Selective Voronoi tessellation as a method to design anisotropic and biomimetic implants

Osseointegration of functionally graded Ti6Al4V porous implants: Histology of the pore network

The additive manufacturing of titanium into porous geometries offers a means to generate low-stiffness endosseous implants with a greater surface area available for osseointegration. In this work, selective laser melting was used to produce gyroid-based scaffolds with a uniform pore size of 300 μm or functionally graded pore size from 600 μm to 300 μm. Initial in vitro assessment with Saos-2 cells showed favourable cell proliferation at pore sizes of 300 and 600 μm. Following implantation into rabbit tibiae, early histological observations at four weeks indicated some residual inflammation alongside neovessel infiltration into the scaffold interior and some early apposition of mineralized bone tissue. At twelve weeks, both scaffolds were filled with a mixture of adipocyte-rich marrow, micro-capillaries, and mineralized bone tissue. X-ray microcomputed tomography showed a higher bone volume fraction (BV/TV) and percentage of bone-implant contact (BIC) in the implants with 300 μm pores than in the functionally graded specimens. In functionally graded specimens, localized BV/TV measurement was observed to be higher in the innermost region containing smaller pores (estimated at 300-400 μm) than in larger pores at the implant exterior. The unit cell topology of the porous implant was also observed to guide the direction of bone ingrowth by conducting along the implant struts. These results suggest that in vivo experimentation is necessary alongside parametric optimization of functionally graded porous implants to predict short-term and long-term bone apposition.

Optimal mechanical properties of Hydroxyapatite gradient Voronoi porous scaffolds for bone applications - A numerical study

Irregular Voronoi-based lattice (IVL) structures in tissue engineering (TE) have significant potential for bone regeneration. These scaffolds can mimic natural human bone interconnectivity by gradually altering strut thickness (ST) and seed point space (PS), which affects both mechanical and biological characteristics. This paper investigates the impact of design parameters, ST and PS, on Hydroxyapatite (HA) ILV structures' mechanical properties (elastic modulus (E) and maximum compressive strength (MCS)) and geometrical characteristics (pore number, size, and distribution, surface area (SA), and surface area-to-volume ratio (SA/VR)). Four types of IVL scaffolds were designed; PC-TC (Constant PS-Constant ST), PC-TG (Constant PS-Gradient ST), PG-TC (Gradient PS-Constant ST), and PG-TG (Gradient PS-Gradient ST). The study, conducted through linear static structural finite element analysis (FEA) with maximum stress criteria, underscores the profound impact of irregularity and morphology on mechanical performance and geometrical features. Regarding SA and SA/VR, a comparison between PC-TC with other proposed scaffolds showed a minor improvement for PC-TG, while higher significant improvements were found for both PG-TG and PG-TC. In terms of pores distribution and number, no noticeable improvement was observed for the PC-TG scaffold compared to PC-TC. In contrast, PG-TC and PG-TG lattices demonstrated a variety of pore distributions and approximately doubled pore numbers. Studying mechanical properties, considering E and MCS, showcases substantial gains for PG-TC. It, however, revealed that for the rest of the scaffolds, no enhancement was observed regarding E. Based on these results, gradient PS proved to be more effective than gradient ST in enhancing mechanical performance and geometrical properties. Due to these improvements, this study holds promise for expediting bone regeneration and reducing postoperative complications in bone replacement applications.

Reliable Methods for Classification, Characterization, and Design of Cellular Structures for Patient-Specific Implants

In our research, our goal was to develop a characterization method that can be universally applied to periodic cell structures. Our work involved the accurate tuning of the stiffness properties of cellular structure components that can significantly reduce the number of revision surgeries. Up to date porous, cellular structures provide the best possible osseointegration, while stress shielding and micromovements at the bone-implant interface can be reduced by implants with elastic properties equivalent to bone tissue. Furthermore, it is possible to store a drug inside implants with a cellular structure, for which we have also prepared a viable model. In the literature, there is currently no established uniform stiffness sizing procedure for periodic cellular structures but also no uniform designation to identify the structures. A uniform marking system for cellular structures was proposed. We developed a multi-step exact stiffness design and validation methodology. The method consists of a combination of FE (Finite Element) simulations and mechanical compression tests with fine strain measurement, which are finally used to accurately set the stiffness of components. We succeeded in reducing the stiffness of test specimens designed by us to a level equivalent to that of bone (7-30 GPa), and all of this was also validated with FE simulation.

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.

Flexural properties of porcupine quill-inspired sandwich panels

This paper presents the bending behaviour of the porcupine quill and bioinspired Voronoi sandwich panels, aiming to explore the effect of geometrical design on the bending performance of the inspired structures. Through the x-ray micro-computed tomography, the internal morphology of the quill is explored. The longitudinal cross-section of the porcupine quill revealed a functionally graded design in the foam structure. Based on this observation, Voronoi sandwich panels are designed by incorporating the Voronoi seed distribution strategy and gradient transition design configurations. Porcupine-inspired sandwich panels with various core designs are fabricated via material jetting technique and tested under three-point bending condition. Results show that the sample failed at the bottom face panels for uniform sandwich panels, whereas graded samples failed in the core panel. The bending behaviour developed via simulation software shows a good agreement with the experimental results. The parametric study provides insights into structural designs for engineering applications, particularly in the aerospace and automobile industries.

A morphological, topological and mechanical investigation of gyroid, spinodoid and dual-lattice algorithms as structural models of trabecular bone

In this study, we evaluate the performance of three algorithms as computational models of trabecular bone architecture, through systematic evaluation of morphometric, topological, and mechanical properties. Here, we consider the widely-used gyroid lattice structure, the recently-developed spinodoid structure and a structure similar to Voronoi lattices introduced here as the dual-lattice. While all computational models were calibrated to recreate the trabecular tissue volume (e.g. BV/TV), it was found that both the gyroid- and spinodoid-based structures showed substantial differences in many other morphometric and topological parameters and, in turn, showed lower effective mechanical properties compared to trabecular bone. The newly-developed dual-lattice structures better captured both morphometric parameters and mechanical properties, despite certain differences being evident their topological configuration compared to trabecular bone. Still, these computational algorithms provide useful platforms to investigate trabecular bone mechanics and for designing biomimetic structures, which could be produced through additive manufacturing for applications that include bone substitutes, scaffolds and porous implants. Furthermore, the software for the creation of the structures has been added to the open source toolbox GIBBON and is therefore freely available to the community.

Definition, measurement, and function of pore structure dimensions of bioengineered porous bone tissue materials based on additive manufacturing: A review

Bioengineered porous bone tissue materials based on additive manufacturing technology have gradually become a research hotspot in bone tissue-related bioengineering. Research on structural design, preparation and processing processes, and performance optimization has been carried out for this material, and further industrial translation and clinical applications have been implemented. However, based on previous studies, there is controversy in the academic community about characterizing the pore structure dimensions of porous materials, with problems in the definition logic and measurement method for specific parameters. In addition, there are significant differences in the specific morphological and functional concepts for the pore structure due to differences in defining the dimensional characterization parameters of the pore structure, leading to some conflicts in perceptions and discussions among researchers. To further clarify the definitions, measurements, and dimensional parameters of porous structures in bioengineered bone materials, this literature review analyzes different dimensional characterization parameters of pore structures of porous materials to provide a theoretical basis for unified definitions and the standardized use of parameters.

Additive Manufacturing and Mechanical Characterization of PLA-Based Skull Surrogates

Several occupational and leisure activities involve a high risk of head impacts, resulting in varying degrees of injuries with chronic consequences that adversely affect life quality. The design and manufacturing of effective head protections rely on proper head simulators to mimic the behavior to impact loading. 3D-printed human skulls are reported herein to address the need for reproducible, cost-effective, anatomically-correct surrogates. To demonstrate the viability of the investigated approach, surrogate bone sections and skulls were mechanically tested under quasi-static loading conditions. The 3D-printed bone sections were flexural tested, elucidating the effect of printing orientations and the sample geometry on their mechanical behavior. The printing orientation minimally influenced the results due to the high infill percentage, while the sample geometry played a major role in the flexural properties because of the change in the section properties. The surrogate skulls were submitted to lateral compression and frontal penetration tests to assess the impact of the sectioning strategy on the overall mechanical performance. Results indicate that PLA-based surrogates reasonably reproduce the behavior of skulls. In addition, the sectioning strategy elucidated the effect of skull sutures, while streamlining the additive manufacturing process. The outcomes lay the foundation for future research seeking a complete surrogate head.

An update of interbody cages for spine fusion surgeries: from shape design to materials

INTRODUCTION

Discectomy and interbody fusion are widely used in the treatment of intervertebral disc-related diseases. Among them, the interbody cage plays a significant role. However, the complications related to the interbody cage, such as nonunion or pseudoarthrosis, subsidence, loosening, and prolapse of the cage, cannot be ignored. By changing the design and material of the interbody fusion cage, a better fusion effect can be obtained, the incidence of appeal complications can be reduced, and the quality of life of patients after interbody fusion can be improved.

AREAS COVERED

This study reviewed the research progress of cage design and material and discussed the methods of cage design and material to promote intervertebral fusion.

EXPERT OPINION

Current treatment of cervical and lumbar degenerative disease requires interbody fusion to maintain decompression and to promote fusion and reduce the incidence of fusion failure through improvements in implant material, design, internal structure, and function. However, interbody fusion is not an optimal solution for treating vertebral instability.Abbreviations: ACDF, Anterior cervical discectomy and fusion; ALIF, anterior lumbar interbody fusion; Axi-aLIF, axial lumbar interbody fusion; BAK fusion cage, Bagby and Kuslich fusion cage; CADR, cervical artificial disc replacement; DBM, decalcified bone matrix; HA, hydroxyapatite; LLIF/XLIF, lateral or extreme lateral interbody fusion; MIS-TLIF, minimally invasive transforaminal lumbar interbody fusion; OLIF/ATP, oblique lumbar interbody fusion/anterior to psoas; PEEK, Poly-ether-ether-ketone; PLIF, posterior lumbar interbody fusion; ROI-C, Zero-profile Anchored Spacer; ROM, range of motion; SLM, selective melting forming; TLIF, transforaminal lumbar interbody fusion or.

Biological Response to Bioinspired Microporous 3D-Printed Scaffolds for Bone Tissue Engineering

The scaffold is a key element in the field of tissue engineering, especially when large defects or substitutions of pathological tissues or organs need to be clinically addressed. The expected outcome is strongly dependent on the cell–scaffold interaction and the integration with the surrounding biological tissue. Indeed, mimicking the natural extracellular matrix (ECM) of the tissue to be healed represents a further optimization that can limit a possible morphological mismatch between the scaffold and the tissue itself. For this aim, and referring to bone tissue engineering, polylactic acid (PLA) scaffolds were 3D printed with a microstructure inspired by the trabecular architecture and biologically evaluated by means of human osteosarcoma SAOS-2 cells. The cells were seeded on two types of scaffolds differing for the designed pore size (i.e., 400 and 600 µm), showing the same growth exponential trend found in the control and no significant alterations in the actin distribution. The microporous structure of the two tested samples enhanced the protein adsorption capability and mRNA expression of markers related to protein synthesis, proliferation, and osteoblast differentiation. Our findings demonstrate that 3D-printed scaffolds support the adhesion, growth, and differentiation of osteoblast-like cells and the microporous architecture, mimicking the natural bone hierarchical structure, and favoring greater bioactivity. These bioinspired scaffolds represent an interesting new tool for bone tissue engineering and regenerative medicine applications.

Application of Computational Method in Designing a Unit Cell of Bone Tissue Engineering Scaffold: A Review

The design of a scaffold of bone tissue engineering plays an important role in ensuring cell viability and cell growth. Therefore, it is a necessity to produce an ideal scaffold by predicting and simulating the properties of the scaffold. Hence, the computational method should be adopted since it has a huge potential to be used in the implementation of the scaffold of bone tissue engineering. To explore the field of computational method in the area of bone tissue engineering, this paper provides an overview of the usage of a computational method in designing a unit cell of bone tissue engineering scaffold. In order to design a unit cell of the scaffold, we discussed two categories of unit cells that can be used to design a feasible scaffold, which are non-parametric and parametric designs. These designs were later described and being categorised into multiple types according to their characteristics, such as circular structures and Triply Periodic Minimal Surface (TPMS) structures. The advantages and disadvantages of these designs were discussed. Moreover, this paper also represents some software that was used in simulating and designing the bone tissue scaffold. The challenges and future work recommendations had also been included in this paper.

Current interpretations on the in vivo response of bone to additively manufactured metallic porous scaffolds: A review

Recent advances in the field of metallic additive manufacturing have expanded production capabilities for bone implants to include porous lattice structures. While traditional models of de novo bone formation can be applied to fully dense implant materials, their applicability to the interior of porous materials has not been well-characterized. Unlike other reviews that focus on materials and mechanical properties of lattice structures, this review compiles biological performance from in vivo studies in pre-clinical models only. First, we introduce the most common lattice geometry designs employed in vivo and discuss some of their fabrication advantages and limitations. Then lattice geometry is correlated to quantitative (histomorphometric) and qualitative (histological) assessments of osseointegration. We group studies according to two common implant variables: pore size and percent porosity, and explore the extent of osseointegration using common measures, including bone-implant contact (BIC), bone area (BA), bone volume/total volume (BV/TV) and biomechanical stability, for various animal models and implantation times. Based on this, trends related to in vivo bone formation on the interior of lattice structures are presented. Common challenges with lattice structures are highlighted, including nonuniformity of bone growth through the entirety of the lattice structure due to occlusion effects and avascularity. This review paper identifies a lack of systematic in vivo studies on porous AM implants to target optimum geometric design, including pore shape, size, and percent porosity in controlled animal models and critical-sized defects. Further work focusing on surface modification strategies and systematic geometric studies to homogenize in vivo bone growth through the scaffold interior are recommended to increase implant stability in the early stages of osseointegration.