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Types of Wound Healing

Wounds are classified by the way the wound closes. A wound can close by primary intention, secondary intention or tertiary intention.

1. Primary Intention

Primary healing involves reepithelialization, in which the skin's outer layer grows closed. Cells grow in from the margins of the wound and out from epithelial cells lining the hair follicles and sweat glands. (Johnstone, Farley,& Hendry, 2005)

Wounds that heal through primary intention are, most commonly, superficial wounds that involve only the epidermis and don't involve the loss of tissue, for example, a first-degree burn. However, a wound that has well-approximated edges (edges that can be pulled together to meet neatly), such as surgical incision, also heals through primary intention. Because there is no loss of tissue and little risk of infection, the healing process is predictable. These wounds usually heal in 4 to 14 days and result in minimal scarring. (Slachta, 2003)

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2. Secondary Intention

A wound that involves some degree of tissue loss heals by secondary intention. The edges of these wounds can't be easily approximated, and the wound itself is described as partial thickness or full thickness, depending on its depth: (Johnstone, Farley,& Hendry, 2005)

During healing, wounds that heal by secondary intention fill with granulation tissue, a scar forms, and reepithelialization occurs, primarily from the wound edges. Pressure ulcers, burns, dehisced surgical wounds, and traumatic injuries are examples of this type of wound. These wounds also take longer to heal, result in scarring, and have a higher rate of complications than wounds that heal by primary intention. (Slachta, 2003)

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3. Tertiary Intention

When a wound is intentionally kept open to allow edema or infection to resolve or to permit removal of exudate, the wound heals by tertiary intention, or delayed primary intention. These wounds result in more scarring than wounds that heal by primary intention but less than wounds that heal by secondary intention. (Johnstone, Farley,& Hendry, 2005)

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Phases of Wound Healing

Normal wound healing occurs in four phases: the hemostasis, the inflammation, the proliferation and the maturation. However, healing rarely occurs in this strict order. Typically, the phases of wound healing overlap. (Slachta, 2003)

1. Hemostasis

Immediately after an injury, the body releases chemical mediators and intercellular messengers called growth factors that begin the process of cleaning and healing the wound. (Harvey, 2005)

When blood vessels are damaged, the small muscles in the walls of the vessels contract, reducing the flow of blood to the injury and minimizing blood loss. Vasoconstriction can last as long as 30 minutes. (Harvey, 2005)

Next, blood leaking from the inflamed, dilated, or broken vessels begin to coagulate. Collagen fibers in the wall of the damaged blood vessels activate the platelets in the blood in the wound. Aided by the action of prostaglandins, the platelets enlarge and stick together to form a temporary plug in the blood vessel, which helps prevent further bleeding. The platelets also release additional vasoconstrictions, such as serotonin, which help to prevent further blood loss. Thrombin forms in a cascade of events stimulated by the platelets, and a clot forms to close the small vessels and stop bleeding. (Johnstone, Farley,& Hendry, 2005)

This initial phase of wound healing occurs almost immediately after the injury occurs and works quickly (within minutes) in small wounds. It's less effective in stopping the bleeding in larger wounds. (Johnstone, Farley,& Hendry, 2005)


2. Inflammation

This phase is both a defense mechanism and a crucial component of the healing process. During this phase, the wound is cleaned and the process of rebuilding begins. This phase is marked by swelling, redness, and heat at the wound site. (Johnstone, Farley,& Hendry, 2005)

Vascular permeability increases, permitting serous fluid carrying small amounts of cell and plasma protein to accumulate in the tissue around the wound (edema). The accumulation of fluid causes the damaged tissue to appear swollen, red, and warm to touch. (Johnstone, Farley,& Hendry, 2005)

During the early phase of the inflammatory process, neutrophils (one type of white blood cell) enter the wound. The primary role of neutrophils is phagocytosis, or the removal and destruction of bacteria and other contaminants. (Harvey, 2005)

As neutrophil infiltration slows, monocytes appear. Monocytes are converted into activated macrophages and continue the job of cleaning the wound. The macrophages play a key role early in the process of granulation and reepithelialization by producing growth factors and by attracting the cells needed for the formation of new blood vessels and collagen. (Johnstone, Farley,& Hendry, 2005)

The inflammatory phase of healing is important for prevent wound infection.


3. Proliferation

During the proliferative phase, the body: (Johnstone, Farley,& Hendry, 2005)

The proliferative phase involves regeneration of blood vessels (angiogenesis) and the formation of connective or granulation tissue. The development of granulation tissue requires an adequate supply of blood and nutrients. Endothelial cells in blood vessels in surrounding tissue reconstruct damaged or destroyed vessels by first migrating and then proliferating to form new capillary beds. As the beds form, this area of the wound takes on a red, granular appearance. (Harvey, 2005)

During this phase, growth factors prompt fibroblasts to migrate to the wound. Fibroblasts are the most common cell in connective tissue; they are responsible for making fibers and ground substance, also known as extracellular matrix, which provides support to cells. At first, fibroblasts populate just the margins of the wound, they later spread over the entire wound surface. (Johnstone, Farley,& Hendry, 2005)

Fibroblasts have the important task of synthesizing collagen fibers which, in turn, produce keratinocyte, a growth factor needed for reepithelialization. This process necessitates a delicate balance of collagen synthesis and lysis (making new and removing old). If the process yields too much collagen, increased scarring results. If the process yields too little collagen, scar tissue is weak and easily ruptured. (Harvey, 2005)

As healing progresses, myofibroblasts and the newly formed collagen fibers contract, pulling the wound edges toward each other. Contraction reduces the amount of granulation tissue needed to fill the wound, thereby speeding the healing process. (Johnstone, Farley,& Hendry, 2005)

Complete healing occurs only after epithelial cells have completely covered the surface of the wound. As this occurs, keratinocytes switch from a migratory mode to a differentiative mode. The epidermis thickens and becomes differentiated, and the wound is closed. Any remaining scab comes off and the new epidermis is toughened by the production of keratin, which also returns the skin to its original colour. (Johnstone, Farley,& Hendry, 2005)


4. Maturation

The final phase of wound healing takes place when wound remodeling occurs. There is a diminution of the vasculature and an enlargement of the collagen fibers, which increase the tensile strength of the repair. This is a gradual, transitional phase of healing that can contribute for months or even years after the wound has closed. (Johnstone, Farley,& Hendry, 2005)

During this phase, fibroblasts leave the site of the wound, the scar shrinks and becomes pale, and the mature scar forms. If the wound involved extensive tissue destruction, the scar won't contain hair, sweat, or subaceous glands. The time frame for wound maturation, under optimal wound healing conditions is 21 days to 2 years. The wound gradually gains tensile strength. In primary intention wounds, tissue will achieve approximately 20% of the original strength between days 1 and 7. When fully healed, tissue will achieve, at best, approximately 70-90% of its original strength. The longer it takes for a wound to heal, the more likely it is for scarring to occur. Scar tissue will always be less elastic than the surrounding skin and is never as strong as the original intact skin. (Johnstone, Farley,& Hendry, 2005)

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Factors that affect Wound Healing

The wound healing process is affected by many factors. The most important influences include: (Slachta, 2003)


Wound-Healing Complications

Unfortunately, not all wounds heal. The most common complications of healing include: (Slachta, 2003)