The Pathophysiology of Wound Healing

Healing

Healing is a complex process that, for descriptive purposes, is arbitrarily divided into three temporally and spatially linked stages (Figure 12):

Figure 12 Schematic diagram showing the phases of wound healing with time.

1. Inflammatory and debridement phase (demarcation).

2. Repair phase (proliferation).

3. Maturation phase (epithelialization and contraction).

Each phase has its local and systemic requirements and will, in turn, influence the others. The clinical objective is to culminate in a closed (healed) wound with a reasonable restoration of both function and cosmesis. The duration of the various phases is variable depending on the site of the wound, the cause of the wound, and the extent of tissue deficits.

Many factors have been identified as having an influence on wound healing; however, any individual factor that adversely (or more rarely beneficially) affects any component of the healing process inevitably carries a penalty (or reward) in the rate and quality of reparative processes (see p. 25).

Inflammatory and Debridement (Demarcation) Phase

Blood and fibrin flow into the wound site and form a fibrocellular clot, comprising mainly fibrin and fibronectin with the normal blood cells enmeshed within it (Figure 13). The clot serves to limit blood loss and provides a scaffold for the formation of a new matrix that will facilitate the migration of cells. The migration of phagocytic cells is vital for the natural debridement of the wound (Figure 14). Foreign matter and bacteria are removed, and non-viable tissue is demarcated and gradually separated from the viable areas.

Figure 13 A fresh laceration on the shoulder of a racehorse showing tissue damage. This represents the earliest stages of the acute inflammatory response with clot formation.

Figure 14 This extensive wound is undergoing natural debridement. Note contraction of the wound.

Repair (Proliferative/Granulation) Phase

This usually commences in the first 12 hours; however, it cannot proceed until any remaining blood clots, necrotic tissue debris, and infection have been eliminated. The process cannot proceed without a good blood supply; angiogenesis is critical to the health of the wound.

Healthy sutured wounds are normally covered in 12–24 hours. Full thickness wounds only epithelialize after formation of a granulating bed, necessitating a lag phase of 4–5 days (Figure 15). Migration of fibroblasts and fibroplasia results in a major gain in tensile strength at 5–15 days in the sutured wound. Granulation tissue comprising of a loose extracellular matrix and increasing numbers of fibroblasts and vascular elements begins to develop 3–6 days postinjury and continues until epithelialization occurs (Figure 16).

Figure 15 The mid repair phase. Note the advancing epithelial margin and the central red granulation tissue bed.

Figure 16 Late repair phase with a healthy epithelial margin and a flat pale granulation tissue bed.

Granulation Tissue

Granulation tissue (Figure 17) is a complex of fibroblasts, vascular endothelial cells (with neovascularization), and macrophages within a collagen and fibrin matrix. Granulation tissue: