Abstract
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.
Highlights
The neutral zone (NZ) is the range over which a spinal motion segment (SMS) moves with minimal resistance
The NZ is seen as play in the SMS when moving from one direction to the other, which is essentially different from the original definition as the zone of minimal stiffness [2]
The NZ magnitude according to the double sigmoid function was not correlated with hysteresis (r = -0.194; p = 0.472), The NZ stiffness determined in extension-flexion and flexion-extension averaged 0.138 (SD 0.060) and 0.133 (SD 0.067) Nm/degree, respectively, without a systematic difference (p = 0.394) and an intra-class correlation (ICC) of 0.971, indicating good correspondence
Summary
The neutral zone (NZ) is the range over which a spinal motion segment (SMS) moves with minimal resistance. One example of such a property is the range of motion (ROM), which is defined as the deflection difference between the maximum applied loads in each direction (Figure 1) Another quantification used in the load-deflection curve is the neutral zone (NZ), described as the range over which a motion segment moves with minimal resistance [1,2]. This range increases with (mild) degeneration of the Panjabi [2] operationally defined the NZ based on the finding that a motion segment does not return to its initial position after loading in a particular direction. Both the maximum load applied, and the loading history, and the point in time at which the residual displacement is measured, are arbitrarily chosen
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