Quantifying intervertebral disc mechanics: a new definition of the neutral zone

T.H. Smit, M.S.L.M. van Tunen, A.J. van der Veen, I. Kingma, J.H. van Dieen

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    Background: The neutral zone (NZ) is the range over which a spinal motion segment (SMS) moves with minimal resistance. Clear as this may seem, the various methods to quantify NZ described in the literature depend on rather arbitrary criteria. Here we present a stricter, more objective definition. Methods. To mathematically represent load-deflection of a SMS, the asymmetric curve was fitted by a summed sigmoid function. The first derivative of this curve represents the SMS compliance and the region with the highest compliance (minimal stiffness) is the NZ. To determine the boundaries of this region, the inflection points of compliance can be used as unique points. These are defined by the maximum and the minimum in the second derivative of the fitted curve, respectively. The merits of the model were investigated experimentally: eight porcine lumbar SMS's were bent in flexion-extension, before and after seven hours of axial compression. Results: The summed sigmoid function provided an excellent fit to the measured data (r
    Original languageEnglish
    Article number38
    JournalBMC Musculoskeletal Disorders
    Publication statusPublished - 2011


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