Biomedical Engineering Reference
In-Depth Information
Fig. 4.11
Pseudotumor close
to a titanium hip stem:
(
a
) black staining by titanium
wear particles;
(
b
) pseudotumor. Courtesy
Prof. J.-P. Simon
(UZ-Pellenberg)
Revision rates for tumors for all patients of a follow-up study after 8 years were
reported to average 4%, but was 0.5% for men, whereas it was 6% for women over
40 years old and 13% for women under 40 years after six years [
170
]! Concentra-
tions of Co and Cr are multiples of what was/is common for modern but even for
older types of total hip replacements [
171
].
The causes are not well defined and a review allowing a reasonable conclusion is
not obvious yet (see f.i. the recent paper [
172
]). The difference in intrinsic proper-
ties as wear and corrosion resistance of the materials used for metal-on-metal THPs
or ASRs cannot account for the present problems. Remains mainly (only?) a biome-
chanical facet: a small margin of error on orientation, i.e., abduction and anteversion
angles, and/or mechanical design constraints may be the cause of excessive wear,
producing an amount of debris and, as inescapable consequence, enhanced metal
ion concentrations beyond the control capabilities of homeostatic mechanisms. One
manufacturer already stopped the production of his ASR. It is reminiscent of another
dramatic case, the Prozyr
R
cups, cited in Chap.
9
.
Scanning diagonally through the literature of the last seven years, two different
conclusions can be drawn. In as far as the enhanced cobalt levels of serum or whole
blood for the second generation of metal-on-metal total hip arthroplasties are con-
cerned, there is reason for concern but no (not yet?) clear proof that it is associated
with hypersensitivity or early osteolysis. The situation is, however, worse for resur-
facing where accurate acetabular positioning seems to be the main complicating
factor [
173
-
179
]. So, wait and see how this issue develops in the years to come.
4.7
And the Answer Is?
The answer to the question in the chapter's heading is a
conditional no
.
We often wondered why implants, subjected to a hostile chemical environment,
to wear and formation of debris, corrosion, systemic delivery to the body fluids of
metal ions intensely interacting with enzymatic systems, ions which are potentially
toxic or cancerogenic and so on and so forth, do relatively speaking so little harm.
