C L I N I C S I NP L A S T I C

S U R G E R Y

Clin Plastic Surg 34 (2007) 621–632

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The Science of Wound BedPreparationJaymie Panuncialman, MDa, Vincent Falanga, MD, FACPa,b,*

- The science of wound bed preparation- Assessment of the patient- Need for debridement- Bacterial burden and biofilms- Moisture balance and dressings- Matrix metalloproteinases and their

inhibitors

- Growth factors- Cellular senescence or near senescence- Monitoring progress in wound healing- Summary- References

The science of wound bed preparation

In recent years, advances in molecular techniqueshave led to increased understanding of the patho-physiologic problems encountered in chronicwounds (Fig. 1). The science of wound bed prepa-ration (WBP) comes from technologic advancesand their use in clinical practice. Technologic ad-vances include new techniques in tissue culture,the development of recombinant growth factors,and tissue engineering. The identification of thecomponents of the extracellular matrix (ECM)and how the balance of proteinases and their inhib-itors affects chronic wound healing has been criticalto our understanding [1]. The development of ge-netically engineered mice or mice models withknock-out features or the presence of induciblegenes provided powerful clues to the actions ofgrowth factors [2]. The cloning of growth factorsand cytokines has opened the doors for gene ther-apy. Gene array technology is providing us withan understanding of the functional pathways and

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interactions of cellular components that carry outor regulate the healing process [3]. In essence,gene array technology allows us to identify the tran-scriptional profile of a large number of genes [4–6].In the near future, this technology also might pro-vide us with a powerful diagnostic tool to identifythe specific processes that are impaired in a certainpatient [7].

The second area involving the science of WBP ishow these new principles or treatments are trans-lated into practice. These are discussed in detail.

Assessment of the patient

To optimize the wound bed, the first step is to ad-dress systemic factors that are present in a patientand that impair healing. For example, one cannot ex-pect healing to take place without controlling edemain patients who have venous leg ulcers. The arterialcomponent must be adequate as well. Tight glycemiccontrol in patients who have diabetes is a priority.The use of off-loading footwear in neuropathic

This study was funded by National Institutes of Health grants (DK067836 and P20RR018757) and the WoundBiotechnology Foundation.a Department of Dermatology and Skin Surgery, Roger Williams Medical Center, Providence, RI, USAb Department of Dermatology and Biochemistry, Boston University, Boston, MA, USA* Corresponding address. Department of Dermatology, Roger Williams Medical Center, 50 Maude Street,Providence, RI 02908.E-mail address: vfalanga@bu.edu (V. Falanga).

ts reserved. doi:10.1016/j.cps.2007.07.003

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Fig. 1. Overview of wound bed preparation, abnormalities present in chronic wounds, and possible correctivemeasures. Dashed lines represent the overlap among different compartments and corrective measures. (Cour-tesy of V. Falanga, MD, Boston, MA. Copyright ª 2007.)

ulcers is imperative. Improving the patient’s nutri-tional status also helps with healing. Evaluationfor an inflammatory cause of nonhealing ulcersshould be done if the ulcers are accompanied bysigns or symptoms suggestive of a connective tissuedisease. Finally, smoking cessation always shouldbe part of the overall management.

Need for debridement

The first hurdle in applying the concepts of WBP isthe confusion that proper WBP can be achievedsolely by debridement. Debridement is definitelyan important part of WBP; however, debridementalone is not enough to sustain healing in a chroniculcer. An old, simplistic way of approaching chronicwounds is to think that one can transform thechronic wound into an acute wound by surgical de-bridement; however, this approach is not enoughfor the other underlying processes in chronicwounds. A better way of understanding the signifi-cance of debridement in chronic wounds is to thinkof debridement as a way to ‘‘introduce’’ an acutewound in a chronic wound [8]. Also, the chronicwound tends to accumulate a ‘‘necrotic burden’’ ofsenescent cells, a corrupt ECM, and inflammatory

enzymes that need to be removed continuouslywithout removing new, healthier tissues [8].

When one is faced with necrotic eschars or gan-grene in the context of life- or limb-threateninginfections, surgical debridement is critical. Debride-ment also gives the clinician the advantage of accu-rately assessing the severity and extent of thewounds [9] Surgical debridement is the fastest wayto debride a wound, but it is not selective becauseit removes viable tissue as well. ‘‘Maintenance de-bridement’’ in between surgical debridement inter-ventions may be achieved by other methods, suchas autolytic, chemical, or biologic means (Fig. 2).

Mechanical methods of debridement, such as wetto dry dressings, hydrotherapy, irrigation, and theuse of dextranomers, is similar to sharp debride-ment. Mechanical debridement is fast, but nonse-lective and painful, and it generally is used forwounds with larger amounts of necrotic tissue [9].In a study done to test the efficacy of growth factorsin diabetic ulcers, ulcers treated with growth factorsand debridement fared better than did those treatedwith growth factors alone [10].

Autolytic debridement occurs by using the body’sown endogenous proteolytic enzymes and phago-cytic cells in clearing up necrotic debris. This process

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Fig. 2. Wound bed debridement. (A) Before autolytic debridement. (B) Three weeks after autolytic debridement.(C) Before surgical debridement. (D) One week after surgical debridement. (E) Before application of Lucila serrica-ta. (F) Two days after application of Lucila serricata. (Courtesy of V. Falanga, MD, Boston, MA. Copyright ª 2007.)

is facilitated by the use of moisture-retentive dress-ings [11] and may take weeks. Chemical debride-ment uses proteolytic enzymes to digest necrotictissue. Enzymatic agents that degrade DNA or pan-creatic enzymes that degrade proteins have beenused in the past. Currently, two enzymatic prepara-tions are in use in the United States: papain-ureaand collagenase. For practical reasons, we focus onthese two agents. Papain comes from the plant Car-ica papaya. Papain attacks and breaks down proteinwith cysteine residues [8]. Urea is added to this sys-tem to potentiate the action of papain by altering thetertiary structure of proteins, thus exposing the cyste-ine residues [12]. This enzyme preparation is activeover a broad pH range (3.0–12) and is associatedwith an intense inflammatory response and break-down of the viable portions of the wound bed.Use of these agents is associated with considerablepain [8]. Another formulation of this enzyme sys-tem was made with the addition of chlorophyllin.Chlorophyllin was added to address the issue ofpain associated with papain-urea. Papain is not ef-fective in debriding denatured collagen becausethe latter has no cysteine residues. In summary, pa-pain is nonselective, but it has a broad spectrum ofaction and is useful in bulk debridement of rela-tively insensate areas [8]. The other enzyme usedcommonly is collagenase derived from Clostridiumhistolyticum. It is a water-based proteinase that spe-cifically attacks and breaks down native collagenand acts in a restrictive pH range of 6 to 8. One ad-vantage of collagenase is that it is extremely gentleon viable cells. Collagenase may be useful in the

‘‘maintenance phase’’ of wound debridement whenone would like to break down tissue gradually [8].

There has been recent interest in biosurgery,which involves the use of the larvae of the greenbottlefly (Lucila serricata). This method has beenused in patients with large ulcers having significantnecrotic material that is not amenable to surgicaldebridement. The larvae digest the necrotic materialand secrete enzymes that are bactericidal. Larvatherapy is reported to be effective against methicil-lin-resistant Staphylococcus aureus (MRSA) andbeta-hemolytic streptococcus [13]. Wounds are de-brided in 2 days, debridement is selective, and sideeffects are limited to patient discomfort. Thismethod of debridement may be done at homewith the services of a trained nurse [14].

The issue with debridement is to know when tostop. Traditional practice in acute wounds hasbeen to debride to the level of bleeding tissue. Ina recently published article, Falanga and colleagues[15] reviewed a method of wound bed scoring inwhich the presence of fibrinous slough did not af-fect healing adversely. In chronic wounds, becauseremoval of the debris is part of a continuous pro-cess, debridement must be done without injuringviable tissue. It seems reasonable to state that thegoal of debridement is to facilitate the efficacy ofother treatments, be it growth factors, bioengi-neered skin, or topical antiseptics.

Bacterial burden and biofilms

The role of bacteria in the healing or nonhealingof chronic wounds has always been a controversial

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topic. Although sterility of the chronic wound bedcannot be achieved, control of bacterial burden,in terms of bacterial density and pathogenicity,is a goal in WBP [16]. An important point in dis-cussing bacterial burden in chronic wounds is theimpact of host response. How one patient reactsto the bacterial burden is different from how thenext patient reacts. Therefore, generalizations inhandling bacterial burden should be made withcaution. The presence of clinical infection is al-ways a deterrent to wound healing. Clinical infec-tion is the presence of multiplying bacteria inbody tissues that results in spreading cellular in-jury as a result of toxins, competitive metabolism,and inflammation [17]. The cardinal features ofan infection, such as heat, swelling, surroundingerythema, and pain, are still the standards bywhich infection is diagnosed; however, in chronicwounds, distinguishing a true infection from col-onization often is difficult. It has been suggestedthat increasing ulcer size, increased exudate, andfriable granulation tissue also are signs thatshould be considered in diagnosing an infection[18]. In light of increasing antibiotic resistance,we are faced with the challenge of antibiotic use.When then, does the presence of bacteria in thewound become a deterrent to healing? All chronicwounds are colonized with bacteria. Colonizationis defined as the presence of replicating bacteriaand adherent microorganisms without tissue dam-age [19]. Critical colonization is a recently definedconcept that has yet to be characterized defini-tively [20]. This novel concept states that the bac-terial burden in the chronic wound does not elicitthe typical symptoms of an infection, but delayshealing [21]. White and Cutting [22] proposedthat this occurs because the bacteria in the wounddo not incite an intense inflammatory responsethrough the production of proteins that makethem evade the immune system effectively; thus,the classic signs of infection are absent, but thereis delayed healing through the inhibition of thegrowth of key cells in healing or by the presenceof biofilms [23]. Biofilms are complex communi-ties of bacteria that have evolved ways to commu-nicate with each other through water channelsand have a protective extracellular polysaccharidematrix covering. Through these communicationchannels, the bacterial colonies are able to up-reg-ulate or down-regulate transcription of genes andprotein products that are beneficial to them anddetrimental to the host by a phenomenon calledquorum sensing. Biofilms have high resistance toantibiotics [24,25]. Bacterial biofilms have beenreported from isolates taken from chronic wounds[26,27]; however, more rigorous studies areneeded.

The importance of the concept of critical coloni-zation in the science of WBP is to encourage clini-cians to pay closer attention to delayed healingand its assessment [22]. In instances in which heal-ing does not take place, despite optimum treat-ment, critical colonization should be considered[22]. Obtaining and interpreting microbiologic di-agnostic data always must be correlated with clini-cal findings. With regard to appropriate samplingmethods, several studies established that properlydone wound swabs were predictive of culturesgrown from tissue biopsies [28–36]. The relevanceof quantitative versus qualitative reports of microbi-ologic data in wound healing has been debated of-ten. A critical bacterial load, synergistic relationshipbetween microorganisms, and the presence of spe-cific pathogens have been implicated in the devel-opment of infections and nonhealing [37].Although studies were done in wounds of differentetiologies, collectively the findings support the ideathat the presence of 106 or more colony-formingunits of bacteria per gram of tissue predicts delayedwound healing and a high risk for developing infec-tions [38–43]. The detrimental effect of specific mi-croorganisms on wound healing also has beenstudied. Most chronic wounds have a polymicrobialflora. It has been suggested that the presence in thewound bed of four or more organisms, rather thantheir type, is a predictor of nonhealing [35,44,45].The reason for this finding is that certain microor-ganisms exhibit synergism. The importance ofanaerobes in chronic wounds has been underesti-mated, possibly because of the difficulty in trans-porting and growing these bacteria [21]. Thepresence of beta-hemolytic streptococcus has beenestablished to cause delayed wound healing orworsening of chronic ulcers [37,44]. The data onStaphylococcus aureus and Pseudomonas aureginosaas major contributors to delayed healing have notbeen consistent; however, other investigatorsshowed that resident microorganisms have little ef-fect on the outcome of healing [46–50].

The indication for the use of topical antimicro-bials is limited. The incidence of antimicrobial re-sistance is attributed, in part, to the use of topicalantimicrobials that have a systemic counterpart. Inaddition, these topical antibiotics are potent sensi-tizers [21,51–53]. Mupirocin is effective in eradicat-ing MRSA; however, because of its widespread use,the incidence of resistance also is increasing. Thus,interest in the use of topical antiseptics has been re-vived. Topical antiseptics have long been used butfell out of favor because of in vitro studies suggest-ing cellular toxicity; however, innovative formula-tions of antiseptics have allayed this fear.Cadexomer iodine and sustained-release silverdressings are examples. Topical antiseptics have

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a broad spectrum of bacterial coverage and deliverhigh concentrations of the drug to the woundbed; they also may be effective against biofilms[17]. In addition, silver dressings are useful in pre-venting cross-contamination of wounds and act asa barrier to the spread of MRSA, which is a usefulfeature in situations in which patients are in closecontact with each other [54]. Debridement is an im-portant adjunct in controlling the bacterial burdenin wounds; it decreases the necrotic material and re-moves possible biofilms in the wounds.

In summary, surface swabbing may be an effec-tive method for identifying and quantifying thebacteria present, but this needs further study.Wounds that are malodorous or exudative shouldbe analyzed for the presence of anaerobic bacteria.Systemic antibiotics or topical antiseptics shouldonly be prescribed in patients who are clinically in-fected or exhibit critical colonization. The routineuse of antibiotics to facilitate wound healing isnot supported by evidence. In the case of criticalcolonization, topical antiseptics may offer a firstline of treatment. If a wound fails to improve afteran initial course of 2 to 3 weeks, continued use isunlikely to be of any benefit. Systemic antibiotictherapy should then be considered [21].

Moisture balance and dressings

A major advance in the way that we treat woundshas been the realization that moist wound healingis beneficial to nonhealing wounds and that occlu-sive dressings do not increase the risk for infection[55,56]. This discovery heralded the production ofa variety of dressings that provide the wound witha moist healing environment and decrease pain,odor, and drainage. In chronic wounds, the woundfluid retards cellular proliferation and angiogenesis[57,58] and contains excessive levels of matrix met-alloproteinases that break down matrix proteins,growth factors, and cytokines [59–61]. There aredifferent types of dressings. An ideal dressing forchronic ulcers is one that provides a moist environ-ment, absorbs exudate, prevents maceration of sur-rounding tissue, is impermeable to bacteria, doesnot cause allergies, does not cause reinjury upon re-moval, and is cost-effective. Unfortunately, no sin-gle dressing can accomplish all of these goals. Thechoice of dressing depends on the characteristicsof the wound bed of a patient at a given time. Fur-thermore, a systematic review reported no thera-peutic advantage to using different dressings forvenous ulcers treated with compression, implyingthat dressings alone will not be able to heal chronicwounds [62]. Nevertheless, dressings are importantbecause their use helps patients with their woundmanagement and may prepare the wound bed for

other therapies. We briefly discuss the types ofdressings available.

Dressings that maintain a moist wound environ-ment are described as being moisture retentive. Thisproperty is measured by the moisture vapor transmis-sion rate (MVTR) [63,64]. A dressing is moisture re-tentive when its MVTR is less than 840 g/m2/24hours [65]. The MVTR of hydrocolloids is less than300 g/m2/24 hours, in contrast to gauze, which hasa moisture transmission rate of 1200 g/m2/24 hours[63]. Depending on whether one wants to createa moist healing environment or if the control ofheavy exudate is the primary goal, the MVTR is a use-ful tool in prescribing dressings. Table 1 summarizesthe properties of each dressing class and itsindications.

An additional therapeutic advantage of dressingsis the incorporation of topical antiseptics in theirformulation. Topical antiseptics play a role in thecontrol of the bacterial burden in the chronicwound. We focus on the newer topical antisepticsavailable: cadexomer iodine and silver. Both anti-septics are broad-spectrum antimicrobials. In addi-tion, research has shown that they may be effectiveagainst biofilms [66,67]; however, in some studies,silver was cytotoxic in vitro to the host’s own cells ina concentration-dependent manner. These in vitrofindings do not necessarily translate to clinical prac-tice. In addition, newer formulations of these anti-septics that are designed to release the drugs intothe wound bed at continuous low concentrationshelps to negate these cytotoxicity concerns. Strikinga balance between controlling the bacterial burdenand potentially harming dividing cells must betaken into consideration.

Iodine is a useful bacteriostatic and bactericidalagent that is effective against MRSA and other path-ogens in vitro. The cadexomer is a polysaccharidestarch lattice containing 0.9% elemental iodine re-leased on exposure to exudates [68]. The antimicro-bial activity of this dressing lasts for days,depending on its formulation. Cadexomer iodineis available in pads or as a gel. Upon absorptionof water the lattice swells, releases iodine into thewound bed, and leads to a visible color change ofthe gel from dark brown to gray. Cadexomer iodinehas been studied extensively and is safe and effec-tive in reducing bioburden [69].

Silver has been used for ages as an antiseptic fromthe formulation of silver nitrate to silver sulfadia-zine. Silver has proven effective as an antisepticwith minimal toxicity. Silver is an element and is in-ert in its metallic form; however, once in contactwith body fluid, it ionizes and becomes reactive[70]. Silver exert its lethal effects, even at low con-centrations. Research suggests that most pathogenicorganisms are killed in vitro at concentrations of

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Table 1: Summary of dressings and their characteristics

Dressing type/material MVTR Properties

Films (polyurethane film) 300–800 No absorptive capacityHydrocolloids (carboxymethyl cellulose,pectin, gelatin, guar gum)

<300 Not for infected wounds or heavilyexudative woundsTraumatic removalHigh incidence of allergic contactdermatitisMay produce offensive odor

Foams (polyurethane) 800–5000 For highly exuding wounds, not for drywoundsUsed under compression bandages

Absorptive wound fillers (calciumalginate; starch copolymers)

Effective in undermined or tunnelingwoundsNontraumatic removalHigh absorptive capacityHemostatic propertiesNontoxic and nonallergenicImprove painMay adhere to dry wounds

Hydrogels (water, polymers,humectants)

For wound hydration, not significantexudate absorptionFor minimally exuding woundsNot for use on a dry ischemic ulcerNonadhesiveRelieve painMay be used to moisten gauze packing

Gauze 1200 For heavily exuding woundsMay adhere to viable areas of thewound bedUseful as a secondary dressing tocontact layers

Data from Seaman S. Dressing selection in chronic wound management. J Am Podiatr Med Assoc 2002;92(1):24–33.

10 to 40 parts per million (ppm), with particularlysensitive organisms susceptible to 60 ppm [71].

The development of nanochemistry has facilitatedthe production of microfine particles that increasesilver’s solubility and the release of silver ions in con-centrations of 70 to 100 ppm [72,73]. Several sus-tained-silver dressings are available. Although thesedressings differ in the technology used, as well asthe characteristics of silver release, all are able to re-lease more than the 10 to 40 ppm needed to achieveantisepsis [72]. Resistance to antiseptics is not a com-mon problem. Data concerning the toxicity of silverions are derived from what has been the experiencewith silver sulfadiazine. Silver absorption through in-tact skin is minimal; however, in full-thicknesswounds, absorption of silver sulfadiazine may be ashigh as 10% [72]. Silver absorbed by epidermal cellsinduces the production of metallotheine, which, inturn, increases the uptake of zinc and copper and ispostulated to increase RNA and DNA synthesis; thismay promote cell proliferation and tissue repair[71,74]. Silver that is absorbed systemically may befound in the liver, brain, kidney, and bone marrowwith apparently little toxic risk. Silver absorbed into

the skin may cause argyria, which is a permanent dis-pigmentation of the skin; however, it is not life threat-ening [72]. There is some concern about the effect ofsilver on the proliferating cells in the bone marrowbecause there have been reports of neutropenia inchildren who were treated with silver sulfadiazine.White blood cell counts returned to normal when sil-ver sulfadiazine was withheld; however, this is a sensi-tizing agent, and patients who developed leukopeniain the past should not be treated with silver again[71]. There has been a paucity of clinical trials evalu-ating the clinical efficacy of silver; thus, no strong con-clusions may be drawn regarding this.

In summary, these dressings are useful tools incontrolling the bacterial burden in wounds, butthey may have cytotoxic effects and their judicioususe is important.

Matrix metalloproteinasesand their inhibitors

Controlled degradation and formation of the ECMare an essential step in wound healing. This facili-tates angiogenesis, migration of keratinocytes,

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reepithelialization, and remodeling of the provi-sional matrix. In addition, the ECM containsgrowth factors and cytokines that must be releasedto be activated. The turnover and remodeling of theECM is tightly regulated [1,75]. The matrix metallo-proteinases (MMPs) are a zinc-dependent group ofenzymes that are capable of breaking down the ma-jor components of the ECM [1,75]. The produc-tion/activation of MMPs are regulated in threeways. First, it is regulated at the level of gene tran-scription, and this is induced by inflammatory cyto-kines and growth factors. Next, the enzymes aresecreted as a zymogen and require activation byproteinases. Lastly, it is inhibited by another groupof enzymes: the tissue inhibitors of metalloprotei-nases (TIMPs) and plasma a macroglobulin [75].MMPs are mostly soluble proteins, except for themembrane-bound metalloproteinase. TIMPs areable to inhibit all unbound MMPs. TIMPs havecell growth–promoting, antiapoptotic, and antian-giogenic activity [76]. The generation of geneticallyengineered knock-out and transgenic mice pro-vided vital information on the physiologic andpathologic functions of MMPs. Mice that lackedMMP-3 showed impaired dermal wound contrac-tion, whereas mice lacking MMP-2 showed im-paired angiogenesis. The most severe woundrepair defect was seen in mice without MMP-7[1]. Conversely, the overexpression of MMP-1 bytransgenic mice results in a hyperproliferative, hy-perkeratotic epidermal phenotype and delayedwound closure [77]. MMPs seem to have overlap-ping substrate specificities and expression patterns.In healing wounds, MMPs follow a well-definedtemporal and spatial profile. MMPs generally arenot detected in normal intact skin; however, uponinjury, MMP-1 is up-regulated, and its levels persistduring healing and decrease with reepithelializa-tion. MMP-1 is expressed in the advancing woundedge, whereas MMP-3 is found in the premigratorybasal keratinocytes [78,79]. MMP-2 and -9 degradethe basement membrane and allow capillaries toform. In addition, MMPs are capable of cleavingand releasing inflammatory cytokines and interleu-kins and are responsible for releasing ECM-boundgrowth factors. MMPs are present in excessiveamounts in chronic wounds, and their temporalpattern of expression is altered. Recent work byNwomeh and colleagues [80] showed that chronic,nonhealing ulcers are characterized by significantlyhigher levels of MMP-1 and -8 and low levels ofTIMP-1. Collagenases are in their inactive formsin normal healing wounds, whereas the activatedenzyme is present at high concentrations in chroniculcers. MMP-1, -2, -8, -9, and -14 were elevated indiabetic foot ulcers, whereas TIMP-2 was decreased[81]. MMP-9 was elevated in venous ulcers, and its

levels correlated with poor healing. An elevated ra-tio of MMP-9/TIMP-1 also was related to nonheal-ing [82]. In summary, this altered expression ofMMPs and TIMPs leads to persistent inflammationand excessive degradation of the ECM and growthfactors.

What can we do to restore the balance of theseenzymes in the chronic wound bed in the contextof WBP? So far, attempts to target MMPs to producehealing in chronic wounds have not been success-ful. Because active MMPs are found in wound fluid,the therapeutic approach has been to develop dress-ings that could bind metalloproteinases and deacti-vate them. A collagen and oxidized regeneratedcellulose dressing was formulated for this purpose.In vitro, Cullen and colleagues [83] showed thatthis dressing was able to decrease the in vitro activ-ity of MMPs in wound fluid from nine patients. Arandomized, double-blind clinical trial in diabeticulcers showed no significant difference in the inci-dence of wound closure with this dressing andmoistened gauze [84]. Targeting MMPs to improvewound healing is difficult because MMPs are essen-tial to wound healing; however, they have to bepresent in the right amount and at the right time.Thus, nonspecific inhibition of MMPs is not a bene-ficial approach. Current areas of research are the de-velopment of selective MMP inhibitors; MMPmodulation by using anti–tumor necrosis factora medications [85]; and the use of activated proteinC, which reduces inflammation and MMP produc-tion by inflammatory cells, but selectively stimu-lates MMP-2 [86]. It is unlikely that medicationsthat address the imbalance of MMPs in chronicwounds will solve the problem entirely; however,we will see more of these strategies in the future.

Growth factors

Growth factors are polypeptides that act in concertin wound healing. Recent research using geneticallyengineered mice confirmed the expression of multi-ple growth factors and their receptors in differentcell types of healing wounds. In addition, highlevels and activation of growth factors during thehealing process may facilitate wound closure.Growth factors have a characteristic spatial andtemporal regulation; changes in this expression pat-tern bring about impaired healing [2]. The im-paired function of one growth factor likely affectsseveral growth factors and other cytokines. Also,there is a significant overlap in the functions ofthese growth factors.

The use of growth factors as therapeutic agents inchronic ulcers is the result of previous work thatshowed that the amount of these growth factors isdecreased markedly in chronic wounds, secondary

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to the presence of high concentrations of proteases[60,87,88]. Conversely, several studies showed thatgrowth factors are present in appropriate amounts,but are trapped within the fibrin cuffs in the sur-rounding capillaries or are bound to the ECM[89–91]. The result of suboptimal concentrationsof growth factors is that cells in the wound bed be-come arrested in the cell cycle. Multiple in vitro andanimal studies have shown the beneficial effect ofgrowth factors, such as platelet-derived growth fac-tors (PDGFs), fibroblast growth factors, and granu-locyte-macrophage colony-stimulating factor, inhealing [92–95]. PDGF is approved for use in dia-betic foot ulcers, as supported by several clinical tri-als that showed statistically significant results. Ina recent study, using a retrospective cohort study,Margolis and colleagues [96] showed that the effec-tiveness of PDGF in clinical trials also was presentin clinical practice. The clinical data on the use ofother growth factors have been mixed. Clinical tri-als of exogenous application of these growth factorsfailed to produce expected significant results. Cur-rently, the focus of research on this treatment is toimprove the delivery system to provide high con-centrations to the wound bed or a more intimate re-lationship with the target cells. Gene therapystudies using adenoviral vectors are being con-ducted. In the future, the best results might be de-rived by using several growth factors that arespecific for different stages of wound healing.

Cellular senescence or near senescence

The realization that the resident cells in chronic ul-cers are phenotypically altered and exhibit

senescence is a new concept. The efficacy of advancedtreatments, such as growth factors and bioengineeredskin, is dependent on the ability of these modalitiesto restore cellular competency in the wound bed[97]. The capacity of resident cells in the woundbed to replicate is critical to wound healing. Fibro-blasts in chronic wounds display a senescent ora near-senescent phenotype. Senescent fibroblastsalso exhibit a decreased mitogenic response togrowth factors attributed to an abnormality in cell-signaling pathways. Chronic wound fibroblasts alsoproduce elevated levels of proteolytic enzymes anddecreased TIMPs. An important concept in cellularsenescence is that cellular senescence is irreversible[97]. Thus, the continued exposure of cells in thewound bed to inflammatory cytokines, reactive oxy-gen species, and bacterial toxins results in an accumu-lation of senescent cells. It is postulated that whenthis population reaches a critical number, woundsare unlikely to heal, even with optimal care [98].

Current treatment strategies to circumvent thisproblem are to remove senescent cells and repopu-late the wound bed with viable nonsenescent fibro-blasts by using tissue-engineered skin [99,100]. Invitro, it was shown that these products could coun-teract the inhibitory effects of wound exudate, pos-sibly by delivery of growth factors in the correcttemporal sequence and adequate concentration[100]. Other sources of nonsenescent fibroblastsare the wound margins and if quiescent fibroblastscould be induced to proliferate [97]. Because a sub-stantial number of patients (up to 50%) are unre-sponsive to treatment, despite available advancedtreatments, the search for new and effective strate-gies goes on.

Fig. 3. Woundbedscoreand its individ-ual features. The individual score foreach characteristic is added to give a to-tal wound bed score. The descriptors inparentheses below represent scores of0, 1, and 2 points, respectively. Percent-age of black eschar present (>25%,1%–25%, 0%); severity of peri-ulcereczema/dermatitis (severe, moderate,none or mild); depth of the wound (se-verely depressed or raised compared toperi-wound skin); severity of scarring(severe, moderate, none or minimal);percentage of pink-colored granula-tion tissue present (<50%, 50%–75%,>75%); severity of edema/swelling(severe, moderate, none/mild); per-centage of regenerating epithelium(healing edges) (<25%, 25%–75%,>75%); severity of exudate/frequencyof dressing changes (severe, moderate,none/mild). (Courtesy of V. Falanga,MD, Boston, MA. Copyright ª 2007.)

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The use of stem cells provides a promising ratio-nal therapeutic approach for chronic wounds. Theethical concerns regarding embryonic stem cell re-search have forced scientists to find more accept-able sources of these cells. Adult autologous bonemarrow stem cells are a potential source of stemcells. These cells have unlimited replicative poten-tial, differentiate into different tissues, and mayprovide needed cytokines and growth factors intothe wound bed. Pilot studies have shown the greatpotential of this treatment; however, validation byrandomized, clinical trials is needed [101,102].

Monitoring progress in wound healing

The ultimate goal of WBP is to achieve completehealing of the wound. One does not expect healingof chronic ulcers to occur overnight. Therefore,there needs to be a way to evaluate the progress ofthese wounds and to be assured that they are ontheir way to closure. A 15% decrease in woundarea weekly has been suggested as the goal of treat-ment. Recently, a wound bed score that has a predic-tive value in wound closure was validated [15]. Thiswound bed score takes into account assessments ofthe wound bed as well as surrounding tissue that re-flect the goals of adequate WBP (Fig. 3). Each pa-rameter receives a score of 0 to 2, which is addedto obtain the total score. The scores are dividedinto four quartiles: 4 to 9, 10 to 11, 12, and 13 to16. With an increase in the wound bed score fromone unit to the next (eg, from 10 to 13), there isa 22.8% increase in the odds of healing. Thiswound bed score will be useful in assessments asa predictor of initial healing and possibly for mon-itoring adequate response to treatment, with theexpectation of achieving quartile increases in thewound bed time.

Summary

The prerequisites for effective therapies are that thewound bed has little necrotic burden, a manageablebacterial load, minimal inflammation, and residentcells that can regenerate needed tissue. Thus, properWBP is the first step toward achieving healing in thechronic wound. Advances in our understanding ofthe underlying problems of impaired healing willbring forth new and innovative therapies. Thewound bed score having a predictive value for heal-ing is useful for monitoring progress in treatment orthe need to reevaluate current treatment strategies.

References

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