Inflammation
Inflammation is vital to successful burn wound healing, and inflammatory mediators (cytokines, kinins, lipids, and so forth) provide immune signals to recruit leukocytes and macrophages that initiate the proliferative phase [
37]. Wound re-epithelialization, or closure, in the proliferative phase via keratinocyte and fibroblast activation, or migration from dedifferentiated hair follicles and other epidermal analogs [
41,
42], is mediated by cytokines recruited in the inflammatory phase. While this indicates that inflammation is essential for wound healing, aberrant inflammatory pathways have also been linked to hypertrophic scarring, and anti-inflammatory treatments could potentially aggravate symptoms and delay wound healing [
40,
43,
44].
Significant edema that is initiated by several factors including vasodilation, extravascular osmotic activity, and increased microvascular permeability often accompanies inflammation [
45]. Excessive or prolonged edema and inflammation exacerbate pain and impair wound healing [
17,
18]. Interestingly, studies suggest that in the absence of infection, inflammation might not be required for tissue repair [
46]. Since inflammation can have both beneficial and detrimental effects on burn wound healing, the clinical challenge becomes management, applying therapeutic intervention only when inflammation and edema become excessive.
Treatment of inflammation in large burns is difficult, as recently discussed in detail elsewhere [
16]. Traditional anti-inflammatory treatments that focus on the inhibition of prostaglandin synthesis, such as nonsteroidal anti-inflammatory drugs or glucocorticoids, impair wound healing [
47]. However, steroid administration has been shown to reduce inflammation, pain, and length of hospital stay in burn patients in several small studies [
48,
49]. Early excision and grafting has become the gold standard for treatment of full and deep partial thickness burns [
50,
51], in part because early excision helps reduce the risk of infection and scarring [
52‐
54]. The timing of debridement coincides with the inflammatory phase of healing, as the burn eschar removed during excision is an inflammatory nidus and a rich pabulum for bacterial proliferation.
Nontraditional anti-inflammatory treatments, such as opioids, have gained considerable attention but have yet to translate promising preclinical results into clinical practice for wound healing. While the majority of animal studies have demonstrated consistent anti-inflammatory effects of opioids on peripheral neurons [
55], clinical studies have shown little to no effect on inflammation [
56]. Furthermore, topical morphine delayed the early inflammatory phase and accelerated the later proliferative phase [
57,
58], which is supported by in vitro studies showing opioid stimulation of keratinocyte migration [
59]. Large-scale clinical trials evaluating opioid efficacy on wound healing have not yet been conducted [
60].
Infection
The skin functions as a barrier to the external environment to maintain fluid homeostasis and body temperature, while providing sensory information along with metabolic and immunological support. Damage to this barrier following a burn disrupts the innate immune system and increases susceptibility to bacterial infection [
61]. Burn wound infection was defined in a rat model with
Pseudomonas aeruginosa [
62,
63], in which the following progression was observed: burn wound colonization; invasion into subjacent tissue within 5 days; destruction of granulation tissue; visceral hematogenous lesions; and leukopenia, hypothermia, and death. Burn patients are at high risk for infection [
64], especially drug-resistant infection [
65], which often results in significantly longer hospital stays, delayed wound healing, higher costs, and higher mortality [
66]. Infection can lead to the development of a pronounced immune response, accompanied by sepsis or septic shock, which results in hypotension and impaired perfusion of end organs, including the skin – all processes that delay wound healing. Furthermore, the leading causes of death following a severe burn are sepsis and multiorgan failure [
67‐
69], so prevention and management of infection is a primary concern in the treatment of burn patients. Early and accurate diagnosis of infection is difficult: C-reactive protein and the white blood cell count are most often used, since the diagnostic power of procalcitonin is questionable in burns [
70]. Consensus definitions of sepsis and infection have recently been proposed that are more relevant to the burn population and are often used clinically but still require validation [
71].
The management of burn wound infections has been extensively reviewed elsewhere [
61,
64‐
66,
72‐
77]. Since the adoption of topical antibiotics, such as mafenide in the 1960s and silver sulfadiazine in the 1970s, and of early excision and grafting in the 1970s and thereafter, systemic infections and mortality have consistently decreased [
68,
72,
78]. However, Gram-positive and Gram-negative bacterial infections still remain one of the most common causes of mortality following burn injury [
73]. Bacterial cultures can aid in the selection of an appropriate antibiotic, especially in cases of bacterial drug resistance, but altered pharmacokinetic parameters in burn patients must be considered and dosing should be adjusted accordingly to maximize antibiotic efficacy [
79]. Importantly, effective topical antimicrobials do not exist for invasive fungal infections, and fungal wound infections are associated with greater mortality rates in large burns (>30 % TBSA) [
80]. Owing to high lethality, suspicion of an invasive burn wound infection mandates rapid diagnosis, often by histopathology, and excision or re-excision of the wound.
Nutrition
Sustained hypermetabolism, hormone elevations, and muscle wasting following severe burn injury all contribute to the clinical outcome, with magnitude and duration that are unique to burns [
81,
82]. Accordingly, reducing the impact of a hypermetabolic state and providing adequate nutrition are key factors that affect burn wound healing and recovery [
83], as has been reviewed elsewhere [
84]. There is a difficult balance between the additional caloric needs to meet the demand from hypermetabolism and the consequences of nutrient overconsumption. Nutritional support following a burn injury is a complex issue. For example, early excision and aggressive feeding in children does not diminish energy expenditure but is associated with decreased muscle protein catabolism, a decreased rate of burn sepsis, and significantly lower bacterial counts from excised tissue [
85]. In adults, early nutritional support is correlated with shorter stays, accelerated wound healing, and decreased risk of infection [
86].
Several nutritional factors must be considered. For example, excess carbohydrate consumption may lead to hyperglycemia [
87] that can exacerbate systemic inflammation and muscle degradation [
88,
89]. Furthermore, excess fat consumption may exaggerate the immunosuppressed state [
90]; and since major burn injuries may also result in immunosuppression [
91], this exaggeration may increase the risk for infection and sepsis. Carbohydrate and fat intake must therefore be closely monitored in burn patients. Guidelines for nutritional support of burn patients vary, but consensus recommendations have been given by the American Burn Association and the American Society for Parenteral and Enteral Nutrition for carbohydrates, proteins, and fats [
84].
In addition to support with amino acids and vitamins [
84], administration of insulin has been shown to decrease healing time by reducing protein catabolism and increasing skeletal muscle protein synthesis [
92‐
96]. More research is needed to optimize insulin delivery, as many recombinant growth factors, such as epidermal growth factor and transforming growth factor, are often cost prohibitive [
93]. Other anabolic agents, such as oxandrolone, have been shown to increase lean body mass recovery, decrease length of stay, and improve overall outcomes, including wound healing [
97‐
100]. Additionally, while conventional theory suggests that hemoglobin levels must be maintained above 10 g/dl to promote wound healing [
101], preliminary evidence suggests that mild to moderate anemia has no effect on graft success if perfusion is maintained with proper circulatory volume [
102]. The results of a multicenter, randomized, controlled trial (ClinicalTrials.gov NCT01079247) comparing blood transfusion with lower volumes (target hemoglobin of 7 to 8 g/dl) and conventional volumes (target hemoglobin >10 g/dl) for a large cohort of patients are expected soon and will allow for more definitive clinical guidelines on blood transfusion volumes.
Resuscitation
Severe thermal injuries over a large area of the skin (>20 % TBSA) require fluid resuscitation for stabilization. Although volume guidelines and fluid compositions vary widely between centers, the goal of fluid resuscitation is to maintain organ perfusion with the least amount of fluid necessary [
12]. Common traditional resuscitation formulas, such as the modified Brooke, and Parkland formulas, employ crystalloids such as lactated Ringer’s that contain sodium, chloride, calcium, potassium, and lactate. During large-volume resuscitations, the addition of colloids (for example, albumin, fresh frozen plasma) as adjuncts has been successful in reducing the total volume [
12]. Despite extensive research into resuscitation fluid compositions and volumes, little is known about the effect of resuscitation on wound healing. A recent meta-analysis showed a positive association between the number of grafting procedures and hypernatremia, suggesting that high serum sodium levels may inhibit graft take [
103]. Additionally, we have recently shown that the rate of wound closure (healing rate) is significantly faster in patients who received lower 24-h fluid resuscitation volumes [
104]. More work is needed to evaluate the effect of resuscitation on wound healing trajectories before clinical recommendations for preferred fluid compositions and volumes can be made.
Wound coverage and grafting
Early excision and grafting has been the standard of care for decades. Most studies have shown that excision within 24 to 48 h after injury is associated with decreased blood loss, infection, length of hospital stay and mortality, and increased graft take [
105‐
108], although mortality reductions may only occur in patients without inhalation injury [
109]. Since one of the main challenges in treating acute thermal injuries is preventing infection, excising the eschar and covering the wound as early as possible are critical. The standard for rapid and permanent closure of full-thickness burns is a split-thickness skin graft from an uninjured donor site on the same patient (autograft). Such grafting provides sufficient coverage without risk of rejection, although meta-analyses have yet to determine the failure rate of split-thickness skin grafts in burn patients. Split-thickness skin grafts can be meshed with variable expansion ratios to increase the coverage area, but concerns remain over the effect that meshing has on range of motion [
110] and the graft site healing rate. On the other hand, donor sites are painful and impose their own wound-healing burden on the patient [
111]. Various dressings have been used to cover donor sites during healing, with variable results [
112].
Patients with more extensive burns often require temporary coverage with an allograft, xenograft, skin substitute, or dermal analog due to insufficient or unavailable donor sites. Allografts, or tissue taken from a living or deceased human donor, and xenografts, taken from a different species, promote re-epithelialization and prepare the wound bed for autograft, increasing the healing rate when compared with traditional dressings [
113]. A recent meta-analysis suggested that since allografts and xenografts appear to be equally effective, xenografts may be a superior choice for their increased safety and reduced price [
114]. However, caution should be exercised in drawing broad conclusions from this meta-analysis because the cited studies lack standardization and critical details such as depth and size of burn, and many studies cited were merely anecdotal. A cadaver allograft is thus widely considered the best material for temporary closure of excised wounds in patients with extensive, life-threatening burns and inadequate donor sites. The cadaver allograft is also the preferred material for protection of widely meshed autografts (3:1 or higher meshing ratios) during healing. In the latter setting, the allograft is applied over the meshed autograft in the manner of a sandwich.
A variety of different skin substitutes and dermal analogs exist [
115‐
119] (Table
2) that can be broadly divided into those which replace the epidermis or replace the dermis [
120,
121]. Epidermal substitutes are normally only a few cell layers thick and lack normal dermal components [
122,
123]. Commercially available dermal substitutes include acellular matrices, commonly from human – for example, Alloderm (LifeCell, Bridgewater, NJ, USA) or GraftJacket (KCI, San Antonio, TX, USA) – or other sources (for example, Integra; Integra LifeSciences, Plainsboro, NJ, USA). Biobrane (Smith & Nephew, London, UK) is a semisynthetic, bilaminar material consisting of a nylon-mesh dermal analog (bonded with porcine collagen) and a silicone epidermal analog. Biobrane is used for temporary closure of superficial burns and donor sites [
124,
125]. Products currently under development integrate the concept of dermal scaffolds that actively promote revascularization by incorporating stem cells and growth factors to recreate a favorable cellular microenvironment [
126,
127].
Table 2
Skin substitutes and coverage options
EpiDex | Autologous | Keratinocyte-based | No (Modex, Lausanne, Switzerland) |
Alloderm | Acellular | Human origin | Yes (LifeCell, Bridgewater, NJ, USA) |
| | Dermal matrix | |
GraftJacket | Acellular | Human origin | Yes (KCI, San Antonio, TX, USA) |
| | Tissue scaffold | |
Integra | Acellular | Bovine/shark origin | Yes (Integra, Plainsboro, NJ, USA) |
| | Bilayer matrix | |
Biobrane | Acellular | Biocomposite dressing, nylon fibers in silicone with collagen | Yes (Smith & Nephew, London, UK) |
Dermagraft | Cellular | Bioabsorbable polyglactin mesh scaffold with human fibroblasts (neonatal origin) | Yes (Organogenesis, Canton, MA, USA) |
Epicel | Cellular | Keratinocyte-based cultured epidermal autograft | Yes (Genzyme, Cambridge, MA, USA) |
Recell | Cellular | Autologous cell suspension of keratinocytes, fibroblasts, Langerhans cells and melanocytes | Yes (Avita, Northridge, CA, USA) |
| | Sprayable after culture | |
Numerous options exist for dressings [
128,
129]. The selection of an appropriate dressing depends on several factors, including depth of burn, condition of the wound bed, wound location, desired moisture retention and drainage, required frequency of dressing changes, and cost. While many factors must be considered in dressing selection, the goals in selecting the most appropriate dressing should include providing protection from contamination (bacterial or otherwise) and from physical damage, allowing gas exchange and moisture retention, and providing comfort to enhance functional recovery. The traditional approach to burn wound care developed at the US Army Burn Center includes alternation of mafenide acetate cream in the morning and silver sulfadiazine cream in the evening, with gauze dressings used over the creams. More recently, silver-impregnated and other dressings have been introduced. Major classes of dressings include: alginate, for example Aquacel (ConvaTec, Bridgewater, NJ, USA), Comfeel (Coloplast, Minneapolis, MN, USA), or Sorbsan (Mylan, Morgantown, WV, USA); antimicrobial, for example Acticoat (Smith & Nephew, London, UK) or Silverlon (Argentum, Geneva, IL, USA); collagen, for example Fibracol (Johnson & Johnson, New Brunswick, NJ) or Puracol (Medline, Mundelein, IL, USA); hydrocolloid, for example Duoderm (ConvaTec, Bridgewater, NJ, USA), Granuflex (ConvaTec, Bridgewater, NJ, USA), or Tegaderm (3M, Maplewood, MN, USA); hydrogel, for example Dermagel (Maximilian Zenho & Co, Brussels, Belgium), SilvaSorb (Medline, Mundelein, IL, USA), or Skintegrity (Medline, Mundelein, IL, USA); and polyurethane foam, for example Allevyn (Smith & Nephew, London, UK) or Lyofoa (Molnycke, Gothenburg, Sweden). Notably, many of these dressings exhibit antimicrobial properties through silver impregnation, but recent studies suggest silver may delay wound healing and should not be routinely used on uninfected donor skin [
130,
131] even though silver dressings may reduce wound pain [
132]. In patients with extensive or deep burns, antimicrobial efficacy should be the first priority in burn wound care.
Alternatively, cell-based techniques for more permanent coverage have made progress. Research on cultured epithelial cells has made advancements, especially with respect to culture time. Culture-based options, such as Epicel (Genzyme, Cambridge, MA, USA), use a small biopsy of the patient’s skin to provide keratinocytes, which are expanded over 2 to 3 weeks (for Epicel, in the presence of proliferation-arrested murine fibroblasts) into a confluent epidermal autograft. Other options, such as ReCell (Avita, Northridge, CA, USA), take a small biopsy of the patient’s skin and prepare a mixture of keratinocytes, melanocytes, and stem cells in a liquid formulation for spraying onto the excised burn wound during the same operation [
133‐
135]. These techniques may reduce the amount of donor skin needed for treatment of large burns, significantly reducing the healing time of both the donor and the burn sites, and increasing overall graft success and scar quality [
136]. More work is needed on cell-based coverage options before widespread implementation can be recommended.
Keratinocytes and stem cells
As mentioned previously, keratinocytes play a vital role in wound closure. Cytokine activation causes keratinocyte migration in the proliferative phase, leading to closure and restoration of a vascular network [
35]. Keratinocytes can also be activated by mu opioid receptor agonists [
59] but the role of these agonists on inflammation and wound closure remains unclear [
57,
58]. Despite positive studies with EpiDex (Modex, Lausanne, Switzerland) – an engineered, fully differentiated autologous skin substitute derived from keratinocytes showing efficacy comparable with split-thickness skin grafts in wound closure and healing [
137] – results have yet to translate into clinically viable options. Studies evaluating expansion of keratinocytes on human fibroblasts following trypsin extraction [
138], and using engineered skin with keratinocytes on a fibrin matrix [
139], have demonstrated improvements in wound healing. Retrospective analyses on autologous keratinocytes showed that cultured allogeneic or autologous keratinocytes may accelerate wound healing [
140,
141]. Taken together, the future impact of keratinocyte-mediated cell coverage options is promising, but more research is needed [
134]. Additionally, keratinocyte-based treatments should be pursued carefully, as overactivation of keratinocytes can contribute to the development of hypertrophic scarring [
43,
142].
The use of adult stem cells, including bone marrow stem cells, hair follicle stem cells, and adipose stem cells, in acute burn care is an exciting topic [
143]. Addition of bone marrow stem cells to nonhealing chronic wounds leads to engraftment of cells and enhanced wound healing [
144,
145]. Moreover, studies have reported that bone marrow stem cells can transdifferentiate towards multiple skin cell types [
146]. Mechanisms of action of bone marrow stem cells in burns are not fully elucidated, but modulation of inflammation has occurred after radiation burns in humans [
147]. Similarly, adipose stem cells accelerate re-epithelialization by paracrine activation of host cells via growth factor secretion [
148,
149]. Also, hair follicle stem cells are capable of generating a stratified epidermis on human burn wounds [
150]. Additionally, the possibility of generating a cellular skin equivalent is being explored. Hair follicle stem cells have been incorporated into products, such as Integra, to investigate wound healing [
151]. A cultured skin substitute using adipose stem cells and keratinocytes has been developed that produces epidermal, dermal, and hypodermal stratification [
152]. Moreover, human adipose stem cells that would normally be discarded have recently been isolated from debrided burn eschar tissue [
153] and used to generate a tri-layered, vascularized construct [
154]. Promising data with nonembryonic stems cells such as these have stimulated interest into future applications and development, and undoubtedly further investigations will produce exciting results.