The effect of facet geometry on the axial torque-rotation response of lumbar motion segments.

The objective of this study is to examine the effect of the facet geometry on the axial torque-rotation response of lumbar motion segments. The three-dimensional facet geometry of 35 L2-3 and 35 L4-5 specimens has been quantified in terms of seven parameters from computed tomographic transverse sections taken at 2-mm intervals. The motion segments have then been subjected to axial torque, for a variety of preload conditions, to establish the axial torque-rotation response. There exists significant variation of facet geometry between the segment levels; however, the degree of facet asymmetry is identical for each level. No significant correlation exists between the lumbar facet geometry and the axial torque-rotation response. The facet joints do, however, act as a "positive stop" to axial rotation, regardless of their orientation. Therefore, axial rotation sufficient to cause disc injury may only be possible after facet failure.