Human lumbar spinal column injury criteria from vertical loading at the base: Applications to military environments

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Abstract

The objective of this study was to determine force-based lumbar spine injury criteria due to vertical impact using Post Mortem Human Surrogate (PMHS) experiments. Mounted personnel in military vehicles sustain loads from the pelvis in combat events such as underbody blast loadings. Forty-three post mortem human subject thoracolumbar spinal columns were obtained, screened for pre-existing trauma, bone mineral densities (BMDs) were determined, pre-test radiological images were taken, fixed at the ends in polymethylmethacrylate, load cells were attached to the ends of the fixation, positioned on custom vertical accelerator device based on a military-seating posture, and impacted at the base. Posttest images were obtained, and gross dissection was done to confirm injuries, classified into single and multilevel groups, groups A and B. Axial and resultant forces at the thoracolumbar (proximal) and lumbosacral (distal) joints were used as response variables to develop lumbar spine injury risk curves using parametric survival analysis. The Brier score metric was used to rank the variables. Age, BMD, column length, and vertebral body and intervertebral disc areas were used as covariates. The optimal metric describing the underlying response to injury was the distal resultant force for group A and proximal axial force for group B specimens. Force-BMD for group A and force-body area for group B were the best combinations. The IRCs with ±95% confidence intervals and quality of risk curves are given in the paper, and they serve as lumbar spine injury criteria. The present human cadaver Injury Risk Curves (IRCs) can be used to conduct matched pair tests to obtain dummy-based injury assessment risk curves/values to predict injury. The present IRCs can be used in human body finite element models. The relationship between covariates and primary forces presented in this study contribute to a better understanding of the role of demographic, geometric, and material factors to impact acceleration loading.

Introduction

Injuries to the human lumbar spine have been reported in civilian and military environments. In the civilian automotive field, compression-related injuries are reported to restrained occupants in frontal crashes (Kaufman et al., 2013). In the military field, recent combat events have resulted in mounted soldiers (sitting in military vehicles) sustaining injuries from underbody blast loading, and this is a unique scenario as the seated posture, vector, and loading rate are different from the civilian automotive field (Vasquez et al., 2018; Danelson et al., 2015). Cited studies are not all inclusive. Vertical loading to the lumbar spinal column is the main mechanical stimulus in underbody blast loading events. Lumbar spinal column studies specific to this load vector are limited. The fundamental requirement to define human tolerance and advance safety in impact loading events is the development of injury criteria in the form of risk curves. They are obtained by subjecting human cadaver specimens, intact or subsystems, to appropriate load vectors. Automotive crashworthiness standards for the civilian environment lack injury criteria for the lumbar spinal column. While the dynamic response index is used as a criterion for aircraft ejection seat designs, its applicability for the underbody blast environment has not been validated (Payne and Stech, 1969). Consequently, to directly address tolerance and human safety in this environment, injury risk curves (IRCs) are needed.

Section snippets

Objectives

The objectives of the present study are to develop IRCs for the human lumbar spine using force and covariate variables under vertical loading and identify the optimal metric for two types of injury outcomes that describe the underlying response using novel statistical techniques.

PMHS procurement

Unembalmed human cadaver specimens were procured for the study. The experimental protocol was approved by the local Institutional Review Board of the institution and by the United States Department of Defense. Before specimen procurement, medical records and x-rays were evaluated, and screened for human immunodeficiency virus, and Hepatitis B and C, to rule out pre-existing trauma to the spine. The specimens selected had no radiological evidence of osteophytes and joint degeneration, considered

General results

The mean age, stature, total body mass, and body mass index (BMI) of the 43 male specimens were: 65 ± 10 years, 1.78 ± 0.06 m, 83 ± 13 kg, and 26 ± 4 kg/m2, respectively. The mean column height, disc and body areas, and BMD were: 17.8 ± 0.9 cm, 18.9 ± 2.8 cm2, 12.4. ± 1.6 cm2, and 140 ± 37 mg/cc, respectively (Table 1). Testing was stopped in three specimens after the first impact because injury was detected or suspected. All injuries sustained in this testing series involved vertebral

Discussion

As stated in the introductory paragraphs, the aim of the study was to describe tolerances of the human lumbar spinal column under vertical impacts. Data are applicable to the military environments as specimens were oriented in the seated soldier posture (Reed, 2013). In addition, impact loading was applied along the inferior-to-superior direction from the base of the preparation, reflecting the modality seen in recent conflicts and the need to understand the biomechanics for injury prevention

Conclusions

Force-based lumbar spine IRCs were developed by subjecting human cadaver thoracolumbar spinal columns to impacts along the inferior to superior direction. The optimal metric describing the underlying response to injury was associated with the distal resultant force for group A and proximal axial force for group B outcomes. For group A, resultant force with trabecular BMD was the optimum, while for the other group, body area was associated with the axial force. The IRCs developed for all

CRediT authorship contribution statement

Narayan Yoganandan: Project administration, Funding acquisition, Supervision, Data curation, Writing - original draft, Writing - review & editing. Jason Moore: Methodology, Resources, Data curation, Writing - review & editing. Nicholas DeVogel: Data curation. Frank Pintar: Project administration, Funding acquisition, Supervision, Resources, Writing - review & editing. Anjishnu Banerjee: Data curation. Jamie Baisden: Formal analysis, Data curation. Jiang Yue Zhang: Project administration.

Declaration of competing interest

None of the authors of this study have any financial or personal relationships with other people or organizations that could inappropriately influence (bias) the presented work, i.e., no conflict of interest for this paper.

Acknowledgments

This research was supported in part by Cooperative Agreement W81XWH-16-01-0010, Department of Veterans Affairs, United States Medical Research, and the Department of Neurosurgery at the Medical College of Wisconsin, United States. This material is the result of work supported with resources and use of facilities at the Zablocki VA Medical Center, Milwaukee, Wisconsin and Medical College of Wisconsin. The first three authors are part time employees of the Zablocki VA Medical Center. The views

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