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Research Papers

Quantitative Analysis of the Nonlinear Displacement–Load Behavior of the Lumbar Spine

[+] Author and Article Information
Andrew D. Hanlon, Daniel J. Cook, Matthew S. Yeager

Department of Neurosurgery,
Allegheny General Hospital,
420 East North Avenue,
Pittsburgh, PA 15212

Boyle C. Cheng

Department of Neurosurgery,
Allegheny General Hospital,
420 East North Avenue,
Pittsburgh, PA 15212
e-mail: boylecheng@yahoo.com

1Corresponding author.

Manuscript received July 11, 2013; final manuscript received May 22, 2014; accepted manuscript posted May 29, 2014; published online June 11, 2014. Assoc. Editor: Brian D. Stemper.

J Biomech Eng 136(8), 081009 (Jun 11, 2014) (7 pages) Paper No: BIO-13-1313; doi: 10.1115/1.4027754 History: Received July 11, 2013; Revised May 22, 2014; Accepted May 29, 2014

There is currently no universal model or fitting method to characterize the visco-elastic behavior of the lumbar spine observed in displacement versus load hysteresis loops. In this study, proposed methods for fitting these loops, along with the metrics obtained, were thoroughly analyzed. A spline fitting technique was shown to provide a consistent approximation of spinal kinetic behavior that can be differentiated and integrated. Using this tool, previously established metrics were analyzed using data from two separate studies evaluating different motion preservation technologies. Many of the metrics, however, provided no significant differences beyond range of motion analysis. Particular attention was paid to how different definitions of the neutral zone capture the high-flexibility region often seen in lumbar hysteresis loops. As a result, the maximum slope was introduced and shown to be well defined. This new parameter offers promise as a descriptive measurement of spinal instability in vitro and may have future implications in clinical diagnosis and treatment of spinal instability. In particular, it could help in assigning treatments to specific stabilizing effects in the lumbar spine.

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References

Figures

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Fig. 1

Radiographic image of an FSU from the nucleus augmentation study in the augmented condition

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Fig. 2

Posterior view of lumbar spine specimen with the Stabilimax device at L4–L5

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Fig. 3

A representative hysteresis loop of a functional spinal unit in flexion–extension in the third cycle of flexibility testing with a spline fit. The fit approximates the flexible region, the stiff region, and the transition between them.

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Fig. 4

A hysteresis loop with the NZ shaded. The NZ slopes were averaged over the shaded region (NZ = neutral zone).

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Fig. 5

A hysteresis loop with the LZ shaded. The LZ slopes were averaged over the shaded region. The zero load extrapolation lines were used to determine the bounds of the LZ as described by Crawford et al. [14] (LZ = lax zone).

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Fig. 6

Average range of motion values with standard errors over the three conditions for the specimens which underwent a nucleus augmentation treatment (FE = flexion–extension and LB = lateral bending)

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Fig. 7

Average maximum slope values with standard errors over the three conditions for the specimens which underwent a nucleus augmentation treatment (F = flexion, E = extension, LLB = left lateral bending, and RLB = right lateral bending)

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Fig. 8

Average range of motion values with standard errors over the five conditions for the index level specimens in the Stabilimax study (FE = flexion–extension, LB = lateral bending, AT = axial torsion, and AC = axial compression)

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Fig. 9

Average maximum slope values with standard errors over the five conditions for the specimens which underwent a Stabilimax treatment (F = flexion, E = extension, LLB = left lateral bending, RLB = right lateral bending, RT = right torsion, and LT = left torsion)

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Fig. 10

A representative derivative of the hysteresis loop for a specimen in the third cycle of flexion. A clear area of high flexibility is seen between−1 and 0.75 Nm. The boundaries of Panjabi's neutral zone and Crawford's lax zone fail to capture that region.

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