Object: The objective of this study was, using a novel intraoperative measurement (IOM) system, to test the hypothesis that an increased facet joint volume is evidence of spinal instability.
Methods: In 29 patients (male/female ratio 13:16; mean age 67.5 years, range 43-80 years)-17 with degenerative spondylolisthesis (DS) of the lumbar spine (Group DS) and 12 with canal stenosis (CS) of the lumbar spine (Group CS)-DICOM (Digital Imaging and Communications in Medicine) data derived from CT scans were transferred to a workstation. A 3D model of facet joint spaces was reconstructed and the average volume of the bilateral facets was calculated. Segmental properties-stiffness, absorption energy (AE), and neutral zone (NZ)-were measured using an IOM system, and values were compared between groups. Linear regression analyses were performed among biomechanical parameters and average volumes.
Results: Stiffness and AE did not differ significantly between groups. The NZ was significantly greater in Group DS than in Group CS (p < 0.05) and significantly positively correlated with the average volume (R(2) = 0.141, p < 0.05). Stiffness tended to negatively correlate with average volume. Absorption energy did not correlate with average volume.
Conclusions: Biomechanical analyses using the IOM system verified that an increased facet joint volume is evidence of spinal instability, represented by NZ, in the degenerative lumbar spine.