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Figure 5-7. The biochemical factors that elicit migration of cells from intravascular compartment into the wound site.

Figure 5-8. Macrophage functions.

Epidermal cells migrate over the scaffold and only after the epithelial bridge is completed, enzymes

are released to dissolve the attachment at the base of the overlying scab that falls off. In response to the

growing need for oxygen and nutrients at the site of healing, the wound microenvironment stimulates

the release of factors needed to bring in a new blood supply (low pH, reduced oxygen tension, and

increased lactate). This process – angiogenesis or neovascularization is stimulated by VEGF, basic

fibroblast growth factor (bFGF), and TGFb. These factors are secreted by several cell types including

vascular endothelial cells, epidermal cells, fibroblasts, and macrophages.

As the proliferative phase progresses the predominant cell in the wound site is the fibroblast. This

multifunctional cell of mesenchymal origin mainly produces and deposits the new matrix for structural

integrity at the level of the wound bed (Fig. 5-9B). ECM production is the defining feature of the

proliferative phase. The ECM is primarily collagen. At least 23 individual types of collagen have been

identified – type I is present mostly in scar tissues.5 Fibroblasts produce collagen via their attachment to

the cables of the provisional fibrin matrix.6 After transcription and processing of the collagen messenger

ribonucleic acid, it is attached to polyribosomes on the endoplasmic reticulum where the new collagen

chains are produced. During this process, there is an important step involving hydroxylation of proline

and lysine residues.7 The collagen molecule transforms itself into the classical triple helical structure

and thereafter its nascent chains are modified through glycosylation8; this procollagen molecule is

released into the extracellular space.9 The hydroxyproline in collagen gives the molecule its stable

helical conformation.10 Whereas fully hydroxylated collagen has a higher stability, unhydroxylated

forms are fragile, similar to collagen produced under anaerobic disease conditions or vitamin C-deficient

states (scurvy), wherein the collagen undergoes denaturation easily and can break.

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Figure 5-9. A: Phase 3 – Proliferation. Vast array of cells are recruited into the wound bed and carry out diverse functions

including proliferation and deposition of ECM. B: Fibroblast functions.

Figure 5-10. Phase 4 – Remodeling and Maturation.

Finally, collagen released into the extracellular space undergoes further processing by cleavage of the

procollagen N- and C-terminal peptides. In the extracellular spaces, an important enzyme, lysyl oxidase,

acts on the collagen to form stable cross-links. As the collagen matures and becomes older, more and

more of these intramolecular and intermolecular cross-links are placed in the molecules. This important

cross-linking step gives collagen its strength and stability over time.11

Remodeling Phase

The remodeling phase is characterized by continued synthesis and degradation of the ECM components

trying to establish a new equilibrium – and the formation of an organized scar (Fig. 5-10). Collagen

degradation occurs

12 via the action of specific collagenases that are secreted by various cells:

fibroblasts, neutrophils, and macrophages each of which can cleave the collagen molecule at differing

but specific locations on all three chains, and break it down to characteristic three-quarter and onequarter pieces. These collagen fragments undergo further denaturation and digestion by other proteases.

Several molecules including TGF play a major role in the remodeling phase. TGF-β-induced intracellular

signaling acts via a set of proteins called the SMAD proteins, which act as direct links between the cell

surface and the nucleus. The recent development of several SMAD pathway specific knockout mice and

transgenic animals has confirmed the pivotal nature of the SMAD pathway in fibrogenesis and

tumorigenesis. Still, several difficulties remain before the TGF-β/SMAD pathway can be efficiently

targeted in situations such as tissue fibrosis or impaired wound healing. In particular, the precise

spatiotemporal role of each TGF-β/SMAD pathway component during the development of excessive

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ECM deposition leading to tissue fibrosis remains to be ascertained.

As the scar matures, late remodeling occurs (that takes up to 1 year); the scar contracts and thins out

(Fig. 5-11).

CLINICAL APPLICATIONS

Surgical Technique

The surgeon equipped with the knowledge of the fundamentals of wound healing is prepared to

insightfully minimize risks of wound healing complications while performing a surgical procedure from

start to finish. The stage is set for healing from the moment the incision is made. The skin and dermis

should be incised perpendicularly to the plane of the surface. Attention to this principle is particularly

important when making an incision on a curved surface. Electrosurgical currents should be used set on

the lowest power settings that accomplish hemostasis. The deep tissues should be handled as

atraumatically as possible. The incision is carried down through deeper layers ensuring that each new

incision is accurately placed in the same line as the previous one. This avoids a saw tooth surface with

devitalized sections. Proper tissue handling techniques in the subcutaneous fat and adjacent soft tissue

are based on well-established wound healing principles, which minimize the risk of infection, seroma,

delayed healing, unnecessary scarring, and other postoperative wound complications. For traumatic

wounds, the first step in treatment is to convert them into controlled surgical wounds by thoughtful

debridement and tissue repair. Treating traumatic wounds minimizing the risk of postoperative wound

complications requires mindfulness of wound healing principles in all of these clinical circumstances.

This is true regardless whether performing repair of a simple laceration or the most complex specialized

procedure. Controlling the degree of injury leads to improved outcomes with fewer complications

related to a failure in the wound healing process. An awareness of the wound healing process informs

proper surgical technique. A clear understanding of wound healing allows the surgeon to advance in

technical skill and achieve continuously improving outcomes throughout a professional career.

Figure 5-11. Contraction of scar – This process occurs over the course of 1 year.

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Figure 5-12. Effects of biofilm on wound healing.

Elective surgery creates controlled tissue injury. Minimizing tissue injury forms the basis of proper

surgical technique. Trauma inures tissues in an uncontrolled fashion. Wounds can occur under special

conditions such as pressure, physiologic impairment (e.g., diabetes) that create traumatic tissue damage

in an uncontrolled injury. Finally, there are wounds that occur under specialized circumstances such as

radiotherapy in cancer treatment.

Biofilm

Biofilm comprises a colony of microorganisms enveloped with a matrix of extracellular polymers.

Estimated biofilm-associated infections costs >$1 billion annually. Both chronic and acute dermal

wounds are susceptible to the formation and propagation of biofilm. Covered in other chapters of this

book, biofilm is relevant to wound healing due to the several inhibitory effects on healing processes

(Fig. 5-12).

Table 5-2 Comparison of Acute and Chronic Wounds

Chronic Versus Acute wounds

Wounds heal within a reasonable time of 4 to 6 week. Those that do not are termed chronic. As shown

in Table 5-2, a variety of factors and disease conditions impair wound healing and result in chronic

nonhealing wounds. Reasons that lead to chronicity instead of normal healing, are complex – but the

salient points are defined in Figure 5-13. The most important concept is a persistent proinflammatory

condition that paradoxically leads to increased degradation of matrix proteins, and an unstable wound

that is recalcitrant to healing.

Comorbid Conditions that Influence Wound Healing

3 Wound healing is a complex biological process that consists of hemostasis, inflammation,

proliferation, and remodeling. Large numbers of cell types – including neutrophils, macrophages,

lymphocytes, keratinocytes, fibroblasts, and endothelial cells – are involved in this process. Multiple

factors can cause impaired wound healing by affecting one or more phases of the process and are

categorized into local and systemic factors (Table 5-3). The influences of these factors are not mutually

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exclusive.13 Single or multiple factors may play a role in any one or more individual phases,

contributing to the overall outcome of the healing process (Fig. 5-14).

Advanced Age and Gender

The elderly population (people over 60 years of age) is growing faster than any other age group (World

Health Organization [WHO, www.who.int/topics/ageing]), and increased age is a major risk factor for

impaired wound healing. Many clinical and animal studies at the cellular and molecular level have

examined age-related changes and delays in wound healing. It is commonly recognized that, in healthy

older adults, the effect of aging causes a temporal delay in wound healing, but not an actual impairment

in terms of the quality of healing.13 Delayed wound healing in the aged is associated with an altered

inflammatory response, such as delayed T-cell infiltration into the wound area with alterations in

chemokine production and reduced macrophage phagocytic capacity.14 Overall, there are global

differences in wound healing between young and aged individuals. A review of the age-related changes

in healing capacity demonstrates that every phase of healing undergoes characteristic age-related

changes, including enhanced platelet aggregation, increased secretion of inflammatory mediators,

delayed infiltration of macrophages and lymphocytes, impaired macrophage function, decreased

secretion of growth factors, delayed re-epithelialization, delayed angiogenesis and collagen deposition,

reduced collagen turnover and remodeling, and decreased wound strength.

Table 5-3 Factors that Affect Wound Healing

Figure 5-13. Comparison of cellular mechanisms in normal and poor wound healing.

Sex hormones play a role in age-related wound healing deficits. Compared with aged females, aged

males have been shown to have delayed healing of acute wounds. A partial explanation for this is that

the female estrogens (estrone and 17β-estradiol), male androgens (testosterone and 5αdihydrotestosterone, DHT), and their steroid precursor dehydroepiandrosterone (DHEA) appear to have

significant effects on the wound healing process.15 It was recently found that the differences in gene

expression between elderly male and young human wounds are almost exclusively estrogen regulated.16

Estrogen affects wound healing by regulating a variety of genes associated with regeneration, matrix

production, protease inhibition, epidermal function, and genes primarily associated with

inflammation.17 Studies indicate that estrogen can improve the age-related impairment in healing in

both men and women, while androgens regulate cutaneous wound healing negatively.

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Figure 5-14. Mechanisms that elicit the effects of comorbid conditions on wound healing.

Several treatments to reduce the age-related impairment of healing have been studied. Interestingly,

exercise has been reported to improve cutaneous wound healing in older adults as well as aged mice,

and the improvement is associated with decreased levels of proinflammatory cytokines in the wound

tissue. Improved healing response may also be due to an exercise-induced anti-inflammatory response in

the wound.18

Nutrition

Malnutrition or specific nutrient deficiencies can have a profound impact on wound healing after trauma

and surgery. Patients with chronic or nonhealing wounds and experiencing nutrition deficiency often

require special nutrients. Energy, carbohydrates, proteins, fat, vitamins, and mineral metabolism all can

affect the healing process.19

Carbohydrates, Proteins, and Amino Acids. Together with fats, carbohydrates are the primary source

of energy in the wound healing process. Glucose is the major source of fuel used to create the cellular

ATP that provides energy for angiogenesis and deposition of the new tissues.20 The use of glucose as a

source for ATP synthesis is essential in preventing the depletion of other amino acid and protein

substrates.

Protein is one of the most important nutrient factors affecting wound healing. A deficiency of protein

can impair capillary formation, fibroblast proliferation, proteoglycan synthesis, collagen synthesis, and

wound remodeling. A deficiency of protein also affects the immune system, with resultant decreased

leukocyte phagocytosis and increased susceptibility to infection. Collagen is the major protein

component of connective tissue and is composed primarily of glycine, proline, and hydroxyproline.

Collagen synthesis requires hydroxylation of lysine and proline, and cofactors such as ferrous iron and

vitamin C. Impaired wound healing results from deficiencies in any of these cofactors.21

Arginine is a semiessential amino acid that is required during periods of maximal growth, severe

stress, and injury. Arginine has many effects in the body, including modulation of immune function,

wound healing, hormone secretion, vascular tone, and endothelial function. Arginine is also a precursor

to proline, and, as such, sufficient arginine levels are needed to support collagen deposition,

angiogenesis, and wound contraction. Arginine improves immune function, and stimulates wound

healing in healthy and ill individuals.22 Under psychological stress situations, the metabolic demand for

arginine increases, and its supplementation has been shown to be an effective adjuvant therapy in

wound healing.

Glutamine is the most abundant amino acid in plasma and is a major source of metabolic energy for

rapidly proliferating cells such as fibroblasts, lymphocytes, epithelial cells, and macrophages. Glutamine

improves nitrogen balance and diminishes immunosuppression and plays a crucial role in stimulating the

early inflammatory phase of wound healing. Major surgery, trauma, and sepsis require supplementation

of glutamine. Oral glutamine supplementation has been shown to improve wound breaking strength and

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