Measuring the neutral zone of spinal motion segments: Comparison of multiple analysis methods to quantify spinal instability

Abstract

PurposeNeutral zone (NZ) parameters in spinal biomechanics studies are sensitive to spinal instability, disc degeneration, and repair. Multiple methods in the literature quantify NZ, yet no consensus exists on applicability and comparability of methods. This study compares five different NZ quantification methods using two different load‐deflection profiles.MethodsRat caudal and lumbar motion segments were tested in axial rotation to generate load‐deflection curves with profiles exhibiting prominent distinction between elastic and NZ regions (ie, triphasic) and profiles that did not (ie, viscoelastic). NZ was quantified using five methods: trilinear, double sigmoid (DS), zero load, stiffness threshold (ST), and extrapolated elastic zone. Absolute agreement and consistency of NZ parameters were assessed using intraclass correlation (ICC), Bland‐Altman analyses, and analysis of variance.ResultsFor triphasic profiles, NZ magnitude exhibited high consistency (methods correlate but differ in absolute values), and only some methods exhibited agreement. For viscoelastic profiles, NZ magnitude showed limited consistency and no absolute agreement. NZ stiffness had high agreement and consistency across most methods and profiles. For triphasic profiles, the linear NZ regions for all methods were not well‐described by a linear fit yet for viscoelastic profiles all methods characterized a linear NZ region.ConclusionThis NZ comparison study showed surprisingly limited agreement and consistency among NZ parameters with approximately 5% to 100% difference depending on the method and load‐deflection profile. Nevertheless, the DS and ST methods appeared to be most comparable. We conclude that most NZ quantification methods cannot be applied interchangeably, highlighting a need to clearly state NZ calculation methods. Future studies are required to identify which methods are most sensitive to disc degeneration and repair in order to identify a “best” method.

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

“…Due to the nonlinear behavior of the disc, multiple approaches have been developed for data analysis (eg, calculating neutral zone and linear region stiffness) 269,270 . Limited consensus regarding data analysis methods has been shown to cause significant differences in reported values 271 . Calculating a neutral zone may not be clinically relevant, as this is the point in vitro where the disc has zero resistance to load between tension and compression.…”

“…269,270 Limited consensus regarding data analysis methods has been shown to cause significant differences in reported values. 271 Calculating a neutral zone may not be clinically relevant, as this is the point in vitro where the disc has zero resistance to load between tension and compression. The magnitude of preload also dictates the linearity of the response to dynamic loading.…”

Tenomodulin and Chondromodulin-1 Are Both Required to Maintain Biomechanical Function and Prevent Intervertebral Disc Degeneration

“…Axial data were analyzed for compressive and tensile elastic zone (EZ) stiffness, range of motion (ROM), and neutral zone (NZ) length from the 20th cycle using a custom MATLAB program 26 (MathWorks; Fig. 1D).…”

“…Axial NZ length was defined as the portion of the load-displacement curve where motion was produced with minimal resistance 27 and was measured from the load versus displacement curve using a custom MATLAB script, employing the double sigmoid method. 25,26,28 Torsional data were analyzed for torque range and torsional stiffness from the 20th cycle (Fig. 1D).…”

Improving reproducibility in spine research

“…This in silico model enables the simulation of clinical parameters which have not been previously possible. Takeoda et al 6 developed and characterized a novel cell culture system using a semipermeable membrane pouch device to test the effects of culturing bovine nucleus pulposus cells under constant and cyclic hydrostatic pressure under high osmolality. This culture system and pouches have potential use for testing the effects of motion on biochemical changes in the nucleus pulposus cells in vitro. Di Pauli et al 7 compare five different methods to determine neutral zone of rat spinal motion segments using two different load deflection profiles demonstrating poor agreement between methodologies, highlighting the need for further work to identify recommended methods which are most sensitive to disc degeneration and can be used in treatment studies.…”