Adjacent segment motion after a simulated lumbar fusion in different sagittal alignments - A biomechanical analysis

Study Design. An in vitro biomechanical study of adjacent segment motion (at L3-L4 and L5-S1) after a simulated lumbar interbody fusion of L4-L5 in different sagittal alignments was carried out. Objectives. To test the hypothesis that an L4-L5 fixation in different sagittal alignments causes increased angular motion at the adjacent levels (L3-L4 and L5-S1) in comparison with the intact spine. Summary of Background Data. Clinical experience has suggested that lumbar fusion in a nonanatomic sagittal alignment can increase degeneration of the adjacent levels. It has been hypothesized that this is the result of increased motion at these levels; however, to the authors' knowledge no mechanical studies have demonstrated this. Methods. Eight fresh human cadaveric lumbar spines (L3-S1) were biomechanically tested. Total angular motion at L3-L4 and L5-S1 under flexion-extension load conditions (7-Nm flexion and 7-Nm extension) was measured. Each specimen was tested intact, and then again after each of three different sagittal fixation angles (at L4-L5): (1) in situ (21degreeslordosis), (2) hyperlordotic (31degreeslordosis), and (3) hypolordotic (7degreeslordosis). The simulated anterior/posterior fusion was performed at L4-L5 with pedicle screws posteriorly, vertebral body screws anteriorly, and an interbody dowel. Results. The averaged values for flexion-extension motion at L3-L4 were as follows: intact specimen 2.0degrees, in situ fixation 4.0degrees, hyperlordotic fixation 1.7degrees, hypolordotic fixation 6.5degrees. The averaged values for flexion-extension motions at L5-S1 were as follows: intact specimen 2.3degrees, in situ fixation 2.6degrees, hyperlordotic fixation 3.6degrees, hypolordotic fixation 2.9degrees. Conclusion. Hypolordotic alignment at L4-L5 caused the greatest amount of flexion-extension motion at L3-L4, and the differences were statistically significant in comparison with intact specimen, in situ fixation and hyperlordotic fixation. Hyperlordotic alignment at L4-L5 caused the greatest amount of flexion-extension motion at L5-S1, and the difference was statistically significant in comparison with intact specimen but not in situ fixation or hypolordotic fixation.