Biomedical Engineering Reference
In-Depth Information
rabbits
[62]
. PAEK and carbon fiber-reinforced
PAEK were only partially encapsulated by fibrous
tissue in interbody spinal fusion of sheep
[2]
and
goats
[61]
, although these implants were augmented
with osteoinductive autograft or rhBMP-2. Retrievals
for failed spinal fusion cages in humans exhibited no
direct bone apposition to carbon fiber-reinforced
PAEK implants
[65]
.
Calcium phosphates, on the other hand, are well
known to be biocompatible and bioactive
[48,66
e
68]
. Bioactivity is the ability of a biomaterial
to elicit or modulate a favorable response (“activity”)
from any part of a biological organism
[10]
. Calcium
phosphates consistently exhibit direct apposition of
bone tissue, without the use of autograft or BMPs, no
matter whether implanted in osseous defects
[48,67]
or non-osseous sites
[68
e
70]
. In the latter case of
subcutaneous or intramuscular implantation, HA
may be considered osteoinductive. The bioactivity of
calcium phosphates in vivo has been attributed to the
release of calcium by dissolution and/or osteoclastic
resorption, as well as an affinity for binding
osteoinductive proteins from the implant site
[66,68]
.
Therefore, a key consideration in the choice of
bioactive phases is the solubility product (
confirming that the HA reinforcements were bioac-
tive and the PEEK matrix was bioinert
[18,19,32]
.
The thickness and surface coverage of the apatite
layer increased with increased HA
[19,32]
or calcium
silicate
[33]
content, as well as for strontium-
substituted HA compared with HA
[32]
. In order to
avoid interference from the bioactive reinforcements
in the underlying composite, the apatite layer should
be removed from the composite surface for charac-
terization using XRD, Fourier transform infrared
spectroscopy (FT-IR), and other surface analytical
techniques
[19]
.
Cell attachment on bioactive PAEK composites
has been demonstrated using fibroblasts
[18]
, human
osteoblasts
[21,32]
, and human fetal osteoblasts
[22,23]
. Osteoblast proliferation and spreading were
reported to be greatest for bioglass (45S5)-reinforced
PEEK, followed by PEEK and then
-TCP-rein-
forced PEEK
[21
e
23]
. However, another study
reported no differences in osteoblast proliferation
and alkaline phosphatase activity (differentiation) for
PEEK, HA-reinforced PEEK, and Sr-HA-reinforced
PEEK
[32]
. HA alone is known to suppress cell
proliferation but enhance differentiation
[73]
. Little
attention seems to have been given to the broad
fluorescence emission spectrum of PEEK, ranging
from 400 to 600 nm
[57]
, and possible interference
with common fluorophores (e.g., alizarin, calcein,
fluorescein isothiocyanate, rhodamine) used for
labeling proteins and mineralization. For example,
increased bioactive reinforcement content could lead
to increased fluorescence from a biochemical assay
with concomitant decreased fluorescence from the
PAEK matrix. Systematic investigations for the
effects of the bioactive reinforcement composition,
content, size, and morphology on cellular behavior
are needed in the future.
There is currently a paucity of data for the in vivo
osteoconductivity or osteointegration of bioactive
PAEK composites. An early study reported bone
ingrowth into a porous HA-reinforced PEEK scaffold
prepared by SLS at 16 weeks postimplantation in pigs
[16]
, but no further details were provided. More
recently, PEEK reinforced with 4 vol%
b
sp
), which
can vary widely (
Table 11.2
). Solubility may aid
bioactivity through calcium release and cellular
signaling, but may also lead to complete degradation
of bioactive reinforcements. Degradation of rein-
forcements may be desirable in the case of degrad-
able polymer composites. However, PAEK polymers
are not degradable and thus PAEK composites are
intended to perform as permanent implants. There-
fore, the complete degradation of bioactive rein-
forcements in PAEK composites could lead to a loss
of biological and/or mechanical function. After
86 weeks in a minipig trabecular bone defect, 97% of
a
b
-TCP bone substitute was completely removed
[71]
. This suggests that for long-term function,
bioactive reinforcements in PAEK composites should
comprise HA, calcium-deficient HA, carbonated HA,
or doped HA. The solubility and bioactivity of HA
are generally increased with increased defects in the
crystal structure, including ionic substitutions, and
decreased particle size
[48,68,72]
.
Investigations of HA-reinforced PAEK compos-
ites to date have mainly focused on in vitro assess-
ment of bioactivity and cytocompatibility (
Table
11.1
). After immersion in simulated body fluid
(SBF), a layer of carbonated apatite was deposited on
HA reinforcements, but not
K
-TCP
prepared by SLS was implanted into 10-mm-diameter
cranial defects in pigs
[24]
. The thickness of the
fibrous
b
tissue
layer
encapsulating the
implant
decreased with increased
-TCP content and time
postimplantation. At 24 weeks postimplantation, there
was direct apposition of bone to
b
b
-TCP reinforce-
the PEEK matrix,
ments but not
the PEEK matrix. Moreover,
the
