Mechanisms of Cervical Spine Disc Injury under Cyclic Loading.

STUDY DESIGN: Determination of human cervical spine disc response under cyclic loading.

PURPOSE: To explain the potential mechanisms of intervertebral disc injury caused by cyclic loading.

OVERVIEW OF LITERATURE: Certain occupational environments in civilian and military populations may affect the cervical spine of individuals by cyclic loading. Research on this mechanism is scarce.

METHODS: Here, we developed a finite element model of the human C4-C5 disc. It comprised endplates, five layers of fibers, a nucleus, and an annulus ground substance. The endplates, ground substance, and annular fibers were modeled with elastic, hyperviscoelastic, and hyper-elastic materials, respectively. We subjected the disc to compressive loading (150 N) for 10,000 cycles at frequencies of 2 Hz (low) and 4 Hz (high). We measured disc displacements over the entire loading period. We obtained maximum and minimum principal stress and strain and von Mises stress distributions at both frequencies for all components. Further, we used contours to infer potential mechanisms of internal load transfer within the disc components.

RESULTS: The points of the model disc displacement versus the loading cycles were within the experimental corridors for both frequencies. The principal stresses were higher in the ground matrix, maximum stress was higher in the anterior and posterior annular regions, and minimum stress was higher along the superior and inferior peripheries. The maximum principal strains were radially directed, whereas the minimum principal strains were axially/obliquely directed. The stresses in the fibers were greater and concentrated in the posterolateral regions in the innermost layer.

CONCLUSIONS: Disc displacement was lower at high frequency, thus exhibiting strain rate stiffening and explaining stress accumulation at superior and interior peripheries. Greater stresses and strains at the boundaries explain disc injuries, such as delamination. The greater development of stresses in the innermost annular fiber layer (migrating toward the posterolateral regions) explains disc prolapse.