In recent years, the rise of bioengineered alternative tissues (BATs) has opened a new avenue toward the management of diabetic foot ulcers. These authors examine the evidence on the efficacy of these advanced wound care products.
The management of chronic diabetic wounds poses a formidable challenge to the healthcare community. In the United States, projections indicate that upward of 30.3 million individuals will be diagnosed with diabetes by 2030. Approximately 15 to 25 percent of these individuals are at risk of developing an ulceration at some point in their lifetime.1,2
With foot ulcers accounting for one-quarter of diabetes-related hospital admissions, preceding 70 to 80 percent of diabetes-related amputations and being associated with a high rate of morbidity and mortality, it stands to reason that heroic efforts must combat this problem.2-5 The potential number of individuals suffering from chronic, recalcitrant wounds becomes even more staggering when factoring in increases in life expectancy and the incidence of comorbid states, the rise in obesity and the likelihood of recurrent ulcerations.
Suffice it to say, this challenge is by no means short lived and treatment of diabetic foot ulcers should continue to be a major public health goal.
Wounds of diabetic neuropathic etiology are difficult to treat even when one follows the fundamentals of good clinical care. Providing a moist wound environment, appropriate offloading of pressure sites, frequent debridement, restoring adequate blood flow to the area and eliminating the risk of infection do not guarantee definitive closure as alterations in the wound’s local biological milieu can impede healing.
The local wound environment and the inflammatory response of a chronic ulceration are much different than that of an acute wound. Aberrations in protease regulation, cytokine release, fibroblast morphology and the composition of the extracellular matrix are all thought to play roles in the impairment of wound healing. Unfortunately, time is of the essence and the longer these ulcerations remain open, the more complicated and costly their treatment becomes and the greater the chance of developing detrimental limb- and life-threatening sequelae.
While numerous advanced wound healing modalities are currently available, practicing clinicians are left wondering what products truly improve mean healing times and what percentage of wounds actually achieve closure in the expected 16 to 20 weeks. New biologic dressings for diabetic neuropathic ulcers seem to be introduced to the market on an almost daily basis. However, not every product has had appropriate evaluation and high-level, evidence-based research supporting or refuting the product’s use may be lacking.
With all these questions looming and clouding physician judgment about appropriate therapeutic selection for their patients, let us take a closer look at various bioengineered alternative tissues and review the published medical literature to determine their efficacy for diabetic foot ulcerations.
The term bioengineered alternative tissue (BAT) is the most current and accurate labeling used to describe these non-autologous skin equivalents. Clinicians have adopted this terminology in recent years as it more clearly defines the composition of the tissue product and the situation(s) most applicable for use. The terms “tissue engineered skin,” “bioengineered skin” and “biological skin equivalents” are misnomers and suggest that the product is an exact replica of human skin comprised of both an epidermis and dermis. In contrast, the name bioengineered alternative tissue implies that the wound product has undergone chemical and biological manufacturing processes to yield a substance that is different than split- and full-thickness skin grafts.
There are growth factors within these engineered tissues that promote granulation and epithelialization, and optimize the wound environment. One can apply bioengineered alternative tissue to ulcerations when serial wound measurements fail to reveal 50 percent improvement in size over a one month duration or when less than 10 to 15 percent wound contraction occurs per week.6
The initial impetus for the development of bioengineered alternative tissues occurred in the 1980s and stemmed from insufficient sources of full-thickness dermal autologous skin grafts available for burn victims and the poor quality of scars after treatment with split-thickness autografts.7 Autologous skin grafts and flaps are highly effective in the treatment of certain wounds. However, the pain and potential complications associated with harvesting the graft are definite drawbacks.
Surgeons and wound care specialists envisioned an “off-the-shelf” product that accelerated tissue regeneration, could be applied in the office, was readily available in large quantities, was not subject to host rejection and was inexpensive. While such an ideal wound product does not currently exist, bioengineered alternative tissues did fill some of the void and provided several of the aforementioned desired clinical characteristics.
Indications for application of these bioengineered alternative tissues have expanded over the past decade. Since many of these alternative tissues rely on allogenic cells from a donor, they are convenient, easily manufactured and close at hand. However, with accessibility, one pays the price for cell persistence as the allogeneic components and cells disappear from the recipient site within two months, making stimulation of healing and production of growth factors only temporary.8
There are three distinct components inherent to the design and construct of bioengineered alternative tissue: the presence of a matrix, a tissue differentiation-inducing substance and a cell source. In order to create the most viable functional product, various combinations of cells, biopolymers and soluble mediators have been subject to testing. Research in this area has shown that replacement of connective tissue may enhance the mechanical strength of the wound and reduce scar tissue formation with subsequent incorporation of fibroblasts into several of the alternative tissues.9
Some bioengineered alternative tissues also contain sheets of biomaterial matrix embedded with allogeneic cells derived from neonatal foreskin. Neonatal foreskin cells have minimal antigenicity, a higher content of keratinocyte stem cells, vigorous cell growth and metabolic activity, and are a relatively convenient and logical source.10
In an article regarding the biological background of dermal substitutes, van der Veen and co-workers described a series of general principles required for adequate function of these advanced wound care technologies.11 Bioengineered alternative tissues need to incorporate various mechanical and physical properties in order for them to meet clinical demands and promote healing. The first of these principles is protection of the wound from fluid loss and infection.
The second characteristic that bioengineered alternative tissues must possess is the ability to remain immunologically inert but structurally intact until the proliferation and migration phases of wound healing are complete. If the product prematurely degrades, angiogenesis and synthesis of neodermal tissue will not occur. Eventual biodegradation of the dermal substitute is required but should take place at the appropriate time.
Enabling cell influx or providing a cell source within the product itself is the third quality. Fibroblast and endothelial cell migration onto the three-dimensional scaffold depend upon pore size and composition of the tissue product. While embedding fibroblasts into the matrix seems like an easy fix, one must take into account concerns about disease transmission and costs.
Lastly, the bioengineered alternative tissue must be resilient and resist shear forces. Ease of handling and application are key factors, especially when these chronic wound sites are located over a joint or on a weightbearing surface.
According to Kim and colleagues, the vast number of bioengineered alternative tissues can be consolidated and understood more simply by dividing them into two broad categories: “living tissue” or “bioactive adjuncts.”12
Those that fall into the living tissue group derive from living cell cultures in which fibroblasts and keratinocytes are implanted into the construct. These products either stimulate the release of growth factors or deliver them directly to the wound when one applies these products. Wounds of a superficial nature tend to respond more favorably to “living tissue” bioengineered alternative tissues as they contain the ingredients needed to replace the currently absent dermis and epidermis.
Tissue substitutes that do not contain living cells are considered members of the bioactive adjunct group and function as a scaffold that supports cellular ingrowth. These products are generally reserved for deeper wounds in which the hypodermis and underlying tissue are compromised in addition to the superficial skin layers. Since the base of the ulceration needs to be built up, a supportive scaffold is in order and the bioactive bioengineered alternative tissue ultimately provides this.
Both categories of bioengineered alternative tissues serve a particular role in wound care management as the depth and the etiology of the ulceration should complement the product design and components. Keep in mind that improper selection and utilization of a bioengineered alternative tissue will result in a lesser degree of predicted success.
While a sizable number of biologics have been released with varied indications, let us take a closer look at the efficacy of the more commonly utilized bioengineered alternative tissues. These are Oasis (Healthpoint Biotherapeutics), Graftjacket (Wright Medical and KCI), Apligraf (Organogenesis) and Dermagraft (Advanced BioHealing). Apligraf and Dermagraft are approved by the Food and Drug Administration (FDA) for diabetic foot ulcers.
Oasis is a porcine xenograft derived from the small intestinal submucosa of swine. It falls under the bioengineered alternative tissue category of “bioactive adjuncts” because this product has components of the dermal extracellular matrix like collagen, elastin, proteoglycans, glycoproteins, glycosaminoglycans and growth factors although no living cells are present. Once harvested, Oasis soaks in antibiotics and bleach, and subsequently gets irradiated to ensure sterility while maintaining the structure of the extracellular molecules. The matrix architecture serves as a scaffold for cell proliferation and adherence. It also helps to alleviate pain, protects vital structures and allows for partial closure and granulation.
Advantages of this product are a long shelf life, immediate availability, ease of application, a relatively low cost in comparison to other bioengineered alternative tissues and similar composition and organization to the native dermis. Some drawbacks include risk of rejection secondary to retained cell remnants and the temporary nature of the product as it will incorporate within the wound or desiccate by three weeks.
With the advent of Oasis, there were questions regarding the persistence and efficacy of growth factors when preserved within a complex matrix for a prolonged period of time. In 2005, Hodde and co-workers investigated the long-term bioactivity of endogenous fibroblast growth factor (FGF-2) in the Oasis wound matrix.13 They found that the product retained its bioactivity when the unstable growth factors were bound within their natural extracellular matrix. Measurement with enzyme-linked immunosorbent assay revealed that the FGF-2 content ranged from 15.3 ng/g to 84.3 ng/g and caused differentiation of cells in culture.
Mostow and colleagues evaluated the efficacy of Oasis for full-thickness chronic venous leg ulcerations.14 In a randomized clinical trial involving 120 patients, they noted that 55 percent experienced complete wound closure with Oasis and compression dressings while only 34 percent healed with compression therapy alone. Additionally, none of the patients treated with Oasis developed a recurrent ulcer by six months.
In a prospective, randomized, controlled multicenter trial, Niezgoda and colleagues compared 12-week healing rates of diabetic ulcers that received either Oasis or becaplermin (Regranex Gel, Healthpoint Biotherapeutics).15 The incidence of complete healing was 49 percent (18/37) for the Oasis group and 28 percent (10/36) for the becaplermin group. Although the difference was not statistically significant due to a small sample size, the study did show that Oasis wound matrix was as effective as Regranex in healing full-thickness diabetic foot ulcers.
Another “bioactive adjunct” is Graftjacket, a bioengineered alternative tissue manufactured from allogenic cadaveric skin that one can apply to both superficial and deep wounds in which tendon and bone are exposed. Like Oasis, this product is acellular and devoid of an epidermis. Processed via glycerol sterilization, Graftjacket retains the natural basket weave pattern of the dermis and contains components of the native papillary side of the basement membrane. These features are believed to promote fibroblast infiltration, ultimately enabling these cells to deposit neodermis in proper orientation. Inherent to the basement membrane are laminin and collagen IV that also aid with wound healing by improving keratinocyte adherence and differentiation.16 Cryogenically stored, this product has an extended shelf life of two years.
As Brigido reported in his 2006 randomized, controlled, prospective study involving 28 patients with chronic diabetic ulcers, one application may result in a significant reduction in wound surface area and depth in comparison to wound gel.17
Winters and colleagues retrospectively evaluated the time to complete healing and the percent resolution of chronic full-thickness diabetic ulcerations that received Graftjacket.12 In 75 people (100 wounds) with comorbidities ranging from soft tissue and bone infection to cardiac disease, researchers found the mean time to heal to be 13.8 weeks and noted that 91 percent of patients went on to full epithelialization. The study authors concluded that this acellular dermal matrix is a viable treatment option for a wide array of diabetic wounds.
Apligraf (graftskin) differs from the two previously described tissue substitutes in that it contains viable allogeneic cells, thereby making it a “living tissue” bioengineered alternative tissue. This biologic consists of two layers that resemble epidermis and dermis. The keratinocytes and fibroblasts that are embedded within the bovine Type 1 collagen of this product originate from neonatal foreskin. By releasing various growth factors and proteins, these cells create a cocktail of cytokines and chemokines that replenishes the deficiencies of the wound and promotes healing.
Initially approved in 1998 by the FDA for chronic venous leg ulcerations, Apligraf received another FDA approval two years later for neuropathic diabetic wounds of three weeks’ or greater duration. Although it has a maximum shelf life of only 10 days and is one of the more expensive bioengineered alternative tissues, Apligraf provides adherence that results in a good cosmetic appearance that is similar to autograft incorporation.18 Complete epithelialization of the ulcer may require multiple weekly applications. However, the overall cost when utilizing Apligraf in comparison to standard wound care practice is often much less due to the fact that the number of office visits and treatment duration is typically fewer.19
A robust and comprehensive study performed by Veves and colleagues evaluated 208 patients with diabetic ulcerations.20 Researchers randomized the patients into either the control group (wet-to-dry dressing) or the graftskin group, and provided treatment for 12 weeks. A maximum of five weekly applications of graftskin was allowed and researchers assessed complete wound healing. The investigators found that 56 percent of graftskin patients and 38 percent of control group patients went on to closure. They determined that the odds ratio for complete healing was 2.14 in comparison to a number for the control group in favor of the bioengineered alternative tissue product. Additionally, the median time to epithelialization was 65 days for graftskin and 90 days for the control group, supporting the view that Apligraf results in a higher healing rate and may be an appropriate adjunctive treatment option for recalcitrant wounds.
Approved by the FDA in 2001 for full-thickness diabetic ulcers of greater than six weeks’ duration, Dermagraft also belongs in the “living tissue” category. This bioengineered alternative tissue has no epidermal component and is for use on superficial wounds as one cannot apply it over tendon, bone, muscle or capsule. It is composed of neonatal foreskin fibroblasts cultured on a bioabsorbable polyglactin mesh. These cells secrete matrix proteins and growth factors that ultimately create a three-dimensional scaffold, which stimulates ingrowth of fibrovascular tissue and epithelialization.
Since Dermagraft is cryopreserved and stored at -75ºC, advantages of this tissue substitute include ease of handling, resistance to tearing and, in most instances, a lack of a foreign body immune response. Some disadvantages are the need for multiple applications, a higher price, the presence of bovine proteins in the medium and a storage solution that could result in hypersensitivity reactions.
In 2003, Marston and colleagues published an article detailing their findings on the efficacy and safety of Dermagraft in comparison to conventional wound therapy.21 Their level 1 therapeutic study evaluated the healing rate of 314 patients suffering with chronic diabetic ulcerations over a 12-week period. The results indicated that Dermagraft is an effective agent for treating recalcitrant diabetic wounds as 30 percent of Dermagraft patients went on to closure versus the 18.3 percent of control patients. Adverse events were similar for both groups, supporting the belief that this bioengineered alternative tissue is safe for use.
The issue of cell binding is unique to products like Dermagraft that contain polymers or non-biological molecules. Although the synthetic polyglactin mesh offers locations for fibroblasts to adhere, the cells need to be told where to migrate. Chemotactic signals direct this activity as well as other cell functions. Since the interaction of fibroblasts with synthetic materials is different than what occurs with native extracellular matrix, manufacturers of these tissue substitutes must integrate protein recognition sequences into the matrices in order to facilitate cell-matrix contact.22 Advances in tissue engineering such as this make it seem as if there is no limit to future technologies and product creations that will solve the ever present problem of chronic non-healing wounds.
While there is a gamut of treatment options currently available for recalcitrant diabetic neuropathic ulcerations, physicians must employ appropriate discretion when selecting a modality. Knowledge of product characteristics and limitations, wound type and depth, and associated patient comorbidities will allow one to make an educated decision, which will result in a favorable outcome.
Bioengineered alternative tissues can play a role in the healing of many chronic diabetic ulcers. However, achieving successful incorporation of these products relies heavily on proper wound preparation and a stable wound environment. Clinicians should not forget this concept.
The preponderance of research presented in this article supports the effectiveness of bioengineered alternative tissues yet these products are not the panacea as complete wound closure occurred in about 30 to 56 percent of patients, a far cry from 100 percent ulcer epithelialization and resolution. Perhaps the key to improving the effectiveness of bioengineered alternative tissues lies in recognizing when standard treatment modalities are just not cutting it. Earlier identification of these non-healing ulcerations may lead to quicker implementation and initiation of bioengineered tissues that could ultimately reduce the time to heal.
It goes without saying that further research is needed to gain a more detailed understanding of the biochemical mechanisms at work within these advanced wound care products. If we can unmask the intricacies of these cellular processes, we can modify products to enhance healing and prevent recurrence.
Dr. Skratsky is an Assistant Professor in the Department of Podiatric Medicine and Radiology at Dr. William M. Scholl College of Podiatric Medicine at Rosalind Franklin University in Chicago.
Dr. Wu is the Director of the Center for Lower Extremity Ambulatory (CLEAR) at the Dr. William M. Scholl College of Podiatric Medicine at Rosalind Franklin University of Medicine and Science in Chicago. She is an Associate Professor for the Center for Stem Cell and Regenerative Medicine at the School of Graduate and Postdoctoral Studies at the Rosalind Franklin University of Medicine and Science. She is also an Associate Professor in the Surgery Department and the Associate Dean of Research at the aforementioned Scholl College of Podiatric Medicine.
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Editor’s note: For further reading, see “A Closer Look At Bioengineered Alternative Tissues” in the July 2006 issue of Podiatry Today, “Essential Insights On Using Skin Substitutes” in the November 2010 issue, “Exploring The Potential Of Bioengineered Alternative Tissues in the September 2006 issue or “When Wounds Stall: Key Considerations To Jump-Start Healing” in the September 2009 issue.