Biomedical Engineering Reference
In-Depth Information
Table 3.2.10-1 Types of implant-tissue response
If the material is toxic, the surrounding tissue dies.
Type 4 (Resorbable)
Type 3
Bioactive
A
Type 2
A
Porous ingrowth
C
B
If the material is nontoxic and biologically inactive (nearly inert),
a fibrous tissue of variable thickness forms.
Type 1
Nearly Inert
D
F
E
G
100
B
If the material is nontoxic and biologically active (bioactive), an
interfacial bond forms.
80
Bioceramics
A. 45S5 Bioglass
B. KGS Cervital
C. 55S4.3 Bioglass
D. A-W Glass Ceramic
E. Hydroxylapatite (HA)
F. KGX Ceravital
G. Al
2
0
3
, Si
3
N
4
60
If the material is nontoxic and dissolves, the surrounding tissue
replaces it.
A
40
B
20
A comparison of the relative chemical activity of the
different types of bioceramics, glasses, and glass-
ceramics is shown in
Fig. 3.2.10-1
. The relative reactivity
shown in
Fig. 3.2.10-1
A correlates very closely with the
rate of formation of an interfacial bond of ceramic, glass,
or glass-ceramic implants with bone (
Fig. 3.2.10-1
B).
Figure 3.2.10-1
B is discussed in more detail in the sub-
section on bioactive glasses and glass-ceramics in this
section.
The relative level of reactivity of an implant influences
the thickness of the interfacial zone or layer between the
material and tissue. Analyses of implant material failures
during the past 20 years generally show failure originating
at the biomaterial-tissue interface. When biomaterials
are nearly inert (type 1 in
Table 3.2.10-2
and
Fig. 3.2.10-1
)
and
D
C
G
F
E
0
3
10
100
1000
Implantation time (Days)
Fig. 3.2.10-1 Bioactivity spectra for various bioceramic
implants: (A) relative rate of bioreactivity, (B) time dependence
of formation of bone bonding at an implant interface.
bonded, there is relative movement and progressive
development of a fibrous capsule in soft and hard tis-
sues.
The
presence
of
movement
at
the
biomater-
ial
tissue interface eventually leads to deterioration in
function of the implant or the tissue at the interface, or
both. The thickness of the non-adherent capsule varies,
depending
d
upon
both
material
(
Fig.
3.2.10-2
)
and
extent of relative motion.
The fibrous tissue at the interface of dense Al
2
O
3
(alumina) implants is very thin. Consequently, as
discussed later, if alumina devices are implanted with
a very tight mechanical fit and are loaded primarily in
compression, they are very successful. In contrast, if
a type 1 nearly inert implant is loaded so that interfacial
movement can occur, the fibrous capsule can become
several hundred micrometers thick, and the implant can
loosen very quickly.
The mechanism behind the use of nearly inert mi-
croporous
the
interface
is
not
chemically
or
biologically
Table 3.2.10-2 Types of bioceramic-tissue attachment and their
classification
Type of attachment
Example
1. Dense, nonporous, nearly
inert ceramics attach by bone
growth into surface
irregularities by cementing
the device into the tissues or
by press-fitting into a defect
(termed ''morphological
fixation'').
Al
2
O
3
(single crystal and
polycrystalline)
materials
(type
2
in
Table
3.2.10-2
and
2. For porous inert implants,
bone ingrowth occurs that
mechanically attaches the
bone to the material (termed
''biological fixation'').
Al
2
O
3
(polycrystalline)
Hydroxyapatite-coated porous
metals
3. Dense, nonporous surface-
reactive ceramics, glasses,
and glass-ceramics attach
directly by chemical bonding
with the bone (termed
''bioactive fixation'').
Bioactive glasses
Bioactive glass-ceramics
Hydroxyapatite
4. Dense, nonporous (or porous)
resorbable ceramics are
designed to be slowly
replaced by bone.
Calcium sulfate (plaster of Paris)
Tricalcium phosphate
Calcium-phosphate salts
Fig. 3.2.10-2 Comparison of interfacial thickness (mm) of
reaction layer of bioactive implants of fibrous tissue of inactive
bioceramics in bone.
