Biomedical Engineering Reference
In-Depth Information
and extrusion rates
[45]
. Another alloplastic material,
HA, is a naturally occurring mineral form of calcium
apatite that is known to support bone ingrowth and
osseointegration. It is available in a powder and paste
form, but can be difficult to work with due to
a tendency for fragmentation and requires substantial
intraoperative sculpting for best results. HA also has
a tendency to cause inflammation and infection when
placed near the paranasal and sinus cavities
[48]
.Itis
expected that this material will find greater accep-
tance with further technical advancements
[48]
.
Finally, adaptable titanium meshes can be used as
noncustomized implants for small and complex
defects. Their use is limited by the size of the defect
due to strength and palpability issues
[48]
. There are
also some issues with thinning of the soft tissues in
delicate regions as well as extrusion. As a rough
guideline, defects of 10 to 50 cm
2
can be recon-
structed with titanium meshes, while larger defects
from 50 cm
2
to over 100 cm
2
need to be treated using
other alloplastic or autogenous graft material.
Customized alloplastic implants are an emerging
option led by advances in three-dimensional CT
scanning techniques. Using specialized software, the
implants can be preoperatively tailored to the exact
defect size and geometry. Customized implants have
“dramatically minimized” the need for major intra-
operative manipulations, making them easier to use
and considerably reducing operative time for some
cases
[46,50]
. The defect-matched fit can guarantee
postoperative stability, providing incomparable and
predictable cosmetic and functional results. Cus-
tomization is especially useful for large defects or
defects of the fronto-orbital region, as these are
difficult and challenging procedures. This option is
only available for delayed reconstructions, as there is
a lead time of the order of 6
e
8 weeks. Also,
computer-assisted designs are inherently more
expensive, ranging from
V
7000 to
V
15,000
[50]
.
High-density polyethylene (HDPE), PEEK, and
titanium craniofacial implants can all be made on
a custom basis. HDPE is good for smaller and more
complex cranial, orbital and temporal defects, as it is
available in a variety of thicknesses, shapes, and
sizes. It is easily modifiable, radiolucent, and will not
cause artifacts during MRI. Further, HDPE has the
ideal pore size for soft tissue ingrowth and integra-
tion
[48]
. The major limitation for use in larger
defects is due to concerns with the strength of the
material. At the other end of the spectrum, titanium is
the strongest material for external force resistance
Figure 15.10
Exoshape by MedShape Solutions. Exo-
shape features sheath and inserter components (top of
figure) made of a proprietary PEEK Altera shape
memory polymer structure. The unique shape memory
material allows the sheath to collapse into a low-profile
device for ease of insertion. When deployed (bottom of
figure), the material is in a zero-strain state, providing
large normal forces against the tunnel wall to fixate
the graft in place. The image is reprinted here with
the permission of the manufacturer.
Figure 15.11
IntraFix PEEK screw and sheath design
marketed by Depuy Mitek. The image is reprinted
here with the permission of the manufacturer.
Figure 15.12
PEEK CF Interference screw marketed
by Parcus Medical. The image is reprinted here with
the permission of the manufacturer.
giving off significant amounts of heat along with
a toxic monomer that can lead to local and systemic
reactions
[48,49]
. Resultantly, there have been
reported thermal injuries along with high infection
