Wound Healing

Contributed by: Khurrum Khan, Baylor College of Medicine, 1 Baylor Plaza, Houston, Tx 77030
Overview
  • Wound healing is a fundamental physiologic process that occurs following injury. The process involves a variety of cell types and can be influenced by local and systemic factors. We first discuss the phases of wound healing then discuss surgical classifications of wounds and their repair.
Wound Healing Physiology
  • Inflammatory stage
    • The inflammatory stage, lasting minutes to hours, is triggered by damage to blood vessels. The extravasated constituents of hemostasis not only limit blood loss, but also help coordinate subsequent wound repair. Platelets release growth factors that stimulate healing and the fibrin clot acts as a physical substrate for migration for neutrophils, macrophages, fibroblasts, and endothelial cells.
    • The second portion of the inflammatory phase involves the phagocytic actions of macrophages and neutrophils which remove bacteria and debris from the wound, minimizing infection. These phagocytes also promote further inflammation and healing by secreting a variety of cytokines and growth factors.
  • Proliferation phase
    • The proliferation phase initiates wound closure and is divided into three main stages: fibroplasia, angiogenesis, and re-epithelialization. Fibroplasia is characterized by fibroblast proliferation, migration to the wound site, and orchestration of extracellular matrix deposition. Angiogenesis generates new vasculature from existing nearby vessels and re-epithelialization involves generation of a new epidermal barrier
  • Remodeling phase
    • The remodeling phase, lasting up to several weeks, involves scar tissue formation and continual tissue remodeling. Type I collagen begins to replace the predominant type III collagen and there is degradation and remodeling of extracellular matrix components. As this phase progresses, wound tensile strength increases.
Surgical Wound Classification
  • Overview
    • Clinical classification of wounds allows for post-operative prediction of infection and other risks. It is primarily based on the amount of bacteria present in the operative site prior to surgery. There are four categories of surgical wounds: clean, clean contaminated, contaminated, and dirty.
  • Clean (Class I)
    • Clean wounds are by definition not infected, not inflamed, and do not communicate with colonized epithelia, such as the respiratory, gastrointestinal, genital, or urinary tracts. The risk of these wounds becoming infected postoperatively is 2%.
  • Clean Contaminated (Class II)
    • Clean contaminated wounds are those that by planned technique communicate with colonized epithelia such as the respiratory, gastrointestinal, genital, or urinary tracts. They must also be uninfected and lack acute inflammatory changes. The risk of these wounds becoming infected postoperatively is 5-15%.
  • Contaminated (Class III)
    • Contaminated wounds encompass a wider range than the first two classes. Any unplanned open wound less than 4 hours old, a wound in an area of non-purulent inflammation, involving gastrointestinal tract content spillage, or a major break in sterile technique falls into this category. The risk of these wounds becoming infected is greater than 15%.
  • Dirty (Class IV)
    • Dirty wounds include those that are greater than 4 hours old, retain dead tissue within them, or those that involve infected areas, or perforated viscera. These wounds have a greater than 30% chance of becoming infected postoperatively.
Closure by Intention
  • Overview
    • There are three methodologies behind closing a wound: primary, secondary, and delayed primary. Each of the three methods has unique aspects which make them applicable to different scenarios, as described below.
  • Primary Intention
    • Primary intention uses sutures or staples to approximate wound edges and allows healing with minimal scar formation. Examples include laceration repair and closure of a surgical wound.
  • Secondary Intention
    • Secondary intention relies on the formation of granulation tissue to fill the space between a wound opening. Wounds take longer to heal by secondary intention with a greater likelihood of scar formation because the edges are not approximated, but the main benefit of healing by secondary intention is that it decreases the chance of wound infection as it allows for wound exudate to easily drain from the wound.
  • Delayed Primary (Tertiary) Intention
    • Delayed primary intention, also known as tertiary intention, is a blend of the above two types of wound closure. Wounds are left open for a few days and granulation tissue is allowed to begin to form and then the edges are approximated with sutures. Scarring from this method of wound closure will be more than from primary intention, but less than from secondary intention.
Further Reading
  • Kaltalioglu K, Coskun-Cevher S. A bioactive molecule in a complex wound healing process: platelet-derived growth factor. Int J Dermatol. 2014 Oct 14.
  • Broughton G 2nd, Janis JE, Attinger CE. Wound healing: an overview. Plast Reconstr Surg. 2006 Jun;117(7 Suppl):1e-S-32e-S.
  • Nichols R. Classification of the surgical wound: a time for reassessment and simplification. Infect Control Hosp Epidemiol. 1993;14(5):253-254.