High-strength, porous additively manufactured implants with optimized mechanical osseointegration

Bio-functional niobium-based metallic biomaterials: Exploring their physicomechanical properties, biological significance, and implant applications

The significance of biomedical applications of bio-functional niobium (Nb)-based metallic biomaterials is underscored by their potential utilization in implant application. Nb-based metallic materials present reliable physicomechanical and biological properties, thus represent materials highly suitable for implant application. This review provides an overview on the advances of pure niobium and Nb-based metallic materials as implant materials over the past 20 years, and highlights the advantages of Nb-based metallic biomaterials for implant application in terms of their physicomechanical properties, corrosion resistance in biological media, magnetic resonance imaging (MRI) compatibility, cell compatibility, blood compatibility, osteogenesis, and bioactivity. An introduction is provided for the production and processing techniques for Nb-based metallic biomaterials, including traditional melting processes like vacuum arc remelting, additive manufacturing like selective laser melting (SLM), electron beam melting (EBM), spark plasma sintering (SPS), and severe plastic deformation like equal channel angular pressing (ECAP), multi-axial forging (MAF), high pressure torsion (HPT), as well as their physicomechanical properties and implant application. Also suggested are the critical issues, challenges, and prospects in the further development of Nb-based metallic biomaterials for implant applications. STATEMENT OF SIGNIFICANCE: Nb-based biomaterials have gained significant interest for bioimplantable scaffolds because of their appropriate mechanical characteristics and biocompatibility. No prior work has been published specifically reviewing bio-functional Nb-based biomaterials for exploring their physicomechanical properties, biological significance, and implant applications. This review provides an overview on the advances of niobium and Nb-based materials as implant materials over the past 20 years, and highlights the advantages of Nb-based biomaterials for implant application. An introduction is provided for the production and processing techniques for Nb-based biomaterials, as well as their physicomechanical properties and implant application. Also suggested are the critical issues, challenges, and prospects in the further development of Nb-based biomaterials for implant applications.

Optimising β-Ti21S Alloy Lattice Structures for Enhanced Femoral Implants: A Study on Mechanical and Biological Performance

The metastable β-Ti21S alloy exhibits a lower elastic modulus than Ti-6Al-4V ELI while maintaining high mechanical strength and ductility. To address stress shielding, this study explores the integration of lattice structures within prosthetics, which is made possible through additive manufacturing. Continuous adhesion between the implant and bone is essential; therefore, auxetic bow-tie structures with a negative Poisson's ratio are proposed for regions under tensile stress, while Triply Periodic Minimal Surface (TPMS) structures with a positive Poisson's ratio are recommended for areas under compressive stress. This research examines the manufacturability and quasi-static mechanical behaviour of two auxetic bow-tie (AUX 2.5 and AUX 3.5) and two TPMS structures (TPMS 2.5 and TPMS 1.5) in β-Ti21S alloy produced via laser powder bed fusion. Micro-CT reveals printability issues in TPMS 1.5, affecting pore size and reducing fatigue resistance compared to TPMS 2.5. AUX 3.5's low stiffness matches cancellous bone but shows insufficient yield strength and fatigue resistance for femoral implants. Biological tests confirm non-toxicity and enhanced cell activity in β-Ti21S structures. The study concludes that the β-Ti21S alloy, especially with TPMS 2.5 structures, demonstrates promising mechanical and biological properties for femoral implants. However, challenges like poor printability in TPMS 1.5 are acknowledged and should be addressed in future research.

Evaluation of biological performance of 3D printed trabecular porous tantalum spine fusion cage in large animal models

Background

The materials for artificial bone scaffolds have long been a focal point in biomaterials research. Tantalum, with its excellent bioactivity and tissue compatibility, has gradually become a promising alternative material. 3D printing technology shows unique advantages in designing complex structures, reducing costs, and providing personalized customization in the manufacture of porous tantalum fusion cages. Here we report the pre-clinical large animal (sheep) study on the newly developed 3D printed biomimetic trabecular porous tantalum fusion cage for assessing the long-term intervertebral fusion efficacy and safety.

Methods

Porous tantalum fusion cages were fabricated using laser powder bed fusion (LPBF) and chemical vapor deposition (CVD) methods. The fusion cages were characterized using scanning electron microscopy (SEM) and mechanical compression tests. Small-Tailed Han sheep served as the animal model, and the two types of fusion cages were implanted in the C3/4 cervical segments and followed for up to 12 months. Imaging techniques, including X-ray, CT scans, and Micro CT, were used to observe the bone integration of the fusion cages. Hard tissue sections were used to assess osteogenic effects and bone integration. The range of motion (ROM) of the motion segments was evaluated using a biomechanical testing machine. Serum biochemical indicators and pathological analysis of major organs were conducted to assess biocompatibility.

Results

X-ray imaging showed that both the 3D-printed and chemical vapor deposition porous tantalum fusion cages maintained comparable average intervertebral disc heights. Due to the presence of metal artifacts, CT and Micro CT imaging could not effectively analyze bone integration. Histomorphology data indicated that both the 3D-printed and chemical vapor deposition porous tantalum fusion cages exhibited similar levels of bone contact and integration at 3, 6, and 12 months, with bone bridging observed at 12 months. Both groups of fusion cages demonstrated consistent mechanical stability across all time points. Serum biochemistry showed no abnormalities, and no significant pathological changes were observed in the heart, liver, spleen, lungs, and kidneys.

Conclusion

This study confirms that 3D-printed and chemical vapor deposition porous tantalum fusion cages exhibit comparable, excellent osteogenic effects and long-term biocompatibility. Additionally, 3D-printed porous tantalum fusion cages offer unique advantages in achieving complex structural designs, low-cost manufacturing, and personalized customization, providing robust scientific support for future clinical applications.

The translational potential of this article

The translational potential of this paper is to use 3D printed biomimetic trabecular porous tantalum spine fusion cage with bone trabecular structure and validating its feasibility in large animal models (sheep). This study provides a basis for further research into the clinical application of the 3D printed biomimetic trabecular porous tantalum spine fusion cage.

Optimization of 3D-printed titanium interbody cage design. Part 1: in vitro biomechanical study of subsidence

BACKGROUND CONTEXT

Cage subsidence is a complication of interbody fusion associated with poor clinical outcomes. 3D-printed titanium interbody cages allow for the alteration of features such as stiffness and porosity. However, the influence of these features on subsidence and their biological effects on fusion have not been rigorously evaluated.

PURPOSE

This 2-part study sought to determine how changes in 3D-printed titanium cage parameters affect subsidence using an in vitro bone model (Part 1) and biological fusion using an in vivo sheep model (Part 2).

STUDY DESIGN

Biomechanical foam block model.

METHODS

In Part 1 of this study, 9 implant types were tested (8 per implant type). The implant types included 7 3D-printed titanium interbody cages with various surface areas, porosities, and surface topographies, along with 1 standard polyetheretherketone (PEEK) cage and 1 solid titanium cage. Subsidence testing was performed in a standardized foam block model using 2 different densities of foam. Digital imaging correlation was used to determine the relative vertical displacement of the interbody cage-foam block construct.

RESULTS

Subsidence decreased as the surface contact area with the bone model increased (all p≤.01). Increased porous surface topography increased subsidence, while a nonporous surface significantly decreased subsidence (all p<.001). Subsidence did not differ based on changes in implant porosity (all p≥.35) or material property/modulus (all p≥.19). Subsidence was significantly decreased with the higher density foam (p<.001). The stiffness of the implant was affected by porosity (all p<.02) and smooth surface topography (p=.01) but not by lumen size (all p≥.15). Stiffness did not differ between porous titanium and PEEK implants (p=.96), which were both less stiff than solid titanium implants (both p<.001). Surface area negatively correlated with subsidence (r=-0.786, p=.012) but was not correlated with stiffness (r=0.560, p=.12).

CONCLUSIONS

Implant surface area and surface topography greatly influenced interbody subsidence. Apparent stiffness, implant porosity, and material property did not affect subsidence in this in vitro model. Higher foam density also led to lower subsidence than low-density foam. Biological response in the in vivo setting likely also influences clinical subsidence, which is evaluated in the companion study (Part 2).

CLINICAL SIGNIFICANCE

This study provides valuable information regarding the new 3D-printed titanium technology. We showed that cage surface area and surface topography were the implant design parameters that had the greatest influence on the development of interbody subsidence. Moreover, bone mineral density was the factor that had the greatest effect on subsidence prevention. These data support patient optimization before surgery and emphasize the importance of endplate protection during surgery.

Comprehensive Overview of Interface Strategies in Implant Osseointegration

With the improvement of implant design and the expansion of application scenarios, orthopedic implants have become a common surgical option for treating fractures and end‐stage osteoarthritis. Their common goal is rapidly forming and long‐term stable osseointegration. However, this fixation effect is limited by implant surface characteristics and peri‐implant bone tissue activity. Therefore, this review summarizes the strategies of interface engineering (osteogenic peptides, growth factors, and metal ions) and treatment methods (porous nanotubes, hydrogel embedding, and other load‐release systems) through research on its biological mechanism, paving the way to achieve the adaptation of both and coordination between different strategies. With the transition of the osseointegration stage, interface engineering strategies have demonstrated varying therapeutic effects. Especially, the activity of osteoblasts runs almost through the entire process of osseointegration, and their physiological activities play a dominant role in bone formation. Furthermore, diseases impacting bone metabolism exacerbate the difficulty of achieving osseointegration. This review aims to assist future research on osseointegration engineering strategies to improve implant‐bone fixation, promote fracture healing, and enhance post‐implantation recovery.

Optimization of 3D-printed titanium interbody cage design. Part 2: An in vivo study of spinal fusion in sheep

BACKGROUND CONTEXT

3D-printed titanium cage designs can incorporate complex, porous features for bone ingrowth and a greater surface area for minimizing subsidence. In a companion study (Part 1), we determined that increased surface area leads to decreased subsidence; however, it remains unclear how increasing the cage surface area, resulting in a smaller graft aperture, influences fusion.

PURPOSE

We evaluated the effects of surface area of 3D-printed titanium cages and the use of autologous bone grafts on spinal fusion in sheep.

STUDY DESIGN

In vivo large animal study in 12 sheep.

METHODS

Interbody fusion was performed in 12 adult sheep at 24 levels (L2-3 and L4-5) using 3D-printed titanium cages with bilateral pedicle screw fixation. The cage designs varied in aperture: standard (low endplate surface area), small (medium endplate surface area), or none (high endplate surface area). These cages were packed with autologous iliac crest bone grafts (ICBG). A fourth group was implanted without bone grafts, using the no-aperture cage. Fusion was evaluated at 16 weeks via manual palpation, microcomputed tomography (microCT), histology, and histomorphometry.

RESULTS

Standard, small, and no-aperture cages packed with ICBG resulted in high fusion rates (80%, 100%, and 83%, respectively) at 16 weeks by manual palpation, and these results were not significantly different. Implantation without ICBG was associated with a significantly lower fusion rate (33%, p<.05). Histological, histomorphometry, and microCT results supported the findings obtained by manual palpation; findings from these modalities showed new bone spanning the vertebral endplates in the spines graded as fused by manual palpation.

CONCLUSIONS

Similar fusion results for standard, small, and no-aperture cage designs packed with ICBG suggest that aperture size does not influence fusion results in the sheep model. However, without ICBG grafting, fusion was significantly decreased, suggesting that graft material is necessary to predictably obtain fusion in this model. When the in vitro subsidence data (companion study, Part 1) is considered with the in vivo fusion data described here, porous 3D-printed titanium cages with maximal surface endplate contact and bone grafting perform favorably, resulting in low subsidence and high fusion rates.

CLINICAL SIGNIFICANCE

3D-printed porous titanium interbody cages are novel devices with increasing clinical use. The study results show that the aperture size of the interbody cage did not influence fusion in a large animal (sheep) model. The use of bone graft material was the most important variable affecting fusion. These data suggest that the clinical use of 3D Ti cages without graft material should be avoided.

Local drug delivery challenges and innovations in spinal neurosurgery

The development of novel therapeutics in the field of spinal neurosurgery faces a litany of translational challenges. Achieving precise drug targeting within the confined spaces associated with the spinal cord, canal and vertebra requires the development of next generation delivery systems and devices. These must be capable of overcoming inherent barriers related to drug diffusion, whilst concurrently ensuring optimal drug distribution and retention. In this review, we provide an overview of the most recent advances in the therapeutic management of diseases and disorders affecting the spine, including systems and devices capable of releasing small molecules and biopharmaceuticals that help eliminate pain and restore the mechanical function and stability of the spine. We highlight material-based approaches and minimally invasive techniques that can be employed to provide control over drug release kinetics and improve retention. We also seek to explore how the newest advancements in nanotechnology, biomaterials, additive manufacturing technologies and imaging modalities can be employed in this translational pursuit. Finally, we discuss the landscape of clinical trials and recently approved products aimed at overcoming the complexities associated with drug delivery to the spine.

Nanocrystal hydroxyapatite carrying traditional Chinese medicine for osteogenic differentiation

Developing biomaterials with high osteogenic properties is crucial for achieving rapid bone repair and regeneration. This study focuses on the application of nanocrystal hydroxyapatite (nHAp) as a drug carrier to load Fu Yuan Huo Xue Decoction (FYHXD), a traditional Chinese medicine derived from Angelica sinensis, aiming to achieve improved efficacy in treating bone diseases such as osteoporosis. Through a facile physical adsorption approach, the FTIR result emerges new characteristic absorption peaks in the range of 1200-950 cm-1, proving the successful absorption of FYHXD onto the nHAp with a loading efficiency of 39.76 %. The modified nHAp exhibits a similar shape to the bone-derived hydroxyapatite nanocrystals, and their diameter increases slightly after modification. The drug release assay implies the rapid release of FYHXD in the first 10 h, followed by a continuously slow release within 70 h. The developed nHAp effectively enhances the adhesion, spreading, and proliferation of MC3T3-E1 cells in vitro, and significantly promotes their osteogenic differentiation, as indicated by increased alkaline phosphatase activity. Overall, the biocomposites hold great promise as active ingredients for integration into bone-associated biomaterials, offering the potential to stimulate spontaneous osteogenesis without requiring exogenous osteogenic factors.

Effect of Structure on Osteogenesis of Bone Scaffold: Simulation Analysis Based on Mechanobiology and Animal Experiment Verification

Porous scaffolds, whose mechanical and biological properties are greatly affected by structure, are new treatments for bone defects. Since bone repair is related to biomechanics, analyzing the osteogenesis in scaffolds based on mechanical stimulation may become a more effective method than traditional biological experiments. A tissue regeneration algorithm based on mechanical regulation theory was implemented in this study to evaluate the osteogenesis of classical scaffolds (Gyroid, I-WP, and Diamond). In vivo experiments were used to verify and supplement the simulation results. Different approaches to describing osteogenesis were discussed. Bone formation was more obvious inside the Gyroid scaffold and outside the I-WP scaffold, while the new bone was more sufficient and evenly distributed in the Diamond scaffold. The osteogenesis pattern of the bone scaffold in the simulation analysis was consistent with the results of animal experiments, and the bone volume calculated by the tissue fraction threshold method and the elastic modulus threshold method was very similar to the in vivo experiment. The mechanical responses mediated by structure affect the osteogenesis of bone scaffolds. This study provided and confirmed a simulation analysis method based on mechanical regulation theory, which is more efficient and economical for analyzing tissue healing in bioengineering.

Islet cell spheroids produced by a thermally sensitive scaffold: a new diabetes treatment

The primary issues in treating type 1 diabetes mellitus (T1DM) through the transplantation of healthy islets or islet β-cells are graft rejection and a lack of available donors. Currently, the majority of approaches use cell encapsulation technology and transplant replacement cells that can release insulin to address transplant rejection and donor shortages. However, existing encapsulation materials merely serve as carriers for islet cell growth. A new treatment approach for T1DM could be developed by creating a smart responsive material that encourages the formation of islet cell spheroids to replicate their 3D connections in vivo and controls the release of insulin aggregates. In this study, we used microfluidics to create thermally sensitive porous scaffolds made of poly(N-isopropyl acrylamide)/graphene oxide (PNIPAM/GO). The material was carefully shrunk under near-infrared light, enriched with mouse insulinoma pancreatic β cells (β-TC-6 cells), encapsulated, and cultivated to form 3D cell spheroids. The controlled contraction of the thermally responsive porous scaffold regulated insulin release from the spheroids, demonstrated using the glucose-stimulated insulin release assay (GSIS), enzyme-linked immunosorbent assay (ELISA), and immunofluorescence assay. Eventually, implantation of the spheroids into C57BL/6 N diabetic mice enhanced the therapeutic effect, potentially offering a novel approach to the management of T1DM.

Implant Strength Contributes to the Osseointegration Strength of Porous Metallic Materials

Creating the optimal environment for effective and long term osseointegration is a heavily researched and sought-after design criteria for orthopedic implants. A validated multimaterial finite element (FE) model was developed to replicate and understand the results of an experimental in vivo push-out osseointegration model. The FE model results closely predicted global force (at 0.5 mm) and stiffness for the 50-90% porous implants with an r2 of 0.97 and 0.98, respectively. In addition, the FE global force at 0.5 mm showed a correlation to the maximum experimental forces with an r2 of 0.90. The highest porosity implants (80-90%) showed lower stiffnesses and more equitable load sharing but also failed at lower a global force level than the low porosity implants (50-70%). The lower strength of the high porosity implants caused premature plastic deformation of the implant itself during loading as well as significant deformations in the ingrown and surrounding bone, resulting in lower overall osseointegration strength, consistent with experimental measurements. The lower porosity implants showed a balance of sufficient bony ingrowth to support osseointegration strength coupled with implant mechanical properties to circumvent significant implant plasticity and collapse under the loading conditions. Together, the experimental and finite element modeling results support an optimal porosity in the range of 60-70% for maximizing osseointegration with current structure and loading.

Porous versus solid shoulder implants in humeri of different bone densities: A finite element analysis

Porous metallic prosthesis components can now be manufactured using additive manufacturing techniques, and may prove beneficial for promoting bony ingrowth, for accommodating drug delivery systems, and for reducing stress shielding. Using finite element modeling techniques, 36 scenarios (three porous stems, three bone densities, and four held arm positions) were analysed to assess the viability of porous humeral stems for use in total shoulder arthroplasty, and their resulting mechanobiological impact on the surrounding humerus bone. All three porous stems were predicted to experience stresses below the yield strength of Ti6Al4V (880 MPa) and to be capable of withstanding more than 10 million cycles of each loading scenario before failure. There was an indication that within an 80 mm region of the proximal humerus, there would be a reduction in bone resorption as stem porosity increased. Overall, this study shows promise that these porous structures are mechanically viable for incorporation into permanent shoulder prostheses to combat orthopedic infections.

Modified Titanium Implants Satisfy the Demands of Diabetic Osseointegration via Sequential Regulation of Macrophages and Mesenchymal Stem Cells

The application of titanium (Ti) implants for patients with diabetes mellitus (DM) is still facing a significant challenge due to obstacles such as hyperglycemia, reactive oxygen species (ROS), and chronic inflammation, which hinders osseointegration. To address this issue, a Ti implant with dual functions of regulating polarization of macrophages and facilitating osseointergration is developed via hydrothermal reaction and hydrogel coating. The reactive oxygen species (ROS) and glucose (Glu) responsive hydrogel coating can locally deliver adenosine (ADO) in the early stage of implantation. The controlled release of ADO regulated the phenotype of macrophages, restored oxidative balance, and enhanced mitochondrial function during the early stages of implantation. Subsequently, strontium (Sr) ions will be released to promote osteogenic differentiation and proliferation of mesenchymal stem cells (MSCs), as the hydrogel coating degraded. It eventually leads to bone reconstruction during the late stages, aligning with the biological cascade of bone healing. The modified Ti implants showed effective osteogenesis for bone defects in DM patients, shedding light on the design and biological mechanisms of surface modification. This research offers promising potential for improving the treatment of bone-related complications in diabetic patients.

An adaptive biodegradable zinc alloy with bidirectional regulation of bone homeostasis for treating fractures and aged bone defects

Healing of fractures or bone defects is significantly hindered by overactivated osteoclasts and inhibited osteogenesis in patients with abnormal bone metabolism. Current clinical approaches using titanium alloys or stainless steel provide mechanical support but have no biological effects on bone regeneration. Therefore, designing and fabricating degradable metal materials with sufficient mechanical strength and bidirectional regulation of both osteoblasts and osteoclasts is a substantial challenge. Here, this study first reported an adaptive biodegradable Zn-0.8 Mg alloy with bidirectional regulation of bone homeostasis, which promotes osteogenic differentiation by activating the Pi3k/Akt pathway and inhibits osteoclast differentiation by inhibiting the GRB2/ERK pathway. The anti-osteolytic ability of the Zn-0.8 Mg alloy was verified in a mouse calvarial osteolysis model and its suitability for internal fracture fixation with high-strength screws was confirmed in the rabbit femoral condyle fracture model. Furthermore, in an aged postmenopausal rat femoral condyle defect model, 3D printed Zn-0.8 Mg scaffolds promoted excellent bone regeneration through adaptive structures with good mechanical properties and bidirectionally regulated bone metabolism, enabling personalized bone defect repair. These findings demonstrate the substantial potential of the Zn-0.8 Mg alloy for treating fractures or bone defects in patients with aberrant bone metabolism.

Advancements in Custom 3D-Printed Titanium Interbody Spinal Fusion Cages and Their Relevance in Personalized Spine Care

3D-printing technology has revolutionized spinal implant manufacturing, particularly in developing personalized and custom-fit titanium interbody fusion cages. These cages are pivotal in supporting inter-vertebral stability, promoting bone growth, and restoring spinal alignment. This article reviews the latest advancements in 3D-printed titanium interbody fusion cages, emphasizing their relevance in modern personalized surgical spine care protocols applied to common clinical scenarios. Furthermore, the authors review the various printing and post-printing processing technologies and discuss how engineering and design are deployed to tailor each type of implant to its patient-specific clinical application, highlighting how anatomical and biomechanical considerations impact their development and manufacturing processes to achieve optimum osteoinductive and osteoconductive properties. The article further examines the benefits of 3D printing, such as customizable geometry and porosity, that enhance osteointegration and mechanical compatibility, offering a leap forward in patient-specific solutions. The comparative analysis provided by the authors underscores the unique challenges and solutions in designing cervical, and lumbar spine implants, including load-bearing requirements and bioactivity with surrounding bony tissue to promote cell attachment. Additionally, the authors discuss the clinical outcomes associated with these implants, including the implications of improvements in surgical precision on patient outcomes. Lastly, they address strategies to overcome implementation challenges in healthcare facilities, which often resist new technology acquisitions due to perceived cost overruns and preconceived notions that hinder potential savings by providing customized surgical implants with the potential for lower complication and revision rates. This comprehensive review aims to provide insights into how modern 3D-printed titanium interbody fusion cages are made, explain quality standards, and how they may impact personalized surgical spine care.

3D Printing and Surface Engineering of Ti6Al4V Scaffolds for Enhanced Osseointegration in an In Vitro Study

Ti6Al4V superalloy is recognized as a good candidate for bone implants owing to its biocompatibility, corrosion resistance, and high strength-to-weight ratio. While dense metal implants are associated with stress shielding issues due to the difference in densities, stiffness, and modulus of elasticity compared to bone tissues, the surface of the implant/scaffold should mimic the properties of the bone of interest to assure a good integration with a strong interface. In this study, we investigated the additive manufacturing of porous Ti6Al4V scaffolds and coating modification for enhanced osteoconduction using osteoblast cells. The results showed the successful fabrication of porous Ti6Al4V scaffolds with adequate strength. Additionally, the surface treatment with NaOH and Dopamine Hydrochloride (DOPA) promoted the formation of Dopamine Hydrochloride (DOPA) coating with an optimized coating process, providing an environment that supports higher cell viability and growth compared to the uncoated Ti6Al4V scaffolds, as demonstrated by the higher proliferation ratios observed from day 1 to day 29. These findings bring valuable insights into the surface modification of 3D-printed scaffolds for improved osteoconduction through the coating process in solutions.

Decoding the "Fingerprint" of Implant Materials: Insights into the Foreign Body Reaction

Foreign body reaction (FBR) is a prevalent yet often overlooked pathological phenomenon, particularly within the field of biomedical implantation. The presence of FBR poses a heavy burden on both the medical and socioeconomic systems. This review seeks to elucidate the protein "fingerprint" of implant materials, which is generated by the physiochemical properties of the implant materials themselves. In this review, the activity of macrophages, the formation of foreign body giant cells (FBGCs), and the development of fibrosis capsules in the context of FBR are introduced. Additionally, the relationship between various implant materials and FBR is elucidated in detail, as is an overview of the existing approaches and technologies employed to alleviate FBR. Finally, the significance of implant components (metallic materials and non-metallic materials), surface CHEMISTRY (charge and wettability), and physical characteristics (topography, roughness, and stiffness) in establishing the protein "fingerprint" of implant materials is also well documented. In conclusion, this review aims to emphasize the importance of FBR on implant materials and provides the current perspectives and approaches in developing implant materials with anti-FBR properties.

Polydopamine-Coated Copper-Doped Co3O4 Nanosheets Rich in Oxygen Vacancy on Titanium and Multimodal Synergistic Antibacterial Study

Medical titanium-based (Ti-based) implants in the human body are prone to infection by pathogenic bacteria, leading to implantation failure. Constructing antibacterial nanocoatings on Ti-based implants is one of the most effective strategies to solve bacterial contamination. However, single antibacterial function was not sufficient to efficiently kill bacteria, and it is necessary to develop multifunctional antibacterial methods. This study modifies medical Ti foils with Cu-doped Co3O4 rich in oxygen vacancies, and improves their biocompatibility by polydopamine (PDA/Cu-Ov-Co3O4). Under near-infrared (NIR) irradiation, nanocoatings can generate •OH and 1O2 due to Cu+ Fenton-like activity and a photodynamic effect of Cu-Ov-Co3O4, and the total reactive oxygen species (ROS) content inside bacteria significantly increases, causing oxidative stress of bacteria. Further experiments prove that the photothermal process enhances the bacterial membrane permeability, allowing the invasion of ROS and metal ions, as well as the protein leakage. Moreover, PDA/Cu-Ov-Co3O4 can downregulate ATP levels and further reduce bacterial metabolic activity after irradiation. This coating exhibits sterilization ability against both Escherichia coli and Staphylococcus aureus with an antibacterial rate of ca. 100%, significantly higher than that of bare medical Ti foils (ca. 0%). Therefore, multifunctional synergistic antibacterial nanocoating will be a promising strategy for preventing bacterial contamination on medical Ti-based implants.

3D Printing Materials and Technologies for Orthopaedic Applications

SUMMARY

3D printing technologies have evolved tremendously over the last decade for uses in orthopaedic surgical applications, including being used to manufacture implants for spine, upper extremity, foot and ankle, oncologic, and traumatic reconstructions. Materials used for 3D-printed orthopaedic devices include metals, degradable and nondegradable polymers, and ceramic composites. There are 2 primary advantages for use of 3D printing technologies for orthopaedics: first, the ability to create complex porous lattices that allow for osseointegration and improved implant stability and second, the enablement of complex geometric designs allowing for patient-specific devices based on preoperative imaging. Given continually evolving technology, and the relatively early stage of the materials and 3D printers themselves, the possibilities for continued innovation in orthopaedics are great.

Fabrication of gelatin-polyester based biocomposite scaffold via one-step functionalization of melt electrowritten polymer blends in aqueous phase

The rapid manufacturing of biocomposite scaffold made of saturated-Poly(ε-caprolactone) (PCL) and unsaturated Polyester (PE) blends with gelatin and modified gelatin (NCO-Gel) is demonstrated. Polyester blend-based scaffold are fabricated with and without applying potential in the melt electrowriting system. Notably, the applied potential induces phase separation between PCL and PE and drives the formation of PE rich spots at the interface of electrowritten fibers. The objective of the current study is to control the phase separation between saturated and unsaturated polyesters occurring in the melt electro-writing process and utilization of this phenomenon to improve efficiency of biofunctionalization at the interface of scaffold via Aza-Michael addition reaction. Electron-deficient triple bonds of PE spots on the fibers exhibit good potential for the biofunctionalization via the aza-Michael addition reaction. PE spots are found to be pronounced in which blend compositions are PCL-PE as 90:10 and 75:25 %. The biofunctionalization of scaffold is monitored through CN bond formation appeared at 400 eV via X-ray photoelectron spectroscopy (XPS) and XPS chemical mapping. The described biofunctionalization methodology suggest avoiding use of multi-step chemical modification on additive manufacturing products and thereby rapid prototyping of functional polymer blend based scaffolds with enhanced biocompatibility and preserved mechanical properties. Additionally one-step additive manufacturing method eliminates side effects of toxic solvents and long modification steps during scaffold fabrication.

Strontium-loaded 3D intramedullary nail titanium implant for critical-sized femoral defect in rabbits

The treatment of critical-sized bone defects has long been a major problem for surgeons. In this study, an intramedullary nail shaped three-dimensional (3D)-printed porous titanium implant that is capable of releasing strontium ions was developed through a simple and cost-effective surface modification technique. The feasibility of this implant as a stand-alone solution was evaluated using a rabbit's segmental diaphyseal as a defect model. The strontium-loaded implant exhibited a favorable environment for cell adhesion, and mechanical properties that were commensurate with those of a rabbit's cortical bone. Radiographic, biomechanical, and histological analyses revealed a significantly higher amount of bone ingrowth and superior bone-bonding strength in the strontium-loaded implant when compared to an untreated porous titanium implant. Furthermore, one-year histological observations revealed that the strontium-loaded implant preserved the native-like diaphyseal bone structure without failure. These findings suggest that strontium-releasing 3D-printed titanium implants have the clinical potential to induce the early and efficient repair of critical-sized, load-bearing bone defects.

The effect of nodal connectivity and strut density within stochastic titanium scaffolds on osteogenesis

Modern orthopaedic implants use lattice structures that act as 3D scaffolds to enhance bone growth into and around implants. Stochastic scaffolds are of particular interest as they mimic the architecture of trabecular bone and can combine isotropic properties and adjustable structure. The existing research mainly concentrates on controlling the mechanical and biological performance of periodic lattices by adjusting pore size and shape. Still, less is known on how we can control the performance of stochastic lattices through their design parameters: nodal connectivity, strut density and strut thickness. To elucidate this, four lattice structures were evaluated with varied strut densities and connectivity, hence different local geometry and mechanical properties: low apparent modulus, high apparent modulus, and two with near-identical modulus. Pre-osteoblast murine cells were seeded on scaffolds and cultured in vitro for 28 days. Cell adhesion, proliferation and differentiation were evaluated. Additionally, the expression levels of key osteogenic biomarkers were used to assess the effect of each design parameter on the quality of newly formed tissue. The main finding was that increasing connectivity increased the rate of osteoblast maturation, tissue formation and mineralisation. In detail, doubling the connectivity, over fixed strut density, increased collagen type-I by 140%, increased osteopontin by 130% and osteocalcin by 110%. This was attributed to the increased number of acute angles formed by the numerous connected struts, which facilitated the organization of cells and accelerated the cell cycle. Overall, increasing connectivity and adjusting strut density is a novel technique to design stochastic structures which combine a broad range of biomimetic properties and rapid ossification.

3D-printed LEGO®-inspired titanium scaffolds for patient-specific regenerative medicine

Despite the recent advances in 3D-printing, it is often difficult to fabricate implants that optimally fit a defect size or shape. There are some approaches to resolve this issue, such as patient-specific implant/scaffold designs based on CT images of the patients, however, this process is labor-intensive and costly. Especially in developing countries, affordable treatment options are required, while still not excluding these patient groups from potential material and manufacturing advances. Here, a selective laser melting (SLM) 3D-printing strategy was used to fabricate a hierarchical, LEGO®-inspired Assemblable Titanium Scaffold (ATS) system, which can be manually assembled in any shape or size with ease. A surgeon can quickly create a scaffold that would fit to the defect right before the implantation during the surgery. Additionally, the direct inclusion of micro- and macroporous structures via 3D-printing, as well as a double acid-etched surface treatment (ST) in the ATS, ensure biocompatibility, sufficient nutrient flow, cell migration and enhanced osteogenesis. Three different structures were designed (non-porous:NP, semi-porous:SP, ultra-porous:UP), 3D-printed with the SLM technique and then surface treated for the ST groups. After analyzing characteristics of the ATS such as printing quality, surface roughness and interconnected porosity, mechanical testing and finite element analysis (FEA) demonstrated that individual and stacked ATS have sufficient mechanical properties to withstand loading in a physiological system. All ATS showed high cell viability, and the SP and UP groups demonstrated enhanced cell proliferation rates compared to the NP group. Furthermore, we also verified that cells were well-attached and spread on the porous structures and successful cell migration between the ATS units was seen in the case of assemblies. The UP and SP groups exhibited higher calcium deposition and RT-qPCR proved higher osteogenic gene expression compared to NP group. Finally, we demonstrate a number of possible medical applications that reveal the potential of the ATS through assembly.

Functionally Tailored Metal-Organic Framework Coatings for Mediating Ti Implant Osseointegration

Owing to their mechanical resilience and non-toxicity, titanium implants are widely applied as the major treatment modality for the clinical intervention against bone fractures. However, the intrinsic bioinertness of Ti and its alloys often impedes the effective osseointegration of the implants, leading to severe adverse complications including implant loosening, detachment, and secondary bone damage. Consequently, new Ti implant engineering strategies are urgently needed to improve their osseointegration after implantation. Remarkably, metalorganic frameworks (MOFs) are a class of novel synthetic material consisting of coordinated metal species and organic ligands, which have demonstrated a plethora of favorable properties for modulating the interfacial properties of Ti implants. This review comprehensively summarizes the recent progress in the development of MOF-coated Ti implants and highlights their potential utility for modulating the bio-implant interface to improve implant osseointegration, of which the discussions are outlined according to their physical traits, chemical composition, and drug delivery capacity. A perspective is also provided in this review regarding the current limitations and future opportunities of MOF-coated Ti implants for orthopedic applications. The insights in this review may facilitate the rational design of more advanced Ti implants with enhanced therapeutic performance and safety.

Revision of Subtalar Joint Arthrodesis: Considerations for Bone Grafting, Fixation Constructs, and Three-Dimensional Printing

Subtalar joint arthrodesis is a commonly used procedure for numerous pathologic conditions in the foot and ankle. Although rarely performed in isolation, this procedure can provide successful resolution of various lower-extremity complaints. There are traditional approaches to isolated subtalar joint arthrodesis; however, when these fail, the authors recommend alternatives to enhance the success of revisional surgery. These include the use of intramedullary nailing, larger harvest of autograft, and metallic wedge.

Latest advances: Improving the anti-inflammatory and immunomodulatory properties of PEEK materials

Excellent biocompatibility, mechanical properties, chemical stability, and elastic modulus close to bone tissue make polyetheretherketone (PEEK) a promising orthopedic implant material. However, biological inertness has hindered the clinical applications of PEEK. The immune responses and inflammatory reactions after implantation would interfere with the osteogenic process. Eventually, the proliferation of fibrous tissue and the formation of fibrous capsules would result in a loose connection between PEEK and bone, leading to implantation failure. Previous studies focused on improving the osteogenic properties and antibacterial ability of PEEK with various modification techniques. However, few studies have been conducted on the immunomodulatory capacity of PEEK. New clinical applications and advances in processing technology, research, and reports on the immunomodulatory capacity of PEEK have received increasing attention in recent years. Researchers have designed numerous modification techniques, including drug delivery systems, surface chemical modifications, and surface porous treatments, to modulate the post-implantation immune response to address the regulatory factors of the mechanism. These studies provide essential ideas and technical preconditions for the development and research of the next generation of PEEK biological implant materials. This paper summarizes the mechanism by which the immune response after PEEK implantation leads to fibrous capsule formation; it also focuses on modification techniques to improve the anti-inflammatory and immunomodulatory abilities of PEEK. We also discuss the limitations of the existing modification techniques and present the corresponding future perspectives.

Multifunctional 3D-printed scaffolds eradiate orthotopic osteosarcoma and promote osteogenesis via microwave thermo-chemotherapy combined with immunotherapy

Tumor recurrence and a lack of bone-tissue integration are two critical concerns in the surgical treatment of osteosarcoma. Thus, an advanced multifunctional therapeutic platform capable of simultaneously eliminating residual tumor cells and promoting bone regeneration is urgently needed for efficient osteosarcoma treatment. Herein, to thoroughly eliminate tumors and simultaneously promote bone regeneration, an intelligent multifunctional therapeutic scaffold has been engineered by integrating microwave-responsive zeolitic imidazolate framework 8 (ZIF-8) nanomaterials loaded with a chemotherapeutic drug and an immune checkpoint inhibitor onto 3D-printed titanium scaffolds. The constructed scaffold features distinct microwave-thermal sensitization and tumor microenvironment-responsive characteristics, which can induce tumor immunogenic death by microwave hyperthermia and chemotherapy. Orthotopic implantation of the nanocomposite scaffold results in an enhanced immune response against osteosarcoma that may effectively inhibit tumor recurrence through synergistic immunotherapy. During long-term implantation, the zinc ions released from the degradation of ZIF-8 can induce the osteogenic differentiation of stem cells. The porous structure and mechanical properties of the 3D-printed titanium scaffolds provide a structural microenvironment for bone regeneration. This study provides a paradigm for the design of multifunctional microwave-responsive composite scaffolds for use as a therapy for osteosarcoma, which could lead to improved strategies for the treatment of the disease.

Dynamic surface adapts to multiple service stages by orchestrating responsive polymers and functional peptides

Control over the implant surface functions is highly desirable to enhance tissue healing outcomes but has remained unexplored to adapt to the different service stages. In the present study, we develop a smart titanium surface by orchestrating thermoresponsive polymer and antimicrobial peptide to enable dynamic adaptation to the implantation stage, normal physiological stage and bacterial infection stage. The optimized surface inhibited bacterial adhesion and biofilm formation during surgical implantation, while promoted osteogenesis in the physiological stage. The further temperature increase driven by bacterial infection induced polymer chain collapse to expose antimicrobial peptides by rupturing bacterial membranes, as well as protect the adhered cells from the hostile environment of infection and abnormal temperature. The engineered surface could inhibit infection and promote tissue healing in rabbit subcutaneous and bone defect infection models. This strategy enables the possibility to create a versatile surface platform to balance bacteria/cell-biomaterial interactions at different service stages of implants that has not been achieved before.

Polyether-Ether-Ketone (PEEK) and Its 3D-Printed Quantitate Assessment in Cranial Reconstruction

Three-dimensional (3D) printing, medical imaging, and implant design have all advanced significantly in recent years, and these developments may change how modern craniomaxillofacial surgeons use patient data to create tailored treatments. Polyether-ether-ketone (PEEK) is often seen as an attractive option over metal biomaterials in medical uses, but a solid PEEK implant often leads to poor osseointegration and clinical failure. Therefore, the objective of this study is to demonstrate the quantitative assessment of a custom porous PEEK implant for cranial reconstruction and to evaluate its fitting accuracy. The research proposes an efficient process for designing, fabricating, simulating, and inspecting a customized porous PEEK implant. In this study, a CT scan is utilized in conjunction with a mirrored reconstruction technique to produce a skull implant. In order to foster cell proliferation, the implant is modified into a porous structure. The implant's strength and stability are examined using finite element analysis. Fused filament fabrication (FFF) is utilized to fabricate the porous PEEK implants, and 3D scanning is used to test its fitting accuracy. The results of the biomechanical analysis indicate that the highest stress observed was approximately 61.92 MPa, which is comparatively low when compared with the yield strength and tensile strength of the material. The implant fitting analysis demonstrates that the implant's variance from the normal skull is less than 0.4436 mm, which is rather low given the delicate anatomy of the area. The results of the study demonstrate the implant's endurance while also increasing the patient's cosmetic value.

Auxeticity as a Mechanobiological Tool to Create Meta-Biomaterials

Mechanical and morphological design parameters, such as stiffness or porosity, play important roles in creating orthopedic implants and bone substitutes. However, we have only a limited understanding of how the microarchitecture of porous scaffolds contributes to bone regeneration. Meta-biomaterials are increasingly used to precisely engineer the internal geometry of porous scaffolds and independently tailor their mechanical properties (e.g., stiffness and Poisson's ratio). This is motivated by the rare or unprecedented properties of meta-biomaterials, such as negative Poisson's ratios (i.e., auxeticity). It is, however, not clear how these unusual properties can modulate the interactions of meta-biomaterials with living cells and whether they can facilitate bone tissue engineering under static and dynamic cell culture and mechanical loading conditions. Here, we review the recent studies investigating the effects of the Poisson's ratio on the performance of meta-biomaterials with an emphasis on the relevant mechanobiological aspects. We also highlight the state-of-the-art additive manufacturing techniques employed to create meta-biomaterials, particularly at the micrometer scale. Finally, we provide future perspectives, particularly for the design of the next generation of meta-biomaterials featuring dynamic properties (e.g., those made through 4D printing).

Comparative Analysis of Bone Ingrowth in 3D-Printed Titanium Lattice Structures with Different Patterns

In this study, metal 3D printing technology was used to create lattice-shaped test specimens of orthopedic implants to determine the effect of different lattice shapes on bone ingrowth. Six different lattice shapes were used: gyroid, cube, cylinder, tetrahedron, double pyramid, and Voronoi. The lattice-structured implants were produced from Ti6Al4V alloy using direct metal laser sintering 3D printing technology with an EOS M290 printer. The implants were implanted into the femoral condyles of sheep, and the animals were euthanized 8 and 12 weeks after surgery. To determine the degree of bone ingrowth for different lattice-shaped implants, mechanical, histological, and image processing tests on ground samples and optical microscopic images were performed. In the mechanical test, the force required to compress the different lattice-shaped implants and the force required for a solid implant were compared, and significant differences were found in several instances. Statistically evaluating the results of our image processing algorithm, it was found that the digitally segmented areas clearly consisted of ingrown bone tissue; this finding is also supported by the results of classical histological processing. Our main goal was realized, so the bone ingrowth efficiencies of the six lattice shapes were ranked. It was found that the gyroid, double pyramid, and cube-shaped lattice implants had the highest degree of bone tissue growth per unit time. This ranking of the three lattice shapes remained the same at both 8 and 12 weeks after euthanasia. In accordance with the study, as a side project, a new image processing algorithm was developed that proved suitable for determining the degree of bone ingrowth in lattice implants from optical microscopic images. Along with the cube lattice shape, whose high bone ingrowth values have been previously reported in many studies, it was found that the gyroid and double pyramid lattice shapes produced similarly good results.

SAXS imaging reveals optimized osseointegration properties of bioengineered oriented 3D-PLGA/aCaP scaffolds in a critical size bone defect model

Healing large bone defects remains challenging in orthopedic surgery and is often associated with poor outcomes and complications. A major issue with bioengineered constructs is achieving a continuous interface between host bone and graft to enhance biological processes and mechanical stability. In this study, we have developed a new bioengineering strategy to produce oriented biocompatible 3D PLGA/aCaP nanocomposites with enhanced osseointegration. Decellularized scaffolds -containing only extracellular matrix- or scaffolds seeded with adipose-derived mesenchymal stromal cells were tested in a mouse model for critical size bone defects. In parallel to micro-CT analysis, SAXS tensor tomography and 2D scanning SAXS were employed to determine the 3D arrangement and nanostructure within the critical-sized bone. Both newly developed scaffold types, seeded with cells or decellularized, showed high osseointegration, higher bone quality, increased alignment of collagen fibers and optimal alignment and size of hydroxyapatite minerals.

Development of a Scaffold-on-a-Chip Platform to Evaluate Cell Infiltration and Osteogenesis on the 3D-Printed Scaffold for Bone Regeneration

Developing a scaffold for efficient and functional bone regeneration remains challenging. To accomplish this goal, a "scaffold-on-a-chip" device was developed as a platform to aid with the evaluation process. The device mimics a microenvironment experienced by a transplanted bone scaffold. The device contains a circular space at the center for scaffold insert and microfluidic channel that encloses the space. Such a design allows for monitoring of cell behavior at the blood-scaffold interphase. MC3T3-E1 cells were cultured with three different types of scaffold inserts to test its capability as an evaluation platform. Cellular behaviors, including migration, morphology, and osteogenesis with each scaffold, were analyzed through fluorescence images of live/dead assay and immunocytochemistry. Cellular behaviors, such as migration, morphology, and osteogenesis, were evaluated. The results revealed that our platform could effectively evaluate the osteoconductivity and osteoinductivity of scaffolds with various properties. In conclusion, our proposed platform is expected to replace current in vivo animal models as a highly relevant in vitro platform and can contribute to the fundamental study of bone regeneration.

Influence of porosity on osteogenesis, bone growth and osteointegration in trabecular tantalum scaffolds fabricated by additive manufacturing

Porous tantalum implants are a class of materials commonly used in clinical practice to repair bone defects. However, the cumbersome and problematic preparation procedure have limited their widespread application. Additive manufacturing has revolutionized the design and process of orthopedic implants, but the pore architecture feature of porous tantalum scaffolds prepared from additive materials for optimal osseointegration are unclear, particularly the influence of porosity. We prepared trabecular bone-mimicking tantalum scaffolds with three different porosities (60%, 70% and 80%) using the laser powder bed fusing technique to examine and compare the effects of adhesion, proliferation and osteogenic differentiation capacity of rat mesenchymal stem cells on the scaffolds in vitro. The in vivo bone ingrowth and osseointegration effects of each scaffold were analyzed in a rat femoral bone defect model. Three porous tantalum scaffolds were successfully prepared and characterized. In vitro studies showed that scaffolds with 70% and 80% porosity had a better ability to osteogenic proliferation and differentiation than scaffolds with 60% porosity. In vivo studies further confirmed that tantalum scaffolds with the 70% and 80% porosity had a better ability for bone ingrowh than the scaffold with 60% porosity. As for osseointegration, more bone was bound to the material in the scaffold with 70% porosity, suggesting that the 3D printed trabecular tantalum scaffold with 70% porosity could be the optimal choice for subsequent implant design, which we will further confirm in a large animal preclinical model for better clinical use.

Silk Fibroin/Collagen/Hydroxyapatite Scaffolds Obtained by 3D Printing Technology and Loaded with Recombinant Human Erythropoietin in the Reconstruction of Alveolar Bone Defects

The fast osteogenesis of the large alveolar fossa and the maintenance of the height of the alveolar ridge after tooth extraction have always been a clinical challenge. Therefore, this work describes the creation of innovative silk fibroin/collagen/hydroxyapatite (SCH) biological scaffolds by 3D printing technology, which are loaded with recombinant human erythropoietin (rh-EPO) for the reconstruction of bone defects. Low-temperature 3D printing can maintain the biological activity of silk fibroin and collagen. The SCH scaffolds showed the ideal water absorption and porosity, being a sustained-release carrier of rh-EPO. The optimized scaffolds had ideal mechanical properties in vitro, and MC3T3-E1 cells could easily adhere and proliferate on it. In vivo experiments in rabbits demonstrated that the composite scaffolds gradually degraded and promoted the accumulation and proliferation of osteoblasts and the formation of collagen fibers, significantly promoting the reconstruction of mandibular defects. In this study, a novel composite biological scaffold was prepared using 3D printing technology, and the scaffold was innovatively combined with the multifunctional growth factor rh-EPO. This provides a new optimized composite material for the reconstruction of irregular mandible defects, and this biomaterial is promising for clinical reconstruction of alveolar bone defects.

Aptamer engineering exosomes loaded on biomimetic periosteum to promote angiogenesis and bone regeneration by targeting injured nerves via JNK3 MAPK pathway

Repairing critical bone defects is a complex problem in the clinic. The periosteum rich in nerve plays a vital role in initiating and regulating bone regeneration. However, current studies have paid little attention to repairing nerves in the periosteum to promote bone regeneration. Thus, it is essential to construct bionic periosteum with the targeted injured nerves in the periosteum. We coupled phosphatidylserine (PS) targeted aptamers with repair Schwann cell exosomes to construct exosome@aptamer (EA). Then through PEI, EA was successfully built on the surface of the electrospun fiber, which was PCL@PEI@exosome@aptamer (PPEA). Through SEM, TEM, and other technologies, PPEA was characterized. Experiments prove in vivo and in vitro that it has an excellent repair effect on damaged nerves and regeneration of vascular and bones. In vivo, we confirmed that biomimetic periosteum has an apparent ability to promote nerve and bone regeneration by using Microcomputer tomography, hematoxylin-eosin, Masson, and Immunofluorescence. In vitro, we used Immunofluorescence, Real-Time Quantitative PCR, Alkaline phosphatase staining, and other tests to confirm that it has central nerve, blood vessel, and bone regeneration ability. The PPEA biomimetic periosteum has apparent neurogenic, angiogenic, and osteogenic effects. The PPEA biomimetic periosteum will provide a promising method for treating bone defects.

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To construct a biomimetic periosteum that can target injured axons and bone regeneration.

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PS targeted aptamer is coupled with repair Schwann cell exosomes.

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PEI self-assembly was used for the PCL electrospun biomimetic membrane loading.

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It targeted and repaired the injured axons and promoted the secretion of CGRP and SP.

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Biomimetic periosteum promotes vascular regeneration and bone regeneration.

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.

Surface Bandgap Engineering of Nanostructured Implants for Rapid Photothermal Ion Therapy of Bone Defects

Bone defects are seriously threatening the health of orthopedics patients and it is difficult for implants to accelerate bone regeneration without using bone growth factors. Herein, a fast photothermal ion therapeutic strategy is developed based on the bandgap engineering of nanostructured TiO₂ through (Si/P)-dual elemental doping by micro-arc oxidation treatment of titanium implants. The (Si/P)-dual doping can tune the surface bandgap structure of TiO₂ by decreasing bandgap and broadening valence band simultaneously, which is confirmed by density functional theory calculations. It not only endows the implants with a mildly photothermal effect under near-infrared (NIR) light irradiation, but also creates an (Si/P) ion-rich microenvironment around the implants. This photothermal ion microenvironment can tune the behaviors of osteoblasts by promoting p38/Smad and ERK signaling pathways of osteoblasts, thus significantly upregulating the expression of osteogenesis genes by the synergistic action of mild photothermal stimulation and increased release of Si/P ions. The in vivo results are also in good agreement with in vitro tests, i.e., under NIR light irradiation, the photothermally responsive TiO₂ enhances the bone formation and osteointegration with implants. Therefore, this kind of photothermal ion strategy is a promising remote and noninvasive therapeutic mode for promoting bone regeneration of Ti implants.

Hybrid Ti6Al4V/Silk Fibroin Composite for Load-Bearing Implants: A Hierarchical Multifunctional Cellular Scaffold

Despite the tremendous technological advances that metal additive manufacturing (AM) has made in the last decades, there are still some major concerns guaranteeing its massive industrial application in the biomedical field. Indeed, some main limitations arise in dealing with their biological properties, specifically in terms of osseointegration. Morphological accuracy of sub-unital elements along with the printing resolution are major constraints in the design workspace of a lattice, hindering the possibility of manufacturing structures optimized for proper osteointegration. To overcome these issues, the authors developed a new hybrid multifunctional composite scaffold consisting of an AM Ti6Al4V lattice structure and a silk fibroin/gelatin foam. The composite was realized by combining laser powder bed fusion (L-PBF) of simple cubic lattice structures with foaming techniques. A combined process of foaming and electrodeposition has been also evaluated. The multifunctional scaffolds were characterized to evaluate their pore size, morphology, and distribution as well as their adhesion and behavior at the metal-polymer interface. Pull-out tests in dry and hydrated conditions were employed for the mechanical characterization. Additionally, a cytotoxicity assessment was performed to preliminarily evaluate their potential application in the biomedical field as load-bearing next-generation medical devices.

Sustained release of BMP-2 using self-assembled layer-by-layer film-coated implants enhances bone regeneration over burst release

Current clinical products delivering the osteogenic growth factor bone morphogenetic protein 2 (BMP-2) for bone regeneration have been plagued by safety concerns due to a high incidence of off-target effects resulting from bolus release and supraphysiological doses. Layer-by-layer (LbL) film deposition offers the opportunity to coat bone defect-relevant substrates with thin films containing proteins and other therapeutics; however, control of release kinetics is often hampered by interlayer diffusion of drugs throughout the film during assembly, which causes burst drug release. In this work, we present the design of different laponite clay diffusional barrier layer architectures in self-assembled LbL films to modulate the release kinetics of BMP-2 from the surface of a biodegradable implant. Release kinetics were tuned by incorporating laponite in different film arrangements and with varying deposition techniques to achieve release of BMP-2 over 2 days, 4 days, 14 days, and 30 days. Delivery of a low dose (0.5 μg) of BMP-2 over 2 days and 30 days using these LbL film architectures was then compared in an in vivo rat critical size calvarial defect model to determine the effect of BMP-2 release kinetics on bone regeneration. After 6 weeks, sustained release of BMP-2 over 30 days induced 3.7 times higher bone volume and 7.4 times higher bone mineral density as compared with 2-day release of BMP-2, which did not induce more bone growth than the uncoated scaffold control. These findings represent a crucial step in the understanding of how BMP-2 release kinetics influence treatment efficacy and underscore the necessity to optimize protein delivery methods in clinical formulations for bone regeneration. This work could be applied to the delivery of other therapeutic proteins for which careful tuning of the release rate is a key optimization parameter.

3D-printed composite scaffold with gradient structure and programmed biomolecule delivery to guide stem cell behavior for osteochondral regeneration

The fabrication of osteochondral scaffolds with both structural and biochemical cues to regulate endogenous bone marrow-derived mesenchymal stem cells (BMSCs) behavior for cartilage and subchondral bone regeneration is still a challenge. To this end, a composite scaffold (BE-PSA) with gradient structure and programmed biomolecule delivery was prepared by fused deposition modeling (FDM) 3D printing and multi-material-based modification. The 3D-printed polycaprolactone (PCL) scaffold included upper pores of 200 μm for cartilage regeneration and lower pores of 400 μm for bone regeneration. For a sequential modulation of BMSCs behavior, fast-degrading sodium alginate (SA) hydrogel was used to deliver a burst release of E7 peptide to enhance BMSCs migration within 72 h, while a slowly-degrading silk fibroin (SF) porous matrix was used to provide a sustained release of B2A peptide to improve BMSCs dual-lineage differentiation lasting for >300 h, depending on the different degradation rates of SA hydrogel and SF matrix. The BE-PSA scaffold had good biocompatibility and could improve the migration and osteogenic/chondrogenic differentiation of BMSCs. Benefiting from the synergistic effects of spatial structures and programmed biomolecule delivery, the BE-PSA scaffold showed enhanced cartilage and subchondral bone regeneration in rabbit osteochondral defect model. This work not only provides a promising scaffold to guide BMSCs behavior for osteochondral regeneration but also offers a method for the fabrication of tissue engineering biomaterials based on the structural and biochemical modification.

Metallic Scaffold with Micron-Scale Geometrical Cues Promotes Osteogenesis and Angiogenesis via the ROCK/Myosin/YAP Pathway

The advent of precision manufacturing has enabled the creation of pores in metallic scaffolds with feature size in the range of single microns. In orthopedic implants, pore geometries at the micron scale could regulate bone formation by stimulating osteogenic differentiation and the coupling of osteogenesis and angiogenesis. However, the biological response to pore geometry at the cellular level is not clear. As cells are sensitive to curvature of the pore boundary, this study aimed to investigate osteogenesis in high- vs low-curvature environments by utilizing computer numerical control laser cutting to generate triangular and circular precision manufactured micropores (PMpores). We fabricated PMpores on 100 μm-thick stainless-steel discs. Triangular PMpores had a 30° vertex angle and a 300 μm base, and circular PMpores had a 300 μm diameter. We found triangular PMpores significantly enhanced the elastic modulus, proliferation, migration, and osteogenic differentiation of MC3T3-E1 preosteoblasts through Yes-associated protein (YAP) nuclear translocation. Inhibition of Rho-associated kinase (ROCK) and Myosin II abolished YAP translocation in all pore types and controls. Inhibition of YAP transcriptional activity reduced the proliferation, pore closure, collagen secretion, alkaline phosphatase (ALP), and Alizarin Red staining in MC3T3-E1 cultures. In C166 vascular endothelial cells, PMpores increased the VEGFA mRNA expression even without an angiogenic differentiation medium and induced tubule formation and maintenance. In terms of osteogenesis-angiogenesis coupling, a conditioned medium from MC3T3-E1 cells in PMpores promoted the expression of angiogenic genes in C166 cells. A coculture with MC3T3-E1 induced tubule formation and maintenance in C166 cells and tubule alignment along the edges of pores. Together, curvature cues in micropores are important stimuli to regulate osteogenic differentiation and osteogenesis-angiogenesis coupling. This study uncovered key mechanotransduction signaling components activated by curvature differences in a metallic scaffold and contributed to the understanding of the interaction between orthopedic implants and cells.

Corrosion Resistance of 3D Printed Ti6Al4V Gyroid Lattices with Varying Porosity

Corrosion of medical implants is a possible failure mode via induced local inflammatory effects, systemic deposition and corrosion related mechanical failure. Cyclic potentiodynamic polarisation (CPP) testing was utilized to evaluate the effect of increased porosity (60% and 80%) and decreased wall thickness in gyroid lattice structures on the electrochemical behaviour of LPBF Ti6Al4V structures. The use of CPP allowed for the landmarks of breakdown potential, resting potential and vertex potential to be analysed, as well as facilitating the construction of Tafel plots and qualitative Goldberg analysis. The results indicated that 60% gyroid samples were most susceptible to the onset of pitting corrosion when compared to 80% gyroid and solid samples. This was shown through decreased breakdown and vertex potentials and were found to correlate to increased lattice surface area to void volume ratio. Tafel plots indicated that despite the earlier onset of pitting corrosion, both gyroid test groups displayed lower rates of corrosion per year, indicating a lower severity of corrosion. This study highlighted inherent tradeoffs between lattice optimisation and corrosion behaviour with a potential parabolic link between void volume, surface area and corrosion being identified. This potential link is supported by 60% gyroid samples having the lowest breakdown potentials, but investigation into other porosity ranges is suggested to support the hypothesis. All 3D printed materials studied here showed breakdown potentials higher than ASTM F2129's suggestion of 800 mV for evaluation within the physiological environment, indicating that under static conditions pitting and crevice corrosion should not initiate within the body.

Improving the Stability of a Hemipelvic Prosthesis Based on Bone Mineral Density Screw Channel and Prosthesis Optimization Design

In pelvic reconstruction surgery, the hemipelvic prosthesis can cause significant changes in stress distribution due to its high stiffness, and its solid structure is not suitable for osseointegration. The purpose of this study was to identify a novel bone mineral density screw channel and design the structure of the prosthesis so as to improve the distribution of stress, promote bone growth, and enhance the biomechanical properties of the prosthesis. The mechanical characteristics of bone mineral density screw and traditional screw were compared by finite element analysis method, and redesigned by topology optimization. The direction of the newly proposed screw channel was the posterolateral entrance of the auricular surface, ending at the contralateral sacral cape. Compared to the original group, the maximum stress of the optimized prosthesis was decreased by 24.39%, the maximum stress of the sacrum in the optimized group was decreased by 27.23%, and the average strain energy density of the sacrum in the optimized group was increased by 8.43%. On the surface of screw and connecting plate, the area with micromotion more than 28 μm is reduced by 12.17%. On the screw surface, the area with micromotion more than 28 μm is reduced by 22.9%. The newly determined screw channel and optimized prosthesis design can effectively improve the biomechanical properties of a prosthesis and the microenvironment of osseointegration. This method can provide a reference for the fixation of prostheses in clinical pelvic reconstruction.

Ultra-Pulsed CO2 Laser Osteotomy: A New Method for the Bone Preparation of Total Knee Arthroplasty

Cementless total knee arthroplasty (TKA) can achieve long-term biological fixation, but its application is limited by the risk of early aseptic loosening. One of the important reasons for early aseptic loosening is that mechanical osteotomy tools cannot achieve ideal bone preparation because of poor accuracy and serious bone tissue damage produced by them. Therefore, we designed an ultra-pulsed CO₂ laser osteotomy system to solve these problems. To reveal the safety at the tissue and cell levels of the ultra-pulsed CO₂ laser osteotomy system, a series of experiments on distal femur osteotomy in animals were performed. Then, the bone surface characteristics were analyzed through scanning electron microscopy, and the bone thermal and mechanical damage was evaluated via histological analysis. Finally, mesenchymal stem cells (MSCs) were inoculated on the bone surfaces prepared by the two osteotomy tools, and the effect of cell adhesion was analyzed through a confocal laser scanning microscope (CLSM). We successfully achieved TKA bone preparation of animal knees with the ultra-pulsed CO₂ laser osteotomy system. Moreover, the biological evaluation results indicated that compared with the traditional mechanical saw, the laser can preserve the natural bone structure and cause no thermal damage to the bone. In addition, CLSM examination results showed that the laser-cut bone surface was more conducive to cell adhesion and infiltration than the bone surface cut by a mechanical saw. Overall, these results indicate that ultra-pulsed CO₂ laser can achieve non-invasive bone cutting, which can be a new option for TKA bone preparation and has the potential to lead in the future.

Additively Manufactured Porous Ti6Al4V for Bone Implants: A Review

Ti-6Al-4V (Ti64) alloy is one of the most widely used orthopedic implant materials due to its mechanical properties, corrosion resistance, and biocompatibility nature. Porous Ti64 structures are gaining more research interest as bone implants as they can help in reducing the stress-shielding effect when compared to their solid counterpart. The literature shows that porous Ti64 implants fabricated using different additive manufacturing (AM) process routes, such as laser powder bed fusion (L-PBF) and electron beam melting (EBM) can be tailored to mimic the mechanical properties of natural bone. This review paper categorizes porous implant designs into non-gradient (uniform) and gradient (non-uniform) porous structures. Gradient porous design appears to be more promising for orthopedic applications due to its closeness towards natural bone morphology and improved mechanical properties. In addition, this paper outlines the details on bone structure and its properties, mechanical properties, fatigue behavior, multifunctional porous implant designs, current challenges, and literature gaps in the research studies on porous Ti64 bone implants.

Biomimetic Hydroxyapatite Nanorods Promote Bone Regeneration via Accelerating Osteogenesis of BMSCs through T Cell-Derived IL-22

Manipulations of morphological properties of nanobiomaterials have been demonstrated to modulate the outcome of osteoimmunomodulation and eventually osteogenesis through innate immune response. However, the functions and mechanisms of adaptive immune cells in the process of nanobiomaterials-mediated bone regeneration have remained unknown. Herein, we developed bone-mimicking hydroxyapatite (HAp) nanorods with different aspect ratios as model materials to investigate the impacts of the nanoshape features on osteogenesis and to explore the underlying mechanisms focusing on the functions of T cells and T cell-derived cytokines. HAp nanorods with different aspect ratios (HAp-0, HAp-30, and HAp-100) were implanted into mouse mandibular defect models. Micro-CT and hematoxylin and eosin staining demonstrated that HAp-100 had the best osteogenic effects. Flow cytometry analysis revealed that HAp-100 increased the percentage of T cells in injured mandibles. The osteogenic effects of HAp-100 were significantly blunted in injured mandibles of TCRβ-/- mice. The Luminex xMAP assay and ELISA showed that HAp-100 induced a marked increase of interleukin (IL)-22 in injured mandibles. In cultured T cells, HAp-100 manifested the best capacity to induce the production of IL-22. Conditioned media from HAp-100-primed T cells promoted osteogenesis and JAK1/STAT3 activation in bone marrow stromal cells, all of which were abolished by neutralizing antibodies against IL-22. In summary, bone-mimicking HAp nanorods with different aspect ratios could regulate osteogenesis through modulation of T cells and IL-22 in the bone regeneration process. These findings provided insights for mediation of the immune response of T cells by nanomaterials on osteogenesis and strategies for designing biomaterials with osteoimmunomodulative functions.

Bioactive Coatings Loaded with Osteogenic Protein for Metallic Implants

Osteoconductive and osteoinductive coatings represent attractive and tunable strategies towards the enhanced biomechanics and osseointegration of metallic implants, providing accurate local modulation of bone-to-implant interface. Composite materials based on polylactide (PLA) and hydroxyapatite (HAp) are proved beneficial substrates for the modulation of bone cells' development, being suitable mechanical supports for the repair and regeneration of bone tissue. Moreover, the addition of osteogenic proteins represents the next step towards the fabrication of advanced biomaterials for hard tissue engineering applications, as their regulatory mechanisms beneficially contribute to the new bone formation. In this respect, laser-processed composites, based on PLA, Hap, and bone morphogenetic protein 4(BMP4), are herein proposed as bioactive coatings for metallic implants. The nanostructured coatings proved superior ability to promote the adhesion, viability, and proliferation of osteoprogenitor cells, without affecting their normal development and further sustaining the osteogenic differentiation of the cells. Our results are complementary to previous studies regarding the successful use of chemically BMP-modified biomaterials in orthopedic and orthodontic applications.