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 steel posterior spinal constructs, and to determine the failure fracture modes. The stainless steel constructs had higher stiffness and yield strength than the titanium constructs, but the ultimate static strength was almost equal for both types of constructs. Titanium constructs, however, exhibited higher variability than the stainless steel constructs. In fatigue tests, the stainless steel constructs were significantly affected by the external load and were frequency independent. It appears from fatigue curves that 500 N can be approximated as the endurance limit for the stainless steel constructs. Titanium constructs were load-frequency dependent, and their endurance limit was somewhere between the 500 and 750 N load levels. There were no differences in performance 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 performance than the stainless steel constructs in the elastic region, and with smaller differences in the plastic region. Most of the failure modes for stainless steel constructs were in screw bending at 16 Hz with a smaller percentage of rod fractures at high loads, with a higher percentage of rod fractures observed for the stainless steel constructs at 4 Hz. Most of the failure modes for titanium constructs occurred in screw bending or fracture.