Assessing The Impact Of Extracellular Matrices

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Biological extracellular matrices may play a valuable role in reducing potential complications and facilitating improved wound healing. Accordingly, these authors review the literature, discuss key considerations with the use of these products in chronic wounds, and share their insights on the potential of an emerging xenograft.

   Chronic wounds in the lower extremities pose a significant healthcare problem. Complications of foot ulcers are the leading cause of hospitalization and amputation in patients with diabetes mellitus.¹ Researchers have estimated that 50 percent of patients suffering from venous ulcers have an ulcer duration of greater than one year. These ulcers have generally proven to be more difficult to manage and also require more time to heal.^2-4

   A variety of treatment modalities are available for the treatment of these difficult wounds.⁵ Topical dressings, ointments, medications, devices and other external products offer varying benefits depending on the nature of the wound environment. Studies that have been published are predicated on the inhibition and/or modulations of biologic markers that are altered in chronic wounds.⁶ Regardless of the treatment modality one utilizes, creating and optimizing the wound environment by facilitating cellular activity may be of significant benefit in promoting wound closure.⁷

   Biological extracellular matrices (ECMs) may consist of allogenic, xenogenic or chemical constructs that act as temporary matrices that the host tissue eventually replaces. These products provide a structure for cell migration and activity that is absent in many full-thickness defects.

   Clinicians have used a variety of these matrices in wound care settings and some authors have had clinical success with regard to reducing the time to healing and improving the wound healing rate.^8,9

   In a prospective, randomized trial, Marston and colleagues found that Dermagraft (Advanced Biohealing), in comparison to conventional therapy, had significant benefit over a 12-week period in patients with chronic diabetic foot ulcerations greater than six weeks in duration.⁸ [5]

   In a randomized multicenter trial involving over 200 patients, with non-infected, neuropathic diabetic foot ulcers, Veves and co-workers compared Apligraf (Organogenesis) to saline-moistened gauze.⁹ Researchers provided extensive surgical debridement and adequate foot offloading to both groups. The study authors found that 56 percent of patients treated with Apligraf achieved complete wound healing at 12 weeks in comparison to 38 percent of the saline-moistened gauze group.

   In a review of collagen-based materials, Ehrenreich and Ruszczak have identified several important attributes that would ensure the best clinical success of a tissue-engineered biologic wound dressing. Although these authors say there is a lack of universal consensus on the mechanism of action of these products, they cite the delivery of growth factors and extracellular matrix components to the wound as “important” in facilitating healing.¹⁰

   Along these lines, we would like to present our experience and approach to treating these chronic wounds with xenografts (Unite Biomatrix, Pegasus Biologics). Accordingly, we will offer insights on the makeup of chronic wounds and how xenografts contribute to the wound healing process.

Key Considerations With The Chronic Wound Environment

   Regardless of the etiology of wounds, once an ulcer develops and does not proceed to healing, the chronic wound environment takes on its own unique characteristics. These characteristics include excessive proteases, increased cellular senescence and increased bacterial bioburden.¹¹ Researchers have noted that, at a cellular level, a higher level of matrix metalloproteinase (MMP) production by fibroblasts is one of the factors that contributes to delayed healing with aging.¹²

   Similarly, the absence of appropriate vascularization is a problem in most patients with chronic wounds. Cells from ischemic or hypoxic skin have higher levels of MMPs. When you combine this with unstable collagen production, the results are continued skin degradation and subsequent ulcerations.¹³ Researchers have pointed out that some of the non-collagen degrading proteases also impair the anchoring of the remodeling cells entering chronic wounds.¹⁴ Conceptually, a matrix that is resistant to premature degradation can provide a better environment to facilitate healing.

What You Should Know About Tissue Stabilization And Sterilization

   Over the past five decades, several methods of tissue stabilization (“cross-linking”) have been in use to increase tissue longevity and enhance its biological effects.¹⁵ These stabilization methods include chemicals, such as glutaraldehyde, isocyanates, sugars, carbodiimides, or physical means such as heat and/or radiation (e.g. ultraviolet and gamma) in the presence of activators.¹⁶

   In general, chemical and heat cross-linking methods tend to stiffen the tissues as the process entails the elimination of water and the tightening of the collagen strands in the treated matrix. In addition to difficult handling and conforming on the wound surface, this tends to leave these materials as poor substrates for subsequent cellular infiltration and remodeling. Also bear in mind that denaturation of collagen in the cross-linked tissues with heat and radiation is a common side effect, which makes the tissue susceptible to early degradation by serine proteases.¹⁷ In addition, some authors have indicated a propensity for continued cytotoxicity and calcification with glutaraldehyde treated tissues.^18,19
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   Tissue sterilization to ensure that the product is free of any harmful organisms could also alter the quality of tissue and possibly result in side effects based on the method that one uses. Chemical sterilants like ethylene oxide (EtO) or glutaraldehyde are known to elicit a chronic inflammatory response.^18,20 However, radiation, which is a commonly used sterilization method, could cause collagen denaturation and tissue accelerated degradation as noted earlier.¹⁷

   In the past five years, Pegasus Biologics utilized a chemical cross-linking stabilization and sterilization method with a water-soluble carbodiimide (EDC) that degrades naturally, resulting in nontoxic and biocompatible residues. This method produces a sterile, flexible, fully cross-linked ECM.^21,22

   While traditional cross-linking methods tend to produce shorter and stiffer cross-links, Nataraj and co-workers have shown this stabilization method yields elastic cross-links and preserves the structure of the tissue.²² In addition, it provides a sterilization method with the same chemical. This results in a sterile, pliable, conforming tissue that resists early enzyme degradation and without the known side effects, such as chronic inflammation and/or calcification.²²

   The Unite Biomatrix product offers a wound dressing that is indicated for a variety of chronic wounds including diabetic wounds and venous leg ulcers. Derived from equine pericardium, these products go through a decellularization process followed by the stabilization (flexible cross-linking) and sterilization methods with EDC. The final product retains the native structural attributes of collagen.²²

A Guide To Clinical Use Of The Xenograft

   After prepping the lower extremity based on standard operating room procedures, one can perform sharp excisional and hydrosurgical debridement (Versajet®, Smith and Nephew). Debridement facilitates wound bed preparation prior to the application of the xenograft. After the wound attains hemostasis, examine the wound bed to determine whether the patient requires the application of other biological materials.

   Then cover the wound with the xenograft. Assess the periwound status in order to select appropriate retention materials. In the majority of cases, you can use staples. When there is fragile periwound tissue, physicians can apply a tincture of benzoin and subsequently employ Steri-Strips (3M) to secure the product to intact skin.

   Then cover the xenograft with a single layer of Adaptic (Systagenix Wound Management), bolstered gauze and Kerlix. When edema or venous disease is present, or when you deem it necessary, cover the xenograft with a compression wrap. Leave all the dressings intact for three days to one week plus or minus one day.

   At the first postoperative visit (week one for outpatients and day three for in-patients), remove all the dressings and examine the underlying xenograft to determine retention over the wound, drainage or any indications of clinical infection. Remove staples at the one-week postoperative visit. If you are using Steri-Strips, leave them on for an additional week. Change the dressings weekly until the xenograft self-detaches and the underlying wound is closed. Dressing changes after week one consist of a single layer of petrolatum-impregnated gauze covered with Kerlix. Apply compression if the patient has edema or venous disease.

Examining Current Concepts With Extracellular Matrices

   The use of acellular products for the treatment of chronic wounds is a recent development although extensive literature is available on the use of these products (cadaver, porcine and equine). While cadaver skin is readily available for clinical use, human tissue still carries the risk of viral transmission, particularly hepatitis and human immunodeficiency virus (HIV).

   In a recent review, Ehrenreich and Ruszczak described the indication of some commonly used products and discussed the ideal attributes of such a product.¹⁰ Some of the requisite attributes discussed include compatibility, the ability to conform to surface irregularities, resistance to fragmentation, storage, ease of use and handling, microbiological safety and minimizing nursing care. Our experience with Unite Biomatrix is in agreement with the existing research and falls in line with these key attributes.

   The use of the term extracellular matrices implies the introduction of a scaffold that assists with cellular activity, leading to cellular proliferation, cellular modulation, granulation and eventual wound closure. Therefore, the scaffold must remain in the wound bed for a sufficient time to allow these activities to occur and eventually lead to tissue repair. A material that rapidly degrades in the wound bed in a matter of days cannot be expected to be a durable scaffold to promote and facilitate these cellular activities.

   While wounds generally undergo debridement to create a healthy wound bed that is free of necrotic tissue and bacterial biofilm, the chronicity of wounds persists due to several altered physiologies at the cellular level including the increased level of degrading proteases. Hypothetically, a biological dressing that consists primarily of protein could bind harmful proteases, facilitate and promote optimal cell and cytokine activity, and thus expedite wound closure.²³ This hypothesis needs to be supported by well-designed and controlled clinical trials.

   The choice of tissue processing methods plays a significant role in the characteristics and the function of the ECM. Processes that can alter the collagen architecture structure of the scaffold can conceivably result in accelerated degradation of collagen. Alternately, those that leave residues or byproducts can result in an abnormal inflammatory response. Either of these two outcomes can lead to possible failure of the biologic dressing’s function and further delays with wound healing.

   In our clinical experience and based on the science of tissue processing, the method used to stabilize and sterilize Unite Biomatrix preserves the nature of collagen and provides a water soluble, non-toxic product. The product is resistant to proteolytic enzymes, has a durable scaffold and provides an optimal environment for cellular activities that lead to wound closure.

   Currently, the mechanism of action of Unite Biomatrix in assisting with wound closure has not been well defined or studied. Scientific assumptions on its clinical benefit are based on available animal data.^22,24-26 When it is not used as an dressing, the material does not appear to act as a true fully incorporated matrix as it eventually becomes displaced from the wound surface. The exception is with deep cavities where cellular invasion and replacement with host cells and tissue eventually occurs.

In Conclusion

   Our clinical experience with Unite Biomatrix is encouraging and suggests that its use as a temporary biological cover and scaffold may be of significant benefit in the treatment of chronic wounds. Clinical and animal studies investigating the cellular and chemical activity are warranted for a better understanding of the mechanism of action in either the surgical or non-surgical setting on full-thickness wounds.

Dr. Mulder is an Associate Professor of Surgery and Orthopedics at the Department of Surgery/Division of Trauma at the University of California-San Diego (UCSD). He is also the Director of the Wound Treatment and Research Center at UCSD. He is a Fellow of the American Professional Wound Care Association.

Dr. Lee is an Assistant Clinical Professor and the Director of Foot and Ankle Surgery in the Department of Orthopaedic Surgery at the University of California in San Diego. He is a Fellow of the American College of Foot and Ankle Surgeons.

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