Biomedical Engineering Reference
In-Depth Information
components falls within the audible region (
15 kHz) and is probably the source of
squeaking. The incidence is less clear but is estimated around 1% [
296
].
<
9.3.2
Low Performance
The mineral phase of bone, 60% by weight, consists preponderately of calcium
hydroxyapatite (HA), stoichiometrically Ca
10
(PO
4
)
6
(OH)
2
. Other ions may partly
substitute hydroxyl or phosphate groups (CO
2
3
,SiO
4
,Mg
C
2
,Sr
C
2
,Na
C
F
and
so on) and the 'biological' form is always a calcium-deficient HA. The ideal Ca/P
ratio is 1.67; phosphates with ratios below 1/1 are not suitable for implantation.
Stoichiometric HA can be sintered to
1,300
ı
C without too much risk for phase
transformation (possible formation of CaO or Ca
3
(PO
4
)
2
). CaO is itself toxic but
reacts to portlandite or carbonate. Because of its similarity to the mineral phase
of bone, it was decades ago the obvious choice as biomedical ceramic. While alu-
mina and zirconia are quoted as inert ceramics, HA is slightly soluble and therefore
bioactive. The rate of dissolution depends on the degree of stoichiometry and the
substitutional or interstitial elements. However, as it becomes clear by inspecting
Tab le
9.5
, hardness, compressive strength, bending strength and certainly fracture
toughness
K
IC
are far below the requirements for joints or weight-bearing appli-
cations; as such it is quasi exclusively used as coating on a metal substrate (see
Sect.
9.5
). More about other phosphates are found in Chap.
10
.
9.4
Glass and Glass-Ceramics
Alumina and zirconia consist of compacted and sintered more or less well ordered,
i.e., crystallized particles. Near the other end of solid state order is the more or
less chaotically organized glassy state. A typical glass former is silicon. As referred
to Sect.
9.1
, the small cations smuggled into the silica array are most commonly
sodium and calcium (window glass; see [
297
]). A characteristic of amorphous
Tabl e 9. 5
Properties of dense hydroxyapatite (HA) ceramics compared to ISO 6474-1 for zirconia
and Y-TZP (Metoxit, cfr. Table
9.4
)
Units
ISO
HA
Y-TZP
kg/m
3
Density
3,156
6,080
Hardness (HV)
GPa
18
5-8
12
Compressive strength
MPa
>
4,000
100-900
Bending strength
MPa
500
20-30
1,200
E modulus
GPa
380
70-120
210
K
IC
MPa m
1=2
2.5
1
8
