Biomechanical Effects of Trunk Flexion Angle and Intervertebral Disc Degeneration on Lumbar Spine under Wheeled Walker-Assisted Gait

Objective To investigate the biomechanical effects of trunk flexion angle on lumbar intervertebral discs with varying degrees of degeneration during walking assisted by a wheeled walker, aiming to provide a quantitative basis for assessing the potential risk of lumbar injury induced by abnormal mechanical loads under walker-assisted gait. Methods A nonlinear finite element model of the lumbar spine (L1-S1) was developed based on computed tomography (CT) images of a healthy male volunteer. Using a validated mathematical model of lumbar spine loading from the literature, axial loads and flexion moments applied to the lumbar spine under upright standing and walker-assisted gait conditions were calculated for trunk flexion angles ranging from 10° to 40°. The biomechanical responses of discs with different degrees of degeneration under these flexion angles were analyzed. Results During upright standing, the lumbar spine was subjected to a follower load of 470 N. The L5-S1 disc, including the annulus fibrosus matrix, nucleus pulposus, and annular fibers, exhibited the highest stress, while the L2-3 disc experienced the lowest stress. The inferior endplate of the L5 segment showed the highest stress, whereas the inferior endplate of the L2 segment had the lowest stress. When the walker provided 20% of body weight support, flexion moments of 27.03, 53.24, 77.83, and 100.05 N·m were generated at trunk flexion angles of 10°, 20°, 30°, and 40°, respectively. When the walker provided 10% of body weight support, a 20° flexion angle produced a flexion moment of 77.16 N·m. Disc stress increased significantly with both increasing trunk flexion angle and greater degeneration severity. The L1-2 and L5-S1 segments exhibited notably higher disc stresses compared to other lumbar segments. The superior endplate of the L2 segment experienced the highest stress, while the inferior endplate of the L5 segment showed the lowest. Conclusions Trunk flexion during wheeled walker-assisted gait markedly increases intervertebral disc stress, particularly in the L1-2 and L5-S1 segments. Degenerated discs are more susceptible to stress concentration.