Second‐intention healing

Wounds on the distal aspect of the limb expand for approximately 11–15 days after wounding as the result of distractive forces applied across the wound during the inflammatory and debridement stages of healing.^5,6 Distractive forces are countered by myofibroblasts formed when granulation tissue develops within the wound. The surface area of wounds healing by second intention is reduced by contraction, brought about by the myofibroblasts in the granulation tissue, and by epithelialization.^7,8 The quicker a granulation bed forms, therefore, the faster the wound begins to contract (Figure 14.1).

Wounds on the distal aspect of the limb heal more slowly than those on the trunk because they have slower rates of epithelialization and contraction, and wounds on the distal aspect of the limb of horses heal more slowly than do wounds on the distal aspect of the limb of ponies because the latter contract more rapidly and to a greater extent than do the former.^8–10

Comparison of histologic analyses of tissue samples from wounds of the distal portion of the limb of horses and of ponies shows that the inflammatory phase of healing is less intense and longer in the wounds of horses than in the wounds of ponies and that myofibroblasts within the wounds of horses are less organized than those within the wounds of ponies.^8,10 The higher concentration of transforming growth factor (TGF)‐β in the wounded tissues of ponies may explain why the wounds of ponies have a more intense inflammatory response and contract to a greater degree than do the wounds of horses, because TGF‐β induces fibroblasts in wounds to differentiate into contractile myofibroblasts.¹¹ The slower rate of epithelialization of wounds of the distal portion of the limb of horses, compared to that of similar wounds of ponies, has been attributed to inhibition of mitotic and migratory activity of keratinocytes by exuberant granulation tissue (EGT), which is much more likely to form in wounds of horses than in wounds of ponies (Figure 14.2).^10,12

Periosteum

Periosteum is a well‐vascularized osteogenic organ possessing two distinct layers, each of which contribute to wound repair. The outer fibrous layer contains fibroblasts, blood vessels, and fibers of Sharpey, and the inner cambium layer contains nerves, capillaries, osteoblasts, and mesenchymal stem cells (MSCs).¹⁷ Periosteal vessels from the fibrous layer contribute to bone healing by nourishing the outer third of the diaphysis and by supplementing the epiphysio‐metaphyseal vessels and the principal nutrient arteries found within the medullary cavity.¹⁸ The cambium layer serves as a reservoir of undifferentiated pluripotent MSCs¹⁹ and as a source of growth factors that play important roles in the healing and remodeling of the outer surface of damaged cortical bone.

Studies have shown that injured periosteum can regenerate cartilage and bone from its progenitor cells;^17,20 in horses, MSCs from the cambium layer were shown to differentiate directly into osteoblasts or into neochondrocytes that produce cartilage, later replaced by bone.²¹

A localized outgrowth of new bone beneath the periosteum, or an exostosis, results from the rapid formation of new or reactive bone, which is produced after progenitors of osteoblasts in the cambium layer are activated by avulsion, laceration, or blunt trauma.²² Depending on its size and the inciting cause, an exostosis may persist or gradually be removed by remodeling.²³ Bone exposed in a wound on the distal portion of the limb (Figure 14.3) can develop an extensive amount of periosteal new bone, which can cause the wound to enlarge, and may result in an enlarged limb, even after the wound has healed.⁸

Formation of bone sequestra

A bone sequestrum may develop as a sequel to a degloving injury that exposes bone and injures the periosteum and is a common cause of delayed healing of wounds on the distal aspect of the limbs of horses.² A sequestrum can result from any insult that interrupts the blood supply to bone. Afferent vessels from the periosteum and medullary cavity provide capillaries that traverse the Haversian canals, which are connected by Volkmann canals, to provide blood to the compact portion of long bones. Outflow, or efferent flow, occurs at the periosteum and endosteum, so periosteal trauma can lead to local vascular stasis by reducing venous outflow.⁴ The blood supply to the cortex of equine long bones is sensitive to trauma because the dense mineralized matrix of the cortex prevents rapid collateralization of vessels after injury.²⁴ Ischemia of the superficial layers of the cortex leads to necrosis of the affected area, and the necrotic bone incites an inflammatory response resulting in accumulation of exudate, which can lead to the formation of a draining tract in a wound that has otherwise healed.³

Periosteum may be avulsed from the bone at the time of injury, or infection or desiccation may cause the periosteum to separate from the bone.^25,26 Loss of periosteal blood supply leaves the bone dependent on endosteal vessels traversing the cortex, resulting in ischemic necrosis of the superficial portion of the cortex.^26,27 Superficial layers of cortical bone devoid of periosteum and exposed to the environment may become desiccated, compounding the effects of ischemia. Small areas of dead bone may be revascularized by microvessels in granulation tissue advancing superficial to the dead bone, or the bone may be revascularized by endosteal vessels of cortical bone deep to the dead bone.²⁸ Avascular cortical bone too thick to revascularize becomes sequestered by granulation tissue produced from viable bone. The body’s attempts to extrude or absorb the dead bone result in a persistent sinus tract and excessive production of exudate. Ischemia caused by loss of periosteal blood supply alone, however, may not be sufficient to provoke formation of a sequestrum;²⁶ bacterial infection of the bone seems to be necessary as well, and ischemia of the bone induced by loss of the periosteum provides an ideal environment for colonization and multiplication of bacteria introduced into the wound from the injury. A sequestrum can delay healing by serving as a focus of continued inflammation and infection, thereby postponing the ensuing phases of repair.

Because the periosteum of young horses plays a greater role in cortical circulation, young horses may be more likely to form a sequestrum than are mature horses.^2,29 Moreover, mature horses are more likely than mature ponies to develop a sequestrum, perhaps because they are more likely to develop infection at the wound, a result of an inefficient inflammatory response and a reduced rate of formation of granulation tissue, and may have a slower onset and longer duration of the periosteal reaction.^8,30 In a retrospective study of 89 ponies and 422 horses with accidental wounds that had been sutured, ponies were found to have suffered less dehiscence and formed fewer sequestra than horses, even though conditions for first‐intention healing of the ponies were less favorable (i.e., wounds were more severe, surgical debridement was less thorough, and systemic antibiotic therapy was administered less commonly).³⁰

Radiographically, a sequestrum appears as a sclerotic segment of bone surrounded by a radiolucent zone of osteolysis (Figure 14.4).^26,27 The sequestrum and zone of osteolysis are surrounded by an envelope of sclerotic bone, termed an involucrum. The opening in the involucrum, through which exudate drains, is termed a cloaca. Extensive periosteal reaction is frequently observed on adjacent, normal bone. A triangular area of new bone that develops adjacent to the cortex from elevation of the periosteum (“Codman triangle”) is sometimes observed at the proximal and distal extents of the involucrum.²⁶ The bone sequestrum contains no new periosteal bone, which aids in its identification.

Exposed bone beneath a fresh wound should be examined radiographically about 10–14 days after injury to determine if a bone sequestrum is developing. Radiographic signs of a sequestrum cannot be detected for at least 7 days after injury but are usually evident within several weeks of injury (Figure 14.4).^26,27 An early radiographic sign that a sequestrum is forming, often seen 10–12 days after injury, is the presence of one or more radiolucent lines within the outer third of the cortex.^26,27 During subsequent radiographic examination, these fine radiolucent lines may be seen to have enlarged and coalesced into one radiolucent band separating viable and non‐viable bone.²⁶ These early radiographic signs of formation of a sequestration sometimes vanish, and the sequestrum fails to form.

Methods used to stimulate the production of granulation tissue to cover exposed bone

Large degloving injuries with exposed bone must granulate to heal. Without granulation tissue to cover the exposed bone, the wound cannot contract, epithelium cannot migrate, and a free skin graft cannot be accepted. Therefore, formation of granulation tissue in the wound should be encouraged, at least initially. Granulation tissue plays an important role in second‐intention healing by shielding underlying tissues from infection and trauma and by providing a moist surface for epithelialization. The delay in healing that occurs when bone is exposed has prompted searches for effective methods to promote coverage of exposed bone with granulation tissue, in people and in companion animals. Even though trauma to the distal aspect of the limb of horses is frequently associated with the presence of exposed bone, methods of stimulating coverage of exposed bone of horses have been poorly investigated. Exposed bone of horses, humans, and dogs has been fenestrated, curetted, or abraded with a file to promote formation of granulation tissue to enhance second‐intention healing or to provide a vascular bed for skin grafting.^7,31,32

Trauma, thermal injury, or oncologic surgery of the head of people often results in exposed bone of the cranium.^31,32 In these cases, to hasten coverage of exposed bone with granulation tissue, the outer cortex of the exposed portion of the cranium is often fenestrated with a drill, burr, or laser to expose the medullary cavity from which granulation tissue can emerge.^31,32 Likewise, the exposed cortex of long bones of people has been fenestrated with a drill to promote formation of granulation tissue.³¹ The fenestrations may promote healing by allowing osteogenic factors from the medullary cavity access to the wound, or the fenestrations may enhance healing of bone and soft tissues by a non‐specific response known as “the regional acceleratory phenomenon,” whereby the rate of remodeling in the region of a bony defect exceeds normal.³³ Cortical fenestration (Figure 14.5), especially when combined with hydrogel dressings to provide moisture to the wound, may accelerate coverage of exposed bone of horses with granulation tissue.⁷

In one study, drilling 1.6‐mm diameter holes through the cortex of the exposed second metacarpal bone in experimentally created wounds of dogs produced a greater amount of blood clot than did curetting the bone. Drilling the second metacarpal bone resulted in clots that protected the exposed bone from desiccation, and the early ingrowth of fibroblasts and capillaries from the surface of the bone and surrounding tissue into the clot sped formation of granulation tissue.³⁴

Exposed bone in experimentally created wounds on the distal aspect of the limb of horses became covered with granulation tissue earlier after undergoing cortical fenestration with a 3.2‐mm drill bit than did exposed bone in control wounds that did not undergo cortical fenestration because granulation tissue formed at the sites of cortical fenestration (Figure 14.6).⁷ When the area of exposed bone was small (i.e., less than 6 × 6 cm), the contribution of the granulation tissue growing directly from the sites of cortical fenestration to overall coverage of the wound was not significant.⁷ Cortical fenestration of exposed bone in wounds may be even more beneficial if fenestration is combined with other methods of promoting formation of granulation tissue.⁷

Autologous platelet‐rich plasma (PRP) has been used with increasing frequency to treat horses for a variety of soft‐tissue and bony defects. Application of PRP gel to experimentally created wounds of horses accelerated the formation of granulation tissue and epithelial differentiation and formation of dermal collagen that was better organized than that found in similar wounds not treated with PRP.³⁵ However, a different study found that topical application of autologous PRP to small granulating wounds on the limbs of horses did not accelerate healing, improve the quality of repair, or prevent the development of EGT.³⁶ The authors of this study suggested that treatment with PRP may be better suited for wounds characterized by massive loss of tissue or chronic wounds in need of a fresh source of mediators to accelerate healing.³⁶ The reader is referred to Chapter 22 for more information about the use of PRP for wound healing.

Wound evaluation and preparation

Using aseptic techniques to cleanse and bandage a wound containing exposed bone is important because exposed bone is susceptible to infection. In one study, the concentration of S. aureus adhered to the periosteum of equine bone was less than that adhered to cortical bone, cut cortical bone, and the endosteal surface of bone, indicating the necessity for meticulous cleansing of wounds with exposed and damaged bone.³⁷

The hair of the skin surrounding the wound should be clipped, and the wound should be irrigated with sterile isotonic saline solution to which an antiseptic (e.g., 0.05% chlorhexidine or 0.1–0.2% povidone–iodine) has been added, using irrigation pressures between 8 and 15 psi.¹² After it is irrigated, the wound should be explored digitally, after donning sterile gloves, to establish the extent of injury and the degree of periosteal damage. Adjacent synovial structures, tendons, and ligaments should be evaluated carefully to determine if they are involved in the injury. The wound should also be examined carefully for the presence of bony fragments or foreign bodies. For a more in‐depth discussion of wound preparation, see Chapter 4.

Distortion of bone underlying a wound suggests that the bone has been fractured, and horses with an open fracture of the distal aspect of the limb have a poor or guarded prognosis for survival. In these cases, radiographic examination of the injured region is indicated to assess osseous damage. The limb need only be bandaged if no fracture is observed, but a splint should be incorporated into the bandage when there is the possibility that a tendon has been lacerated or a synovial structure invaded. For more information regarding splinting, see Chapter 7.

Surgical management

Surgical debridement remains the technique of choice for removing devitalized tissues and tissues heavily contaminated by dirt and bacteria. Small, relatively clean wounds may be debrided with the horse standing after desensitizing the wound using local or, preferably, regional anesthesia. Large, heavily contaminated/infected wounds are often best debrided with the horse anesthetized. The wound is sharply debrided until only apparently healthy tissue remains (Figure 14.7). See Chapters 4 and 8 for more information regarding preparation of a wound to reduce the likelihood of infection.

If a flap of skin remains attached to a wound in which bone is exposed, the flap, or at least a portion it, can often be apposed to surrounding skin, using monofilament suture, to cover all or a portion of the exposed bone.¹² That portion of the wound that cannot be covered by the skin flap is allowed to heal by second intention. Because the loss of periosteum may cause ischemia of the outer third of the cortex, superficial ostectomy to remove ischemic bone, performed at the time of wound debridement, may prevent a sequestrum from forming or at least minimize its extent. A shallow portion of exposed bone can be excised with an osteotome,³⁸ a bone rasp, or an air drill until punctate hemorrhage or a yellow, serum‐like fluid begins to exude from the cortex. Hemorrhage exudes if the horse’s bone is immature, and serum‐like fluid exudes if the horse’s bone is mature. Granulation tissue grows directly from viable, exposed bone; therefore, exposing fresh, bleeding bone speeds proliferation of granulation tissue from the bone.³⁹ If the uncovered area of bone is larger than 36 cm², fenestrating its cortex, using a 3.2‐mm drill bit, to facilitate the formation of granulation tissue may be useful.⁷ Drilling small holes in exposed bone promotes healing by allowing osteogenic factors from the medullary cavity access to the wound.⁴⁰ The holes should be placed in a diamond‐shaped pattern, so that they are equidistant from each other, and should be separated from one another by 12–15 mm to avoid substantially weakening the bone.⁷ The cortex can be rasped in areas where drilling the full thickness of the cortex is contraindicated, such as in wounds where hairline fractures of the cortical bone are observed.

Because bacterial infection of the bone contributes to the formation of a sequestrum, efforts should be made to prevent exposed bone from becoming infected.^25,41 To prevent septic osteitis, the horse should receive parenteral, broad‐spectrum, antimicrobial therapy soon after injury. Regional limb perfusion with an antimicrobial drug, used alone or in conjunction with systemic administration of an antimicrobial drug, may be indicated. A hydrogel dressing should be applied to the exposed bone to prevent desiccation, and the wounded portion of the limb should be bandaged to protect it from contamination and to absorb exudate. Non‐steroidal anti‐inflammatory drugs (NSAIDs) have been reported to have an adverse effect on the incidence of infection by negatively affecting migration of leukocytes, so they should be used only when indicated to relieve swelling and pain.^42,43

Removing the sequestrum is the most effective treatment, but occasionally a sequestrum is shed spontaneously (Figure 14.8), and, rarely, a small sequestrum may be resorbed. Resorption of the sequestrum is hampered because osteoclasts are unable to migrate to the avascular sequestrum. The horse may respond initially to parenterally administered antimicrobial therapy with a decrease in swelling and discharge of exudate from the wound,² but treating an affected horse with antimicrobial therapy alone is ineffective because the lack of blood supply to the sequestrum isolates bacteria harbored by the sequestrum from antimicrobial drugs. Culture of exudate from the draining sinus, or even from around the sequestrum, is unlikely to be helpful in determining the organism(s) involved in formation of a sequestrum because secondary pathogens rapidly colonize an open wound or sinus.

Often, a sequestrum can be removed easily with the horse standing, provided that at least one edge of the sequestrum is not covered with granulation tissue (Figure 14.9). A sequestrum covered by tissue or embedded within the marrow cavity is best removed with the horse anesthetized (Figure 14.10). The sequestrum should be removed as soon as its separation from viable bone becomes radiographically apparent. A large sequestrum embedded within the marrow cavity may have to be divided to remove it through the cloaca (Figure 14.11). In these cases, a cast should be applied to the limb prior to recovering the horse from anesthesia to reduce stress concentration at the cloaca, thereby decreasing the risk of a complete fracture.

Primary closure of the wound may be possible if the wound from which the sequestrum was removed is small,²⁹ but if a large amount of soft tissue was avulsed, the wound must heal by second intention or by skin grafting. Broad‐spectrum, perioperative antimicrobial treatment should be administered before surgery if primary closure is planned. At surgery, the sequestrum is removed, and the membrane lining the involucrum is excised by curettage. Curettage is continued until punctate hemorrhage or serum‐like fluid from the underlying cortical bone is observed. The curetted involucrum is irrigated with a dilute antiseptic solution, after which new sterile gloves are donned, and a new set of sterile instruments is used to close the wound. Proliferative bone, if present, can be removed prior to closing the wound (Figure 14.12).