Wound healing process

1. Hemostasis

• Injury causes vasoconstriction to reduce bleeding.
• Platelets adhere and aggregate, forming an initial plug.
• The coagulation cascade creates a fibrin mesh, stabilizing the clot.
• Platelets release growth factors, including PDGF and TGF-beta, which recruit inflammatory cells and fibroblasts.

2. Inflammatory phase

• Initially, blood vessels become more permeable, producing redness, warmth, swelling, and mild pain.
• Neutrophils arrive first and help remove bacteria and necrotic material.
• Macrophages then become predominant. They clear debris and release cytokines and growth factors that drive repair.
• Excessive or prolonged inflammation, such as with infection or poor perfusion, delays healing.

3. Proliferative phase

• Angiogenesis: New capillaries grow into the wound, giving granulation tissue its pink-red, granular appearance.
• Fibroplasia: Fibroblasts produce extracellular matrix and predominantly type III collagen.
• Epithelialization: Keratinocytes migrate from wound edges and skin appendages to cover the surface.
• Contraction: Myofibroblasts pull the wound edges inward, especially in wounds healing by secondary intention.

4. Maturation or remodeling phase

• Type III collagen is progressively replaced by stronger, more organized type I collagen.
• Collagen fibers cross-link and align along stress lines.
• Vascularity decreases, so the scar becomes flatter and paler.
• A healed wound does not regain the tensile strength of unwounded skin. The mature scar typically reaches about 70-80% of original tensile strength.

Types of wound healing

1. Primary intention
   • Clean wound with edges approximated, such as a sutured surgical incision.
   • Minimal tissue loss, less granulation tissue, faster healing, and generally a smaller scar.
2. Secondary intention
   • Wound is left open because of tissue loss, contamination, or inability to approximate edges.
   • Healing occurs through granulation, contraction, and epithelialization.
   • Slower healing and larger scar.
3. Tertiary intention or delayed primary closure
   • Wound is initially left open, often because it is contaminated or infected.
   • It is closed later once contamination and inflammation are controlled.

Factors that delay wound healing

• Infection or foreign body
• Poor blood supply, ischemia, edema, or hypoxia
• Diabetes mellitus
• Malnutrition, especially protein, vitamin C, or zinc deficiency
• Smoking
• Corticosteroids, chemotherapy, and immunosuppression
• Anemia, advanced age, obesity
• Excess tension, repeated trauma, pressure, or poor wound care