Effects of torso flexion on fatigue failure of lumbosacral motion segments.
Gallagher-S; Marras-WS; Litsky-AS; Burr-D
SpineWeek - Annual Meeting of the International Society for the Study of the Lumbar Spine, Spine Society of Europe, and Cervical Spine Research Society-European Section, May 30-June 5, 2004, Porto, Portugal. Toronto, ON, Canada: The International Society for the Study of the Lumbar Spine (ISSLS), 2004 May; :505
Twelve fresh, frozen lumbosacral spines (average 81 + 9 years) were dissected into three motion segments (L1-L2, L3-L4, L5-S1). Care was taken to reproduce the postures, spinal loads, and loading rates associated with lifting a 9-kg box in three torso flexion angles. An EMG-assisted model was used to develop loads and load rates at three torso flexion angles (0, 22.5, and 45 deg). Motions segments were randomly assigned to torso flexion postures using a partially balanced incomplete block design. Specimens were potted in trays containing polymethylmethacrylate, with proper flexion angles being confirmed using multiple radiographs during fixation. Motion segments were placed in a humidified environmental chamber at 37 deg C, creep loaded for 15 min, and then repetitively loaded at 0.33 Hz (up to 10,000 cycles) using an MTS servohydraulic test frame (Bionix 858, MTS Systems, Eden Prairie, MN). Failure was defined as displacement of the specimen by 10 mm (after creep loading). Torso flexion angle had an immense impact on fatigue life (p<0.0001). Motion segments loaded in the 0 deg condition averaged 8,253 cycles to failure (+2,895), specimens at 22.5 deg lasted 3,257 (+4,443), while those at 45 deg lasted an average of 263 (+646) cycles. No differences in fatigue life were observed by lumbar level or flexion level interaction (p>0.05). Logistic regression uncovered associations between specific damage patterns and loading and/or motion segment characteristics. As examples, stellate endplate fractures were associated with less degenerated discs (p<0.01) and increased shear forces (p<0.05), while lateral endplate fractures were seen in larger segments (p<0.01). Damage to facets was more common at 0 deg torso flexion (p<0.01). Results of this study imply greatly increased risk of fatigue failure of spinal tissues while lifting in flexed torso postures.
Fatigue; Motion-studies; Manual-lifting; Injuries; Back-injuries; Low-back-pain; Biomechanics; Spine-loading; Vertebral-fractures; Musculoskeletal-system; Musculoskeletal-system-disorders
NIOSH Pittsburgh Research Laboratory, P.O. Box 18070, Pittsburgh, PA 15236
Annual Meeting of the International Society for the Study of the Lumbar Spine, Spine Society of Europe, and Cervical Spine Research Society-European Section, Porto, Portugal, May 30-June 5, 2004