Gallagher Sean, Marras William S, Litsky Alan S, Burr Deborah, Landoll John, Matkovic Velimir
NIOSH Pittsburgh Research Laboratory, Pittsburgh, PA 15236-0070, USA.
Spine (Phila Pa 1976). 2007 Aug 1;32(17):1832-9. doi: 10.1097/01.brs.0000259812.75138.c0.
Survival analysis techniques were used to compare the fatigue failure responses of elderly motion segments to a middle-aged sample.
To compare fatigue life of a middle-aged sample of lumbosacral motion segments to a previously tested elderly cohort. An additional objective was to evaluate the influence of bone mineral content on cycles to failure.
A previous investigation evaluated fatigue failure responses of 36 elderly lumbosacral motion segments (average age, 81 +/- 8 years) subjected to spinal loads estimated when lifting a 9-kg load in 3 torso flexion angles (0 degrees, 22.5 degrees, and 45 degrees). Results demonstrated rapid fatigue failure with increased torso flexion; however, a key limitation of this study was the old age of the specimens.
Each lumbosacral spine was dissected into 3 motion segments (L1-L2, L3-L4, and L5-S1). Motion segments within each spine were randomly assigned to a spinal loading condition corresponding to lifting 9 kg in 3 torso flexion angles (0 degrees, 22.5 degrees, or 45 degrees). Motion segments were statically loaded and allowed to creep for 15 minutes, then cyclically loaded at 0.33 Hz. Fatigue life was taken as the number of cycles to failure (10 mm displacement after creep loading).
Compared with the older sample of spines, the middle-aged sample exhibited increased fatigue life (cycles to failure) in all the torso flexion conditions. Increased fatigue life of the middle-aged specimens was associated with the increased bone mineral content (BMC) in younger motion segments (mean +/- SD, 30.7 +/- 11.1 g per motion segment vs. 27.8 +/- 9.4 g). Increasing bone mineral content had a protective influence with each additional gram increasing survival times by approximately 12%.
Younger motion segments survive considerably longer when exposed to similar spine loading conditions that simulate repetitive lifting in neutral and flexed torso postures, primarily associated with the increased bone mineral content possessed by younger motion segments. Cycles to failure of young specimens at 22.5 degrees flexion were similar to that of older specimens at 0 degrees flexion, and survivorship of young specimens at 45 degrees flexion was similar to the older cohort at 22.5 degrees.
采用生存分析技术比较老年运动节段与中年样本的疲劳失效反应。
比较腰骶运动节段中年样本与先前测试的老年队列的疲劳寿命。另一个目的是评估骨矿物质含量对失效循环次数的影响。
先前的一项研究评估了36个老年腰骶运动节段（平均年龄81±8岁）在3种躯干屈曲角度（0度、22.5度和45度）下提起9千克重物时估计的脊柱负荷下的疲劳失效反应。结果表明，随着躯干屈曲增加，疲劳失效迅速；然而，该研究的一个关键局限性是样本年龄较大。
将每个腰骶脊柱解剖成3个运动节段（L1-L2、L3-L4和L5-S1）。每个脊柱内的运动节段被随机分配到对应于在3种躯干屈曲角度（0度、22.5度或45度）下提起9千克重物的脊柱负荷条件。对运动节段进行静态加载并使其蠕变15分钟，然后以0.33赫兹进行循环加载。疲劳寿命以失效循环次数（蠕变加载后10毫米位移）表示。
与老年脊柱样本相比，中年样本在所有躯干屈曲条件下均表现出疲劳寿命增加（失效循环次数）。中年样本疲劳寿命的增加与年轻运动节段骨矿物质含量（BMC）的增加有关（平均±标准差，每个运动节段30.7±11.1克对27.8±9.4克）。骨矿物质含量每增加1克，生存时间约增加12%，对疲劳寿命有保护作用。
当暴露于模拟中立和屈曲躯干姿势下重复提起重物的类似脊柱负荷条件时，年轻运动节段的存活时间长得多，这主要与年轻运动节段骨矿物质含量增加有关。22.5度屈曲时年轻样本的失效循环次数与0度屈曲时老年样本相似，45度屈曲时年轻样本的存活率与22.5度时老年队列相似。
