The influence of lumbar disc height and cross-sectional area on the mechanical response of the disc to physiologic loading.

STUDY DESIGN The influence of lumbar disc height and cross-sectional area on the mechanical response of the disc to physiologic loading was determined using a finite element model. OBJECTIVES To identify which geometric characteristics are potentially related to motion segment mechanical response to applied load, such as flexibility, fiber stress, disc bulge, and nucleus pressure. SUMMARY OF BACKGROUND DATA The height and area of the lumbar disc varies within the disc itself, between disc levels, between people, between men and women, with aging, and during the day. Mechanical theory dictates that the height and area influence the mechanical response of the disc to loading. This could have important consequences in risk of injury. METHODS Three-dimensional finite-element models representing three disc heights (5.5 mm, 8.5 mm, and 10.5 mm) and three disc areas (1060 mm2, 1512 mm2, and 1885 mm2) were generated. The effect of disc geometry on the mechanical properties of the disc were studied for four moment loads (magnitude, 7.5 Nm) with compressive preload (400 N) and for three different direct forces. Commercially available finite-element software was used. RESULTS Discs with a ratio of small disc area to disc height were more prone to larger motion, higher anular fiber stresses, and larger disc bulge. When the disc height alone was increased by a factor, its flexibility also increased, either by the same amount or by a much larger ratio. CONCLUSIONS Discs with the most height and smallest area are exposed to much higher risk of failure than other combinations of disc height and geometry.