Tissue and Fascial Expansion of the Abdominal Wall

Tissue expansion works by mechanisms termed creep and stress relaxation. Creep is the reaction tissues have in response to stretch. There are two types of creep—biologic and mechanical. When tissue is acutely stretched, collagen fibers straighten and realign in a parallel fashion with the vector of force, elastic fibers microfragment, water is displaced from the ground substance, and adjacent tissue is recruited. This is mechanical creep. When tissue is chronically stretched, cellular growth is initiated, and stretch-induced signal transduction pathways lead to increased production of collagen, angiogenesis, fibroblast mitosis, and epidermal proliferation. This is biologic creep. Stress relaxation is a phenomenon whereby the force required to maintain tissue elongation decreases over time.

Well-known changes occur in the layers of the skin and soft tissue as a result of tissue expansion ( Table 11.1 ). The epidermis thickens as a result of increased mitotic activity, hyperkeratosis, and acanthosis. After 6 months, the epidermis normalizes. The dermis has an increased number of fibroblasts and increased activity of myofibroblasts. Consequently, the dermis thins. Normalization of the dermal thickness occurs 2 years after expansion. Muscle thickness and mass decrease; however, there is no effect on function. Adipose tissue is the most sensitive to expansion, with the number of fat cells decreasing 30%–50% with tissue expansion.

In addition to the above-mentioned tissue changes, a capsule forms around the expander within days, which essentially is a foreign body reaction against the expander. The capsule is composed of thick collagen bundles oriented parallel to the surface of the expander with elongated fibroblasts and a few myofibroblasts. Angiogenesis occurs rapidly in expanded tissues. The number of arterioles and venules increases, with the highest density of vessels forming at the junction of the capsule and the host tissue, making this highly vascular.

When choosing an expander, numerous variables need to be considered, including the type of injection port, base style, surface texture, and shape. Injection ports can be either remote or integrated ( Fig. 11.1 ). A remote port is connected to the tissue expander with tubing and is placed in the subcutaneous tissue. This port is palpated and accessed percutaneously. A remote port can also be placed externally, avoiding the need for skin puncture. This is especially beneficial in pediatric patients, in whom needle sticks (especially if occurring frequently) may be intolerable. A remote port, either subcutaneous or external, decreases the risk of tissue expander perforation during filling. However, complications can occur with remote ports; for example, the ports can flip, migrate, or shift position, preventing access. Additionally, the tube connecting the port to the expander can become occluded.

An integrated port is incorporated into the surface of the tissue expander. This port can be palpated, or a magnetic finder can be used to locate the port for access. Unintended perforation of the tissue expander is a risk of integrated port use; however, with integrated ports, there is no need for a remote port site or separate incision, there is no kinking or twisting of the tubing, and none of the other remote port challenges mentioned earlier are present.

The base of the tissue expander can be either soft or stable. A soft base allows for expansion in all directions. A stable base permits unidirectional expansion, up and outward from the rigid back.

The surface of tissue expanders can be either smooth or textured. A textured surface can allow for ingrowth of the capsule into the device. This is helpful in abdominal wall tissue expansion because this decreases migration of the expander. A smooth surface allows for easier placement; however, a more precise pocket dissection is needed because the expander can potentially migrate.

Tissue expanders are available in many shapes or can be custom designed. The shape influences the amount of surface area gained, with round expanders providing 25% of calculated tissue expansion, crescent expanders providing 32%, and rectangular expanders providing 38%. However, newer tissue expanders have been designed with a higher profile for the same base width. These expanders provide a greater percentage of calculated tissue expansion. The size of expander should be calculated based on the defect size. The base diameter of the expander should be two to two and a half times the diameter of the defect to be covered, either as a single expander or taken together if multiple expanders are used.