Biomedical Engineering Reference
In-Depth Information
behave as it is intended. This is where material
properties such as stiffness and viscoelasticity come
into play. If a device is intended to stabilize the spine
for fusion, but the material is not stiff enough to
provide adequate stability, then a new material may
be needed. If a device is expected to bear load over
a lifetime, but has viscoelastic properties that cause it
to essentially change its geometry (i.e., creep) over
time, then the material choice may need to be
reassessed.
over the service life of the device, and what the
expected size and morphology of this debris will be.
Wear is a biological concern because, depending on
the size, material and amount of debris, the body's
ability to isolate and tolerate the debris could vary,
and as with other orthopedic joints, the physiologic
response to debris could lead to other unwanted
biologic reactions such as
osteolysis
, or degradation
of the surrounding bone. It is important that a sponsor
evaluate how susceptible their device is to the
generation of wear debris and justify whether the
wear debris created will be tolerated by the human
body without causing an unacceptable biological
response. This evaluation is typically conducted
through a combination of mechanical testing (to
generate wear debris and evaluate the wear rate) and
animal testing (to estimate the biological response of
the wear debris).
17.8.2 Biocompatibility
Biocompatibility of materials is another extremely
important element of device safety. It is important,
prior to implanting a device in humans, to ensure that
the device's materials will not cause unwanted bio-
logical reactions. There are recognized standard test
methods for biocompatibility testing such as ISO
10993 that outline animal and benchtop tests that
should be completed prior to putting a material into
humans. As with mechanical testing, it is preferred
that these tests be performed using the final, sterilized
device materials. However, a company may be able
to justify that testing performed on the raw device
material is representative of the final, sterilized form.
FDA scientists do consider the biocompatibility of
the bulk device material, but it is also important to
investigate the specific processing and finishing steps
used on the material in a particular device, and also to
assess any by-products created by the device either
chemically or mechanically. Chemical material
extracts are used in many of the ISO 10993 tests,
which allow for evaluation of any chemicals that may
potentially leach from the device materials inside the
body. Another common type of “by-product” of
a device, especially in orthopedics and spine, is wear
debris. Particulate wear debris is generated when two
or more bearing surfaces within the device articulate
against each other under a load in vivo. This articu-
lation may be by design (in the case of total disc
replacements or other mobile devices) or uninten-
tional (in cases where components may become loose
and wear against each other). Although there are
materials that are more wear resistant than others,
and although wear rate is affected by many variables
such as load and lubrication regime, any two mate-
rials that articulate under a load will eventually
experience wear through adhesive, abrasive, or
fatigue mechanisms (or a combination of all three).
The question is how much debris will be generated
17.9 Current Uses of PEEK in FDA-
Approved Spinal and Orthopedic
Implants
As discussed in Chapter 14, PEEK is used to
manufacture several different spinal fusion implants.
In fact, the vast majority of medical devices currently
on the market that are manufactured from PEEK are
spinal implants.
17.9.1 Intervertebral Body Fusion
Devices
IBFDs, also referred to as “cages” or “spacers,”
are used to aid in fusion procedures that are intended
to treat patients with degenerative disc disease. In
these procedures, the degenerated disc thought to be
a source of pain is partially or fully replaced by an
IBFD that is filled with and surrounded by bone graft.
The IBFD is intended to stabilize the spinal segment
in order to promote the formation of a solid fusion
mass across the diseased motion segment. When
successful, the fusion and subsequent lack of motion
relieve any pain caused by the removed diseased
anatomy as well as any mechanical sources of pain
caused by motion of the segment.
Many IBFDs aremanufactured from titaniumalloy,
which has a long history of use in spinal applications
and is probably the most common material used
to manufacture spinal fusion devices. However,
PEEK has become a very popular material in the
