A comparative investigation on the mechanical properties and cytotoxicity of Cubic, Octet, and TPMS gyroid structures fabricated by selective laser melting of stainless steel 316L

Static and dynamic optimisation of fluid-filled responsive orthotic insoles

This study was focused on developing an optimisation-based methodology to create customised solid-liquid composite (SLC) orthotic insoles. The goal was to reduce peak plantar pressures through gait through a dynamic numerical optimisation. A gait simulation was developed through a series of numerical models with increasing complexity. These models were validated against experimental analyses. The insole was designed based on numerical optimisation techniques that regionally tailored the insole with the aim to reduce temporal peak pressures. A prototype of the optimised insole was created using additive manufacturing and tested experimentally. The numerical gait simulation showed good correlation with experimental results. The largest differences are attributed to the bone geometry adopted from a previous study from a subject of different age, gender and size demographics. The optimisation process showed significant reductions in peak plantar pressures in the static peak pressures by approximately 8% and in the summation of dynamic peak pressures by 50%. Experimental validation confirmed the numerical predictions, highlighting the effectiveness of the optimised insole. The findings suggest that the optimised insoles can improve plantar pressure distributions and reduce peak pressures, making them a viable alternative to traditional orthotic insoles. Future research should focus on more accurate geometry for the numerical models and clinical trials.

The mechanical response of polymeric gyroid structures in an optimised orthotic insole

This study aims to explore the mechanical behaviour of polymeric gyroid structures under compression within the context of orthotic insoles, focussing on custom optimisation for lower peak plantar pressures. This research evaluates the compressive response of gyroid structures using a combination of experimental testing and numerical modelling. Stereolithography was used to manufacture gyroid samples for experimental tests, and explicit finite element analysis was used to model the gyroid's response numerically. Hyperfoam, first-order polynomial, and second-order polynomial hyperelastic constitutive models were considered to homogenise the mechanical response of the structure. The homogenised properties of the structure were then implemented in an optimisation algorithm to obtain the optimal gyroid structure for a given subject by minimising the standard distribution of plantar pressures. Findings indicate that the compressive response polymeric gyroid structures can be represented with a homogeneous material. The hyperfoam model was chosen due to its accuracy and interpolation quality. The optimisation process successfully identified configurations that maximise the mechanical advantages of gyroid lattices, demonstrating significant improvements in plantar pressure distributions. The optimised insole showed a 30% reduction in the standard deviation of the plantar pressure and a 10% reduction in the peak stress. The optimisation method reduced peak pressures by 12.2 kPa compared to a traditional medium-density Poron orthotic insole, and 94.3 kPa compared barefoot conditions. The mechanical response of gyroid structures has successfully been modelled, analysed and homogenised. The study concludes that custom gyroid-based orthotic insoles offer a promising solution for personalised foot care.

Definition, Fabrication, and Compression Testing of Sandwich Structures with Novel TPMS-Based Cores

Triply periodic minimal surfaces (TPMSs) constitute a type of metamaterial, deriving their unique characteristics from their microstructure topology. They exhibit wide parameterization possibilities, but their behavior is hard to predict. This study focuses on using an implicit modeling method that can effectively generate novel thin-walled metamaterials, proposing eight shell-based TPMS topologies and one stochastic structure, along with the gyroid acting as a reference. After insights into the printability and design parameters of the proposed samples are presented, a cell homogeneity analysis is conducted, indicating the level of anisotropy of each cellular structure. For each of the designed metamaterials, multiple samples were printed using a stereolithography (SLA) method, using a constant 0.3 relative density and 50 µm resolution. To provide an understanding of their behavior, compression tests of sandwich-type specimens were performed and specific deformation modes were identified. Furthermore, the study estimates the general mechanical behavior of the novel TPMS cores at different relative densities using an open cell mathematical model. Alterations of the uniform topologies are then suggested and the way these modifications affect the compressive response are presented. Thus, this paper demonstrates that an implicit modeling method could easily generate novel thin-walled TPMSs and stochastic structures, which led to identifying an artificially designed structure with superior properties to already mature topologies, such as the gyroid.

Immunotoxicity of stainless-steel nanoparticles obtained after 3D printing

This study aims to investigate the in vitro effects of nanoparticles (NPs) produced during the selective laser melting (SLM) of 316 L stainless steel metal powder on the immune response in a human blood model. Experimental data did not reveal effect on viability of 316 L NPs for the tested doses. Functional immune assays showed a significant immunosuppressive effect of NPs. There was moderate stimulation (117%) of monocyte phagocytic activity without significant changes in phagocytic activity and respiratory burst of granulocytes. A significant dose-dependent increase in the levels of the pro-inflammatory cytokine TNF-a was found in blood cultures treated with NPs. On the contrary, IL-8 chemokine levels were significantly suppressed. The levels of the pro-inflammatory cytokine IL-6 were reduced by only a single concentration of NPs. These new findings can minimise potential health risks and indicate the need for more research in this area.

3D printing metal implants in orthopedic surgery: Methods, applications and future prospects

Background

Currently, metal implants are widely used in orthopedic surgeries, including fracture fixation, spinal fusion, joint replacement, and bone tumor defect repair. However, conventional implants are difficult to be customized according to the recipient's skeletal anatomy and defect characteristics, leading to difficulties in meeting the individual needs of patients. Additive manufacturing (AM) or three-dimensional (3D) printing technology, an advanced digital fabrication technique capable of producing components with complex and precise structures, offers opportunities for personalization.

Methods

We systematically reviewed the literature on 3D printing orthopedic metal implants over the past 10 years. Relevant animal, cellular, and clinical studies were searched in PubMed and Web of Science. In this paper, we introduce the 3D printing method and the characteristics of biometals and summarize the properties of 3D printing metal implants and their clinical applications in orthopedic surgery. On this basis, we discuss potential possibilities for further generalization and improvement.

Results

3D printing technology has facilitated the use of metal implants in different orthopedic procedures. By combining medical images from techniques such as CT and MRI, 3D printing technology allows the precise fabrication of complex metal implants based on the anatomy of the injured tissue. Such patient-specific implants not only reduce excessive mechanical strength and eliminate stress-shielding effects, but also improve biocompatibility and functionality, increase cell and nutrient permeability, and promote angiogenesis and bone growth. In addition, 3D printing technology has the advantages of low cost, fast manufacturing cycles, and high reproducibility, which can shorten patients' surgery and hospitalization time. Many clinical trials have been conducted using customized implants. However, the use of modeling software, the operation of printing equipment, the high demand for metal implant materials, and the lack of guidance from relevant laws and regulations have limited its further application.

Conclusions

There are advantages of 3D printing metal implants in orthopedic applications such as personalization, promotion of osseointegration, short production cycle, and high material utilization. With the continuous learning of modeling software by surgeons, the improvement of 3D printing technology, the development of metal materials that better meet clinical needs, and the improvement of laws and regulations, 3D printing metal implants can be applied to more orthopedic surgeries.

The translational potential of this paper

Precision, intelligence, and personalization are the future direction of orthopedics. It is reasonable to believe that 3D printing technology will be more deeply integrated with artificial intelligence, 4D printing, and big data to play a greater role in orthopedic metal implants and eventually become an important part of the digital economy. We aim to summarize the latest developments in 3D printing metal implants for engineers and surgeons to design implants that more closely mimic the morphology and function of native bone.

Morphologies, mechanical and in vitro behaviors of DLP-based 3D printed HA scaffolds with different structural configurations

In the field of bone engineering, porous ceramic scaffolds are in great demand for repairing bone defects. In this study, hydroxyapatite (HA) ceramic scaffolds with three different structural configurations, including the body-centered cubic (BCC), the face-centered cubic (FCC), and the triply periodic minimal surface (TPMS), were fabricated through digital light processing (DLP) based 3D printing technologies. The effects of the structural configurations on the morphologies and mechanical properties of the DLP-based 3D printed HA scaffolds were characterized. Furthermore, in vitro evaluations, including in vitro cytocompatibility, bone alkaline phosphatase (ALP) activity assay, and protein expression, were conducted to assess HA scaffold behavior. Finally, we evaluated the effects of structural configurations from these aspects and selected the most suitable structure of HA scaffold for bone repair.

Novel 3D printable PEEK-HA-Mg2SiO4 composite material for spine implants: biocompatibility and imaging compatibility assessments

PURPOSE

To develop a novel 3D printable polyether ether ketone (PEEK)-hydroxyapatite (HA)-magnesium orthosilicate (Mg2SiO4) composite material with enhanced properties for potential use in tumour, osteoporosis and other spinal conditions. We aim to evaluate biocompatibility and imaging compatibility of the material.

METHODS

Materials were prepared in three different compositions, namely composite A: 75 weight % PEEK, 20 weight % HA, 5 weight % Mg2SiO4; composite B: 70 weight% PEEK, 25 weight % HA, 5 weight % Mg2SiO4; and composite C: 65 weight % PEEK, 30 weight % HA, 5 weight % Mg2SiO4. The materials were processed to obtain 3D printable filament. Biomechanical properties were analysed as per ASTM standards and biocompatibility of the novel material was evaluated using indirect and direct cell cytotoxicity tests. Cell viability of the novel material was compared to PEEK and PEEK-HA materials. The novel material was used to 3D print a standard spine cage. Furthermore, the CT and MR imaging compatibility of the novel material cage vs PEEK and PEEK-HA cages were evaluated using a phantom setup.

RESULTS

Composite A resulted in optimal material processing to obtain a 3D printable filament, while composite B and C resulted in non-optimal processing. Composite A enhanced cell viability up to ~ 20% compared to PEEK and PEEK-HA materials. Composite A cage generated minimal/no artefacts on CT and MR imaging and the images were comparable to that of PEEK and PEEK-HA cages.

CONCLUSION

Composite A demonstrated superior bioactivity vs PEEK and PEEK-HA materials and comparable imaging compatibility vs PEEK and PEEK-HA. Therefore, our material displays an excellent potential to manufacture spine implants with enhanced mechanical and bioactive property.

Design and 3D printing of novel titanium spine rods with lower flexural modulus and stiffness profile with optimised imaging compatibility

PURPOSE

To manufacture and test 3D printed novel design titanium spine rods with lower flexural modulus and stiffness compared to standard solid titanium rods for use in metastatic spine tumour surgery (MSTS) and osteoporosis.

METHODS

Novel design titanium spine rods were designed and 3D printed. Three-point bending test was performed to assess mechanical performance of rods, while a French bender was used to assess intraoperative rod contourability. Furthermore, 3D printed spine rods were tested for CT & MR imaging compatibility using phantom setup.

RESULTS

Different spine rod designs generated includes shell, voronoi, gyroid, diamond, weaire-phelan, kelvin, and star. Tests showed 3D printed rods had lower flexural modulus with reduction ranging from 2 to 25% versus standard rod. Shell rods exhibited highest reduction in flexural modulus of 25% (~ 77.4 GPa) and star rod exhibited lowest reduction in flexural modulus of 2% (100.8GPa). 3D printed rod showed reduction in stiffness ranging from 40 to 59%. Shell rod displayed highest reduction in stiffness of 59% (179.9 N/mm) and gyroid had least reduction in stiffness of 40% (~ 259.2 N/mm). Rod bending test showed that except gyroid, other rod designs demonstrated lesser bending difficulty versus standard rod. All 3D printed rods demonstrated improved CT/MR imaging compatibility with reduced artefacts versus standard rod.

CONCLUSION

By utilising novel design approach, we successfully generated a spine rod design portfolio with lower flexural modulus/stiffness profile and better CT/MR imaging compatibility for potential use in MSTS/other conditions such as osteoporosis. Thus, exploration of new rod designs in surgical application could enhance treatment outcome and improve quality of life for patients.

A biomechanical evaluation on Cubic, Octet, and TPMS gyroid Ti6Al4V lattice structures fabricated by selective laser melting and the effects of their debris on human osteoblast-like cells

Lattice structures are widely used in orthopedic implants due to their unique features, such as high strength-to-weight ratios and adjustable biomechanical properties. Based on the type of unit cell geometry, lattice structures may be classified into two types: strut-based structures and sheet-based structures. In this study, strut-based structures (Cubic & Octet) and sheet-based structure (triply periodic minimal surface (TPMS) gyroid) were investigated. The biomechanical properties of the three different Ti6Al4V lattice structures fabricated by selective laser melting (SLM) were investigated using room temperature compression testing. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were used to check the 3D printing quality with regards to defects and quantitative compositional information of 3D printed parts. Experimental results indicated that TPMS gyroid has superior biomechanical properties when compared to Cubic and Octet. Also, TPMS gyroid was found to be less affected by the variations in relative density. The biocompatibility of Ti6Al4V lattice structures was validated through the cytotoxicity test with human osteoblast-like SAOS2 cells. The debris generated during the degradation process in the form of particles and ions is among the primary causes of implant failure over time. In this study, Ti6Al4V particles with spherical and irregular shapes having average particle sizes of 36.5 μm and 28.8 μm, respectively, were used to mimic the actual Ti6Al4V particles to understand their harmful effects better. Also, the effects and amount of Ti6Al4V ions released after immersion within the cell culture media were investigated using the indirect cytotoxicity test and ion release test.