THE SPINAL LAX ZONE AND NEUTRAL ZONE - MEASUREMENT TECHNIQUES AND PARAMETER COMPARISONS

The neutral zone (NZ) is a well-established parameter that describes t he loose region of the spinal range of motion (ROM) where the spine mo ves easily with minimal applied force. The loose region is of interest clinically because in vitro studies show that with the onset of insta bility, this region increases before the ROM increases. The upper boun dary of the NZ has been described as the displacement at which ligamen tous resistance just begins. However, we hypothesized that a different parameter that we have named the lax zone (LZ) more accurately descri bes the region of ligamentous laxity. We hypothesized that the NZ is a smaller subset of the LZ that is governed by frictional properties at the joint. Methods for determining the spinal LZ experimentally are i ntroduced. To demonstrate that the LZ is distinct from the NZ, both pa rameters were quantified in six normal cadaveric human C5-C6 specimens for three different preload magnitudes and three different preload di rections. LZ variations with changing preload closely matched variatio ns in the location of the load-deformation curve elbow, whereas NZ val ues followed specimen resting position instead, verifying parameter in dependence. Reproducibility and interspecimen variability values were approximately equivalent for both parameters. Compared in a separate d ata set, NZ and LZ were both more sensitive than ROM in response to ce rvical discectomy. With discectomy, the absolute change in LZ was larg er and the percentage change in LZ was smaller than in NZ. The NZ may be a more appropriate measure when investigating the biomechanical eff ect of alterations to joint articulations and when the loading environ ment is well controlled. The LZ may be a more appropriate measure when investigating the effect of ligament/disc alterations and when the lo ading environment cannot be controlled. Future work is needed to compa re the sensitivities of the LZ and NZ in detecting spinal instability onset for various injuries.