Introduction
Wounds can heal primarily or secondarily. In primary wound healing, the wound edges adapt themselves or are adapted tension-free by sutures. The wounds usually heal very quickly and almost without scarring.
All that remains is a fine, barely visible scar. The prerequisites for primary wound healing are smooth wound edges, non-irritating wounds and no infections. Typically, these prerequisites are given after operations, in the case of wounds caused by sharp objects or after larger superficial wounds (e.g. abrasions).
- Crush wound
- Laceration
- Laceration
Secondary wound healing usually does not take place without complications. The edges of the wound are not smooth and cannot adapt well to each other or cannot be adapted without tension by sutures. The wound heals from depth by granulation, contraction and epithelialization.
The wound remains open until the end so that pus and wound secretions can drain off. Secondary wound healing occurs due to infection or poor circulation (e.g. gangrenous foot in diabetes mellitus). The healing process here takes much longer than in primary wound healing and a wider scar remains.
Stages of wound healing
The closure of a tissue defect can be achieved either by regeneration or repair of the tissue. During physiological regeneration or in the case of superficial injuries (e.g. skin abrasions), the tissue is completely replaced by the original tissue. No scars are left behind and the tissue is as functional after healing as it was before the injury.
The epidermis and mucous membranes in particular possess this ability to regenerate. However, the majority of injuries, especially deeper injuries of the skin, heal by repair. This results in the formation of inferior replacement tissue (scar tissue).
This is less functional. It merely closes the defect, but is not capable of all cellular differentiation forms. This means that no new skin appendages such as hair or sweat glands can be formed.
Repair is divided into four main phases. Overall, the wound is most sensitive in the period between the removal of the necrosis and the formation of the granulation tissue. Mechanical stress in this phase can lead to serious complications and greatly impair wound healing.
Once collagen synthesis has begun, the mechanical load and tear resistance of the wound increases continuously. Rough time estimates can be given as a guide: After about 1 week of wound healing, the tensile strength of the wound is about 3%, after 3 weeks about 20% of the maximum. This maximum of the tensile strength of a scar is about 80% and is reached after about 3 months.
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- In the exudation phase of wound healing (1st to 8th hour after injury), the capillaries are initially constricted to keep blood loss as low as possible, coagulation sets in and hemostasis occurs. The vessels then dilate, causing white blood cells and platelets to be transported to the site of injury. The wound is filled with wound secretion, dead collagen particles are removed and growth-promoting cytokines are released.
Fibrin formation occurs. This closes the wound defect mechanically and makes it resistant to mechanical stress.
- On the first to fourth day after the injury, the resorption phase of the wound healing occurs. This is characterized by the body’s own defense system.
Bacteria are fended off, necrotic tissue is cleared away and the fibrin is dissolved again. The entire resorption phase is thus characterized by the cleansing and defense of foreign bodies in order to protect the wound from infection and prepare it for the ingrowth of new cells.
- Following the resorption phase, the proliferatin phase of wound healing follows (3rd to 10th day). In this phase, new capillaries are formed (angiogenesis).
In addition, new epithelial cells and fibroblasts are activated. These close the wound defect mechanically. The strongly capillarized connective tissue grows from the edge of the wound into the wound until the defect is completely filled.
Due to the strong capillarization, the wound appears granular (= granulum, lat- the granule) and is therefore also called granulation tissue.
- The differentiation phase of wound healing begins from about day 7 onwards. This can last for months and consists of the actual scar formation.The number of connective tissue cells in the area of the wound decreases, as does the number of capillaries. This results in an increase in fibrous connective tissue.
- Wound healing concludes with epithelization.
During this process, marginal epithelial cells migrate into the fibrous connective tissue and the actual scar is formed. The resulting scar tissue is initially raised and impressed with a reddish color. After a few weeks, the scar tissue adapts to the skin level and the color fades.
A white scar develops. Since the pigment cells (melanocytes) cannot be regenerated, the scar becomes lighter in color than the rest of the skin surface.
Only minutes after a wound has developed, the body begins to close the injury. Depending on the author, three to five phases of wound healing are distinguished, which overlap in time.
The sequence of events is as follows: If one speaks of only three phases, the first and last phases are omitted. The latency phase describes the period of time between the development of the injury and the onset of wound healing; this period of time is called the latency period. Immediately after the wound has developed, a blood clot is formed from escaping blood from injured vessels, so that major blood loss can be prevented by closing the vessels again as quickly as possible.
This is followed by the exudation phase. In medicine, exudation refers to a leakage of fluid. In this case, the exudate consists of a liquid that is squeezed out of the passing blood, or more precisely blood serum, and is then called wound secretion.
The purpose of the wound secretion is to flush foreign bodies out of the wound. The secretion also contains cells of our immune system, in particular macrophages and white blood cells (especially granulocytes), which kill bacteria and absorb dead material and remove it from the wound. For example, dead skin parts and coagulated blood are removed from the wound to make room for the newly growing tissue.
The immune cells also produce signal substances that stimulate the cells to grow, which should later close the wound again. If there are too many bacteria in a wound, many, many immune cells can produce pus from the wound secretion and an inflammatory reaction occurs. If only a few germs are present, the inflammation is hardly noticeable.
The wound secretion also contains fibrin, a kind of endogenous glue. It is part of the blood clotting system and on the other hand, fibrin seals the wound edges as well as possible by sticking together. The wound secretion usually dries up over the course of a few days, so that the typical scab develops on the surface.
This acts like the body’s own plaster and underneath it the healing process can proceed undisturbed.
- Rest or latency phase
- Exudation phase
- Granulation or proliferation phase
- Regeneration phase
- Maturation phase.
If the wound conditions are properly established, new tissue can completely close the wound. This is done in the granulation or proliferation phase.
Proliferation means cell growth. This occurs through intact cells at the edges of the wound. These begin to divide continuously and thus produce new tissue.
If the edges of the wound, such as superficial incisions, fit together optimally, the tissue can grow back together with the original tissue. Larger wounds must first be filled with granulation tissue. Granulation tissue describes a network of connective tissue and in-growing blood vessels that must first be gradually stabilized and converted into the desired tissue.
Since this tissue looks granular (lat = granule: granules), this has given the phase its name. If the original tissue can no longer be restored exactly, scar tissue is formed. This tissue does not have the same properties as the original tissue and is therefore less resilient.
In addition, there is a lack of hair, sweat glands, pigment cells and nerve tracts for sensitivity to pain, for example. New blood vessels for the supply of nutrients are also absolutely necessary for the new tissue. These sprout into the granulation tissue during tissue proliferation and supply the new tissue with oxygen and nutrients.
The uppermost skin layer is also regenerated. This happens in the regeneration or repair phase. On the one hand, new skin is formed, on the other hand, wound edges contract and thus reduce the wound area. The final scar tissue only develops over many months up to two years in the maturation phase (maturation = maturation). It adapts to local requirements, but always remains less resilient than original tissue.This is also the reason why surgical treatments should induce the smallest possible scars.
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