Surgical Wounds

Classification

• By depth
  • Superficial (epidermis and dermis only)
    • Heal without granulation
    • e.g. superficial burns, graze, split skin graft (donor site)
  • Deep (deep to dermis)
    • Heal with migration of fibroblasts and formation of granulation tissue
• By mechanism
  • Incisions
    • Surgical or traumatic (e.g. knife/glass stabbing)- sharp pierce
  • Lacerations
    • Blunt trauma/tearing
    • Usually adjacent soft tissue damage +/- vascular impairment
  • Abrasions
    • Friction causing sloughing of superficial skin layers
  • Degloving injury
    • A form of laceration- when shearing forces pulls a layer of tissue away from adjacent layer
    • Vascular damage common
  • Burns
• By contamination
  • Clean (e.g. hernia repair)
  • Clean contaminated (e.g. cholecystectomy)
  • Contaminated wound (e.g. colonic resection)
  • Dirty wound (e.g. laparotomy for peritonitis)

Wound healing

• Intention
  • Primary
    • Uncontaminated wounds, with minimal tissue loss, to allow for close approximation of the wound edges with sutures/staples etc without excessive tension.
  • Secondary
    • Usually when there is substantial tissue loss, when the wound edges cannot easily be apposed, or when there is extensive contamination and the wound requires regular cleaning.
    • The wound is left open and is allowed to heal from the deep tissue up.  This takes longer and promotes a larger inflammatory response.  Scars are usually larger.
  • Tertiary
    • The wound is closed several days after incision
    • Usually when risk of infection is high and requires reduction, or where there is oedema preventing primary closure initially.
• Pathophysiology
  • Acute inflammatory phase and coagulation
  • Proliferation phase
    • Involves the recruitment and proliferation of a number of cells
      • Epithelial cells (within hours)- migration and proliferation of epithelial cells at the defect; closure usually by 48 hours
      • Fibroblasts (over days)- migrate, proliferate and synthesise extracellular matrix components e.g. collagen and ground substance.  Myofibroblast action also causes the wound to contract.
      • Endothelial cells (over days)- development of new vessels occurs simultaneously with fibroblast activity.  Heavily dependent on the action of metalloproteases.
  • Maturation phase
    • Matrix remodelling (months)
      • Devascularisation
      • Increase in strength due to collagen modifications
        • Note that it can take several months for a wound to recover strength.  In particular, incisions across muscle fibres may need 3-4 months of continuous suture support (use long-lasting and strong sutures e.g. nylon or PDS)

Some Specific Healing capacities of different tissues

• Some tissues are essentially regenerative
  • skin
  • liver
    • stimulation for regeneration is a decrease in liver mass to body mass ratio or the loss of liver functional capacity
    • all components of the liver (hepatocytes, biliary and fenestrated epithelial cells, and Kupffer’s cells) can regenerate
      • initially just the hepatocytes around the portal triads- spreading to other areas/cells by 24 hours.
  • Bowel
    • Bowel mucosa heals by ‘restitution’- a process largely mediated by inflammatory cytokines causing rapid migration of mucosal cells to the site of damage.  Whilst the rest of bowel tissues e.g. muscle layer, heals by inflammation etc, the mucosa can heal much faster thanks to this process.

 

 

Skin Structure/Function

Four main functions

• Protection – provides a physical barrier against infection/trauma etc
• Sensation – contains the majority of receptors responsible for sensation (touch, pressure, pain and temperature)
• Thermoregulation – insulation, sweating and changes in blood flow to the skin (heat transfer) controls body temperature
• Metabolism – e.g. vitamin D synthesis; triglyceride storage

There are three main layers

• Epidermis
  • Most superficial layer- relatively thin (thickness varies between body sites)
  • Consists of stratified squamous epithelial cells
    • Mainly keratinocytes
      • Become more ‘mature’ as they migrate superficially from the basement membrane- losing their nuclei and becoming keratinised
        • Four ‘layers’ (from superficial to deep)- stratum corneum (‘corneocytes’- dead, dried out, anuclear cells); stratum granulosum (contain basophilic granules); stratum spinosum (intercellular desmosomes give a spindle appearance); stratum basale (columnar cells with regenerative capacity)
    • Melanocytes
      • Found in the basal layer; produce melanin which contributes to skin colour and absorption of UV light
    • Langerhans cells
      • Immune cells- responsible for idenitifying, fragmenting and presenting allergens/foreign material to the lymphatic system and potentially activate the immune system
    • Merkel cells
• Dermis
  • Fibrous connective tissue under the epidermis- consists of mainly collagen and elastin, bound together by ground substance.  Adnexal structures (e.g. hair follicles and sweat glands; nerve endings etc) are found here
• Hypodermis/Subcutis/Subcutaneous layer
  • Contains the adipose tissue immediately under the skin.

A note about skin lines…

• Blaschko’s lines
  • Thought to represent the pattern of epidermal skin cell migration and proliferation during development.  They may be visible in inherited conditions.
• Langer’s lines (wrinkle lines)
  • These outline the arrangement of fibrous tissue in the dermis and hypodermis.  They are primarily longitudinal in the limbs and horizontal in the trunk (with exception of the limb flexors which are also horizontal)
  • They may be important when deciding where to make an incision, as cutting across them may result in increased/uneven tension across the wound and poorer healing.