Clinical results for unilateral variable screw placement instrumentation in isolated lumber fusion have been reported to be as good as those for bilateral instrumentation. However, unilateral instrumentation may not be recommended for multilevel fusion but unilateral fixation is not stable enough to prevent fusion cage migration in some patients who undergo TLIF. We showed that the unilateral pedicle screw system offers only uneven fixation. This results in dispersion of rigidity depending on the direction of bending and rotation. The bilateral pedicle screw system, however, allows excellent fixation in all directions.