Institut für Zoologie und Evolutionsforschung, Friedrich-Schiller-Universität Jena, Erbertstraße 1, 07743 Jena, Germany.
Institut für Zoologie und Evolutionsforschung, Friedrich-Schiller-Universität Jena, Erbertstraße 1, 07743 Jena, Germany.
Acta Biomater. 2021 Jan 15;120:146-155. doi: 10.1016/j.actbio.2020.05.001. Epub 2020 May 16.
Tendon insertions to bone are heavily loaded transitions between soft and hard tissues. The fiber courses in the tendon have profound effects on the distribution of stress along and across the insertion. We tracked fibers of the Achilles tendon in mice in micro-computed tomographies and extracted virtual transversal sections. The fiber tracks and shapes were analyzed from a position in the free tendon to the insertion. Mechanically relevant parameters were extracted. The fiber number was found to stay about constant along the tendon. But the fiber cross-sectional areas decrease towards the insertion. The fibers mainly interact due to tendon twist, while branching only creates small branching clusters with low levels of divergence along the tendon. The highest fiber curvatures were found within the unmineralized entheseal fibrocartilage. The fibers inserting at a protrusion of the insertion area form a distinct portion within the tendon. Tendon twist is expected to contribute to a homogeneous distribution of stress among the fibers. According to the low cross-sectional areas and the high fiber curvatures, tensile and compressive stress are expected to peak at the insertion. These findings raise the question whether the insertion is reinforced in terms of fiber strength or by other load-bearing components besides the fibers. STATEMENT OF SIGNIFICANCE: The presented study is the first analysis of the 3D fiber tracks in macroscopic tendon samples as determined by a combination of cell-maceration, phase-contrast µCT and template-based tracking. The structural findings change the understanding of the tendon-bone insertion and its biomechanics: (1) The insertion is not reinforced in terms of fiber numbers or sizes. Its robustness remains unexplained. (2) The orientation of fibers in the tendon center is higher than in the margins. This arrangement could inspire material development. (3) Fibers inserting at a protrusion of the insertion area stem from a distinct portion within the tendon. The results show that fibrous structure analysis can link macro- to micromechanics and that it is ready for the application to complete muscle-tendon units.
肌腱插入骨骼是软组织和硬组织之间负荷较重的过渡部位。肌腱中的纤维走向对插入部位的应力分布有深远的影响。我们在小鼠的微计算机断层扫描中追踪了跟腱的纤维，并提取了虚拟的横切面。从自由肌腱的位置到插入部位分析了纤维轨迹和形状。提取了力学相关参数。沿着肌腱，纤维数量基本保持不变。但是纤维的横截面积向插入部位减小。纤维主要通过肌腱扭转相互作用，而分支只在沿着肌腱的低发散水平上形成小的分支簇。最高的纤维曲率出现在未矿化的腱骨结合处纤维软骨内。在插入区域的突起处插入的纤维形成了肌腱内的一个明显部分。肌腱扭转有望有助于纤维之间的应力均匀分布。根据低的横截面积和高的纤维曲率，可以预期拉伸和压缩应力在插入部位达到峰值。这些发现提出了一个问题，即插入部位是否通过纤维强度或除纤维以外的其他承载部件得到了加强。
本研究首次对宏观肌腱样本中的 3D 纤维轨迹进行了分析，这是通过细胞粉碎、相衬µCT 和基于模板的跟踪相结合来确定的。结构研究改变了对肌腱-骨插入及其生物力学的理解：（1）插入部位没有在纤维数量或大小方面得到加强。其坚固性仍未得到解释。（2）肌腱中心纤维的方向高于边缘。这种排列方式可能激发材料的发展。（3）插入插入区域突起的纤维来自肌腱内的一个明显部分。结果表明，纤维结构分析可以将宏观力学与微观力学联系起来，并且已经准备好应用于完整的肌肉-肌腱单元。
