Wound healing

Healing is the process of replacing dead and damaging tissue with healthy tissue; this may occur through regeneration or repair. 

The regenerative capacity of cells can be categorized in three main ways: 

• Labile cells are constantly dividing and have a good capacity for regeneration, this allows the replacement of ageing tissue such as the surface epithelia of the skin, gastrointestinal tract and uterus; blood cells are derived from labile cells of the bone marrow. 
• Stable cells slowly replicate e.g., cells of liver, renal tubular epithelium, endocrine glands. 
• Permanent tissues: incapable of division and cannot be regenerated e.g., brain cells, myocardial cells, skeletal muscle cells. 

Healing by primary intention:

• Healing of a clean, uninfected surgical incision approximated by surgical sutures. 
• Day-1: Neutrophilic infiltration + fibrin clot 
• Day-2: Epithelial cells from both edges have begun to migrate and proliferate along the dermis, yielding a thin but continuous epithelial layer. 
• Day-3: Macrophages replace neutrophils Appearance of granulation tissue. Type III collagen deposition begins but do not bridge the incision. 
• Day 5: Abundant granulation tissue Collagen fibrils bridge the incision. 
• Neovascularization is maximum. 
• Week-2: Dense scar tissue produced from granulation tissue contains type III collagen (weak collagen) that must be remodeled. Remodeling increases the tensile strength of scar tissue. 
• 1 month: Replacement of collagen type III with collagen type I (has greater tensile strength) due to action of collagenase enzyme. 
• The predominant collagen in adult skin is type I, whereas in early granulation tissue, it is type III. 

Healing by secondary intention

Occurs in large wounds abscess formation and ulceration. 

Main features: 

• A larger clot or scab 
• Inflammation: more intense Granulation tissue: much larger amount 
• Scar tissue: greater mass 
• Wound contraction: by myofibroblast e.g. Within 6 weeks, large skin defects may be reduced to 5% to 10% of their original size, largely by contraction. 

Tertiary Intention: 

• Contaminated wound is initially treated with debridement and antibiotics followed by surgical wound closure (suture, skin graft replacement, flap) 
• Note: Regarding the neat surgical scar after 2 months of surgery fibroblasts have predominant role 
• Most important for delayed wound healing is due to infection. 

Factors affecting wound healing: 

Complications of Healing 

• Hypertrophic scar 
• Keloid formation 
• Failure to heal (abscess or empyema formation) 
• Failure to unite (skin, muscle or fascia wound breakdown) 
• Fracture complications 

Stages of bone/fracture healing: 

• Hematoma formation (Immediately in Day 1) 
• Inflammation and proliferation (8 hours to 1-2 weeks) predominant cells neutrophils and macrophages 
• Callus formation (2-3 weeks) 
• Consolidation (2 to 3 months) 
• Re-modelling (2 months to years) Osteoclasts are responsible. 

Factors that prevent efficient healing: 

• Malnutrition 
• Infection 
• Corticosteroids therapy 
• Poor blood supply