Risk of lumbar spine injury from cyclic compressive loading.

STUDY DESIGN: Survival analyses of a large cohort of published lumbar spine compression fatigue tests. OBJECTIVE: To produce the first large-scale evaluation of human lumbar spine tolerance to repetitive compressive loading and to evaluate and improve guidelines for human exposure to whole-body vibration and repeated mechanical shock environments. SUMMARY OF BACKGROUND DATA: Several studies have examined the effects of compressive cyclic loading on the lumbar spine. However, no previous effort has coalesced these studies and produced an injury risk analysis with an expanded sample size. Guidelines have been developed for exposure limits to repetitive loading (e.g., ISO 2631-5), but there has been no large-scale verification of the standard against experimental data. METHODS: Survival analyses were performed using the results of 77 male and 28 female cadaveric spinal segment fatigue tests from 6 previously published studies. Segments were fixed at each end and exposed to axial cyclic compression. The effects of the number of cycles, load amplitude, sex, and age were examined through the use of survival analyses. RESULTS: Number of cycles, load amplitude, sex, and age all are significant factors in the likelihood of bony failure in the spinal column. Using a modification of the risk prediction parameter from ISO 2631-5, an injury risk model was developed, which relates risk of vertebral failure to repeated compressive loading. The model predicts lifetime risks less than 7% for industrial machinery exposure from axial compression alone. There was a 38% risk for a high-speed planing craft operator, consistent with epidemiological evidence. CONCLUSION: A spinal fatigue model which predicts the risk of in vitro lumbar spinal failure within a narrow confidence interval has been developed. Age and sex were found to have significant effects on fatigue strength, with sex differences extending beyond those accounted for by endplate area disparities.