Biomedical Engineering Reference
In-Depth Information
implicating ADAMTS-4 as the major source of aggrecan destruction in the degen-
erative process. In addition, ECM synthesis can be dramatically hampered due to
cell senescence, apoptosis, necrosis, and altered cell phenotype within degenerated
IVDs [
43
].
Taken together, these biochemical and cellular transformations within the
degenerating IVD result in macroscopic changes manifest as a dehydrated and
fibrous NP, disorganized AF lamellae, and an inconspicuous boundary between
the NP and AF. Concentric and/or radial annular tears, and NP fissure formations
are also observed within the severely degenerated IVD, and an overall reduction in
the central disc height may occur [
11
,
31
]. Additionally, blood vessels and nerve
endings have been found to infiltrate into the center of the disc, which may harbor
inflammatory cells and their mediators.
6 Functional Consequences of IDD
Degenerative changes within the IVD, and in particular the NP, result in functional
changes to the disc and adjacent spinal structures. The NP has been shown to
transition from a fluid or gel-like material to that of a solid as indicated by an
increase in shear modulus (range: 5-60 kPa) and a diminished capacity to dissipate
energy [
44
]. As proteoglycan content of the NP decreases, the water content,
swelling capacity and thus the hydrostatic nature of the NP is lost [
45
]. This in
turn affects the IVD's ability to distribute stress evenly across the disc and causes
stress concentrations to be most prevalent in the postero-lateral AF. As a result, the
AF directly supports compressive spinal loads, which together with loss of hydro-
static pressure of the NP may lead to increased shear stresses between AF lamellae
and thus contribute to their structural failure, increasing the risk of disc herniation
[
46
,
47
]. Concomitantly, circumferential and radial strains have been observed to be
greater in the postero-lateral region of degenerated IVD, resulting in disc bulging
and reduction in disc height [
48
].
7 Nucleus Pulposus Replacement with Polymeric Materials:
A Potential Treatment Option for Symptomatic IDD
Surgical options commonly employed to combat the effects of IDD range in
invasiveness and depend on the extent and indication of the degenerative process.
While approximately two-thirds of patients undergo successful pain relief using
conservative treatments such as bed rest, anti-inflammatory drugs, chiropractic
adjustment, and physical therapy, the remainder will require more invasive surgical
intervention. These procedures include the removal or obliteration of herniated
or degenerate NP material, or complete removal of the disc, which requires
