WOUND HEALING

Impact on Wound Healing

Impact on Wound Healing

Introduction

Wound healing involves a broad range of overlapping cellular and metabolic processes that are orchestrated as a fundamental homeostatic response to injury. An understanding of these concepts is essential to care for wounds in all disciplines of surgery. Plastic surgeons are often consulted by other practitioners to deal with difficult, nonhealing, compromised wounds. Therefore, an understanding of the basic science of wound healing allows one to identify the variables involved in a given wound, and ultimately modulate the process to restore the structure and function of the injured tissue.

Classically, wound healing is divided into three distinct phases: inflammatory, proliferative and remodeling (Table 1.1). Even though each phase is described as a separate event, there is a large degree of temporal overlap and variability in these phases. Factors that influence the timing and length of these events include ischemia, age of the host, nutrition, radiation, smoking, systemic diseases such as diabetes, contamination or infection, desiccation, and the amount of devitalized or necrotic tissue in the wound.

Wound healing process

Inflammatory Phase

Immediately after injury, bleeding occurs as a result of disruption of the blood vessels. Hemostasis is obtained by initial transient vasoconstriction and subsequent platelet plug and clot formation. Platelet degranulation of alpha and dense granules releases various substances, including platelet-derived growth factor (PDGF) and transforming growth factor-ß (TGF-ß), which ignites the chemotaxis and proliferation of inflammatory cells that characterize this phase of wound healing. Following the period of vasoconstriction, the migration of cells to the site of injury is aided by vasodilation and increased endothelial permeability (mediated by histamine, prostacyclin and other substances).

The first cells to arrive are the polymorphonuclear leukocytes (PMNs), which increase in numbers over the first 24 hours. These cells aid in the process of clearing the wound of debris and bacteria. Over the next 2-3 days, macrophages replace the PMNs as the predominant cell type. Macrophages have several critical roles in healing wound, including phagocytosis, release of multiple growth factors and cytokines, and recruitment of additional inflammatory cells. The importance of macrophages is exemplified by studies that have shown that wound healing is significantly impaired without their participation. In contrast, blocking or destroying PMNs during the inflammatory phase still results in a normally healing wound in the absence of bacteria. Finally, lymphocytes populate the wound, although their direct role in wound healing requires further investigation.

Proliferative Phase

The clot formed during the inflammatory phase provides the provisional matrix and scaffolding for the proliferation of the dominant cell type during this phase— the fibroblast. In addition, growth factors stimulate angiogenesis and capillary ingrowth by endothelial cells. The capillaries and fibroblasts form a substrate recognized clinically and histologically as granulation tissue. Fibroblasts produce collagen, which is the principal structural molecule in the final scar.

Initially, type III collagen is produced in relative abundance in the healing wound; the normal adult 4:1 ratio of type I to type III collagen is gradually restored during the remodeling ...