Definitions
A wound is defined as an injury or tear on the skins surface due to physical, chemical, mechanical, and/or thermal damage. A more scientific definition is a disruption of the normal anatomic structure and function of the skin (8).
There are two categories of wounds: acute and chronic wounds. Acute wounds usually heal within 8 to 12 weeks. These wounds can be caused by mechanical damage induced by the stabbing action of hard objects. Acute wounds can also form by exposure to extreme heat, irradiation, electrical shock, and/or irritated with corrosive chemicals.
Chronic wounds are slow healing injuries, produced as a result of specific diseases such as diabetes, tumours and persistent infections, as well as patient specific factors e.g. old age. Healing can take more than 12 weeks and recurrence of the wound is common (8).
One can also define and classify wounds according to their appearance: Epithelializing (clean, medium to high exudates) (a), granulating (clean, exudating) (b), slough-covered (c) and necrotic (dry) wounds (d), respectively (image 3)(9). Wounds are also classified based on the number of skin layers and area of skin affected. Injury affecting just the epidermis is called a superficial wound, whilst injury involving both the epidermis and the deeper dermal layers including the blood vessels, sweat glands and hair follicles are called partial thickness wounds. Full thickness wounds occur when the underlying subcutaneous fat or deeper tissues are damaged, in addition to the above layers (9).
There are two categories of healing. Healing by primary intention is typical for simple surgical wounds. The wound edges are kept together with sutures or staples and healing occurs by wound epithelialisation and connective tissue deposition. These wounds usually heal quickly (approximately 30 days) if an infection isn't present. Healing by secondary intention is typical for chronic wounds like venous leg ulcers. The wound is left open, and healing occurs by granulation tissue formation, contraction of the wound edges and then epithelialisation. They heal slowly due to the volume of connective tissue required to fill the defect. Since there is no epidermal barrier, the risk of an infection is significantly higher in these wounds (18).
Image 2: Animation of normal wound healing (7)
Image 3: Wounds according to their appearance (9)
The Wound Healing Process
This process can be considered a dynamic process, occuring in all tissues and organs of the body. Cellular and matrix components act to re-establish the integrity of damaged tissue and replace lost tissue. It consists of complex overlapping stages which last for variable lengths of time. However, any stage may be prolonged due to local factors such as ischemia or poor nutritional intake. Tissue repair is arbitrarily divided into phases, in order to aid understanding of the physiological processes happening in the wound and surrounding tissue (2,6).
The cascade of carefully and precisely regulated steps and events, correlate with the appearance of various cell types in the wound bed during distinct phases of healing. Separate parts of a wound may be at different stages of healing at any one time. Timing and interactions between the components taking part in the wound healing process differ for acute and chronic wounds, although the main phases remain the same (2,6).
Regardless of the source or the extent of tissue damage, in normal conditions wound healing occurs in a predictable fashion as four overlapping stages: inflammation, migration, proliferation and maturation a.k.a remodelling. Healing is considered to be complete when the skin surface has reformed and has regained its tensile strength. (6)
Inflammation is the body’s initial response to injury and involves both cellular and vascular response. The release of histamine and other cell-mediated factors into the wound results in vasodilation, increased capillary permeation and stimulation of pain receptors. Release of a protein-rich exudate containing phagocytes and other materials from the blood capillaries onto the wound surface engulfs the debris of dead cells and bacteria (autolytic debridement). Fibrinogen in the exudate initiates clotting, producing a clot or scab on the wound which causes bleeding to stop. It also gives strength and support to the injured tissue. This stage usually occurs within minutes to 24 hours of injury, when the wound will be red, inflamed, painful and moist. (2,6)
Migration growth factors in the wound exudate promote the growth and migration of epithelial cells, fibroblasts and keratinocytes to the injured area to replace damaged and lost tissue. These cells regenerate from the margins, rapidly growing over the wound under the dried scab. This epithelial thickening and basal cell proliferation lasts for 2–3 days. (2,6)
Proliferation: This phase involves the development of new tissue, and occurs simultaneously or just after the migration phase. It lasts 5 - 20 days. Granulation tissue is formed by the infiltration of blood capillaries and lymphatic vessels into the wound, and by the supporting collagen network synthesized by fibroblasts (a process known as granulation). The network is important for developing the tensile strength of the skin. As proliferation continues, further epithelial cell migration across the wound occurs, providing closure and visible wound contraction. During this stage, the wound is typically beefy red in colour and moist, but not exuding. (2,6)
Maturation: This final phase of wound healing (also called the ‘remodelling phase’) involves the process of the vasculature and enlargement of collagen fibres, which increase the tensile strength of the repair. The timescale for wound repair is from about 3 weeks - 2 years. The tensile strength of the final scar usually reverts to 70–90% of that of the pre-injured tissue. (2,6)
