Background:​ Low back pain is experienced by 80% of the population at some point in their lives. Approximately 40% of these cases are attributed to internal disc disruption, which is characterized by damage to the internal structure of the intervertebral disc (IVD) and is a precursor to herniation. 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 to the needle pressure sensor for measuring intradiscal pressure over time, and 2) to characterize changes in intradiscal pressure, moments, and axial deformation using a FEC loading protocol.

Study 1 summary:​ In the first study, technical specifications of the bore-screw pressure sensor system were compared to the needle pressure sensor, which has been used in previous spine in vitro studies. 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 (no lag and good correlation) compared to the needle pressure sensor. Several factors have been identified for consideration when conducting in vitro tests using this bore-screw pressure sensor:​ baseline pressure, test duration, hydration, loading protocol, specimen choice, and ambient temperature.

Study 2 methods:​ The bore-screw pressure sensor system was successfully instrumented in 14 porcine specimens (C34 and C56). The FEC loading protocol consisted of 3600 cycles of 1 Hz flexion-extension movement (one hour) while applying a 1500 N compressive load. The four dependent variables collected were intradiscal pressure, moment, axial deformation (specimen height loss), and angular displacement. In each flexion-extension cycle, average, maximum, minimum, and difference between maximum and minimum values were identified.

Study 2 results:​ Intradiscal pressure and specimen height decreased by 45 % and 62 %, respectively, and the peak moment and axial deformation increased by 102 % following the FEC loading protocol. There was a strong negative correlation between average intradiscal pressure and peak moment and a strong positive correlation between average intradiscal pressure and average axial deformation. All variables exhibited significant initial changes, except the angle at maximum pressure, which demonstrated a significant difference after 2700 cycles (p < 0.01). There were no sequential changes in pressure difference after 2100 cycles (p > 0.05), whereas moments and axial deformation were significantly different throughout the protocol. Of the 14 specimens, 12 specimens showed partial herniation (85.7%);​ however, the injury type was not correlated to any of the pre-post dependent variable changes.

Conclusions:​ Changes in intradiscal pressure were successfully characterized over time, in conjunction with previously studied measures such as moments and axial deformation. Average intradiscal pressure decreased and the difference between maximum and minimum pressure in each cycle increased over time during a FEC loading protocol. Although the pre-post change in the 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. Another measure to be examined further in future studies is the angle where maximum pressure occurred, which shifted significantly towards the end of the protocol, indicating that substantial structural change in passive tissues may have occurred.