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Controlled Tissue Expansion in Facial Reconstruction
INTRODUCTION
During the past three decades, many reconstructive surgical procedures have been developed or improved, providing a functional and aesthetic benefit. Among these, tissue expansion offers a unique potential to preserve both form and function. One of the major benefits of tissue expansion for correction of cutaneous defects is the ability to use tissue optimally matched in color, texture, thickness, and hair-bearing qualities. It may also obviate the need for multiple flaps or grafts and avoids the creation of a second defect at a distant area to correct the primary deformity. Expansion of tissue is a dynamic process producing a dividend of tissue available for flap transfer. A basic understanding of the physiologic changes that occur with skin expansion and the techniques of expansion is imperative for any surgeon performing reconstructive procedures of the head and neck. Other than rapid intraoperative tissue expansion, the term tissue expansion is used in this chapter to refer to the prolonged expansion of skin during an interval of days to weeks.
HISTORY
Tissue expansion can be a natural physiologic process encountered in normal life, with pregnancy and obesity being the most obvious examples. This process has been used for centuries in certain cultures to modify facial features as illustrated on the pages of National Geographic magazine. Modern use of tissue expansion was first introduced in the early 1900s. A technique was described that used progressively larger objects to expand tissue with multiple operations required. Because of the number of procedures required and the high morbidity, the procedure was abandoned. The practical application of tissue expansion was not possible until the introduction of plastic materials later in the century. In 1957, Neumann published “The Expansion of an Area of Skin by Progressive Distension of a Subcutaneous Balloon.” This article described the use of a latex balloon to expand the skin for reconstruction of an ear. He placed the balloon above the ear in a subcutaneous tissue pocket and brought a polyethylene tube out through an independent skin incision. He gradually injected air into the implant through a stopcock. His procedure was a success, but the potential impact of his work was not recognized at the time. No other work was performed on tissue expansion for almost 20 years.
In the early 1970s, Radovan and Austed independently began studying the use of tissue expansion to repair soft tissue defects. Radovan’s early expanders consisted of a balloon and two separate tubing systems for injection and removal of saline. The entire system was placed under the skin. His first experiments were performed in 1976 and were met with skepticism when his results were presented to the medical community later that year. This was most likely because of his unscientific methodology. He was not discouraged by the skepticism and continued his clinical trials. The results of Radovan’s experiments met wider acceptance 3 years later accompanied by Austed’s work, which used an osmotically driven, self-inflating expander. Austed, unaware of Neumann’s and Radovan’s work, first suggested the idea of implanting a device beneath the skin to induce tissue expansion in 1975. His work established the basis for understanding of the tissue response to expansion. His early laboratory experience and histologic studies confirmed that tissue expansion is safe and effective.
PHYSICAL PROPERTIES AND MORPHOLOGIC CHANGES
It is important for surgeons performing tissue expansion to understand the physiologic and morphologic changes associated with the process. This knowledge will help the surgeon understand the potential benefits, limitations, and complications of this procedure. When the technique of tissue expansion was first introduced, the unanswered question was whether skin is stretched or new skin is created. If skin is simply stretched, a permanent gain in terms of surface area would not occur. Gibson studied the properties of skin and described its inherent extensibility. He noted that skin could stretch beyond its inherent extensibility and called this mechanical creep. The beneficial effects of mechanical creep are attributed to the displacement of interstitial fluids and mucopolysaccharide ground substance, the parallel alignment of randomly oriented collagen fibers, and the migration of tissue into the field by the stretching force.
He referred to biologic creep as the gradual stretching of skin overlying a slowly expanding subcutaneous structure. This is not simple stretching because the skin is affected by metabolic activity, including the creation of epithelium, blood vessels, nerves, lymphatics, collagen, and elastin fibers. All of this occurs during sustained prolonged expansion of the skin.
The first experimental studies of skin expansion were performed by Austed et al and Pasyk et al using guinea pigs. They used self-inflating expanders, causing gradual uninterrupted expansion. Unlike humans, guinea pigs have loose skin, allowing it to move freely over the muscle beneath and to be picked up in large folds. These qualities make the guinea pig model less than ideal for understanding the effects of expansion on human skin. Nevertheless, even with these limitations, their studies provided valuable information about tissue expansion. Because pig skin lacks a well-developed panniculus carnosus and has elastic tissue comparable with that of humans, it has been used in place of guinea pig skin for subsequent laboratory studies of skin expansion.
Epidermis
The epidermis is composed of stratified squamous epithelium of the keratinized type. The epidermis shows no thinning during or after expansion. In fact, most studies have noted an increase in thickness during skin expansion. The active basal layer of the epidermis increases in density and thickness. The basal layer normally composes 10% of the thickness of the epidermis, but during expansion it increases to 40%. Van Rappard et al described the histologic changes found in expanded pig skin. They observed that during skin expansion, epidermal thickness increases; however, there is no histologic change in the layers of the epidermis. Other studies have also demonstrated that epidermal thickness increases or remains unchanged during expansion in spite of an obvious increase in surface area ( Fig. 25.1 ). These findings would indicate that the epidermis is not merely being stretched. Using tritiated thymidine labeling, Austed et al demonstrated an increase in the cellular mitotic rate in the stratum basale of expanded skin and showed a net gain in donor tissue. Van Rappard et al confirmed these findings in the pig model. They demonstrated that the number of epidermal cells for a given surface area remained the same in expanded and no expanded skin . Initial stretching of the epidermis from inflation of an expander increases the intercellular spaces, which in turn probably signals the cells to increase mitotic activity and restore the epidermis to a normal cellular density and thickness. The increase in mitotic activity caused by tissue expansion eventually returns to normal but remains active for as long as 2 months after expansion is discontinued.
Dermis
The dermis, which is a sheet of connective tissue underlying and supporting the epidermis, is composed of the papillary layer and the reticular layer. The papillary layer contains finer collagen fibers than the reticular layer and contains numerous elastic fibers. The reticular layer is composed of dense, irregular collagen fibers. The reticular layer is responsible for the lines of tension. The dermis does not tolerate expansion as well as the epidermis does. During tissue expansion, a rapid decrease in dermal thickness is observed and averages 20%. Total restoration of dermal thickness after skin expansion has not been demonstrated. The amount of thinning depends on the rate of expansion. Greater thinning is noted with more rapid expansion. The expanded dermis contains large bundles of collagen fibers and active fibroblasts. An increase in collagen synthesis occurs in the dermis during tissue expansion. Normal collagen fibers contain bundles cross-linked at angles of 70°. With expansion, these angles gradually decrease until the collagen bundles are almost parallel to the skin surface. This effect is noted for 6 months after cessation of the expansion process. Melanin production increases during skin expansion and can cause a temporary hyperpigmentation of the skin. This disappears, in most cases, within a few months. Olenius et al described an increase in the amount of melanin pigment granules in the basal layers of expanded human skin.
Subcutaneous Tissue
The subcutaneous tissue is composed of fat; loose connective tissue; collagen; arteries; veins; nerves; lymphatics; hair follicles; and sebaceous glands. The thickness of the subcutaneous layer is diminished by as much as 50% during prolonged expansion. Nevertheless, Pasyk et al noted the return to greater than normal thickness of the subcutaneous tissue within 2 years after removal of expanders in a series of three patients. During skin expansion, there is a marked decrease in the number of fat cells within the subcutaneous tissue, and those that remain have less lipid content ( Fig. 25.2 ). There is also an increase in the interlobular spaces and an increase in the collagen content within these spaces. A varying amount of fat necrosis is also noted. The more rapid the skin expansion, the greater the fat necrosis. Restoration of normal-appearing fat cells after skin expansion is not observed for as long as 12 months after expansion.
Hair follicle morphology remains the same during and after skin expansion. The follicles become separated during expansion, but the number of follicles remains unchanged. Nevertheless, hair shock is common and is manifested by conversion of follicles to their telogen phase, which results in temporary alopecia. These phenomena may take several months to recover. Sebaceous glands in the subcutaneous tissue show little if any change in response to expansion; however, expansion can cause blockage of some of the glandular ducts.
Skeletal Muscle
Skeletal muscle is sensitive to tissue expansion. Ultrastructural changes of expanded muscle occur. An increase in the number and size of mitochondria, number of vesicles, and amount of sarcoplasm is observed. The number of muscle cells remains unchanged, but significant thinning of muscle occurs during tissue expansion. The striated pattern of muscle cells decreases, and some cells may become necrotic. Hyalinization of muscle cells is also observed and may be followed by calcification in some cases. The significance of these changes is not fully understood, but expansion may lead to a decrease in muscle function and strength in the region of tissue expansion. Sasaki has reported visible muscle atrophy and weakness after tissue expansion, but permanent sequelae are rare.
Blood Vessels
Blood vessels undergo some of the most important changes during tissue expansion. Tissue expansion stimulates the proliferation of blood vessels, creating highly vascular tissue ( Fig. 25.3 ). Distention of capillaries is first noted during tissue expansion, followed by an increase in the number of venules and arterioles within a few days. Barium angiography demonstrates a dense vascular pattern in expanded tissue. Stark et al demonstrated a rapid elongation of arteries and veins with no loss of vessel wall diameter or intimal integrity. They also found no difference in tolerance to tissue expansion between arteries and veins. Studies of flap survival, which is dependent on skin vascularity, revealed that flaps raised from expanded tissue have survival rates comparable to those of delayed flaps. Cherry et al demonstrated that flaps raised from expanded tissue have a 117% increase in survival length compared with random pattern flaps raised in no expanded skin. Other authors have described similar findings. , In essence, expanded skin has characteristics similar to delayed flaps. These studies indicate that tissue expansion enhances the vascularity of skin and improves flap survival.
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