wounds of the hoof capsule later in this chapter). A wound on the pastern quickly becomes contaminated with manure and dirt because the pastern is close to the ground. A laceration incurred while the horse is at pasture may go unrecognized for days because it is hidden from view by grass. The heel bulb is the region of the pastern most susceptible to injury. A heel bulb laceration, especially one caused by barbed wire, often extends in an arc from the quarter of the hoof wall to the depression between the heel bulbs (Figure 4.2a); occasionally, the arc extends from one quarter to the other (Figure 13.6). In either case, the wound often gapes when the horse bears weight on the limb. The deeper the laceration, the greater is the likelihood that one or more critical structures in the pastern or foot may be damaged. Lacerations of the pastern that course deep to the hoof wall may involve one or more collateral ligaments, the capsule of the proximal or distal interphalangeal joint, the deep digital flexor tendon and its digital flexor tendon sheath, a collateral cartilage of the distal phalanx, and/or the navicular bone and its bursa and ligaments (Figure 13.18, Figure 13.19; see Figure 16.6c). Occasionally, a laceration of the pastern is accompanied by fracture of the middle or distal phalanx. Figure 13.18 Vascular injury to the pastern that resulted from the limb becoming caught between the rails of a cattle guard. The hoof wall sloughed 2 weeks following the injury. Figure 13.19 An extensive laceration of the pastern that entered the proximal and distal interphalangeal joints, transected the lateral collateral ligaments of both joints, and lacerated the collateral cartilage of the distal phalanx, as well as the coronary band. Rope burns are found most commonly on a pastern, usually that of a hindlimb, and result most commonly from entanglement of the limb in a rope while the horse is picketed. A rope burn is a combination of abrasion and thermal damage caused by friction. A rope burn may be superficial partial‐thickness (first‐degree burn) or deep partial‐thickness (second‐degree burn), or it may extend through all layers of the skin (full‐thickness or third‐degree burn) (Figure 13.1). A horse with a pastern wound is frequently lame, but the degree of lameness depends on the duration of the injury and the structures involved and whether or not the wound has become infected. Generally, the deeper the laceration, the greater the lameness, but lameness may be attenuated if a digital nerve has been severed. Even if a vital structure is not involved, the horse may be reluctant to fully bear weight on the injured limb, especially if the wound is infected. Laceration of the digital artery and vein causes severe hemorrhage, sometimes resulting in cardiovascular shock. To avoid exacerbating hypotension caused by severe hemorrhage, the horse’s cardiovascular status should be assessed prior to administering a sedative or tranquilizer to facilitate examination. Administration of a phenothiazine‐derivative tranquilizer, in particular, should be avoided because it may result in severe hypotension if the horse is hypovolemic. Hemorrhage from the digital vessels can usually be controlled by applying a pressure bandage for 1 or 2 hours, but if the wound requires immediate examination, the digital vessels are best ligated. Anesthetizing the ipsilateral palmar/plantar digital nerve at the level of the proximal sesamoid bones may facilitate ligation by preventing pain caused by inadvertently disturbing the digital nerve, which courses next to the digital vessels. The blood supply to the foot is maintained if one of the arteries remains intact because of the terminal arch of the digital arteries.29 Damage to deep structures may be difficult to recognize visually, and, if suspected, the wound should be carefully palpated. To discriminatively inspect an acute laceration, the wound should be carefully palpated after the hair has been clipped and the wound irrigated with an antiseptic solution. A dilute solution of povidone–iodine [0.1% (1 mL of the 10% concentrate/L)] may be safer than a dilute solution of chlorhexidine diacetate [0.05% (25 mL of the 2% concentrate/975 mL]) for irrigation of any laceration that might involve a synovial structure because a solution of chlorhexidine diacetate can damage synovial structures. Indeed, one study examining the effects of lavaging the tarsocrural joint of horses with a solution of chlorhexidine diacetate found that a 0.05% solution caused synovial ulceration, inflammation, and deposition of fibrin.30 Devitalized tissue should be excised, and gross contaminants removed using irrigation/cleansing. After donning sterile gloves, the depths of the wound are palpated to detect damage to deeper structures. Discerning damage to structures deep in the wound using digital palpation may be difficult if granulation tissue has already formed. For a more complete discussion of the approaches used for examining wounds, see Chapter 4. Laxity and instability of the middle or distal interphalangeal joint indicate disruption of a collateral ligament, and a sucking noise may indicate that the proximal or distal interphalangeal joint has been opened. A puncture wound of the coronary band may, deceptively, appear unimportant, but it should be inspected closely for the presence of wood splinters (Figure 13.3). A laceration or puncture to the dorsal aspect of the coronary band may involve the dorsal pouch of the distal interphalangeal joint, which extends proximad to the hoof capsule on the dorsal surface of the pastern (Figure 13.20). Figure 13.20 Wound to the coronary band that extended into the dorsal proximal pouch of the distal interphalangeal joint. The distal interphalangeal joint is the synovial structure most commonly involved in lacerations of the heel bulbs.25 If the laceration extends completely through the collateral cartilage of the distal phalanx, the distal interphalangeal joint is likely to be breached because the collateral cartilage adjoins the capsule of this joint (Figure 13.21). If articular cartilage can be palpated or if synovial fluid is seen, penetration of a joint capsule is certain. Discerning penetration of the digital flexor tendon sheath may be more difficult, but if a laceration in the deep digital flexor tendon can be palpated, penetration of the sheath is certain. A laceration over the palmar/plantar surface of the pastern is highly likely to have entered the digital flexor tendon sheath, which lies superficially in this region (Figure 13.22). Figure 13.21 Extensive laceration of the pastern that extended through the collateral cartilage of the distal phalanx into the distal interphalangeal joint. Figure 13.22 This wound over the palmar surface of the pastern entered the digital flexor tendon sheath, which lies superficially in this region. Determining the depth of a partial‐thickness rope burn of the pastern may be difficult because swelling of the remaining skin may confound estimates of tissue loss. Superficial partial‐thickness rope burns epithelialize within 3 weeks, and piliation at the site of injury is good. Deep partial‐thickness rope burns may take months to heal, and the site may remain scarred and hairless. A full‐thickness rope burn of the palmar/plantar surface of the pastern may breach the digital flexor tendon sheath resulting in unremitting lameness and failure of the wound to heal (Figure 13.1). The lacerated region should be examined radiographically to help ascertain the status of the bones and joints and to determine whether a radiopaque foreign body is present (Figure 13.23). The proximity of the laceration to critical structures, such as the navicular apparatus, can be determined by examining radiographs of the pastern and foot taken after inserting a sterile probe into the depths of the wound. If a laceration of a collateral ligament of the distal interphalangeal joint is suspected, the region can be examined radiographically with the collateral ligament of the lacerated side stressed to determine if the bones of the joint are shifted abnormally relative to each other. To obtain a stress radiograph of the distal interphalangeal joint, mediolateral force is applied to the joint as the region is radiographed, to determine if the middle and distal phalanges shift abnormally relative to one another. Force can be applied by standing the horse on a small block of wood placed beneath the side of the hoof contralateral to the wound. Figure 13.23 Radiographic examination of a deep wound to the pastern region might reveal the presence of a foreign body deep within the wound. This barb was not identified by visual or digital inspection of the wound. A contrast study may be helpful if penetration of a synovial structure is suspected but cannot be confirmed by digital palpation. Air in the wound inhibits accurate ultrasonograph examination of the injured pastern, but, occasionally, ultrasonography may be valuable in detecting damage to ligaments or tendons or the presence of a foreign body. Doppler imaging may assist in determining the integrity of the vascular supply to the region. The structures involved in the laceration can sometimes be appreciated only when the wound is explored and debrided while the horse is anesthetized. Suturing a laceration in the pastern region and applying a distal limb cast (i.e., one that encompasses the foot and extends to the carpus or tarsus) or a phalangeal cast (i.e., foot–pastern, or slipper cast) usually provides the best cosmetic and functional outcomes. An acute laceration may be sutured without delay if the laceration is clean, tissue damage is minimal, and no synovial structure has been penetrated. In this case, systemic administration of an antimicrobial drug is often not necessary, but administration of a non‐steroidal, anti‐inflammatory drug (NSAID) may be indicated to reduce pain and inflammation. Closure of an acute wound and application of a cast should be delayed for 2–3 days after injury if the exposed tissue is grossly contaminated or severely traumatized but should be performed before granulation tissue develops (i.e., delayed primary closure). The horse is best treated by second‐intention healing or by delayed secondary closure of the wound (closure delayed until granulation tissue has formed) if presented after the wound appears infected (Figure 13.6). To prepare a contaminated or infected wound for suturing, hair surrounding the wound is removed with clippers. Sterile, water‐soluble lubricating gel or sterile gauze soaked in physiologic saline solution (authors’ preference) can be inserted into the wound to prevent clipped hair from falling into it. Skin surrounding the wound should be cleansed with an antiseptic scrub or skin cleanser containing surfactant(s), but application of either to the wound itself should be avoided because the detergent, which is a surfactant, is cytotoxic and increases the wound’s susceptibility to infection.31,32 The use of hydrogen peroxide to irrigate the wound should also be avoided because, at its usual 3% concentration, it is cytotoxic and only weakly bactericidal.32 The use of a dilute antiseptic solution [e.g., povidone iodine 0.1–0.2% (authors’ preference)] or chlorhexidine (0.05%) using irrigation delivered at 10–12 psi is preferred. Various commercial wound cleansers can be used as an alternative – see Table 5.1 for more information. The wound is debrided; an antimicrobial solution, cream, or ointment is applied topically to the cleansed/irrigated wound to reduce the concentration of microorganisms within it; and the wound is bandaged. If the wound is infected and exudative or if it is difficult to debride, a debriding dressing may be applied and changed daily for several days – the reader is referred to Chapter 6 for more information regarding wound dressings. The bandage is usually changed daily or every other day to assess healing after application of the debriding dressing is discontinued. When the bandage is changed, the wound and surrounding skin and hoof wall are cleansed, and, if necessary, the wound is again debrided. This treatment is continued until the wound no longer requires debridement and appears healthy enough to heal by delayed primary closure. The horse should receive broad‐spectrum, antimicrobial therapy systemically, but if a synovial structure has been penetrated, antimicrobial therapy should also be delivered intrasynovially or by regional limb perfusion. When a synovial structure has been penetrated, the horse should receive antimicrobial therapy only after a sample of synovial fluid has been obtained for bacterial culture and sensitivity testing of bacterial isolates. The type of antimicrobial drugs administered should be adjusted, if necessary, according to the results of sensitivity testing of bacteria isolated by culture. The duration of systemic administration of antimicrobial therapy is dictated by the horse’s response to therapy. Administration of a NSAID to reduce pain and inflammation is usually indicated. When a synovial structure has been penetrated, closure of the wound and application of a cast should be delayed for several days, even if the wound is clean. Heavily contaminated or badly damaged tissue surrounding the open synovial structure should be debrided, and synovial lavage should be executed through a needle or cannula inserted at a site remote to the wound, using 3–6 L of sterile, isotonic saline solution or a balanced electrolyte solution. Using arthroscopic equipment to lavage an infected synovial structure not only ensures thorough lavage but also allows the structure to be examined directly, aiding in predicting prognosis (see Figure 16.3). After the wound has been debrided and the synovial structure lavaged, the wound is protected under a sterile bandage, and the horse is confined to a dry area. To aid resolution of synovial infection, an appropriate antimicrobial drug should be delivered to the infected tissue in a concentration greater than the minimum inhibitory concentration. Although broad‐spectrum antimicrobial therapy should be administered systemically, vascular injury and thrombosis may limit delivery of drugs to the wound. Regional limb perfusion circumvents these limitations by delivering the antimicrobial drug, under pressure through the venous system, in a high concentration. For more information, see Chapter 16, which covers treatment of horses with a wound involving a synovial structure. The wound is ready for delayed primary closure when it appears clean and contains little exudate, or it is ready for delayed secondary closure when healthy, pink, granulation tissue fills the wound (Figure 13.24a). If the horse remains or becomes severely lame, the wound should be reassessed to determine whether vital structures have been damaged. Provided that the horse is docile and the wound is uncomplicated, the wound can be sutured with the horse sedated after desensitizing the pastern and foot using regional anesthesia, but in most cases, the wound is best sutured with the horse anesthetized. The hoof should be cleansed and trimmed, especially if a cast is to be applied, either before or after the horse is anesthetized. Lightly rasping the hoof wall distal to the wounded region on the pastern reduces surface contamination. After the hoof has been scrubbed with antiseptic soap using a brush, a sterile glove is applied to the foot to protect the wound from contamination, and the wound is isolated with drapes. Applying a tourniquet proximal to the pastern may speed surgery by reducing hemorrhage and improving visibility. Figure 13.24 Same wound as in Figure 13.6, following 5 days of treatment. (a) Healthy, pink, granulation tissue fills the wound, indicating that it is ready for delayed secondary closure. (b) Excessive granulation tissue has been excised from the wound. (c) A combination of vertical mattress tension and simple interrupted sutures were used to close this wound. (d) A distal limb cast. (e) Excellent healing was observed when the cast was removed after 2 weeks. Granulation tissue should be removed before the wound is sutured (Figure 13.24b) because doing so reduces the concentration of contaminating bacteria, exposes the depth of the wound for examination, and enhances mobility of the wound’s margin for suturing. Care should be exercised to avoid damaging the digital vessels and unwounded portions of the coronary region while excising the granulation tissue. The debrided wound is closed with size 0 or 1 sutures of monofilament nylon or polypropylene placed in a vertical mattress pattern (tension‐relieving sutures) after which the skin edges are apposed with 2‐0 monofilament nylon or polypropylene placed in either a simple interrupted or an interrupted vertical mattress pattern (Figure 13.24c). The edges of a wound on the coronary band should be aligned properly to prevent or minimize the development of a defect in the hoof capsule. Sutures placed through the coronary band do not impair formation of the hoof capsule distal to the laceration. Sutures can be placed through the lacerated hoof capsule, if necessary, to stabilize a wound that extends into the capsule. Thinning the wall with a rasp or burr and bandaging the foot with a wet dressing prior to surgery eases placement of sutures through the hoof wall (see the section on laceration and avulsion wounds of the hoof capsule, hereafter). The sutured wound is covered with a sterile, semi‐occlusive dressing, which is maintained in position with sterile, elastic gauze. The distal portion of the limb, including the foot, is then enclosed within a bandage or cast (Figure 13.24d,e). Protecting an unsutured wound in the pastern area with only a bandage results in a substantially longer time for the wound to heal and a poorer cosmetic outcome, especially if the laceration involves a heel bulb, than does immobilization of the foot and pastern within a cast (Figure 13.25). Protecting the wound with a bandage can be more costly than protecting it with a phalangeal cast, considering the quantity of bandage material that must be used over time. To minimize distractive forces on a sutured or unsutured laceration at a heel bulb, a cast that encompasses the foot and extends to the mid‐pastern region is usually sufficient. To decrease distractive forces on a heel bulb wound that extends to the proximal third of the pastern, a cast that encompasses the foot and extends to the carpus or tarsus (i.e., a distal limb cast) may be necessary. Distal limb casts are more likely to cause cast sores than are phalangeal casts, which do not incorporate the fetlock region, a frequent site of sores caused by a distal limb cast. Figure 13.25 Failure to immobilize the distal portion of the limb of a horse that has incurred a heel bulb laceration is likely to result in movement, which in turn leads to production of EGT. The cast enhances healing of a laceration to the pastern by protecting the wound from the environment, limiting movement of the wound’s edges, relieving tension on the wound, and providing a moist environment for epithelialization. The time at which the cast is removed depends on the amount of tension required to close the wound or the deficit of tissue that must heal by second intention. The greater the tension on the sutured wound or the larger the deficit of tissue, the longer the cast should be maintained. Leaving the cast in place for 14–21 days is usually sufficient to facilitate healing of most wounds to the pastern. Treatment of a horse with a recently acquired rope burn involves cleansing the wound, administering local and systemic anti‐inflammatory drug therapy, and bandaging. Hair is clipped from the area in preparation for cleansing/irrigation. Skin surrounding the wound should be cleansed with an antiseptic soap, but application of soap to the wound itself should be avoided for the aforementioned reasons. An ointment can be applied to the wound for its effect as an emollient, and a topical anti‐inflammatory drug, such as dimethyl sulfoxide (DMSO) or diclofenac sodium (1%) cream (SurpassTM, Idexx Laboratories), should be applied liberally to the edematous tissues surrounding the wound (but not to the wound itself). The wounded pastern is protected with a bandage between treatments to prevent the wound from becoming contaminated and desiccated, and damaged by excessive movement. The bandage should extend to the carpus or tarsus to prevent the limb from swelling and to minimize movement of the wound. The horse should receive a NSAID to attenuate pain and decrease swelling. With proper treatment, a horse with a pastern laceration not involving a supporting structure or extending into a synovial structure has a good prognosis for a satisfactory cosmetic outcome and return to soundness. In a study of 101 horses that had incurred a heel bulb laceration, the laceration of 17 horses involved a synovial structure.25 Of the 61 horses for which follow‐up information was available, 94% (48 of 51) of horses, in which the laceration did not involve a synovial structure, were able to return to their intended use. Treatment was more problematic, and the results were poorer, when a synovial structure was involved. Only 30% of horses (3 of 10 for which follow‐up information was available) in which the laceration involved a synovial structure were able to return to their intended use. Contrasting with the results of that study are the results of a more recent study of 49 horses with a wound of the pastern and foot region that were managed with a phalangeal cast.33 In that study, 89.5% of the horses returned to soundness and had a good cosmetic outcome, regardless of whether or not a synovial structure was involved. The prognosis for soundness is poor if the wound is accompanied by subluxation of the distal or proximal interphalangeal joint because osteoarthritis of the affected joint is likely to develop. Infection of a collateral cartilage may result in lameness and development of one or more chronic draining sinuses at the coronary band (i.e., quittor). To prevent infection of a collateral cartilage, discolored portions of cartilage should be excised when the wound is debrided. Formation of a painful neuroma after nerve transection rarely may cause lameness necessitating excision of the neuroma. Healing of a deep, partial‐ or full‐thickness rope burn is usually protracted, and the wound often heals with a blemish that resembles a keloid. The degree of dermal destruction governs the time required for complete healing. The deeper the destruction into the dermis, the fewer the adnexa remaining to provide keratinocytes for epithelialization, and the longer the time for healing. Full‐thickness rope burns to the palmar/plantar surface of the pastern may require skin grafting.
Management of Wounds of the Distal Extremities
Summary
Introduction
Wound categories
Wounds involving the fetlock, metacarpus/metatarsus, and/or carpus/tarsus
Causes
Closed injuries
Diagnosis
Treatment
Prognosis
Open (full‐thickness) injuries
Diagnosis
Treatment
Prognosis
Wounds involving the pastern
Causes
Diagnosis
Physical examination
Imaging
Treatment
Prognosis
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