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.