Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 314

paired with most other orthopaedic alloys such as cobalt chromium and
titanium and is particularly prone to crevice corrosion.
PROBLEM 12.12
Two different implant manufacturers market a stainless steel humeral
nail with comparable nail and screw geometries, and which both adhere
to ASTM F138 in terms of elemental composition. It is assumed that the
corrosion response of both implants will be similar. True or false?
ANSWER:
False. While the final components may, in fact, have the same corro-
sive potential, there are other variables that can dramatically affect this
outcome. For example, if the material is improperly heat treated by the
suppliers, intergranular corrosion may be exacerbated if chromium is
depleted from regions near the grain boundaries.
PROBLEM 12.13
A modular hip arthroplasty implant is composed of a cobalt chrome
stem, a cobalt chrome head, a polyethylene insert, and a titanium ace-
tabular shell. Would the following forms of corrosion be of primary
concern?
1. Crevice corrosion
2. Intergranular corrosion
3. Galvanic corrosion
4. Fretting corrosion
5. Stress corrosion
ANSWER:
Yes, No, No, Yes, Yes
Because of the modularity of the cobalt chromium stem/head pair,
crevice corrosion and fretting corrosion will be relevant. Stress cor-
rosion is always a concern in a load-bearing implant. Intergranular
corrosion is less of a concern with titanium and cobalt chromium
implants than those composed of stainless steel. Also, because there
is a polyethylene insert separating the titanium shell and the cobalt
chromium head, galvanic corrosion will be limited barring significant
rim impingement or wear through of the liner, which would present
other concerns as well.
annotated bibliography
1. Drummond JL: Degradation of ceramic materials in physiological
media. In Rubin LR (ed): Biomaterials in Reconstructive Surgery . CV
Mosby, St. Louis, 1983.
A brief but useful review.