Silicon

Silicon is an essential element in humans, found in significant concentrationsin hair, bone, epidermis and dental enamel. It is also the second most abundantelement on the earths crust, constituting about 28% by weight.(cite) Manyfoods and beverages, including certain vegetables, grains, rice, and beer havebeen shown to contain significant amounts of silicon. Silicone is a syntheticform of silicon and includes 40% silicon by weight. The silicones are syntheticpolymers and are not therefore found naturally. They have a linear, repeatingsilicon-oxygen backbone akin to silica.

However, organic groups attacheddirectly to the silicon atoms by carbon-silicon bonds prevent formation of thethree-dimensional network found in silica. These types of compound are alsoknown as polyorganosiloxanes. Certain organic groups can be used to link two ormore of these silicon-oxygen backbones and the nature and extent of this crosslinking enables a wide variety of products to be manufactured.(cite) The mostimportant materials used in medical implants are fluids, gels and rubbers (elastomers)whose physical and chemical properties include, amongst others, a high degree ofchemical inertness, thermal stability and resistance to oxidation. Silicone isused by many prosthesis, medical devices, and pharmaceutical products. The manysilicone containing medical devices include artificial heart valves, artificialjoints, Norplant contraceptive implants, pacemaker wires, and dialysis tubing.Of course silicone is probably best known for its use in breast implants. In1992 the FDA pulled silicone-gel filled breast implants off the market as theywere alleged to cause connective-tissue disorders such as systemic lupuserthematosus, rheumatoid arthritis and scleroderma, a hardening of theskin.

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(cite) Recent studies have disproven this, showing thatconnective-tissue diseases were no more common in women with implants than thosewithout. Also a study by the U.S. National Cancer Institute showed a lowercnacer risk amoung women However, tests looking with reliable, validatedanalytical techniques for the dissemination of silicones from implants in thebody, including breakdown products of the polymers, have shown either nodissemination, or the presence of only very small amounts at distant sitesfollowing rupture of gel-filled implants, or after deliberate injection of thegel.(cite) The risks of these implants, as shown in laboratory studies aswell as in real life, are local inflammatory and scarring reactions, and localinfection, as around any foreign body in the tissues. If a silicone fluid isreleased from a ruptured gel-containing implant, the inflammatory and fibroticreaction will affect a wider area. There is no evidence of any type ofsystemic reaction, or of abnormalities of the immune system in subjects whohave received implants.

(cite) Perhaps one of the best known biomaterialstoday 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 commonalloy 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 thisalloy is corrosion resistant but not ware-resistant and has a higher strengththan when in its pure form. The major drawback of this alloy is in its long-termusage. The vanadium is biocompatible only in the short term.(3,pg. 2) There arefour 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 yieldand 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 toerosion, cavitation or impingement attack. Titanium is at least 20 times moreerosion resistant than the coppernickel alloys.(cite) The low density oftitanium makes it significantly lighter when compared to the stainless steelsand cobalt-alloys. The densities of titanium-based alloys range between .160lb/in3 and .175 lb/in3. Titanium also has a higher fatigue strength than manyother metals. Yield strengths range from 25,000 psi commercially pure(CP) Grade1 to above 200,000 psi for heat treated beta alloys.

(cite) The combination ofhigh strength and low density results in exceptionally favorablestrength-to-weight ratios for titanium-based alloys. These ratios are superiorto almost all other metals and become important in such applications as thesurgical implants in the plastic and reconstructive surgery fields of medicine.Titaniums higher strength permits the use of thinner walled equipment. Due tothe difficulty in electropolishing titanium, it is anodized, this is anelectrochemical process which increases the thickness of the oxide film thatlies on titanium. Here is where the colors that are associated with titanium,most often gold, is produced. In addition, the unique qualities of titaniumprove to be MRI and CT compatible. Titanium is a standard material fororthopedic devices such as hip joints, knee joints, bone screws, bone plates,and dental implants.

Titanium screws and plates are devices which haverevolutionized the reconstruction of the crainiofacial skeleton. The plates arereasonably easy to bend, but provide rigid fixation for bony tissues. It is useddue to the outstanding strength-to-weight ratio of the material and its immunityto body fluids. The body readily accepts titanium since it is more biocompatiblethan stainless steel or cobalt chrome, thus making it an ideal bioimplant.

Yetthere have been some problems. For example in crainiofacial surgery, titaniumplates are used. Usually these titanium fixation plates are not removed afterosteosynthesis, because of the high biocompatability and high corrosionresistance characteristics, mentioned earlier.

But, experiments with laboratoryanimals, and limited studies of analyses of human tissues, have reportedevidence of titanium release into local and distant tissues.(cite) Energydispersive x-ray analysis, scanning electron microscopy, and electrothermalatomic absorption spectrophotometry were used to detect trace amounts oftitanium in surrounding soft tissues. A single metal inclusion was detected byscanning electron microscopy and energy dispersive x-ray analysis in onepatient, whereas, electrothermal atomic absorption spectrophotometry analysesrevealed titanium present in three of four specimens in levels ranging from 7.92to 31.8 g/gm of dry tissue.(cite) Results studies revealed trace amounts oftitanium in tissues surrounding the crainiofacial plates. Perhaps this willcause us to rethink the supposed high biocompatability of titanium over time.

Ifa titanium or silicon implant shifts slightly out of alignment, a secondoperation my be necessary to replace it in its proper position. Every surgerycarries risks, and should be avoided if possible. As with any surgery,complications can occur. These may include infection; excessive bleeding, suchas hematomas (pooling of blood beneath the skin); significant bruising andwound-healing difficulties; and problems related to anesthesia and surgery.

Infection can occur with any operation. If infection were to occur around aimplant and did not clear up after treatment with antibiotics, the implant mighthave to be temporarily removed and replaced at a later time. There are a numberof factors that may increase the risk of complications in the actual healing. Ingeneral, 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 radiationtherapy; has decreased circulation to the surgical area; has HIV or an impairedimmune system; or has poor nutrition. If you regularly take aspirin or someother medication that affects blood clotting, your risks are further increased.Chemistry