Examining changes in intradiscal pressure during intervertebral disc herniation
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Background: Approximately 40% of low back pain cases are attributed to internal disc disruption. Since mechanical loading directly affects intradiscal pressure and the stresses that the inner annulus fibrosus experiences, the mechanism that leads to disruption of the inner annulus fibrosus may be linked to changes in intradiscal pressure. Hence, there is a need to examine how intradiscal pressure changes over time during a flexion extension cyclic (FEC) loading protocol known to induce internal disc disruption. Purpose: 1) To determine whether a bore-screw pressure sensor system could be used as an alternative sensor for measuring intradiscal pressure, and 2) to characterize changes in intradiscal pressure, moments, and axial deformation using a FEC loading protocol. Study 1 summary: Technical specifications of the bore-screw pressure sensor system were compared to the needle pressure sensor. The error projected at a static compressive load of 1500 N was approximately eight percent and the bore-screw pressure sensor had an excellent dynamic response compared to the needle pressure sensor. Study 2 methods: The bore-screw pressure sensor system was successfully instrumented in 14 porcine specimens. The FEC loading protocol consisted of 3600 cycles of 1 Hz flexion-extension movement while applying a 1500 N compressive load. The four dependent variables collected were intradiscal pressure, moment, axial deformation, and angular displacement. Study 2 results: Intradiscal pressure and specimen height decreased by 45 % and 62 %, respectively, and the peak moment increased by 102 % following the FEC loading protocol. There were strong correlations between average intradiscal pressure and both peak moment and average axial deformation. The angle where maximum pressure occurred demonstrated a significant difference after 2700 cycles. There were no sequential changes in pressure difference after 2100 cycles. Twelve out of 14 specimens showed partial herniation (85.7%); however, the injury type was not correlated to the pressure change. Conclusions: Changes in intradiscal pressure were successfully characterized over time using a new pressure sensor system. Although the change in pressure difference was not predictive of an injury type, its increasing trend over time suggested that the inner annulus fibrosus failure mechanism may be related to fatigue.