Motion segment stiffness measured without physiological levels of axial compressive preload underestimates the in vivo values in all six degrees of freedom
Gardner-Morse, M.G.; Stokes, I.A.; Churchill, D.; Badger, G.
Studies in Health Technology and Informatics 91: 167-172
2002
ISSN/ISBN: 0926-9630 PMID: 15457717 Document Number: 544204
Axial preload is known to alter the mechanical properties of spinal motion segments. The objective of this study was to compare the experimentally measured load-displacement behavior (stiffness, hysteresis and linearity) of porcine lumbar motion segments in vitro with physiological axial compressive preloads of 0, 200 and 400 N equilibrated in a physiological fluid environment. At each preload, displacements in each of six degrees of freedom (+/- 0.3 mm AP and lateral translations, +/- 0.2 mm axial translation, +/- 1 degrees lateral bending and +/- 0.8 degrees flexion/extension and torsional rotations) were imposed. The resulting forces and moments were recorded. Tests were repeated after removal of posterior elements. Using least squares, the forces at the vertebral body center were related to the displacements by a symmetric 6x6 stiffness matrix. The stiffness, hysteresis area and linearity of six diagonal and two off-diagonal load-displacement relationships were examined for differences between preload conditions. Mean values of the diagonal terms of the stiffness matrix for intact porcine motion segments increased significantly by an average factor of 2.2 and 2.9 with 200 and 400 N axial compression respectively (p<0.001). Increases for isolated disc specimens averaged 4.6 and 6.9 times with 200 and 400 N preload (p<0.001). Changes in hysteresis correlated with the changes in stiffness. The load-displacement relationships were progressively more linear with increasing preload (R2 = 0.82, 0.97 and 0.98 at 0, 200 and 400 N axial compression respectively). Motion segment and disc load-displacement behaviors were stiffer, more linear and had greater hysteresis with axial compressive preloads.