The equivalence of multi-axis spine systems: Recommended stiffness limits using a standardized testing protocol

Timothy Holsgrove, Dhara Amin, Sonia Ramos Pascual, Boyin Ding, William Welch, Sabina Gheduzzi, Anthony Miles, Beth Winkelstein, John Costi

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The complexity of multi-axis spine testing often makes it challenging to compare results from different studies. The aim of this work was to develop and implement a standardized testing protocol across three six-axis spine systems, compare them, and provide stiffness and phase angle limits against which other test systems can be compared. Standardized synthetic lumbar specimens (n = 5), comprising three springs embedded in polymer at each end, were tested on each system using pure moments in flexion–extension, lateral bending, and axial rotation. Tests were performed using sine and triangle waves with an amplitude of 8 Nm, a frequency of 0.1 Hz, and with axial preloads of 0 and 500 N. The stiffness, phase angle, and R2 value of the moment against rotation in the principal axis were calculated at the center of each specimen. The tracking error was adopted as a measure of each test system to minimize non-principal loads, defined as the root mean squared difference between actual and target loads. All three test systems demonstrated similar stiffnesses, with small (<14%) but significant differences in 4 of 12 tests. More variability was observed in the phase angle between the principal axis moment and rotation, with significant differences in 10 of 12 tests. Stiffness and phase angle limits were calculated based on the 95% confidence intervals from all three systems. These recommendations can be used with the standard specimen and testing protocol by other research institutions to ensure equivalence of different spine systems, increasing the ability to compare in vitro spine studies.
Original languageEnglish
Pages (from-to)59-66
Number of pages8
JournalJournal of Biomechanics
Early online date14 Sep 2017
Publication statusPublished - 21 Mar 2018



  • Multi-axis
  • Six-axis
  • Spine simulator
  • Spine testing
  • Test machines
  • Test systems

ASJC Scopus subject areas

  • Biophysics
  • Orthopedics and Sports Medicine
  • Biomedical Engineering
  • Rehabilitation

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