Álvaro Cuadra, Bruno Dagnino

Abstract

Reconstruction of soft tissue defects may be achieved by different methods. Skin grafts and flaps, either local, regional, or free, are all valid alternatives, depending on the case. However, they all produce varying degrees of morbidity on the donor area, may lead to a poor functional or cosmetic result in the case of skin grafts, and may be technically demanding. Also, in some cases, even these techniques may not provide enough healthy tissue for coverage. Tissue expansion is based on the use of a silicone device that is placed under the skin and gradually inflated with saline, thus causing stretching of the overlying tissue (skin, subcutaneous fat, fascia, muscle). Such forces favor cell proliferation and angiogenesis as well as the recruitment of neighboring skin so that at the end of the expansion process newly generated, well-vascularized tissue is available for reconstruction. Tissue expansion presents a spectrum of treatment options, including expansion of a skin graft donor site, expansion of neighboring skin to be used as a local flap, and expansion of distant skin to be used as a free flap. Regarding its clinical applications, tissue expansion may be used to treat soft tissue defects secondary to trauma, burns, oncologic resection, and congenital conditions, with postmastectomy breast reconstruction being one the most frequent indications. This chapter describes the main aspects of tissue expansion with regard to changes in tissues, surgical technique, potential complications, and clinical applications.

Keywords: biological creep, implant, infection, stress relaxation, tissue expansion

The capacity of tissues to stretch and expand gradually over time has been observed in physiologic and pathological circumstances, as well as being part of cultural ornaments. Some examples of physiologic changes of the skin and soft tissues may be seen during pregnancy, massive weight loss, and cranial development during the first years of life. Skin and soft tissues may also experience stretching and expansion as a consequence of pathological growth from underlying tumors, as may happen with soft tissue sarcomas and benign tumors from the skin or subcutaneous tissue. Classical examples of tissue expansion as cultural ornaments are seen in the Chad tribes, where people undergo lip stretching, or the Padaung tribe from Burma, famous for their elongated necks from the use of metallic rings.

The history of tissue expansion did not begin with skin or soft tissue, but with the expansion of bones. In 1905 Codevilla and then Magnuson described bone lengthening using traction devices. In 1921, Putti, in Italy, demonstrated that sustained traction for a period of several months could lead to bone lengthening and also stretching of surrounding noble structures, such as nerves and blood vessels, preceding the method later described by Ilizarov.

In 1957 Neumann used a rubber ball placed underneath the skin to obtain skin for cutaneous coverage of a subtotal ear amputation. Nevertheless, his results did not have much impact at that time. Two decades later, in 1976 and 1978, Radovan reported the use of a subcutaneous tissue expander that could be inflated percutaneously with normal saline through a remote valve. In 1979 and 1982, Austad and Rose described the use of a device that employed the osmotic gradient of sodium chloride for its inflation, which avoided the need for repeated injections. Despite this advantage, the long periods of expansion and the risks related to exposure of surrounding tissues to hypertonic solutions in case of rupture, made Radovan’s the preferred method for tissue expansion, which has remained to our days.

During tissue expansion the surgeon tries to optimize stress in order to produce the maximum stretching that the skin can tolerate. Accordingly the region under the curve between B and C must be considered, without exceeding the tensile strength of the skin, which would lead to its rupture, represented by area C, D of the curve.

Skin can be stretched because of its inherent elasticity and as a result of biological and mechanical creep. Mechanical creep occurs when skin is stretched at a constant force, leading to extrusion of fluids from the interstitium of the collagen chain and subsequent skin stretching beyond its inherent elasticity. Apparently blood supply is not compromised during this process. A consequence of this phenomenon is stress relaxation, produced by the fact that if skin is stretched at a constant length the force required to maintain this state gradually reduces. On the other hand, biological creep is the slow and gradually expansion seen during pregnancy, weight increase, or tissue expansion, in which continuous stretching and tissue thinning stimulate production of new epithelium, collagen, elastic fibers, and neovasculogenesis as a means to reduce stress (stress relaxation).

Upon expansion, different tissues (e.g., skin, muscle, bone, and the vascular network) experience a number of changes, which are summarized as follows:

• Epidermal thickening and dermal thinning.

• Formation of a capsule around the expander.

• Muscle thinning and reduction in mass.

• Bone thinning and new bone formation peripherally.

• Mechanical strain triggering DNA synthesis and cellular proliferation.

• Increased vascularity.

The epidermis becomes thicker during tissue expansion. Initially this occurs as a consequence of tissue edema, which lasts for approximately 4 weeks; nevertheless, even after edema subsides, the epidermis remains thickened for several months. Conversely, the dermis becomes thinner, and changes in the dermal thickness last for at least 36 weeks after expansion has ceased.

Total skin area increases from recruitment of adjacent skin and an increase in the mitotic rate. Hair follicles and appendages may become compressed but do not degenerate, and there is an increase in melanocytic activity, which reverts once the expander is removed.

Following expander placement, a dense fibrous capsule is formed around the device, which shows increased vascularity and reduced cellularity as expansion progresses.

Muscle always atrophies regardless of whether the tissue expander is placed above or below. Animal studies have shown that expansion induces growth of muscle cells and an increase in the total number of sarcomeres for each fiber of striated muscle. Once the expander is removed, muscle recovers its normal architecture, blood supply, and function.

Applying a mechanical stress on living cells affects a series of cellular structures and signaling pathways that lead to cell proliferation, growth, and tissue regeneration, which explains the formation of new tissue during the process of expansion. Mechanical deformation triggers a series of cellular responses involving the cytoskeleton, extracellular matrix, enzymatic activity, second messengers, and ionic channels. The cytoskeleton plays a major role in transforming extracellular forces into intracellular events, which are also transmitted to neighbor cells.

Several growth factors essential for normal cell growth also appear to be involved in cell proliferation induced by stretching. These include epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), transforming growth factor (TGF) family, platelet-derived growth factors (PDGFs), and angiotensin II. Activation of EGF receptors (EGFRs) generates an ionic influx that induces membrane ruffling, a phenomenon commonly seen previous to cellular division. TGF increases fibroblast growth and stimulates extracellular matrix production. In the absence of exogenous proteins, mechanical stretching by itself induces upregulation of growth factors, such as PDGF and angiotensin II, suggesting that tissue elongation may have an autocrine function too.

The extracellular matrix is also essential for cell proliferation. Under mechanical forces there is an increase in DNA and collagen synthesis. In addition, the mechanical stress deforms the matrix, altering the configuration of adhesion proteins (e.g., integrins), which act as mechanochemical transducers, transmitting this extracellular signal to the intracellular environment by producing changes in the cytoskeleton. Several cellular morphological changes also occur as a result of rearrangement of actin filaments induced by mechanical stress. Actin also plays a key role in cell to cell signaling. Other families of transmembrane proteins, such as protein G, are also relevant for mechanotransduction in that their strain-induced conformational changes are supposed to initiate second-messengers signaling cascades and cellular growth.

Ionic channels sensitive to mechanical stimulus favor the passage of several cations as Na⁺, K⁺, and Ca⁺⁺ in response to strain, which promotes a diversity of secondary chemical reactions that determine cellular responses, including depolarization, cellular contraction, and generation of myogenic tone.

Protein kinases, especially protein kinase C, are strong transducers of cellular signaling also favoring membrane ruffling.

Expanders with integrated valves minimize the risk associated with the remote injection port and connection tubing, but in turn have a risk of puncture of the reservoir during injection when the integrated port is not adequately located. However, this has been simplified by incorporating metallic ports that can be readily identified by ultrasound or magnetic devices, the latter being very common in breast reconstruction.

In 1982 Austad and Rose created a self-inflating device consisting of a silicone balloon filled with hypertonic sodium chloride (NaCl), creating an osmotic gradient that would allow its filling without the disadvantages associated with injection ports. These devices are now infrequently used because they require long periods of inflation between 8 and 14 weeks and have high rates of early rupture and tissue necrosis. K. Günter Wiese and then Shaheel Chummun developed expanders using an osmotically active hydrogel with the capacity to expand more than 10 times its original size. They consist of a polymeric matrix and an aqueous component, which absorbs fluid from the extracellular space causing edema of the hydrogel. They have the advantage of requiring fewer visits to the office, reduced pain, and faster expansion. However, their main downside is that expansion cannot be stopped in case of complications or doubts about tissue vitality.

The main factors to consider during the preoperative planning of tissue expansion include the type, shape, and size of the expander, as well as the expected volume of expansion.

Almost any kind of tissue is susceptible to expansion, but those with underlying bone tissue will have better results due to the transmission of the tensile strength of the expansion, preferably to the more compliant overlying skin, and not to the rigid bone.

The presence of active infection, heavy smoking, and a previous history of radiotherapy are contraindications to tissue expansion given the unacceptable risk of complications. In addition, children under 7 years of age, expanders in the extremities, and integrated valve expanders in children have been identified as risk factors for complications; hence close surveillance during the expansion process is recommended in these cases. The use of expansion in trauma cases is advocated as a delayed rather than an immediate method of reconstruction.