The development of a dynamic, six-axis spine simulator

Background Context

Although a great deal of research has been completed to characterize the stiffness of spinal specimens, there remains a limited understanding of the spine in 6 df and there is a lack of data from dynamic testing in six axes.
Purpose

This study details the development and validation of a dynamic six-axis spine simulator.
Study Design/Setting

Biomechanical study
Methods

A synthetic spinal specimen was used for the purpose of tuning the simulator, completing positional accuracy tests, and measuring frequency response under physiological conditions. The spine simulator was used to complete stiffness matrix tests of an L3-L4 lumbar porcine functional spinal unit. Five testing frequencies were used, ranging from quasistatic (0.00575 Hz) to dynamic (0.5 Hz). Tests were performed without an axial preload, and with an axial preload of 500 N.
Results

The validation tests demonstrated that the simulator is capable of producing accurate positioning under loading at frequencies up to 0.5 Hz using both sine and triangle waveforms. The porcine stiffness matrix tests demonstrated that the stiffness matrix is not symmetrical about the principal stiffness diagonal. It was also shown that whilst an increase in test frequency generally increased the principal stiffness terms, axial preload had a much greater effect.
Conclusions

The spine simulator is capable of characterising the dynamic biomechanics of the spine in six axes, and provides a means to better understand the complex behaviour of the spine under physiological conditions.