Surgical physiology of wound healing: a review.AK Deodhar, RE Rana
The healing of wounds caused by accident, assault, welfare and surgical operations has always been a central consideration in surgical practice because any breach in continuity of skin or mucous membrane exposes the deeper tissues to the danger of infections. The understanding of the mechanism of wound healing has increased dramatically during last few years. Today wound healing abnormalities are among the greatest causes of disability and deformity. "I dressed the wound, God healed it" (Ambroise Pare) wound healing involves multiple complicated events. It is the amount and quality of scar tissue and ultimately its remodelling that is of greater importance. The understanding of this process of wound healing and factors affecting it forms the basis of any surgical procedure.
Keywords: Human, Wound Healing, physiology,
Cutaneous healing may be defined broadly as the interaction of a complex series of phenomena that eventuates in the resurfacing, reconstitution and proportionate restoration of tensile strength of wounded skin.
Phases of wound healing
1. Vascular and inflammatory phase
3. Granulation tissue formation
4. Fibroplasia and matrix formation
5. Wound contraction
7. Matrix and collagen remodelling
Immediately after the tissue injury blood vessels and lymphatics are disrupted. An initial 5-10 minutes period of vasoconstriction is followed by more persistent vasodilatation. Blood components are extravasated into wound cavity. Endothelial cells retract and loose their attachments with adjoining cells, exposing subendothelial factor VII related Von Willebrand factor and fibrillar collagen in the injured tissue. Platelets adhere to these surfaces to form platelet plug and are activated to release the components within their intracellular granules. Hageman factor (XII) is activated, leading to subsequent events of intrinsic coagulation pathway. These events are important in the formation of a fibrin clot to coapt the wound edges initially. Platelets upon activation also secrete soluble modulators of wound healing upon activation and release their granular contents. These include chemotactic and growth factors such as platelet derived growth factor (PDGF), proteases and vasoactive substances such as serotonin and histamine.
The cellular elements important in the inflammatory phase of wound healing are the polymorphonuclear leucocyte (PMN) and the monocyte or macrophage. The PMN is short lived, and though initially the predominant cell type, is largely replaced by the macrophage by the fifth day after wounding. The prime PMN function is one of the phagocytosis and killing of contaminant bacteria. Macrophage helps in bacterial phagocytosis and tissue debridement. This cell has an important function in directing the subsequent course of wound healing. After activation in the wound, these cells release proteases and vasoactive peptides as well as growth and chemotactic factors for fibroblasts and endothelial cells.
Inflammation and particularly macrophages therefore play a critical role in the wound healing scenario. Vasoactive, chemotactic, proliferative and other factors are produced from a variety of inflammatory pathways (such as kinins and complement cascades) and activated cells (such as platelets and macrophages). These factors are active in stimulating chemotaxis and proliferation of fibroblasts, endothelial and other cells.
The process of epithelial resurfacing is critical in order for the wound to be considered `healed’. The initial event in epithelisation is the migration of undamaged epidermal cells from the wound margins and from the epithelium of hair follicles and other adnexal structures, if the defect is superficial enough. This process occurs within hours of wounding and is a directed event that doesn’t require an initial increase in cellular proliferation. After migration has begun, an increase in epithelial proliferation at the wound margins occurs to provide the additional cells needed for wound cover. Proliferation is maximal at 48 to 72 hours after wounding and is reflected by a 17 fold increase in mitosis and epithelial hyperplasia at the wound edges.
Keratinocytes assist in the process of reepithelisation by producing fibronectin, collagenases, plasminogen activator, neutral proteases and type V collagen. Fibronectin is an important matrix component that promotes adhesion of keratinocytes and assist in their guidance across the wound base. Collagenase and other proteases are important in debridement of devitalised tissue.
Role of fibronectin in wound healing is as follows :
1. Cross links with fibrin to provide matrix for cell adhesion and migration.
2. Functions as an early component of the extracellular matrix.
3. Binds to collagen and interacts with matrix glycosaminoglycans.
4. Has chemotactic properties for macrophages, fibroblasts and endothelial and epidermal cells.
5. Promotes opsonization and phagocytosis.
6. Forms a component of the fibronexus.
7. Forms scaffolding for collagen deposition.
Plasminogen activator initiates fibrinolytic mechanisms that assist in dissolving the clot. Type V collagen is synthesized by epidermal cells and may be required during the ongoing process of reepithelisation.
The plane of movement of epidermal cells is determined in part by the water content of the wound bed. Theepithelium seeks a plane of migration with a critical humidity. Open, dessicated, superficial wound epithelises much more slowly than occluded wound. Occlusive and semiocclusive dressings optimally promote reepithelisation postoperatively.
Fibroplasia and matrix formation
Granulation tissue consists of inflammatory cells, fibroblasts and new vasculature in a hydrated matrix of glycoproteins, collagen and glycosaminoglycans. Its formation begins within 3-5 days after wounding and overlaps with the preceding inflammatory phase.
The fibroblast is the critical cell in the formation of granulation tissue. It not only produces collagen but also forms elastin, fibronectin, sulfated and non-sulfated glycosaminoglycans and proteases such as collagenases that are important in wound remodelling.
Shortly after wounding resident skin fibroblasts and perivascular mesenchymal cells differentiate into a phenotypically different cell the “Myofibroblast”. The migration and proliferation of this cell is directed in large part by a host of chemotactic and growth factors. Fibronectin is important because it functions not only as a chemoattractant but also when cross linked with fibrin acts as an adhesive scaffolding upon which the myofibroblast can migrate and later synthesize collagen.
Growth factors important in wound healing are
* Epidermal growth factor
* Macrophage derived growth factor (MDGF)
* Platelet derived growth factor (PDGF)
* Certain lymphokines
In a mature scar or normal skin, type I & II collagen make up the majority of collagen present and type I predominates. In contrast, early granulation tissue is composed largely of type III collagen. After a 5 day lag period these fibers are laid down on a framework of fibronectin and the pre-existence of fibronectin may be critical for subsequent collagen deposition.
Glycosaminoglycans are also important during granulation tissue formation. During the early phases (first 4 days) of granulation tissue formation hyaluronic acid is a prominent component of the matrix. In the later stages of granulation tissue formation hyaluronic acid is replaced by a variety of proteoglycans. These include chondroitin-4-sulfate, dermaten sulfate, heparin sulfate and others. They are important in contributing to tissue resilience and may play a role in regulating collagen synthesis.
A) Wound contraction
It is defined as the centripetal movement of the edges of a full thickness wound in order to facilitate closure of the defect.
Contraction is maximal between 5 and 15 days after wounding and is mediated to a great extent by the myofibroblast and its specialized connections with the surrounding extracellular matrix. Isolated granulation tissue strips when stimulated using 5HT, angiotensin, vasopressin and other mediators, appear to contract as a unit, much as muscle does. This phenomenon led to theory that there were specialised connections between the contractile myofibroblasts and the other elements of granulation tissue. These connections would mediate the forces of contraction in a united fashion throughout the open wound.
Singer has described such a specialised structure as the “Fibronexus”. It consists of a very intimate association across the cytoplasmic membrane of the myofibroblast of intracellular actin microfilaments and extracellular fibronectin fibers. They form connections with collagen fibers and other myofibroblasts. Thus as the actin filaments within the myofibroblast contract, the fibronexus transmits this force to the surrounding matrix and mediates the clinical phenomenon of wound contraction.
The initial event in this process is the directed migration of endothelial cells. There is an initial fragmentation of the venule basement membrane possibly mediated by collagenase and plasminogen activator elaborated by the endothelial cells. The endothelial cells develop pseudopodia that protrude through disrupted basement membrane and the entire cell migrates into the perivascular space.
Hypoxic conditions appear to stimulate migration of cells and this phenomenon is mediated in part by a macrophage derived angiogenic factor elaborated during times of low oxygen tension (PO2). There is an increase in fibronectin surrounding the neovasculature of the healing wound. Because of its adhesive properties it acts as scaffolding upon which these cells can migrate. Fibronectin has also been shown to be chemotactic for endothelial cells.
This process continues for months after reepithelisation has occurred and may continue indefinitely. The events in remodelling are responsible for the increase in tensile strength, decrease in erythema and scar tissue bulk, and the final appearance of the healed scar.
Fibroblast is the most important cell behind the synthesis of collagen. Rough endoplasmic reticulum in the fibroblast is the site of collagen synthesis. Hydroxylation of proline and lysin is important in collagen synthesis, and it occurs with the help of various co-factors like iron, copper, vitamin C etc. The collagen molecule that is secreted by the fibroblast, after hydroxylation, is referred to as procollagen. After the amino and carboxy terminal peptides have been removed, the molecule is termed collagen. The primary collagen molecules crosslink with each other to form the fibrils and fibers that provide the tensile strength to the wound.
As collagen is laid down, fibronectin gradually disappears. The nonsulfated glycosaminoglycans, hyaluronic acid is replaced by more resilient proteoglycans such as chondroitin-4-sulfate. In addition, water is gradually reabsorbed from the scar allowing collagen fibers and other matrix components to lie closer together. This facilitates cross linking of collagen fibers mediated by lysyl oxidase that provides the increase in scar tensile strength.
Collagen bundles grow in bulk and become progressively reoriented from a random pattern to like parallel to the skin surface. Type I collagen becomes the major collagen present in the remodelled scar, reversing the earlier type III collagen predominance.
Important local factors affecting wound healing are discussed below.
It is the most common local cause for prolonged healing. All wounds are contaminated postoperatively by resident bacterial flora, however clinical infection occurrs when a critical number of pathogenic organisms are present. Bacteria prolong healing by activating the alternate complement pathway and detrimentally exaggerating and prolonging the inflammatory phase of wound healing. They also elaborate toxins and proteases that can be damaging to cells. Finally, they compete for oxygen and nutrients in the wound milieu. Lactic acid is produced in this hypoxic state, that further stimulates the release of damaging proteolytic enzyme.
Formation of excessive devitalised tissue, increased tension in the wound, hematoma and seromas, foreign bodies in the wound, all these factors predispose for bacterial secondary infection. All these can be avoided by proper surgical techniques.
2. Surgical Technique
The rough handling of tissue or the use of inappropriately bulky instrumentation can lead to crushed skin edges and subsequent devitalization of tissue, leading to increase in inflammatory reaction and risk of secondary infection with increased scarring.
Wound closed with inappropriately reactive suture material may increase the chances of a foreign body reaction and subsequent infection. Skin sutures tied too tightly may lead to tissue ischaemia and predispose to infection.
3. Haematoma Formation
Excessive bleeding and the formation of a hematoma within the wound not only can mechanically disrupt the wound closure but also can serve as an excellent culture medium for micro organisms.
4. Foreign body reaction
A foreign body in the wound serves as an appropriate surface for the activation of the alternate complement pathway and the generation of a prolonged inflammatory response, which interferes with the subsequent stages of wound repairs. Wounds containing foreign materials are characterised by low pH and low PO2. These factors significantly slow down wound repair.
5. Tissue ischaemia
Local factors such as foreign bodies, infection or strangulating sutures significantly slow healing by promoting tissue ischemia. Local hypoxia is detrimental to cellular proliferation, resistance to infection and collagen production. The cumulative effect is delayed healing.
6. Topical medications and dressings
Occlusive or semiocclusive dressings promote faster reepithelization. They may also alter certain aspects of dermal repair. They also provide the moist environment needed for optimal wound repair, they may also help to prevent bacterial invasion and wound infection.
Local medicaments applied to the wound may affect wound repairs. Even the bases in which these agents are compounded may accelerate or diminish the rates of epithelization. Triamcinolone acetonide ointment (0.1%) nitrofurazone, benzoyl peroxidase cream, silver sulfadiazine ,neosporin ointment are examples of the drugs that affect epidermal migration.
1. Deficiency states
a. Metabolism : Aberrant carbohydrate and fat metabolism slows wound repair. Glucose may be unavailable or fail to enter cell properly. Insulin may act as a fibroblast growth factor and its deficiency leads to suppress collagen deposition in the wound.
Negative nitrogen balance and relative protein deficiency may occur after major trauma or during sepsis. Fibroplasia and all aspects of matrix formation are delayed, wound remodelling is also impaired. Cellular and humoral immune responses are blunted and bacterial phagocytosis and killing are defective. Protein deficiency may lead to an increased propensity for infection.
b. Vitamins : Vitamin A deficiency has been associated with slowed reepithelization, decreased collagen synthesis and stability and an increased susceptibility to infection.
Vitamin C (Ascorbic acid) is an essential cofactor during collagen biosynthesis. In scurvy, the collagen formed is unhydroxylated, relatively unstable and subject to collagenolysis.
Vitamin K deficiency results in a deficiency in the production of vitamin K dependent clotting factors (factors II, VII, IX and X) resulting in bleeding diathesis, hematoma formation and secondary detrimental effects on wound healing.
c. Trace elements and minerals : These are required as cofactors for various enzymes during wound healing. These include zinc, copper, iron, manganese etc. Zinc deficiency however is more important clinically, as it is a constituent of multiple important metalloenzymes including collagenase and DNA and RNA polymerases. Its deficiency results in impaired immune responses, decreased protein and collagen synthesis, decreased lysyl oxidase activity and interfere with vit. A transport.
Physiologic aging diminishes virtually all phases of wound healing. Disease status associated with accelerated aging (such as Werner’s syndrome) may be characterized by recalcitrant cutaneous ulcerations and impaired healing.
The mechanism behind lack of scarring in fetal wounds are unknown, but probably relate to the control of collagen fibrillogenesis. The role of collagen in the fetal wound matrix is controversial. Longaker and coworkers found that collagen was deposited in fetal wounds much more rapidly than in adults. Collagen deposition occurred in a normal dermal and mesenchymal pattern in second and early third trimester in fetal lambs. These findings are consistent with the observation that fetal wounds heal faster and without scar formation.
3. Disease states
Some of the most important diseases leading to impaired wound healing are listed here.
Disease states associated with impaired wound healing
Ehlers-Danlos syndrome More Details, Prolidase deficiency
Hemophilia, Von Willebrand’s disease, Factor XIII deficiency, Hypofibrinogenemia
Congestive heart failure, Atherosclerosis, Vasculitis
Chronic renal failure
Immunologic deficiency states
Chronic pulmonary disease
Chronic liver disease (cirrhosis)
Myelofibrosis and other chronic hematologic disorders associated with thrombocytopenia
Other chronic illness
The immunologic deficiency states may impair healing by predisposing the wound to infection and diminishing the inflammatory phase of wound healing.
Some of the drugs causing impaired wound healing are
Zinc sulfate (high doses)
Beta amino proprionitrile
Better documented are the effects of anticoagulants and glucocorticosteroids. Anticoagulants indirectly interfere with healing by increasing the chances of bleeding and hematoma formation. Corticosteroids in contrast, directly inhibit wound healing. They diminish inflammation, decrease protein and collagen synthesis, decrease epidermal proliferation, interfere with host defense mechanisms and promote the hypercatabolism of existing collagen. In addition they may lead to relative tissue ischaemia by means of their vasoconstrictive properties.