Silicon Silicon is an essential element in humans, found in significant concentrations in hair, bone, epidermis and dental enamel. It is also the second most abundant element on the earths crust, constituting about 28% by weight.(cite) Many foods and beverages, including certain vegetables, grains, rice, and beer have been shown to contain significant amounts of silicon. Silicone is a synthetic form of silicon and includes 40% silicon by weight. The silicones are synthetic polymers and are not therefore found naturally. They have a linear, repeating silicon-oxygen backbone akin to silica.
However, organic groups attached directly to the silicon atoms by carbon-silicon bonds prevent formation of the three-dimensional network found in silica. These types of compound are also known as polyorganosiloxanes. Certain organic groups can be used to link two or more of these silicon-oxygen backbones and the nature and extent of this cross linking enables a wide variety of products to be manufactured.(cite) The most important materials used in medical implants are fluids, gels and rubbers (elastomers) whose physical and chemical properties include, amongst others, a high degree of chemical inertness, thermal stability and resistance to oxidation. Silicone is used by many prosthesis, medical devices, and pharmaceutical products. The many silicone containing medical devices include artificial heart valves, artificial joints, Norplant contraceptive implants, pacemaker wires, and dialysis tubing. Of course silicone is probably best known for its use in breast implants.
In 1992 the FDA pulled silicone-gel filled breast implants off the market as they were alleged to cause “connective-tissue disorders such as systemic lupus erthematosus, rheumatoid arthritis and scleroderma, a hardening of the skin.”(cite) Recent studies have disproven this, showing that connective-tissue diseases were no more common in women with implants than those without. Also a study by the U.S. National Cancer Institute showed a lower cnacer risk amoung women However, tests looking with “reliable, validated analytical techniques for the dissemination of silicones from implants in the body, including breakdown products of the polymers, have shown either no dissemination, or the presence of only very small amounts at distant sites following rupture of gel-filled implants, or after deliberate injection of the gel.”(cite) The risks of these implants, as shown in laboratory studies as well as in real life, are local inflammatory and scarring reactions, and local infection, as around any foreign body in the tissues. If a silicone fluid is released from a ruptured gel-containing implant, the inflammatory and fibrotic reaction will affect a wider area. There is no evidence of any type of”systemic reaction, or of abnormalities of the immune system in subjects who have received implants.”(cite) Perhaps one of the best known biomaterials today is titanium and its alloys.
Commercially pure titanium, also known as F67, is non-magnetic, and there is no harmful additives or alloying. The most common alloy used is called F136, or Ti-6Al-4V. This alloy is an alpha-beta alloy, meaning the properties will vary depending on treatments. However usually this alloy is corrosion resistant but not ware-resistant and has a higher strength than when in its pure form. The major drawback of this alloy is in its long-term usage.
The vanadium is biocompatible only in the short term.(3,pg. 2) There are four grades of titanium, 1-4 with four being the strongest but least ductile. The amount of oxygen in the CP titanium is a major force on how strong the yield and fatigue strengths will be, and also determines the grade of the alloy. Titanium demonstrates exceptional resistance to a broad range of acids, alkalis, natural waters and industrial chemicals. It also offers superior resistance to erosion, cavitation or impingement attack. Titanium is at least 20 times more erosion resistant than the coppernickel alloys.(cite) The low density of titanium makes it significantly lighter when compared to the stainless steels and cobalt-alloys.
The densities of titanium-based alloys range between .160 lb/in3 and .175 lb/in3. Titanium also has a higher fatigue strength than many other metals. Yield strengths range from 25,000 psi commercially pure(CP) Grade 1 to above 200,000 psi for heat treated beta alloys. (cite) The combination of high strength and low density results in exceptionally favorable strength-to-weight ratios for titanium-based alloys. These ratios are superior to almost all other metals and become important in such applications as the surgical implants in the plastic and reconstructive surgery fields of medicine. Titaniums higher strength permits the use of thinner walled equipment.
Due to the difficulty in electropolishing titanium, it is anodized, this is an electrochemical process which increases the thickness of the oxide film that lies on titanium. Here is where the colors that are associated with titanium, most often gold, is produced. In addition, the unique qualities of titanium prove to be MRI and CT compatible. Titanium is a standard material for orthopedic devices such as hip joints, knee joints, bone screws, bone plates, and dental implants. Titanium screws and plates are devices which have revolutionized the reconstruction of the crainiofacial skeleton. The plates are reasonably easy to bend, but provide rigid fixation for bony tissues.
It is used due to the outstanding strength-to-weight ratio of the material and its immunity to body fluids. The body readily accepts titanium since it is more biocompatible than stainless steel or cobalt chrome, thus making it an ideal bioimplant. Yet there have been some problems. For example in crainiofacial surgery, titanium plates are used. Usually these titanium fixation plates are not removed after osteosynthesis, because of the high biocompatability and high corrosion resistance characteristics, mentioned earlier.
But, experiments with laboratory animals, and limited studies of analyses of human tissues, have reported evidence of titanium release into local and distant tissues.(cite) Energy dispersive x-ray analysis, scanning electron microscopy, and electrothermal atomic absorption spectrophotometry were used to detect trace amounts of titanium in surrounding soft tissues. A single metal inclusion was detected by scanning electron microscopy and energy dispersive x-ray analysis in one patient, whereas, electrothermal atomic absorption spectrophotometry analyses revealed titanium present in three of four specimens in levels ranging from 7.92 to 31.8 g/gm of dry tissue.(cite) Results studies revealed trace amounts of titanium in tissues surrounding the crainiofacial plates. Perhaps this will cause us to rethink the supposed high biocompatability of titanium over time. If a titanium or silicon implant shifts slightly out of alignment, a second operation my be necessary to replace it in its proper position. Every surgery carries risks, and should be avoided if possible.
As with any surgery, complications can occur. These may include infection; excessive bleeding, such as hematomas (pooling of blood beneath the skin); significant bruising and wound-healing difficulties; and problems related to anesthesia and surgery. Infection can occur with any operation. If infection were to occur around a implant and did not clear up after treatment with antibiotics, the implant might have to be temporarily removed and replaced at a later time. There are a number of factors that may increase the risk of complications in the actual healing. In general, a patient is considered to be a higher risk if he or she is a smoker; has a connective-tissue disease; has areas of damaged skin from radiation therapy; has decreased circulation to the surgical area; has HIV or an impaired immune system; or has poor nutrition. If you regularly take aspirin or some other medication that affects blood clotting, your risks are further increased.