The Effect Of Lumbar Pedicle Screw Fixation On Segmental Motion And Intradiscal Pressure Of Adjacent Segments

[Objective] To study the effect of rigid fixation on range of motion and intradiscal pressure of adjacent segment, and evaluate the respective role in the mechanism of accelerated degeneration of the segment adjacent to a lumbar fusion according to the changes of the two factors.[Methods] Seven calf lumar spine specimens(T13~S2) were biomechanically tested under pure moment of 10Nm loading with Tenor pedicle screw system as the fixation device. Range of motion and intradiscal pressure were measured in all unfused segments and compared among the control group (intact spine), the single-segment-fixed group (L4/5) and the double-segment-fixed group (L3~L5), and the correlation analysis was performed.[Results] After instrumentation, the segmental motion of the upper adjacent segments increased obviously in all directions, and increased as the range of fixation increased. The segmental motion of the lower adjacent segment increased only in direction of axial rotation. Comparatively, the changes of intradiscal pressure were extremely small, neither in single-segment-fixed group nor in double-segment-fixed group, and in whatever directions. Positive correlation between the increase of segmental motion and intradiscal pressure was shown in correlation analysis.[Conclusion] Accelerated disc degeneration of adjacent segment after rigid fixation is the result of compromise of kinematic changes and increase of intradiscal pressure together with some other factors. The stress concentration and compensative motion increase as well as loss of stability of adjacent segments are the most direct and primary biomechanical changes due to rigid fixation which may lead to accelerated disc degeneration. The intradiscal pressure will also increase after instrumentation secondarily, but appears to be just a minor aspect.