Surgical wounds, wound healing and surgical site infection

• Every operation creates a wound, and the body repairs it through a predictable sequence: an initial inflammatory phase, a proliferative phase that fills the gap, and a long maturation phase that remodels the scar. ^[4]
• A surgical site infection (SSI) is the single most common complication after surgery in many series; it ranges from a mildly inflamed superficial wound to deep or organ-space infection that needs re-operation.
• Prevention matters more than treatment. The strongest, best-supported measures are: appropriate antibiotic prophylaxis, chlorhexidine-alcohol skin preparation, avoiding shaving, maintaining the patient's temperature and oxygenation, and good glycaemic control.
• Stitch lines can fail. Wound dehiscence most often occurs while the wound is still gaining strength; it is driven by infection, coughing, obesity, diabetes, poor nutrition and technical failure, and is a surgical emergency when deep.
• Pressure injuries are a related but distinct problem of sustained load on skin over bone; they are staged 1 to 4 and are prevented by repositioning, pressure redistribution and skin care.

By the end of this page you should be able to:

1. Describe the phases of wound healing from haemostasis to scar maturation, and name the key cells and signals in each.
2. List local and systemic factors that impair healing and explain how each acts.
3. Compare common suture materials and choose a closure for a given wound.
4. Recognise wound dehiscence, explain why it occurs and how it is managed.
5. Stage a pressure injury and describe its prevention.
6. Define a surgical site infection and classify it as clean, clean-contaminated, contaminated or dirty, and as superficial incisional, deep incisional or organ/space.
7. Set out an evidence-based prevention bundle and state where the CDC 2017, WHO 2018 (2nd edition) and NICE NG125 guidelines agree and differ.
8. Manage an established SSI, including sampling, antibiotics and when to reopen the wound.

A surgical wound is any break in the skin or mucosa made, deliberately or traumatically, during an operation. A clean surgical wound is made under sterile conditions, with no inflammation present and without entering the respiratory, alimentary or genitourinary tract. Whether such a wound becomes infected depends on the balance between the number and virulence of organisms reaching it, and the patient's local and systemic defences.

Wound healing is the organised sequence by which the body restores structure and, when possible, function after injury. Healing can be described as three overlapping phases: haemostasis and inflammation, proliferation, and remodelling. It can also be described by intention:

• Healing by primary intention occurs when a wound has been sutured after an operation and heals to leave a minimal, cosmetically acceptable scar.
• Healing by secondary intention occurs when a wound is deliberately left open at the end of an operation, usually because of heavy bacterial contamination or devitalised tissue; it heals from the base up and is later sutured, or left to heal naturally.

This matches the NICE NG125 definitions of these terms.^[3]

SSI is the most commonly measured hospital-acquired infection in surgical patients in many national surveillance programmes. It occurs most often after operations on the gastrointestinal tract, obstetric and gynaecological procedures, and implant surgery. Surveillance definitions demand that the infection occurs within a defined postoperative window, extended when an implant is left in place, and that it has the usual features of infection: erythema, heat, swelling, pain, discharge, or a raised white cell count. Definitions of the severity of SSI vary, which should be taken into account when comparing reported rates.^[3]

The consequences are substantial even when an infection is superficial: extra days in hospital, delayed return to work, higher cost, and a worse cosmetic scar. Deep and organ-space infections carry real mortality and may require re-operation. The global burden falls heaviest in low- and middle-income countries, where health-care-associated infection more broadly is the most frequent adverse event affecting patient safety worldwide.^[2]

Risk flows from the patient, the operation and the environment.

Patient factors. Older age, obesity, diabetes, smoking, malnutrition (protein and micronutrient deficit), immunosuppression (steroids, chemotherapy, biologics), anaemia, peripheral vascular disease and remote infection (urinary, dental, skin) all reduce local defences. Smoking in particular lowers tissue oxygen tension and delays collagen synthesis.

Operative factors. Long duration, heavy blood loss, emergency surgery, repeat procedures, contaminated or dirty wounds, a foreign body or implant, tissue ischaemia from tight sutures, and any break in sterile technique. Each hour of extra operative time adds risk.

Environmental and process factors. Skin flora at the incision site, the theatre environment, and staff movement. The most important single contaminating organism is Staphylococcus aureus, including meticillin-resistant strains.

Within minutes of wounding, vasoconstriction and platelet aggregation form a clot. Platelets degranulate and release growth factors that call inflammatory cells to the wound. Neutrophils arrive first and are replaced over the next few days by macrophages, which phagocytose debris and bacteria and secrete cytokines that drive the next phase. Inflammation is necessary for healing, but if it is prolonged it becomes destructive, as in diabetes and chronic wounds.

Fibroblasts migrate into the wound, lay down a collagen-rich matrix, and support new small blood vessels (angiogenesis), so that granulation tissue forms. Epithelial cells migrate across the wound from the edges and from hair follicles to resurface it, a process called re-epithelialisation. Contraction of the wound by myofibroblasts reduces its area.

The scar's collagen is reorganised from type III to type I, cross-linked, and aligned along lines of tension. Tensile strength rises slowly: a healing wound at one week has only a small fraction of the strength of normal skin, and even at three months it has not returned to baseline. Hypertrophic and keloid scarring represent failure of normal regulation in this phase.

A wound should be closed only when clean enough to heal by primary intention. The choice of material follows time to dissolution, infection risk and cosmetic need.

Principles worth applying: use the smallest effective suture; take wide, shallow bites rather than deep narrow ones; avoid tension; evert the skin edges; match absorbable material to the tissue layer; remove sutures early from cosmetically important sites and late from high-tension sites; and never close a heavily contaminated wound primarily without delaying or draining it. NICE suggests considering sutures rather than staples for skin closure after caesarean birth to reduce superficial wound dehiscence, and considering triclosan-coated antimicrobial sutures, especially in paediatric surgery, to reduce SSI.^[3]

Wound dehiscence is the separation of the edges of a wound, either superficial (skin and subcutaneous tissue) or deep (fascia and deeper). Deep dehiscence with protrusion of bowel or omentum is wound evisceration and is a surgical emergency.

It most often occurs while the wound is still gaining strength and before collagen deposition has secured it. The typical patient is obese, diabetic, a smoker or malnourished, often with pre-operative remote infection, or has had emergency surgery. Precipitating factors are infection of the wound, increased intra-abdominal pressure from coughing, vomiting, ileus or respiratory effort, and technical failure (sutures placed under tension, wrong suture choice, incorrect knotting).

Presentation ranges from a small discharge of serosanguinous fluid through the incision to sudden opening with loops of bowel. A persistent cough after abdominal surgery, or a sudden gush of fluid from the wound, should raise suspicion.

Management depends on depth:

• Superficial separation with a clean base can be managed with local dressings and watchful waiting; secondary healing is accepted, or delayed primary closure once clean.
• Fascial dehiscence without evisceration needs surgical re-closure, usually with retention sutures or mesh, together with treatment of infection and correction of the precipitating factor.
• Evisceration needs immediate protection of the exposed bowel (cover with a moist sterile saline-soaked towel), nil by mouth, intravenous resuscitation and antibiotics, and urgent return to theatre.

Prevention follows the bundle described below: prevent SSI, control cough and vomiting, support the wound after a heavy cough, optimise nutrition and glucose, and avoid closing under tension.

A pressure injury (formerly pressure ulcer or bedsore) is localised damage to the skin and/or underlying soft tissue, usually over a bony prominence, caused by intense and/or prolonged pressure or pressure combined with shear. It is related to, but separate from, a surgical wound SSI: it is a failure of pressure relief and skin care, not of asepsis.

The NPUAP pressure injury staging system is widely used. The stages are: (1) non-blanchable erythema of intact skin; (2) partial thickness loss of skin with exposed dermis; (3) full thickness loss of skin with visible adipose tissue; (4) full thickness loss with exposed fascia, muscle, tendon, ligament, cartilage or bone; plus unstageable injury (slough or eschar obscures the wound bed) and deep tissue pressure injury (persistent non-blanchable deep red, maroon or purple discoloration).^[5]

Risk assessment uses the Braden scale or similar (sensory perception, moisture, activity, mobility, nutrition, friction and shear). High-risk patients include those with impaired mobility, incontinence, malnutrition, reduced consciousness or critical illness.

Prevention: reposition regularly (more often if evidence of damage), use pressure-redistributing mattresses and cushions, keep skin clean and dry, protect against friction and shear with lifting rather than dragging, ensure adequate protein and calories, inspect skin daily, and involve tissue viability staff early. A heel or sacral pressure injury in a postoperative patient often starts in theatre or the recovery room on a hard trolley.

A surgical site (wound) infection is a surgical wound with local signs and symptoms of infection: heat, redness, pain, swelling, and in more serious cases systemic signs such as fever or a raised white cell count. Infection may prevent healing, cause the wound edges to separate, or produce an abscess in the deeper tissues.^[3]

CDC surveillance categories, used in the CDC 2017 guideline and in most national programmes, classify a wound by the tissues involved:

• Superficial incisional: infection of skin and subcutaneous tissue only.
• Deep incisional: infection of fascia or muscle.
• Organ/space: infection of any part of the body opened or manipulated during the operation other than the incision itself.

Surgical wound classification by contamination, as used in NICE NG125, divides the operation itself:

• Clean: no inflammation encountered, no break in sterile technique, respiratory/alimentary/genitourinary tracts not entered.
• Clean-contaminated: these tracts entered under controlled conditions without contamination encountered.
• Contaminated: major break in sterile technique, gross spillage from the gastrointestinal tract, or acute non-purulent inflammation encountered; open traumatic wounds more than 12 to 24 hours old. ^[3]
• Dirty or infected: viscus perforated, or acute inflammation with pus encountered (for example faecal peritonitis); delayed or faecally contaminated traumatic wounds.

The wound class is decided at the end of the operation and determines whether antibiotic prophylaxis is needed, and the expected baseline infection rate.^[3]

The three major guidelines are broadly in agreement on the important measures, and differ mainly in detail and in how strongly they phrase some recommendations. The WHO 2018 2nd edition guideline covers preoperative, intraoperative and postoperative care; the CDC 2017 guideline is the reference update for individual recommendations; NICE NG125 sets out a complete perioperative pathway with implementation detail.^[1][2][3]

The main differences worth noting. NICE makes the most explicit, prescriptive recommendations on antibiotic prophylaxis (single IV dose, repeat by half-life) and on postoperative dressing change. The CDC guideline frames many recommendations as strong or weak and stresses normothermia and oxygenation in colorectal and major surgery. The WHO guideline is broader and designed for worldwide use, including low-resource settings, and strongly supports alcohol-based chlorhexidine skin preparation and structured perioperative care.^[1][2][3]

Antibiotic prophylaxis in brief. Give a single intravenous dose on starting anaesthesia, choosing the agent by the likely organisms and local resistance, and repeat it when the operation lasts longer than the half-life of the drug. Do not use prophylaxis routinely for clean, non-prosthetic, uncomplicated surgery. Treat dirty or infected wounds with antibiotics as well as prophylaxis. Tell the patient if antibiotics were given.^[3]

Treat the patient, not just the wound. Picture the depth.

1. Assess the wound and the patient's general state. Local signs plus any fever, tachycardia or raised white cell count indicate deep or spreading infection.
2. Sample before antibiotics when possible. Take a wound swab from clean granulation tissue after debridement, or aspirate deep (stitch) abscesses, and send for culture and sensitivity.
3. Open and drain any collection or abscess. Superficial collections can be opened at the bedside; deep or organ-space collections need ultrasound guidance or re-operation.
4. Antibiotics chosen to cover the likely organisms, guided by the operation (skin flora, enteric flora), local resistance and culture results. Keep courses short and reviewed daily.
5. Debride devitalised tissue. Do not use Eusol and gauze, dextranomer or enzymatic debridement for this (NICE). Sterile saline is appropriate for cleansing in the first 48 hours.
6. Secondary healing with an interactive dressing is accepted for many superficial infections; escalate if there is systemic upset, deep infection, a foreign body present or non-healing.
7. Remove and manage hardware (prostheses, mesh) with the operating team when infection involves an implant.

Red flags needing senior or surgical review: rapidly spreading cellulitis, systemic sepsis, necrotising soft-tissue infection, infected prosthesis, or SSI in an immunocompromised patient.

• Delayed healing and wound breakdown leading to dehiscence and evisceration.
• Deep or organ-space infection requiring re-operation.
• Abscess formation in wound or deep tissues.
• Sepsis and death from major SSI.
• Poor cosmetic outcome and painful scar.
• Functional loss from contractures after secondary healing.

Most superficial SSIs heal with antibiotics, drainage and dressings, but they extend length of stay and convalescence. Deep infection around an implant often requires removal of the implant and carries significant morbidity. Prevention therefore remains the priority: it is far more effective and cheaper than treating infection once established.

• Children. SSI prophylaxis doses and durations are weight-based; triclosan-coated sutures are specifically considered for paediatric surgery. Infection presents more subtly in the very young, so reassess early.
• Pregnancy. Caesarean section is the most common source setting for postoperative wound infection in obstetrics; NICE specifically addresses prophylaxis timing and caesarean closure. Infection in pregnancy also risks endometritis.
• Older and frail adults. thinner skin, comorbidity and reduced physiological reserve mean lower thresholds for early surgical review.
• Immunocompromised patients. Presentations are atypical; consider infection earlier and have a lower threshold for imaging.
• Implant surgery. Infection here is a catastrophe; prophylaxis, skin preparation, asepsis and draping matter even more, and implant removal is often required.

• Focus on the high-yield, low-cost measures: soap shower before surgery, no shaving, skin preparation with available antiseptic, single-dose perioperative antibiotic when affordable, hand hygiene, instrument sterility, temperature (extra blankets and warm fluids), and repositioning for pressure injury prevention.
• NICE and WHO recommendations on irrigation and lavage are reassuring in settings where fluids are limited: do not irrigate to prevent SSI.
• SSI definition and culture may be unavailable; use clinical assessment and, when possible, pus sampling for Gram stain and culture.
• Pressure-redistribution mattresses may not be available; use turning schedules, padding and family involvement.

• A surgical wound heals by three overlapping phases: inflammation, proliferation and remodelling.
• Clean surgical wounds have a low but real chance of infection; contaminated and dirty wounds have a high one.
• The SSI bundle with the strongest evidence is: correct antibiotic prophylaxis, alcohol-based chlorhexidine skin preparation, no shaving, temperature control, oxygenation and glycaemic control.
• NICE NG125 recommends a single IV dose of prophylaxis on starting anaesthesia, repeated by antibiotic half-life, and gives explicit postoperative dressing and bathing advice (sterile saline to 48 hours, shower at 48 hours).
• The WHO 2018 2nd edition guideline frames prevention across all three phases of care and is designed for worldwide implementation.
• The CDC 2017 guideline is the reference update for individual recommendations; it classifies SSI as superficial incisional, deep incisional or organ/space.
• Wound dehiscence peaks at postoperative day 5 to 10; deep dehiscence with evisceration is a surgical emergency.
• Pressure injuries are staged 1 to 4 (plus unstageable and deep tissue) and are prevented by repositioning, pressure redistribution and skin care.
• An established SSI needs sampling, drainage of any collection, sensible antibiotics, debridement and daily reassessment.
• No SSI is trivial: even superficial infection extends stay and worsens the scar.