Cumulative effect of repeated impacts on lumbar intervertebral disc mechanics: A diurnal cycle study

High speed boat operators in the U.S. Navy frequently suffer from low back pain and accelerated intervertebral disc degeneration, stemming from the large accelerations experienced while navigating on the open seas. Previous work has shown that these accelerations produce high-magnitude axial loads on the disc, causing the tissue to behave with increased stiffness and lower energy dissipation compared to more physiological quasistatic loading. The aim of this project is to further investigate impact loading events and their cumulative effect on disc mechanical behavior during a 24-hr diurnal (sleep-wake) cycle. Lumbar discs from bovine spines were loaded in compression to 400N for 16 hrs to simulate moderate loading activity; this load was reduced to 50N to represent overnight loading for 8 hrs. Samples were divided into two groups: a control which was tested as previously described, and an impact group, which had one hour of repeated impact events within the 16-hr functional loading period. Results showed disc height lost during the 16-hr period and regained during the 8-hr period was not significantly different between the two groups. Stiffness, energy dissipation, and neutral zone were measured at 0, 16, and 24 hrs; no significant differences were shown in the two groups among these time periods. Future efforts may involve testing throughout several diurnal cycles to see if there is an effect.