Fatigue Behavior of Stainless Steel, Titanium, and Cobalt Chromium Molybdenum Spinal Rods

The objective of this study was to measure and compare the fatigue behavior of 316L stainless steel, titanium (Ti-6Al-4V), and cobalt chromium molybdenum (CoCrMo) spinal rods in vitro. Spinal rods are used to immobilize the spine while fusion of the vertebrae occurs (spinal arthrodesis). Implanted spinal rods are subjected to cyclic loading and are therefore susceptible to fatigue failure if fusion does not occur sufficiently quickly. A significant number of spinal rod fatigue failures have been observed between six months to one year following surgical implantation. On average, the spine will experience about 3 million cycles per year. Stress overloads can result in permanent deformation or immediate failure of the rod, however these overloads are seldom the root cause of failure—rods typically fail by fatigue [1].