Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2024 Apr 23;7(2):e1332.
doi: 10.1002/jsp2.1332. eCollection 2024 Jun.

Comparison of four in vitro test methods to assess nucleus pulposus replacement device expulsion risk

Tamanna Rahman  1   2 , Matthew J Kibble  1 , Gianluca Harbert  1 , Nigel Smith  3 , Erik Brewer  4 , Thomas P Schaer  5 , Nicolas Newell  1
Affiliations

Comparison of four in vitro test methods to assess nucleus pulposus replacement device expulsion risk

Tamanna Rahman et al. JOR Spine. .

Abstract

Background: Nucleus replacement devices (NRDs) are not routinely used in clinic, predominantly due to the risk of device expulsion. Rigorous in vitro testing may enable failure mechanisms to be identified prior to clinical trials; however, current testing standards do not specify a particular expulsion test. Multiple methods have therefore been developed, complicating comparisons between NRD designs. Thus, this study assessed the effectiveness of four previously reported expulsion testing protocols; hula-hoop (Protocol 1), adapted hula-hoop (Protocol 2), eccentric cycling (Protocol 3), and ramp to failure (Protocol 4), applied to two NRDs, one preformed and one in situ curing.

Methods: Nucleus material was removed from 40 bovine tail intervertebral disks. A NRD was inserted posteriorly into each cavity and the disks were subjected to one of four expulsion protocols.

Results: NRD response was dependent on both the NRD design and the loading protocol. Protocol 1 resulted in higher migration and earlier failure rates compared to Protocol 2 in both NRDs. The preformed NRD was more likely to migrate when protocols incorporated rotation. The NRDs had equal migration (60%) and expulsion (60%) rates when using unilateral bending and ramp testing. Combining the results of multiple tests revealed complimentary information regarding the NRD response.

Conclusions: Adapted hula-hoop (Protocol 2) and ramp to failure (Protocol 4), combined with fluoroscopic analysis, revealed complimentary insights regarding migration and failure risk. Therefore, when adopting the surgical approach and animal model used in this study, it is recommended that NRD performance be assessed using both a cyclic and ramp loading protocol.

Keywords: biomechanical testing; expulsion; hydrogel; intervertebral disk; nucleus replacement; spine.

PubMed Disclaimer

Conflict of interest statement

Thomas P. Schaer was involved in research support as Principal Investigator and the patenting of the hydrogel‐based technologies for ReGelTec, Inc. and Johnson and Johnson DePuy‐Synthes. Additionally, Thomas P. Schaer is a paid consultant and stock supplier for ReGelTec, Inc. Nigel Smith was involved in the development and patenting of hydrogel‐based technologies for Synthes Spine as Director of Non‐Fusion Technologies. Nigel Smith and Erik Brewer were involved in the development and patenting with ReGelTec Inc. of the Hydrafil™ Injectable hydrogel for minimally invasive treatment for treating degenerative disk disease. Nigel Smith and Erik Brewer are current shareholders of ReGelTec Inc. (https://regeltec.com/) that is active in the clinical evaluation of Hydrafil™. Additionally, Erik Brewer receives research funding from ReGelTec, Inc.

Figures

FIGURE 1
FIGURE 1
Axial view of transversely cut bovine disks following insertion of (A) Hydrogel A and (B) Hydrogel B. Hydrogel A was a thin, cylindrical (1.5 mm diameter) preformed nucleus replacement device (NRD) that bundled to fill the disk cavity. Hydrogel B was an in situ curing NRD that, once injected, conformed to fill the disk cavity prior to curing.
FIGURE 2
FIGURE 2
Schematic showing the different testing subgroups. Protocol 1: hula‐hoop; Protocol 2: adapted hula‐hoop; Protocol 3: unilateral cyclic loading; and Protocol 4: ramp to failure.
FIGURE 3
FIGURE 3
Schematic depicting the four expulsion testing protocols investigated. In all protocols, an eccentric load was applied at a moment arm of 30 mm from the coronal midline of the disk. In Protocols 1, 2, and 3, a sinusoidal load between 50 and 250 N was applied at 5 Hz until 100 000 cycles were completed or nucleus replacement device extrusion or expulsion occurred. In Protocol 4, an off‐axis ramp to failure was applied at 2 mm/min.
FIGURE 4
FIGURE 4
Unconfined compression results from Hydrogel A (preformed) and Hydrogel B (in situ curing) compared to the human cadaveric NP values reported by Cloyd et al. (A) Average stress–strain curves 20 obtained using mean hydrogel values fit according to σ = A(e βε−1). (B) Average toe and linear region Young's moduli. Error bars represent one standard deviation. Significant differences are denoted by an asterisk (p < 0.05).
FIGURE 5
FIGURE 5
Images of bovine specimens following (A) extrusion, (B) expulsion, and (C) migration. The purple arrows point to nucleus replacement devices (NRDs) following failure. NRD migration was observed using fluoroscopic images (C). Extrusion was defined as NRD protrusion exceeding 2 mm through the annular defect, while expulsion was a protrusion of more than 10 mm. Migration was defined as an observable geometric change or internal displacement of the NRD, plus minor protrusions (less than 2 mm) not meeting the criterion for failure via extrusion or expulsion. VB, vertebral bodies.
FIGURE 6
FIGURE 6
(A) Bar graph summarizing the effects of using different loading protocols on the two nucleus replacement devices (NRDs). (B) Lateral view fluoroscopic images of two typical instances of NRD geometry change. The yellow arrows indicate the direction in which the NRD shape has changed. n = 5 of each hydrogel was tested for each protocol.
FIGURE 7
FIGURE 7
Logarithmic dot‐plot of the cycles to nucleus replacement devices extrusion or expulsion (note data from tests where migration was seen are not included here) when using Protocols 1, 2, or 3. Median values are reported with bars spanning the range.
FIGURE 8
FIGURE 8
Dot‐plot graph of intervertebral disk heights at the various stages of testing. Significant differences are denoted by asterisks (p < 0.05).
See this image and copyright information in PMC

References

    1. Daffner SD, Hymanson HJ, Wang JC. Cost and use of conservative management of lumbar disc herniation before surgical discectomy. Spine J. 2010;10:463‐468. - PubMed
    1. Rajaee SS, Bae HW, Kanim LEA, Delamarter RB. Spinal fusion in the United States: analysis of trends from 1998 to 2008. Spine. 2012;37:67‐76. - PubMed
    1. Cheh G, Bridwell KH, Lenke LG, et al. Adjacent segment disease following lumbar/thoracolumbar fusion with pedicle screw instrumentation. Spine. 2007;32:2253‐2257. - PubMed
    1. Helgeson MD, Bevevino AJ, Hilibrand AS. Update on the evidence for adjacent segment degeneration and disease. Spine J. 2013;13:342‐351. - PubMed
    1. Min J‐H, Jang J‐S, Lee S‐H. Comparison of anterior‐ and posterior‐approach instrumented lumbar interbody fusion for spondylolisthesis. J Neurosurg Spine. 2007;7:21‐26. - PubMed