REDUCTION IN DISK AND FIBER STRESSES BY AXIAL DISTRACTION IS HIGHER IN CERVICAL DISK WITH FIBERS ORIENTED TOWARD THE VERTICAL RATHER THAN HORIZONTAL PLANE: A FINITE ELEMENT MODEL ANALYSIS

Objective: The purpose of this study was to quantify the biomechanical changes that occur in a compressed cervical disk with the application of axial distraction when the annular fiber orientation angles are varied between the horizontal and vertical planes. Methods: A 3-dimensional finite element (FE) model of a cervical motion segment was developed. From this model, 3 FE models were developed and validated corresponding to 3 different fiber angles relative to the end plate-disk interface: +/- 25 degrees (oriented toward the horizontal plane), +/- 45 degrees (midway between the horizontal and vertical planes), and +/- 65 degrees (oriented toward the vertical plane). Compression (50N), followed by an axial distraction (17N), was simulated. Annulus and nucleus stresses, von-Mises fiber stresses, annulus radial bulging, and nucleus radial displacement were computed. Hard tissue (cortical and cancellous bones and end plate) stresses were also quantified. Results: With increasing fiber angle (toward vertical), axial segmental stiffness increased, whereas annulus and nucleus stresses, fiber stresses, annulus radial bulging, and nucleus radial displacement decreased. Similar outcomes were observed when axial distraction was applied to the compressed segment. Hard tissues were not affected with varying fiber angles; however, their mechanics changed when axial distraction was applied on the compressed segment. We noted lower disk stress in axial distraction than in compression. Conclusions: The results confirmed the hypothesis that fibers oriented toward the vertical plane reduce disk and fiber stresses and disk bulging. By aligning annular fibers toward the vertical plane axial distraction may help reduce disk and fiber stresses. Axial disk stresses decrease radially from outside to inside under compressive loading and that the anterior annulus is more stressed than the posterior-lateral annulus during both compression and distraction. Stresses decreased in both the annulus tissue matrix and fibers with increasing fiber angles and increasing fiber slope to 90 degrees (vertical fibers) is further anticipated to reduce the compressive disk stresses. The fibers in tension apply compression to the annulus tissue matrix, thus, decreasing annulus stresses in the axial and circumferential directions. (J Manipulative Physiol Ther 2010;33:252-260)