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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...
Context in source publication
Context 1
... was used to grind the top and bottom of samples before the experiment, which can ensure that the samples and the platens have a flatter contact surface during the compression test. The compression test of a universal testing machine is shown in Figure 4. The samples were tested under compression with a strain rate of 1 × 10 −4 s −1 at room temperature by using the Instron 5582 (Instron Inc, Norwood, MA, USA) universal testing machine equipped with a 100 kN load cell and Instron 2601 Linear Variable Differential Transformer (LVDT) displacement transducer, as shown in Figure 5. ...
Citations
... This mitigates the risk of post-operative migration and significantly reduces the likelihood of complications. Our team has conducted previous studies on bone defects [32,33]. ...
This study proposed a composite tibia defect scaffold with radial gradient porosity, utilizing finite element analysis to assess stress in the tibial region with significant critical-sized defects. Simulations for scaffolds with different porosities were conducted, designing an optimal tibia defect scaffold with radial gradient porosity for repairing and replacing critical bone defects. Radial gradient porosity scaffolds resulted in a more uniform stress distribution, reducing titanium alloy stiffness and alleviating stress shielding effects. The scaffold was manufactured using selective laser melting (SLM) technology with stress relief annealing to simplify porous structure fabrication. The study used New Zealand white rabbits’ tibia defect sites as simulation parameters, reconstructing the 3D model and implanting the composite scaffold. Finite element analysis in ANSYS-Workbench simulated forces under high-activity conditions, analyzing stress distribution and strain. In the simulation, the titanium alloy scaffold bore a maximum stress of 122.8626 MPa, while the centrally encapsulated HAp material delivered 27.92 MPa. The design demonstrated superior structural strength, thereby reducing stress concentration. The scaffold was manufactured using SLM, and the uniform design method was used to determine a collection of optimum annealing parameters. Nanoindentation and compression tests were used to determine the influence of annealing on the elastic modulus, hardness, and strain energy of the scaffold.
... In the dynamic tests, the runout load of compression, shear and torsion tests were all increased; while for the stretching test, the tensile strength was significantly decreased, and the yield strength and stretching rate were significantly increased mechanotransduction and bone remodeling; both factors lead to a successful intervertebral fusion. High stiffness of the cage may cause stress concentration and a stress shielding effect between the vertebral bones and the cages; then the stress shielding effect easily causes damage and leading to a higher risk of reoperation [31]. Design strategies such as contact area, open architecture (i.e., pores) to allow for multidirectional bone ingrowth, conformity, and direct loading of the graft material had been shown to accelerate bone healing and interbody fusion formation [32]. ...
... Design strategies such as contact area, open architecture (i.e., pores) to allow for multidirectional bone ingrowth, conformity, and direct loading of the graft material had been shown to accelerate bone healing and interbody fusion formation [32]. A porous structure for the spinal fusion device can effectively reduce the stiffness to obtain more comparative strength for the surrounding tissue, this results in uniform distribution of the stress and strain of the devices with the human bone and reduce the stress concentration [31]. With the help of selective laser melting (SLM) technology, Additive manufacturing technology is possible to quickly produce fully functional and complexly-shaped parts, which are often not produced by other conventional technologies. ...
Background
Degenerative disc disease is one of the most common ailments severely affecting the quality of life in elderly population. Cervical intervertebral body fusion devices are utilized to provide stability after surgical intervention for cervical pathology. In this study, we design a biomimetic porous spinal cage, and perform mechanical simulations to study its performances following American Society for Testing and Materials International (ASTM) standards before manufacturing to improve design process and decrease cost and consumption of material.
Methods
The biomimetic porous Ti-6Al-4 V interbody fusion devices were manufactured by selective laser melting (laser powder bed fusion: LPBF in ISO/ASTM 52900 standard) and subsequently post-processed by using hot isostatic pressing (HIP). Chemical composition, microstructure and the surface morphology were studied. Finite element analysis and in vitro biomechanical test were performed.
Findings
The post heat treatment can optimize its mechanical properties, as the stiffness of the cage decreases to reduce the stress shielding effect between two instrumented bodies. After the HIP treatment, the ductility and the fatigue performance are substantially improved. The use of HIP post-processing can be a necessity to improve the physical properties of customized additive manufacturing processed implants.
Interpretation
In conclusion, we have successfully designed a biomimetic porous intervertebral device. HIP post-treatment can improve the bulk material properties, optimize the device with reduced stiffness, decreased stress shielding effect, while still provide appropriate space for bone growth.
Clinical significance
The biomechanical performance of 3-D printed biomimetic porous intervertebral device can be optimized. The ductility and the fatigue performance were substantially improved, the simultaneously decreased stiffness reduces the stress shielding effect between two instrumented bodies; while the biomimetic porous structures provide appropriate space for bone growth, which is important in the patients with osteoporosis.
... 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 Pan et al., 2021). ...
Disc degenerative problems affect the aging population, globally, and interbody fusion is a crucial surgical treatment. The interbody cage is the critical implant in interbody fusion surgery; however, its subsidence risk becomes a remarkable clinical complication. Cage subsidence is caused due to a mismatch of material properties between the bone and implant, specifically, the higher elastic modulus of the cage relative to that of the spinal segments, inducing subsidence. Our recent observation has demonstrated that endplate volumetric bone mineral density (EP-vBMD) measured through the greatest cortex-occupied 1.25-mm height region of interest, using automatic phantomless quantitative computed tomography scanning, could be an independent cage subsidence predictor and a tool for cage selection instruction. Porous design on the metallic cage is a trend in interbody fusion devices as it provides a solution to the subsidence problem. Moreover, the superior osseointegration effect of the metallic cage, like the titanium alloy cage, is retained. Patient-specific customization of porous metallic cages based on the greatest subsidence-related EP-vBMD may be a good modification for the cage design as it can achieve biomechanical matching with the contacting bone tissue. We proposed a novel perspective on porous metallic cages by customizing the elastic modulus of porous metallic cages by modifying its porosity according to endplate elastic modulus calculated from EP-vBMD. A three-grade porosity customization strategy was introduced, and direct porosity-modulus customization was also available depending on the patient’s or doctor’s discretion.
... 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. ...
Bone replacement using porous and solid metallic implants, such as Ti-alloy implants, is regarded as one of the most practical therapeutic approaches in bone tissue engineering. The bone is a complex tissue with various mechanical properties based on the site of action. Patient-specific Ti-6Al-4V constructs may address the key needs in bone treatment for having customized implants that mimic the complex structure of the natural tissue and diminish the risk of implant failure. This review focuses on the most promising methods of fabricating such patient-specific Ti-6Al-4V implants using additive manufacturing (AM) with a specific emphasis on the popular subcategory, which is powder bed fusion (PBF). Characteristics of the ideal implant to promote optimized tissue-implant interactions, as well as physical, mechanical/chemical treatments and modifications will be discussed. Accordingly, such investigations will be classified into 3B-based approaches (Biofunctionality, Bioactivity, and Biostability), which mainly govern native body response and ultimately the success in implantation.
... Metal 3D printer was strongly recommended to print molds or dies with CCC using maraging steel powder. Therefore, investigation on the coolant temperature difference between the core and cavity inserts fabricated by metal 3D printing techniques such as vacuum diffusion bonding, selective laser sintering, selective laser melting [21], and selective electron beam melting is also an key research issue. 3D printed conformally cooled molds or dies can be employed for plastic injection molding, blow molding, metal injection molding, powder metallurgy, die casting, hot extrusion, injection-compression molding, rotational molding, thermoforming, transfer molding, and hot stamping. ...
Conformal cooling channels (CCCs) are widely employed in the plastic injection molding (IM) due to uniform cooling in the cooling stage. IM is a process that molten materials are pushed into the mold cavity. The cooling stage is an important part in the IM process since it takes most of the cycle time. According to practice experience, it is very difficult to improve the cycle time and quality of injection molded part simultaneously in the conventional straight drilled cooling system. Thus, improving warpage and cooling time simultaneously of the injection molded parts is a critical item in the IM. In this study, an effective method for reducing both warpage and cooling time of the wax patterns was proposed by changing the coolant temperature difference between the core and cavity inserts. It was found that both core insert with series connection CCCs and cavity insert with parallel connection CCCs is a good combination in the IM mold. The cooling efficiency of the core insert is increased from 42 to 54%, while the coolant temperature difference between the core and cavity inserts is 2 °C. The average deformation of the injection molded parts can be improved by 75.2%. The cooling time of the injection molded parts can be further reduced by 6%. The cooling time of the injection molded parts can be saved by about 30%, and the average deformation of the injection molded parts can be improved by about 60% compared with IM mold embedded with conventional cooling channel. The mechanism to minimize the amount of warpage of injection molded parts using coolant temperature difference between the core and cavity inserts is presented. Finally, the proposed method is also verified by practical implementation and comparison with experimental data. The experimental results found that the improvement rate of the average deformation of the molded parts is up to 74.5% and the cooling time of the molded parts can be reduced by approximately 15.7%. The variances compared with the simulation results are approximately 0.7% and 9.7%, respectively.
... And the porous fusion cages obtained by topology optimization method could reduce the risk of poor fusion caused by the subsidence of the cage and stress shielding, which further promoted the fusion between the cage and the adjacent end-plates. This finding is consistent with previous findings in the optimization design of lumbar fusion cages (Pan et al. 2021). In addition, the maximum von Mises stresses of the D-unit cell cage were bigger under loadings of flexion, extension and left bending than the O-unit cell cage, while the former was smaller under loading of right bending, left rotation and right rotation than the latter. ...
The porous interbody fusion cage could provide space and stable mechanical conditions for postoperative intervertebral bone ingrowth. It is considered to be an important implant in anterior cervical discectomy and internal fixation. In this study, two types of unit cells were designed using topology optimization method and introduced to the interbody fusion cage to improve the biomechanical performances of the cage. Topology optimization under two typically loading conditions was first conducted to obtain two unit cells (O-unit cell and D-unit cell) with the same volume fraction. Porous structures were developed by stacking the obtained unit cells in space, respectively. Then, porous interbody fusion cages were obtained by the Boolean intersection between the global structural layout and the porous structures. Finite element models of cervical spine were created that C5-C6 segment was fused by the designed porous cages. The range of motion (ROM) of the cervical spine, the maximum stress on the cage and the bone graft, and the stress and displacement distributions of the cage were analyzed. The results showed the ROMs of C5-C6 segment in D-unit cell and O-unit cell models were range from 0.14° to 0.25° under different loading conditions; the cage composed of the D-unit cells had a more uniform stress distribution, smaller displacement on cage, a more reasonable internal stress transfer mode (transmission along struts of the unit cell), and higher stress on the internal bone graft (0.617 MPa). In conclusion, the optimized porous cage is a promising candidate for fusion surgery, which would avoid the cage subsidence, and promote the fusion of adjacent endplates.
... The process used an optical fiber laser to melt the Ti6-Al-4V powder (EOS, Krailling, Germany), which was dedicated to the EOSINT M system, and then melted and molded the scaffolds layer by layer according to the design. The prepared samples were subjected to a stress relief annealing process with the parameters shown in Table 3. Table 3. Optimal stress relief annealing parameters [26]. ...
... 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. ...
The tibia of New Zealand White rabbits was used as a model of critical bone defects to investigate a new design of composite scaffold for bone defects composed of dual materials. The all-in-one design of a titanium alloy (Ti-6Al-4V) scaffold comprised the structure of a bone plate and gradient porosity cage. Hydroxyapatite (HAp), a biodegradable material, was encapsulated in the center of the scaffold. The gradient pore structure was designed with 70%-65%-60%-55%-50% porosity, since the stresses could be distributed more uniformly when the all-in-one scaffold was placed on the bone contact surface. By covering the center of the scaffold with a low strength of HAp to contact the relatively low strength of bone marrow tissues, the excessive stiffness of the Ti-6Al-4V can be effectively reduced and further diminish the incidence of the stress shielding effect. The simulation results show that the optimized composite scaffold for the 3D model of tibia had a maximum stress value of 27.862 MPa and a maximum strain of 0.065%. The scaffold prepared by selective laser melting was annealed and found that the Young’s coefficient increased from 126.44 GPa to 131.46 GPa, the hardness increased from 3.9 GPa to 4.12 GPa, and the strain decreased from 2.27% to 1.13%. The result demonstrates that the removal of residual stress can lead to a more stable structural strength, which can be used as a reference for the design of future clinical tibial defect repair scaffolds.
In biomedical applications, various additive manufacturing (AM) techniques such as fused deposition modeling (FDM), inkjet, stereolithography (STL), direct powder extrusion (DPE), and selective laser sintering (SLS), as well as other digitally controlled 3D printing (3DP) techniques, are used. Advances in AM methods have led to the development of tissues, microdevices, artificial organs, personalized prostheses and orthoses, dental and various bone implants, biopharmaceutical applications and drug delivery system (DDS), and patient-specific surgical models, etc. that require multiscale structures, materials and functions. It enables the three-dimensional (3D) design and manufacturing of biomedical products with complex geometries. Additionally, it enables the modeling and 3DP using the biomimetic approach for applications that require lightweight and durable structures as well as biocompatibility. The purpose of this study is to review macro-to-nano multiscale AM technologies, design and modeling status, materials, and applications used for biomedical applications. Additionally, recommendations are given on what needs to be done to overcome the current limitations and challenges of micro/-nano printing in current AM technologies.
Mechanical performance is crucial for biomedical applications of scaffolds. In this study, the stress distribution of six lattice-inspired structures was investigated using finite element simulations, and scaffolds with pre-designed structures were prepared using selective laser sintering (SLS) technology. The results showed that scaffolds with face-centered cubic (FCC) structures exhibited the highest compressive strength. Moreover, scaffolds composed of polylactic acid/anhydrous calcium hydrogen phosphate (PLA/DCPA) showed good mechanical properties and bioactivity. An in vitro study showed that these scaffolds promoted cell proliferation significantly and showed excellent osteogenic performance. Composite scaffolds with FCC structures are promising for bone tissue engineering.
