Biomedical Engineering Reference
In-Depth Information
Tabl e 4. 4
In vitro dissolution of polished discs of Ti6Al4V at pH
D
7.6 in (a) Hanks' (H), (b) H
C
EDTA, (c) H
C
l-leucine, (d) H
C
citrate was followed over a period of 12 weeks; exposed
surface was 5 mm
2
/ml supernatant; concentrations of the supernatant solution at weeks 2 and 12
are given in ppb
Metal
a
b
c
d
Ti
<
4
23-234
<
4
156-752
Al
<
2
79-238
<
2
51-339
V
<
2
<
2
<
2
6-357
fluid
(SBF).
14
But in an in vitro simulation experiment, addition to Hanks' of a
selection of complex forming agents changes the picture completely. EDTA has no
biologic roots but is a 'wide spectrum' complex former; l-leucine is an essential
amino acid and present in plasma; citrate is an intermediate in the citric acid cycle
and a good complex former.
15
The results are summarized in Table
4.4
.
Simulated body fluid was and still is today widely used to simulate in vitro
the chemical behavior of materials in vivo. It is instructive to analyze the data of
Tab le
4.4
: (1) they demonstrate that different dissolution chemistry is provoked
by subtle changes in composition of the simulated body fluid, so do not trust too
much tests based on too simple simulations; (2) the dissolution rates are very much
dependent on the ligands the alloy is in contact with and thus, time is an important
parameter. This explains the in vivo observation that the relative concentrations in
solution do not reflect the composition of the alloys.
Aluminum is a nonessential element but there are enough question marks about
its interactions with the body to remain wary when it circulates in our body. Healthy
individuals have formidable barriers toward aluminum absorption. At physiological
pH, it exists as the highly insoluble Al(OH)
3
and at first sight unavailable for inter-
action with the body. A characteristic is that, aside from most essential elements,
the rate of ligand exchange in and out the coordination sphere is slow, for instance
ten times slower than Fe
3C
or a ten million times slower than Ca
2C
. Once linked
to a ligand it starts circulating in the body. The most likely vehicles are citrate and
transferrin. Transferrin has the highest affinity for Al and its ligand sites are only for
30% occupied by iron. This does not preclude that citrate acts as a go-between and
presents Al to transferrin. It accumulated, however, in bone tissue of renal failure
patients. Already in 1972, neurological disorders in chronic hemodialysis patients,
termed
dementia dialysis
, were recognized as a consequence of aluminum accu-
mulation in the dialysate. The elevated occurrence of dementia in England, United
States and Scotland were also attributed to aluminum contaminated tapwater [
160
-
164
]. The most intriguing aspect of the element's toxicity is the potential implication
in the pathogenesis of Alzheimer's disease. However, the active role of aluminum
in this pathogenesis is not confirmed though not entirely excluded either.
14
The composition of Hanks' is tabulated in Appendix D
.
15
The citric acid cycle is the only metabolic pathway that the heart can use for oxidative
metabolism.
