## 4 Discussion  
We feel that we successfully met our research goal of confirming the feasibility and repeatability of quantifying cervical IVD pressure in physiological motions of the cervical spine. To our knowledge, this is the first study to implement the use of miniature pressure sensors in whole-body PMHS cervical spine IVDs to assess pressure changes during non-injurious quasistatic cervical motion for biomechanical response. The initial analysis of our data showed promising parallels to the two most relevant data sets related to the cervical IVD pressure response reported in the literature [23,24]. Our cervical IVD pressure results show similar trends and comparable pressure values to those reported in the literature despite previous studies involving different specimen/subject types (e.g., FSUs and/or component cervical PMHS spines versus whole-body PMHS as used in this study), investigating varying cervical IVD levels, utilizing specimens with documented clinical cervical disc disease [23] and employing differing pressure sensor instrumentation. The similarity of values reported in the literature from research involving live human patients [23] and component PMHSs [24] suggests that the PMHS is indeed a viable and beneficial surrogate [16,51], and the selected pressure sensor is appropriate for this type of biomechanical experimentation. We recognize that there were notable differences between the previously reported pressure response values and those obtained in this study; however, as this was a feasibility study, our primary goal was to ensure that the pressure sensors were able to detect and capture changes in cervical IVD pressure. Our secondary goal was to evaluate if our methodology and instrumentation (which differed substantially from many of the previous studies) proffered pressure values that were within the realm of previously reported values. PMHS results from the current feasibility study were closely aligned to previously established pressure ranges (often within a range of approximately +/\(-\)1 SD) (Tables 3 and 4). [PERSON] [23] provided the first in vivo cervical disc pressure measurements for neutral and non-neutral head/neck positions and established positionally/motion-related cervical IVD pressures in normal/abnormal discs. An important distinction between the Hattori study and the current study is the sample type and size. The Hattori study included 80 cervical IVDs (5% C3/C4, 23% C4/C5, 45% C5/C6, and 28% C6/C7) from 48 pre-operative live patients. Only half of the Hattori cervical IVD levels were directly comparable to those included in this study (levels C3/C4 and C5/C6), while the other IVD levels (C4/C5, C6/C7) were anatomically close but not directly comparable. Additionally, and perhaps \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**C3/C4**} & \multicolumn{2}{c}{**C5/C6**} & \multicolumn{2}{c}{**C7/T1**} \\ \hline
**Motion** & **Mean** & **+/\(-\)** & **Mean** & **+/\(-\)** & **Mean** & **+/\(-\)** \\ \hline Gentle Traction & 28.01 & 6.53 & 14.22 & 14.62 & 76.93 & 3.86 \\ \hline Flexion & 39.47 & 14.82 & 21.24 & 19.80 & 165.71 & 44.53 \\ \hline Extension & 9.99 & 4.30 & 10.20 & 1.99 & 135.96 & 16.75 \\ \hline Lateral Bending—Left & 29.42 & 3.16 & 24.71 & 16.74 & 21.49 & 16.05 \\ \hline Lateral Bending—Right & 32.77 & 11.62 & 23.32 & 16.67 & 34.07 & 17.84 \\ \hline Axial Rotation—Left & 25.15 & 5.62 & 26.31 & 15.58 & 10.42 & 7.21 \\ \hline Axial Rotation—Right & 15.84 & 1.05 & 35.45 & 24.08 & 8.23 & 0.40 \\ \hline Forced Tension & 32.83 & 10.60 & 28.52 & 31.00 & 89.59 & 12.80 \\ \hline Forced Compression & 18.38 & 15.96 & 32.75 & 33.28 & 115.84 & 12.37 \\ \hline Gentle Traction & 26.61 & 13.62 & 26.85 & 31.39 & 82.54 & 0.94 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of mean absolute pressure values (PSI) for all trials for both specimens.