Department of Mechanical Engineering, University of British Columbia, Vancouver, Canada; ICORD, University of British Columbia, Vancouver, Canada.
ICORD, University of British Columbia, Vancouver, Canada.
J Mech Behav Biomed Mater. 2021 Jun;118:104446. doi: 10.1016/j.jmbbm.2021.104446. Epub 2021 Mar 15.
Passive mechanical properties of the paraspinal muscles are important to the biomechanical functioning of the spine. In most computational models, the same biomechanical properties are assumed for each paraspinal muscle group, while cross-sectional area or fatty infiltration in these muscles have been reported to differ between the vertebral levels. Two important properties for musculoskeletal modeling are the slack sarcomere length and the tangent modulus. This study aimed to investigate the effect of vertebral level on these biomechanical properties of paraspinal muscles in a rat model.
The left paraspinal muscles of 13 Sprague-Dawley rats were exposed under anesthesia. Six muscle biopsies were collected from each rat: three from multifidus (one per each of the L1, L3, and L5 levels) and similarly three from longissimus. Each biopsy was cut into two halves. From one half, two to three single muscle fibers and two to six muscle fiber bundles (14 ± 7 fibers surrounded in their connective tissue) were extracted and mechanically tested in a passive state. From the resulting stress-strain data, tangent modulus was calculated as the slope of the tangent at 30% strain and slack sarcomere length (beyond which passive force starts to develop) was recorded. The other half of each biopsy, which represented the muscle at the fascicle level, was snap frozen, sectioned, stained for Collagen I and its area fraction was measured. To evaluate the effect of spinal level on these biomechanical properties of multifidus and longissimus, one-way repeated measures ANOVA (p < 0.05) was performed for tangent modulus and slack sarcomere length, while for collagen I content linear mixed-models analysis was adopted.
In total, 192 fibers and 262 fiber bundles were mechanically tested. For both muscle groups, no significant difference in tangent modulus of the single fibers was detected between the three spinal levels (p = 0.9 for multifidus and p = 0.08 for longissimus). Similarly, the tangent modulus values for the fiber bundles were not significantly different between the three spinal levels (p = 0.13 for multifidus and p = 0.49 for longissimus). In both muscle groups, the slack sarcomere lengths were not different among the spinal levels except for multifidus fibers (p = 0.02). Collagen I area fraction in muscle fascicles averaged 6.8% for multifidus and 5.3% for longissimus and was not different between the spinal levels.
The results of this study highlighted that the tangent modulus, slack sarcomere length, and collagen I content of the lumbar paraspinal muscles are independent of spinal level. This finding provides the basis for the assumption of similar mechanical properties along a paraspinal muscle group.
脊柱旁肌肉的被动力学特性对于脊柱的生物力学功能很重要。在大多数计算模型中，假设每个脊柱旁肌肉群具有相同的生物力学特性，而这些肌肉的横截面积或脂肪浸润已被报道在不同的椎体水平之间存在差异。对于肌肉骨骼建模来说，两个重要的特性是松弛肌节长度和切线模量。本研究旨在调查在大鼠模型中，椎体水平对脊柱旁肌肉这些生物力学特性的影响。
在麻醉下暴露 13 只 Sprague-Dawley 大鼠的左侧脊柱旁肌肉。从每只大鼠身上采集 6 块肌肉活检：三块来自多裂肌（每个 L1、L3 和 L5 水平各一块），同样的还有三块来自最长肌。每个活检被切成两半。从一半中，提取两到三根单根肌纤维和两到六根肌纤维束（被其结缔组织包围的 14±7 根纤维），并在被动状态下进行机械测试。从产生的应力-应变数据中，计算切线模量作为 30%应变处切线的斜率，并且记录松弛肌节长度（超过该长度，被动力开始发展）。每个活检的另一半，代表着肌纤维束水平的肌肉，被快速冷冻，切片，用 I 型胶原染色，并测量其面积分数。为了评估脊柱水平对多裂肌和最长肌这些生物力学特性的影响，采用单向重复测量方差分析（p<0.05）来评估切线模量和松弛肌节长度，而对于胶原 I 含量则采用线性混合模型分析。
总共对 192 根纤维和 262 根纤维束进行了机械测试。对于两组肌肉，在三个脊柱水平之间，单根纤维的切线模量没有显著差异（多裂肌的 p=0.9，最长肌的 p=0.08）。同样，纤维束的切线模量值在三个脊柱水平之间也没有显著差异（多裂肌的 p=0.13，最长肌的 p=0.49）。在两组肌肉中，除了多裂肌纤维外（p=0.02），松弛肌节长度在脊柱水平之间没有差异。多裂肌和最长肌的肌纤维束中的胶原 I 面积分数平均为 6.8%和 5.3%，在脊柱水平之间没有差异。
本研究结果表明，腰椎脊柱旁肌肉的切线模量、松弛肌节长度和胶原 I 含量与脊柱水平无关。这一发现为假设脊柱旁肌肉群具有相似的力学特性提供了基础。
