Department of Industrial Engineering, Università di Bologna, Bologna, Italy.
IRCCS Istituto Ortopedico Rizzoli, Movement Analysis Laboratory, Bologna, Italy.
J Mech Behav Biomed Mater. 2021 Sep;121:104608. doi: 10.1016/j.jmbbm.2021.104608. Epub 2021 May 25.
One of the main biomechanical causes for aseptic failure of orthopaedic implants is the stress shielding. This is caused by an uneven load distribution across the bone normally due to a stiff metal prosthesis component, leading to periprosthetic bone resorption and to implant loosening. To reduce the stress shielding and to improve osseointegration, biocompatible porous structures suitable for orthopaedic applications have been developed. Aim of this study was to propose a novel in-vitro model of the mechanical interaction between metal lattice structures and bovine cortical bone in compression. Analysis of the strain distribution between metal structure and bone provides useful information on the potential stress shielding of orthopaedic implants with the same geometry of the porous scaffold. Full density and lattice structures obtained by the repetition of 1.5 mm edge cubic elements via Laser Powder Bed Fusion of CoCrMo powder were characterized for mechanical properties using standard compressive testing. The two porous geometries were characterized by 750 μm and 1000 μm pores resulting in a nominal porosity of 43.5% and 63.2% respectively. Local deformation and strains of metal samples coupled with fresh bovine cortical bone samples were evaluated via Digital Image Correlation analysis up to failure in compression. Visualization and quantification of the local strain gradient across the metal-bone interface was used to assess differences in mechanical behaviour between structures which could be associated to stress-shielding. Overall stiffness and local mechanical properties of lattice and bone were consistent across samples. Full-density metal samples appeared to rigidly transfer the compression force to the bone which was subjected to large deformations (2.2 ± 0.3% at 15 kN). Larger porosity lattice was associated to lower stiffness and compressive modulus, and to a smoother load transfer to the bone. While tested on a limited sample size, the proposed in-vitro model appears robust and repeatable to assess the local mechanical interaction of metal samples suitable for orthopaedic applications with the bone tissue. CoCrMo scaffolds made of 1000 μm pores cubic cells may allow for a smoother load transfer to the bone when used as constitutive material of orthopaedic implants.
一种导致骨科植入物无菌失效的主要生物力学原因是应力屏蔽。这是由于刚性金属假体部件导致的骨负荷分布不均匀引起的，导致假体周围骨吸收和植入物松动。为了减少应力屏蔽并改善骨整合，已经开发出适用于骨科应用的生物相容性多孔结构。本研究的目的是提出一种新的用于模拟金属晶格结构与牛皮质骨在压缩过程中机械相互作用的体外模型。对金属结构与骨骼之间应变分布的分析为具有相同多孔支架几何形状的骨科植入物的潜在应力屏蔽提供了有用信息。通过 CoCrMo 粉末的激光粉末床熔合，通过重复 1.5mm 边缘立方单元获得的全密度和晶格结构，使用标准压缩测试对其机械性能进行了表征。通过数字图像相关分析，对两种多孔结构的特征进行了评估，直到压缩失效，分析了新鲜牛皮质骨样品与金属样品的局部变形和应变。通过在金属-骨界面上可视化和量化局部应变梯度，来评估结构之间机械行为的差异，这种差异可能与应力屏蔽有关。整体刚度和晶格及骨骼的局部力学性能在各样本之间具有一致性。全密度金属样品似乎将压缩力刚性地传递给骨骼，导致骨骼发生大变形（在 15kN 时为 2.2±0.3%）。较大的孔隙晶格与较低的刚度和压缩模量相关，并与骨骼的更平滑的负载传递相关。虽然在有限的样本量上进行了测试，但所提出的体外模型似乎具有稳健性和可重复性，可用于评估适合骨科应用的金属样品与骨骼组织的局部机械相互作用。用作骨科植入物的组成材料时，由 1000μm 孔隙立方细胞组成的 CoCrMo 支架可能允许更平滑地向骨骼传递负载。
