Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA.
Department of Integrative Anatomical Sciences, Keck School of Medicine, University of Southern California, Los Angeles, CA, USA.
J Anat. 2019 Nov;235(5):873-882. doi: 10.1111/joa.13056. Epub 2019 Aug 2.
The human clavicle (i.e. collarbone) is an unusual long bone due to its signature S-shaped curve and variability in macrostructure observed between individuals. Because of the complex nature of how the upper limb moves, as well as due to its complex musculoskeletal arrangement, the biomechanics, in particular the mechanical loadings, of the clavicle are not fully understood. Given that bone remodeling can be influenced by bone stress, the histologic organization of Haversian bone offers a hypothesis of responses to force distributions experienced across a bone. Furthermore, circularly polarized light microscopy can be used to determine the orientation of collagen fibers, providing additional information on how bone matrix might organize to adapt to direction of external loads. We examined Haversian density and collagen fiber orientation, along with cross-sectional geometry, to test whether the clavicle midshaft shows unique adaptation to atypical load-bearing when compared with the sternal (medial) and acromial (lateral) shaft regions. Because fractures are most common at the midshaft, we predicted that the cortical bone structure would show both disparities in Haversian remodeling and nonrandomly oriented collagen fibers in the midshaft compared with the sternal and acromial regions. Human clavicles (n = 16) were sampled via thin-sections at the sternal, middle, and acromial ends of the shaft, and paired sample t-tests were employed to evaluate within-individual differences in microstructural or geometric properties. We found that Haversian remodeling is slightly but significantly reduced in the middle of the bone. Analysis of collagen fiber orientation indicated nonrandom fiber orientations that are overbuilt for tensile loads or torsion but are poorly optimized for compressive loads throughout the clavicle. Geometric properties of percent bone area, polar second moment of area, and shape (I /I ) confirmed the conclusions drawn by existing research on clavicle macrostructure. Our results highlight that mediolateral shape changes might be accompanied by slight changes in Haversian density, but bone matrix organization is predominantly adapted to resisting tensile strains or torsion throughout and may be a major factor in the risk of fracture when experiencing atypical compression.
人的锁骨（即锁骨）因其独特的 S 形曲线和个体之间宏观结构的可变性而成为一种不寻常的长骨。由于上肢运动的复杂性质以及其复杂的肌肉骨骼排列，锁骨的生物力学，特别是机械载荷，尚未完全了解。鉴于骨重塑可能受到骨应力的影响，哈弗氏骨的组织学结构提供了一种关于骨骼承受的力分布的反应假说。此外，圆偏振光显微镜可用于确定胶原纤维的取向，提供有关骨骼基质如何组织以适应外部载荷方向的更多信息。我们检查了哈弗氏密度和胶原纤维取向以及横截面几何形状，以测试锁骨中轴是否与胸骨（内侧）和肩峰（外侧）轴区域相比，对异常承重表现出独特的适应性。由于骨折最常见于中轴，我们预测与胸骨和肩峰区域相比，皮质骨结构在中轴处会出现哈弗氏重塑的差异和非随机定向的胶原纤维。通过对轴的胸骨、中部和肩峰端进行薄切片取样，对 16 个人类锁骨进行了取样，并采用配对样本 t 检验评估了个体内微观结构或几何特性的差异。我们发现，骨骼中间的哈弗氏重塑略有但显著减少。胶原纤维取向分析表明，纤维取向是非随机的，对拉伸或扭转负荷过度构建，但对整个锁骨的压缩负荷优化不足。骨面积百分比、极二次矩和形状（I / I ）的几何特性证实了现有锁骨宏观结构研究的结论。我们的结果强调，内外侧形状变化可能伴随着哈弗氏密度的轻微变化，但骨骼基质组织主要适应抵抗拉伸应变或扭转，这可能是在经历异常压缩时骨折风险的主要因素。
