The effect of spinal instrumentation on lumbar intradiscal pressure

The purpose of this study was to investigate the effect of spinal instrumen tation on the intradiscal pressure (IDP) within the fixed motion segment. I n vitro biomechanical testing was performed in six single functional spinal units of fresh calf lumbar spines using a pressure needle transducer. Vari ous loads were applied by a materials testing system device. In addition to intact spine (control), anterior spinal instrumentation (AXI) and pedicle screw fixation (PS) constructs, as well as destabilized spine were tested. Relative to the control, the destabilized spine tended to have an increased IDP; by 15% in axial compression and by 9-36% in flexion-extension Compare d to the control, PS decreased the IDP by 23% in axial loading and 51% in e xtension loading and increased it by 60% in flexion for each loading. AXI d ecreased the IDP by 32%, in flexion and 1% in extension. Lateral bending pr oduced symmetrical changes of IDP in the control and destabilized spine, bu t no change in the PS construct. The IDP of the AST construct was decreased by 77% in ipsilateral bending and increased by 22% in contralateral bendin g. These results demonstrated that eccentric loading from the spinal instru ments increased IDP and significant disc pressure may still exist despite a n increase in motion segment stiffness after lumbar stabilization.