Analysis of the mechanical behavior of the lumbar spine under high impact loading

High speed boat (HSB) crewman in the U.S. navy often suffer from lower back pain and accelerated intervertebral disc (IVD) degeneration, due to the high-G impacts experienced during typical missions, resulting in significant time on limited duty status. The relationship between high impact loads and disc biomechanics is not well understood. We seek to analyze the changes in mechanical behavior, particularly compressive stiffness, of anterior column units (ACU) of the lumbar spine as a function of duration of impact. Lumbar spine sections from four donors with mean age 63 ± 8.8 years were used, yielding n=5 ACUs. The discs were put under quasi-static loading to simulate normal loading conditions and determine the neutral zone and quasi-static loading stiffness. Each ACU then underwent an impact loading sequence, compressing a distance equal to its neutral zone. Each sequence consisted of six impact loads, with varying duration of compression-relaxation (10, 20, 40, 80, 160, and 320 ms). The data shows there is an increase in stiffness as the impact duration decreased. The sole exception was the 10-ms impact, which yielded stiffness values smaller than that of the 20- or 40-ms impacts. Stiffness values for all impact durations, however, were significantly higher (p < 0.05, Student's t-test) than that of the quasi-static conditions. The data supports our theory that the IVD is more stiff under high impact loading conditions than normal loading, and that this stiffness increases with shortened impact duration. This shows that the highly demanding environment of naval crewman is deleterious to the IVD, and may provide insight into the high incidence of back pain and accelerated disc degeneration for this population.