Effectiveness of transfixation and length of instrumentation on titanium and stainless steel transpedicular spine implants

This study compares the effectiveness of transfixation on the stiffness of two pedicle screw-rod constructs of different manufacture, implant design, and alloy, applied in one- and two-level instability. Four screws composed of either stainless steel or Titanium were assembled in pairs to two polyme thylmethacrylate blocks to resemble one- and two-level corpectomy models an d the construct underwent nondestructive torsional, extension, and flexion loading. In every loading test, each construct was tested using stainless s teel or titanium rods of 4.9-mm diameter in two different lengths (short, 1 0 cm; long, 15 cm), not augmented or augmented with different transfixation devices or a pair of devices. The authors compared the stiffness of stainl ess steel and titanium constructs without cross-link with the stiffness of that reinforced with single or double Texas Scottish Rite Hospital (TSRH) c ross-link, closed new-type cross-link (closed NTC), or open new-type cross- link (open NTC). The results showed that augmentation or no augmentation of short rods conferred significantly more stiffness than that of long rods o f the same material in all three loading modes. The closed NTC provided the greatest increase of torsional, extension, and flexion stiffness, and sing le TSRH provided the least amount of stiffness. Torsional stiffness of shor t stainless steel rods augmented or not augmented was significantly greater than that of their titanium counterparts. Torsional stiffness of long tita nium rods was always greater than that of their stainless steel counterpart s. Extension stiffness of short nonaugmented titanium rods was superior to that of long titanium rods, whereas extension stiffness of nonaugmented sho rt and long stainless steel rods was similar. Nonaugmented short titanium r ods showed greater flexion stiffness than that of long titanium rods. Long stainless steel rods displayed significantly greater flexion stiffness than did their titanium counterparts. This nondestructive study showed that cro ss-links increase the torsional stiffness significantly but less so the fle xion and extension stiffness of both titanium and stainless steel posterior transpedicular constructs. This increase was proportional to the cross-sec tional diameter of the cross-link. Titanium constructs showed more torsiona l stiffness when used in two-level instability and steel showed more torsio nal stiffness in one-level instability, particularly when they are reinforc ed. Stainless steel constructs showed greater flexion stiffness when they w ere used in two-level and titanium showed greater flexion stiffness in one- level instability, particularly when they were reinforced with stiff cross- links. The effect of transfixation on extension forces was obvious when thi ck cross-links were used.