Background

The number of women in the United States who have breast implants is unknown, but current estimates derived from national surveys range up to more than 6 million. This represents more than 5% of the adult female population. The American Society of Plastic Surgeons (ASPS) collects information annually on plastic surgery procedures performed by its members. In 2008, approximately 307,000 women received breast implants for cosmetic breast augmentation and 111,000 for reconstruction of congenital or postmastectomy deformities.^[1] These data do not include those procedures (mostly cosmetic) performed by non plastic surgeons such as otolaryngologists, general surgeons, gynecologists, and others.

Following adverse publicity in the early 1990s, interest in the procedure fell significantly, especially for gel-filled devices, but it seems to have recovered incrementally. According to the ASPS, in 2008, 53% of total breast implants were filled with saline; 47% were filled with silicone.^{[1, 2]}

Prior to 1963, various plastic foam materials were used; for breast augmentation, however, it became apparent that the air cells would collapse and, combined with tissue ingrowth, shrink and harden the device. These materials were wrapped in plastic film to minimize this effect to no avail. Amazingly, some of these implants were so well tolerated that they have stayed in place to this day.

The modern silicone breast implant has been available since 1963 and has gone through an evolution of change and improvement. Several types of devices, with many variations, shapes and styles within each class, are now available or under testing for US Food and Drug Administration (FDA) approval. Basic to all implants is a silicone rubber (elastomer) shell, which can be single or double, smooth or textured, barrier-coated, or covered with polyurethane foam. The foam-covered devices have not been available in the United States since 1990 but are still marketed in Europe.

The contents are either factory-filled with silicone gel of various consistencies or inflated at surgery with normal saline. One brand that was manufactured overseas was prefilled with saline at the factory. It was briefly marketed in the United States but was later withdrawn when the FDA denied approval.

The double-lumen devices consist of concentric balloons that contain silicone in one chamber and saline in the other. The only one still in use is the Becker, which has an outer layer of gel and an inner balloon that is valved to permit postoperative gradual inflation with saline. This is termed a “permanent tissue expander,” since it permits gradual and temporary overinflation to create the pocket and then can be left in as a permanent implant after the size is adjusted appropriately. At this writing, the Becker devices are not generally available in the United States until current FDA mandated studies are evaluated.^[3] For more information on expandable implants, see eMedicine article Uses of the Postoperatively Adjustable Implant in Aesthetic Breast Surgery.

In 1990, the FDA placed a moratorium on gel-filled implant use for cosmetic augmentation. They remained available for reconstruction and replacement, but mandated extensive record keeping, follow-up, and IRB approval were required for use. In 2006, after extensive study and analysis, the FDA deemed the device safe for all augmentation and reconstructive purposes, but they continue to require tracking of patients.

Saline Implants

Saline filled implants are available as empty silicone balloonlike devices to be filled with normal saline at the time of surgery. This permits subtle size adjustments to compensate for asymmetry between the breasts. They are less popular than silicone implants, as they often may have a less natural feel. If the patient has very little breast tissue or only a skin covering after mastectomy, unsightly wrinkles and folds of the device may be visible on the breast. This is more common when the surface is textured.

Silicone Gel Implants

Three generations of basic design of this device have been created, with many variations within each type.

First generation

The first models to be marketed had envelopes of thick, smooth-walled silicone elastomer made in 2 sections, filled with a viscous silicone gel material (dimethylsiloxane) and glued together. They were available in only three sizes: small, medium, and large. In the first few years, surgeons believed that the device required attachment to the tissues to prevent migration. Scar ingrowth for fixation was accomplished by patches of material (eg, Dacron mesh or perforated silicone) attached to the back of the device or by an outer covering of polyurethane foam. The Dacron and silicone patches were subsequently found to be unnecessary; they actually detracted from the quality of the result. Some patches or tabs created a stress point that led to tears of the envelope. Fixation patches were eliminated in the early 1970s.

Second generation

Manufacturers varied the gel consistency and shell thickness in an attempt to improve performance. Beginning in the mid-1970s, the shells were made thinner and the gel less viscous (ie, more “responsive”), primarily in an ill-conceived attempt to control hardening from scar shrinkage (capsular contracture.) This trend reversed in the early 1980s when it was recognized as not effective in reducing contracture and as resulting in a more fragile device. Most were broken 10 years later.

Third generation

New formulations of the shell and gel contents became available that were stronger and had a second barrier coat of diphenyl silicone. This coating almost totally eliminated so-called “gel bleed” or diffusion of small amounts of the silicone oil through the implant shell. The gel content also was made more viscous and cohesive.

In 1989, textured-surface shells that many surgeons hoped would minimize the incidence of unwanted firmness from capsular contracture became available. Recent studies are somewhat confusing regarding whether this was effective. The textured implants had the disadvantage of a higher rupture rate than the more traditional smooth shells and often produced visible wrinkles in the breast in women with very little overlying tissue to mask the ripples. Because of these shortcomings and lack of solid evidence that these devices were softer, they have become much less popular in recent years.

Polyurethane-covered implants

In the late 1960s, a variation of the device was developed containing a polyurethane sponge coating over an otherwise standard gel-filled implant. Although the coating originally was planned as a fixation layer, many surgeons came to believe that the foam cover resulted in a decreased incidence (or at least a delayed onset) of capsular contracture. These implants also evolved in shape and design, culminating in the early 1980s with the Meme and Optimam styles. In April 1991, the manufacturer voluntarily withdrew the foam-covered implants from the market.^[4]

One style, the MemeME, had a unique construction. It had no true shell, but a skin of sorts was formed in situ by spraying the surface with silicone containing extra catalyst prior to curing. This increased the crosslinking of the surface to create a shell-like membrane. The polyurethane foam was then shaped and sealed over the surface. Implants of this particular type were known to occasionally extrude some of their gel contents through the foam when squeezed. This is a possible explanation for reports of blood being found within the substance of the gel in apparently intact implants. The MemeME model was marketed from 1983-1988.

Other filler materials

While silicone remains the only available shell material, new filler substances were in use in Europe and South America and, at one time, were under development or in experimental trial in the United States. Various hydrogels and a pure form of triglycerides were the 2 main formulations. The major advantage of the triglyceride formulation (Trilucent) was that it had a Z number (measure of radiolucency) similar to that of fat, thus resulting in little or no compromise of mammography. Another fill substance, polyvinyl pyrrolidone in saline, was briefly available, under the trade name Misty Gold. None of these products is currently available in the United States. At this time, only silicone gel or saline-filled models are available for use in the United States.

Recent developments

In late 2006, a new formulation of silicone gel filler called MemoryGel (Mentor Corp, Santa Barbara, Calif) gained FDA approval.^[5] This gel implant is thicker and more cohesive so as to minimize gel spread in the case of rupture and to resist scar shrinkage that would deform its contour. When cut, the gel retains its shape and doesn’t run. This device has a doughy feel to it.

Currently available devices in the United States are saline- or silicone-filled implants with either textured or smooth surfaces. They come in round or tear drop shapes with a choice of 3 different projections. Only 2 companies, Mentor Corporation and Allergen (a successor to McGhan and Inamed), have FDA approval to market these devices in the United States.

The implants produced currently are much improved devices compared to earlier units. The shell is still made of an outer layer of a mix of dimethyl siloxane and amorphous silica with an inner barrier coat of diphenyl siloxane to minimize silicone gel bleed. The shells, on testing for breakage, exceed the American Society for Testing and Materials (ASTM) requirements by more than 300%. The gel is more cohesive, varying from a standard 60% crosslinking to 80% for the more cohesive type nicknamed “gummy bear” (because of is consistency similar to the candy).

As evidenced by sales figures prior to the moratorium, and now following their release for cosmetic purposes, approximately 80-85% of surgeons and patients prefer the quality of results obtained by gel implants, making them the implants of choice. In 1997, sales figures for Europe, where usage was unrestricted, show a distribution of 70% for gel, 15% for saline, and 15% for alternate fills such as triglycerides (then still available) and hydrogels for cosmetic use.

Safety

Silicone is probably the most studied implantable material available today. After over 35 well-conducted studies from many countries, it seems certain that this material does not cause disease. The results of more than 7 long-term follow-up studies show that women with implants have a reduced incidence of breast cancer than is otherwise expected in the general population. No hard evidence reveals that a broken implant is harmful. Almost all of the problems that can occur with breast implants, such as infection, hardening, extrusion, and malposition are related to the surgical procedure or the patient’s own biology, not the device.