Tissue expansion relies on the capability of skin and soft tissues to generate in response to tension. In plastic surgery, tension is generated by implanting a subcutaneous balloon (expander) that is inflated over a period of weeks; new tissue is generated in response to the constant stretch caused by the progressive inflation of this expander. This tissue may be utilized to reconstruct extirpative or traumatic defects such as those encountered after mastectomy, burn excision, or removal of giant nevi.
A number of studies support the concept that the increase in skin surface region after expansion is due to the generation of new tissue instead of the stretching of existing skin. In culture, mechanical tension induces fibroblast and epidermal hyperplasia. These cells preserve their phenotype with out malignant degeneration. This observation is supported by the fact that there has never been a reported case of skin malignancy secondary to tissue expansion.
From a histological standpoint, adult and pediatric skin responds to expansion in the same manner. Within 1 week of expansion, the epidermis begins to thicken and also the dermis thins. The skin appendages do not change. The subcutaneous fat and muscle atrophy. Cellular proliferation reduces the resting tension of the skin over time, enabling an additional round of expansion to take place.
Once the process is complete, the expanded skin eventually returns to its baseline thickness. The vessels of the skin and subcutaneous tissue also resume their pre-expanded size and number; however anecdotally, some flaps demonstrate increased vascularity.
In general terms, expansion of tissue is utilized to enhance rotation, transposition or advancement of local or regional flaps, or to increase the harvest of full-thickness skin grafts. Recently, tissue expansion has been successfully applied to myocutaneous and totally free flaps. In adults, aside from their use in breast reconstruction, tissue expanders are utilized primarily for secondary burn and trauma reconstruction in the head and neck region. In the pediatric population, expanders have been utilized in a multitude of reconstructive procedures.
The most typical indication in kids would be to reconstruct defects left by excision of giant congenital nevi. Tissue expansion is contraindicated in infected skin. Even though expansion is feasible in radiated or scarred tissue, it is associated with a much higher complication rate and should be avoided whenever possible.
Expanders come in a variety of shapes and sizes, and there is no absolute ideal expander for a given website or condition. Expanders can have either internal or external filling ports. Most experienced surgeons recommend using remote ports. These should be placed away from the expander. Internal ports have both a higher failure rate and a higher incidence of accidental expander rupture. In kids, the use of internal ports is associated with a higher rate of exposure of the expander because of the pressure exerted on the skin by the port.
Whenever feasible, the incision should be placed within tissue destined to be excised, as in the case of congenital nevi. Straight incisions along the border of the defect should be avoided because this will enlarge the defect and may interfere with flap coverage. An alternative would be to use a U- or V-shaped incision that is hidden and remote from the defect.
Such incisions should be perpendicular to the direction of expansion in order to maximize skin blood supply. When doing serial expansion, longitudinal blood supply should be preserved. This holds true especially in the trunk and extremities.
The expander should be placed on top of the deep fascia (or subgaleal in the scalp), unless the plan would be to incorporate muscle into the expanded flap. The pocket should always be larger than the base diameter of the expander. Blunt dissection in a single fascial plane is safest for preserving blood supply. Most surgeons overinflate tissue expanders beyond the manufacturer's recommended maximum capacity. Studies have demonstrated that significant overinflation is possible before weakening or rupturing. The rate of inflation is variable and largely based on surgeon preference.
Patient comfort and signs of tissue perfusion, like tension, color, and capillary refill, guide the filling rate. Filling is usually initiated one week after surgery. Tissue expansion ought to continue until the expanded area is larger than the defect, because of the length that is lost upon advancement and inset of the flap. The use of rotation and transposition flaps enables the transfer of tension from the tip of the flap more proximally to its base.
A single or double back-cut may be performed prior to inset in order to gain additional length. Lastly, the donor website should be closed in layers after the implant capsule is excised. Pre-expansion of distant pedicle- or free-flaps facilitates closure of otherwise tight donor sites.
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