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.

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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.