The dynamic, six-axis stiffness matrix testing of porcine spinal specimens

Background contextComplex testing protocols are required to fully understand the biomechanics of the spine. There remains limited data concerning the mechanical properties of spinal specimens under dynamic loading conditions in six axes.PurposeTo provide new data on the mechanical properties of functional spinal unit (FSU) and isolated disc (ISD) spinal specimens in 6 df.Study designDynamic, six-axis stiffness matrix testing of porcine lumbar spinal specimens.MethodsThe stiffness matrix testing of lumbar porcine FSU (n=6) and ISD (n=6) specimens was completed in a custom six-axis spine simulator using triangle wave cycles at a frequency of 0.1 Hz. Specimens were first tested without an axial preload, then with an axial preload of 500 N, with equilibration times of both 30 and 60 minutes.ResultsThe stiffness matrices were not symmetrical about the principal stiffness terms. The facets increased all the principal stiffness terms with the exception of axial compression-extension. Significant differences were detected in 15 stiffness terms because of the application of an axial preload in the ISD specimens, including an increase in all principal stiffness terms. There were limited differences in stiffness because of equilibration time of 30 and 60 minutes.ConclusionsThe assumption of stiffness matrix symmetry used in many previous studies is not valid. The biomechanical testing of spinal specimens should be completed in 6 df, at physiologic loading rates, and incorporate the application of an axial preload. The present study has provided new data on the mechanical properties of spinal specimens and demonstrates that the dynamic stiffness matrix method provides a means to more fully understand the natural spine and quantitatively assess spinal instrumentation.