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Research Papers

The Use of a Novel Injectable Hydrogel Nucleus Pulposus Replacement in Restoring the Mechanical Properties of Cyclically Fatigued Porcine Intervertebral Discs

[+] Author and Article Information
Christian Balkovec

University of Waterloo,
Waterloo, ON N2L 3G1, Canada

Jennifer Vernengo

Rowan University,
Glassboro, NJ 08028

Stuart M. McGill

University of Waterloo,
Waterloo, ON N2L 2G1, Canada
e-mail: mcgill@uwaterloo.ca

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received July 5, 2012; final manuscript received March 7, 2013; accepted manuscript posted April 22, 2013; published online May 9, 2013. Assoc. Editor: Yener N. Yeni.

J Biomech Eng 135(6), 061004 (May 09, 2013) (5 pages) Paper No: BIO-12-1270; doi: 10.1115/1.4024285 History: Received July 05, 2012; Revised March 07, 2013; Accepted April 22, 2013

Repeated flexion and extension of an intervertebral disc has been shown to affect the angular stiffness of spinal motion segments and is a barometer of the mechanical integrity of the disc. A degenerated disc that loses height causes higher levels of stress on the annulus and facet joints which may increase its level of degeneration; restoring disc height may therefore help to slow this degenerative cascade. Previous research has indicated that nucleus implants have the potential to improve the mechanical characteristics of a disc and an implant that is custom-fit to the intervertebral disc yields the best results with respect to decreasing annular degeneration. Two groups of porcine spinal motion segments were exposed to repeated flexion and extension. One group was then injected with a novel hydrogel while the other group was used as a control. Both groups were then exposed to another round of cyclic flexion and extension to examine the effect that the hydrogel had on restoring the original mechanics to the motion segments. Angular stiffness was restored to the group which received the hydrogel injection in addition to a significant improvement in specimen height. No significant changes were seen in the group which did not receive an injection. It would appear that use of the novel injectable hydrogel is able to restore angular stiffness to cyclically fatigued spinal motion segments. It is also important to note that continued repetition of the event causing specimen fatigue after performing hydrogel injection will result in an eventual return to the same fatigued state.

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Figures

Grahic Jump Location
Fig. 1

(a) Heating jacket used to bring the specimens to sufficient temperature for the hydrogel to change from a liquid to a gel state. Rubber tubing was attached circumferentially and had heated water cycled through it. (b) The jacket itself was then wrapped around the metal cups used to secure the specimens. This ensured that the heating jacket was not in physical contact with the specimen and did not interfere with the flexion or extension motions of the segment while still providing heat. (c) Entire heating jacket configuration secured around the specimen while being cyclically fatigued.

Grahic Jump Location
Fig. 2

Custom-built apparatus used to provide traction to specimens in order to achieve sufficient endplate separation facilitating hydrogel injection. Specimens were suspended while a 5 kg weight was hung to the carriage holding the bottom disc in place. A sleeve with tubing running heated water on its periphery provided heat to the specimens without interfering with flexion or extension motions.

Grahic Jump Location
Fig. 3

Angular stiffness values before and after the hydrogel injection for the experimental group of specimens reveal successful restoration of their mechanical behavior. Even after going through 8000 cycles of repetitive flexion and extension the fatigued specimens still closely follow the same pathway as a fresh specimen. Statistical tests revealed the pre- and post-conditions for the experimental group to not be significantly different from each other (p = 1.00).

Grahic Jump Location
Fig. 4

Angular stiffness values before and after the sham intervention protocol for the control group reveal statistically significant differences between the pre- and post-conditions. This indicates that the mechanical profile was not restored to the specimen after the control intervention (p = 0.041). Controls went through the same traction protocol as experimental specimens and were unloaded for the same amount of time, but they did not receive a hydrogel injection.

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