Biomedical Engineering Reference
In-Depth Information
applications. the low roughness of ceramics is also limiting. As mentioned
in the previous section, the metals have, on average, a fracture toughness 20
times that of the ceramics. in particular the toughness of hA, the bioceramic
most similar in composition to the inorganic component of bone, is far below
that of cortical bone. Although bone contains a considerable amount of the
inorganic 'ceramic' phase, the collagen fibrils which run longitudinally through
it greatly improve its toughness, preventing the type of catastrophic failure
that plagues biological glasses and calcium phosphates. the high stiffness
of ceramics might also pose a problem in a load-bearing situation owing to
modulus mismatch with surrounding bone tissue.
in compositional considerations and resulting
in vivo
behaviour, ceramics
encompass a range of performances. Some, such as alumina and zirconia, are
bioinert, having neither an adverse nor beneficial effect on the surrounding
tissue. Others, like hA, are so similar in composition to the inorganic phase
of bone that they can create a stable bond with it and are termed bioactive.
Still others are bioresorbable, like tCP, dissolving over time as they donate
their ions towards new bone growth. Bioactive glasses and glass ceramics,
including Bioglass
®
, developed by hench
et al
. (1971), and apatite-wollastonite
(A-W) glass ceramic, encompass a wide range of biological behaviour,
depending on the exact proportions of CaO, SiO
2
, Na
2
O and P
2
O
5
.
Were it not for their poor mechanical properties, the compositional similarity
of some bioceramics to bone tissue would make them an ideal replacement
material. With the current state of technology, materials like hA and bioactive
glasses are limited to powders to fill in bony defects, non-major load-bearing
applications, like middle ear implants and alveolar ridge reconstruction,
and coatings on orthopaedic, dental and maxillofacial prosthetics. While
the poor strength and toughness of hA and bioactive glasses limits their
use, the significantly higher mechanical properties of glass ceramics enable
their use in higher load-bearing situations in porous, granular and even bulk
forms, mainly in the spinal area. Alumina and zirconia, on the other hand,
with their superior mechanical properties compared with other bioceramics,
can be used in situations requiring both strength and wear resistance, such
as femoral heads and acetabular cups.
5.5.3 Polymers
the properties of polymers can vary considerably, depending on their degree
of crystallinity and crosslinking and their molecular weight. Amorphous,
rubbery polymers are soft and ductile, have a low modulus and can extend
hundreds of times their original length. Semi-crystalline polymers, on the
other hand, have much higher moduli and lower extensibilities. Common
to many polymers is their high toughness. While some, such as PMMA,
exhibit brittle behaviour, the vast majority used in biomedical applications
