Biomedical Engineering Reference
In-Depth Information
Other clinical applications of alumina prostheses
reviewed by Hulbert
et al.
(1987) include knee prosthe-
ses; bone screws; alveolar ridge and maxillofacial re-
construction; ossicular bone substitutes; keratoprostheses
(corneal replacements); segmental bone replacements;
and blade, screw, and post dental implants.
their subsequent decrease in strength, requires bone in-
growth to stabilize the structure of the implant. Clinical
results for non-load-bearing implants are good (Schors
and Holmes, 1993).
Bioactive glasses and
glass-ceramics
Porous ceramics
Certain compositions of glasses, ceramics, glass-
ceramics, and composites have been shown to bond to
bone (Hench and Ethridge, 1982; Gross
et al.
, 1988;
Yamamuro
et al.
, 1990; Hench, 1991; Hench and Wilson,
1993). These materials have become known as bioactive
ceramics. Some even more specialized compositions of
bioactive glasses will bond to soft tissues as well as bone
(Wilson
et al.
, 1981). A common characteristic of bio-
active glasses and bioactive ceramics is a time-dependent,
kinetic modification of the surface that occurs upon
implantation. The surface forms a biologically active
carbonated HA layer that provides the bonding interface
with tissues.
Materials that are bioactive develop an adherent in-
terface with tissues that resist substantial mechanical
forces. In many cases, the interfacial strength of adhesion
is equivalent to or greater than the cohesive strength of
the implant material or the tissue bonded to the bioactive
implant.
Bonding to bone was first demonstrated for a compo-
sitional range of bioactive glasses that contained SiO
2
,
Na
2
O, CaO, andP
2
O
5
in specific proportions (Hench
et al.,
1972) (
Table 3.2.10-5
). There are three key
compositional features to these bioactive glasses that
distinguish them from traditional soda-lime-silica
glasses: (1) less than 60 mol% SiO
2
, (2) high Na
2
O and
CaO content, and (3) a high CaO/P
2
O
5
ratio. These
features make the surface highly reactive when it is ex-
posed to an aqueous medium.
Many bioactive silica glasses are based upon the for-
mula called 45S5, signifying 45 wt.% SiO
2
(S
¼
the net-
work former) and 5 :1 ratio of CaO to P
2
O
5
. Glasses with
lower ratios of CaO to P
2
O
5
do not bond to bone. How-
ever, substitutions in the 45S5 formula of 5-15 wt.%
B
2
O
3
for SiO
2
or 12.5 wt.% CaF
2
for CaO or heat treating
the bioactive glass compositions to form glass-ceramics
has no measurable effect on the ability of the material to
form a bone bond. However, adding as little as 3 wt.%
Al
2
O
3
to the 45S5 formula prevents bonding to bone.
The compositional dependence of bone and soft tissue
bonding on the Na
2
O-CaO-P
2
O
5
-SiO
2
glasses is illus-
trated in
Fig. 3.2.10-5
. All the glasses in
Fig. 3.2.10-5
contain a constant 6 wt.% of P
2
O
5
. Compositions in
the middle of the diagram (region A) form a bond with
bone. Consequently, region A is termed the bioactive
bone-bonding boundary. Silicate glasses within region B
The potential advantage offered by a porous ceramic
implant (type 2,
Table 3.2.10-2
,
Figs. 3.2.10-1 and
3.2.10-2
) is its inertness combined with the mechanical
stability of the highly convoluted interface that develops
when bone grows into the pores of the ceramic. The
mechanical requirements of prostheses, however, se-
verely restrict the use of low-strength porous ceramics to
nonload-bearing applications. Studies reviewed by
Hench and Ethridge (1982), Hulbert
et al.
(1987), and
Schors and Holmes (1993) have shown that when load-
bearing is not a primary requirement, porous ceramics
can provide a functional implant. When pore sizes exceed
100
m
m, bone will grow within the interconnecting pore
channels near the surface and maintain its vascularity and
long-term viability. In this manner, the implant serves as
a structural bridge or scaffold for bone formation.
Commercially available porous products originate from
two sources: HA converted from coral (e.g., Pro Osteon)
or animal bone (e.g., Endobon). Other production routes;
e.g., burnout techniques and decomposition of hydrogen
peroxide (Peelen
et al.
, 1978; Driessen
et al.
, 1982) are
not yet used commercially. The optimal type of porosity is
still uncertain. The degree of inter-connectivity of pores
may be more critical than the pore size. Eggli
et al.
(1988)
demonstrated improved integration in interconnected 50-
100
m
m pores compared with less connected pores with
a size of 200-400
m
m. Similarly K¨hne
et al.
(1994)
compared two grades of 25-35% porous coralline apatite
with average pore sizes of 200 and 500
m
m and reported
bone ingrowth to be improved in the 500
m
m pore sized
ceramic. Holmes (1979) suggests that porous coralline
apatite when implanted in cortical bone requires in-
terconnections of osteonic diameter for transport of nu-
trients to maintain bone ingrowth. The findings clearly
indicate the importance of thorough characterisation of
porous materials before implantation, and Hing
et al.
(1999) has recommended a range of techniques that
should be employed.
Porous materials are weaker than the equivalent bulk
form in proportion to the percentage of porosity, so that
as the porosity increases, the strength of the material
decreases rapidly. Much surface area is also exposed, so
that the effects of the environment on decreasing the
strength become much more important than for dense,
nonporous materials. The aging of porous ceramics, with
