Thoracic and lumbar spinal impact tolerance.

INTRODUCTION: Thoracolumbar injuries resulting from motor vehicle accidents, falls, and assaults have a high risk of morbidity and mortality. However, there are no biomechanically based standards that address this problem. METHODS: This study used four cadaveric porcine specimens as a model for direct spinal impact injuries to humans to determine an appropriate injury tolerance value. The anthropometric parameters of these specimens are compared with values found in a large human cadaveric dataset. Each specimen was subjected to five impacts on the dorsal surface of the lower thorax and abdomen. RESULTS: The injuries ranged from mild spinous process fractures to endplate fractures with anterior longitudinal ligament (ALL) transactions with a maximum AIS=3. The average peak reaction force for the thoracic failure tests was 4720+/-1340 N, and the average peak reaction force for the lumbar failure tests was 4650+/-1590 N. DISCUSSION: When scaled to human values using anthropometric parameters determined in this study, the force at which there is a 50% risk of injury is 10,200+/-3900 N. This value favorably compares to that found in the existing literature on isolated vertebral segments. SUMMARY: After demonstrating that the porcine model can be used as a spinal impact model for the human, the resulting injury risk value can be used in determining new standards for human injury risk or in guiding the design of safety equipment for the back.

Viscoelastic properties of the cervical spinal ligaments under fast strain-rate deformations.

The mechanical response of ligaments under fast strain-rate deformations is a necessary input into computational models that are used for injury assessment. However, this information frequently is not available for the ligaments that are routinely injured in fast-rate loading scenarios. In the current study, experiments were conducted at fast strain rates for the cervical spinal ligaments: the anterior longitudinal ligament, the posterior longitudinal ligament and the ligamentum flavum. Bone-ligament-bone complexes at three spine levels were harvested for mechanical testing. Displacement-controlled sub-failure uniaxial tensile tests were performed in both load-relaxation and sinusoidal conditions. A nonlinear (separable) viscoelastic model was used to examine the experimental data. An unexpected result of the modeling was that the instantaneous elastic functions could be approximated as linear for these strain rates. A five-parameter model was sufficient to characterize the ligament viscoelastic responses and had good predictive capacity under different applied loading conditions.

Failure properties of cervical spinal ligaments under fast strain rate deformations.

STUDY DESIGN: The failure responses of the anterior longitudinal ligament, posterior longitudinal ligament, and ligamentum flavum were examined in vitro under large strain-rate mechanical loading. OBJECTIVE: To quantify the failure properties for 3 cervical spinal ligaments at strain rates associated with traumatic events. SUMMARY OF BACKGROUND DATA: There exists little experimentation literature for fast-rate loading of the cervical spine ligaments. The small amount of available information is framed only in extensive experimental coordinates, and not in the context of strains. METHODS: Bone-ligament-bone complexes were strained at fast rates, in an incrementally increasing loading protocol using a servohydraulic mechanical test frame. Failure loads and displacements were converted to engineering and true stress and strain values, and compared for the different ligaments (anterior longitudinal ligament, posterior longitudinal ligament, and ligamentum flavum), spinal levels (C3-C4, C5-C6, and C7-T1), and for male versus female specimens. RESULTS: There were no significant differences in force or true stress for gender or spinal level. There was a significant difference in force and true stress for ligament type. A difference was found between the posterior longitudinal ligament and ligamentum flavum for failure force, and between the ligamentum flavum and both the anterior and posterior longitudinal ligaments for failure true stress. No significant differences were found in true strain for ligament, gender, or spinal level. The mean ligament failure true strain was 0.81. Failure true strains were approximately 57% of the failure engineering strains. CONCLUSIONS: Once the injury mechanisms of the cervical spine are fully understood, computational models can be employed to understand the potentially traumatic effects of clinical procedures, and mitigate injury in impact, falls, and other high-rate scenarios. The soft tissue failure properties in this study can be used to develop failure tolerances in fast-rate loading scenarios. Failure properties of the anterior and posterior longitudinal ligaments were similar, and the same properties can be used to model both ligaments.