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 earth’s 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.
Titanium’s 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 µg/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.