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 determine d using a finite element model. Objectives. To identify which geometric characteristics are potentially rel ated to motion segment mechanical response to applied load, such as flexibi lity, fiber stress, disc bulge, and nucleus pressure. Summary of Background Data. The height and area of the lumbar disc Varies w ithin 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 he ights (5.5 mm, 8.5 mm, and 10.5 mm) and three disc areas (1060 mm(2), 1512 mm(2), and 1885 mm(2)) were generated. The effect of disc geometry on the m echanical properties of the disc were studied for four moment loads (magnit ude, 7.5 Nm) with compressive preload (400 N) and for three different direc t forces. Commercially available finite-element software was used. Results. Discs with a ratio of small disc area to disc height were more pro ne to larger motion, higher anular fiber stresses, and larger disc bulge. W hen the disc height alone was increased by a factor, its flexibility also i ncreased, either by the same amount or by a much larger ratio. Conclusions. Discs with the most height and smallest area are exposed to mu ch higher risk of failure than other combinations of disc height and geomet ry.