The importance of mechanism of injury was initially introduced by Holdsworth who made the supposition that all fractures are created when the spine is subject to one of 5 types of violence. It has been our experience that similar injury mechanisms can lead to variable fracture patterns. Alternatively, different injury mechanisms can lead to the same fracture pattern.

Purpose: To evaluate the variation in fracture patterns when a single and uniform force vector is applied to the spine with variable degrees of spinal flexion. Finite element modeling was used for this analysis.

Methods: Three different finite element models were created to represent each accident situation. The straight spine was modeled as a simple column with alternating vertebrae and disc segment. The moderately flexed and significantly flexed spines were modeled as curved cylinders sectioned into vertebrae and discs, then bent around a solid cylinder representing the abdomen. A 1000 N compressive load was applied vertically to the top of the spine. The model was restrained along all bottom surfaces, and the interface between the spine and abdomen sections was defined as frictionless. The model is fixed at the lower end and the area of greatest interest is the transition zone from the most rigid to the less rigid portion. Although no specific area of the spine is intended for purposes of the model, this composition is much like the thoracolumbar junction – the location of the majority of spinal injuries.

Results: The straight spine showed pure compression throughout the length of the spine, while the moderately curved spine showed the posterior elements of the region of interest in tension and the anterior elements in compression. The significantly curved spine was found to be in tension in both posterior and anterior elements.

Conclusion: In a situation where the patient is sitting upright with a straight spine, a compressive load will cause a burst fracture. When the patient is partially bent over, such as with a shoulder seat belt, a flexion distraction injury will occur with the posterior aspect of the spine failing in tension and the anterior in compression. When the patient is fully bent over, such as with a laponly seat belt, a purely distractive fracture can occur.