Injury risk for research subjects with spina bifida occulta in a repeated impact study: a case review.

Spina bifida occulta (SBO) occurs in 18-34% of the normal U.S. population. Recently, 16.5% of normal, asymptomatic male soldier volunteer candidates in a U.S. Army Aeromedical Research Laboratory ride motion study were excluded from the study because they had SBO at one vertebral level. Disqualifying this percentage of screened research subject candidates threatened the timely completion of the schedule-intense protocol. Although one study suggests that SBO at spinal level S1 has a higher incidence of posterior disc herniation, the preponderance of clinical literature reports that spina bifida occulta is not a medical problem. The impact literature indicates that lumbosacral vertebral bodies fracture at 7.14 kN in static compression and 20+ G during dynamic vertical impacts. In this paper, we examined the human data observed in ejection seat incidents, the rationale for excluding volunteers with single level SBO and the path of axial load transmission through the lumboscral spine. Based on the findings, we concluded that research volunteers with single level SBO are not at increased risk for injury and recommended inclusion of these volunteers in future studies involving repeated axial impacts due to ride motion.