Effects of short-segment posterior pedicle fixations on adjacent intervertebral discs: a robotic-based biomechanical research

Abstract

Objective To compare the effects of three-level fixations by 4, 5 and 6 pedicle screws into the adjacent upper and lower vertebrae on the strain changes at the adjacent intervertebral discs in the treatment of thoracolumbar burst fractures. Methods Six human cadaveric spines were dissected to prepare specimens of T11-L3. The inferior half of the L1 vertebral bodies and L1-L2 discs were resected tosimulate an unstable L1 burst fracture without support of anterior column. Biomechanical tests were performed in groups of intact condition and three-level fixations by 4, 5 and 6 pedicle screws. The strain changes at T11/12, L2/3 disc were measured by a robotic system of 6 degrees of freedom under loadings at flexion, extension, the left and right lateral bendings, and the left and fight axial torsions. One-way statistical analysis was used to compare the data collected. Results With the increase in number of screws, the strain at the adjacent intervertebral discs (T11/12, L2/3) was gradually increasing. The strains at T11/12 disc in fixations by 5 and 6 screws were significantly lager than in fixation by 4 screws only under axial rotation (P 0.05). The strains at L2/3 disc were not significantly different among the 3 groups under any loading (P>0.05). Conclusions In the three-level fixations by 4, 5 and 6 pedicle screws to treat thoracolumbar burst fractures, strains at the adjacent intervertebral discs may be gradually increasing with the increase in number of screws. Since the strain at the adjacent upper intervertebral disc is increased more markedly, the upper disc is more likely to degenerate than the lower disc. Key words: Thoracic vertebrae; Lumbar vertebrae; Fracture fixation, internal; Biomechanics