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Research Papers

Nucleus Implantation: The Biomechanics of Augmentation Versus Replacement With Varying Degrees of Nucleotomy

[+] Author and Article Information
Marco Cannella, Shanee Allen, Argjenta Orana

Department of Material Science and Engineering,
Drexel University,
3141 Chestnut Street,
Philadelphia, PA 19104

Jessica L. Isaacs

Mem. ASME
Department of Mechanical Engineering
and Mechanics,
Drexel University,
3141 Chestnut Street,
Philadelphia, PA 19104

Edward Vresilovic

Department of Orthopedic Surgery,
Milton S. Hershey Medical Center,
Penn State University,
500 University Dr.,
Hershey, PA 17033

Michele Marcolongo

Department of Material Science and Engineering,
Drexel University,
3141 Chestnut Street,
Philadelphia, PA 19104
e-mail: marcolms@drexel.edu

1Corresponding author.

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical Engineering. Manuscript received May 2, 2013; final manuscript received January 24, 2014; accepted manuscript posted March 6, 2014; published online April 10, 2014. Assoc. Editor: James C. Iatridis.

J Biomech Eng 136(5), 051001 (Apr 10, 2014) (9 pages) Paper No: BIO-13-1212; doi: 10.1115/1.4027056 History: Received May 02, 2013; Revised January 24, 2014; Accepted March 06, 2014

Nucleus pulposus replacement and augmentation has been proposed to restore disk mechanics in early stages of degeneration with the option of providing a minimally invasive procedure for pain relief to patients with an earlier stage of degeneration. The goal of this paper is to examine compressive stability of the intervertebral disk after either partial nucleus replacement or nuclear augmentation in the absence of denucleation. Thirteen human cadaver lumbar anterior column units were used to study the effects of denucleation and augmentation on the compressive mechanical behavior of the human intervertebral disk. Testing was performed in axial compression after incremental steps of partial denucleation and subsequent implantation of a synthetic hydrogel nucleus replacement. In a separate set of experiments, the disks were not denucleated but augmented with the same synthetic hydrogel nucleus replacement. Neutral zone, range of motion, and stiffness were measured. The results showed that compressive stabilization of the disk can be re-established with nucleus replacement even for partial denucleation. Augmentation of the disk resulted in an increase in disk height and intradiskal pressure that were linearly related to the volume of polymer implanted. Intervertebral disk instability, evidenced by increased neutral zone and ranges of motion, associated with degeneration can be restored by volume filling of the nucleus pulposus using the hydrogel device presented here.

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Figures

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Fig. 1

Schematic of experimental protocol for partial denucleation experimental group

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Fig. 2

Schematic of experimental protocol for nucleus augmentation group

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Fig. 3

IVDs were denucleated using the Nucleotome (Clarus-Medical, Minneapolis, MN) in 5 min increments (until 20 min) then implanted with some volume of hydrogel nucleus replacement to restore to the intact disk height (DH ± 0.9%) and 5% above the intact disk height (+5DH ± 1.5%). There is a nonlinear implantation of material with a 17.5 and 7.8 min time constant, respectively. A third data set represents the augmented samples that were restored to 5% above intact disk height (+5%DH: Augmented). Average implant volumes are depicted with corresponding standard error means.

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Fig. 7

(a) Normalized NZ, tROM and cROM for augmented specimens. (b) Normalized stiffness for augmented specimens. (Average values are depicted with corresponding standard error means. p < 0.05: *statistically different from intact, +statistically different from +2.5%DH implantation).

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Fig. 6

Intact IVDs were augmented by adding some percentage of hydrogel into the nucleus pulposus. Percentage volume implanted versus (a) percentage disk height (normalized to intact) and (b) percentage pressure (normalized to intact) were both fit by the method of least squares.

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Fig. 5

Normalized (a) NZ, tROM, and cROM and (b) stiffness for partially denucleated specimens when injected with implant to (i) 5% over intact disk height and (ii) intact disk height. (Average values are depicted with corresponding standard error means. p < 0.05: *statistically different from intact, +statistically different from 20 min of denucleation, @statistically different from 15 and 20 min of denucleation).

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Fig. 4

IVDs were denucleated at time steps (5, 10, 15, and 20 min) then injected with some volume of hydrogel implant to restore to initial disk height. Percentage volume implanted versus (a) disk height normalized to intact and (b) change of pressure from intact were both fit by the method of least squares for: (i) the specimens after 20 min of denucleation and (ii) a representative anterior column unit at four time points.

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Fig. 8

Comparison of augmented and denucleated states when restored to 105% of the intact disk height for normalized (a) NZ, tROM, and cROM and (b) stiffness. (Average values are depicted with corresponding standard error means. p < 0.05: *statistically different from intact, +statistically different from augmented implantation).

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