A non-linear multiaxial fatigue damage model for the cervical intervertebral disc annulus

A significant portion of the military population develops severe neck pain in the course of their duties. It has been hypothesized that neck pain is a consequence of accelerated degeneration of the intervertebral discs in the cervical spine, but more occupational and mechanistic-based tools and research are needed to positively confirm the link between neck pain and accelerated disc degeneration. Heavy head-supported mass including helmets and accessories worn by military personnel may subject the intervertebral discs of the cervical spine to complex cyclic loading profiles. In addition, some military operational travel which includes riding on high speed planing boats has also been reported to result in high magnitude cyclic loading on cervical spine discs. In this article, we present a methodology to computationally predict fatigue damage to cervical intervertebral discs over extended periods of time, by integrating kinematicsbased biomechanical models with a continuum damage mechanics-based theory of disc degeneration. Through this computational approach, we can gain insights into the relationship between these military activities and possible accelerated fatigue degeneration of cervical intervertebral discs and provide a quantitative prediction tool for decade-long time ranges. The four significant improvements this computational framework adds to the area of modeling intervertebral disc degeneration are the following: (a) it addresses the non-linear nature of fatigue damage evolution, (b) it includes the effect of aging and damage recovery to accurately simulate biological phenomena, (c) it computes fatigue damage taking into account the multiaxial stress state in the disc, and (d) it correlates the computational damage parameter with established clinical grading systems for disc degeneration.