Skin consists of epidermis and dermis. The epidermis is a layer of keratinized, stratified squamous epithelium (Fig. 18.2) that sends three appendages (hair follicles, sweat glands and sebaceous glands) into the underlying dermis. Because of their deep location, the appendages escape destruction in partial-thickness burns and are a source of new cells for reconstitution of the epidermis. The basal germinal layer of the epidermis generates keratin-producing cells (keratinocytes), which become increasingly keratinized and flattened as they migrate to the surface, where they are shed. The basal layer also contains pigment cells (melanocytes) that produce melanin, which is passed to the keratinocytes and protects the basal layer from ultraviolet light.

Fig. 18.2 Structure of skin.

The dermis is composed of collagen, elastic fibres and fat. It supports blood vessels, lymphatics, nerves and the epidermal appendages. The junction between the epidermis and the dermis is undulating where dermal papillae push up towards the epidermis.

The three types of epidermal appendage extend into the dermis and, in some places, into the subcutaneous tissues. Hair follicles produce hair, the colour of which is determined by melanocytes within the follicle. The sebaceous glands secrete sebum into the hair follicles, which lubricates the skin and hair. The sweat glands are coiled tubular glands lying within the dermis and are of two types; eccrine sweat glands secrete salt and water on to the entire skin surface, while apocrine glands secrete a musty-smelling fluid in the axilla, eyelids, ears, nipple and areola, genital areas and the perianal region. Hidradenitis suppurativa affects the latter.

The nails are flat, horny structures composed of keratin. They arise from a matrix of germinal cells, which can be seen as a white crescent (lunula) at the nail base. If a nail is avulsed, a new nail grows from this matrix. If the matrix is destroyed, nail regeneration is impossible, and the layer of epidermal cells covering the nailbed thickens to form a keratinized protective layer.

Wounds

A wound may be defined as disruption of the normal continuity of bodily structures due to trauma, which may be penetrating or non-penetrating. In both cases, inspection of the body surface may give little indication of the extent of underlying damage.

Types of wound

Wounds can be classified according to the mechanism of injury:

• Incised wounds. A sharp instrument causes these; if there is associated tearing of tissues, the wound is said to be lacerated

• Abrasions. These result from friction damage and are characterized by superficial bruising and loss of a varying thickness of skin and underlying tissue. Dirt and foreign bodies are frequently embedded in the tissues and can give rise to traumatic tattooing

• Crush injuries. These are due to severe pressure. Even though the skin may not be breached, there can be massive tissue destruction. Oedema can make wound closure impossible. Increasing pressure within fascial compartments can cause ischaemic necrosis of muscle and other structures (compartment syndrome)

• Degloving injuries. These result from shearing forces that cause parallel tissue planes to move against each other: for example, when a hand is caught between rollers or in moving machinery. Large areas of apparently intact skin may be deprived of their blood supply by rupture of feeding vessels

• Gunshot wounds. These may be low-velocity (e.g. shotguns) or high-velocity (e.g. military rifles). Bullets fired from high-velocity rifles cause massive tissue destruction after skin penetration

• Burns. These are caused not only by heat but also by electricity, irradiation and chemicals.

Principles of wound healing

The essential features of healing are common to wounds of almost all soft tissues, and result in the formation of a scar. Soft tissue healing can be subdivided into three phases (Table 18.1) according to the development of tensile strength (Fig. 18.3).

Table 18.1 Phases of wound healing

Lag phase (2–3 days)

• Inflammatory response

Incremental or proliferative phase (approximately 3 weeks)

• Fibroblast migration

• Capillary ingrowth (granulation tissue)

• Collagen synthesis with rapid gain in tensile strength

• Wound contraction

Plateau or maturation phase (approximately 6 months)

• Organization of scar

• Slow final gain in tensile strength (80% of original strength)

Fig. 18.3 Phases of wound healing.

Keloids

Epidermis

Epithelium heals by regeneration and not by scar formation. Epithelial cells at the edge of the wound lose their adhesion to each other and migrate across the wound until they meet cells from the other side. As they migrate, they are replaced by new cells formed by the division of basal cells near the wound edge. The cells that have migrated undergo mitosis and the new epithelium thickens, eventually forming normal epithelial cover for the scar produced by the dermis.

Primary and secondary intention

Wounds may heal by primary intention if the edges are closely approximated: for example, by accurate suturing. Epithelial cover is quickly achieved and healing produces a fine scar (Fig. 18.4). If the wound edges are not apposed, the defect fills with granulation tissue and the restoration of epidermal continuity takes much longer. The advance of epithelial cells across the denuded area may be hindered by infection. This is known as healing by secondary intention and usually results in delayed healing, excessive fibrosis and an ugly scar (Fig. 18.5). If a wound has begun to heal by secondary intention, it may still be possible to speed healing by excising the wound edges and bringing them into apposition, or by covering the defect with a skin graft.

Fig. 18.4 Wound healing.

A Healing by primary intention. B Healing by secondary intention, showing shrinkage of the wound.

Fig. 18.5 Healing by secondary intention.

Summary Box 18.1 Classification of wound healing

• Healing by first intention is the most efficient method and results when a clean incised surgical wound is meticulously apposed and heals with minimal scarring

• Healing by second intention occurs when wound edges are not apposed and the defect fills with granulation tissue. In the time taken to restore epithelial cover, infection supervenes, fibrosis is excessive and the resulting scar is unsightly

• The term ‘healing by third intention’ describes the situation where a wound healing by second intention (e.g. a neglected traumatic wound or a burn) is treated by excising its margins and then apposing them or covering the area with a skin graft. The final cosmetic result may be better than if the wound had been left to heal by second intention.

Factors influencing wound healing

Many of the factors influencing healing are interrelated: for example, the site of the wound, its blood supply, and the level of tissue oxygenation. Although some adverse factors, such as advanced age, cannot be influenced, others, such as surgical technique, nutritional status and the presence of intercurrent disease, can be modified or eliminated.

Blood supply

Wounds in ischaemic tissue heal slowly or not at all. They are prone to infection and frequently break down. When this occurs, the wound may not be able to sustain the metabolic demands of healing by second intention. Arterial oxygen tension (PaO₂) is a key determinant of the rate of collagen synthesis. Anaemia may not affect healing if the patient has a normal blood volume and arterial oxygen tension. Poor surgical technique, such as crushing tissue with forceps, approximating wound edges under tension and tying sutures too tightly, can make well vascularized tissue ischaemic and lead to wound breakdown.

Infection

The general risks of wound infection depend upon age, the presence of intercurrent infection, steroid administration, diabetes mellitus, disordered nutrition, and cardiovascular and respiratory disease. Local factors are also important. Bacterial contamination can be minimized by careful skin preparation and aseptic technique, but some wounds are more likely to be contaminated than others. Bacteria may enter wounds from the atmosphere, from internal foci of sepsis or from the lumen of transected organs. In some cases, contamination occurs in the postoperative period. Provided contamination is not gross and local blood supply is good, natural defences are usually able to prevent and contain overt infection. Devitalized tissues, haematomas and the presence of foreign material such as sutures and prostheses favour bacterial survival and growth. Common infecting organisms are staphylococci, streptococci, coliforms and anaerobes. Overcrowding of wards and excessive use of operating theatres increase the bacterial population of the atmosphere and hence the risk of wound infection. The failure of medical and nursing staff to wash their hands before and after touching and examining each patient is perhaps the greatest source of cross-contamination.

When wound contamination is anticipated, topical antibacterial chemicals or topical and systemic antibiotics can be used prophylactically. For example, a single dose of systemic antibiotic is normally used to reduce the risk of infection during gastrointestinal surgery and when prosthetic material (hip joint, cardiac valves, arterial bypass) is being inserted. In acute traumatic wounds, tetanus prophylaxis is routine, but antibiotics are not normally necessary provided prompt and thorough surgical treatment is undertaken. However, if there has been a delay in the treatment of such a wound, antibiotic prophylaxis may be necessary.

Age

Wounds in the elderly may heal poorly because of impaired blood supply, poor nutritional status or intercurrent disease. However, as mentioned above, they tend to form ‘good’ scars.

Site of wound

Surgical incisions placed in the lines of least tissue tension are subject to minimal distraction and should heal promptly, leaving a fine scar. On the face, these lines run at right angles to the direction of underlying muscles and form the lines of facial expression.

Nutritional status

Malnutrition has to be severe before healing is affected. Protein availability is most important, and wound dehiscence and infection are common when the serum albumin is low. Healing problems should be anticipated if recent weight loss exceeds 20%. Vitamin C is essential for proline hydroxylation and collagen synthesis. The number of fibroblasts is not reduced in scorbutic states. Zinc is a co-factor for important enzymes involved in healing, and its deficiency retards healing. Supplements of ascorbic acid and zinc are effective in patients with known deficiencies, but do not improve healing in normal subjects.

Intercurrent disease

Healing may be affected by the disease itself or by its treatment. Cachectic patients with severe malnutrition (as seen in advanced cancer) have marked impairment of healing. Diabetes mellitus impairs healing by reducing tissue resistance to infection and by causing peripheral vascular insufficiency and neuropathy. Haemorrhagic diatheses increase the risk of haematoma formation and wound infection. Obstructive airway disease lowers arterial PO₂ and so affects healing. Abdominal wound dehiscence is more common in patients with respiratory disease because of the strain put on the wound during coughing. Corticosteroid therapy reduces the inflammatory response, impairs collagen synthesis and decreases resistance to infection. The effect of steroids on wound healing is most marked if they are given within 3 days of injury. Immunosuppressive therapy and chemotherapy impair healing by reducing resistance to infection. As radiotherapy greatly reduces the vascularity of the tissues, the healing of wounds in irradiated areas is often impaired.

Surgical technique

Where possible, skin incisions are placed in the line of least tissue tension. Aseptic technique, gentle handling and accurate apposition of wound edges favour healing by first intention. Dead spaces must be avoided, as the accumulation of blood and exudate encourage infection. Correct suturing of the deeper layers avoids dead space and often allows the skin edges to fall together without tension, so that superficial sutures or adhesive tape can achieve skin apposition. Drains should be used in contaminated wounds and those where exudate is expected. Drains may be connected to a suction apparatus or allowed to empty by gravity. The drain site is a potential portal of entry for infection and drains should be removed as soon as possible, especially when prosthetic material has been implanted.

Choice of suture and suture materials

Foreign material in the tissues predisposes to infection. The finest sutures that will hold the wound edges together should be used. Whereas 5/0 or 6/0 sutures are appropriate for the face, stronger ones (3/0 or 4/0) are needed for incisions near joints and still stronger ones for the abdominal wall. The suture should be strong enough to support the wound until tensile strength has recovered sufficiently to prevent breakdown. Absorbable materials are preferred for buried layers.

Wound infection

Classification

Surgical procedures can be classified according to the likelihood of contamination and wound infection as ‘clean’, ‘clean-contaminated’ and ‘contaminated’:

• Clean procedures are those in which wound contamination is not expected and should not occur. An incision for a clean elective procedure should not become infected. In clean operations, the wound infection rate should be less than 1%.

Summary Box 18.2 Factors affecting wound healing

The site of the wound and its orientation relative to tissue tension lines are major determinants of healing

Wounds with a good blood supply (e.g. head and neck wounds) heal well

Infection is a major adverse factor and the risk of infection is influenced by:

• general factors such as the patient’s age, presence of intercurrent infection, nutritional status and cardiorespiratory disease

• local factors including bacterial contamination, antibacterial prophylaxis, aseptic technique, degree of trauma, presence of devitalized tissue, haematoma and foreign bodies.

Intercurrent disease may impair healing. Important factors include:

• malnutrition

• diabetes mellitus

• haemorrhagic diatheses

• hypoxia (e.g. obstructive airways disease)

• corticosteroid therapy

• immunosuppression

• radiotherapy.

Surgical technical factors that have a major influence on wound healing include:

• gentle tissue handling

• avoidance of undue trauma

• accurate tissue apposition

• meticulous haemostasis

• appropriate choice of suture material.

• Clean-contaminated procedures are those in which no frank focus of infection is encountered but where a significant risk of infection is nevertheless present, perhaps because of the opening of a viscus, such as the colon. Infection rates in excess of 5% may suggest a breakdown in ward and operating theatre routine.

• Contaminated or ‘dirty’ wounds are those in which gross contamination is inevitable and the risk of wound infection is high; an example is emergency surgery for perforated diverticular disease, or drainage of a subphrenic abscess.

Antibiotic prophylaxis is appropriate for the latter two types of operation.

Clinical features

Wound infection usually becomes evident 3–4 days after surgery. The first signs are usually superficial cellulitis around the margins of the wound, or swelling of the wound with some serous discharge from between the sutures. Fluctuation is occasionally elicited when there is an abscess or liquefying haematoma. Crepitus may be present if gas-forming organisms are involved. In some cases of deep infection, there are no local signs, although the patient may have pyrexia and increased wound tenderness. Systemic upset is variable, usually amounting to only moderate pyrexia and leucocytosis. Toxaemia, bacteraemia and septicaemia can complicate serious wound infection, especially where there is an accumulation of pus. The differential diagnosis includes other causes of postoperative pyrexia, wound haematoma and wound dehiscence. Wound haematoma may result from reactive bleeding during the first 24–48 hours after an operation. It causes swelling and discomfort, but only minimal pyrexia and few systemic signs.

Prevention

The risk of wound infection is reduced by careful patient preparation, the prophylactic use of antibiotics in high-risk patients, and meticulous attention to good operating theatre techniques. Severely contaminated wounds are sometimes best closed by delayed primary suture; most gunshot wounds are treated in this way. Skin sutures may be inserted at this time but are not tied for several days, by which time it should be clear that infection has been avoided. Antibiotic therapy is essential for grossly contaminated wounds. The aim is to achieve high tissue concentrations as soon as possible. The choice of antibiotic is determined by the nature of the infection. Topical agents such as povidone-iodine may also be used to combat infection in contaminated wounds. Radical excision of the wound margins, thorough mechanical cleansing and delayed suture may also be required.

Management

A wound swab or specimen of pus is routinely sent for bacteriological culture and sensitivity determination. In urgent cases, a Gram stain may be useful. The state of immunity against tetanus is assessed and appropriate action taken. Trivial superficial cellulitis can be managed expectantly. The area of redness is ‘mapped out’ with an indelible pen so that its extent can be monitored. Spreading cellulitis is an indication for antibiotic therapy. Many infected wounds heal rapidly without further surgery, particularly if the original skin incision is placed in the line of least tissue tension. The problem is often to keep the wound open, rather than to achieve closure. If it appears that spontaneous wound closure will take a long time, secondary suture or skin grafting can be considered to speed healing, but only once it is clear that infection has been eradicated. The presence of clean healthy granulation tissue in the wound is usually a good indication that closure can be undertaken.

Involvement of other structures

All wounds must be inspected carefully in good light to assess the extent of devitalization and injury to other structures. However, it is important to appreciate that a small, apparently innocent wound may conceal extensive damage to deeper structures. Body cavities may have been penetrated, or tendons, nerves and blood vessels divided. Damage to muscles, tendons or nerves is assessed by checking relevant motor and sensory function. If the injury

Summary Box 18.3 Principles of management of contaminated traumatic wounds

• Contaminated wounds should be debrided under general anaesthesia

• The contaminated wound and its margins must be cleansed thoroughly, and grit, soil and foreign bodies/materials removed

• Devitalized tissue is formally excised until bleeding is encountered

• Primary closure is avoided if there has been gross contamination and when treatment has been delayed for more than 6 hours. Inappropriate attempts to achieve primary closure increase the risk of wound infection and expose the patient to the risks of anaerobic infection (tetanus and gas gangrene)

• Wounds left open may be suitable for delayed primary suture after 2–3 days, or for later excision and secondary suture (with or without skin grafting)

• Appropriate protection against tetanus must be afforded and the use of antibiotics should be considered.

involves a limb, the distal circulation must be checked. Where appropriate, X-rays will help to establish whether peritoneal, pericardial or pleural cavities have been entered, and whether there is underlying bony injury.

Provided there is no deep damage, small, relatively uncontaminated wounds can be treated under local anaesthesia in the A&E department. The wound margins are cleaned with a mild antiseptic such as cetrimide and the wound is irrigated with sterile saline. Any devitalized tissue is removed, deep tissues are sutured with absorbable material and the skin margins are closed.

More extensive or severely contaminated wounds usually require inpatient treatment, with exploration and debridement under general anaesthesia. The wound and its margins are cleansed and all obvious foreign material picked out. Devitalized tissue is trimmed back until bleeding occurs. In areas of poor vascularity such as the leg, or if there is severe contamination, crushing or a fracture, the wound margins are formally excised (Fig. 18.6). Bleeding from the wound margin is not a certain indication of its ultimate survival, as impaired venous drainage can lead to progressive necrosis, particularly after a crushing or degloving injury. If there is any doubt, the wound should not be sutured and a ‘second-look’ dressing change should be undertaken under anaesthesia after 48 hours.