Biomedical Engineering Reference
In-Depth Information
it replaces phosphate. Although similar to molybdenum, W acts antagonistically to
Mo, decreasing the sulfite and xanthine oxidase activity. It is also proved to accel-
erate the development of mammary cancer in rats but is not, in as far as we know,
documented for humans [
159
].
We are not aware of recent epidemiological studies in humans or other studies on
tungsten that allow to make assumptions on implant-related adverse effects. Both
Mo and W have a quick turn-over in tissue and blood. Problems with these metals
when oxidized from an implant alloy are not expected. They were discussed at some
length here because, in particular, molybdenum is part of an important enzyme co-
factor and tungsten acts antagonistically to molybdenum.
Manganese.
This element is in stainless steel as well as in CoCr alloys a minor
key element but present in concentration below 2%. It is biologically an essential
element and acts as a cofactor in many enzymes. Overt signs of intoxication occur
after months to years and it is known and extensively documented for persons who
get intoxicated by inhalation of airborne manganese particles.
13
The initial expres-
sion of intoxication is not nothing and characterized by psychiatric disorders known
as
manganism
, often ending in an irreversible brain damage. Most of the data on
levels of tolerance are relative to inhalation. Data on
T
1=2
after intravenous admin-
istration are not known. Accumulation in the body or incidence of any deleterious
effect attributed to implant-related manganese remains for the time being unclear.
4.5.3
Titanium Alloys
As discussed in Chap.
2
, the contact of tissue with titanium implants is with the
oxide surface layer. Wear debris is composed mainly of TiO
2
. A serious chemical
effort is needed to dissolve this oxide but, as already stated, the body is an effi-
cient chemical war machine. The attack is occasionally wear-assisted in load bearing
implants and, in case of long-term implants, the body has ample time to act.
Titanium is not an essential element and is considered as biologically neutral. It
is the ninth most abundant element in nature and is present everywhere in nature
and in our body at low concentration. So, it is not surprising that the daily intake
is something like 0.3-1 mg. The tissue concentration of patients bearing titanium
implants is enhanced but the element has no intrinsic toxicity. The main if not the
only concern is wear debris, but that will be discussed later. So, no intrinsic toxicity
problem with implants made of pure titanium.
Aluminum.
Aluminum is another history. Till today, the most widely used tita-
nium alloys contain aluminum: Ti6Al4V, Ti6Al7Nb (for composition, see Table
2.3
).
Ion release in vitro varies strongly with composition of contacting solution. A pol-
ished surface of Ti6Al4V is quasi-inert in contact with Hanks', a
simulated body
13
Manganese dust in mines, as oxide or hydroxy-oxides, but a recent airborne threat was the use
of a manganese containing compound as antiknock agent in gasoline.
