Wound healing is the process of the body replacing devitalised and/or missing tissue in order to fill a cavity and repair damaged skin. This typically occurs in a coordinated fashion along a healing continuum, a process taking up to two years.  During this time wounds are vulnerable to repeated trauma and breakdown and should be protected where possible. Across the spectrum of health care settings health professionals face the challenge of  difficult, hard to heal wounds in addition to the uncomplicated wound that heals as expected. In order to facilitate healing it is important for nurses to not only recognise the stage of healing but to recognise when a wound is failing to heal. In this article we will look at the types of healing intention, some of the factors that influence wound healing and revisit the stages of wound healing.

Types of healing
Nurses will come across three types of healing intention during their clinical practice - healing by primary or secondary closure and delayed primary closure (or tertiary closure).1 ,4 Some might be familiar with the terms first, second and third intention healing respectively.  In the acute care setting, the majority of wounds heal by primary intention where wound edges are easily approximated and held in place with various closure materials such as sutures, staples, glue or steri-strips. These wounds are generally clean, uncontaminated or non-infected wounds with minimal tissue defects.  Surgical wounds, clean cuts and lacerations are examples of such wounds. These wounds have minimal scarring as there is no tissue defect and new dermal tissue is only required to fill the gap across the closely aligned wound edges.

Wounds healing by secondary intention are typically chronic wounds, wounds with large tissue defects or wounds that cannot be easily closed due to the degree of skin loss. Healing occurs more slowly by granulation (growth of new tissue to fill a cavity), wound contraction (to close the wound) and re-epithelialisation (growth of new skin) of the wound surface.1,4 These wounds usually result in lager scar tissue formation as new skin must grow across a larger area. Pressure injuries, diabetic ulcers, leg ulcers and dehisced wounds are examples of wounds healing by secondary intention.


Some wounds may require delayed closure due to infection, the need for debridement or formation of new granulation tissue to cover exposed fascia, bone or tendon prior to definitive or primary closure. This type of healing is commonly referred to a delayed primary closure or tertiary closure. 1 The need for a skin graft is an example where delayed primary closure may be required. Primary closure may occur up to 7 days after cleansing or debridement of the wound bed and where infection is adequately treated.

Factors affecting wound healing
A myriad of factors can slow down or impair healing notwithstanding age, illness, complex disease processes, medications, psychological or social factors, or a combination of any of these. Any condition that reduces perfusion to the wound bed will impair healing  for example  vascular disease (arterial disease or chronic venous insufficiency) and chronic airways disease (e.g. emphysema, asthma or lung cancer) while rheumatoid arthritis, diabetes and age impact wound healing through altered cellular mechanisms involved in the healing process. The  peri-operative period, surgical procedure and post-operative pain in particular can create stress and anxiety which induce physiological responses that interfere with healing processes. Similarly, psychosocial factors such as the lack of social and family support networks, loss of independence, depression and poverty  initiate the same stress responses that  delay healing. Locally wound characteristics also influence reparative processes. Tissue type, infection and inflammation, moisture balance, wound edges, wound temperature and the capacity of the microcirculation to deliver oxygen to the wound bed and surrounding tissues all impact on healing.

The wound healing process
Regardless of the type of wound the same basic physiological principles apply to how wounds heal. Generally there are three stages in the wound healing process although some authors tend to separate haemostasis and inflammation. The wound healing occurs though a continuum and comprises haemostasis, inflammation, proliferation and maturation (or remodelling). 2,3

Immediately post injury blood vessels at the site of the injury will firstly constrict to reduce blood flow to the area.2,3 Collagen fibres are exposed at the site of damaged vascular tissue which trap platelets, which in turn release chemicals that make nearby platelets sticky and clump together to form a platelet plug resulting in clot formation.2,3  The inflammatory phase occurs simultaneously where vasoconstriction is followed by vasodilation increasing blood flow to the damaged site with an influx of macrophages and neutrophils which begin to clean the wound removing debris, bacteria, damaged cells and devitalised tissue readying the site for the next stage of healing. 2 The inflammatory phase is characterised by the cardinal signs heat, pain, redness and swelling and usually will lasts from zero to three days depending on the nature of the wound and other patient characteristics but will continue until the wound bed has been adequately prepared for the next stage of the healing, the proliferative phase.

The proliferative phase usually occurs from between three days to approximately three weeks (again depending on the nature of the wound and patient factors that may limit healing). 2,3 It is during this phase that new blood vessels are created (angiogenesis) and new tissue in the form of  extra cellular matrix, primarily composed of collagen and elastin, is produced to fill the wound cavity over which new skin will grow (epithelialisation).2,3  Collagen and elastin together provide tensile strength and elasticity to newly formed skin.2,3 Once the wound is repaired the final phase of wound healing, the maturation phase begins.2,3  This phase, lasting up to two years involves wound contraction and remodelling of newly formed collagen to produce scar tissue. 2.3 Remember  it is during this phase the wound remain  vulnerable to breakdown though repeated insult especially wounds healing by secondary intention such as dehisced wounds, pressure ulcers, diabetic foot ulcers and leg ulcers and subsequently should be protected at all times.

Conclusion
It is important for nurses understand the type of healing intention and to recognise the stage of reparation and its progression to gauge whether wounds are healing as they should. Knowledge of  the clinical signs of inflammation and/or infection such as new or increased pain, prolonged inflammation, new or spreading erythema, an increase in exudate or change in exudate colour with or without odour, change in colour of the wound or increased temperature will ensure prompt review of the patient. Infected wounds and wounds stuck in the inflammatory stage are unable to progress through the healing continuum. Thorough wound assessment and investigations are required to determine the reason why wounds won’t heal. Accurate identification of the stage of healing allows nurses to develop appropriate wound management plans to protect the wound and facilitate healing. If in doubt always refer your patient to the wound specialist and/or request a clinical review.

References
1. Benbow, M. (2007). Healing and wound classification. Journal of Community Nursing, 21(9), 26-32.
2. Johnstone, C., Farley, A., & Hendry, C. (2005). The physiological basics of wound healing. Nursing Standard, 19(43), 59-65.
3. Lloyd-Jones, M. (2007). Tissue viability: the physiology of wound healing. British Journal of Health Care Assistants, 1(4), 181-184.
4. Streker-McGraw, M., Jones, T., & Baer, D. (2007). Soft tissue wounds and principles of healing. Emergency Medicine Clinics of North America, 25, 1-22.