Influence of morphological variations on cervical spine segmental responses from inertial loading

Objectives: The objective of this study was to investigate the influence of morphological variations in osteoligamentous lower cervical spinal segment responses under postero-anterior inertial loading. Methods: A parametric finite element model of the C5–C6 spinal segment was used to generate models. Variations in the vertebral body and facet depth (anteroposterior), posterior process length, intervertebral disc height, facet articular process height and slope, segment orientation ranging from lordotic to straight, and segment size were parameterized. These variations included male–female differences. A Latin hypercube sampling method was used to select parameter values for model generation. Forces and moments associated with the inertial loading were applied to the generated model segments. The 7 parameters were grouped as local or global depending on the number of spinal components involved in the shape variation. Four output responses representing overall segmental and soft tissue responses were analyzed for each model variation: response angle of the segment, anterior longitudinal ligament stretch, anterior capsular ligament stretch, and facet joint compression in the posterior region. Pearson's correlation coefficient was used to compute the correlations of these output responses with morphological variations. Results: Fifty models were generated from the parameterized model using a Latin hypercube sampling technique. Variation in response angle among the models was 4° and was most influenced by change in the combined dimension of vertebral body and facet depth, followed by size of the segment. The maximum anterior longitudinal ligament stretch varied between 0.1 and 0.3 and was strongly influenced by the change in the segment orientation. The anterior facet joint region sustained tension, whereas the posterior region sustained compression. For the anterior capsular ligament stretch, the most influential global variation was segment orientation, whereas the most influential local variations were the facet height and facet angle parameters. In the case of posterior facet joint compression, segment orientation was again most influential, whereas among the local variations, the facet angle had the most influence. Conclusion: Shape variations in the intervertebral disc influenced segmental rotation and ligament responses; however, the influence of shape variations in the facet joint was confined to capsular ligament responses. Response angle was most influenced by the vertebral body depth variations, explaining greater segmental rotations in female spines. Straighter spine segments sustained greater posterior facet joint compression, which may offer an explanation for the higher incidence of whiplash-associated disorders among females, who exhibit a straighter cervical spine. The anterior longitudinal ligament stretch was also greater in straighter segments. These findings indicate that the morphological features specific to the anatomy of the female cervical spine may predispose it to injury under inertial loading.研究目的：本研究旨在探究骨韧带性下颈段脊柱节段在前后向惯性载荷下的响应规律，分析其受形态学变异的影响机制。 研究方法：本研究以C5-C6脊柱节段的参数化有限元模型（parametric finite element model）为基础构建衍生模型。对椎体与关节突前后向深度、后突长度、椎间盘高度、关节突关节高度与倾角、节段曲度（从脊柱前凸至平直状态）以及节段尺寸等7项参数进行了参数化设置，涵盖男女之间的解剖学差异。采用拉丁超立方抽样（Latin hypercube sampling）方法选取参数值以生成各类模型。将与惯性载荷对应的力与力矩施加于生成的模型节段。根据形态变异涉及的脊柱组件数量，将7项参数划分为局部参数与全局参数两类。针对每一种模型变异，分析4项表征节段整体与软组织响应的输出指标：节段响应角、前纵韧带牵张量、前关节囊韧带牵张量以及后方区域的关节突关节压应力。采用皮尔逊相关系数（Pearson's correlation coefficient）计算上述输出指标与形态学变异之间的相关性。 研究结果：通过拉丁超立方抽样技术，从参数化模型中共生成50组模型。各组模型的节段响应角变异范围为4°，其受椎体与关节突深度的联合尺寸变化影响最大，其次为节段尺寸。最大前纵韧带牵张量介于0.1至0.3之间，其受节段曲度变化的影响最为显著。前方关节突关节区域承受牵张力，而后方区域则承受压应力。针对前关节囊韧带牵张量而言，影响最大的全局变异为节段曲度，而影响最大的局部变异为关节突高度与关节突倾角参数。对于后方关节突关节压应力而言，节段曲度仍为影响最大的因素，而在局部变异中，关节突倾角的影响最为显著。 研究结论：椎间盘的形态变异会影响节段旋转与韧带响应，但关节突关节的形态变异仅对关节囊韧带响应产生影响。节段响应角受椎体深度变异的影响最大，这解释了女性脊柱节段旋转角度更大的现象。脊柱节段曲度越平直，其后方关节突关节承受的压应力越大，这或许可以解释女性颈段脊柱更平直，因此挥鞭样相关疾患（whiplash-associated disorders）发病率更高的现象。曲度越平直的节段，其前纵韧带牵张量也越大。上述研究结果表明，女性颈段脊柱特有的解剖学形态特征可能使其在惯性载荷作用下更易发生损伤。