- Revise biology of wound healing.
- Partial thickness wounds regenerate more rapidly than full thickness wounds.
- Injury results in haemostasis, inflammation, proliferation, and repair.
- Keratinocytes migrate and proliferate in a single layer before keratinisation.
- Dermal injury is repaired by granulation tissue, then a collagen matrix (scar).
- Damaged hair follicles, sweat glands and melanocytes may not be replaced.
- Chronic wounds are those that persist for longer than 4 weeks.
- A moist environment promotes wound healing.
- Patient factors delaying wound healing include smoking, diabetes, poor nutritional state and other illness.
- Local factors delaying wound healing include inadequate perfusion, wound infection and skin disease.
Skin generally heals rapidly to restore barrier function after injuries such as cuts, abrasions, thermal burns, puncture wounds and blisters.
The severity of a wound depends on its surface area and depth. Partial thickness wounds can recruit new keratinocytes from adjacent adnexal tissue and can regenerate much more rapidly than full thickness wounds of the same surface area where regeneration of epithelium arises only from keratinocytes on the wound edges. Damage to the dermis can be repaired but results in altered tissue i.e. a scar.
Injury results in epidermal and dermal repair: haemostasis, inflammation, proliferation, and repair. Haemostasis creates a protective wound scab beneath which cell migration and movement of the wound edges can occur. Inflammation brings nutrients to the area of the wound, removes debris and bacteria, and provides chemical stimuli for wound repair. Repair begins immediately after wounding and proceeds rapidly through the processes of epithelialisation, fibroplasia, and capillary proliferation into the healing area.
Wounds can be classified as either acute or chronic. Acute wounds are sometimes defined as those that follow the normal phases of healing; they are expected to show signs of healing in less than 4 weeks and include postoperative wounds. Chronic wounds are those that persist for longer than 4 weeks and are often of complex poorly understood origin.
Acute surgical wound after curettage of pyogenic granuloma
Chronic pressure ulceration
Venous ulceration healing with granulation tissue
- Vasoconstriction to reduce blood loss.
- Blood clotting forms a scab.
- Injured keratinocytes releases interleukin-1 (IL-1) drawing lymphocytes to the wound and activating healthy keratinocytes, releasing tumour necrosis factor-a (TNF-a) and other cytokines
- Migration and proliferation of keratinocytes to form a single layer.
- Restoration of basement membrane
- The scab sloughs off and epidermis begins to keratinise.
- Re-epithelialisation maintains a moist clean environment for dermal healing.
- Chemotactic factors attract neutrophils (first 3 days) and monocytes, which remove foreign material, bacteria and cellular debris by phagocytosis.
- Complement activation results in vasodilatation and inflammation.
- Nitric acid regulates cellular activities.
- Growth factors are released by platelets and macrophages.
- Granulation tissue is evident at about 5 days due to proliferating macrophages, fibroblasts, smooth muscle and endothelial cells.
- Wound contraction is mediated by myofibroblasts, which contain abundant actin filaments.
- Fibroblasts produce fibronectin, which acts as a scaffold for the fibroblasts and then for collagen fibrils.
- Fibroblasts also produce ground substance.
- The collagen matrix slowly transforms granulation tissue into scar tissue.
- Remodelling of the collagen matrix may continue for months or years, with collagenase degrading the collagen and fibroblasts creating more and stronger parallel fibres of collagen. Despite this, the scar is rarely as strong as the tissue it replaced.
- As vascularization decreases, the colour of the wound fades.
- Damaged hair follicles, sweat glands and melanocytes may not be replaced so the scar has a smooth uniform surface.
Moist wound healing
A moist environment is beneficial compared with a dry wound. Moist wounds:
- Prevent further tissue loss by desiccation
- Promote activity of lytic enzymes
- Accelerate the cellular phase of collagen matrix deposition
- Facilitate keratinocyte migration.
Delayed wound healing is more likely in some patients.
- Poor nutritional state
- Pre-existing illness
- Certain medications
- Immune deficiencies
- Coagulation defects
- Abnormal collagen
- Arterial insufficiency
- Venous insufficiency
- Inadequate perfusion for other reasons
- Exposure to radiation
- Wound infection
- Skin disease arising in wound