Biomedical Engineering Reference
In-Depth Information
subsequent spinal fusion or total disc arthroplasty [
11
,
49
]. Despite the availability
of these treatments, they are palliative solutions at best. Fusion and disc arthroplasty
are considered to be last resort options for patients with severe symptomatic IDD.
Patients must wait until the degeneration and resulting pain and instability is severe
enough to warrant the removal of the disc, only to be replaced with stiff immobile
metallic fusion hardware or articulating joints. Consequences arising from these
treatments include spinal stenosis, accelerated degeneration in adjacent spinal
segments, altered range of motion and intradiscal pressure in adjacent levels,
subsidence, wear debris generation, and implant migration [
50
-
55
]. From this, it
is clear that current surgical options have serious limitations and that a gap in
treatment options exists [
56
].
An alternative surgical solution, nucleus pulposus replacement (NPR), is being
investigated as a minimally invasive, early-stage intervention for patients with mild
to moderate IDD. As degeneration first manifests within the NP, it is believed that
removal of the degenerated NP and its subsequent replacement with an alternative
mimetic material can mitigate the progression of IDD, while relieving pain and
restoring anatomic and biomechanical function in select patients [
57
]. To date, most
NPRs have been developed from in situ forming or pre-formed synthetic and
biologic polymers, which are currently in different stages of development or
clinical evaluation. We will briefly describe select devices and the polymeric
materials from which they have been developed. For a more in depth review of
commercial device designs, and pre-clinical, and clinical outcomes the readers are
directed to the following resource: “Motion Preservation of the Spine: Advanced
Techniques and Controversies” [
58
].
In situ curing materials offer the advantageous characteristics of being implanted
using minimally invasive techniques and allow for complete filling of the NP region
of the IVD. Detrimental characteristics of this class of materials include the
potential for exothermic reactions during curing which can elicit native tissue/
cellular destruction and migration of these flowable materials away from the NP
region prior to curing. One copolymer material developed in consideration of these
characteristics is a methylenediphenyldiisocyanate-polyurethane mixture, which is
injected into an expandable polycarbonate urethane containment balloon tailored to
the central region of the patient's IVD [
59
]. Once cured, this material forms a
resilient and incompressible material that can re-establish pre-degeneration disc
heights.
Copolymers of polyethylene glycol dimethacrylate (PEGDM) and poly
(
N
-isopropylacrylamide) (PNIPAAM) have also been investigated as potential
injectable materials for NPR [
60
]. Coupling together the thermoresponsive nature
of PNIPAAM with the hydrophilicity of PEGDM, investigators were able to form
hydrogels that exhibited the capacity to gel at 32
C, which allows for gelation within
the human body. These hydrogels exhibit mechanical properties that can be tailored
and an elastic characteristic that allows for the recovery of 85-98% of hydrogel
original height by 55 min following stress relaxation testing [
60
].
As an alternative to purely synthetic injectable materials, an NPR system
composed of a recombinant protein block copolymer of silk and elastin has been
