Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, Japan.
Proc Inst Mech Eng H. 2022 Sep;236(9):1357-1364. doi: 10.1177/09544119221114199. Epub 2022 Jul 23.
An ankle-foot orthosis (AFO) constructed as a single piece of isotropic elastic material is a commonly used assistive device that provides stability to the ankle joint of patients with spastic diplegic cerebral palsy. The AFO has asymmetric stiffness that restricts plantarflexion during the swing phase while it is flexible to allow dorsiflexion during the stance phase with a large deflection, including buckling originating from geometric nonlinearity. However, its mechanical implications have not been sufficiently investigated. This study aims to develop a computational model of an AFO considering geometric nonlinearity and examine AFO stiffness asymmetry during plantarflexion and dorsiflexion using physical experiments. Three-dimensional AFO mechanics with geometric nonlinearities were expressed using corotational triangle-element formulations that obeyed Kirchhoff-Love plate theory. Computational load tests for plantarflexion and dorsiflexion, using idealised AFOs with two different ankle-region designs (covering or not covering the apexes of the malleoli), showed that plantarflexion moment-ankle angle relationships were linear and dorsiflexion moment-ankle angle relationships were nonlinear; increases in dorsiflexion led to negative apparent stiffness of the AFO. Both ankle-region designs resisted both plantarflexion and dorsiflexion, and out-of-plane elastic energy was locally concentrated on the lateral side, resulting in large deflections during dorsiflexion. These findings give insight into appropriate AFO design from a mechanical viewpoint by characterising three-dimensional structural asymmetry and geometric nonlinearity.
一种由各向同性弹性材料制成的一体式踝足矫形器（AFO）是一种常用的辅助设备，可为痉挛性双瘫脑瘫患者的踝关节提供稳定性。AFO 具有不对称的刚度，在摆动阶段限制跖屈，而在支撑阶段则具有较大的柔性，允许背屈，包括由几何非线性引起的屈曲。然而，其力学特性尚未得到充分研究。本研究旨在开发一种考虑几何非线性的 AFO 计算模型，并通过物理实验研究 AFO 在跖屈和背屈时的刚度不对称性。使用遵循 Kirchhoff-Love 板理论的共旋转三角形单元公式表达具有几何非线性的三维 AFO 力学。使用两种不同的踝关节区域设计（覆盖或不覆盖外踝顶点）的理想化 AFO 进行跖屈和背屈的计算负荷测试表明，跖屈力矩-踝关节角度关系呈线性，背屈力矩-踝关节角度关系呈非线性；背屈的增加导致 AFO 的表观刚度为负。两种踝关节区域设计都能抵抗跖屈和背屈，并且弹性能量在平面外集中在外侧，导致背屈时产生较大的变形。这些发现通过对三维结构不对称性和几何非线性进行特征化，从机械角度为合适的 AFO 设计提供了深入的了解。
