Biomedical Engineering Reference
In-Depth Information
phenotype of the marrow stromal cells used in this
study. The porous PEEK-OPTIMA showed excellent
cytocompatibility and also demonstrated the ability
to maintain the cells in their differentiated state.
18-month animal as a comparison, the mechanical
properties at 4 weeks were also improved over the
older animals at 4 weeks. This improvement at
the same time point is likely to be attributable to the
enhanced healing ability of younger animals.
12.5.4 InVivo Study
To assess the biocompatibility of the processing
and to determine the potential for osseoconductivity
of the porous PEEK-OPTIMA, the implants were
machined to dowel forms (25 mm long and 6 mm in
diameter) for placement into cortical and cancellous
bone sites in a sheep model (age 4
e
5 years). The
porous PEEK implants and solid, unfilled PEEK
control samples were steam sterilized. Cancellous
bone was evaluated by placement of one implant in
the medial distal femoral condyle and one in the
proximal tibia. Cortical bone was evaluated from
three bicortical implants placed on the anteromedial
aspect of the tibias. Each implant was press fitted into
the site and placed 15 mm apart. After recovery, the
sheep were allowed to mobilize and fully weight
bear. At 4- and 12-week timepoints, solid and porous
PEEK samples were retrieved and analyzed using
photographs, Faxitron radiographs, mechanical
testing (cortical sites only), histology and SEM. All
implants were well tolerated in the body with no
adverse reactions or infection. Anteroposterior and
lateral faxitron radiograph assessment was made at 4
and 12 weeks postsurgery and revealed no adversities
such as inflammatory reactions. The implant sites and
radiolucent PEEK samples can be clearly seen on the
radiograph (
Fig. 12.4
).
12.5.6 Bone Ongrowth
QuantificationdCortical and
Cancellous Sites
Tissue sections were prepared from the cortical
and cancellous
magnification on an environmental electron micro-
scope. The captured images were then subjected to
a custom MatLab analysis technique in order to
calculate the percentages of bone ongrowth. Those
areas of bone intimately in contact with the PEEK
samples are denoted green, while those areas with no
ongrowth are marked red (see
Fig. 12.6
). The limited
sample size (
n ¼
samples and examined at 40
3) meant that there was no signifi-
cant difference between solid and porous, but there
was a noticeable effect of difference in time (4 vs.
12 weeks) in the amount of bone ongrowth observed
(
Fig. 12.7
).
12.5.7 Histology
Sample sections were assessed for histology at
12 weeks using methylene blue
e
fuchsin staining and
the results demonstrated that there was adjacent
tissue integration and bone ongrowth on the porous
PEEK samples, which increased over time.
Figure 12.8
shows histological samples from the
lateral cortical and medial cortical sites for solid and
porous PEEK at 12 weeks.
The gap observed on the images showing the
interface between the solid PEEK implant and the
tissue (pink) is exaggerated as a result of the embed-
ding process and sectioning during the histology
preparation. This gap was not filled with any fibrous
tissue, which implies that the solid PEEK implant was
in intimate contact with the bone but had little direct
ongrowth at that time. The porous PEEK presented to
the tissue a rougher (
R
a
¼
12.5.5 Mechanical Testingd
Cortical Sites Only
For porous and control samples postsurgery, the
implant
e
bone interface shear strength was assessed
by a standard push-out test using a calibrated servo-
hydraulic testing machine. The load was applied
from the endosteal side with the sample being pushed
out toward the periosteal side of the bone. After
mechanical testing and PMMA embedding, the
cortical thickness of each specimen was measured at
two locations and the mean was used for the shear
stress calculations. For the aged animals (4
e
5 years)
the porous samples that had been in situ for longer
time (12 weeks) had better mechanical properties
than those in place for 4 weeks (
Fig. 12.5
). When
porous PEEK samples were implanted into an
6
m
m) and lessuniform
surface than the machined surface (
R
a
¼
3
m
m), which
may have some implications for improved bone
response. There appeared intimate implant
e
bone
contact at the interface of the porous PEEK and
surrounding tissue, as observed by the pink fuchsin
staining into the porous domains of the material. With
these samples, no fibrous interface was obvious.
