Wound healing for 5 mark
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"wound healing" phases mechanisms
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I now have enough comprehensive content from Schwartz's Principles of Surgery to write an excellent 5-mark answer. Let me compile it.
Wound Healing
(Source: Schwartz's Principles of Surgery, 11e; Bailey & Love's Short Practice of Surgery, 28e)
Definition
Wound healing is a complex, coordinated cellular and biochemical cascade that restores tissue integrity and function following injury.
Types of Wound Healing
| Type | Description |
|---|---|
| Primary intention | Clean wound edges approximated immediately (sutures, staples). Minimal scarring. |
| Secondary intention | Wound left open; heals from base upward via granulation tissue, contraction, and epithelialization. Larger scar. |
| Tertiary intention (Delayed primary closure) | Wound cleaned and left open initially; closed later (e.g., after 4-5 days). Used for contaminated wounds. |
Phases of Wound Healing
1. Hemostasis (Immediate - within minutes)
- Tissue injury exposes subendothelial collagen, triggering platelet aggregation and activation of the coagulation cascade.
- Fibrin clot forms - acts as a provisional matrix and reservoir for growth factors (PDGF, TGF-beta, etc.).
- Vasoconstriction (first 5-10 min) followed by vasodilation.
2. Inflammatory Phase (Day 1-4)
- Neutrophils are the first cells to arrive (within hours); they phagocytose bacteria and debris, release proteases and reactive oxygen species.
- Macrophages (appear by day 2-3) are the "master regulators" of healing; they phagocytose debris, produce growth factors (PDGF, TGF-beta, VEGF, FGF), and recruit fibroblasts.
- Clinically manifests as rubor, calor, dolor, tumor (redness, warmth, pain, swelling).
3. Proliferative Phase (Day 4 - Day 21)
Three overlapping processes:
- Fibroplasia and Matrix Deposition: Fibroblasts migrate into the wound and synthesize type III collagen (initially), fibronectin, and proteoglycans. Type III collagen is later replaced by stronger type I collagen.
- Angiogenesis: New capillaries sprout into the wound (stimulated by VEGF, bFGF), forming granulation tissue (pink, moist, granular appearing tissue).
- Epithelialization: Basal cells at wound edges migrate across the provisional matrix in a "leapfrog" fashion, stimulated by EGF, TGF-beta, bFGF, and PDGF. Complete in <48 hours for clean incisions; much longer for open wounds.
- Wound contraction: Myofibroblasts (modified fibroblasts containing alpha-smooth muscle actin) pull wound edges together, reducing wound size.
4. Remodeling / Maturation Phase (Day 21 - up to 2 years)
- Collagen remodeling: type III collagen replaced by type I; cross-linking increases tensile strength.
- Maximum tensile strength = 80% of original skin (never fully restored).
- At 3 weeks: ~20% tensile strength. At 3 months: ~80%.
- Matrix metalloproteinases (MMPs) degrade excess collagen; balance with TIMPs (tissue inhibitors of MMPs).
- Wound vascularity decreases; scar matures and pales.
Key Cells and Their Roles
| Cell | Role |
|---|---|
| Platelets | Hemostasis; release PDGF, TGF-beta |
| Neutrophils | Debridement, antimicrobial defense |
| Macrophages | Phagocytosis, growth factor secretion, coordination |
| Fibroblasts | Collagen synthesis, ECM production |
| Myofibroblasts | Wound contraction |
| Endothelial cells | Angiogenesis |
| Keratinocytes | Epithelialization |
Complications of Wound Healing
- Hypertrophic scar: Excess collagen within wound boundaries; responds to silicone, steroids.
- Keloid: Collagen extends beyond wound margins; common in darker skin types; sites: ear lobe, deltoid, presternal. Resistant to treatment.
- Wound dehiscence: Failure of wound closure, often due to infection or technical error.
- Chronic non-healing wound: Persists >3 months; associated with ischemia, infection, diabetes, malnutrition.
Factors Impairing Wound Healing
Local: infection, foreign body, ischemia, radiation, repeated trauma.
Systemic: diabetes mellitus, malnutrition (especially vitamin C and zinc deficiency), corticosteroids, chemotherapy, old age, jaundice, anemia, uremia.
Exam tip: The 5-mark question typically expects: types of healing (1 mark) + phases with key cells/events (3 marks) + complications or factors (1 mark). The macrophage is the single most important cell in wound healing overall.
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