8. Wound healing

1. Definition

• Wound healing → restoration of tissue continuity after injury, by regeneration or repair
• Regeneration → replacement with identical tissue (epidermis, liver, bone)
• Repair → replacement with connective tissue → scar

2. Phases of healing: tissue processes

A. Haemostasis

• Minutes to hours
• Vasoconstriction, platelet plug, fibrin clot
• Platelets release growth factors → platelet-derived growth factor (PDGF), transforming growth factor beta (TGF-β)

B. Inflammation (days 1–4)

• Vasodilatation, increased permeability → redness, heat, swelling, pain
• Neutrophils → first 24–48 h; kill bacteria, remove debris
• Macrophages → days 2–4; main cell of healing; phagocytosis, growth factors
• Lymphocytes
• Macroscopic → oedema and hyperaemia of the edges, exudate
• Result → wound cleansed of dead tissue (biological cleansing)

C. Proliferation (day 4 to week 3)

• Fibroblasts → collagen type III
• Angiogenesis → new capillary loops
• Granulation tissue → new capillaries, fibroblasts, macrophages in a loose matrix
• Granulation tissue appearance → red, moist, granular, bleeds easily on touch
• Epithelialisation → keratinocytes migrate from the edges and skin appendages; 1 mm per day
• Wound contraction → myofibroblasts pull the edges together

D. Remodelling (maturation) (week 3 to 1–2 years)

• Collagen type III replaced by type I
• Collagen cross-linking, realignment along tension lines
• Fewer vessels and cells → scar turns from red to pale
• Tensile strength:
  • 20% of normal at 3 weeks
  • 80% maximum at 3 months; never reaches 100%

3. Types of healing

Primary intention (per primam)

• Clean wound, edges apposed (sutured surgical incision)
• Minimal granulation tissue
• Epithelial bridge within 24–48 h
• Heals in 6–8 days → thin, linear scar

Secondary intention (per secundam)

• Wound left open: tissue loss, infection, gaping edges
• Cavity fills with granulation tissue
• Marked wound contraction
• Slow epithelialisation from the edges
• Result → wide, irregular, weak scar; contracture risk

Tertiary intention (delayed primary closure)

• Contaminated wound left open 3–5 days, then sutured once clean

Healing under a scab (sub crusta)

• Superficial wounds, abrasions, superficial burns
• Epithelium grows under the dried crust

4. Scar

• Normal scar → flat, pale, within the wound borders after maturation
• Hypertrophic scar → raised, within the wound borders; appears within weeks; regresses with time
• Keloid → grows beyond the wound borders; does not regress; recurs after excision; more common in dark skin, earlobe, sternum, shoulder
• Contracture → shortening of a scar across a joint
• Marjolin's ulcer → squamous cell carcinoma in a chronic scar or ulcer
• Treatment of abnormal scars → pressure, silicone gel, steroid injection, excision with radiotherapy (keloid)

5. Factors affecting healing

Local

• Infection → most common cause of delayed healing
• Poor blood supply, ischaemia
• Haematoma, seroma, foreign body, necrotic tissue
• Tension, movement
• Previous radiotherapy
• Wound site → head and neck heal fastest

General

• Age
• Malnutrition → protein, vitamin C (collagen cross-linking), zinc
• Diabetes mellitus
• Anaemia, hypoxia, smoking
• Steroids, chemotherapy, immunosuppression
• Jaundice, uraemia, malignancy
• Obesity

6. Complications of healing

• Wound infection
• Dehiscence → burst abdomen
• Incisional hernia
• Chronic wound → ulcer, sinus, fistula
• Hypertrophic scar, keloid, contracture