Volume 8, Issue 7 (July 2011)
Symposium on Static and Dynamic Spinal Implants: Are We Evaluating Them Appropriately?
Although the standard characterizes intervertebral fusion devices with simple symmetric shapes, modern devices may contain more complex geometries that may require a pocket to fully characterize their strength. Test fixture design has proven to significantly affect the static shear result and failure mode of intervertebral fusion devices with a bullet-nose (rounded) design. An initial test using fixtures without pockets was unsuccessful in determining the shear strength of the fusion device bodies. Revised fixtures with a pocket to force shear loading through the center of the device were designed to create a functional failure. For this study, testing was performed using the ASTM recommended 45° Z axis with a spherically attached actuator pushrod in 37 degrees C phosphate buffered saline. The initial stainless steel fixtures featured mating tooth geometry and no pocket, which forced failures to the device teeth only, resulting in expulsion of the device from the testing fixtures. The revised stainless steel fixtures featured the mating tooth geometry and incorporated a larger anterior/posterior contact surface; thus creating a pocket to constrain the bullet-nose device that prevented the device from expelling. The fusion devices tested with the revised fixture design produced a compression-shear failure of permanent plastic deformation of the body of the device resulted in a 90.2 % increase in the mean yield strength as compared to the fusion devices tested with the initial no-pocket design fixture that produced failures of only the device teeth. The results of this study revealed how the static shear result and failure mode can change significantly with minor changes in fixture design.