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

Abstract

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 8Nm, a frequency of 0.1Hz, and with axial preloads of 0 and 500N. The stiffness, phase angle, and R value of the moment against rotation in the principal axis were calculated at the center of each specimen. The tracking error was adopted asa 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.

Influence of testing environment and loading rate on intervertebral disc compressive mechanics: An assessment of repeatability at three different laboratories

“…Previous studies demonstrated the importance of disc hydration, 3‐5 preload, 6‐9 and test rate 10‐15 . More recently, studies have compared biomechanical testing between laboratories, to investigate the application of pure moments to multi‐level spinal specimens 16 and to assess differences between six‐axis testing systems 17 . These studies have highlighted the importance of consistent methods for data processing, yet there is still a lack of consistency in specimen preparation and testing conditions used throughout the spinal community 18 .…”

Spine biomechanical testing methodologies: The controversy of consensus vs scientific evidence

“…While a 400 N axial preload is commonly applied to the lumbar spine during in vitro ROM testing, a range of loads have been used depending on the in vivo loading condition being replicated. Axial compressive preloads may range from 0 to 250 N, 23,145,160‐166 350 to 500 N, 11,43,55,126,137,142,145,148,165,167,168 and greater than 500 N 26,55,92,95,150,151,155,167,169‐173 . For conditions to simulate in vivo bending of the lumbar spine, axial compressive preloads above 500 N are generated 144 depending on disc cross‐sectional area.…”

“…Finally, an analysis of repeatability of methodologies, within or between laboratories is important to consider. 168,177,193,311 A number of these aspects were explored in the survey, which are summarized below.…”

“…American male of weight 900 N, 159 (Table 2). 144 T A B L E 1 Axial preload applied to the spine is a result of the weight of the head in the cervical spine and torso in the lumbar spine 23,145,[160][161][162][163][164][165][166] 350 to 500 N, 11,43,55,126,137,142,145,148,165,167,168 and greater than 500 N. 26,55,92,95,150,151,155,167,[169][170][171][172][173] For conditions to simulate in vivo bending of the lumbar spine, axial compressive preloads above 500 N are generated 144 depending on disc cross-sectional area. Low magnitude axial loads (0-250 N) may simulate lying down, but several studies have applied pure moments with no axial compressive preload.…”

“…in vivo translations and rotations. There is substantial work based on both the application of pure moments, either with axial loads 128,136,137,142,150,155,160,161,163,164,[166][167][168]171,179,197,[203][204][205][206] or without axial loads, 7,39,63,134,137,162,[174][175][176][177][207][208][209][210][211][212][213][214] and with using displacement control/stiffness test methods also being commonly used. 23,32,43,125,126,148,165,[215][216][217][218] Th...…”