Biomedical Engineering Reference
In-Depth Information
c) Biocompatibility
Titanium is a very reactive metal. Fortunately, it quickly reacts with oxygen to
form, self healing, hard titanium oxide film which inhibits corrosion. Thus,
titanium is the most corrosion resistant non noble metal approaching the overall
corrosion resistance of platinum. It is particularly corrosion resistant to sea water
as is its Ti-6Al-4Va-ELI alloy used in medical applications [5]. This superior
corrosion resistance produces a high level of biocompatibility.
Further, titanium ions are not toxic even in large quantities [2]. It is estimated
that almost a milligram of titanium is ingested by humans each day but most is
excreted without being absorbed.
The authors have noted in the retrieval of titanium and Co-Cr Implants fixed by
biological ingrowth that titanium ingrowth occurs over almost the entire fixation
surfaces and that there is direct contact between the titanium and bone. Ingrowth
on the Co-Cr alloy fixation surfaces, on the other hand, is limited to a few small
areas, like “spot welding”. Even in these areas microscopic examination shows no
direct contact of Co-Cr alloy and bone, but rather, a thin fibrous tissue layer is
interposed between metal and bone. Titanium and its alloys are the most
biocompatible of the materials used for orthopaedic implants.
1.5.1.2 Cobalt Chromium Alloys
a) Introduction
Cobalt alloys have been used in the body at least since the 1930's, after an alloy
called Vitallium, a variation of the alloy called “Haynes Stellite” was developed in
the 1920's. There are three types of cobalt alloys in general use. There are wear
resistant, temperature resistant and corrosion resistant alloys. This section will
deal only with the corrosion resistant alloys, such as Vitallium.
Parts made from such alloys were generally made as investment castings since
originally it was not practical to machine such alloys due to their hardness and
brittleness. Recently, however, wrought alloys, cutting tools and methods have been
developed so that now machining of cobalt alloys is practical. Parts are routinely
now machined from cobalt alloys. Still the cost of Co-Cr parts is relatively high when
one considers the combined effects of the costs of materials processing.
b) Physical Properties
Cobalt, like titanium has a hexagonal crystal structure at room temperature. At
elevated temperatures the crystal structure becomes face centered cubic. Thus, the
addition of alloying elements can produce a multi phase material and make possible
precipitation hardening allowing, with proper processing, high strength and hardness
[9]. The stiffness and density of these alloys are similar to those of stainless steels.
Thus they do not offer the advantages of lower stiffness and weight for use in
implants associated with titanium. The principal advantage of Co-Cr alloys is their
very high hardness and abrasion resistance. Thus these materials make an excellent
counterface material in a metal against plastic articulating couple.
