Tissue expansion has become a major reconstructive modality over the past 30 years. It has become more and more widespread, particularly in the fields of breast reconstruction, burn surgery, and pediatric plastic surgery.  In many cases, tissue expansion can be said to have revolutionized plastic surgery.
The lack of available soft tissues is a common challenge facing the reconstructive surgeon. The phenomenon of tissue expansion of the skin and underlying soft tissues has been observed commonly in pregnancy, slow-growing tumors, and fluid collections, where the local tissue expands and enlarges in response to the tension generated by the increased volume of the mass. This response has been found to be a metabolically active process with increased mitotic activity and vascularity of the expanded skin and has been applied clinically as an important skill in the armamentarium of the reconstructive surgeon. 
Tissue expansion has numerous advantages. While it provides skin with a near-perfect match in color and texture, minimal donor site morbidity and scarring occur. [3, 4, 5] It also can be used in various parts of the body to provide tissue with specialized sensory function or adnexal characteristics. Examples include the superior sensation of the skin flaps in breast reconstruction and the hair-bearing flaps designed in the treatment of male pattern baldness developed with expanders. [6, 7] In addition, expanded flaps are more resistant to bacterial invasion than random cutaneous flaps.  This technique has been extended to other types of tissue, including bladder reconstruction, vascular elongation, and nerve lengthening.
Disadvantages include temporary cosmetic deformity during the expansion phase, prolonged period of expansion, the need for multiple procedures, and complications associated with the implant and placement.
Tissue expansion induces changes in the vascularity and cellular activity in involved skin. Alterations in microcirculation have been shown to create hardier flaps in expanded versus nonexpanded skin. Histologic evidence supports expansion as a type of delay. Cherry and others showed that in the animal model, expanded skin demonstrated increased vascularity on microangiography. These flaps had significantly increased survival length when compared with acutely raised random-pattern flaps.  The capsule that forms around the prosthesis is involved in the increased vascularity and has been shown to have a circulation exceeding that of the subdermal plexus.  Removal of this capsule compromises the integrity of the expanded tissue, so it is often unnecessary and sometimes risky.
Studies of the skin surrounding an expander indicate that the epidermis initially thickens slightly while the dermis demonstrates rapid thinning during the first 3 weeks.  Skeletal muscle atrophies under expansion but retains its activity. However, adipose tissue undergoes permanent atrophy of 30-50% with loss of fat cells. [11, 12]
Increased epidermal mitotic activity demonstrated by Austad and others and increased numbers of radiolabeled keratinocytes suggest that new skin elements form in the expanded flaps. When compared with intraoperative tissue expansion as described by Sasaki, tissue expansion over a period of several weeks attains nearly 4 times the surface area and 3 times the arc of rotation.  Intraoperative expansion depends on the viscoelastic properties of skin in response to load cycling.  This increase in length in response to an applied force is known as "creep" and is distinct from the biological response of the replicating epidermal cells noted in animal model flaps developed over several weeks. Austad and others concluded that this was consistent with a net "dividend" of tissue gained. 
The technique of tissue expansion can be used in a number of clinical applications wherever soft tissue coverage is needed in reconstructive surgery. The most common indication is in breast reconstruction where the use of expanders accounted for 70-80% of postmastectomy breast reconstructions in 1995. Recently, we have assisted in the rise in the number of reconstructions performed with autologous tissue. The timing of the reconstruction may be either immediate or delayed, occurring several weeks to years following the ablative procedure. Patients who require postoperative radiation therapy are not good candidates for tissue expansion because of the high rate of capsular contracture and wound complications. However, this is not an absolute contraindication, as shown by Cordeiro et al. [16, 17]
In selected patients, place the expander in a submuscular pocket beneath the pectoralis major and serratus anterior muscles, carefully providing total or near-total muscular coverage over the expander. [18, 19] The advantages are lower contracture and infection rates demonstrated in the submuscular prosthetic experience. The implant is then inflated intermittently over several weeks postoperatively. The reconstructed breast generally is expanded larger than the contralateral breast to allow for the creation of ptosis, with modification of the inframammary fold possibly performed at a later procedure, if desired. Then the expander is replaced with a permanent prosthesis or left in place, if a Becker-type permanent unit is used. This technique has been expanded to reconstruct the breast in congenital hypoplasia of the breast (as in Poland syndrome) as well as in burn victims. 
Face, head, and neck
The face, head, and neck are other areas of reconstruction in which tissue expansion has been particularly useful. In addition to treatment of male pattern baldness, expansion has been used to eliminate large scalp defects  and large burn scars. In every case, hair-bearing tissue can be expanded to approximately twice its size without a noticeable deficit in hair density. Unilateral forehead flaps were designed using expansion to reconstruct contralateral defects of the forehead  and nose. Neck contractures have been managed with flaps developed with an implant in the supraplatysmal plane. Correction of congenital microtia has been described, where tissue expanders were used to provide local skin coverage over the cartilaginous graft framework. 
Cleft lip repair
Cleft lip repair also has been described with excellent cosmetic results in the presence of severe deformity and a tight lip.  Intraoperative tissue expansion also has been used to facilitate rhytidectomy.  In pediatric plastic surgery, the use of tissue expanders has also been extremely helpful in the treatment of giant melanocytic nevi, allowing a better surgical planning of the serial excisions. 
Alternative to reconstructive methods
Tissue expansion appears to be useful in almost all regions as an alternative to other reconstructive methods. Primary closure of the soft tissue has been shown to be possible in complicated abdominal wounds using tissue expanders.
The technique has been extended to cover lower extremity defects as well,  although this remains a very problematic region. In the upper extremity, the dorsal skin of the hand is also amenable to expansion when skin grafting is a less desirable option.
The use of tissue expanders does have limitations, particularly related to the placement of the expander. An optimal expansion is obtained if the process is done against a hard surface (eg, bone), while an expansion is more complicated if performed against other soft tissues (eg, neck, abdomen). In the latter case, careful monitoring of the progress of the expansion is necessary.
The expansion itself may also create some new edges of tissue, sometimes hard solid, which eventually need to be addressed during the final surgery.
The technical advances and the improvement of the materials reached in the last few years have now widened the possible use of tissue expanders to an incredible range of clinical situations. Tissue expanders may be custom-made and adjusted to particular and specific use. Some tissue expanders use the osmotic process to fill automatically when inserted in the human body (eg, recreating the orbital socket). Another future project is the creation of a tissue expander with an antibiotic-impregnated capsule or capable of releasing specific growth factors.
In recent years, the use of tissue expanders has reached particular notoriety with the public because of the extensive coverage of the expander's use in several cases of separation of conjoined twins.
While Neumann originally had used a simple rubber balloon gradually filled with air,  recent technical advancements in the prostheses greatly have increased their reliability and the reproducibility of results. Austad and others described a self-inflating tissue expander composed of a sealed, semipermeable membrane shell containing hypertonic saline that caused continuous expansion using a relative osmotic gradient between the implant and the surrounding tissues. [3, 4]
However, most modern implants are made of silicone elastomers filled with interval injections of saline. These function as reservoirs that contain self-sealing injection ports and protective semirigid backings that permit percutaneous filling. While expanders with external reservoirs have been described as successful with few complications,  most are of the internal reservoir type. The ports often can be externalized to reduce pain and anxiety during filling, which seems to be tolerated particularly well in pediatric patients. In addition, while most expanders ultimately are removed or replaced with a permanent prosthesis (as in the case of breast reconstruction), implants that function to both expand the tissues and act as the final prosthesis have been developed (eg, Becker-type implant). 
Implants are available in a variety of volumes and shapes (eg, croissant shaped, rectangular, spherical) to provide the maximal surface area and direction of the flap developed for a specific region of the body or shape of the defect. The actual amount of tissue realized is only a fraction (approximately 35%) of the mathematically expected increase in surface area.  However, a clear difference was observed in the amount of tissue gained depending on the shape of the expander. In all cases, the importance of preoperative planning with special regard to flap design and consideration of the region of the body undergoing expansion together with its particular characteristics of the skin and adnexal structures to minimize risk and morbidity should be emphasized.
An example of a spherical tissue expander is shown in the image below.
Once an appropriate expander has been chosen and the proposed flaps raised, place the implant under the normal tissue, which lies adjacent to the defect. The flaps are developed in a variety of planes depending on the location and indication and may be submuscular, subgaleal, or subcutaneous. In general, the diameter of the base of the expander should be the same as that of the defect, while the pocket that is dissected must be larger than the unit. Most surgeons use blunt dissection (eg, fingers, urethral sounds, Metzenbaum tips) to develop the pocket to preserve the longitudinal blood supply vulnerable to sharp dissection. 
When available, use existing scars or incisions to minimize scarring and place them perpendicularly to the direction of expansion to reduce tension across the incision once it is closed and expansion begun. This allows for earlier expansion because the risk of dehiscence is reduced. In all cases, meticulous hemostasis must be achieved to prevent hematoma formation, which may contribute to pressure necrosis, possible infection, and the need to remove the expander.
Some surgeons prefer to fill the expander with a small amount of fluid at the initial placement, ensuring no tension on the suture line. The expanders then are filled periodically beginning approximately 2-3 weeks following placement and wound healing. Patient discomfort is a good indicator of the maximum volume with which an implant can be filled at one time. Other indicators used to guide the amount of filling include clinical examination of the overlying skin and the use of pressure transducers when patient feedback is unavailable. The latter method has been used in animal studies but not in clinical practice.
Rapid expansion performed according to accelerated schedules or continuously with pressure transducer monitoring can result in flaps sized similarly to those of traditional expansion in a shortened span of time.  Continue expansion until enough soft tissue to cover approximately twice the area of the defect has been achieved. Waiting 2-3 weeks after the goal volume has been achieved is customary to allow the expanded skin to soften. This may help reduce the amount of contraction encountered at the time of flap transposition. Interestingly, 2-3 times the recommended fill volumes may be instilled safely without adversely affecting the devices. In some cases, always following the recommendations of the manufacturer, the implant is hyperinflated to a few times the designated volume in order to achieve more expansion.
In any case, an adequate final outcome always depends on careful planning, meticulous technique, close follow-up, and patient compliance. The images below depict the stages of expansion and closure.
The overall rate of complications in tissue expansion has been relatively low. Complications result from the wound, the device itself, the process of expansion, the quality of the available tissues, and the execution of the wrong surgical technical plan.  Infection, hematoma, and seroma formation may occur in the wound, the device may fail during placement or filling, and the skin flaps can necrose. 
Most hematomas result from inadequate hemostasis at the time of placement but may be associated with the erosion of blood vessels with expansion. Seromas most commonly occur with breast reconstruction but may be minimized with partial insufflation of the implant at the time of placement to eliminate dead spaces in the dissected submuscular pocket. Drains commonly are placed intraoperatively and are removed as soon as the drainage becomes minimal. Similar criteria to removal as those used when discontinuing drains of other techniques of flap surgery may be applied.
The rate of infection with tissue expansion is relatively low. Despite the enhanced ability of expanded flaps to resist infection compared to other types of flaps, the presence of a foreign body predisposes that area to infection. This also may occur late in the process of expansion during the installation process. Severity ranges from a mild infection that may be treated nonoperatively, allowing completion of the expansion process, to a fulminant case necessitating removal of the prosthesis. Perioperative antibiotics routinely are given.
Exposure of the implant with or without flap necrosis is an indication to abort the procedure and remove the implant. This usually results from inadequate dissection of the implant pocket with later suture line breakdown. Healing is allowed to occur and the process is repeated at a later time when indicated. However, some surgeons may opt to continue the expansion process if they determine that the infectious process does not jeopardize the site. This practice is not standard and is based only on anecdotal experience and subjective interpretation. Minor complications include pain with filling of the prosthesis and temporary cosmetic deformity during the expansion phase.