5.a Wound Healing

Wound healing is a complex process involving a highly regulated cascade of biochemical and cellular events designed to achieve restoration of tissue integrity following injury. The repair process in human tissue combines both aspects of tissue repair and regeneration in response to tissue damage or loss. In tissue repair scar is generated at the site of repair.


Types and forms of healing

Surgeons usually divide the wound healing into repair by first, second and third intention.

Primary or first intention healing occurs when tissue is cleanly incised and re-approximated and healing occurs without complications. The incisional defect re-epithelizes rapidly and matrix deposition seals the defect.

Secondary or second intention healing occurs in open wounds. When the wound edges are not approximated and it heals with formation of granulation tissue, contraction and eventual spontaneous migration of epithelial cells.

Third intention healing (delayed primary) occurs when a wound is allowed to heal open for a few days and then closed as if primarily. Such wounds are left open initially because of gross contamination.


Phenomena of wound healing

Wound healing is a concert of simultaneously occurring processes rather than a series of discrete steps, which for convenience sake can be divided into early, intermediate, late and final phases.


Early wound healing events

Coagulation

Any injury results in disruption of blood vessels leading to haemorrhage which is controlled by clot formation which contains fibrin mesh with aggregated platelets embedded in it. Fibrin is the end product of coagulation pathway and besides causing heamostasis it is also the primary component of the provisional tissue matrix seen in the early phases of wound healing. It provides a scaffold for the migration of inflammatory and mesenchymal cells.


Platelet aggregation is a vicious cycle and leads to release of cytokines, which includes PDGF, TGF-a, FGFb, PDEGF. These cytokines influence wound healing directly or indirectly.


The processes of clot formation and platelet aggregation terminate when stimuli for clot formation dissipate.


Lysis of clot starts along with clot formation and is mediated by plasminogen activator, which converts plasminogen to plasmin.


Inflammation

Tissue trauma stimulates the inflammatory response. Immediately after injury intense local vasoconstriction occurs, mediated by circulating catecholeamines and prostaglandins released by injured cells. This is followed by vasodilatation and increased capillary permeability resulting in local edema. This is mediated by histamine, kinins, prostaglandins, leukotrienes, and endothelial cell products.


Neutrophils are the first leukocytes to be found in wounded tissues. They phagocytose damaged tissue or bacteria. Neutrophils themselves are phagocytosed by macrophages. Pain in the area of injury is due to changes in pH due to break down of tissues and bacteria along with swelling and decreased tissue oxygenation due to disruption of blood vessels. Neutrophil count of the wound increases for 24-48 hrs and then declines unless wound contamination has occurred.


Monocytes transform into macrophages as they migrate from capillaries into extra vascular space. Macrocytes phagocytose bacteria and tissue debris and secrete enzymes (collagenase and elastase) responsible for breaking down damaged matrix. They also cytokines, P.G.s, oxygren free radicals and other regulators of wound healing.


Lymphocytes produce various factors like HB-EGF(Heparin binding epidermal growth factor), basic fibroblast growth factor and they are also involved in cellular and humoral immunity.


Initially for 24-48 hrs neutrophils dominate but 48-72hrs later they are outnumbered by macrophages which persist for few days. After 5-7 days fibroblasts are the predominant cell type.


Intermediate wound healing events

Mesenchymal cell migration and proliferation

The fibroblasts from the adjoining undamaged tissues migrate into the wound matrix under the influence of cytokines. The movement of cells is possible due to their ability to bind and release fibronectin, fibrin and vitronectin and also proteolytic enzymes (matrix metaloproteinase 1-3) help by creating a path through the matrix for cell migration. There is also proliferation of native as well as newly arrived mesenchymal cells. These fibroblasts secrete collagen and proteoglycans of connective tissue matrix that unites wound edges together by assuming polymeric form.


Angiogenesis

Stimulated by raised lactate levels, decreased pH and tissue hypoxia, capillaries sprout from existing venules. These capillary sprouts grow by proliferation of endothelial cells and in primarily closed wounds the sprouting vessels soon meet their counter parts from the other side of the wound thus re-establishing blood flow across the wound. In unclosed wounds, the new capillaries fuse only with neighbors migrating in the same direction and so forming granulation tissue. Cytokines including FGF, TGF-a, EGF, TGF-b, PDECGF, VEGF, angiogenin, interlukin-8, wound fluids, P.G.s and adipocyte lipids stimulate angiogenesis.


Epithelialization

Epithelialization alone is enough to provide total healing in partial thickness wounds. Incisional wounds are usually completely re-epithelized in 24-48 hrs.


Epithelialization can be divided into separate cellular events including cell dedifferentiation, mitosis, migration and proliferation, which begin within hours of injury and results in resurfacing any denuded area. Thickening of the basal cell layer at the wound edge is the earliest aspect of epithelialization process. The marginal basal cell layer then elongates and detaches from the basement membrane with subsequent migration into the wound. These cells migrate as a single layer in a leap frog fashion and usually orient themselves along collagen fibres exhibiting contact guidance till they meet similar cell types when adhesions occur the entire process reverts to a resting stage, the phenomenon called contact inhibition. Cells of the mono-layer then differentiate into multi-layer. A new basement membrane is generated beginning at the wound edge. Cellular proliferation continues as a multi-layered epithelium is re-established. Subsequently new surface cells begin to keratinize. Cytokines are involved in all aspects of epithelialization and they include EGF, TGF-a, HB-EGF, IGF and members of FGF.


Unfortunately regenerated epithelium does not retain all the functional advantages of normal epithelium. These include fewer basal cells, abnormal interface between dermis and epidermis and thin epithelium in the mid portion of the re-epithelized wound.


Late wound healing events

Collagen sythesis

Once the fibroblast has migrated into the wound, they switch there major function to protein synthesis. Collagen is the major component of the normal skin, granulation tissue and mature scar and is synthesized primarily by the fibroblasts. This activity starts by 3-5 days post injury and the rate of synthesis increases rapidly and continues at an accelerated rate for 2-4 weeks and starts declining after 4 weeks eventually becoming equal to rate of collagen destruction by collagenase. Collagen provides structural configuration, strength and matrix for cellular mobility in the wound.


Matrix components-collagen fibre lysis and contraction

Replacement of extra-cellular matrix is a complex process as it contains components other than collagen including proteoglycans, fibronectin and elastin.


Proteoglycans are synthesized primarily by fibroblasts and consists of protein core covalently linked to glycosaminoglycans including chondroitin sulphate, dermatan sulphate, heparin & heparin sulphate, keratan sulphate and hyaluronic acid.


Fibronectin are mainly attachment proteins and important in various phases of wound healing.


Elastin is not synthesized in response to injury and hence the absence of elasticity in scar tissue.


Wound contraction

Starts 4-5 days after injury, and is represented by centripetal movement of wound edges towards the centre of the wound. The average rate of wound contraction is 0.6-0.75 mm/day. Myofibroblasts in the injured area are thought to be responsible for wound contraction.


Final wound healing events

Scar remodeling is the hallmark of the final phase in healing process. Collagen degradation is in a finely controlled equilibrium with collagen synthesis. The process of scar remodeling dramatically increases the wound bursting strength, by 6 weeks after the injury, the wound has reached 80-90% of its eventual strength. This process continues for 6-12 months and is visible as change in color, texture and thickness of healing wound.



CHRONIC WOUNDS

Most of chronic wounds are associated with a small number of well defined clinical entities like chronic venous stasis, pressure necrosis and diabetes mellitus, and these entities contribute for about 70% of chronic wounds. Besides the chronic wounds attributable to specific causes, there are several pathophysiologic elements purposed for their roles in the failure of wounds to heal.


Pathophysiologic mechanisms

The smoothness and orderliness of healing process frequently is disrupted by some underlying abnormality that prolongs the inflammatory phase there by generating a cascade of tissue responses that perpetuate the non-healing state.


Repeated trauma, foreign bodies, pressure necrosis, infection, ischaemia, tissue hypoxia are few of the factors proposed for promoting a chronic inflammatory state characterized by increased number of inflammatory cells- neutrophils, macrophages and lymphocytes. Subsequently dead tissue debris, bacterial products, foreign bodies are powerful chemo-attractants capable of sustaining a continuous influx of inflammatory cells. These wounds are characterised by high levels of inflammatory cytokines like TNF and interleukins, activated collagenase as well as matrix degrading enzymes including elastase. So chronic wound microenvironment is characterized by an imbalance between matrix degrading enzymes and their inhibitors, with the former gaining upper hand. Such a wound microenvironment results in degradation of all protein elements found in the tissues. So the matrix deposition does not gain a foothold and epithelium proceeds slowly. Thus sets in a vicious cycle capable of propagating wound chronicity. Any effective intervention must include steps for disrupting this cycle and resetting the wound on a path towards healing.



PROBLEM SCARS

Wound healing occurs by scar formation which restores the structural integrity of tissues but can be problematic if they are too weak, too strong or too abundant.


Hypertrophic scar

The standard term defines it as a raised, erythematous, pruritic lesion that remains within the confines of the original scar. Most of the scars at least temporarily appear hypertrophic but usually under regression with scar remodeling and maturation. So within 6 weeks to 6 months after wound closure, normal scar begins to fade and regress in size though some may persist the same for years.


Histology reveals collagen fibres arranged in nodules containing mybroblasts and increased density of blood vessels


Following conditions predispose the wonds for hypertrophic scar formation:-

Histology reveals collagen fibres arranged in nodules containing mybroblasts and increased density of blood vessels


Keloids

They are erythematous and pruritic lesions which spread in dermis and adjacent subcutaneous tissues. They may develop upto 1 year after injury and rarely regress on their own. Keloid scaring represents true genetic abnormality in wound healing as there is a positive family history.


Keloids are locally invasive benign neoplastic scar tissue.


Histology shows:-


Treatment of scar remodelling

Prevention

Early wound clsure

Gentle tissue handling

Planned skin incisions to avoid skin tension lines.


Conventional strategies include thicker skin grafts, use of skin flaps, scar massage, static splinting, casting, ultrasound heat therapy and scar compression.


Biophysical therapies include compression pressure, silicone gel sheets, ultrasonic tissue heating, low dose ionizing radiation.


Pharmacological therapies include:-


Surgical measures include reorienting unfavourably placed scars or complete removal of abnormal scar lesions and wound closure using meticulous surgical tecnique. Addition of adjunctive therapies may help reduce the recurrence rate.



FUTURE- SCARLESS HEALING

Although adult skin wounds heal by scar formation, early gestational fetal wounds heal without scar and thus may hold the key to scar-less repair. Important concepts central to the fetal wound healing include:-


Once the biology of fetal wound healing is fully determined, attempts to manipulate the adult wound will progress rapidly and scarless repair may become a clinical reality in children and adults.