Biomedical Engineering Reference
In-Depth Information
Fig. 9.13
Damaged ball head of the femoral component of an all glass hip prosthesis: salient
aspect of broken glass surface.
Right
: detail; arrows point to conchoidal edges
materials is their conchoidal fracture with very sharp cutting edges.
6
Therefore, it
was used for scalpels for surgery and cutting blades in microtomes. Amorphous sil-
ica occurs in nature as obsidian (volcanic glass) or flint (silex). It permitted Stone
Age men to produce tools, a substantial achievement in the technological evolution
of mankind. Less successful was its use in hip implants. The conchoidal charac-
ter of fracture is nicely demonstrated by Fig.
9.13
, conform with theory, nice for
the physicist, however, not for the patient having implanted the hip prosthesis (left
picture).
9.4.1
Bioactive Glasses
Ordinary glass is corroding in a wet environment and small surface defects deter-
mine and/or co-determine the fracture of glass. A consequence is that the strength
depend on the size of the test specimen, similar to the behavior of high performance
ceramics (Weibull statistics). And the same holds for bioactive glasses (BAG). On
the one hand, the matching of
moduli of bone and BAG were an attractive
mechanical feature (Table
9.6
) but, on the other hand, all other properties are simply
unsuitable for any load bearing application.
So, end of the story?
It all started in a most appealing way in the late 1960s, early 1970s. Why? It
is easily explained in Fig.
9.14
, where the dependence of interaction with tissue on
composition is demonstrated. In the composition domain around E both bone and
soft tissue bonding happens, while in A bonding to bone is stimulated. Thus, mod-
ulating composition allowed to tailor a material for a specific purpose. Moreover,
the success was supported by a few spectacular results: a segmental bone replace-
ment in a monkey made of 45S5 Bioglass
R
E
-ceramic was broken in the implant
6
From
o
(greek) or
concha
(latin) mollusk shell.
