Biomechanical study of thoracolumbar junction fixation devices with different diameter dual-rod systems.

Object: Advances in the design of a smaller-diameter rod system for use in the thoracolumbar region prompted the authors to undertake this biomechanical study of two different thoracolumbar implants.

Methods: In vitro biomechanical testing was performed using human cadaveric spines. All specimens were loaded to a maximum moment of 5 Nm with 300-N axial preload in six modes of motion. Two types of anterior implants with different rod diameters were applied to intact T10-12 specimens in two groups. The loading was repeated and the range of motion (ROM) was measured. A T-11 corpectomy was then performed and a strain gauge-mounted carbon fiber stackable cage was implanted. The ROM and compression force on the cage were measured, and the mean values were compared between these two groups. With stabilization of the intact spine, ROM decreased least in extension and greatest in bending compared with the intact specimens. After corpectomy and stabilization, ROM increased in extension by 104.89 +/- 53.09% in specimens with a 6.35-mm rod insertion and by 83.81 +/- 16.96% in those with a 5.5-mm rod, respectively; in flexion, ROM decreased by 26.98 +/- 27.43% (6.35 mm) and by 9.59 +/- 15.42% (5.5 mm), respectively; and in bending and rotation, both groups each showed a decrease in ROM. The load sharing of the cage was similar between the two groups (the 6.35-mm compared with 5.5-mm rods): 47.44 and 44.73% (neutral), 49.16 and 39.02% (extension), 61.90 and 56.88% (flexion), respectively.

Conclusions: There were no statistical differences in the ROM and load sharing of the cage when either the 6.35-or 5.5-mm-diameter dual-rod was used.