Biomedical Engineering Reference
In-Depth Information
Fig. 5.1
Worn prosthesis stem. (
a
) Normal image. (
b
) Image under UV-light with fluorescein dye
retained on the rougher nonworn surface and absent on the worn-polished surface [183]
analysis of metallographic images of the fracture surfaces as shown in Fig. 2.5 would
allow the number of critical load cycles to be estimated, say the number of steps
climbing the stairs, if not hurrying to the latest bus. Two other cases were analyzed
in Chap. 3 and also here catastrophic fracture was excluded. It was demonstrated that
the strength of the device by design and materials used far outweighed the in vivo
stresses expected from patient's normal or even extravagant activities. Nevertheless,
the stem broke.
What then should we look for...extrahighstrength?AsuperficiallookataMate-
rials Property Chart offers us an instant solution: ceramics such as Al
2
O
3
or ZrO
2
.
Plotting elasticity modulus E vs. tensile strength one finds the collection of ceram-
ics top right of Fig.
5.2
(left). Moreover, ceramics exhibit an extraordinary resistance
to corrosion and wear, chemically inert, nontoxic. But remember that we stressed
above that the fractures under scrutiny did happen noncatastrophically. Our enthu-
siasm about ceramics is immediately tempered by inspecting Fig.
5.2
(right) where
fracture toughness is plotted vs. strength, a plot similar to toughness vs. E.
1
Metals
occupy now the top right position of the plot. Moreover, the lower E for metals is
mechanically beneficial because it results in less stress shielding.
A stress-strain curve for ceramics does not show the upper end plateau-like
behavior before fracture as in Fig. 1.6. This stress-strain behavior tells us the same
story as the Materials Properties Charts: ceramics fracture catastrophically; without
any warning your ceramic hip prosthesis can fracture descending the stairs on your
way to breakfast.
1
For details, the reader is advised to read the concise paper on philosophy and use of Materials
Properties Charts by Ashby [184].
