## 4 Discussion

Table 3: Comparison of previously established pressure ranges from prior research involving live human patients [23] and results of this study. Pressure values are expressed in PSI. of maximizing the anatomical gift for future experiments; however, plans for future studies do include post-test autopsy to assess disc morphology and clinically grade the level of disc degeneration. Despite these limitations, our results suggest that obtaining cervical IVD pressure readings is possible, even in mature PMHSs. [PERSON] et al. [24] was the first to measure IVD pressure in the cervical spine in an in vitro/experimental setting and sought to establish normal values for cervical IVD pressures under \"physiological conditions\" with and without simulated muscle forces. Data obtained from intact (pre-discectomy) component cadaveric specimens (C2-C7) sans muscle force simulation were reported as median values and ranges. Trends in pressure response during motion were similar to our data set in that higher pressure values were reported for flexion/extension in comparison with axial rotation and lateral bending. Converse to our results, higher pressures were obtained in the upper segment (C3/C4) than in the lower segment (C5/C6) for flexion/extension and axial rotation. There are certain limitations associated with component PMHSs (i.e., FSUs and component head/neck specimens), as the procurement of these specimen types often disrupts the integrity of the primary and surrounding anatomical structures, negatively and potentially artificially affecting the overall PMHS kinematics/kinetics [21, 59]. In the [PERSON] et al. study, both the cranial- and caudal-most vertebrae were potted in polymethylmethacrylate and affixed to the test device, which differed greatly from our use of whole-body PMHSs and motion methodology and could have potentially affected the reported pressures. We feel that employing the use of pressure sensors in a whole-body PMHS provided a more realistic head/complex response that we hoped would act as a firm experimental base for our feasibility study. Previous in vitro and in vivo cervical IVD studies also noted marked interindividual differences in pressure readings between experimental positions/motions [23, 24, 26, 27]; while our two study subjects displayed relatively similarly trended pressure values, there was some variability between the subjects, confirming interindividual differences and suggesting that subject-specific postural configurations (e.g., cervical sagittal balance) and/or subject sex differences may play a role in results [60]. Continued analysis of the 3D motion tracking data from this study is anticipated to provide additional information on subject-specific kinematics and pressure responses. It is also a well-known phenomenon that sex differences exist in both cervical anatomy and biomechanical response (e.g., anatomically, males typically exhibit larger neck cross-sectional area/muscular than females, males are typically stronger than females in both neck flexor and extensor strengths, females typically display a greater neck range of motion than males, and females exhibit lower average neck loads at failure than do males [8, 61, 62]). Generally, the differences in cervical IVD pressure response between males and females may be affected by simple anatomical differences in which the cervical IVDs in females are smaller than males and thus result in higher IVD pressure readings, an outcome that could be attributed to the principle that disc pressure for a given force is inversely proportional to disc cross-sectional area [63]. As this was a feasibility study, our primary goal was to ensure that the pressure sensors were able to capture changes in cervical IVD pressure during head/neck motion.