## 4 Discussion
As a result, any large variations noted between the pressure values of the PMHSs may be attributed to this progressive improvement in test methodology. Additionally, the pressure sensor at the C5/C6 level in PMHS 2 was erroneously/manually displaced and replaced between the first and second motion trials, which could have affected the resulting pressure data and is the most likely reason behind the larger standard deviations noted at this IVD level. Limitations were observed in this feasibility study. PMHS anatomy and the degree of natural disc degeneration inherent with ageing within the population also greatly affect the potential for ideal sensor placement, as well as the tissue characteristics of the discs themselves [52]. Previous whole-body and component PMHS studies have shown that lumbar IVD pressure data successfully detected changes in loading condition; however, the load signal magnitude and resultant pressure data were deemed to be highly sensitive to the positioning/location of the pressure gauge within the disc [38; 52]. The fluid behaviour of the nucleus pulposus yielded improved pressure response data as compared to that from the solid material behaviour exhibited by the annulus fibrosis. Given the extremely small size of the cervical IVD (~15 mm diameter, ~5 mm anterior thickness [65]), perfect placement of the sensor in the centre of the nucleus pulposus proved difficult (Figure 6). Variations in sensor positioning within the three anatomical planes may have affected the recorded pressure outputs. The nucleus pulposus, at the IVD centre, is composed of a homogeneous semi-fluid/gelatinous matrix and has been found to act hydrostatically. While sensor orientation should be of no consequence for a truly hydrostatically responsive tissue, rotational orientation of the pressure sensor within the cervical IVD may have affected the recorded pressure. Attempts were made to orient the pressure sensors with the active measuring face perpendicular to the vertical IVD axis, and, while the high resolution of our CT images (i.e., 0.625 mm slice interval and thickness) made it possible to locate the sensor location within the three anatomical planes of the cervical IVD, the resolution was not sufficient to confirm its degree of axial rotation. The pressure sensors utilized in this study have one active measuring face and one inactive face; however, per the manufacturer, the direction of the measuring face is unimportant when utilized in a static fluid material (e.g., the hydrostatic nucleus pulposus), as the pressure gradients over the miniature pressure sensor are reported to be negligible. While it is possible that the sensors rotated within the cervical IVD after initial placement, sensor wires were strain relieved to surrounding anatomical structures to mitigate any sensor movement during the motion trials; thus, no shift in sensor location (nor pressure response) was anticipated. In addition to variation in sensor placement, discrepancies in pressure sensor outputs may be related to basic anatomy. The head/neck complex can be viewed in a simplified manner as a column with a fixed base (i.e., the neck attachment to the torso). In normal cervical motion, there is a greater degree of motion in the upper portion of the cervical spine than in the lower, and there is independent motion/variability between the individual intervertebral levels due to the segmented nature of the spine. As a result of the lower cervical spine attachment to the torso via the ribcage, the lower cervical spine is more rigid, and motion is more structurally arrested. This increase in arrested motion plus the weight of the head and upper cervical spine may result in increased restraining forces, correlating with increased cervical IVD pressure in the lower cervical spine as compared to the higher instrumented disc levels [21, 59].