Biomedical Engineering Reference
In-Depth Information
Fig. 3.2.9-13 Microstructure of wrought and mill-annealed Ti-6Al-4V, showing small grains of alpha (light) and beta (dark). (Photo
courtesy of Zimmer USA, Warsaw, IN.)
evaluating the structure-property relationships of porous-
coated or plasma-sprayed implants. Again, as in the case
of the cobalt-based alloys, there is the technical problem
of successfully attaching the coating onto the metallic
substrate while maintaining adequate properties of both
coating and substrate. Optimizing the fatigue properties
of Ti-6Al-4V porous-coated implants becomes an in-
terdisciplinary design problem involving not only metal-
lurgy but also surface properties and fracture mechanics.
is true that inadequate attention to material properties can
doom a device to failure. However, it is also true that even
with the best material, a device can fail because of faulty
structural properties, inappropriate use of the implant,
surgical error, or inadequate mechanical design of the im-
plant in the first place. As an illustration of this point,
Fig. 3.2.9-14 shows a plastically deformed 316L stainless
steel Harrington spinal distraction rod that failed
in vivo
by
metallurgical fatigue. An investigation of this case con-
cluded that failure occurred not because 316L cold-
worked stainless steel had poor fatigue properties per se,
but rather due to a combination of factors: (a) the surgeon
bent the rod to make it fit a bit better in the patent, but this
increased the bending moment and bending stresses on the
rod at the first ratchet junction, which was a known
problem area; (b) the stress concentrations at the ratchet
end of the rod were severe enough to significantly increase
stresses at the first ratchet junction, which was indeed the
eventual site of the fatigue fracture; and (c) spinal fusion
did not occur in the patient, which contributed to relatively
persistent loading of the rod over several months post-
implantation. Here the point is that all three of these fac-
tors could have been anticipated and addressed during the
initial design of the rod, during which both structural and
material properties would be considered in various stress
analyses related to possible failure modes. It must always
be recalled that implant design is a multifaceted problem in
which materials selection is only a part of the problem.
Concluding remarks
It should be evident that metallurgical principles guide
understanding of structure-property relationships and
inform judgments about implant design, just as they would
in the design process for any well-engineered product. Al-
though this section's emphasis has been on mechanical
properties (for the sake of specificity), other properties, in
particular surface texture, are receiving increasing attention
in relation to biological performance of implants. Timely
examples of this are (a) efforts to attach relevant bio-
molecules to metallic implant surfaces to promote certain
desired interfacial activities; and (b) efforts to texture im-
plant surfaces to optimize molecular and cellular reactions.
Another point to remember is that the intrinsic material
properties of metallic implantsdsuch as elastic modulus,
yield strength, or fatigue strengthdare not the sole de-
terminant of implant performance and success. Certainly it
