Fibroplasia

Soon after injury, fibroblasts move out of the connective tissue adjacent to the wound and migrate across the protein lattice formed by the clot. Fibroblasts replace damaged dermis with ground substance and collagen. This collagen forms scar tissue that is structurally altered and functionally inferior to the original dermis.

Granulation

Granulation tissue is highly vascular connective tissue that functions to nourish the epithelial cells, fibroblasts, and macrophages. By the process of angiogenesis, endothelial cells migrate and anastomose to revascularize the wounded tissue. The mechanisms that control endothelial cell and fibroblast proliferation in the formation of granulation tissue are ill-defined but include a plethora of growth factors produced by platelets, macrophages, fibroblasts, and endothelial cells. Hypoxia also seems to play an important role.

Epithelialization

After injury, the epithelial layer is capable of regenerating. Epithelial cells at the wound margin proliferate and migrate over the granulation tissue toward the center of the wound. Epithelialization is complete when apposing margins meet. Contact inhibition prevents further epithelial cell division.

Contraction

Wound contraction is the centripetal closure of the total wound area. The full thickness of skin is pulled together. Wound contraction reduces the defect so less scar tissue and epithelialization occurs. For contraction to proceed, select fibroblasts differentiate into myofibroblasts that are capable of contracting to pull the wound margins together. Contraction stops when the wound margins are apposed or when tension across the wound exceeds the potential strength of myofibril contraction.

Foreign Bodies/Contamination

Foreign matter and organic debris pose a physical barrier to wound healing. They are also infection-potentiating factors, the presence of organic debris reduces the minimum concentration of bacteria necessary to cause infection.

Necrotic Tissue

Presence of necrotic debris prevents formation of granulation tissue and provides a substrate for bacterial growth.

Infection

Open wounds expose the fibrin clot to the environment. Pathogenic or opportunistic bacteria take advantage of the same binding sites and scaffolding used by neutrophils and fibroblast to adhere to the wound. Some bacteria produce toxins that increase tissue damage and further the ability of bacteria to colonize and infect the tissue. Wounds with significant tissue damage and decreased perfusion, such as blunt trauma injuries, are at increased risk of infection because the devitalized tissue provides an optimal environment for bacterial proliferation and growth.

Movement

Wounds located over joints or tendons are subject to excessive motion, which delays wound healing. Tension against the wound and recurrent irritation may cause chronic inflammation and exuberant granulation tissue development.

pH

A slightly acid environment has been shown to enhance wound healing. This may be the result of enhanced oxygen release from hemoglobin in acidic environments. The acidic environment may also inhibit bacterial growth. However, too low a pH causes cell damage and delayed healing.

Blood Supply

An adequate blood supply is essential for wound healing. Blood delivers nutrients, oxygen, neutrophils, and macrophages and removes cellular wastes.

Temperature

Wounds heal faster at higher temperatures, possibly because secondary vasodilation increases perfusion. Body temperature of the distal limbs is generally slightly lower then core body temperature and may contribute to delayed wound healing and exuberant growth of granulation tissue in these locations.

Oxygen Tension

The presence of oxygen is necessary to meet the aerobic demands of metabolically active cells involved in healing. However, some degree of hypoxia stimulates wound healing. Low oxygen tension stimulates longevity and multiplication of fibroblasts. Hypoxia also induces the secretion of growth factor and angiogenic substances.

Nutrition

Poor nutrition can result in compromised immune function and deficiency in vitamins and building blocks necessary for protein synthesis and wound healing.

Second Intention Closure

Second intention closure relies on the wound’s natural ability to contract and epithelialize. It is applicable to wounds that are heavily contaminated, have excessive soft tissue damage (contusions), dead space, or extensive tissue loss that prevents closure of the defect. Second intention healing provides maximum drainage and is optimal for deep, penetrating wounds. Wounds healing by second intention require greater time to heal and may be unsightly and disturbing to the owner, but the final result is often a surprising, aesthetically acceptable return to function.

Once the decision has been made to allow a wound to heal by second intention, the wound is cleaned and debrided. Topical treatments may be used, and a bandage is applied. Petroleum jelly applied to the skin ventral to the wound helps prevent serum scald. If the injury lies over a highly mobile location on a limb, immobilizing the area with a splint or cast may be necessary.

If tissue contraction is insufficient to close the wound, the final result is epithelialization. Epithelial cells extend across the lesion but are structurally weaker and more prone to recurrent injury than full thickness skin is. If extensive epithelialization results, reconstructive surgery or grafting may be necessary.

Primary Wound Closure

With primary wound closure, the defect is closed by apposing and suturing skin edges. The best example is closure of a surgical incision. Primary wound closure provides the greatest rate of healing and the most normal return to function. It is the ideal route of healing because it results in the most physiologically normal tissue and the least amount of scarification. It should be used when the wound is very recent or the result of sharp incision with minimum contamination or soft tissue damage.

If necessary, the wound should be lavaged and debrided. Then the wound edges are apposed and sutured. To minimize the inflammatory reaction, nonreactive suture material and the minimum number of sutures necessary to close the defect are used. A bandage or cast may be applied over the closed wound to minimize contamination and restrict motion.

When excessive dead space remains, a drain should be placed alongside the suture line to facilitate drainage; it should not be directly beneath the sutured wound edge. Drains should be sutured proximally and distally and exit the skin through incisions separate from the sutured wound. Where the drain is exposed, it should be protected by a sterile bandage or stent; otherwise it may wick bacteria and contaminants into the wound. Drains should remain in place for 24 to 48 hours or until drainage ceases.

With primary wound closure, drainage is reduced, and contamination trapped in the wound site could cause abscessation. Also, excessive tension across the wound can cause pressure necrosis, suture failure, or can interrupt circulation and inhibit wound healing. If necessary, tension-relieving sutures can be placed initially and can be followed by simple interrupted sutures placed between the tension-relieving bites (Figure 8.1-1). Interrupted near-far-far-near tension-relieving sutures placed at regular intervals along the incision starting in the center work well to bring skin edges into apposition. For wounds under a lot of tension, using penetrating towel clamps to help align tissues may be necessary. Rubber stents or “quills” placed on the tension sutures help prevent tissue necrosis under the suture material. After 3 to 4 days, the tension sutures are removed, and the simple interrupted sutures are left for 10 to 14 days.

Figure 8.1-1 Primary wound closure using tension-relieving sutures placed at regular intervals to appose wound edges. A, Placement of a near-far-far-near tension-relieving suture, with 1-4 indicating order the bites are placed and 5 indicating the knot. B, Two simple interrupted sutures placed next to a near-far-far-near tension-relieving suture.

Delayed Primary or Secondary Closure

These approaches to wound closure should be applied to wounds that did not receive immediate attention, had topical therapies that should not be closed into the wound applied, or are moderately contaminated.

Delayed primary closure makes use of lavage and manual debridement to clean a contaminated wound before it is closed. The wound is sutured several days after the injury but before granulation tissue develops.

Secondary closure allows a healthy bed of granulation tissue to develop before the wound is sutured. Excessive granulation tissue is sharply excised and the skin edges apposed over the granulation tissue.

These methods are particularly applicable to wounds of the distal limb that have degloving injuries or tissue flaps. When the viability of a tissue flap is uncertain, the skin is temporarily tacked down with a minimal number of sutures. This protects the skin flap and the underlying tissue from further damage, contamination, and desiccation. In 24 to 48 hours, the skin flap can be assessed. Viable tissue will be warm and pliable and will have good blood flow. The nonviable tissue is sharply debrided and the wound closed.

Povidone Iodine

At a 0.1% concentration (10 cc of 10% stock solution in 1 liter of saline), povidone iodine is antibacterial and does not interfere with healing. At concentrations less than 0.1%, povidone iodine has a stimulatory effect on neutrophils but minimal bactericidal effect. At higher concentrations, povidone iodine is toxic to fibroblasts. Povidone iodine is inactivated by organic debris and has little residual effect.

Chlorhexidine Diacetate

Chlorhexidine solution at a 0.05% dilution (25 cc of 2% stock solution in 1 liter saline) has antibacterial effects without inhibiting wound healing, Chlorhexidine diacetate has a longer residual effect than povidone iodine and has shown greater efficacy in the presence of organic debris.

Hydrogen Peroxide

Despite its impressive foaming action, hydrogen peroxide has not shown effective bactericidal activity and is cytotoxic at concentrations greater than 10%. However, hydrogen peroxide maintains sporicidal activity and may be indicated for treatment of potential spore-forming bacterial infections.

Adherent Bandages

Adherent bandages are used during the inflammatory and debridement phases of wound healing. Mesh gauze is used as the contact layer and may be alternated “wet-to-dry” with each bandage change. Necrotic tissue and debris adhere to the gauze and are removed when the bandage is changed. Once granulation tissue develops debridement is no longer necessary, and adherent bandages are no longer used.

Semiocclusive, Nonadherent Bandages

Semiocclusive, nonadhesive bandages are used once granulation tissue has developed. Being semiocclusive, the bandage maintains hydration of the wound but retains some absorbency to prevent exudate buildup. The contact layer should be nonadhesive material. Numerous nonadhesive pads are available commercially. They can also be made by coating gauze with triple anti-biotic ointment, povidone-iodine ointment, or other topical ointments to prevent gauze adhesion to the wound.

Amnion has been used as a semiocclusive, nonadherent wound dressing. Qualitative assessment of wound healing indicates amnion increases wound healing and diminishes the amount of exuberant granulation tissue on wounds of the distal limb. The exact mechanism by which amnion potentiates wound healing is unknown. Because it clings tightly to the wound margin without adhering to developing granulation or epithelial tissue, amnion may simply be acting mechanically to prevent bacterial contamination. It is possibly less abrasive than synthetic pads and causes less irritation. The natural, low immunogenicity of amnion may induce less immune reaction when it contacts the wound surface.

Amnion can be stored for up 6 months at 4° C in a 0.05% chlorhexidine diacetate solution in water. The benefits and availability of amnion make it an affordable alternative to nonadherent pads.

Occlusive Bandages

Occlusive bandages prevent absorption of exudates and impede gas exchange across the wound surface. They are used extensively in human medicine but are of limited value in veterinary medicine. They have been shown to inhibit wound healing and promote excessive granulation in horses.

Aloe Vera Extract

Aloe vera extract has been shown to have both antibiotic and antiinflammatory properties. In combination with allantoin, aloe vera extract has been shown to increase the rate of wound healing.

Triple Antibiotic Ointment

Triple antibiotic ointment of bacitracin zinc, neomycin sulfate, and polymixin B sulfate has been shown to increase reepithelialization by 25%.

Silver Sulfadiazine

Silver sulfadiazine is commonly used in human medicine to treat burn victims and promote wound healing.

Nitrofurazone

Nitrofurazone (a potential carcinogen) use in food animals is strictly prohibited by the Food and Drug Administration. Even topical administration has resulted in systemic detection of drug residue.

Povidone Iodine Ointment

Application of povidone iodine ointment decreases bacterial concentrations and infection rates, but it may also decrease tensile strength of the wound.

Petroleum

Although commonly used in nonadherent bandages, petroleum has no apparent antibacterial properties and decreases the rate of wound contraction.

Topical Antibiotic

Application of topical antibiotics such as penicillin to a wound or incision decreases infection rate and does not suppress wound healing.

Tetanus Toxoid

Tetanus prophylaxis boosters should be administered to animals with significant tissue damage or penetrating wounds at risk of anaerobic bacterial growth. Tetanus vaccines should be used in accordance with manufacturers’ directions and species specificity. If the animal has not been vaccinated before injury, tetanus antitoxin may be administered. However, anaphylactic reactions or acute hepatic disease (Theiler’s Disease in horses) are potential side effects to antitoxin.