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tvpjournal.com | July/August 2015 | TodAy's VeTerinAry PrAcTice MoisT Wound HeAling: THe neW sTAndArd of cAre Peer reviewed 33 WOUND CLOSURE Wound closure is performed after all contaminants and nonviable tissue have been removed, and it can be orchestrated by the animal's body or clinician. Physiology of Wound Closure The body typically accomplishes closure over 2 to 4 weeks via the repair (proliferative) phase of wound healing. 2 During this phase, skin coverage of the wound is achieved through 2 simultaneous, yet independent, means—epithelialization and contraction. This process is accomplished by creation of: • Granulation tissue: As wound bed debridement by WBCs progresses, cells on the periphery of the wound receive signals to move in and fll the cleaned-out defect with granulation tissue. Granulation tissue is built by fbroblasts, which secrete new extracellular matrix molecules (eg, collagen, elastin) and endothelial cells, which build new blood vessels (Figures 2 and 3). • Epidermis: Epithelial cells on the skin edge then migrate onto the granulation tissue, which provides the oxygen, moisture, and surface required for epithelial cells to proliferate, cross the wound, and create a new epidermis (Figure 4). • Myofbroblasts: Wound contraction occurs when the fbroblasts that formed the granulation tissue meet in the center of the wound and develop characteristics similar to smooth muscle. Now called myofibroblasts, these cells—linked to each other by intercellular connections and to the wound edges via their attachments to the granulation tissue—contract and exert centripetal force on the skin edges, drawing them toward the center of the wound (Figure 4). Surgical Closure of Wounds Surgical closure can be: 1 • Primary: Immediate • Delayed primary: Before granulation tissue formation • Secondary: After granulation tissue formation. Surgical closure should be reserved for wounds that contain only viable tissue, are free of contamination, and can be closed in a tension-free manner. Regardless of timing, surgical closure is preceded by one or more rounds of debridement and lavage, and, except in the case of primary closure, initial management as an open wound. Gradation of Wound Effusion Wound fuid, a derivation of plasma that leaks out of blood vessels due to increased capillary permeability in response to infammation, 3 contains many benefcial factors for healing, including oxygen, nutrients, cytokines, growth factors, chemotactic factors, WBCs, enzymes that aid selective debridement, and systemic antibiotics, if the animal is receiving them. 3-6 Wound fuid is serosanguineous in appearance. Exudate, which has a more cloudy appearance, consists of wound fuid plus liquefed necrotic tissue created by WBCs via autolytic debridement and edema caused by infammation, decreased patient mobility, and inadequate lymphatic or venous drainage. 6 Exudate levels are proportionate to the amount of contamination, infection, and tissue damage in a wound, and should subside as the wound transitions from the infammatory/ debridement phase to the repair phase. Persistent exudate is an indication that more aggressive surgical debridement may be needed to remove foreign material, sites of infection, and/or nonviable tissues. FIGURE 3. Patient from Figure 1; day 11 after injury: Granulation tissue is moving in from the periphery of the wound, while the central area is still undergoing debridement by WBCs. FIGURE 4. Patient from Figure 1; week 6 after injury: Granulation tissue is flling the wound. A rim of pink epithelial cells is migrating inward across the granulation tissue, building new skin. Contraction has pulled pre-existing skin inward, diminishing the size of the wound.