Biomedical Engineering Reference
In-Depth Information
developed [
61
]. Genetically altered
Escherichia coli
bacteria produce the copoly-
mer, which is subsequently modified such that a diisocyante-based crosslinker can
be used to stabilize the system. The result is a cured material that has similar water,
protein, and mechanical properties to the native NP [
61
]. An in situ polymerizing
glutaraldehyde crosslinked albumin-based hydrogel has also been investigated as
protein-based NPR; however, information on this particular material is limited [
62
].
Pre-formed NPRs should exhibit the ability to be compressed or dehydrated
to minimize their profile for minimally invasive insertion and for preservation of
surrounding anatomic structures. Moreover, the dimensional constraints of the
patient's NP must be considered pre-operatively in order to determine appropriate
implant sizing and optimal fit. Theoretically, as these materials swell, the pressure
generated aids in restoring/maintaining IVD height while the application of com-
pressive loading during daily activities would result in partial load transmission to
the AF by the implant. One of the most widely investigated pre-formed devices is
composed of a hydrogel pellet core of polyacrylonitrile and polyacrylamide copol-
ymer that is surrounded by a woven jacket of ultrahigh molecular weight poly-
ethylene (UHMWPE). As the hydrophilic core swells and absorbs body fluid, the
surrounding jacket prevents excessive expansion that could damage the adjacent
endplates. This device has exhibited excellent compressive fatigue-resistant pro-
perties and optimistic clinical outcomes [
63
]. Similarly, an acrylic copolymer
layered with dacron mesh that prohibits excessive lateral expansion has been
developed and exhibits the innate ability to absorb water while concomitantly
generating an axial lift force of up to 400 N [
64
].
Polyvinyl alcohol (PVA)/polyvinyl pyrrolidone (PVP) copolymers have also been
investigated for use as pre-formed NPRs. This material can be formed by successive
freeze-thaw cycles that create physical crosslinks via interchain hydrogel bonding
between the two components [
65
-
67
]. It was theorized that this material can be
partially dehydrated for implantation and that, following insertion and rehydration,
it would recapitulate the intradiscal pressure observed within the normal NP [
66
].
Unconfined compressive fatigue testing of the copolymer showed that there was no
change in polymer content, indicating the stability of the material [
65
]. However,
after 10 million testing cycles a permanent set was indicated by an un-recoverable
decrease in sample height and an increase in sample diameter compared with original
values [
65
]. Hydrogels composed of PVA alone have also been developed that
exhibit osmotic swelling pressure profiles similar to those of human NP [
68
].
In consideration of the clinical importance of being able to visualize any implant
material within the body, radio-opaque hydrogels for NPR have been formulated
[
69
]. Copolymers of iodobenzoyl-oxo-ethyl methacrylate (4IEMA) and hydrophilic
PVP or hydroxylethyl methacrylate (HEMA) exhibit appropriate swelling
characteristics, viscoelastic mechanical properties, and excellent cytocompatibility
[
69
]. Moreover, inclusion of the covalently attached iodine molecules allowed for
hydrogel visualization via X-ray in a porcine cadaveric spine model [
69
].
Although many designs and materials to date have shown promise, some
limitations are becoming evident. Pre-formed synthetics fail to fill the entire void
space following removal of the NP, leaving open the potential for device migration
