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

BMC Musculoskeletal Disorders

Research Article

Idsart Kingma¹  Jaap H van Dieën¹  Theodoor H Smit²  Manon SLM van Tunen³  Albert J van der Veen³ 

[1] Research Institute MOVE, Faculty of Human Movement Sciences, VU University, Amsterdam, The Netherlands;VU University Medical Center, Department of Orthopaedic Surgery, Research Institute MOVE, Amsterdam, The Netherlands;VU University Medical Center, Department of Physics and Medical Technology, Research Institute MOVE, Amsterdam, The Netherlands;

关键词: Intervertebral Disc;    Axial Compression;    Sigmoid Function;    Spinal Segment;    Neutral Zone;   

DOI  :  10.1186/1471-2474-12-38

 received in 2010-05-12, accepted in 2011-02-07,  发布年份 2011

来源: Springer

【 摘 要 】

BackgroundThe 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.MethodsTo 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.ResultsThe summed sigmoid function provided an excellent fit to the measured data (r2 > 0.976). The NZ by the new definition was on average 2.4 (range 0.82-7.4) times the NZ as determined by the more commonly used angulation difference at zero loading. Interestingly, NZ consistently and significantly decreased after seven hours of axial compression when determined by the new definition. On the other hand, NZ increased when defined as angulation difference, probably reflecting the increase of hysteresis. The methods thus address different aspects of the load-deflection curve.ConclusionsA strict mathematical definition of the NZ is proposed, based on the compliance of the SMS. This operational definition is objective, conceptually correct, and does not depend on arbitrarily chosen criteria.

【 授权许可】

CC BY   
© Smit et al; licensee BioMed Central Ltd. 2011