Biomedical Engineering Reference
In-Depth Information
be extrapolated to predict clinical performance
because of the complexity of the in vivo loading
environment compared with well-controlled labora-
tory conditions. As mentioned above, although
a method should provide repeatability (high precision)
and minimize variation between investigators using
the method (low bias), numerical estimates of these
variability measures are often unavailable. The
consensus process does not usually deliver rapid
results, and it can therefore be a challenge to insure that
standards are updated frequently enough to reflect the
current technology innovations and breadth of designs
for a particular category of device. Occasionally,
a duplication of effort exists when multiple standards
organizations publish methods for the same purpose
but with different techniques or parameters. Finally,
because of the diversity in designs even within a single
category of device, standards must often be adapted or
modified by the user to fit a particular device. Although
this modification may be unavoidable, it is the
responsibility of the user to analyze and justify how the
interpretation of results may need to be adjusted to fit
the changes in the method. Despite these limitations,
however, standards provide not only useful practical
information about how to set up an experiment but also
effective communication tools for researchers with
similar areas of interest.
device submission types including 510(k)s, PMAs,
and IDEs, and for a number of device types including
bone screws and plates, VBRs, and pedicle screw-
based spinal fusion systems
[10]
. Recognition means
that a company may, in their submission to the FDA,
include a “declaration of conformity” that is intended
to streamline the review process because the FDA is
already familiar with the content of the standard, and
this familiarity should therefore enhance under-
standing of the device's material. In other words, the
standards community, and not the FDA, has written
the standard because their consensus is that medical-
grade PEEK used for devices should have the char-
acteristics defined in the standard. By recognizing
that standard, the FDA is not specifically mandating
that PEEK used in a medical device must conform to
the standard; it is only stating that by declaring
conformity to the standard, the manufacturer may not
need to submit lengthy test reports on
basic material
properties
because it is already understood that the
material meets the specifications in the standard.
A database of recognized standards, with details
about the extent and applicability of recognition to
different submission types, can be accessed on the
FDA's website
[11]
.
17.12 Summary and Conclusions
17.11.4 ASTM Standard
Specification for PEEK
ASTM International has published a standard
material specification for PEEK titled “F2026-10
Standard Specification for Polyetheretherketone
(PEEK) Polymers for Surgical Implant Applications”
[9]
. This specification lists several properties that the
virgin resin must possess in order for the material to
conform to the standard, as well as a list of suggested
materials testing standards and a table of “typical”
properties of fabricated PEEK forms. The required
properties of the resin include the glass transition
temperature (
T
g
), crystallization temperature (
T
c
),
and melting temperature (
T
m
), the viscosity, and
the total content of heavy metals such as lead.
The properties for which typical values are listed in
the standard include the density, tensile yield
strength, percent elongation, flexural strength, flex-
ural modulus, and impact strength.
The FDA, through their standards process, has
recognized this standard
d
specifically, the 2008
version of it
d
as appropriate for use in a number of
For any medical device, manufactured from PEEK
or from other materials, the developer should always
begin planning a testing regimen with a basic risk
analysis of the device in mind. This risk analysis
should take into account all the types of loading that
the device will be subjected to and all the potential
failure mechanisms: mechanical, chemical, and/or
biological. The investigator should always keep in
mind that the all-important goal of testing is to
evaluate the device's ability to withstand physiologic
conditions without failure.
To date, several orthopedic and spinal devices
manufactured from PEEK have been cleared or
approved by the FDA, and as is always the case, there
are more spinal devices manufactured from PEEK
that are currently still under development or investi-
gation. Thus far, the overall strength and durability of
the material for applications such as IBFDs and
vertebral replacements appears to be sufficient. Other
applications, such as the use of PEEK as a bearing
material, have a very limited history at this time and
thus the long-term clinical performance of
the
