Validation of a Noninvasive Dynamic Spinal Stiffness Assessment Methodology in an Animal Model of Intervertebral Disc Degeneration

Study Design. An experimental in vivo ovine model of intervertebral disc degeneration was used to quantify the dynamic motion response of the lumbar spine. Objective. The purpose of this study was to: (1) compare invasively measured lumbar vertebral bone acceleration responses to noninvasive displacement responses, and (2) determine the effects of a single level degenerative intervertebral disc lesion on these responses. Summary of Background Data. Biomechanical techniques have been established to quantify vertebral motion responses, yet their invasiveness limits their use in a clinical setting. Methods. Twenty-five Merino sheep were examined; 15 with surgically induced disc degeneration at L1-L2 and 10 controls. Triaxial accelerometers were rigidly fixed to the L1 and L2 spinous processes and dorsoventral (DV) mechanical excitation (20-80 N, 100 milliseconds) was applied to L3 using a spinal dynamometer. Peak force and displacement and peak-peak acceleration responses were computed for each trial and a least squares regression analysis assessed the correlation between L3 displacement and adjacent (L2) segment acceleration responses. An analysis of covariance (ANCOVA) was performed to test the homogeneity of slopes derived from the regression analysis and to assess the mean differences. Results. A significant, positive, linear correlation was found between the DV displacement of L3 and the DV acceleration measured at L2 for both normal (R(2) = 0.482, P < 0.001) and degenerated disc groups (R(2) = 0.831, P < 0.001). The L3 DV displacement was significantly lower (ANCOVA, P < 0.001) for the degenerated group (mean: 10.39 mm) in comparison to the normal group (mean: 9.07 mm). Mean peak-peak L2-L1 DV acceleration transfer was also significantly reduced from 12.40 m/s(2) to 5.50 m/s(2) in the degenerated animal group (ANCOVA, P < 0.001). Conclusion. The findings indicate that noninvasive displacement measurements of the prone-lying animal can be used to estimate the segmental and intersegmental motions in both normal and pathologic spines.