Biomedical Engineering Reference
In-Depth Information
a soft tissue when considering magnetic resonance
imaging (MRI). A porous version could make the
already accepted biomaterial osseoconductive, which
may open up new applications and opportunities for
medical device design engineers. PEEK can also be
modified through a variety of surface modifications
such as coatings, plasma, or wet chemistry. There-
fore, there is the potential option for companies to
modify porous PEEK with osseointegrative or
proprietary treatments.
This chapter summarizes current research efforts
to develop porous PEEK biomaterials for medical
device applications. This chapter first briefly
provides an overview of porous biomaterials in
existing medical devices. Then, it reviews technolo-
gies for introducing porosity into polymers for
industrial applications and porous PEEK biomate-
rials. This chapter also includes three case studies
that illustrate applications of porous PEEK develop-
ment. References in the scientific literature have been
cited when applicable; however, many of the
advances in manufacturing appear only in the patent
literature, which was also consulted for this chapter.
Biorthex's (Canada) Actipore
is a porous Nitinol
(nickel
e
titanium) material targeted at anterior
cervical fusion devices. The porous metal has an
interconnected porosity of 65% and pore diameter
of 200
m
m, and it is produced in-house from metal
powders using self-propagating high-temperature
synthesis. The material is available for anterior
cervical fusion implants. Zimmer (Warsaw, Indiana)
manufacture Trabecular Metal
(tantalum) after
acquiring the technology from Implex in 2004.
Trabecular Metal is an 80% porous structure targeted
as a structural replacement for bone. Applications
utilizing Trabecular Metal technology include
osteonecrosis intervention implant (trauma), cervical
vertebral body replacement (spine), humeral stem,
reverse shoulder, hips (acetabular cup, revision
shells, acetabular restrictors, and modular systems),
and knees (tibial monoblock, patella). Zimmer also
produce porous titanium as the Cancellous-
Structured Titanium (CSTi
) range that markets the
benefits of surface porosity achieved by sinter
coating with commercially pure titanium (CPTi)
powder. Example application areas are devices
relying on fixation such as cementless hip stems,
knee, and acetabular components. Biomet (Warsaw,
USA) Regenerex porous titanium Ti
e
6Al
e
4V alloy
has been positioned as a direct competitor to
Zimmer's Trabecular Metal. Stryker (Mahwah, NJ)
have likewise developed a porous metal technology
using CPTi with the trade name Tritanium.
Porous biomaterials are now widely used in CMF
applications. Here, titanium accounts for 95% of the
implants (as of 2003). Porous materials offer
a middle-ground between titanium and the biode-
gradable materials, allowing structure and tissue
infiltration. Porosity is seen as a benefit in CMF as it
encourages osteointegration and reduced movement
of the implant. A well-fixed implant through
osteointegration may reduce inflammation and
possible wear debris. The main drivers away from
porous metal implants for CMF applications include
impedance of growth and migration in pediatrics,
heat transfer, weight, color, and imaging. Migration
of devices in nonporous implants is 15% as opposed
to 1% with porous polyethylene. Coralline
hydroxyapatite is naturally porous but has a rough
surface that can damage the conjuncta. From 1981 to
2001, there was a 5% complication rate in 393
coralline hydroxyapatite cases
[7]
. Porous
polyethylene is available from Ceramed (Lisbon,
Portugal) and Porex (now Stryker CMF, Michigan,
12.2 Porous Biomaterials in
Existing Implants
Porosity already appears in medical devices as
permanent or degradable biomaterial forms and for
structural or filling applications. In orthopedics,
porous biomaterials applications are currently
dominated by porous tantalum and titanium in the
case of uncemented arthroplasty. These porous
metals are often found in the form of a coating to
a metal substrate, for example in the case of unce-
mented fixation of components in the acetabulum and
the knee. In craniomaxillofacial (CMF) applications,
porous materials appear for mid-face reconstruction
(such as orbital floor) as high-density polyethylene
(Medpor, Porex) and hydroxyapatite ceramic
[4,5]
.
Porous titanium dominates the porous materials in
dental.
The porous metals are widely used in spinal and
orthopedic applications with load-bearing and fixa-
tion requirements. Synthes (West Chester, USA)
have the in-house capability to sinter titanium
powder for their porous PlivioPore lumbar fusion
cages. The “space holder” method used by them has
been described with parts then cut from the porous
metal using wire electrical discharge machining
[6]
.
