Demirtas Ahmet, Curran Erin, Ural Ani
Department of Mechanical Engineering, Villanova University, 800 Lancaster Avenue, Villanova, PA, United States.
Department of Mechanical Engineering, Villanova University, 800 Lancaster Avenue, Villanova, PA, United States.
Bone. 2016 Oct;91:92-101. doi: 10.1016/j.bone.2016.07.015. Epub 2016 Jul 20.
The recent reports of atypical femoral fracture (AFF) and its possible association with prolonged bisphosphonate (BP) use highlighted the importance of a thorough understanding of mechanical modifications in bone due to bisphosphonate treatment. The reduced compositional heterogeneity is one of the modifications in bone due to extensive suppression of bone turnover. Although experimental evaluations suggested that compositional changes lead to a reduction in the heterogeneity of elastic properties, there is limited information on the extent of influence of reduced heterogeneity on fracture resistance of cortical bone. As a result, the goal of the current study is to evaluate the influence of varying the number of unique elastic and fracture properties for osteons, interstitial bone, and cement lines on fracture resistance across seven different human cortical bone specimens using finite element modeling. Fracture resistance of seven human cortical bone samples under homogeneous and three different heterogeneous material levels was evaluated using a compact tension test setup. The simulation results predicted that the crack volume was the highest for the models with homogeneous material properties. Increasing heterogeneity resulted in a lower amount of crack volume indicating an increase in fracture resistance of cortical bone. This reduction was observed up to a certain level of heterogeneity after which further beneficial effects of heterogeneity diminished suggesting a possible optimum level of heterogeneity for the bone tissue. The homogeneous models demonstrated limited areas of damage with extensive crack formation. On the other hand, the heterogeneity in the material properties led to increased damage volume and a more variable distribution of damage compared to the homogeneous models. This resulted in uncracked regions which tended to have less damage accumulation preventing extensive crack propagation. The results also showed that the percent osteonal area was inversely correlated with crack volume and more evenly distributed osteons led to a lower amount of crack growth for all levels of material heterogeneity. In summary, this study developed a new computational modeling approach that directly evaluated the influence of heterogeneity in elastic and fracture material properties on fracture resistance of cortical bone. The results established new information that showed the adverse effects of reduced heterogeneity on fracture resistance in cortical bone and demonstrated the nonlinear relationship between heterogeneity and fracture resistance. This new computational modeling approach provides a tool that can be used to improve the understanding of the effects of material level changes due to prolonged BP use on the overall bone fracture behavior. It may also bring additional insight into the causes of unusual fractures, such as AFF and their possible association with long term BP use.
近期有关非典型股骨骨折（AFF）及其与长期使用双膦酸盐（BP）可能存在关联的报道，凸显了全面了解双膦酸盐治疗导致骨力学改变的重要性。成分异质性降低是由于骨转换受到广泛抑制而导致的骨改变之一。尽管实验评估表明成分变化会导致弹性性能异质性降低，但关于异质性降低对皮质骨抗骨折能力的影响程度的信息有限。因此，本研究的目的是使用有限元模型，评估改变骨单位、骨间质和黏合线的独特弹性和骨折性能数量对七种不同人类皮质骨标本抗骨折能力的影响。使用紧凑拉伸试验装置评估了七种人类皮质骨样本在均匀和三种不同非均匀材料水平下的抗骨折能力。模拟结果预测，材料性能均匀的模型裂纹体积最大。异质性增加导致裂纹体积减少，表明皮质骨抗骨折能力增强。在达到一定异质性水平之前，这种减少都能观察到，之后异质性的进一步有益影响减弱，这表明骨组织可能存在一个最佳异质性水平。均匀模型显示损伤区域有限，有广泛的裂纹形成。另一方面，与均匀模型相比，材料性能的异质性导致损伤体积增加，损伤分布更具变化性。这导致未开裂区域的损伤积累较少，从而防止了广泛的裂纹扩展。结果还表明，骨单位面积百分比与裂纹体积呈负相关，并且在所有材料异质性水平下，骨单位分布更均匀会导致裂纹扩展量减少。总之，本研究开发了一种新的计算建模方法，直接评估弹性和骨折材料性能的异质性对皮质骨抗骨折能力的影响。结果建立了新的信息，显示了异质性降低对皮质骨抗骨折能力的不利影响，并证明了异质性与抗骨折能力之间的非线性关系。这种新的计算建模方法提供了一种工具，可用于增进对长期使用BP导致的材料水平变化对整体骨骨折行为影响的理解。它还可能为异常骨折的原因，如AFF及其与长期使用BP的可能关联带来更多见解。
