Biomedical Engineering Reference
In-Depth Information
a clinically acceptable alternative (these three take approximately five years to complete),
and finally, postmarket surveillance (gathered from product use by the general public after
FDA approval). Prior to initiating a clinical trial, a manufacturer must obtain an investiga-
tional device exemption (IDE) that includes all manufacturing and quality control proce-
dures, the plan for the clinical study, and the lists of the review boards that have reviewed
the proposed plan (see Chapter 2). The FDA has 30 days to approve or disapprove the
IDE. If the PMA application is considered complete, the FDA has 180 days to approve or dis-
approve the application. If approved, the product can be marketed for human use for the
purposes declared in the application, which are to be described in the product labeling.
Another FDA submission for review is required prior to legally marketing the product for
a new use (off-label use). For those interested in further information on regulation by CDRH
or any of the other FDA centers, FDA guidance documents that cover all aspects of regulatory
approval are readily available online (www.fda.gov).
5.7 APPLICATION-SPECIFIC STRATEGIES FOR THE DESIGN
AND SELECTION OF BIOMATERIALS
5.7.1 Musculoskeletal Repair
The design and selection of the biomaterials components for any implant should be
based on restoring the biological function of the damaged or diseased tissue. The principal
function of musculoskeletal tissues is to provide a framework to support the organs and to
provide a means of locomotion. Bone, cartilage, tendons, ligaments, and muscles are all part
of the group of musculoskeletal tissues; however, they have different functions and differ-
ent biological properties. Each musculoskeletal tissue must be considered individually in
terms of implant design and biomaterials selection.
Bone is the only tissue capable of undergoing spontaneous regeneration. It is constantly
in a state of remodeling, always optimizing its structure to best meet the needs of the
body. This ongoing cellular activity is why astronauts rapidly lose bone mass during zero
gravity conditions and why jaw bone rapidly resorbs when teeth are pulled. Unlike bone,
cartilage is acellular and has a very limited capacity for repair. Therefore, damage to
cartilage is often permanent and often progressive. Cartilage provides an articulating
surface enabling low-friction movements between opposing bone surfaces. Ligaments
may appear on the surface to be easier to replace due to their “simple” function, but they
are not simply passive joint restraints; they also provide electromechanical signals for
joint-stabilizing muscle contractions.
Replacement of damaged or diseased tissues or organs is best accomplished by autograft
or allograft donor tissue, but they have limited availability, and a biomimetic synthetic sub-
stitute is the next best alternative. Biomimetic calcium phosphate materials (hydroxyapa-
tite) have been shown clearly to enhance bone cell activity and are either used alone or in
combination with collagen or other polymers as bone graft substitutes. Hydroxyapatite
not only influences bone cell attachment, but it also appears to control the differentiation
of stem cells to bone-forming cells. This is particularly important for tissue engineering
approaches that aim to not only restore function but also to restore the actual biological
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