Wound Healing.docx

**[Wound Healing]** Moderator : Dr Akshay Samagani Presenter : Dr Amrita Kumari [Introduction] **[Structure and Function of the Skin]** - The skin provides a life-protective barrier between the body and the external environment against physical damage, pathogens, fluid loss, and has i...

**[Wound Healing]** Moderator : Dr Akshay Samagani Presenter : Dr Amrita Kumari [Introduction] **[Structure and Function of the Skin]** - The skin provides a life-protective barrier between the body and the external environment against physical damage, pathogens, fluid loss, and has immune-neuroendocrine functions that contribute to the maintenance of body homeostasis. - Its structure is composed of two layers: the epidermis and the dermis. The epidermis contains keratinocytes, melanocytes, dendritic cells, Langerhans cells and other immune cells, sensory axons, and the epidermal-dermal basement membrane. - The dermis has the skin appendages, mast cells, fibroblasts, antigen presenting dermal cells, resident and circulating immune cells. - Additionally, the dermis includes the extracellular matrix complex that provides support to intercellular connections, cellular movement, and regulates cytokine and growth factors' functions. - Skin innervation consists of a dense network of sensory and autonomic fibers that form tight junctions with keratinocytes and transmit sensations of pain, temperature, pressure, vibration, and itch. - Skin circulation is composed of parallel arterial-venous thermoregulatory shunt circulation controlled by tonic adrenergic sympathetic vasoconstrictor and vasodilator nerves that give origin to a subepidermal capillary network that provide oxygen and nutrients to the epidermis and remove CO2 and waste products. - The lymphatic vessels of the skin consist of lymph capillaries that run horizontally under the epidermis, followed by precollector vessels located deeper in the dermis and lymph collecting vessels in the subcutaneous fat layer. - Lymph vessels are connected to the skin local draining lymph nodes, and lymph vessels that exit these lymph nodes converge to the regional sentry lymph nodes before reaching the thoracic duct. **[The Healing Process]** - After injury, skin integrity must be promptly restored in order to maintain its functions. In this process, peripheral blood mononuclear cells, resident skin cells, extracellular matrix, cytokines, chemokines, growth factors, and regulatory molecules participate in the wound healing process. - The intricate skin repair process has been organized in three sequential and overlapping steps: the inflammatory phase, the proliferative phase, and the remodelling phase. - The inflammatory phase includes cutaneous neurogenic inflammation and hemostasis; these early events start in the first seconds after injury and last approximately 1 hour. Followed by the fast recruitment of neutrophils to the injured tissue during the first 24 hours and its posterior decline during the subsequent week. - The progressive infiltration of inflammatory monocytes-macrophages to the wound starts the second day after injury and continues to increase, reaching its maximum during the proliferative phase, starting its decline during the following two weeks, becoming the dominant mononuclear cell in the tissue repair process. - Circulating lymphocytes migrate to the skin early after injury reaching a plateau by day 4 and their presence continues for two more weeks before declining. The last phase starts in the second week after injury and includes remodeling the tissue previously formed in the proliferation phase and the organization of a scar in order to restore the skin integrity. - This last stage could last for months. This review provides present day information regarding the central role of the resident and peripheral immune cells as well as the microenvironment and their interactions during the wound healing process. **[Definition]** Dorland's medical dictionary defines wound as "disruption in the normal continuity of the body structure". Even though the conventional definition specifies a wound as disruption of skin layer, wounds secondary to dermatology procedures can be superficial. The healing process depends on local wound factors, systemic mediators, the underlying disease and the type of injury. Chronic wounds are the result of an inadequate repair process that is unable to restore anatomic and functional integrity in an appropriate length of time. **[Phases in Wound Healing ]** The three phases in wound healing are : +-----------------------------------+-----------------------------------+ | **\ | | | \ | | | 1. Inflammatory phase** | | | | | | **2. Proliferative phase** | | | | | | **3. Remodeling phase** | | +-----------------------------------+-----------------------------------+  **[Inflammatory Phase ]** ***[Cutaneous Neurogenic Inflammation]*** ***[Platelets Hemostasis]*** ***[The Role of Peripheral Blood Mononuclear Cells during the Inflammatory Phase --]*** - ***Neutrophils*** - ***Monocytes*** - ***Monocyte-Macrophages*** - ***Lymphocytes*** - ***Innate Lymphocytes*** The initial phase after cutaneous injury is dominated by inflammatory reactions mediated by cytokines, chemokines, growth factors and their actions on cellular receptors. The first step in this phase is vascular response and hemostasis. Hemostasis consists of two phases: (1) formation of fibrin clot and (2) coagulation. - Following tissue injury, hemostasis occurs to minimize hemorrhage. When the blood vessels constrict, platelets are activated by binding to the exposed collagen in the extracellular matrix. - The platelets then release various factors such as fibronectin, thrombospondin and von Willebrand factor, which promote further platelet activation and aggregation.3 The activated factors result in the formation of a fibrin matrix which functions as a provisional matrix to stabilize the wound site. The aggregated platelets then become trapped in the fibrin matrix, thus forming a stable clot within the provisional matrix. - Platelets release several important mediators which are responsible for the initiation and progression of wounds through the subsequent phases of wound healing. The degranulation of alpha granules and dense bodies causes release of mediators such as platelet-derived growth factor (PDGF) and transforming growth factor-β (TGF-β). - Transforming growth factor-α (TGF-α) and PDGF recruit various cells such as neutrophils and macrophages to enter the wound. Platelet-derived growth factor (PDGF) also recruits fibroblasts to the wound and activates the production of collagen and glycosaminoglycans by fibroblasts, which are important for the repair of the extracellular matrix. - The next phase of wound healing is inflammation, which begins within the first 24 hours after an injury. The vasoactive substances released by the damaged cells and clot breakdown mediate this response. - The dilated capillaries become leaky and thus allow the efficient movement of neutrophils from the vasculature to the site of injury. Leucocyte migration occurs by diapedesis and margination. As various chemical signals are released from the wound site, the endothelial cells in the nearby vessels are activated and begin to express specialized cell adhesion molecules (CAMs) called selectins. - These CAMs cause the neutrophils to bind to the endothelial cell surface by a process called pavementing. The adherent neutrophils begin to roll along the endothelial cell lining and then by a process called diapedesis, they squeeze through the cell junctions that have been made leaky by the mast cell mediators. - The primary function of the neutrophils is phagocytosis of any foreign materials and to kill bacteria by the powerful battery of enzymes and reactive oxygen species, which they can generate. The neutrophils actually initiate the first stage of the proliferative phase by releasing IL1 and tumor necrosis factor (TNF-α) to begin the activation of fibroblasts and epithelial cells. - During the inflammatory phase, activated macrophages also play a key role in the regulation of wound healing. Wound macrophages are derived from fixed tissue monocytes that originate from circulating monocytes. - The macrophages function to remove residual bacteria, foreign bodies and remaining necrotic tissue. Macrophages also secrete a number of growth factors and cytokines, such as PDGF, TGF-β, TNF-α, fibroblast growth factor (FGF), insulin like growth factor-1, and IL-6, which then recruit fibroblasts and endothelial cells to the wound site for matrix deposition and neovascularization. - The macrophages also produce tissue inhibitors of matrix metalloproteinases which inhibit the enzymatic destruction of tissues.8 The result of the inflammatory phase is a clean wound bed.  **[Proliferative Phase ]** - ***Fibroplasia*** - ***Reepithelialization*** - ***Angiogenesis*** - ***Peripheral Nerve Repair*** - The proliferative phase is characterized by fibroblast proliferation and collagen deposition to provide a stable extracellular matrix at the wound site. Within a few days after wounding, the fibroblasts start synthesizing collagen. Deposition of collagen gradually enables the skin integrity to be restored. - The new extracellular matrix consists of collagen, proteoglycans and fibronectins. In addition, angiogenesis occurs such that new blood vessels replace the previously damaged capillaries and provide nourishment for the matrix. - **[The hallmark of proliferation stage is formation of granulation tissue.]** Granulation tissue consists of a dense network of blood vessels and capillaries, elevated cellular density of fibroblasts and macrophages and randomly organized collagen fibers. Fibroblasts migrate into the wound in response to mediators released from the platelets and macrophages and move through the extracellular matrix. - The fibroblasts also secrete matrix metalloproteinases (MMPs), which facilitate their movement through the matrix and help with the removal of damaged matrix components. Once the fibroblasts have entered the wound, they produce collagen, proteoglycans and other components. - Neoangiogenesis is stimulated by angiogenic factors secreted by keratinocytes at the wound site. Endothelial cells are activated by TNF-α and basic FGF (bFGF) to initiate angiogenesis. The newly formed blood vessels invade the matrix and promote blood flow to support the high metabolic activity in the newly deposited tissue. - Angiogenesis is regulated by a combination of local stimulatory factors, such as vascular endothelial cell growth factor (VEGF) and antiangiogenic factors, such as angiostatin, endostatin, thrombospondin and pigment epithelium-derived growth factor. Local factors that stimulate angiogenesis include low oxygen tension, low pH and high lactate levels. **[Remodeling Phase ]** - The last phase of wound healing is the remodeling phase in which granulation tissue matures to form a scar. The tissue remodeling phase starts as early as a few days after injury and lasts up to 2 years. The capillaries aggregate to form larger blood vessels and cell density and overall metabolic activity of the wound decrease. - The collagen fibers become organized resulting in increased tensile strength of the wound. Initially, there is an increased deposition of type III collagen, also referred to as reticular collagen, that is gradually replaced by type I collagen, the dominant fibrillar collagen in skin.11 As the wound continues to remodel, changes in collagen organization increases the tensile strength to a maximum of about 80% of normal tissue. - An important group of proteinases are matrix metalloproteinases (MMPs) which play a central role in wound healing as they degrade certain constituents of provisional wound tissue, such as collagen I, III, IV, and VII.12 They also facilitate epithelial cell migration into the wound, angiogenesis and overall tissue remodeling. - Matrix metalloproteinases (MMPs) are secreted by epidermal cells and modulate tissue inhibitors of metalloproteinases (TIMPs) as well as degrade other growth factors.13,14 Similarly, a disruption in the balance between MMPs and their inhibitors has been reported in diabetic and venous stasis ulcers. - In addition, MMPs can be found in increased levels in chronic wounds.16 MMP expression is regulated by TGF-β. Transforming growth factor-β also minimizes matrix degradation by downregulating protease secretion and stimulating synthesis of TIMP. - The process of collagen synthesis and degradation usually continues for a long time as the matrix strives to achieve the original highly organized structure that was present before the wound injury. The scar tissue is always weaker than the normal surrounding matrix and can only achieve about 80% of the tensile strength that was present initially. If matrix synthesis is greater than degradation, then a hypertrophic scar may form. - Conversely, if matrix degradation is greater than synthesis or if synthesis is inhibited by pharmacologic agents such as steroids or cancer chemotherapeutic agents, the scar becomes too weak and wound dehiscence can occur.  **[Factors Resulting in Chronicity of Wound]** **These include both local and systemic factors as shown in Table 5-1.** **TABLE 5-1: Factors affecting wound healing and resulting in chronicity of wound** **[Primary Versus Second Intention Healing]** - When an acute wound, such as one resulting from a surgical excision, is allowed to heal on its own, this is termed second (secondary) intention healing. In primary intention healing, closure of the wound is accomplished by approximating the wound edges; the latter includes side-to-side closures, flaps, and grafts. Both primary and second intention forms of wound healing proceed through the three phases of healing. - In second intention healing, the time interval until complete re-epithelialization depends upon several factors. These include wound depth, anatomic location (e.g. facial wounds heal faster than acral wounds), any secondary infection, vascular supply, and geometric shape (for a given area, a more narrow diameter will heal faster). For smaller wounds, especially those that are partial-thickness, primary and second intention healing can produce similar cosmetic results. Once wounds have a diameter \>8 mm, primary intention healing leads to better cosmetic results. - Tertiary intention healing refers to wounds that are closed with the goal of primary intention healing, but dehiscence occurs and the wound is then allowed to heal by second intention. **[Common Complications of Normal Skin Acute Wound Healing Process]** **[Keloids and Hypertrophic Scars ]** - The presence of TGF-β isoforms, receptors and activity modulators, may have a major role in the development of both keloids and hypertrophic scarring. Similar results were detected with co-culturing of keloid derived keratinocytes and fibroblasts, suggesting that keratinocytes might have an important role in keloid and hypertrophic scar pathogenesis by producing signals that stimulate the fibroblasts in the underlying dermis to proliferate or produce more extracellular matrix (ECM). - Increased numbers of mast cells have been reported during the active period of hypertrophic and keloid scar formation. Clinically, the release of histamine by these cells contributes complaint of itchiness. In addition, the vasodilatory effect of histamine may promote erythema and leakage of plasma proteins into the regional tissues. - The major effectors of ECM degradation and remodeling belong to a family of structurally-related enzymes called MMPs. The levels of MMP expression in normal cells are low and allow healthy connective tissue remodeling. - Several MMPs have been shown to mediate the breakdown of type I and III collagen, the most abundant types of collagen in the skin ECM. Interestingly, hypertrophic scars and keloids were found to have high levels of MMP-2 and low levels of MMP-9. -.The low levels of decorin,a small proteoglycan, found in hypertrophic scarring may account for their irregular collagen organization as well as increased ECM production. - Other abnormalities seen in keloids include delayed apoptosis of myofibroblasts and prolongation of scar maturation. **[Acne Scars ]** - The pathogenesis of acne is currently attributed to multiple factors such as increased sebum production, alteration of the quality of sebum lipids and rogen activity, proliferation of Propionibacterium acnes (P. acnes) within the follicle and follicular hyperkeratinization. - The inflammatory process occurs in the infundibular region which result in follicular rupture and perifollicular abscess formation. These initiate a cascade of wound healing events. Holland et al. found that the inflammatory reaction at the pilosebaceous gland was stronger and had a longer duration in patients with scars than those without; in addition, the inflammatory reaction was slower in those with scars than patients who did not develop scars. - They showed a strong relationship between severity and duration of inflammation and the development of scarring, suggesting that treating early inflammation in acne lesions may be the best approach to prevent acne scarring. - Fibroblasts and keratinocytes produce enzymes including those that determine the architecture of the MMPs and tissue inhibitors of MMPs. As a consequence, an imbalance in the ratio of MMPs to tissue inhibitors of MMPs results in the development of atrophic or hypertrophic scars. **[Wound Healing After Procedures ]** - **[Cryosurgery\ ]** - Freezing of tissue with liquid nitrogen results in blister formation in a span of 2--3 days. Rupture of the blister results in eschar formation. Post-procedural depigmentation can occur in dark skinned patients. - **[Chemical Peeling ]** - Chemical peels are utilized to produce damage to the epidermis and superficial dermis. This results in epidermal regeneration and postinflammatory neocollagen formation. - **[Dermabrasion ]** - Dermabrasion creates a partial thickness wound to heal by secondary intention. In partial thickness wounds, the adnexal structures are preserved and act as a reservoir for epidermal proliferation. Transforming growth factor-β, keratinocyte growth factor and epidermal growth factor stimulate re-epitheliazation from both the appendageal structures and epithelial cells. - **[Fractional Lasers ]** - The mechanisms of the action of laser resurfacing include tissue ablation, immediate collagen shrinkage, and dermal collagen remodeling. Fractional laser may result in macroscopic epidermal regeneration. - Collagen contraction induced by fractional reticular dermal ablation was thought to be the underlying factor leading to clinically observed skin tightening. The improvement resulting from the ablation process may be seen soon after the epidermal regeneration phase is complete, whereas the clinical benefit arising from the dermal remodeling process normally starts months after treatment and continues over years after the initiation of the cutaneous wound healing cascade. - A recent in vivo study in human skin revealed that neocollagenesis was persistent for a minimum of 3 months after a CO2 ablative fractional resurfacing treatment as confirmed by a constant up-regulation of the heat shock protein expression. - **[Dressing]** - The ideal dressing should create an optimal environment for wound healing. The wound bed should be kept moist for migration of epithelial cells. Generally, the following rules apply: desiccated wounds should be hydrated; absorbent dressings are well-suited for exudative wounds; necrotic crusts should be debrided; if a wound is infected, directed antimicrobial treatment should be initiated. - **[Gauze ]** - Gauze consists of woven or non-woven fibers and is permeable, non-occlusive, absorbent, inexpensive and readily available. Wet-to-dry dressings mechanically debride tissue; however, more recent evidence shows this technique causes pain to the patient and impairs healing by non-selectively removing healthy granulation tissue. - Advantages of petroleum impregnated gauze such as Xeroform Gauze® (Coviden) or Adaptic Gauze® (Johnson and Johnson) include their non-adherent nature, painless removal and ability to hydrate the wound. - Disadvantages include the inability to absorb exudates and the higher cost compared to standard gauze. Studies show that semiocclusive dressings such as petrolatum-impregnated gauze did not reduce either time for wound healing or total cost of wound care compared to non-occlusive gauze in surgical patients. - Gauze may also be impregnated with antibacterial agents such as iodine, bismuth, zinc, silver, sulfadiazine, framycetin and chlorhexidine. In order to increase absorption and control odor, activated charcoal may be added as well. - **[Transparent Film Dressings ]** - Transparent film dressings (TegadermTM, 3M and OpsiteTM, Smith and Nephew) are composed of polyurethane and are impermeable to bacteria and water but permeable to oxygen, CO2 and water vapor. - However, transparent films are not absorptive, and the occlusion provides an ideal environment for bacterial growth in infected wounds. - **[Foam Dressings ]** - Foam dressings are semiocclusive, bilaminate with a hydrophilic foam layer and a hydrophobic polyurethane or silicone top layer. Foam dressings have been used successfully for surgical wounds healing by secondary intention following soft tissue tumor excision. - Advantages include providing a moist environment to the wound bed, absorption exudates and providing protection against bacterial infection. However, foam dressings are too absorptive for dry wounds, are not absorptive enough for very exudative wounds and necessitate frequent dressing changes. - **[Hydrogels ]** - Hydrogels such as FlexigelTM (Smith and Nephew) contain a hydrophilic crosslinked organic polymer with up to 95% water content and are available as sheets, amorphous gels and impregnated dressings. - They are particularly effective for transmitting water to dry and necrotic wounds. They also thermally insulate wounds and decrease postoperative pain. A limitation of hydrogels is that they are not as effective for highly exudative or bleeding wounds. Also, hydrogels are permeable to water and gases, and thus provide a less effective barrier against bacteria than more occlusive dressings. - Another disadvantage is possible maceration of surrounding healthy skin. Hydrogels are used in partial-thickness wounds, pressure ulcers, burns and after cosmetic procedures such as dermabrasion, laser resurfacing and chemical peels. Although literature in the postsurgical wound population is limited, hydrogels with silver were shown to decrease scar length and lower complication rate in primarily closed surgical wounds.. - **[Hydrocolloids ]** - A colloid is defined as one phase of matter that is uniformly dispersed within another phase of mater. This dispersion is possible because of mutually attracted charges between the two materials. Hydrocolloid dressings such as DuoDERM® (Convetec) contain an inner layer composed of hydrophilic colloid materials (pectin, karaya, gelatin or carboxymethyl cellulose) that form a gel that is able to absorb water and expand. The hydrocolloids are available as sheets with a semipermeable outer layer and adhesive bottom layer. - These dressings provide a moist environment, promote autolytic debridement of the tissue, protect from infection, do not require secondary dressing and can be left in place up to 7 days. Other advantages include conformation to the wound, a cushioning effect that relieves pressure and waterproof properties. Disadvantages include possible maceration of normal skin at wound edges, a yellowish color and malodor of the gel during dressing changes and possible incorporation of gel material into the wound causing granulomatous inflammation with delayed epithelialization - **[Alginates ]** - Alginates are exceedingly absorptive, water soluble and biodegradable dressings derived from brown seaweed. Alginate dressings are most appropriate for highly exudative wounds and are left in place for several days, during which a hydrophilic yellowish gel forms - This gel provides a non-adherent cover and maintains a moist environment. Another unique advantage of alginates is their hemostatic properties. Disadvantages are excessive desiccation of dry wounds and the yellowish gel and malodor may be confused with purulence. - Although alginates are especially useful in highly exudative venous stasis ulcers, there may not be as much clinical benefit in postsurgical wounds. For instance, in a study comparing occlusive paraffin gauze and alginates plus polyurethane film in granulating split-thickness skin graft donor site wounds, there was no clinical advantage and cost was higher in the alginate group. - **[Hydrofibers ]** - Hydrofiber dressings such as Aquacel (Convatec Ltd.) are structurally similar to alginates, and thus have similar properties and indications. They are composed of sodium carboxymethyl cellulose and form a gel when placed in contact with exudative fluid. Like alginates, hydrofiber dressings are indicated for highly exudative and/or infected wounds. - In a study comparing Aquacel with paraffin gauze in split-thickness skin graft donor sites, the Aquacel group had more rapid healing and superior cosmetic appearance of the final scar. - **[Silicone Dressings ]** - Silicone sheeting such as Cica-Care (Smith and Nephew) has been shown to soften and prevent hypertrophic and keloid scars. Fibroblasts activity is slowed and collagen deposition is decreased with silicone sheeting. - The mechanism of action is not known, but there are theories that the silicone improves hydration of the scar. Unfortunately, data are limited by small sample sizes, short follow up times and lack of controls. - **[Dressings with Antimicrobial Properties ]** - Silver has broad-spectrum activity against fungi, viruses and bacteria, including MRSA and VRE, and is available in conjunction with almost any other type of dressing. Silver ions control infection by inhibiting enzymes involved in bacterial cell wall synthesis and gene transcription. - Silver has been shown to decrease the risk of burn wound-related sepsis. - For wound healing, there is a benefit because of decreased bacterial load, however conflicting studies report concerns for cytotoxicity to healthy granulation tissue with povidone-iodine. Interestingly, the newer cadexomer iodine has not been shown to have cytotoxic effects on keratinocytes.  **[REFERENCES]** 1. Srinivas C R,Sekhar sanmuga C;In: Sacchinand S, Venkatraman M, Nagaraju U, editors. Textbook on cutaneous and astheticsurgery 1^ST^ ed. New Delhi: Jaypee Brother Medical Publishers, 2012 p62-267. 2. Newsom E, Connolly K, Nehal K, In Moschella and Hurley's Dermatology Fourth ed. Babar K Rao, New Delhi: Jaypee Brother Medical Publishers, 2020 p1627-1630. 3. Hafner A, Sprecher E , In Jean L. Bolognia J L, Julie V. Schaffer, Lorenzo Cerroni, Fourth ed. Babar K Rao, New Delhi: Jaypee Brother Medical Publishers, 2018 p1834. 4. O'Toole E, Mellerio J E Wound Healing. In: Griffiths CEM, Barker J, editors. Rook\'s Textbook of Dermatology.8^th^ ed. UK: John Wiley & Sons, Ltd; 2016. p. 14.1- 14.19 **Thank you**