Excerpt from Implants, Soft Tissue, AlloDerm

Synonyms, Key Words, and Related Terms: AlloDerm implants, acellular dermis, allograft skin, cross-linked porcine skin, allograft rejection, transplantation, cadaveric skin, cadaveric skin, acellular matrix, acellular human dermis

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Throughout history, multiple approaches have been used to replace lost, damaged, or diseased tissues. Some methods use synthetics, biosynthetic constructs, cross-linked biological materials, or preserved allografts. Investigations using these approaches have supplied increasing evidence that the matrix component of tissue replacement must be complex. The physician must understand the normal structure of the integument and how this arrangement is either preserved or slightly altered by the fabrication methods of these allograft materials. These concepts become increasingly important when considering the remodeling of the allograft material after implantation into the patient. Factors to be considered by the physician while selecting the allograft material include the patient’s medical history, the anatomic site of implantation, the desired result, and the chosen material type.

Integument structure

Skin consists of 2 essential layers: the epidermis, which provides a barrier against the environment, and the dermis, which provides strength, durability, and elasticity. The dermis of the skin is a complex multicomponent matrix. In contrast to skin of other species, human skin lacks a panniculus, or underlying muscle sheath. Because many allograft materials are xenografts, this anatomic fact is important to remember. Consequently, the human dermis has evolved as a multilayered complex organ able to deal with the stress patterns resulting from the relative immobility of human skin. The layers of the dermis include the papillary dermis and the reticular dermis. The reticular dermis transitions into the deep reticular-subcutaneous junction.

The important components of the dermis that contribute to its function include the basement membrane complex at the dermal-epidermal junction, collagen, elastin, proteoglycans, and a distinctive vascular plexus. The basement membrane complex at the dermal-epidermal junction contains type IV collagen, laminin, and highly specialized type VII collagen. Type VII collagen forms anchoring fibrils and filaments, which ensure strong physical bonding of the epidermis to the dermis.

The bundle orientation of collagen and elastin differs between the papillary dermis and the reticular dermis. Collagen bundle orientation is random in the papillary dermis, but it is perpendicular to the lines of tension in the deeper reticular dermis. Similarly, elastin fibers are sparse and finely reticular in the papillary dermis, whereas they are thicker and form a complex 3-dimensional array in the reticular dermis.

The dermal vasculature forms a distinct plexus in the papillary dermis. This plexus configuration plays an important role in the remodeling process because collagen deposition tends to occur along the pathways of neovascularization. If the plexus is absent, collagen remodeling occurs along the pathways of an altered vascular pattern, as evident in granulation tissue in scar formation. Proteoglycans of the dermis provide a reservoir for growth factors (eg, basic fibroblast growth factor [FGF] binds to heparin sulfate). Dermal proteoglycans also direct the assembly of collagen (eg, decorin, tenascin) or are involved directly in angiogenesis and the regulation of cellular functions (eg, hyaluronic acid, chondroitin sulfate). Each of these factors plays a significant role in the ability of the allograft material to maintain volume persistence over time.

Dermal function depends on the intricate and complex organization of the extracellular matrix components and their interactions. Matrix components include collagen fibrils oriented perpendicular to the lines of stress, an elastin network consisting of elastin and microfibrillar fibers, diverse and compartmentalized proteoglycan species, and a structurally unique basement membrane complex.

Wound healing

Following full-thickness skin injury, the epidermis heals rapidly by regeneration. However, the .....