The influence of material and design features on the mechanical properties of transpedicular spinal fixation implants

This study describes the finding and performance of mechanical strength and corrosion testing procedures for comparative examination of multiple internal transpedicular spine fixators. Seven different implant models from five different manufacturers were compared regarding their bending strength and fatigue resistance. Because of the unacceptably high levels of time and material that they require, ISO and ASTM testing standards are not applicable to comparative testing. In addition, there is a lack of knowledge about clinically defined and proven strength-limit values. Therefore, actual standard testing procedures have been modified and extended to corrosion testing. Overall, the effort necessary to obtain reproducible comparative data has been reduced significantly. Although a reduced number of implants of each type were available for destructive testing, the results revealed fundamental differences in the tested implants between different materials and design features. During fatigue testing some of the implants showed poor corrosion properties. Because spinal fixation implants tend to be used as long-duration implants, corrosion testing as well as comparative strength testing with clinically successful implant models should be performed as preclinical evaluations.