An acute wound results from the sudden loss of anatomic structure in tissue following the transfer of kinetic, chemical, or thermal energy. Functionally, an acute wound should pass predictably through the phases of wound healing to result in complete and sustained repair. Acute wounds typically occur in recently uninjured and otherwise normal tissue. Acute wound healing is timely and reliable, completing the entire process within 6-12 weeks. Most surgical wounds are acute wounds.
Wound healing fails in a chronic wound. The process of tissue repair is prolonged and pathologic. The usual mechanism is dysregulation of one of the phases of normal acute wound healing. Most often, healing arrest occurs in an inflammatory phase. This prolonged inflammatory phase may be due to wound infection or another form of chronic irritation. Tissue and wound hypoxia is the other important mechanism for the development of a chronic wound. Failed epithelialization due to repeat trauma or desiccation may also result in a chronic partial thickness wound. Surgeons may sharply convert a chronic wound into an acute wound.
Surgeons often describe wound healing as primary or secondary. Primary healing occurs when tissue is cleanly incised and anatomically reapproximated. It is also referred to as healing by primary intention, and tissue repair usually proceeds without complication. Secondary healing occurs in wounds left open through the formation of granulation tissue and eventual coverage of the defect by migration of epithelial cells. Granulation tissue is composed of new capillaries, fibroblasts, and a provisional extracellular matrix that forms at the base of the early wound. This process is also referred to as healing by secondary intention. Most infected wounds and burns heal in this manner. Primary healing is simpler and requires less time and tissue synthesis than secondary healing. A wound healing primarily repairs a smaller volume than an open wound healing secondarily. The principles of primary and secondary healing are combined in delayed primary closure, when a wound is left open to heal under a carefully maintained, moist wound healing environment for approximately 5 days and is then closed as if primarily. Wounds treated with delayed primary closure are less likely to become infected than if closed immediately because bacterial balance is achieved and oxygen requirements are optimized through capillary formation in the granulation tissue.
The Mechanism of Wound Healing
The complex process of wound healing normally proceeds from coagulation and inflammation through fibroplasia, matrix deposition, angiogenesis, epithelialization, collagen maturation, and finally wound contraction (Figure 6–1). Wound healing signals include peptide growth factors, complement, cytokine inflammatory mediators, and metabolic signals such as hypoxia and accumulated lactate. Many of these cellular signaling pathways are redundant and pleiotropic.