Tissue expansion is a technique used by plastic, maxillofacial and reconstructive surgeons to cause the body to grow additional skin, bone, or other tissues. Other biological phenomena such as tissue inflammation can also be considered expansion (see tissue inflammation below).
Skin expansion Skin expansion is a common surgical procedure to grow extra skin through controlled mechanical overstretch. It creates skin that matches the color, texture, and thickness of the surrounding tissue, while minimizing scars and risk of rejection. When skin is stretched beyond its physiological limit, mechanotransduction pathways are activated. This leads to cell growth as well as to the formation of new cells. In some cases, this may be accomplished by the implantation of inflatable balloons under the skin. By far the most common method, the surgeon inserts the inflatable expander beneath the skin and periodically, over weeks or months, injects a saline solution to slowly stretch the overlaying skin. The growth of tissue is permanent, but will retract to some degree when the expander is removed. Topically applied tissue expansion devices also exist. These have the benefit of being inexpensive and do not require a surgical procedure to implant them under the skin. Breast reconstruction surgery, for example, can use this technique when the mammary gland is removed by surgery (mastectomy). Later, a more permanent breast implant filled with saline or silicone gel is inserted under the expanded skin pocket. In other applications, excess skin is grown purposely by expansion on the back or the buttocks, so that it can be harvested later for transplantation to another site where skin was lost due to trauma, extensive wounds, surgery, burns, etc.
Mechanics of skin expansion Stretching the skin beyond normal expansion invokes several mechanotransduction pathways which increase mitotic activity and promote collagen synthesis. As a result, the skin surface area increases. Continuum mechanics approaches can be used to model skin growth during tissue expansion and non-linear finite element methods can be used to computationally simulate different tissue growth scenarios. Tissue growth due to skin expansion can be modeled as anisotropic surface area growth as described by the following equations:
F = F e ⋅ F g {\displaystyle F=F^{e}\cdot F^{g}\,}
where F e {\displaystyle F^{e}} is elastic area stretch that is reversible and F g {\displaystyle F^{g}} is irreversible area growth described by:
F g = θ g I + [ 1 − θ g ] n 0 ⊗ n 0 {\displaystyle F^{g}={\sqrt {\theta ^{g}}}\mathbb {I} +[1-{\sqrt {\theta ^{g}}}]n_{0}\otimes n_{0}\,}
where n 0 {\displaystyle n_{0}} is a vector in the direction of skin thickness. We assume that the skin does not grow in the thickness direction for area growth is equal to volume growth or θ g = d e t ( F g ) = J g {\displaystyle \theta ^{g}=det(F^{g})=J^{g}} . We also assume that the newly created skin will have the same density, stiffness, and microstructure as the original, non-expanded skin.
Radial forearm free flaps Recent studies have demonstrated that using topical tissue expansion can reduce the need for a split thickness skin graft after harvesting a forearm free flap. The authors noted that this results in less pain as well as reduced healing time. This method has also been shown to be cost effective as well as improve cosmetics.
In scalp reconstruction Tissue expansion has been used on the scalp for treating scalp scarring and baldness, in lieu of hair transplantation when there is insufficient donor hair to transplant on the scar or the scar tissue is not vascularized to support hair growth. For instance, in a patient who had melanomas removed from the scalp resulting in alopecia defects (hair loss), tissue expansion can be used to allow for the removal of scars and complete hair coverage. The two main indications for choosing tissue expansion over hair grafting are the size and shape of the defect relative to potential supply of donor hair, and the quality and thickness of the scar tissue. Areas of significant scarring and/or tissue atrophy, which is likely to make hair grafting unsuccessful, are best excised and replaced by normal expanded scalp skin. According to research from Dr Jeffrey Stuart Epstein while he was a professor at University of Miami, theoretically there is no limit to the amount of tissue that can be created with tissue expansion, provided the process is conducted gradually.
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