BIOMECHANICAL ASSESSMENT OF TITANIUM AND STAINLESS-STEEL POSTERIOR SPINAL CONSTRUCTS - EFFECTS OF ABSOLUTE RELATIVE LOADING AND FREQUENCY ON FATIGUE LIFE AND DETERMINATION OF FAILURE MODES/

The goal of this study was to examine the effects of absolute/relative loads and frequency on the fatigue life of titanium and stainless ste el posterior spinal constructs, and to determine the failure fracture modes. The stainless steel constructs had higher stiffness and yield s trength than the titanium constructs, but the ultimate static strength was almost equal for both types of constructs. Titanium constructs, h owever, exhibited higher variability than the stainless steel construc ts. In fatigue tests, the stainless steel constructs were significantl y affected by the external load and were frequency independent. It app ears from fatigue curves that 500 N can be approximated as the enduran ce limit for the stainless steel constructs. Titanium constructs were load-frequency dependent, and their endurance limit was somewhere betw een the 500 and 750 N load levels. There were no differences in perfor mance between the stainless steel and titanium constructs at 16 Hz. At 4 Hz, titanium constructs performed as well or better than stainless steel constructs. Also, the titanium constructs resulted in better per formance than the stainless steel constructs in the elastic region, an d with smaller differences in the plastic region. Most of the failure modes for stainless steel constructs were in screw bending at 16 Hz wi th a smaller percentage of rod fractures at high loads, with a higher percentage of rod fractures observed for the stainless steel construct s at 4 Hz. Most of the failure modes for titanium constructs occurred in screw bending or fracture.