Treatment of Deep Full-Thickness Wounds Containing Exposed Muscle, Tendon, and/or Bone Using a Bioactive Human Skin Allograft

A Large Cohort Case Series

Michael S. Flood, MD, FACS; Blake Weeks, DPM; Kenneth O. Anaeme, MD; Heather Aguirre, DO, MS; Kimberlee B. Hobizal, DPM, MHA; Sandi E. Jiongco, MSN; Robert J. Klein, DPM; Andrew Lemoi, DPM; Rafael Rafols, MD; Adam S. Landsman, DPM, PhD

Disclosures

Wounds. 2020;32(6):164-173. 

In This Article

Abstract and Introduction

Abstract

Background: Deep wounds with exposed muscle, tendon, and/or bone structures are especially difficult to treat, often requiring a multifaceted approach. Bioactive human skin allograft (BSA) has been proven to be effective in the treatment of deep wounds, but the mechanism of action and clinical use in the real-world setting is not as well known.

Objective: The aim of this case series is to study deep wounds treated with BSA to better understand how it is used in real-world patients and discuss its mechanism of action.

Materials and Methods: A total of 51 deep wounds of various etiologies and locations were included from 10 sites across the United States. To be included, patients must have failed wound care without BSA for at least 30 days, with more than 50% reduction in size prior to BSA application.

Results: The mean wound area was 50.37 cm2 and average wound duration was 3.67 months. The mean time to closure was 15.33 weeks, achieved with an average of 4.24 BSA applications. Many patients received adjunctive therapies either prior to or in combination with BSA.

Conclusions: This study demonstrates the effectiveness of BSA in the treatment of deep wounds of various etiologies. The authors provide clinical information on using BSA either alone or in conjunction with other advanced modalities and offer insight into the hypothesized mechanism of action in which these grafts become incorporated. Ultimately, this information can guide best practices in the treatment of full-thickness wounds to improve outcomes.

Introduction

A full-thickness wound, or deep wound, is defined as a loss of continuity of the skin and is associated with tissue loss of the epidermis and dermis.[1] When full-thickness wounds extend past the subdermal layers of the skin and involve exposed underlying structures such as tendons, muscle, and bone, they are sometimes referred to as cavity wounds.[1] The challenge with these full-thickness wounds is that the exposed structures are frequently dysvascular and a great number of obstacles must be overcome in order to achieve wound closure. Wound closure must occur in layers, with deep tissue reconstruction mimicking the normal morphology found beneath the dermis.[2] Healing in these wounds can be further obstructed by deep infections, an abundance of synovial fluid exudate, tissue maceration, and disruption of delicate neodermis from muscle, tendon, joint, and bone motion compounding this process. Diminished vascular supply in the wound bed and adjacent tissues certainly complicates matters further. Deep cavity wounds in patients with diabetes are of particular importance because they are associated with higher amputation risk.[3]

Closure of deep wounds is achieved with a multifaceted approach that includes moist (but not macerated) wound care, debridement, edema control, offloading, immobilization, optimization of blood flow, reduction of bioburden, and appropriate medical management.[4] Simply providing the raw materials to facilitate healing of these complex wounds is often insufficient, and many patients will require primary closure through flap reconstruction and potentially lengthy microsurgery. Unfortunately, flap reconstruction and microsurgery are typically implausible in this patient population that is highly complex and typically has innumerable comorbidities.[5] Given the morbidity, mortality, and costs associated with these wounds, it is critical to find alternative advanced modalities that encourage rapid granulation over the exposed structures and achieve wound closure when good wound care alone is not enough.

A commonly used advanced treatment modality for deep cavity wounds is negative pressure wound therapy (NPWT) in which a reticulated foam dressing is applied to the wound bed and connected to a vacuum pump. Often, NPWT alone is not enough to stimulate granulation over exposed deep structures, and a tissue scaffold that can support the migration of tissues over the deep, poorly vascularized, and otherwise hostile environment of exposed tendon or bone is needed.[6] Clinically, these wounds appear to have excellent granulation tissue to either side of the exposed tissue, but with no bridging over the exposed structure (Figure 1).[6]

Figure 1.

Granulation tissue requires a bridge to carry it over the exposed structure. (A) After 1 month of treatment with negative pressure wound therapy, significant granulation tissue built up to either side of the exposed tendon, but did not bridge the structure; (B) bioactive human skin allograft was applied; (C) at 4 days following application, there were areas where the graft turned pink, representing areas of vascularization and bridging over the exposed tendon; and (D) wound went on to become epithelialized.

Options for the treatment of these complex wounds using biologics, xenografts, synthetics, or bioengineered tissues are limited, because the majority of these materials and medical devices have never been studied under these conditions or are not cleared by the United States Food and Drug Administration for use over exposed structures.[7] In addition, many of the aforementioned tissues have differing properties (eg, variations in scaffold thickness or absence of a scaffold altogether) that limit their ability to rapidly vascularize and granulate over the exposed structure.[7]

This study focused on a bioactive human split-thickness skin allograft (BSA; TheraSkin; Misonix), which is cleared for the treatment of deep, full-thickness, and cavity wounds, and has been shown to be effective in previous studies for these types of wounds.[7–10] A BSA is a minimally manipulated human skin allograft that has been uniquely cryopreserved to maintain cellular viability.[11] The BSA is a donated human tissue, which meets similar strict criteria to any other donated human organ.[7] Following a careful screening of the donors to determine they meet the quality and safety criteria, the skin is procured from organ donors in less than 24 hours postmortem.[7] After cleansing and cryopreservation, it is ready for implantation on a recipient wound. Previously, it has been shown[11] that about 70% of the living cells are viable at the time of implantation, and produce cytokines, growth factors, and collagen typically found in healing skin. Thus, BSA is considered bioactive because it contains the elements necessary for stimulating the healing process, and it preserves the architectural structure of the human extracellular matrix (ECM).[11] This is an important distinction to make when comparing the science of various skin substitutes whose purpose it is to replicate the properties of native human skin. The common commercially available skin substitutes can be described as striving to provide the wound with some, or many of the components found in human skin, without achieving actual parity.[11]

In 3 real-world studies,[7,8,10] BSA was shown to be clinically effective in healing wounds involving exposed deep structures, especially in large wounds of long duration in a medically complex patient population. Of the 3 studies, 2 were large, well-conducted matched cohort studies.[7,8] The studies[7,8] included thousands of patients who reported higher healing rates with BSA in full-thickness wounds when compared with the standard of care (SOC) group, but they also found that wound recidivism rates were significantly reduced compared with SOC alone. Both matched cohort studies[7,8] used rigorous methodologies to create identically matched groups from data drawn from several hundred sites that used the same evidence-based clinical practice. One study[8] involving close to 4000 patients with wounds below the knee, reported fewer lower extremity amputations in BSA-treated patients as compared with SOC alone, including a large number of wounds with exposed structures. Similarly, in the other propensity matched cohort study[7] looking at nearly 1600 diabetic wounds, the authors observed significant improvement in healing rates and less recidivism in cavity wounds treated with BSA versus SOC alone. Both matched cohort studies[7,8] accounted for the concomitant use of hyperbaric oxygen therapy (HBOT) and NPWT and found no impact on the outcomes reported, based, in part, on the small number of patients who received both therapies. A third study by Wilson et al[10] consisted of a case series of 15 diabetic foot ulcers treated with BSA. In their study,[10] they reported an average of 5 weeks to achieve granulation over exposed bone or tendon and a total of 2 graft applications to achieve wound closure.

All wounds, especially complex full-thickness wounds, must first heal vertically (ie, granulate to fill in the defect) before they can heal horizontally (ie, close through epithelialization). Deep wounds treated with BSA heal vertically though granulation and angiogenesis (via vascularization of the ECM scaffold which occurs within 72 hours after application) followed by epithelialization aided by the presence of living cells, relevant growth factors, and an intact basement membrane.[12] The process of BSA incorporation is very similar to split-thickness skin autografts; and so, in order to appreciate the benefit of BSA, one must first explore the similarities of BSA to autologous split-thickness skin grafts (STSGs).

Graft Incorporation

When an autologous STSG is placed on a properly prepared wound bed, imbibition takes place during the first 24 hours.[13] During this phase, the STSG "imbibes" plasmatic fluid carrying oxygen and nutrients into the graft and onto the wound surface through an osmotic gradient.[13] A fibrin seal is formed between the graft and wound surface to ensure good contact and prevent shearing.[13] During the next 24 hours (ie, 48 hours after application of the graft), inosculation occurs, whereby the capillary network from the wound bed and the capillary network from the graft "kiss" and share nutrients in preparation for the final phase of graft incorporation, known as revascularization. Revascularization occurs about 72 hours after initial application of the graft when blood flows through the implanted capillary network. Once revascularization has occurred, the graft will eventually become fully incorporated.

This entire process of autograft incorporation is not only dependent on the existence of living cells that produce the various growth factors that stimulate the process, but also a considerable supply of type 1 and type 3 collagen that encompass the established capillary network of the ECM.[14] This collection of materials will support the exchange of fluids, cytokines, and growth factors between the recipient site and the applied graft.[14] Ideally, the skin autograft will become fully incorporated and integrated into the wound site. When applied to a properly prepared wound bed, BSA and an autologous STSG will vascularize in the same way due to the existence of a fully developed capillary network.[15]

Tendon and Bone. Historically, it has been very difficult to get split-thickness skin autografts to adhere and grow over exposed tendon and bone because the tendon and bone are significantly less vascularized than soft tissues, such as skin or muscle.[6] In fact, in many cases, as tendons tend to dry out, they are often removed to facilitate wound coverage, leading to loss of function. Tendons easily glide within their sheaths and are coated with slippery synovial fluid that collectively disrupts the attachment process, even with small amounts of movement. The sheaths that surround the tendons are also relatively dysvascular.

Similarly, exposed bone is also a poor support structure for overlying soft tissues because bone takes about a third of its nutrition from the periosteum. This tough, membranous material is poorly vascularized compared with adjacent soft tissues and can act as a barrier to soft tissue attachment to the bone. During clinical treatment, the periosteum does not exist or is in poor condition, and the surface of the bone is often debrided just prior to application of the graft, to stimulate bleeding and facilitate the vascularization (imbibition, inosculation, and revascularization) process and secure the graft.[16]

Muscle. Muscle is a well-vascularized tissue that can often be surgically manipulated to cover a site of exposed bone. Once positioned, it can be grafted over, or the tissues may be allowed to granulate in over its surface. The movement of muscle tissue may be disruptive to the graft application process,[17] so immobilization is often necessary, in order to improve the chances of graft attachment.

Bioburden and Necrosis. Bioburden, including bacteria and biofilms, and necrosis result in enzymatic digestion of tissues on the wound bed. Matrix metalloproteases (MMPs) are desirable in small quantities as they can help to stimulate angiogenesis within the graft and at the graft interface.[18] However, in chronic wounds, MMP levels can become high enough that they actually digest the wound bed, which prevents healing and increases the size of the wound.[18] They also are elevated in the presence of inflammation.[18] Similarly, necrotic tissue within the wound bed attracts bacteria and increases drainage as the grafted tissue is broken down, additionally causing local tissue hypoxia.[19] Skin autografts can be very helpful in the presence of MMPs because the human collagen matrix is a competitive inhibitor of MMPs and can help to reduce inflammation and wound bed digestion; unfortunately, the autograft is often destroyed in the process.[18]

Autografts Versus BSA

There are clearly benefits to autografts, which can be summed up in the availability of living cells along with collagen, cytokines, growth factors, and a fully developed capillary network within the ECM.[20] Not only do these materials facilitate the growth of skin to achieve wound closure, but with an autograft, the expectation is for the graft to take permanently by incorporating native cells into the wound bed. In the case of deeper wounds, however, autografts are less effective because vertical healing and granulation over the exposed structure are needed first. The additional disadvantages of a skin autograft are typically focused on donor site morbidity, limited donor sites, and the questionable value of harvesting skin from patients who are elderly or diseased to treat themselves with their own skin.[20]

A cryopreserved living skin allograft (eg, BSA) provides the next-best alternative to autografts, because it contains all of the components of human skin, including large quantities of signaling molecules (cytokines and growth factors), living cells, and the fully developed capillary network within the ECM.[13] Although implanted living cells are allogenic and do not incorporate into the wound bed, the other materials delivered to the wound site (ie, cytokines, growth factors, collagen) are not immunogenic and function in an identical fashion to those delivered by an autograft.[15] Furthermore, the implanted living cells are not recognized by the recipient upon contact but rather take 7 to 10 days before sloughing, leaving behind all of the materials they manufactured from the time of application.[15]

When comparing autologous STSG to BSA for deep full-thickness wounds, the greatest asset of BSA is that there is no donor site morbidity, and BSA can be a readily available and recurrent source for the necessary collagen, cells, and growth factors. For deep wounds where vertical healing and timely granulation over the exposed structure are imperative, allografts are readily available if they do not incorporate on first application. On the contrary, there are significant benefits in using the collagen portion of the allograft as a competitive inhibitor of enzymatic degradation from MMPs, which thereby reduces inflammation and digestion of the wound bed while acting as a scaffold for new vascular in-growth.[18] In many cases, the BSA is used to prepare the wound bed and serve as a bridge to an STSG autograft, especially in cases in which there are profound tissue defects that must be filled in prior to autografting.[21]

The objective of this research was to study a series of 51 unique cases of deep full-thickness wounds with exposed muscle, tendon, and/or bone that were treated with BSA in order to understand the role of BSA to achieve healing where good wound care alone failed. The authors propose a mechanism of action to help understand why BSA is effective in healing these wounds and hope the findings of this large case series will be informative for clinicians caring for complex wounds in which the speed to granulation over the exposed structure and healing is critical. Lastly, the authors hope this case series will stimulate some interest in using BSA as a cost-effective alternative to autografts and other biologically active materials in the treatment of deep wounds in a complex environment.

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