Wound Healing and Its Impact on Dressings and Postoperative Care

Chapter Summary

Dressings cover the wound, absorb drainage, and provide a moist environment.
Dressings should be selected to keep the wound moist, but not too wet nor too dry.
Moist wound healing enhances epithelial migration, stimulates angiogenesis, helps in retention of growth factors, facilitates autolytic debridement and fibrinolysis, protects against exogenous organisms, and maintains voltage gradients.
Healing of acute wounds is accelerated in a moist environment. In chronic wounds, moist dressings can relieve pain, promote autolytic debridement, and decrease the frequency of dressing changes.
Various types of dressings, as well as skin grafts and skin substitutes, are currently available as dressing options.

Introduction

Dressings

A dressing is defined as a covering applied to a wound. This simple definition belies the importance of dressings in wound healing and the complexities of choosing the correct dressing for a particular indication. At present, there is a myriad of categories, subcategories, and types of dressings with different functions, structural compositions, and physical and chemical characteristics, some common and some very unique.

Historical perspective

Even the ancient Egyptians had a keen interest in wound healing. They formulated home-made concoctions of lint, grease, and honey as topical therapy for wounds, or soaked strips of bandage material in oils and resins to use as dressings. They even used raw, fresh meat to cover wounds during the first day of healing.

In 1867, the first antiseptic dressings were introduced by Lister, who soaked lint and gauze in phenol and then applied them to wounds. In general, before the twentieth century, it was believed that wounds healed best when left open and dry as advocated by Pasteur. This view began to change in 1958, when Odland observed that a blister healed faster when left unbroken. Winter's landmark study on swine in 1962 showed that superficial wounds kept moist with a film healed twice as fast as those exposed to the air. Hinman and Maibach repeated Winter's study in humans and found a similar increase in epithelialization rate for occluded wounds. These studies revolutionized the approach to wound care by demonstrating the importance of moist wound healing. A multitude of sophisticated occlusive dressings have been formulated, studied, and become commercially available.

Functions of Dressings

Dressings serve several basic functions (see Table 8.1 ).

Table 8.1

Basic functions of wound dressings

Function	Benefit
Cover wound	Protection from trauma and contamination from bacteria and foreign materials
Cover wound	Minimize fluid and heat loss
Absorb wound drainage	Keep wound moist, but not wet
Absorb wound drainage	Minimize maceration
Compression	Increase hemostasis
	Minimize edema and hematoma formation
	Prevent dehiscence
Provide moist environment	Facilitate healing of acute wounds
Provide moist environment	Reduce pain in chronic wounds

Acute wounds versus chronic wounds

Acute wounds

Acute wounds are wounds with no underlying healing defect that heal in an orderly and timely fashion, passing through well-defined phases of inflammatory response, granulation tissue formation, and remodeling. In acute wounds, maintaining a moist environment is critical in facilitating healing. In fact, acute wounds have been shown to heal 40% faster in a moist environment, than when air exposed. The specific effects of a moist environment and occlusion on wound healing are well-established in this wound type.

Enhancement of epithelial migration

Rovee established that in moist wound healing of acute wounds, wound resurfacing occurs more rapidly because keratinocytes begin to migrate sooner, and not because of a higher rate of mitosis.

Stimulation of angiogenesis

Moist wound healing promotes a greater rate of vascularization. The accumulation of angiogenesis-stimulating factors, such as tumor necrosis factor and heparin, under the dressing partly accounts for this. In addition, because hypoxia often stimulates angiogenesis, the dressing establishes a steep oxygen gradient, which stimulates capillary growth toward the more hypoxic center.

Retention of growth factors

Acute wound fluid beneath occlusive dressings stimulates proliferation of fibroblasts, keratinocytes, and endothelial cells. The growth factors involved in this are platelet-derived growth factor (PDGF), basic fibroblast growth factor (bFGF), transforming growth factor (TGF)-β, epidermal growth factor (EGF), and interleukin (IL)-1. PDGF is a powerful mitogenic, chemotactic, and angiogenic factor. EGF is important in epidermal cell growth, survival, and differentiation. TGF-β induces angiogenesis, fibrosis, differentiation, and proliferation.

Facilitation of autolytic debridement

Retained water and proteolytic enzymes interact and achieve painless wound debridement of necrotic tissue.

Protection against exogenous organisms

Although the bacterial count is higher in occlusive dressings than non-occlusive dressings, this does not predispose to infection. Overall infection rate is 2.6% for occlusive dressings versus 7.1% for non-occlusive dressings. As well as acting as a physical barrier, occlusive dressings allow neutrophils to infiltrate and function more actively. Occlusion is also associated with the presence of higher levels of lysozymes and globulins. Lastly, occlusion maintains a mildly acidic pH, which is inhibitory to the growth of some bacteria, especially Pseudomonas and Staphylococcus spp.

Maintenance of voltage gradients

Moist wound healing helps in the maintenance of an electric field, which is essential in keratinocyte migration. Also, an increase in the synthesis of growth factors by human fibroblasts has been demonstrated during in-vitro electrical stimulation.

Chronic wounds

In chronic wounds, the normal process of healing has been disrupted at one or more points in the phases of hemostasis, inflammation, proliferation, and remodeling. In this wound type, there is usually an underlying pathology, which produces a delay in the healing process. The effect of occlusion in these wound types is not as well established because there is a dearth of randomized controlled trials for chronic wounds.

In contrast to acute wound fluid, chronic wound fluid was found to be inhibitory to epithelialization, and to contain degradation products of vitronectin and fibronectin, which inhibit keratinocyte migration. Further, when chronic wound fluid is added to cultures of keratinocytes, fibroblasts, or endothelial cells, it fails to stimulate DNA synthesis directly, contrasting the DNA-synthesizing ability of acute wound fluid. Another important biochemical difference in chronic wounds is that they exhibit considerably higher protease activity than acute wounds.

A study on occluded versus non-occluded venous ulcers showed that the difference in the number of wounds healed at the end of 12 weeks was not statistically significant; however, the rate of healing was more rapid in the occlusive dressing group. For patients with chronic wounds, moisture-retentive dressings do offer the advantages of pain relief, painless wound debridement, containment of wound exudates, reduction in the incidence of complications, and improved quality of life.

Types of Dressings

Dressings may be classified based on their clinical functions as well as their physical appearance and composition ( Box 8.1 ).

Box 8.1

Types of dressings

Non-adherent fabrics

Absorptive
- Gauze
- Foams
- Alginates

Occlusive/moisture-retentive dressings

Non-biologic – traditional
- Foams
- Films
- Hydrocolloids
- Hydrogels
- Alginates
Non-biologic – new
- Hydrofiber dressings
- Collagen dressings
- Hyaluronic acid dressings
Biologic – grafts
- Split-thickness skin grafts (STSGs)
- Full-thickness skin grafts (FTSGs)
- Composite grafts
Biologic/biosynthetic – skin substitutes
- Cultured epidermal grafts
- Dermal replacements
- Composite skin substitutes
Antimicrobial dressings

Non-adherent fabrics

Non-adherent fabrics are derived from a combination of fine mesh gauze and tulle gras, commonly impregnated with chemicals to potentiate the dressing's occlusive or non-adherent characteristics, its ability to facilitate healing, or its antimicrobial properties. They may be subdivided into hydrophobic and hydrophilic types. Hydrophobic fabrics have greater occlusive capability, but hinder fluid drainage through them. These include Vaseline gauze (The Kendall Co, Mansfield, MA), Xeroform (The Kendall Co), and Telfa (The Kendall Co). In contrast, the hydrophilic dressings are less occlusive, but have the ability to readily facilitate the drainage of fluids and exudates into overlying dressings. Examples are Xeroflo (The Kendall Co), Mepitel (Mölnlycke Health Care, Gothenburg, Sweden), Adaptic (Johnson & Johnson Medical, Arlington, TX), and N-Terface (Winfield Laboratories, Dallas, TX).

Absorptive dressings

Gauze is one of the most commonly used absorptive dressings. It is excellent at drawing fluids and exudates away from the wound surface, but loses its efficacy when saturated. It is usually used to cover non-occlusive, non-adhering fabric dressing materials and absorb discharge, which drains through them. It may also be used over occlusive dressings as a secondary dressing to fix them in place. Wide mesh gauze is usually not placed in direct contact with wounds because it adheres to the surface of the wound, resulting in pain on removal. The only exception is when mechanical debridement is desired. Foam dressings and alginates are classified as both absorptive and occlusive/moisture-retentive dressings.

Occlusive/moisture retentive

A moist wound environment is provided by a dressing that transmits moisture vapor at a rate lower than that at which a wound loses moisture. This is measured as moisture vapor transmission rate (MVTR) through the dressing when it is left in place for 24 h. MVTR of intact normal skin is about 200 g/m ² per day, while that of wounded skin is 40 times higher. Dressings with an MVTR of <35 g/m ² per hour are defined as occlusive or moisture retentive.

Non-biologic occlusive dressings

Traditional occlusive dressings are classified into five basic categories ( Table 8.2 ).

Table 8.2

Types and characteristics of occlusive/moisture-retentive wound dressings

Type	Advantages	Disadvantages	Indications	Examples
Foams	Absorbent, conform to body contours	Opaque, require secondary dressing	Partial-thickness wounds, moderately to heavily exudative wounds, pressure relief	Allevyn Flexzan Hydrasorb Lyofoam Vigifoam
Films	Transparent, create bacterial barrier, adhesive without secondary dressing	May adhere to wounds, can cause fluid collection	Donor sites, superficial burns and ulcers, partial-thickness wounds with minimal exudates	Tegaderm Bioclusive BlisterFilm Omniderm Transeal
Hydrocolloids	(+) autolytic debridement, enhance angiogenesis, absorbent, create bacterial and physical barrier	Opaque, gel has unpleasant smell, expensive	Partial- or full-thickness wounds, mildly to moderately exudative wounds, pressure ulcers, venous ulcers, donor sites, acute surgical wounds	Duoderm Nu-Derm Comfeel Cutinova Replicare
Hydrogels	Semitransparent, soothing, do not adhere to wounds, hydrating	Require secondary dressing, frequent dressing changes	Painful wounds, partial-thickness wounds, wounds after laser, dermabrasion or chemical peel, donor sites	Vigilon Tegagel Curagel ClearSite Curafil Elasto-Gel SoloSite wound gel 2nd Skin
Alginates	Highly absorbent, hemostatic, do not adhere to wounds, fewer dressing changes	Require secondary dressing, gel has unpleasant smell	Highly exudative wounds, partial- or full-thickness wounds, after surgery	AlgiDerm AlgiSite Algisorb Kaltostat Curasorb Polymem SeaSorb Sorbsan

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