Page 382 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOCERAMICS 359

15.2 BIOINERT CERAMICS

Ceramics are fully oxidized materials and are therefore chemically stable and less likely to elicit an
adverse biological response than metals, which only oxidize at their surface. Three types of “inert”
ceramics are of interest in musculoskeletal applications: carbon, alumina, and zirconia.

15.2.1 Carbon

The benign biological reaction to carbon-based materials, along with the similarity in stiffness and
strength between carbon and bone, made carbon a candidate material for musculoskeletal recon-
struction almost 40 years ago (Bokros et al., 1972). Carbon has a hexagonal crystal structure that is
formed by strong covalent bonds. Graphite has a planar hexagonal array structure, with a crystal size
of approximately 1000 Å (Bokros, 1978). The carbon-carbon bond energy within the planes is large
(114 kcal/mol), whereas the bond between the planes is weak (4 kcal/mol) (Hench and Ethridge,
1982). Therefore, carbon derives its strength from the strong in-plane bonds, whereas the weak bonding
between the planes results in a low modulus, near that of bone (Bokros, 1978).
Isotropic carbon, on the other hand, has no preferred crystal orientation and therefore possesses
isotropic material properties. There are three types of isotropic carbon: pyrolytic, vitreous, and vapor
deposited. Pyrolytic carbons are formed by the deposition of carbon from a fluidized bed onto a sub-
strate. The fluidized bed is formed from pyrolysis of hydrocarbon gas between 1000 to 2500°C
(Hench and Ethridge, 1982). Low temperature isotropic (LTI) carbons are formed at temperatures
below 1500°C. LTI pyrolytic carbon possesses good frictional and wear properties, and incorpora-
tion of silicon can further increase hardness and wear resistance (Bokros, 1978). Vitreous carbon is
a fine-grained polycrystalline material formed by slow heating of a polymer. Upon heating, the more
volatile components diffuse from the structure and only carbon remains (Hench and Ethridge, 1982).
Since the process is diffusion mediated and potentially volatile, heating must be slow and dimensions
of the structure are limited to approximately 7 mm (Bokros, 1978). Salient properties of all three
forms of carbon are summarized in Table 15.1.
Deposition of LTI coatings onto metal substrates is limited by the brittleness of the coatings and
propensity for coating fracture and coating/substrate debonding (Hench and Ethridge, 1982). Carbon
may also be vapor deposited onto a substrate by the evaporation of carbon atoms from a high-
temperature source and subsequent condensation onto a low temperature substrate (Hench and
Ethridge, 1982). Vapor-deposited coatings are approximately 1 μm thick, allowing properties of the
substrate to be retained. More recently, diamondlike carbon (DLC) coatings have been studied, as a
means of improving fixation to bone (Koistinen et al., 2005; Reikeras et al., 2004) and wear resis-
tance (Allen et al., 2001). Carbon-based thin films are produced from solid carbon or liquid/gaseous
hydrocarbon sources, using ion beam or plasma deposition techniques, and have properties interme-
diate to those of graphite and diamond (Allen et al., 2001).
With the advent of nanotechnology, interest in carbon has been rekindled, in the form of carbon
nanotubes (CNTs). CNTs have been proposed as scaffolds to support osteoconductivity (Zanello
et al., 2006) and as a second phase in polymer scaffolds (Shi et al., 2006).

15.2.2 Alumina

High-density, high-purity, polycrystalline alumina is used for femoral stems, femoral heads, acetabular
components, and dental implants (Kohn and Ducheyne, 1992; Boutin et al., 1988; Heimke et al., 1978;
Tohma et al., 2006; Nizard et al., 2008). More recently, ion-modified and nanostructured Al O have
2 3
been synthesized, to make these bioceramics stronger and more bioactive (Webster et al., 2000; Zreiqat
et al., 1999). In addition to chemical stability and relative biological inertness, other attributes of alu-
mina are hardness and wear resistance. Therefore, a main motivation for using alumina in orthopedic
surgery is to increase tribological properties, and many total hip replacements are now designed as
modular devices, that is, an alumina femoral head is press-fit onto the neck of a metal femoral stem.