SKIN SUBSTITUTES IN RECONSTRUCTION SURGERY: THE PRESENT AND FUTURE PERSPECTIVES

^INTRODUCTION

The progress in critical and intensive care in the field of burn management in the 21^st century has significantly reduced mortality in patients with critical burn injuries. This progress has shifted the focus of burn care from simple survival to the quality of life after the burn trauma, in particular to healing of defects caused by full-thickness burns, subsequent maturation, characteristics and appearance of the scars. The application of autologous skin grafts in case of skin loss injuries is a well-established method of defect healing, it is however not entirely satisfactory where the subsequent scar properties are concerned. A simple application of skin grafts cannot fully replace the lost skin. The aim of the development of skin substitutes is substitution of the dermis responsible for mechanical elasticity and pliability. The benefits of application of skin substitutes include elimination of excessive scarring, hypertrophic and keloid scars formation and resulting contractures, representing up to 67 % of morbidity in surviving patients after a critical injury¹. Formation of aesthetically and functionally inferior scars leads to functional as well as psychological and social consequences limiting the patient’s quality of life after a burn injury.

Skin substitutes are three-dimensional biomatrices designed to mimic the dermis. In principle, they are scaffolds demonstrably improving and directing the healing of acute defects caused by full-thickness burns, they also optimise the properties of the resulting scar including its flexibility and elasticity^2,3,4.

^{EVALUATION OF THE TOPIC}

Dermal (or skin) substitutes are designed to mimic the properties of the extracellular matrix, allowing the formation of neodermis through gradual cellularization. Subsequently, it assumes the role of the skin in restoration of anatomical and physiological functions. The properties of the dermal matrix reduce the duration of the defect healing, contractility, elasticity of the resulting scar, as well as the formation of hypertrophic and keloid scars⁵. Biocompatibility of the dermal scaffold depends on its acceptance by and integration into the surrounding tissues, immunocompatibility, and biodegradation. The biocompatibility clinically demonstrates as capillary growth into the scaffold and subsequent fibroblast infiltration and collagen production resulting in the formation of neodermis⁶. The proliferation of capillaries into the scaffold depends, among others, on its pore size. Although the scaffold is termed “permanent”, the resulting structure of the neodermis is in effect determined by the invading cellular colonies of fibroblasts and collagen. A controlled gradual biodegradation of the dermal scaffold resulting in formation of non-toxic low-molecular weight metabolites without any inflammatory reaction or foreign body reaction is desirable⁷. The susceptibility of the dermal scaffold to vascularisation is its crucial characteristic, given both by the aforementioned pore size and by the used material. The speed of vascularisation determines also the speed of the subsequent application of the autologous dermal-epidermal graft on the new epidermis, which can be either transplanted at a later time (e.g. Integra^® skin substitute), or immediately with the application of the scaffold (such as in the Matriderm^® dermal substitute)^7,8. Biopolymers, mainly collagen, are most commonly used for production of skin substitutes. Besides collagen, hyaluronic acid, polylactones, elastin, chondroitin-sulphate, etc. can be used for this purpose. The final resistance of the formed neoepidermis to friction is another important characteristic of dermal substitutes, as are the financial affordability and simplicity of storage of the matrix prior to application⁹.

Acellular dermal allografts are produced by de-epithelisation of cadaverous skin. The skin is subject to processes such as decellularization or removal of the infectious and antigenic elements¹⁰. The result is a freeze-dried matrix that can be simply stored for many months. One of the most widely used acellular dermal matrices is Alloderm^® (LifeCell, Branchburg, N.J.). Alloderm^® is used as a dermal substitute in both partial and full thickness burns and allows subsequent application of dermo-epidermal graft¹¹. Alloderm^® was successfully used also in other applications, such as the replacement of soft tissues in prosthesis covering, lip augmentation, covering of defects of abdominal wall or vaginal prolapse^12,13. DermaMatrix^® (Synthes, West Chester, Pa.) offers a possible alternative to Alloderm^®. In a comparative study on an animal model, DermaMatrix^® preserved its original structure and consistency three months after implantation while Alloderm^® showed marks of structural deterioration. 12 months after implantation, only a thin layer was found after DermaMatrix^® application while a thick-walled dense capsule surrounded by inflammatory reaction was observed after Alloderm^® use. Although the results of this animal study favoured DermaMatrix^®, no firm conclusions can be drawn as clinical data are not available^14,15.

Acellular xenodermal matrices contain cross-linked bonds, which makes them slightly less favourable than allografts in clinical practice. Products from this group include porcine skin derivatives Permacol^® (Tissue Science Laboratories, Hampshire, UK), and EZ-Derm^® (Mölnlycke Health Care AB, Gothenburg, Sweden). The clinical use of Permacol^® has been already abandoned, the results of EZ-Derm^® use are, according to the available studies, not convincing^15,16,17.

Xenodermal matrices without chemically induced cross-linked bonds can be used also for soft tissue defects of other origin than burn injury¹⁸. The OASIS Wound matrix^® (Healthpoint, Fort Worth, Tex.) is a dermal matrix derived from the submucosal part of the porcine intestine and consists predominantly of glycosaminoglycans, collagen, fibronectin and growth factors (FGF2, TGF2). One of the biggest advantages of this matrix is its storage convenience (up to 
2 years at room temperature), which facilitates its immediate use¹⁹. The principal application of this matrix is treatment of ulcerations. A randomised controlled trial with 120 patients proved a higher percentage of treatment of lower limb venous defects using compression therapy combined with OASIS Wound matrix^® compared to compression therapy alone²⁰. In patients with lower limb venous and arterial defects, a complete closure of the defect was achieved in 82% of patients when using OASIS treatment, compared with 46% when using Hyaloskin^® preparation treatment (Apeldoorn, The Netherlands) based purely on hyaluronic acid²¹. Treatment benefits included lower pain and better patient comfort during treatment. Good results were achieved even in diabetic patients where 49% of defects healed after 12 weeks, compared with a 29% success rate of the Regnarex^® gel with platelet derivative (Johnson & Johnson Wound Management, Somerville, USA)²².

Derivatives based on human amniotic membrane should also be mentioned here. An example of such product is Neox^® (Amniox Medical, Marietta, Ga.), primarily containing collagen and fibronectin without chemically induced cross-links. Its preferred use is in the treatment of thermal injuries where it prevents bacterial contamination and wound infection, as well as desiccation of the defect. Patches containing human amniotic membrane should be changed every two days if possible^23,24.

Synthetic acellular dermal substitutes consist of natural or synthetic polymers or of their combinations. Natural polymers include e.g. collagen, elastin, glycosaminoglycans, chitosan, fibrin, or silk^25,26,27.

A major advantage of natural polymers is their low antigenic capacity and in the fact that they do not provoke major inflammatory reactions²⁵. A disadvantage is, however, in their low biostability and low mechanical durability, contributing to scar contraction. To improve the mechanical stability, cross-links are chemically induced, both within the natural polymers and between natural and synthetic polymers²⁵. Natural cross-linked polymers are successfully used in clinical applications where increased strength and durability are required, such as tendon substitutes, hernia reconstruction, or in fillings where material durability and biocompatibility are preferred to cellularization. They are therefore less suitable for wound and defect healing²⁸.

Examples of absorbable synthetic polymers include polycaprolactone, polylactic acid, polyglycolic acid, lactic/glycolic acid polymer (PLGA), poly(ethylene glycol)/poly(butylene terephthalate), and polyethylene glycol. To name but a few examples of non-absorbable synthetic polymers, we can mention polyurethanes, nylon, or polytetrafluoroethylene (PTFE)^25,26. Synthetic polymers can be mass-produced by available technologies as well as customised and tailor suited for achieving required properties, in particular where enhanced mechanical properties are required. Their lower biocompatibility however represents a drawback. Synthetic polymers are being used in suture and mesh materials such as nylon (Ethilon^®, Ethicon, Edinburgh, UK), PLGA (Vycril^®, Ethicon), polyglycolic acid (Dexon^®Davis & Geck), or polycaprolactone (Monocryl^®, Ethicon). They are also often used as wound dressing, such as polyurethanes Tegaderm^® (3M Healthcare, St Paul, Minn., USA) and Opsite^® (Smith and Nephew Healthcare, London, UK)^29,30,31.

Some dermal substitutes have a detachable semipermeable upper silicone layer preventing the desiccation of the wound as well as excessive permeability of the base layer and infection of the wound. Such skin matrices are called acellular bilaminate skin substitutes. Integra^® (Integra Life Science Corporation, Plainsboro, NJ, USA) is one of such substitutes^{31, 32}.

^{INTEGRA vs. MATRIDERM – SINGLE STEP OR TWO STEP STRATEGY}

Integra^® (Integra Life Science Corporation, Plainsboro, NJ, USA) is currently the most accessible skin replacement and is the most commonly used in reconstruction of full-thickness or partial thickness burns³³. Integra^® is also used to support healing of chronic skin defects and acute traumatic defects with exposed bone. Integra was first introduced in 1981 ³² and approved by FDA for use in burn trauma and contracting scars. Integra^® contains bovine type I collagen and shark glycosaminoglycans, chemically cross-linked with glutaraldehyde³⁴. The dermal matrix of Integra^® is porous and the pore size ranges from 70–200 micrometres. The pore size is crucial for the migration of autologous fibroblasts and endothelial cells as well as for the neovascularisation of the dermal matrix. The collagen in the matrix is invaded by fibroblasts from the bottom of the defect and the matrix is gradually degraded. The full decomposition of the matrix lasts 30 days, vascularisation sufficient for application of a skin graft is nevertheless achieved by the 21^st day after application. Integra® is produced and supplied as a bilaminate substitute, containing the above described bottom (internal) layer and an outer silicone layer. The outer layer is removed after 21 days and a thin dermo-epidermal graft is placed on the already vascularised dermal matrix³⁴. The dermo-epidermal graft is then neovascularised from the dermal matrix. The thickness of the dermal matrix itself is 2 mm. The manufacturer (Integra Life Science Corporation, Plainsboro, NJ, USA) also introduced IntegraSL^®, a 1.3mm thick dermal matrix, facilitating single step application on the defect together with the dermo-epidermal graft. Integra^® is at present the most widely used dermal matrix. Figures 1 to 5 describe the two-step application process of Integra^® after a burn trauma.

Matriderm^® (Skin and Health Care AG, Billerbeck, Germany) is a porous dermal matrix consisting of bovine type I, III and V collagen with an addition of alpha-elastin hydrolysate. Matriderm^® is applied as a monolaminate dermal matrix with 1 mm thickness, immediately followed by application of the skin graft (a single-step application)¹⁰. This dermal substitute is used in treatment of soft tissue defects as well as of partial and full-thickness burns. Due to its resulting elasticity-viscosity characteristics after healing, its use is advantageous where the resulting cosmetic effect is important, in the region of joints and in children³⁵. Matriderm^® was in the Czech Republic first used in clinical practice at the Department of Burns and Reconstructive Surgery at the University Hospital Brno. Figures 6 to 9 show a single step application of Matriderm in a patient after a burn trauma (Figure 6, 7, 8, 9).

Schneider et al. applied both Matriderm^® and Integra^® in their study on a small animal model and subsequently covered them with a dermo-epidermal graft according to the manufacturer’s instructions³⁶. They reported that there was no difference in the neodermis thickness, in healing of the dermo-epidermal graft and in the resulting vascularisation of the dermis between the two skin substitutes. A difference in the neodermis thickness was observed between each of the substitutes and a control group where only the dermo-epidermal graft was applied. In effect, they therefore conclude that a single step procedure is more suitable as the patient is spared an additional surgical procedure and the hospitalisation can be shortened. Inhoff et al. compared the total costs of reconstruction of the complex defects of the scalp using an allogenous graft of fascia lata, dermal matrix and negative pressure wound therapy. The authors state that considering the shorter duration of the treatment, smaller number of re-dressings and improved comfort for the patients, the cost of dermal matrix treatment is bearable, despite being more economically demanding³⁷.

Attempts for combining dermal grafts with a scaffold cast with cells resulting in so-called living skin-equivalent grafts have been made. The pioneering work in the development of such combined skin substitutes was done by Bell et al.³⁸, who published the first successful result of the application of a skin substitute consisting of a superficial layer of autologous keratinocytes and from fibroblasts cast in a collagen matrix on a rat model in 1981. PermaDerm^® (Regenicis, New York, USA), is a skin substitute prepared by cultivation of autologous keratinocytes and fibroblasts in a collagen matrix with a substrate. Due to the duration of cultivation of the autologous cells in the matrix, this product is not yet available for clinical application, it has however been experimentally used for burn trauma, resulting in the formation of the basal membrane as soon as 9 days after application³⁹. To prevent hypopigmentation, melanocytes were added into the keratinocyte cell culture; the resulting pigmentation was however notably variable.

“Like with like replacement”, i.e., substitution of a missing tissue with a tissue of the same or similar properties (mechanical, of texture and pigmentation) is one of the basic principles of plastic surgery. Acellular dermal allografts such as Alloderm^® or DermaMatrix^® show very good results of resulting neodermis properties after healing. Application of acellular xenogenous substitutes is another method of choice, its results are however less satisfactory than those of allografts. Products with chemically induced polymer cross-links are less suitable for use as skin substitutes due to their cytotoxicity; they are more suitable for use as mechanical substitutes where the strength and mechanical integrity are preferred to the incorporation into the tissue. A promising way of research is development of dermal matrices with prefabricated vascular network⁴⁰. A major drawback of the current application of skin substitutes is the absence of the subcutaneous fatty tissue layer as the current methods focus on the reconstruction of the dermis and epidermis only. This leads to reduction in mobility of the newly formed skin and the dipped contour of the healed defect is also apparent. The development of autologous cellularized dermal substitutes is significantly more expensive than that of acellular ones, which is reflected in their price. For comparison, Alloderm^® or Integra^® cost between 15–30 USD per square centimetre, cellularized substitutes are approximately four times more expensive⁴¹.

^CONCLUSION

Skin substitutes became an integral part of both critical and long-term care in the management of burn trauma. Although numerous case reports have been published in the literature, a uniform strategy for comparison of results of their application as well as of long-term results is missing so far. The presented paper provided an overview of the currently used skin substitutes and discussed the pros and cons of their use in clinical practice.

Role of authors: Martin Knoz: first author. Jakub Holoubek: corresponding author. Břetislav Lipový, Ivan Suchánek, Ivona Kaloudová, Tomáš Kempný, Zdeněk Dvořák: consulting authors. All authors have read and approved the final version of the manuscript. All authors declare that this paper or its part is not concurrently under review in another journal or publication.

Conflict of interest statements: All authors declare that they have no conflict of interest.

Compliance with ethical requirements: All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. For this type of study, formal consent is not required.

Funding: The research is financially supported by the Ministry of Education, Youth and Sports of the Czech Republic, projects CEITEC 2020 (LQ1601), LO1503 and LO1218, as well as the Ministry of Health of the Czech Republic, grant No. 17-29874A.

List of abbreviations:

PLGA – poly(lactic-co-glycolic) acid

PTFE – polytetrafluoroethylene

FDA – Food and Drug Administration

Corresponding author:

Jakub Holoubek, MD

Department of Burns and Plastic Surgery

University Hospital Brno

Jihlavská 20

625 00 Brno, Czech Republic

E-mail: holoubek.jakub@fnbrno.cz