Design of Customize Interbody Fusion Cages of Ti64ELI with Gradient Porosity by Selective Laser Melting Process

Abstract

Intervertebral fusion surgery for spinal trauma, degeneration, and deformity correction is a major vertebral reconstruction operation. For most cages, the stiffness of the cage is high enough to cause stress concentration, leading to a stress shielding effect between the vertebral bones and the cages. The stress shielding effect affects the outcome after the reconstruction surgery, easily causing damage and leading to a higher risk of reoperation. A porous structure for the spinal fusion cage can effectively reduce the stiffness to obtain more comparative strength for the surrounding tissue. In this study, an intervertebral cage with a porous gradation structure was designed for Ti64ELI alloy powders bonded by the selective laser melting (SLM) process. The medical imaging software InVesalius and 3D surface reconstruction software Geomagic Studio 12 (Raindrop Geomagic Inc., Morrisville, NC, USA) were utilized to establish the vertebra model, and ANSYS Workbench 16 (Ansys Inc, Canonsburg, PA, USA) simulation software was used to simulate the stress and strain of the motions including vertical body-weighted compression, flexion, extension, lateral bending, and rotation. The intervertebral cage with a hollow cylinder had porosity values of 80–70–60–70–80% (from center to both top side and bottom side) and had porosity values of 60–70–80 (from outside to inside). In addition, according to the contact areas between the vertebras and cages, the shape of the cages can be custom-designed. The cages underwent fatigue tests by following ASTM F2077-17. Then, mechanical property simulations of the cages were conducted for a comparison with the commercially available cages from three companies: Zimmer (Zimmer Biomet Holdings, Inc., Warsaw, IN, USA), Ulrich (Germany), and B. Braun (Germany). The results show that the stress and strain distribution of the cages are consistent with the ones of human bone, and show a uniform stress distribution, which can reduce stress concentration.

Endplate volumetric bone mineral density biomechanically matched interbody cage

“…Differences in elastic moduli between the implant and bone cause stress shielding, finally leading to the weakening of the bone ( Murr, 2020 ). In cases of osteoporotic or osteopenia, a porous cage with one standard modulus may not a suitable choice due to the differences in the moduli ( Li et al, 2019 ; Pan et al, 2021 ).…”

A New Design of Porosity Gradient Ti-6Al-4V Encapsulated Hydroxyapatite Dual Materials Composite Scaffold for Bone Defects

“…The results show that the average Young's coefficient for the original specimen before annealing was 126.44 GPa and the average hardness value was 3.9 GPa, as shown in Figure 16a. However, these values are lower than those of the annealed specimens, as shown in Figure 16b, with an average Young's coefficient of 131.46 GPa and an average hardness value of 4.12 GPa, which confirmed that the average Young's modulus and the average hardness could be increased after annealing treatment due to the removal of residual stress [22,26]. A comparison of the data is shown in Table 9.…”

A review onin vitro/in vivoresponse of additively manufactured Ti–6Al–4V alloy

“…Therefore, to improve the mechanical properties, and maximizing biological properties, creating a graded/gradient porosity structure is one of the most effective solutions. [216][217][218][219] The design pattern of these implants imitates bone structure. The bone has a functionally graded structure where the outer part, called cortical, which is dense and has a modulus of elasticity is B15-25 GPa, the inner part is called trabecular with a porous structure and its modulus of elasticity is B0.1-4.5 GPa.…”