The spinal lax zone and neutral zone: measurement techniques and parameter comparisons.

The neutral zone (NZ) is a well-established parameter that describes the loose region of the spinal range of motion (ROM) where the spine moves easily with minimal applied force. The loose region is of interest clinically because in vitro studies show that with the onset of instability, this region increases before the ROM increases. The upper boundary of the NZ has been described as the displacement at which ligamentous resistance just begins. However, we hypothesized that a different parameter that we have named the lax zone (LZ) more accurately describes 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 introduced. To demonstrate that the LZ is distinct from the NZ, both parameters were quantified in six normal cadaveric human C5-C6 specimens for three different preload magnitudes and three different preload directions. LZ variations with changing preload closely matched variations in the location of the load-deformation curve elbow, whereas NZ values followed specimen resting position instead, verifying parameter independence. Reproducibility and interspecimen variability values were approximately equivalent for both parameters. Compared in a separate data set, NZ and LZ were both more sensitive than ROM in response to cervical discectomy. With discectomy, the absolute change in LZ was larger and the percentage change in LZ was smaller than in NZ. The NZ may be a more appropriate measure when investigating the biomechanical effect of alterations to joint articulations and when the loading environment is well controlled. The LZ may be a more appropriate measure when investigating the effect of ligament/disc alterations and when the loading environment cannot be controlled. Future work is needed to compare the sensitivities of the LZ and NZ in detecting spinal instability onset for various injuries.