Emerging Concepts In Healing Diabetic Foot Ulcers

Continued research in the arena of diabetic foot ulcers has led to a greater understanding of the etiologies of these wounds as well as the mechanisms of healing. With this in mind, this author raises questions on wound debridement, weighs in on the impact of quorum sensing and offers insights on the roles of allografts and advanced biologic therapies.

As our understanding of the etiology of diabetic foot ulcers (DFUs) has improved, the rate of amputations has continued to decrease. The early work of Brand and colleagues identified a link among neuropathy, repetitive mechanical stress and ulcer formation, and launched the concept of total contact casting to relieve mechanical pressure.1 Remarkably, research showed that the control of mechanical forces led to wound closure in some cases and this served as one of the first viable alternatives to amputation.2

   After identifying the link among mechanical forces, neuropathy and ulceration, the focus on healing of DFUs shifted toward the physiology of wounds. In an effort to achieve higher closure rates, clinicians began to address barriers to cellular proliferation. For many years, the debate raged on about the role of diminished blood flow in the development of DFUs as well.

   Differentiation between calcification of the intima media versus luminal blockages and the presence of basement membrane thickening was a subject of wide discussion. The argument between the impact of macroscopic and microscopic vascular disease was widely debated as well, and ultimately led to strong affiliations between vascular and podiatric surgeons.

A Closer Look At Innovations In The Treatment Of Diabetic Foot Wounds

Over the last 15 years, our understanding of wound healing has expanded. There was a realization that growth factors played a critical role in angiogenesis, mitogenesis and chemotaxis within the wound bed. This resulted in development of the first widely used, cloned, growth factor becaplermin (Regranex, Systagenix). Simultaneously, the impact of bacterial infection and colonization has led to a variety of wound dressings designed to control bacterial load. New antibiotics have also emerged in order to combat methicillin-resistant Staph aureus (MRSA) and a variety of other resistant bacteria that developed in the presence of chronic infections.

   The science of wound healing changed dramatically as both patients and clinicians realized there were alternatives to amputation. While some focused on the issues of the wound itself, others addressed the underlying bony problems. Prophylactic surgery and reconstruction of feet devastated by Charcot neuroarthropathy emerged as a new approach for managing the complex array of mechanical forces that can lead to ulceration. As our understanding of these complex deformities improved, our tools have also increased in complexity. Circular frames based on the principles of Ilizarov provided a way to reconstruct even the most severely deformed feet while allowing the clinician to evaluate and manage the foot as needed.

   Furthermore, in some cases, the patients could even remain ambulatory during the healing process. Frame designs such as the Taylor Spatial Frame (Smith and Nephew) allowed for complex readjustments over time in order to optimize the final architecture of the foot.

   Most recently, the advanced biologics have moved to the forefront as a substitution for skin grafts and human allografts, which have been used for decades on patients with burns and other complex types of wounds. Apligraf (Organogenesis) and Dermagraft (Advanced BioHealing) became the only bioengineered skin substitutes to gain FDA approval for the treatment of diabetic foot ulcers. Although neither of these products can duplicate the complex array of collagen, cytokines and growth factors found in real skin, they do provide a readily available source of growth factors that one can apply to a stagnant wound.

   Decellularized collagen products are now more readily available as well. As their name implies, these products consist primarily of extracellular matrix collagen without the cellular components. They provide a scaffold to support cellular migration from the wound bed and may contribute growth factors stored within their matrix. Collagen materials can be processed in a variety of ways to amplify various attributes such as tensile strength or the ability to incorporate into a wound bed. Research has shown both collagen and cellular components to be beneficial in coaxing DFUs to heal.3 Previously, I published a small study that demonstrated similar rates of healing when comparing decellularized collagen to a bioengineered skin substitute.4 This supports the idea that both a developed extracellular matrix and cellular components are needed to achieve wound closure.

Determining A Standard Of Care For Diabetic Foot Wounds

With so much progress occurring in the management of DFUs, the clinician can quickly become overwhelmed. The need to adopt advanced biologics should be based on one’s personal experiences as well as the support of the scientific literature rather than broad statements that imply it is the “standard of care.”

   So what exactly is the standard of care? Consensus statements supported by industry lead us to consider many different possibilities. A single treatment plan has not emerged but there is one significant exception. Sheehan and colleagues published a very widely cited study, showing that diabetic foot wounds that do not close by nearly 50 percent in the first four weeks of treatment have less than a 10 percent chance of closing by week 12.5

   This observation has been used to amplify the importance of re-evaluation of wounds on a regular basis. Frequent re-evaluation helps the clinician to catch developing complications at their earliest stages when they are easiest to treat. Furthermore, it gives the clinician a chance to redirect the focus of the treatment when a wound is not responding as expected. In any case, regularly reassess stagnant wounds to make sure you have addressed the most fundamental potential difficulties. Blood supply, glucose control, infections, offloading, wound characteristics and adherence with treatment recommendations are all important considerations.

Keys To Improving Angiogenesis And Vascularity

Without adequate blood flow, ischemia and infection will dominate the wound environment. Most formal wound care programs today involve strong collaboration between podiatric physicians and vascular surgeons. However, 20 years ago, there were few vascular surgeons willing to attempt bypasses as far distally as the dorsalis pedis or posterior tibial arteries. Time has demonstrated the value of more distal bypasses and angioplasty, and many more surgeons are able to do this work. Restored blood flow enhances our ability to close many difficult wounds.

   Growth factors such as platelet-derived growth factors (PDGF) also increase angiogenesis at the microscopic level. Following debridement, platelets in the vicinity of the wound will degranulate to release PDGF and other growth factors that stimulate chemotaxis and mitogenesis in addition to angiogenesis. It is important to note that wounds that appear to be progressing will demonstrate increased vascularity within the wound bed in the form of granulation tissue.

   Other authors have reported the benefits of hyperbaric oxygen therapy (HBOT) and wound irrigation with superoxygenated solutions such as Microcyn (Oculus Innovative Sciences) to stimulate increased vascularity of the wound bed.6,7 Alternatively, negative pressure wound therapy (NPWT) has emerged as a tremendous adjunct for improving wound bed vascularity and is frequently used a prelude to skin grafting or delayed primary wound closure. Some studies indicate that fluctuating pressure within the wound bed may lead to increased cellular proliferation.8

   Although the debate continues about whether micro-ischemia and basement membrane thickening play a strong role in the development of ulcers, there is no doubt that angiogenesis and macroscopic blood flow are key elements to wound healing.

What You Should Know About Quorum Sensing

High bacteria loads can inhibit wound healing, particularly when the count rises above 105/mm3.9 Furthermore, the chronic treatment of colonized wounds (i.e. wounds contaminated by bacteria without corresponding evidence of a clinical response) with antibiotics has certainly contributed to the explosion of resistant bacteria such as MRSA and vancomycin resistant Enterococcus (VRE).

   Alternatively, topical treatments have also run into a barrier, literally, with the discovery of biofilms, a tough glycocalyx coating, which protects bacteria from topical agents and maintains their presence within the wound bed.10 Although debridement can penetrate the biofilm, we now believe that the biofilm harbors a much more devastating problem.

   Quorum sensing describes the ability of bacteria to know when they are in the vicinity of other bacteria of the same species.10 Some bacteria, such as E. coli and P. aeruginosa, can secrete signaling inducer molecules that other bacteria detect. These inducers travel by diffusion and will become more concentrated when a lot of bacteria are present. When a sufficient number of inducers reach the bacterial receptor, this activates gene transcription, which leads to a positive feedback loop and coordinated production of more bacteria. In this way, the bacteria can sit in a dormant stage until their number is sufficient to cause a clinically significant infection. Specific inducers such as oligopeptides are widely used by gram-positive bacteria while N-acyl homoserine lactones are more common to gram-negative bacteria.

   The principle of quorum sensing is controversial because it implies an understanding of the benefits and motives by the bacteria. Leading researchers do not question the existence of the activity but rather the justification for it. Many have returned to the term “autoinduction” to remove the implication that bacteria may have cognitive ability.11

   New antibiotics have focused on the disruption of quorum sensing, forcing the bacteria to reproduce more slowly and thereby allowing the host to use natural immunologic responses to detect and destroy bacteria. This diminishes the evolutionary benefits of bacteria with genetic variants that make them resistant to antibiotics. Certainly, the study of quorum sensing and how it helps us to fight bacterial contamination within wounds represents a paradigm shift in the management of DFUs.

Why Less Wound Debridement Appears To Be Better

The benefits of wound debridement have been widely discussed in the past. Debridement results in the removal of necrotic tissue and reduces surface contamination. It eliminates edge effects around the perimeter of the wound by increasing flexibility and reducing shear forces. Although these are all critical aspects of wound debridement, the one that seems to be the most important is the stimulation of bleeding within the wound bed. As described above, bleeding stops with the deposition of platelets, which subsequently degranulate to release growth factors essential for stimulating angiogenesis, mitogenesis and chemotaxis.

   Over the years, surgeons have performed debridement with greater frequency. However, debridement can also be detrimental if it is overutilized.

   During the normal course of wound healing, debridement “converts” chronic DFUs to acute wounds. These acute wounds will then go through a complex series of coordinated events to stimulate healing. During the first two days, the wound goes through an inflammatory phase in which platelets collect and degranulate, releasing growth factors to stimulate angiogenesis, mitogenesis and chemotaxis. This is followed by a cellular proliferation phase, which does not reach its peak until approximately 10 to 12 days after initial debridement. Wounds that undergo debridement after seven days have only reached approximately 20 percent of the maximum proliferation rate. Early debridement prevents the wound from developing the young neodermis, which forms the basis for new skin formation.

   Over the past 19 years, I have had the opportunity to be involved in more than 30 clinical trials involving DFUs. However, only in the last three years have I begun to notice that clinical studies now routinely recommend less debridement of the wound bed but continue to support the debridement of periwound hypertrophic callus.12

   I also have noticed a trend toward wound dressings with non-adherent contact layers since even simple dressing changes with adherent materials will inadvertently damage the delicate neodermis that has formed on the wound surface. Non-adherent dressings and those dressings that regulate moisture will help to reduce destruction of the neodermis and diminish the risk of maceration within the wound bed.

Pertinent Pointers On Split Thickness Skin Grafts And Advanced Biologic Modalities

The purpose of an advanced biologic treatment is to provide adjunctive materials for wounds that fail to progress. Although bioengineering has provided wound care specialists with a variety of options including cloned growth factors, decellularized collagen materials and bioengineered skin substitutes containing fibroblasts and keratinocytes, the field really goes back to the treatment of wounds with skin autografts and allografts.

   Split thickness skin grafts (STSG) have always served as the gold standard in this area because they provide the patient with the most complete material possible. Living split thickness autografts provide a fully developed array of growth factors, extracellular matrices and one’s own cells. Although the cellular component is important, many would argue that it is the extracellular matrix that is the key to STSG success. The extracellular matrix acts as a reservoir for over a dozen types of growth factors and provides the collagen scaffold necessary for supporting cellular migration.

   Although STSGs have clear advantages over other advanced biologics, there are also some significant disadvantages to their use. First and foremost is donor site morbidity. When you take tissue from one site and add it to another, you create a new wound. There are limitations to how much tissue one can capture and where the tissue must come from. In addition, there are cosmetic concerns due to scarring and hyperpigmentation, which frequently occurs at the donor site.

   As an alternative, physicians today have many options. Cryopreserved human allograft STSG (TheraSkin, Soluble Systems) is now readily available. This cadaveric STSG is harvested within hours of death and is frozen in a manner that preserves some of the living cells as well as the fully developed extracellular matrix with growth factors and collagen.13 Concerns about communicable diseases have widely fallen by the wayside as surgeons have implanted literally millions of pieces of banked tissues in patients with no known incidence of a disease or infection transmitted to a tissue recipient in over 10 years.

   Alternatively, clinicians can opt to apply a variety of biologics that offer some unique component of natural skin. Growth factors such as PDGF are now available by prescription and patients can apply them topically on a daily basis. As I noted earlier, decellularized collagen materials are also readily available from a variety of sources and provide a scaffold for the migration of cellular components within the wound bed. Some interesting innovations in this area include the use of fetal bovine tissue, enriched with higher quantities of type III collagen (TEI Biosciences), which may enhance the rapid uptake and incorporation of collagen in the wound bed. Some collagen materials are processed and fractionated, and even paired with silicone sheeting (Integra, Integra Life Sciences) to act as a full wound treatment unit.

   The bioengineered skin substitutes, Apligraf and Dermagraft, have also provided the clinician with a way to apply living cells to wounds. Dermagraft consists of fibroblasts grown on an absorbable mesh substrate while Apligraf consists of fibroblasts and keratinocytes grown on a bovine collagen substrate.

   When one applies these modalities, a large percentage of the cells found in these materials remain alive and release growth factors to the wound bed. Similar to the cryopreserved STSG allograft, the cells eventually die by apoptosis and disassemble to provide additional building blocks to the wound bed.

   In our clinic, we have moved towards the cryopreserved STSG allograft as our standard treatment due to the complete array of growth factors and cellular components that it provides. However, we have had good successes with all of the advanced biologics. Although many consider advanced biologics to be part of the standard of care for wounds that do not show rapid progress towards healing, one must use this technology in the context of a comprehensive approach to all aspects of wound care.

When Should You Consider Surgical Intervention?

Surgical treatment of wounds certainly includes complex skin flaps but usually the focus is on control of mechanical forces, slowing or reversing osseous deformity, and resection of infected tissues. Although many reconstructive procedures are possible, there has to be total agreement between the physician and the patient on the reasonable expectations of success and tolerance for the very long recovery time associated with many reconstructive procedures.

   Furthermore, these procedures often carry a higher level of risk due to the presence of open and dirty wounds at or near the surgical site. New surgical hardware such as retrograde nails, specialized plates, external fixators and techniques for bone growth stimulation have resulted in great successes in many cases when a decade ago, the only option would have been amputation. The area of surgical intervention for prevention and treatment of ulcers remains complex, and cannot be adequately covered here except to say that there are many options now in comparison to a decade ago.

In Conclusion

The treatment of DFUs has evolved to become an entire subspecialty within podiatric medicine and surgery. The fundamentals of biomechanics help us to appreciate the complex mechanical forces that are so integral to the development of foot ulcers. Our knowledge about the complex environment of wounds has become enhanced with time as we consider the impact of infections as well as the roles that growth factors, collagen and other essential building blocks play in getting wounds to close.

   However, experience has become one of the most important tools we have when addressing DFUs. The literature continually evolves to teach us what works and what does not. With time, we gain a better appreciation for what to expect from wounds and this helps us to redirect our energies when wounds do not progress as expected.

   Dr. Landsman is an Assistant Professor of Surgery at Harvard Medical School and the Chief of the Division of Podiatric Surgery at Cambridge Health Alliance. He is the Director of Research at the California School of Podiatric Medicine at Samuel Merritt University. Dr. Landsman is a Fellow of the American College of Foot and Ankle Surgeons.

References
1. Coleman WC, Brand PW, Birke JA. The total contact cast: a therapy for plantar ulceration on insensitive feet. J Am Pod Assoc 1984; 74(11):548-52.
2. Frykberg RG, Zgonis T, Armstrong DG, Driver VR, Giurini JM, Kravitz SR,Landsman AS, Lavery LA, Moore JC, Schuberth JM, Wukich DK, Andersen C, Vanore JV. Diabetic foot disorders. A clinical practice guideline (2006 revision). J Foot Ankle Surg. 2006;45(5Suppl):S1-66.
3. Brem H, Young J, Tomic-Canic M, Isaacs C, Erlich HP. Clinical efficacy and mechanism of bilayered living human skin equivalent (HSE) in treatment of diabetic foot ulcers. Surg Technol Int. 2003; 11:23-31.
4. Landsman A, Taft D, Riemer K. The role of collagen bioscaffolds, foamed collagen and living skin equivalents in wound healing. Clin Pod Med Surg. 2009; 26(4):525-33.
5. Sheehan P, Jones P, Caselli A, Guirini JM, Veves A. Percent change in wound area of diabetic foot ulcers over a four-week period is a robust predictor of complete healing in a 12-week prospective trial. Diabetes Care. 2003; 26(6):1879-82.
6. Roeckl-Wiedmann I, Bennett M, Kranke P. Systematic review of hyperbaric oxygen in the management of chronic wounds. Br J Surg. 2005; 92(1):24-32.
7. Sauer K, Thatcher E, Northey R, Gutierrez AA. Neutral super-oxidised solutions are effective in killing P. aeruginosa biofilms. Biofouling. 2009; 25(1):45-54.
8. Nasca MR, Shih AT, West DP, Martinez WM, Micali G, Landsman AS. Intermittent pressure decreases human keratinocyte proliferation in vitro. Skin Pharm and Physio. 2007; 20(6):305-12.
9. Robson MC. Wound infection. A failure of wound healing caused by an imbalance of bacteria. Surg Clin North Am. 1997; 77(3):637-50.
10. Davis SC, Martinez L, Kirsner R. The diabetic foot: the importance of biofilms and wound bed preparation. Curr Diab Rep. 2006; 6(6):439-45.
11. Sharif DI, Gallon J, Smith CJ, Dudley E. Quorum sensing in Cyanobacteria: N-octanoyl-homoserine lactone release and response, by the epilithic colonial cyanobacterium Gloeothece PCC6909. ISME J. 2008; 2(12):1171-82.
12. Cardinal M, Eisenbud DE, Armstrong DG, Zelen C, Driver V, Attinger C, Phillips T, Harding K. Serial surgical debridement: a retrospective study on clinical outcomes in chronic lower extremity wounds. Wound Repair Regen. 2009;17(3):306-11.
13. Landsman AS, Cook J, Cook E, et al. A retrospective clinical study of 188 consecutive patients to examine the effectiveness of a biologically active cryopreserved human skin allograft (TheraSkin) on the treatment of diabetic foot ulcers and venous leg ulcers. Foot Ankle Spec. 2010 Dec 6. [Epub ahead of print]