Intradiscal pressure recordings in the cervical spine

OBJECTIVE: Experimental investigations analyzing the biomechanics of the ce rvical spine are less common than similar studies of other regions of the s pine. There are no reports on cervical intradiscal pressure (PID) measureme nts in vitro. We therefore wanted to establish normal values for PID under physiological conditions by simultaneous muscle force simulation. Moreover, the impact of ventral cervical fusion should be elucidated, because in cli nical studies, it is a well-known phenomenon that the adjacent segments oft en show increased degenerative changes. We present a pilot study. METHODS: Seven human cervical spine specimens were tested biomechanically i n a specially developed spine tester. Only pure moments were used for flexi on/extension, axial rotation, and lateral bending (maximal moment +/- 0.5 N m). PID was measured simultaneously in C3-C4 and C5-C6. The specimens were tested as intact specimens and after discectomy and fusion in C4-C5. Both t est situations were repeated with simulation of muscle forces. RESULTS: We found characteristic load-pressure curves for each of the three motion axes. In neutral position, PID correlated well with former publishe d data from in vivo measurements. After fusion of C4-C5, there was a marked increase of PID in both adjacent segments (e.g., less than or equal to 180 % for axial rotation). With muscle force simulation, the increase was even higher (e.g., less than or equal to 400% for axial rotation). CONCLUSION: For the first time, PID could be measured in the cervical spine in an experimental setting. The results obtained using normal specimens un der physiological conditions confirmed those reported in two clinical studi es. After cervical fusion, a marked increase in PID could be found in both adjacent segments. Presuming that an increase in PID had a negative effect on metabolism of the intervertebral disc, our results may help to explain w hy progressive degeneration occurs in these segments.