wound healing

General pathology



Types of wound healing in the skin:
1- Healing by First Intention (primary union )
The least complicated example of wound repair is the healing of a clean, uninfected surgical incision approximated by surgical sutures, such healing is referred to as primary union or healing by first intention. The incision causes death of a limited number of both epithelial and connective tissue cells. The narrow incisional space immediately fills with clotted blood containing fibrin and blood cells; dehydration of the surface clot forms the well-known scab that covers the wound.
Within 24 hours, neutrophils appear at the margins of the incision, moving toward the fibrin clot. The epidermis at its cut edges thickens as a result of mitotic activity of basal cells, and spurs of epithelial cells from the edges both migrate and grow along the cut margins of the dermis. They fuse in the midline beneath the surface scab, thus producing a continuous but thin epithelial layer.
By day 5, the incisional space is filled with granulation tissue. Collagen fibrils become more abundant and begin to bridge the incision.
During the second week, there is continued accumulation of collagen and proliferation of fibroblasts, while the leukocytic infiltrate, edema, and vascularity have largely disappeared.
By the end of the first month, the scar comprises a cellular connective tissue devoid of inflammatory infiltrate, covered now by intact epidermis.
Note :- Although most skin lesions heal efficiently, the end product may not be functionally perfect. Epidermal appendages do not regenerate, and there remains a dense connective tissue scar in place of the mechanically efficient meshwork of collagen in the unwounded dermis.

2- Healing by Second Intention (Wounds With Separated Edges(.
This type of wound healing seen in case of extensive loss of cells and tissue, as occurs in inflammatory ulceration, abscess formation, and surface wounds that create large defects, the reparative process is more complicated. The common denominator in all these situations is a large tissue defect that must be filled. Regeneration of parenchymal cells cannot completely reconstitute the original architecture. Abundant granulation tissue grows in from the margin to complete the repair. This form of healing is referred to as secondary union or healing by second intention.
Secondary healing differs from primary healing in several respects:
1- Inevitably, large tissue defects initially have more fibrin and more necrotic debris and exudate that must be removed, so the inflammatory reaction is more intense.
2- Much larger amounts of granulation tissue are formed.
3- the phenomenon of wound contraction, which occurs in large surface wounds.

Note: - Whether a wound heals by primary or secondary intention is determined by the nature of the wound, rather than by the healing process itself.

Wound Strength
When sutures are removed, usually at the end of the first week, wound strength is approximately 10% of the strength of unwounded skin, but it increases rapidly over the next 4 weeks. This rate of increase then slows at approximately the third month after the original incision and then reaches a plateau at about 70 to 80% of the tensile strength of unwounded skin, which may persist for life. The recovery of tensile strength results from increased collagen synthesis exceeding collagen degradation during the first 2 months and by replacement of collagen type III by collagen type I, which is the main type of interstitial collagen of the skin.
Factors That Influence Wound Healing
A- Systemic factors include the following:
1- Nutrition has profound effects on wound healing. Protein deficiency, for example, and particularly vitamin C deficiency inhibit collagen synthesis and retard healing.
2- Metabolic status can change wound healing. Diabetes mellitus, for example, is associated with delayed healing.
3- Circulatory status can regulate wound healing. Inadequate blood supply usually caused by arteriosclerosis or venous abnormalities that retard venous drainage also impair healing.
4- Hormones, such as glucocorticoids, have well-documented anti-inflammatory effects that influence various components of inflammation and additionally, these agents inhibit collagen synthesis.
B-Local factors that influence healing include the following:
1-Infection is the single most important cause of delay in healing.
2- Mechanical factors, such as early motion of wounds, can delay healing.
3- Foreign bodies, such as unnecessary sutures or fragments of steel, glass, or even bone, constitute impediments to healing.
4-Size, location, and type of wound influence healing. Wounds in richly vascularized areas, such as the face, heal faster than those in poorly vascularized ones, such as the foot. As we have discussed, small injuries produced intentionally heal faster than larger ones caused by blunt trauma.
Pathologic Aspects of Wound Repair
Complications in wound healing can arise from abnormalities in any of the basic repair processes. These aberrations can be grouped into three general categories:
(1) deficient scar formation.
Inadequate formation of granulation tissue or a scar can lead to two types of complications: wound dehiscence and ulceration.
Dehiscence or rupture of a wound is most common after abdominal surgery and is due to increased abdominal pressure. This mechanical stress on the abdominal wound can be generated by vomiting, coughing, or ileus.
Ulceration of wounds can occur because of inadequate vascularization during healing, for example, lower extremity wounds in individuals with atherosclerotic peripheral vascular disease typically ulcerate. Non-healing wounds also form in areas devoid of sensation, these neuropathic ulcers are occasionally seen in patients with diabetic peripheral neuropathy .
(2) excessive formation of the repair components.
Excessive formation of the components of the repair process can also complicate wound healing. The accumulation of excessive amounts of collagen may give rise to a raised tumorous scar known as a keloid, or hypertrophic scar. Keloid formation appears to be an individual predisposition, and for reasons unknown this aberration is somewhat, it is more common in blacks. The mechanisms of keloid formation are still unknown.
(3) formation of contractures.
Contraction in the size of a wound is an important part in the normal healing process. An exaggeration of this process is called a contracture and results in deformities of the wound and the surrounding tissues. Contractures are particularly prone to develop on the palms, the soles, and the anterior aspect of the thorax. Contractures are commonly seen after serious burns and can compromise the movement of joints, to a degree than required plastic surgery to restore function of the parts affected.