MULTICYCLE MECHANICAL PERFORMANCE OF TITANIUM AND STAINLESS-STEEL TRANSPEDICULAR SPINE IMPLANTS

Study Design. This was a prospective in vitro study comparing titanium alloy and stainless steel alloy in transpedicular spine implants from two different manufacturers. Objective. To compare the multicycle mec hanical performance of these two alloys, used in each of two different implant designs. Summary of Background Data. Transpedicular spine imp lants primarily have been manufactured from stainless steel, but titan ium alloy offers imaging advantages. However, the notch sensitivity of titanium alloy has caused concern regarding how implants made from th is material will compare in stiffness and fatigue life with implants m ade from stainless steel. Methods. Twenty-four implants (two alloys, t wo designs, six implants per group) were mounted in machined polyethyl ene wafers and repetitively loaded (up to 1 million cycles) from 80 N to 800 N using a 5-Hertz sinusoidal waveform. Load and displacement da ta were automatically and periodically sampled throughout the entire t est. Results. Implant stiffness increased with cycle load number, reac hed a steady state, then declined just before fatigue failure. Stiffne ss varied less in titanium transpedicular spine implants than in their stainless counterparts. All stainless steel implant types were stiffe r (steady-state value, P < 0.0001) than their titanium alloy counterpa rts. One titanium implant design failed with fewer (P < 0.05) load cyc les than its stainless steel counterpart, whereas a stainless steel im plant of another design failed with fewer (P = 0.002) load cycles than its titanium counterpart. Overall, fatigue life, i.e., the total numb er of load cycles until failure, was related to implant type (P < 0.00 01), but not to implant material. Conclusions. A transpedicular spine implant's fatigue lifetime depends on both the design and the material and cannot be judged on material alone. Stainless steel implants are stiffer than titanium alloy implants of equal design and size; however , for those designs in which the fatigue life of the titanium alloy ve rsion is superior, enlargement of the implant's components can compens ate for titanium's lower modulus of elasticity and result in an implan t equally stiff as its stainless steel counterpart. Such an implant ma de from titanium alloy would then be clinically preferable because of titanium's previously reported imaging advantages.