Biomedical Engineering Reference
In-Depth Information
fixed but are subjected to small-amplitude sliding (
micro-movements
) as we already
pointed out. The movement gives rise to mostly mild wear. The worn areas are
many square millimeters if not square centimeters wide and, although present on,
mechanically speaking, “logic” sites, it is not what is commonly understood by
localized corrosion (matter of scale!). Most cemented hip stems are sandblasted
and the said sites are polished accompanied by removal of oxide. One should expect
fast progress of corrosion but it does not. We consider concentrations below or less
than ten times the normal standard levels as a balance between slow dissolution of
the passive metal and evacuation. We come back on evacuation times after removal
of the implant and cases of reactions to ions or debris in Chap.
4
. The monitoring of
highly worn CoCr implants is another story (values in bold). These cases concerned
patients having a broken ceramic head, replaced afterward by a metallic one with-
out replacement of the polyethylene acetabulum. Remaining ceramic wear debris
provoked high wear and subsequent extraordinary dissolution.
Monitoring patients having a titanium alloy THR showed for at least some time-
enhanced but constant blood levels of titanium, vanadium and aluminium without
visible localized effect on the stems. This should also attributed to the balance
between constant supply of these ions by uniform corrosion and evacuation.
Is the foregoing discussion becoming obsolete after so many years of research
and use of successful alloys such as CoCrMo and titanium, as well as progres-
sively more positive implant results? Still today an estimated 20% of human patients
require revision surgery for various reasons, aseptic loosening being the most fre-
quent one. The answer is “no” and this statement is refueled by recent studies. One
of them is the evaluation of CoCrMo implants by Hodgson and colleagues in sheep
and in simulated body fluid: their results are completely aligned with our own expe-
rience and with the results of Hildebrand's study 11 years earlier [
95
,
96
]. Metal
trace levels in blood and tissues are enhanced but not in the same ratio as in the
alloy. From well-fixed to fixed implants, the ratio for cobalt with respect to un-
implanted sheep ranges from 10 to 5,000, the loose from 7,000 to 50.000. Cobalt
concentrations always exceed largely the reference values, followed by chromium
and molybdenum. The results are explained in terms of uniform corrosion combined
with tribocorrosion. The nonstoichiometric dissolution of cobalt can be simulated in
vitro by cyclic polarization between cathodic and anodic potentials. This observa-
tion fits the continuous activation-repassivation cycles in vivo and completes the
picture.
Localized Corrosion
We just made reference to dissolved oxygen. Oxygen pressure above a liquid, and
consequently its concentration in the liquid, is dominating the equilibrium of the
cathodic reaction of (
3.13
), hence, an important partner in this business. In the
second point, at the start of this chapter, we stated that:
Corrosion observed on one spot can simply be the result of a wrong environment at the
distant end of a metal object. Metals are highways for electric currents.
