Changes in Cervical Canal Spinal Volume During In Vitro Flexion‐Extension

Study Design. Quasistatic flexion and extension loads were applied in vitro to lower cervical spines. The flexion‐extension motion produced was checked for physiologic relevance. Objectives. To examine the changes in the volume of the cervical spinal canal in flexion‐extension motion. Summary of Background Data. Many papers have been published concerning the cervical canal volume as inferred from standard lateral radiographs. This study compares the inferred (radiographic) volumes and their changes to the physical changes within the spinal canal. Methods. The lower cervical spines (C2‐C7) from 10 cadavers were subject to stepwise flexion and extension in a purpose‐built rig. Before this testing, the spinal cord was removed from the canal space of each specimen and replaced by a thin latex tube stoppered and secured at the opening of the canal (at C2) so that the volume of liquid displaced from the tube could be measured. This was done at each loading stage by means of a graduated glass column, and a radiograph of the spine was also taken to allow angular and displacement readings to be taken from C2 to C7. Results. The average recorded change in volume of the spinal canal with flexion‐extension motion was 1.9 ml, and showed a significant linear correlation with the dynamic canal width (r = 0.868, P < 0.05) and also with the total angle of flexion or extension (r = 0.979, P < 0.005). The volume of liquid displaced from the canal in lateral bending was much lower than that in flexion‐extension motion, and only amounted to about 0.2 ml. The angular ranges of motion produced at each level were compared to previous results obtained in vivo, and no significant differences between the angular displacements found in vivo and in vitro under this experimental arrangement were seen. Conclusions. The loading regime described in this study causes angular displacements similar to those in vivo, and on this basis is a physiologically relevant loading pattern. The change in the volume of the spinal canal between C2 and C7 shows linear relationships with the angle of flexion and the dynamic canal width.