Biomedical Engineering Reference
In-Depth Information
mediated and the responsible enzymes are hyaluronidases. The degradation products are oligo-
saccharides and low molecular weight hyaluron. HA was fi rst developed as a biomedical product
in the 1970s and used for eye surgery in the 1980s. Healon was used for corneal transplantation,
cataract surgery, and retinal detachment.
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It has been used in the treatment of osteoporosis and
is generally administered as a course of injections.
97,98
This topic has been eloquently reviewed by
Allison & Grande-Allen and further details are not within the scope of this chapter.
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15.4 APPLICATIONS
Degradable polymers have found widespread use as biomaterials for biomedical applications.
Biomaterials have been used for skin, bone, cartilage, vascular, nerve, and drug delivery.
100
15.4.1 O
RTHOPEDICS
The use of bioabsorbable materials has become commonplace in orthopedic surgery. Screws, suture
anchors, meniscal repair devices, bone plates, staples, and simple fracture fi xation devices have
expanded the armamentarium of the orthopedic surgeon, and are increasingly used for anterior cru-
ciate ligament reconstruction, shoulder surgery, meniscal repair, and fracture care.
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-
103
The three
most commonly used polymers in clinical applications are PGA, PLA, and polydioxanone.
104
These
polymers are α-polyesters or poly(α-hydroxy) acids and can be used for stabilization of fractures,
osteotomies, bone grafts, and fusions, as well as for reattachment of ligaments, tendons, meniscal
tears, and other soft tissue structures. The advantage of such devices is their resorbability, thus the
need for a removal operation is overcome and long-term interference with tendons, nerves, and
the growing skeleton is avoided. The polymeric devices also provide lower moduli, thus, problems
associated with periprosthetic stress shielding and infections are reduced.
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Poly-l-lactic bioab-
sorbable interference screws have been reported to provide clinical results, which are similar to
metal interference screw for fi xation of a central third bone-patellar tendon-bone graft in ACL
reconstruction.
15.4.2 T
ISSUE
E
NGINEERING
AND
D
EGRADABLE
P
OLYMERS
The fi eld of tissue engineering is a rapidly emerging fi eld that aims to address a diverse range of clin-
ical needs for the replacement of damaged or diseased tissues. Biomaterials, on its own, are able to
address certain clinical needs through the development of prosthetic hearts, valves, blood vessels,
and artifi cial breasts; however, only few perform adequately over the life span of a patient. Tissue
engineering aims to create three-dimensional tissues and organs using autologous cells. The three
main components of tissue engineering are namely the scaffold, appropriate cells, and the pres-
ence of appropriate growth factors. Thus, the scaffold is generally intended to provide the shape of
the construct, which then is eliminated by the body, while the natural tissues regrow. Scaffolds
thus form an integral part of the tissue-engineered construct and must be able to direct the ar range-
ment of cells in an appropriate three-dimensional confi guration and present molecular signals in
appropriate spatial and temporal manner
.
Growth factors are increasingly employed to promote
tissue regeneration with various biomaterial scaffolds.
In vitro
release kinetics of protein growth
factors from tissue engineering scaffolds are often investigated in aqueous environment, which
is signifi cantly different from
in vivo
environment. Biodegradable polymers, thus form the basis
of a large number of scaffolds that are used for tissue engineering; however, detailed discussion
is outside the scope of this chapter. Nerve tissue engineering is expected to be one of the most
promising methods for the restoration of central nerve system damages in health care. Spinal cord
injury, particularly when it involves partial or complete transection of the cord, is devastating,
as transected axons, in the absence of further intervention, do not regenerate. Research has been
mainly focused on the development of regimes to overcome this lack of axon growth, and limited
but signifi cant strides have been made within the past decade. Three-dimensional distribution and
