Abstract

The human thoracolumbar spinal column sustains axial loading under physiological and traumatic loading situations. Clinical studies have focused on the former scenario, and the investigation of low back pain issues and spinal stabilization using artificial devices such as arthroplasty are examples. Investigative studies have largely used quasi-static and vibration loading on the spine segment(s) and spinal columns. The traumatic loading scenario is relatively less researched, and it is a dynamic event. Injuries under this scenario occur in sports, automotive, and combat environments. Impact vectors include flexion-extension modes in automotive crash events. Vertical or caudal to cephalad oriented impacts have been identified in both automotive and military scenarios. Frontal impacts to restrained occupants in the automotive and underbody blast impacts from improvised explosive device in combat situations are examples of the vertical loading vector. Although some studies have been conducted using whole body human cadavers and isolated spinal columns, determinations have not been made of the injury risks and stress and strain responses for a variety of accelerative pulses. The aims of the present investigation were to delineate the internal biomechanics of the spinal column under this impact vector and assess the probability of injury. Male and female whole-body human finite element models were used in the study. The occupants were restrained and positioned on the seat, and caudo-cephalad impacts were applied to the base. Different acceleration-time profiles (pulse durations ranging from 50 to 200 ms and peak accelerations varying from 11 g to 46 g) were used as inputs in both male and female models. The resulting stress-strain profiles in the cortical and cancellous bones were evaluated at different vertebral levels. Using the peak transmitted forces at the thoracolumbar disc level as the response variable, the probability of injury for the male spine was obtained from experimental risk curves for the various accelerative pulses. Results showed that the shorter pulse durations and rise times impart greater loading on the thoracolumbar spine. The analysis of von Mises stress and strain distributions showed that the compression-related fractures of vertebrae are multifaceted with contributions from both the cortical and cancellous bony components of the body. Profiles are provided in the body of the paper for different spinal levels. The intervertebral disc may be involved in the fracture mechanism, because it acts as a medium of load transfer between adjacent vertebrae. Injury risks for the shortest pulse was sixty-three percent, and for the widest pulse it was close to zero, and injury probabilities for other pulses are given. The present computational modeling study provides insights into the mechanisms of the internal load transfer and describe the injury risk levels from caudal to cephalad impacts.

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