Silicon: Silicon is a metal in the same column as carbon in the periodic table. It is the most abundant element on earth and does not occur naturally in its pure metallic state.
Silica: Silica in its crystalline form is common sand, marble, or quartz. It also occurs in an amorphous form. Very fine, extremely pure, amorphous silica is used as a filler to strengthen solid silicone, such as in the shell of an implant. Each grain of silica is encapsulated tightly in silicone so that even when the elastomer is abraded or torn, no silica is exposed to the body.
Silicate: In one form, its hydroscopic properties are used to keep the contents of containers dry.
Silicone: Substances known as silicones are polymers of silicon and oxygen. Silicone has as many forms as its carbon-based sister, oil. Like salad oils versus motor oils, not all are fit for human consumption. Dimethylsiloxane is the building block for most medical-grade silicone products, including breast implants. It can be made extremely pure and modified into products with a multitude of characteristics (see the image below). The molecular structure of silicone.
Keep in mind the difference between elemental silicon and the polymer silicone. Medical-grade silicone is usually a specific, very pure polymer of silicon and oxygen with methyl side groups (dimethylsiloxane). It is one of the least bioreactive materials available for use in medical devices. The shell is made of a rubberlike membrane of fully polymerized silicone with an amorphous (noncrystalline) silica filler added for strength.
Until the moratorium, most implants used were filled with a silicone gel, the physical form of which can be likened to a spongelike matrix or 3-dimensional net filled with various chain-length silicone oils. These form a physical chemical bond resulting in a gel. The shell membrane is slightly permeable to the oils.
Depending upon the brand, age, characteristics, and environmental mechanics of a particular device, small amounts of the oil diffuse or bleed through the shell. For most implants, this is a matter of a few grams. Newer barrier coat devices introduced in the early 1980s bleed at as little as one tenth the rate of the older materials.
This leakage of silicone should be viewed in perspective. Medical-grade silicone is ubiquitous in the environment, and probably everyone in the civilized world has some form of silicone in his or her body. For example, every disposable needle and syringe, as well as intravenous tubing, is lubricated with silicone. (The FDA permits up to 1 mg/cm2 of barrel surface.)
Medications in stoppered vials contain residual silicone from its use in the manufacturing process. Silicone is hydrophobic and lipophilic; thus, various amounts may be injected along with the medication depending upon the lipid characteristics of the drug used. Because insulin binds to silicone, extrapolative calculations suggest that patients with type I diabetes may inject as much as 25-30 g of silicone over a lifetime.
In its solid form, silicone elastomers are used for pacemaker coatings, tubing, prosthetic joints, hydrocephalus shunts, penile implants, and as the envelope for Norplant and other implanted drug delivery systems. Some testicular and chin implants are similar to breast implants, since both usually are made of a silicone gel in a silicone envelope.
More than 1000 medical products contain silicone as either a component or as a residuum from use in the manufacturing process. Silicone is a nonspecific term for a class of compounds, some of which are highly reactive or toxic. The generic term silicone is similar to the generic term oil, which can include both salad oil and motor oil. Within the subclass of medical-grade material, the formulations vary to some degree with intended use. The body may react differently to some of these formulations.
The designation methicone (as in simethicone or dimethicone) as an ingredient in any medication is simply silicone formulated to comply with FDA regulations for human consumption in items such as medication, foods, and cosmetics. Calcium carbonate, magnesia, simethicone antacid (Di-Gel), and oral simethicone (Mylicon) are examples of medications containing silicone that are marketed over the counter, even in pediatric formulations, with FDA approval. Silicones are used in lipstick, hairspray, food processing, skin creams, and cosmetics and are known to be absorbed through both the bowel and the lungs.
Biologically, medical-grade silicones invoke a straightforward, nonspecific foreign body response, resulting in typical macrophage invasion, giant cell formation, and eventual scarring. Several animal studies suggest that relatively huge volumes of gel injected into the peritoneal cavity of rodents may stimulate an immune response. This is not observed with the oil or solid elastomers. It can be demonstrated only by emulsifying the gel, a condition not seen in the implant. Intact gel does not lend itself to these test procedures.
Despite the many reports in the media, exhaustive evaluations by multiple prestigious scientific bodies such as the Institute of Medicine, the British Ministry of Health, the Spanish Government, a committee of the European Union (EQUAM), Harvard University, the Mayo Clinic, and multiple panels of experts established by various courts have confirmed that no evidence exists of any known or new systemic illness definitively attributed to silicones.