Loading of the Lumbar Spine During Transition from Standing to Sitting: Effect of Fusion versus Motion Preservation at L4-L5 and L5-S1.

BACKGROUND CONTEXT Transition from standing to sitting significantly decreases lumbar lordosis with the greatest lordosis-loss occurring at L4-S1. Fusing L4-S1 eliminates motion and thus the proximal mobile segments maybe recruited during transition from standing to sitting to compensate for the loss of L4-S1 mobility. This may subject proximal segments to supra-physiologic flexion loading. PURPOSE Assess effects of instrumented fusion versus motion preservation at L4-L5 and L5-S1 on lumbar spine loads and proximal segment motions during transition from standing to sitting. STUDY DESIGN Biomechanical study using human thoracolumbar spine specimens. METHODS A novel laboratory model was used to simulate lumbosacral alignment changes caused by a person's transition from standing to sitting in eight T10-sacrum spine specimens. The sacrum was tilted in the sagittal plane while constraining anterior-posterior translation of T10. Continuous loading-data and segmental motion-data were collected over a range of sacral slope values, which represented transition from standing to different sitting postures. We compared different constructs involving fusions and motion preserving prostheses across L4-S1. RESULTS After L4-S1 fusion, the sacrum could not be tilted as far posteriorly compared to the intact spine for the same applied moment (P<0.001). For the same reduction in sacral slope, L4-S1 fusion induced 2.9 times the flexion moment in the lumbar spine and required 2.4 times the flexion motion of the proximal segments as the intact condition (P<0.001). Conversely, motion preservation at L4-S1 restored lumbar spine loads and proximal segment motions to intact specimen levels during transition from standing to sitting. CONCLUSIONS In general, sitting requires lower lumbar segments to undergo flexion, thereby increasing load on the lumbar discs. L4-S1 fusion induced greater moments and increased flexion of proximal segments to attain a comparable seated posture. Motion preservation using a total joint replacement prosthesis at L4-S1 restored the lumbar spine loads and proximal segment motion to intact specimen levels during transition from standing to sitting.