D. Pienkowski et al., MULTICYCLE MECHANICAL PERFORMANCE OF TITANIUM AND STAINLESS-STEEL TRANSPEDICULAR SPINE IMPLANTS, Spine (Philadelphia, Pa. 1976), 23(7), 1998, pp. 782-788
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.