The dynamic, six-axis stiffness matrix testing of porcine spinal specimens

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Abstract

Background Context
Complex testing protocols are required to fully understand the biomechanics of the spine. There remains limited data concerning the mechanical properties of spinal specimens under dynamic loading conditions in six axes.

Purpose
To provide new data on the mechanical properties of functional spinal unit (FSU), and isolated disc (ISD) spinal specimens in six degrees of freedom.

Study Design/Setting
Dynamic, six-axis stiffness matrix testing of porcine lumbar spinal specimens.

Methods
The stiffness matrix testing of lumbar porcine FSU (n=6) and ISD (n=6) specimens was completed in a custom six-axis spine simulator using triangle wave cycles at a frequency of 0.1 Hz. Specimens were first tested without an axial preload, and with an axial preload of 500 N with equilibration times of both 30 and 60 minutes. The study was supported through an institutional grant, and the authors are not aware of any conflicts of interest related to this research.

Results
The stiffness matrices were not symmetrical about the principal stiffness terms. The facets increased all the principal stiffness terms with the exception of axial compression-extension. Significant differences were detected in 15 stiffness terms due to the application of an axial preload in the ISD specimens, including an increase in all principal stiffness terms. There were limited differences in stiffness due to an equilibration time of 30 and 60 minutes.

Conclusions
The assumption of stiffness matrix symmetry used in many previous studies is not valid. The biomechanical testing of spinal specimens should be completed in 6 degrees of freedom, at physiological loading rates, and incorporate the application of an axial preload. The present study has provided new data on the mechanical properties of spinal specimens, and demonstrates that the dynamic stiffness matrix method provides a means to more fully understand the natural spine, and quantitatively assess spinal instrumentation.
Original languageEnglish
Pages (from-to)176-184
Number of pages9
JournalThe Spine Journal
Volume15
Issue number1
Early online date16 Sep 2014
DOIs
Publication statusPublished - 1 Jan 2015

Keywords

  • spine biomechanics
  • dynamic
  • stiffness matrix

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