Selective laser melting-produced porous titanium scaffolds regenerate bone in critical size cortical bone defects

Mechanical, bioactive, and long-lasting antibacterial properties of a Ti scaffold with gradient pores releasing iodine ions

The ideal bone implant would effectively prevent aseptic as well as septic loosening by minimizing stress shielding, maximizing bone ingrowth, and preventing implant-associated infections. Here, a novel gradient-pore-size titanium scaffold was designed and manufactured to address these requirements. The scaffold features a larger pore size (900 μm) on the top surface, gradually decreasing to small sizes (600 μm to 300 μm) towards the center, creating a gradient structure. To enhance its functionality, the additively manufactured scaffolds were biofunctionalized using simple chemical and heat treatments so as to incorporate calcium and iodine ions throughout the surface. This unique combination of varying pore sizes with a biofunctional surface provides highly desirable mechanical properties, bioactivity, and notably, long-lasting antibacterial activity. The target mechanical aspects, including low elastic modulus, high compression, compression-shear, and fatigue strength, were effectively achieved. Furthermore, the biofunctional surface exhibits remarkable in vitro bioactivity and potent antibacterial activity, even under conditions specifically altered to be favorable for bacterial growth. More importantly, the integration of small pores alongside larger ones ensures a sustained high release of iodine, resulting in antimicrobial activity that persisted for over three months, with full eradication of the bacteria. Taken together, this gradient structure exhibits obvious superiority in combining most of the desired properties, making it an ideal candidate for orthopedic and dental implant applications.

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.

Research progress and perspective of metallic implant biomaterials for craniomaxillofacial surgeries

Craniomaxillofacial bone serves a variety of functions. However, the increasing number of cases of craniomaxillofacial bone injury and the use of selective rare implants make the treatment difficult, and the cure rate is low. If such a bone injury is not properly treated, it can lead to a slew of complications that can seriously disrupt a patient's daily life. For example, premature closure of cranial sutures or skull fractures can lead to increased intracranial pressure, which can lead to headaches, vomiting, and even brain hernia. At present, implant placement is one of the most common approaches to repair craniomaxillofacial bone injury or abnormal closure, especially with biomedical metallic implants. This review analyzes the research progress in the design and development of degradable and non-degradable metallic implants in craniomaxillofacial surgery. The mechanical properties, corrosion behaviours, as well as in vitro and in vivo performances of these materials are summarized. The challenges and future research directions of metallic biomaterials used in craniomaxillofacial surgery are also identified.

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.

Selective Laser Melting and Spark Plasma Sintering: A Perspective on Functional Biomaterials

Achieving lightweight, high-strength, and biocompatible composites is a crucial objective in the field of tissue engineering. Intricate porous metallic structures, such as lattices, scaffolds, or triply periodic minimal surfaces (TPMSs), created via the selective laser melting (SLM) technique, are utilized as load-bearing matrices for filled ceramics. The primary metal alloys in this category are titanium-based Ti6Al4V and iron-based 316L, which can have either a uniform cell or a gradient structure. Well-known ceramics used in biomaterial applications include titanium dioxide (TiO2), zirconium dioxide (ZrO2), aluminum oxide (Al2O3), hydroxyapatite (HA), wollastonite (W), and tricalcium phosphate (TCP). To fill the structures fabricated by SLM, an appropriate ceramic is employed through the spark plasma sintering (SPS) method, making them suitable for in vitro or in vivo applications following minor post-processing. The combined SLM-SPS approach offers advantages, such as rapid design and prototyping, as well as assured densification and consolidation, although challenges persist in terms of large-scale structure and molding design. The individual or combined application of SLM and SPS processes can be implemented based on the specific requirements for fabricated sample size, shape complexity, densification, and mass productivity. This flexibility is a notable advantage offered by the combined processes of SLM and SPS. The present article provides an overview of metal-ceramic composites produced through SLM-SPS techniques. Mg-W-HA demonstrates promise for load-bearing biomedical applications, while Cu-TiO2-Ag exhibits potential for virucidal activities. Moreover, a functionally graded lattice (FGL) structure, either in radial or longitudinal directions, offers enhanced advantages by allowing adjustability and control over porosity, roughness, strength, and material proportions within the composite.

In vitro fatigue behavior and in vivo osseointegration of the auxetic porous bone screw

The incidence of screw loosening, migration, and pullout caused by the insufficient screw-bone fixation stability is relatively high in clinical practice. To solve this issue, the auxetic unit-based porous bone screw (AS) has been put forward in our previous work. Its favorable auxetic effect can improve the primary screw-bone fixation stability after implantation. However, porous structure affected the fatigue behavior and in vivo longevity of bone screw. In this study, in vitro fatigue behaviors and in vivo osseointegration performance of the re-entrant unit-based titanium auxetic bone screw were studied. The tensile-tensile fatigue behaviors of AS and nonauxetic bone screw (NS) with the same porosity (51%) were compared via fatigue experiments, fracture analysis, and numerical simulation. The in vivo osseointegration of AS and NS were compared via animal experiment and biomechanical analysis. Additionally, the effects of in vivo dynamic tensile loading on the osseointegration of AS and NS were investigated and analyzed. The fatigue strength of AS was approximately 43% lower while its osseointegration performance was better than NS. Under in vivo dynamic tensile loading, the osseointegration of AS and NS both improved significantly, with the maximum increase of approximately 15%. Preferrable osseointegration of AS might compensate for the shortage of fatigue resistance, ensuring its long-term stability in vivo. Adequate auxetic effect and long-term stability of the AS was supposed to provide enough screw-bone fixation stability to overcome the shortages of the solid bone screw, developing the success of surgery and showing significant clinical application prospects in orthopedic surgery. STATEMENT OF SIGNIFICANCE: This research investigated the high-cycle fatigue behavior of re-entrant unit-based auxetic bone screw under tensile-tensile cyclic loading and its osseointegration performance, which has not been focused on in existing studies. The fatigue strength of auxetic bone screw was lower while the osseointegration was better than non-auxetic bone screw, especially under in vivo tensile loading. Favorable osseointegration of auxetic bone screw might compensate for the shortage of fatigue resistance, ensuring its long-term stability and longevity in vivo. This suggested that with adequate auxetic effect and long-term stability, the auxetic bone screw had significant application prospects in orthopedic surgery. Findings of this study will provide a theoretical guidance for design optimization and clinical application of the auxetic bone screw.

3D printing for bone regeneration: challenges and opportunities for achieving predictability

3D printing offers attractive opportunities for large-volume bone regeneration in the oro-dental and craniofacial regions. This is enabled by the development of CAD-CAM technologies that support the design and manufacturing of anatomically accurate meshes and scaffolds. This review describes the main 3D-printing technologies utilized for the fabrication of these patient-matched devices, and reports on their pre-clinical and clinical performance including the occurrence of complications for vertical bone augmentation and craniofacial applications. Furthermore, the regulatory pathway for approval of these devices is discussed, highlighting the main hurdles and obstacles. Finally, the review elaborates on a variety of strategies for increasing bone regeneration capacity and explores the future of 4D bioprinting and biodegradable metal 3D printing.

Porous metal implants: processing, properties, and challenges

Porous and functionally graded materials have seen extensive applications in modern biomedical devices-allowing for improved site-specific performance; their appreciable mechanical, corrosive, and biocompatible properties are highly sought after for lightweight and high-strength load-bearing orthopedic and dental implants. Examples of such porous materials are metals, ceramics, and polymers. Although, easy to manufacture and lightweight, porous polymers do not inherently exhibit the required mechanical strength for hard tissue repair or replacement. Alternatively, porous ceramics are brittle and do not possess the required fatigue resistance. On the other hand, porous biocompatible metals have shown tailorable strength, fatigue resistance, and toughness. Thereby, a significant interest in investigating the manufacturing challenges of porous metals has taken place in recent years. Past research has shown that once the advantages of porous metallic structures in the orthopedic implant industry have been realized, their biological and biomechanical compatibility-with the host bone-has been followed up with extensive methodical research. Various manufacturing methods for porous or functionally graded metals are discussed and compared in this review, specifically, how the manufacturing process influences microstructure, graded composition, porosity, biocompatibility, and mechanical properties. Most of the studies discussed in this review are related to porous structures for bone implant applications; however, the understanding of these investigations may also be extended to other devices beyond the biomedical field.

Influence of β-Stabilizer Element on Microstructure and Mechanical Behavior of Porous Titanium Alloy Synthesized by Liquid Metal Dealloying

The metallic implant materials for load-bearing applications typically possess a significantly higher stiffness when compared with that of human bone. In some cases, this stiffness mismatch leads to a stress-shielding effect and eventual loosing of the implant. Porous metallic materials are suitable candidates to overcome this problem. In this study, we synthesized low modulus open porous TiFe alloy by liquid metal dealloying of the precursor Ti47.5Fe2.5Cu50 (at.%) material in liquid Mg. Upon liquid metal dealloying, Cu was selectively dissolved from the precursor, and the remaining Ti and Fe elements were reorganized into a bicontinous porous structure. The synthesized TiFe alloy is composed of α-titanium and β-titanium phases. The average measured ligament size is in the micrometer range. It was found that a higher dealloying temperature leads to a pronounced coarsening of the microstructure. The open porous TiFe alloy possesses a low elastic modulus of about 6.4-6.9 GPa. At the same time, its yield strength value reaches about 185 MPa due to the α + β microstructure. Its attractive mechanical properties for biomedical applications, together with its open porous structure, indicate the potential of porous TiFe alloys to be used as implants.

An overview of 3D printed metal implants in orthopedic applications: Present and future perspectives

With the ability to produce components with complex and precise structures, additive manufacturing or 3D printing techniques are now widely applied in both industry and consumer markets. The emergence of tissue engineering has facilitated the application of 3D printing in the field of biomedical implants. 3D printed implants with proper structural design can not only eliminate the stress shielding effect but also improve in vivo biocompatibility and functionality. By combining medical images derived from technologies such as X-ray scanning, CT, MRI, or ultrasonic scanning, 3D printing can be used to create patient-specific implants with almost the same anatomical structures as the injured tissues. Numerous clinical trials have already been conducted with customized implants. However, the limited availability of raw materials for printing and a lack of guidance from related regulations or laws may impede the development of 3D printing in medical implants. This review provides information on the current state of 3D printing techniques in orthopedic implant applications. The current challenges and future perspectives are also included.

3D printed PEKK bone analogs with internal porosity and surface modification for mandibular reconstruction: An in vivo rabbit model study

Polyetheretherketone (PEEK) and its derivative polyetherketoneketone (PEKK) have been used as implant materials for spinal fusing and enjoyed their success for many years because of their mechanical properties similar to bone and their chemical inertness. The osseointegration of PEEKs is datable. Our strategy was to use custom-designed and 3D printed bone analogs with an optimized structure design and a modified PEKK surface to augment bone regeneration for mandibular reconstruction. Those bone analogs had internal porosities and a bioactive titanium oxide surface coating to promote osseointegration between native bone and PEKK analogs. Our workflow was 3D modeling, bone analog designing, structural optimization, mechanical analysis via finite element modeling, 3D printing of bone analogs and subsequently, an in vivo rabbit model study on mandibular reconstruction and histology evaluation. Our results showed the finite element analysis validated that the porous PEKK analogs provided a mechanical-sound structure for functional loadings. The bone analogs offered a perfect replacement for segmented bones in the terms of shape, form and volume for surgical reconstruction. The in vivo results showed that bioactive titanium oxide coating enhanced new bone in-growth into the porous PEKK analogs. We have validated our new approach in surgical mandibular reconstruction and we believe our strategy has a significant potential to improve mechanical and biological outcomes for patients who require mandibular reconstruction procedures.

Laser Powder Bed Fusion of Dissimilar Metal Materials: A Review

The laser powder bed fusion (LPBF) technique is used to manufacture complex and customised components by exploiting the unique advantages of two types of metal materials to meet specific performance requirements. A comprehensive overview of LPBF-processed dissimilar metal materials, a combination of different single metals or alloys, is developed. The microstructure in the fusion zone and the corresponding mechanical properties of LPBF-processed dissimilar metal materials are summarised. The influence of processing factors on the mechanism of defect formation, wetting properties and element diffusion behaviour at the interface between different materials and their typical cases are scientifically investigated in detail. Particular attention is paid to energy input, Marangoni convection and interfacial bonding behaviour. The underlying science of the metallurgical structure and properties of the LPBF-processed dissimilar metal materials is revealed. The build quality and efficiency could be further improved by designing machine structures and predicting the process-property relationship. This review provides a significant guide for expanding the industrial application of LPBF-processed dissimilar metal materials.

Study of the Effect of Anodic Oxidation on the Corrosion Properties of the Ti6Al4V Implant Produced from SLM

Additive technologies allowed for the development of medicine and implantology, enabling the production of personalized and highly porous implants. Although implants of this type are used clinically, they are usually only heat treated. Surface modification using electrochemical methods can significantly improve the biocompatibility of biomaterials used for implants, including printed ones. The study examined the effect of anodizing oxidation on the biocompatibility of a porous implant made of Ti6Al4V by the SLM method. The study used a proprietary spinal implant intended for the treatment of discopathy in the c4–c5 section. As part of the work, the manufactured implant was assessed in terms of compliance with the requirements for implants (structure testing—metallography) and the accuracy of the pores produced (pore size and porosity). The samples were subjected to surface modification using anodic oxidation. The research was carried out for 6 weeks in in vitro conditions. Surface topographies and corrosion properties (corrosion potential, ion release) were compared for unmodified and anodically oxidized samples. The tests showed no effect of anodic oxidation on the surface topography and improved corrosion properties. Anodic oxidation stabilized the corrosion potential and limited the release of ions to the environment.

The presence of 3D printing in orthopedics: A clinical and material review

The field of additive manufacturing, 3D printing (3DP), has experienced an exponential growth over the past four decades, in part due to increased accessibility. Developments including computer-aided design and manufacturing, incorporation of more versatile materials, and improved printing techniques/equipment have stimulated growth of 3DP technologies within various industries, but most specifically the medical field. Alternatives to metals including ceramics and polymers have been garnering popularity due to their resorbable properties and physiologic similarity to extracellular matrix. 3DP has the capacity to utilize an assortment of materials and printing techniques for a multitude of indications, each with their own associated benefits. Within the field of medicine, advances in medical imaging have facilitated the integration of 3DP. In particular, the field of orthopedics has been one of the earliest medical specialties to implement 3DP. Current indications include education for patients, providers, and trainees, in addition to surgical planning. Moreover, further possibilities within orthopedic surgery continue to be explored, including the development of patient-specific implants. This review aims to highlight the use of current 3DP technology and materials by the orthopedic community, and includes comments on current trends and future direction(s) within the field.

Improving Biocompatibility for Next Generation of Metallic Implants

The increasing need for joint replacement surgeries, musculoskeletal repairs, and orthodontics worldwide prompts emerging technologies to evolve with healthcare's changing landscape. Metallic orthopaedic materials have a shared application history with the aerospace industry, making them only partly efficient in the biomedical domain. However, suitability of metallic materials in bone tissue replacements and regenerative therapies remains unchallenged due to their superior mechanical properties, eventhough they are not perfectly biocompatible. Therefore, exploring ways to improve biocompatibility is the most critical step toward designing the next generation of metallic biomaterials. This review discusses methods of improving biocompatibility of metals used in biomedical devices using surface modification, bulk modification, and incorporation of biologics. Our investigation spans multiple length scales, from bulk metals to the effect of microporosities, surface nanoarchitecture, and biomolecules such as DNA incorporation for enhanced biological response in metallic materials. We examine recent technologies such as 3D printing in alloy design and storing surface charge on nanoarchitecture surfaces, metal-on-metal, and ceramic-on-metal coatings to present a coherent and comprehensive understanding of the subject. Finally, we consider the advantages and challenges of metallic biomaterials and identify future directions.

Enhanced bone tissue regeneration using a 3D-printed poly(lactic acid)/Ti6Al4V composite scaffold with plasma treatment modification

The mechanical and biological properties of polylactic acid (PLA) need to be further improved in order to be used for bone tissue engineering (BTE). Utilizing a material extrusion technique, three-dimensional (3D) PLA-Ti6Al4V (Ti64) scaffolds with open pores and interconnected channels were successfully fabricated. In spite of the fact that the glass transition temperature of PLA increased with the addition of Ti64, the melting and crystallization temperatures as well as the thermal stability of filaments decreased slightly. However, the addition of 3-6 wt% Ti64 enhanced the mechanical properties of PLA, increasing the ultimate compressive strength and compressive modulus of PLA-3Ti64 to 49.9 MPa and 1.9 GPa, respectively. Additionally, the flowability evaluations revealed that all composite filaments met the print requirements. During the plasma treatment of scaffolds, not only was the root-mean-square (Rq) of PLA (1.8 nm) increased to 60 nm, but also its contact angle (90.4°) significantly decreased to (46.9°). FTIR analysis confirmed the higher hydrophilicity as oxygen-containing groups became more intense. By virtue of the outstanding role of plasma treatment as well as Ti64 addition, a marked improvement was observed in Wharton's jelly mesenchymal stem cell attachment, proliferation (4',6-diamidino-2-phenylindole staining), and differentiation (Alkaline phosphatase and Alizarin Red S staining). Based on these results, it appears that the fabricated scaffolds have potential applications in BTE.

Porous Scaffolds Based on Polydopamine/Chondroitin Sulfate/Polyvinyl Alcohol Composite Hydrogels

In this paper, porous scaffolds based on composite hydrogels were fabricated using polydopamine (PDA), chondroitin sulfate (CS), and polyvinyl alcohol (PVA) via the freezing/thawing method. Different characteristics of the prepared composite hydrogels, including the pore sizes, compression strength, lap shear strength, mass loss, and cytocompatibility were investigated. Scanning electron microscope images (SEM) displayed the hydrogel pore sizes, ranging from 20 to 100 μm. The composite hydrogel exhibited excellent porosity of 95.1%, compression strength of 5.2 MPa, lap shear strength of 21 kPa on porcine skin, and mass loss of 16.0%. In addition, the composite hydrogel possessed good relative cell activity of 97%. The PDA/CS/PVA hydrogel is cytocompatible as a starting point, and it can be further investigated in tissue engineering.

Optimal microstructure and mechanical properties of open-cell porous titanium structures produced by selective laser melting

Three-dimensional printing technology enables the production of open cell porous structures. This has advantages but not only in terms of weight reduction. In implant structures, the process of osseointegration is improved, mechanical integration is better, the open cell porous structures resemble a trabecular structure that mimics bone tissue. In this work, we investigated titanium structures made porous by cutting spheres. Based on the patterns of different types of crystal models we created porosity with different strategies. We have shown that there are significant differences in mechanical properties between the porous structures formed with different strategies. We determined the structure that loses the least load-bearing capacity compared to the solid structure, with the same porosity levels and mechanical stresses. We characterized the possibility location and environment of becoming an open cell structure. We performed the calculations with mechanical simulations, which were validated experimentally. The quality of the three-dimensional printing of samples was checked by computed tomography reconstruction analysis.

Surgical Classification for Preclinical Rat Femoral Bone Defect Model: Standardization Based on Systematic Review, Anatomical Analysis and Virtual Surgery

Though surgical techniques profoundly influence in vivo experiments, significant heterogeneity exists in current surgeries for inducing rat femoral bone defects. Such variations reduce the reproducibility and comparability of preclinical studies, and are detrimental to clinical translation. The purposes of this study were: (1) to conduct a systematic review of rat femoral defect models, summarizing and analyzing the surgical techniques; (2) to analyze surgical design and potential pitfalls via 3D anatomy and virtual surgeries for fostering future precision research; and (3) to establish a surgical classification system, for improving the reproducibility and comparability among studies, avoiding unnecessary repetitive experiments. The online database PubMed was searched to identify studies from January 2000 to June 2022 using keywords, including rat, femur, bone defect. Eligible publications were included for a review of surgical methods. Anatomical analysis and virtual surgeries were conducted based on micro-CT reconstruction of the rat femur for further investigation and establishment of a classification system. A total of 545 publications were included, revealing marked heterogeneity in surgical methods. Four major surgical designs were reported for inducing defects from the proximal to distal femur: bone tunnel, cortical window, segmental defect, and wedge-shaped defect. Anatomical analysis revealed potential pitfalls hindering efficient clinical translation. A classification system was established according to the anatomical region, surgical design, and fixation devices. This systematic review in combination with 3D analysis and virtual surgery provides a general overview of current surgical approaches to inducing femoral defects in rats, and establishes a surgical classification facilitating preclinical research of quality and translational value.

A comparison of cemented and cementless intra-neck curved stem use during hip-preserving reconstruction following massive femoral malignant tumor removal

Background

Patients who undergo massive femoral malignant tumor (MFMT) resection often exhibit shortened femoral metaphyseal juxta-articular segments. The use of a customized femoral endoprosthesis (CFE) with an intra-neck curved stem (INCS) has emerged as a viable reconstructive surgical strategy for these individuals. Relative to a cemented INCS, it remains unclear as to whether cementless INCS use is associated with improvements in functionality or reconstructive longevity. As such, the present study was conducted to compare functional outcomes, endoprosthetic survival, and endoprosthesis-related complication rates in patients undergoing cemented and cementless INCS implantation.

Methods

A total of 24 patients undergoing lower limb salvage and reconstructive surgical procedures utilizing cemented or cementless INCS endoprostheses were retrospectively included. Patient-functional outcomes, endoprosthetic survival, and complication rates were compared as a function of age; diagnosis; the length of femoral resection; residual proximal femur length; Musculoskeletal Tumor Society (MSTS) scores; visual analog scale (VAS) scores; and the rates of implant breakage, periprosthetic infections, periprosthetic fractures, and aseptic loosening.

Results

The mean follow-up was 56 months. Significant differences in the length of femoral resection (p<0.001) and residual proximal femur length were observed (p<0.001) between the cemented and cementless INCS groups. There were no differences in overall patient survival and aseptic loosening-associated endoprosthesis survival in the cemented and cementless groups. None of the included patients experienced periprosthetic fractures, infections, or implant breakage. Average respective MSTS and VAS scores did not differ between groups.

Conclusion

For patients undergoing treatment for MFMTs, the use of a CFE with an INCS has emerged as a viable approach to hip-preserving reconstructive surgery. With appropriately designed individualized rehabilitative programs, good functional outcomes can be achieved for these endoprostheses, which are associated with low complication rates. Moreover, the selection between cemented or cementless INCS in the clinic should be made based on patient-specific factors, with cementless INCS implementation being preferable in younger patients with good-quality bone, the potential for long-term survival, and the osteotomy site near the lesser trochanter, whereas cemented INCS use should be favored for individuals who are older, have a shorter life expectancy, or have poor bone quality.

An in silico model predicts the impact of scaffold design in large bone defect regeneration

Large bone defects represent a clinical challenge for which the implantation of scaffolds appears as a promising strategy. However, their use in clinical routine is limited, in part due to a lack of understanding of how scaffolds should be designed to support regeneration. Here, we use the power of computer modeling to investigate mechano-biological principles behind scaffold-guided bone regeneration and the influence of scaffold design on the regeneration process. Computer model predictions are compared to experimental data of large bone defect regeneration in sheep. We identified two main key players in scaffold-guided regeneration: (1) the scaffold surface guidance of cellular migration and tissue formation processes and (2) the stimulation of progenitor cell activity by the scaffold material composition. In addition, lower scaffold surface-area-to-volume ratio was found to be beneficial for bone regeneration due to enhanced cellular migration. To a lesser extent, a reduced scaffold Young's modulus favored bone formation. STATEMENT OF SIGNIFICANCE: 3D-printed scaffolds offer promising treatment strategies for large bone defects but their broader clinical use requires a more thorough understanding of their interaction with the bone regeneration process. The predictions of our in silico model compared to two experimental set-ups highlighted the importance of (1) the scaffold surface guidance of cellular migration and tissue formation processes and (2) the scaffold material stimulation of progenitor cell activity. In addition, the model was used to investigate the effect on the bone regeneration process of (1) the scaffold surface-area-to-volume ratio, with lower ratios favoring more bone growth, and (2) the scaffold material properties, with stiffer scaffold materials yielding a lower bone growth.

Selective Laser Melting Fabrication of Porous Ti6Al4V Scaffolds With Triply Periodic Minimal Surface Architectures: Structural Features, Cytocompatibility, and Osteogenesis

The relationship between pore architecture and structure performance needs to be explored, as well as confirm the optimized porous structure. Because of the linear correlation between constant C and pore architecture, triply periodic minimal surface (TPMS) based porous structures could be a controllable model for the investigation of the optimized porous structure. In the present work, three types of TPMS porous scaffolds (S, D and G) combined with four constants (0.0, 0.2, 0.4 and 0.6) were designed, and built successfully via the selective laser melting (SLM) technology. The designed feature and mechanical property of porous scaffolds were investigated through mathematical method and compression test. And the manufactured samples were co-cultured with rMSCs for the compatibility study. The results indicated that the whole manufacturing procedure was good in controllability, repeatability, and accuracy. The linear correlation between the porosity of TPMS porous scaffolds and the constant C in equations was established. The different TPMS porous scaffolds possess the disparate feature in structure, mechanical property and cell compatibility. Comprehensive consideration of the structure features, mechanical property and biology performance, different TPMS structures should be applied in appropriate field. The results could guide the feasibility of apply the different TPMS architectures into the different part of orthopedic implants.

Additive Manufacturing in Orthopedics: A Review

Additive manufacturing is an advanced manufacturing manner that seems like the industrial revolution. It has the inborn benefit of producing complex formations, which are distinct from traditional machining technology. Its manufacturing strategy is flexible, including a wide range of materials, and its manufacturing cycle is short. Additive manufacturing techniques are progressively used in bone research and orthopedic operation as more innovative materials are developed. This Review lists the recent research results, analyzes the strengths and weaknesses of diverse three-dimensional printing strategies in orthopedics, and sums up the use of varying 3D printing strategies in surgical guides, surgical implants, surgical predictive models, and bone tissue engineering. Moreover, various postprocessing methods for additive manufacturing for orthopedics are described.

Mesh Ti6Al4V Material Manufactured by Selective Laser Melting (SLM) as a Promising Intervertebral Fusion Cage

Intervertebral cages made of Ti6Al4V alloy show excellent osteoconductivity, but also higher stiffness, compared to commonly used polyether-ether-ketone (PEEK) materials, that may lead to a stress-shielding effect and implant subsidence. In this study, a metallic intervertebral fusion cage, with improved mechanical behavior, was manufactured by the introduction of a three-dimensional (3D) mesh structure to Ti6Al4V material, using an additive manufacturing method. Then, the mechanical and biological properties of the following were compared: (1) PEEK, with a solid structure, (2) 3D-printed Ti6Al4V, with a solid structure, and (3) 3D-printed Ti6Al4V, with a mesh structure. A load-induced subsidence test demonstrated that the 3D-printed mesh Ti6Al4V cage had significantly lower tendency (by 15%) to subside compared to the PEEK implant. Biological assessment of the samples proved that all tested materials were biocompatible. However, both titanium samples (solid and mesh) were characterized by significantly higher bioactivity, osteoconductivity, and mineralization ability, compared to PEEK. Moreover, osteoblasts revealed stronger adhesion to the surface of the Ti6Al4V samples compared to PEEK material. Thus, it was clearly shown that the 3D-printed mesh Ti6Al4V cage possesses all the features for optimal spinal implant, since it carries low risk of implant subsidence and provides good osseointegration at the bone-implant interface.

Hip-Preserved Reconstruction Using a Customized Cementless Intercalary Endoprosthesis With an Intra-Neck Curved Stem in Patients With an Ultrashort Proximal Femur: Midterm Follow-Up Outcomes

Background: Hemiarthroplasty is widely used for proximal femoral reconstruction after tumor resection. However, complications of hemiarthroplasty include infection, hip dislocation, and acetabular wear. This study aimed to: (1) evaluate the reliability and validity of a customized cementless intercalary endoprosthesis (CCIE) with an intra-neck curved stem (INCS) to reconstruct femoral diaphyseal defects with an ultrashort proximal femur (UPF); (2) assess the lower extremity function after reconstruction with this endoprosthesis; and (3) identify the postoperative complications associated with the use of this endoprosthesis.

Methods: Between October 2015 and May 2019, 13 patients underwent reconstruction with a CCIE with an INCS. The distance from the center of the femoral head to the midline of the body and the apex of the acetabulum was measured preoperatively. Additionally, the distance from the tip of the INCS to the midline of the body and the apex of the acetabulum was measured postoperatively. The femoral neck–shaft angle was also measured pre- and postoperatively. After an average follow-up duration of 46 months, the radiological outcomes of the CCIE with an INCS were analyzed. Function was evaluated with the Musculoskeletal Tumor Society (MSTS) score. Pain was measured using a paper visual analog scale (VAS) pre- and postoperatively, and complications were recorded.

Results: Compared with our preoperative design, we found no significant difference in the postoperative distance from the tip of the INCS to the body midline (p = 0.187) and the apex of the acetabulum (p = 0.159), or in the postoperative femoral neck–shaft angle (p = 0.793). Thus, the INCS positions were deemed accurate. The average MSTS score was 26 (range: 24–28), and the VAS score was significantly decreased postoperatively compared with preoperatively (p < 0.0001). No patients developed aseptic loosening, infection, periprosthetic fracture, or prosthetic fracture as of the last follow-up.

Conclusion: The CCIE with an INCS was a valid and reliable method for reconstructing femoral diaphyseal defects with a UPF following malignant tumor resection. Postoperative lower extremity function was acceptable, with an appropriate individualized rehabilitation program, and the incidence of complications was low.

3D-printed porous Ti6Al4V scaffolds for long bone repair in animal models: a systematic review

BACKGROUND

Titanium and its alloys have been widely employed for bone tissue repair and implant manufacturing. The rapid development of three-dimensional (3D) printing technology has allowed fabrication of porous titanium scaffolds with controllable microstructures, which is considered to be an effective method for promoting rapid bone formation and decreasing bone absorption. The purpose of this systematic review was to evaluate the osteogenic potential of 3D-printed porous Ti6Al4V (Ti64) scaffold for repairing long bone defects in animal models and to investigate the influential factors that might affect its osteogenic capacity.

METHODS

Electronic literature search was conducted in the following databases: PubMed, Web of Science, and Embase up to September 2021. The SYRCLE's tool and the modified CAMARADES list were used to assess the risk of bias and methodological quality, respectively. Due to heterogeneity of the selected studies in relation to protocol and outcomes evaluated, a meta-analysis could not be performed.

RESULTS

The initial search revealed 5858 studies. Only 46 animal studies were found to be eligible based on the inclusion criteria. Rabbit was the most commonly utilized animal model. A pore size of around 500-600 µm and porosity of 60-70% were found to be the most ideal parameters for designing the Ti64 scaffold, where both dodecahedron and diamond pores optimally promoted osteogenesis. Histological analysis of the scaffold in a rabbit model revealed that the maximum bone area fraction reached 59.3 ± 8.1% at weeks 8-10. Based on micro-CT assessment, the maximum bone volume fraction was found to be 34.0 ± 6.0% at weeks 12.

CONCLUSIONS

Ti64 scaffold might act as a promising medium for providing sufficient mechanical support and a stable environment for new bone formation in long bone defects. Trail registration The study protocol was registered in the PROSPERO database under the number CRD42020194100.

Biodegradable magnesium alloy WE43 porous scaffolds fabricated by laser powder bed fusion for orthopedic applications: Process optimization, in vitro and in vivo investigation

Laser powder bed fusion (L-PBF) of magnesium (Mg) alloy porous scaffolds is expected to solve the dual challenges from customized structures and biodegradable functions required for repairing bone defects. However, one of the key technical difficulties lies in the poor L-PBF process performance of Mg, contributed by the high susceptibility to oxidation, vaporization, thermal expansion, and powder attachment etc. This work investigated the influence of L-PBF energy input and scanning strategy on the formation quality of porous scaffolds by using WE43 powder, and characterized the microstructure, mechanical properties, biocompatibility, biodegradation and osteogenic effect of the as-built WE43 porous scaffolds. With the customized energy input and scanning strategy, the relative density of struts reached over 99.5%, and the geometrical error between the designed and the fabricated porosity declined to below 10%. Massive secondary phases including intermetallic precipitates and oxides were observed. The compressive strength (4.37–23.49 MPa) and elastic modulus (154.40–873.02 MPa) were comparable to those of cancellous bone. Good biocompatibility was observed by in vitro cell viability and in vivo implantation. The biodegradation of as-built porous scaffolds promoted the osteogenic effect, but the structural integrity devastated after 12 h by the immersion tests in Hank's solution and after 4 weeks by the implantation in rabbits' femur, indicating an excessively rapid degradation rate.

•

In vitro and in vivo investigations were performed on WE43 porous scaffolds.

•

Reliable fusion quality and dimensional accuracy were achieved.

•

The compressive strength and Young modulus ranged 4.37–23.49 and 154.40–873.02 MPa.

•

Good biocompatibility and improved osteogenic effect were observed.

•

The massive secondary phases as well as the enlarged specific surface resulted to a rapid degradation rate.

The contribution of pore size and porosity of 3D printed porous titanium scaffolds to osteogenesis

Porous titanium implants were popularly fabricated to promote bone formation. A desirable porous scaffold was recommended to be with porosity of >60% or/and pore size of >300 μm for better osteointegration. However, whether the pore size and porosity could be randomly selected within the recommended values? And what is the correlation between pore size and porosity for accelerating osteointegration? In this study, porous titanium with cubic cell structure was produced by selective laser melting. The designed porosities of scaffolds with 700-μm pore size were 40%, 70% and 90%; and the pore sizes of scaffolds with 70% porosity were 400, 700 and 900 μm. The in vitro osteogenic potential and in vivo bone formation were investigated. Results showed that porosity and pore size could be tuned by altering strut size, which was further directly responsible for mechanical properties. Besides, pore size and porosity synergistically contributed to osteogenic activity in vitro and new bone formation in vivo. In regard to pore sizes herein, the optimized one for better osteogenic response and bone forming ability was ~600-700 μm (p70). Too smaller or too larger pore size might more or less hinder cellular behaviors and bone regeneration, even if both pore size (300-900 μm) and porosity (70%) were within the recommended value range. At a constant pore size (~600-700 μm), p70 and p90 with higher porosity was more conductive to biological effects, compared with p40. As a result, pore-size variation revealed more significant influence on osteogenesis, compared with variation of porosity within recommended values. However, the applicable porosity within recommended values should be designed with the consideration of specific load-bearing conditions. This study helps to provide guidance for designing porous scaffolds with appropriate mechanical strengths and effective bone-forming ability, so as to develop better custom-made bone substitutes.

Comprehensive Review on Design and Manufacturing of Bio-scaffolds for Bone Reconstruction

Bio-scaffolds are synthetic entities widely employed in bone and soft-tissue regeneration applications. These bio-scaffolds are applied to the defect site to provide support and favor cell attachment and growth, thereby enhancing the regeneration of the defective site. The progressive research in bio-scaffold fabrication has led to identification of biocompatible and mechanically stable materials. The difficulties in obtaining grafts and expenditure incurred in the transplantation procedures have also been overcome by the implantation of bio-scaffolds. Drugs, cells, growth factors, and biomolecules can be embedded with bio-scaffolds to provide localized treatments. The right choice of materials and fabrication approaches can help in developing bio-scaffolds with required properties. This review mostly focuses on the available materials and bio-scaffold techniques for bone and soft-tissue regeneration application. The first part of this review gives insight into the various classes of biomaterials involved in bio-scaffold fabrication followed by design and simulation techniques. The latter discusses the various additive, subtractive, hybrid, and other improved techniques involved in the development of bio-scaffolds for bone regeneration applications. Techniques involving multimaterial printing and multidimensional printing have also been briefly discussed.

Femoral Stems With Porous Lattice Structures: A Review

Cementless femoral stems are prone to stress shielding of the femoral bone, which is caused by a mismatch in stiffness between the femoral stem and femur. This can cause bone resorption and resultant loosening of the implant. It is possible to reduce the stress shielding by using a femoral stem with porous structures and lower stiffness. A porous structure also provides a secondary function of allowing bone ingrowth, thus improving the long-term stability of the prosthesis. Furthermore, due to the advent of additive manufacturing (AM) technology, it is possible to fabricate femoral stems with internal porous lattices. Several review articles have discussed porous structures, mainly focusing on the geometric design, mechanical properties and influence on bone ingrowth. However, the safety and effectiveness of porous femoral stems depend not only on the characteristic of porous structure but also on the macro design of the femoral stem; for example, the distribution of the porous structure, the stem geometric shape, the material, and the manufacturing process. This review focuses on porous femoral stems, including the porous structure, macro geometric design of the stem, performance evaluation, research methods used for designing and evaluating the femoral stems, materials and manufacturing techniques. In addition, this review will evaluate whether porous femoral stems can reduce stress shielding and increase bone ingrowth, in addition to analyzing their shortcomings and related risks and providing ideas for potential design improvements.

Data related to architectural bone parameters and the relationship to Ti lattice design for powder bed fusion additive manufacturing

The data included in this article provides additional supporting information on our publication (McGregor et al. [1]) on the review of the natural lattice architecture in human bone and its implication towards titanium (Ti) lattice design for laser powder bed fusion and electron beam powder bed fusion. For this work, X-ray computed tomography was deployed to understand and visualize a Ti-6Al-4V lattice structure manufactured by laser powder bed fusion. This manuscript includes details about the manufacturing of the lattice structure using laser powder bed fusion and computed tomography methods used for analyzing the lattice structure. Additionally, a comprehensive literature review was conducted to understand how lattice parameters are controlled in additively manufactured Ti and Ti-alloy parts aimed at replacing or augmenting human bone. From this literature review, lattice design information was collected and is summarized in tabular form in this manuscript.

CAD/CAM scaffolds for bone tissue engineering: investigation of biocompatibility of selective laser melted lightweight titanium

The objective of the current in-vitro study was to evaluate the biocompatibility of a new type of CAD/CAM scaffold for bone tissue engineering by using human cells. Porous lightweight titanium scaffolds and Bio-Oss® scaffolds as well as their eluates were used for incubation with human osteoblasts, fibroblasts and osteosarcoma cells. The cell viability was assessed by using fluorescein diazo-acetate propidium iodide staining. Cell proliferation and metabolism was examined by using MTT-, WST-Test and BrdU-ELISA tests. Scanning electron microscope was used for investigation of the cell adhesion behaviour. The number of devitalised cells in all treatment groups did not significantly deviate from the control group. According to MTT and WST results, the number of metabolically active cells was decreased by the eluates of both test groups with a more pronounced impact of the eluate from Bio-Oss®. The proliferation of the cells was inhibited by the addition of the eluates. Both scaffolds showed a partial surface coverage after 1 week and an extensive to complete coverage after 3 weeks. The CAD/CAM titanium scaffolds showed favourable biocompatibility compared to Bio-Oss® scaffolds in vitro. The opportunity of a defect-specific design and rapid prototyping by selective laser melting are relevant advantages in the field of bone tissue engineering and regenerative medicine.

The three-dimensional shape symmetry of the lunate and its implications

We studied the three-dimensional (3-D) shape variations and symmetry of the lunate to evaluate whether a contralateral shape-based approach to design patient-specific implants for treatment of Kienböck's disease is accurate. A 3-D statistical shape model of the lunate was built using the computed tomography scans of 54 lunate pairs and shape symmetry was evaluated based on an intraclass correlation analysis. The lunate shape was not bilaterally symmetrical in (1) the angle scaphoid surface - radius-ulna surface, (2) the dorsal side and the length of the side adjacent to the triquetrum, (3) the orientation of the volar surface, (4) the width of the side adjacent to the scaphoid, (5) the skewness in the coronal plane and (6) the curvature of bone articulating with the hamate and capitate. These findings suggest that using the contralateral lunate to design patient-specific lunate implants may not be as accurate as it is intended.

Nature-inspired materials and structures using 3D Printing

Emulating the unique combination of structural, compositional, and functional gradation in natural materials is exceptionally challenging. Many natural structures have proved too complex or expensive to imitate using traditional processing techniques despite recent advances. Recent innovations within the field of additive manufacturing (AM) or 3D Printing (3DP) have shown the ability to create structures that have variations in material composition, structure, and performance, providing a new design-for-manufacturing platform for the imitation of natural materials. AM or 3DP techniques are capable of manufacturing structures that have significantly improved properties and functionality over what could be traditionally-produced, giving manufacturers an edge in their ability to realize components for highly-specialized applications in different industries. To this end, the present work reviews fundamental advances in the use of naturally-inspired design enabled through 3DP / AM, how these techniques can be further exploited to reach new application areas, and the challenges that lie ahead for widespread implementation. An example of how these techniques can be applied towards a total hip arthroplasty application is provided to spur further innovation in this area.

3D Printing of Micro- and Nanoscale Bone Substitutes: A Review on Technical and Translational Perspectives

Recent developments in three-dimensional (3D) printing technology offer immense potential in fabricating scaffolds and implants for various biomedical applications, especially for bone repair and regeneration. As the availability of autologous bone sources and commercial products is limited and surgical methods do not help in complete regeneration, it is necessary to develop alternative approaches for repairing large segmental bone defects. The 3D printing technology can effectively integrate different types of living cells within a 3D construct made up of conventional micro- or nanoscale biomaterials to create an artificial bone graft capable of regenerating the damaged tissues. This article reviews the developments and applications of 3D printing in bone tissue engineering and highlights the numerous conventional biomaterials and nanomaterials that have been used in the production of 3D-printed scaffolds. A comprehensive overview of the 3D printing methods such as stereolithography (SLA), selective laser sintering (SLS), fused deposition modeling (FDM), and ink-jet 3D printing, and their technical and clinical applications in bone repair and regeneration has been provided. The review is expected to be useful for readers to gain an insight into the state-of-the-art of 3D printing of bone substitutes and their translational perspectives.

Regenerating Critical Size Rat Segmental Bone Defects with a Self-Healing Hybrid Nanocomposite Hydrogel: Effect of Bone Condition and BMP-2 Incorporation

The aim of the current study is to assess the biological performance of self-healing hydrogels based on calcium phosphate (CaP) nanoparticles and bisphosphonate (BP) conjugated hyaluronan (HA) in a critical size segmental femoral bone defect model in rats. Additionally, these hydrogels are loaded with bone morphogenetic protein 2 (BMP-2) and their performance is compared in healthy and osteoporotic bone conditions. Treatment groups comprise internal plate fixation and placement of a PTFE tube containing hydrogel (HA_BP -CaP) or hydrogel loaded with BMP-2 in two dosages (HA_BP -CaP-lowBMP2 or HA_BP -CaP-highBMP2). Twelve weeks after bone defect surgery, bone formation is analyzed by X-ray examination, micro-CT analysis, and histomorphometry. The data show that critical size, segmental femoral bone defects cannot be healed with HA_BP -CaP gel alone. Loading of the HA_BP -CaP gel with low dose BMP-2 significantly improve bone formation and resulted in defect bridging in 100% of the defects. Alternatively, high dose BMP-2 loading of the HA_BP -CaP gel does not improve bone formation within the defect area, but leads to excessive bone formation outside the defect area. Bone defect healing is not affected by osteoporotic bone conditions.

The Clinical Application of Porous Tantalum and Its New Development for Bone Tissue Engineering

Porous tantalum (Ta) is a promising biomaterial and has been applied in orthopedics and dentistry for nearly two decades. The high porosity and interconnected pore structure of porous Ta promise fine bone ingrowth and new bone formation within the inner space, which further guarantee rapid osteointegration and bone-implant stability in the long term. Porous Ta has high wettability and surface energy that can facilitate adherence, proliferation and mineralization of osteoblasts. Meanwhile, the low elastic modulus and high friction coefficient of porous Ta allow it to effectively avoid the stress shield effect, minimize marginal bone loss and ensure primary stability. Accordingly, the satisfactory clinical application of porous Ta-based implants or prostheses is mainly derived from its excellent biological and mechanical properties. With the advent of additive manufacturing, personalized porous Ta-based implants or prostheses have shown their clinical value in the treatment of individual patients who need specially designed implants or prosthesis. In addition, many modification methods have been introduced to enhance the bioactivity and antibacterial property of porous Ta with promising in vitro and in vivo research results. In any case, choosing suitable patients is of great importance to guarantee surgical success after porous Ta insertion.

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.

Functional repair of critically sized femoral defects treated with bioinspired titanium gyroid-sheet scaffolds

Despite the innate ability for bone to remodel and repair, its regeneration has a limit. In these cases of critically sized bone defects (CSBD), the bone deficit must be repaired using reconstructive techniques that support immediate load bearing and encourage bone bridging across the defect. High-strength porous titanium implants offer a solution for treatment of CSBD in which the scaffold can support physiological loads, provide a matrix to guide ingrowth, and carry graft materials and/or biologics. Fabrication of titanium meta-materials via additive manufacturing (AM) has unlocked the potential to modulate mechanical and biological performance to achieve a combination of properties previously unachievable. Meta-material scaffolds with topology based on triply periodic minimal surfaces (TPMS) have gained increasing interest for use in biomedical applications due to their bioinspired nature. Despite enthusiasm for TPMS-based titanium scaffolds due to their high strength to stiffness ratio, high permeability, and curvature similar to trabecular bone, there is little preclinical evidence to support their in vivo response in bone. The present study sought to evaluate the performance of gyroid-sheet titanium scaffolds produced via AM to repair a critically size femoral cortical bone defect in rats. Empty gyroid-sheet scaffolds were shown to repair segmental defects with up to 38% of torsional strength and 54% torsional stiffness of the intact femur (control) at 12-weeks. Gyroid-sheet scaffolds carrying recombinant bone morphogenic protein-2 demonstrated bridging bone growth across the length of the defect, with torsional strength and stiffness superior to that of the intact controls.

A systematic review of preclinical in vivo testing of 3D printed porous Ti6Al4V for orthopedic applications, part I: Animal models and bone ingrowth outcome measures

For Ti6Al4V orthopedic and spinal implants, osseointegration is often achieved using complex porous geometries created via additive manufacturing (AM). While AM porous titanium (pTi) has shown clinical success, concerns regarding metallic implants have spurred interest in alternative AM biomaterials for osseointegration. Insights regarding the evaluation of these new materials may be supported by better understanding the role of preclinical testing for AM pTi. We therefore asked: (a) What animal models have been most commonly used to evaluate AM porous Ti6Al4V for orthopedic bone ingrowth; (b) What were the primary reported quantitative outcome measures for these models; and (c) What were the bone ingrowth outcomes associated with the most frequently used models? We performed a systematic literature search and identified 58 articles meeting our inclusion criteria. We found that AM pTi was evaluated most often using rabbit and sheep femoral condyle defect (FCD) models. Additional ingrowth models including transcortical and segmental defects, spinal fusions, and calvarial defects were also used with various animals based on the study goals. Quantitative outcome measures determined via histomorphometry including ''bone ingrowth'' (range: 3.92-53.4% for rabbit/sheep FCD) and bone-implant contact (range: 9.9-59.7% for rabbit/sheep FCD) were the most common. Studies also used 3D imaging to report outcomes such as bone volume fraction (BV/TV, range: 4.4-61.1% for rabbit/sheep FCD), and push-out testing for outcomes such as maximum removal force (range: 46.6-3092 N for rabbit/sheep FCD). Though there were many commonalities among the study methods, we also found significant heterogeneity in the outcome terms and definitions. The considerable diversity in testing and reporting may no longer be necessary considering the reported success of AM pTi across all model types and the ample literature supporting the rabbit and sheep as suitable small and large animal models, respectively. Ultimately, more standardized animal models and reporting of bone ingrowth for porous AM materials will be useful for future studies.

Chitosan/hydroxyapatite composite coatings on porous Ti6Al4V titanium implants: in vitro and in vivo studies

Purpose

Titanium implants are widely used in the treatment of dentition defects; however, due to problems such as osseointegration failure, peri-implant bone resorption, and peri-implant inflammation, their application is subject to certain restrictions. The surface modification of titanium implants can improve the implant success rate and meet the needs of clinical applications. The goal of this study was to evaluate the effect of the use of porous titanium with a chitosan/hydroxyapatite coating on osseointegration.

Methods

Titanium implants with a dense core and a porous outer structure were prepared using a computer-aided design model and selective laser sintering technology, with a fabricated chitosan/hydroxyapatite composite coating on their surfaces. In vivo and in vitro experiments were used to assess osteogenesis.

Results

The quasi-elastic gradient and compressive strength of porous titanium implants were observed to decrease as the porosity increased. The in vitro experiments demonstrated that, the porous titanium implants had no biological toxicity; additionally, the porous structure was shown to be superior to dense titanium with regard to facilitating the adhesion and proliferation of osteoblast-like MC3T3-E1 cells. The in vivo experimental results also showed that the porous structure was beneficial, as bone tissue could grow into the pores, thereby exhibiting good osseointegration.

Conclusions

Porous titanium with a chitosan/hydroxyapatite coating promoted MC3T3-E1 cell proliferation and differentiation, and also improved osseointegration in vitro. This study has meaningful implications for research into ways of improving the surface structures of implants and promoting implant osseointegration.

Complete Osseointegration of a Retrieved 3-D Printed Porous Titanium Cervical Cage

Introduction: Porous 3D-printed titanium has only recently been introduced for spinal applications. Evidence around its use is currently limited to animal studies and only few human case series. This study describes the histological findings of a retrieved EIT cervical cage, explanted 2 years after insertion.

Materials and Methods: The patient underwent a double level C4/C5 & C5/C6 anterior cervical decompression using EIT cervical cages without an anterior plate. Two years later the C6/7 level degenerated and began to cause myelopathic symptoms. In order to address the kyphotic imbalance of the cervical spine and fix the C6/7 level, the surgeon decided to remove the C5/6 cervical cage and bridge the fusion from C4 to C7 inclusive. The retrieved cage was histologically evaluated for bone ingrowth and signs of inflammation.

Results: MRI demonstrated spinal canal stenosis at C6/C7. Plain radiographs confirmed well-integrated cervical cages at 2 years postoperative. The peroperative surgical need to use a chisel to remove the implant at C5/C6 reconfirmed the solid fusion of the segment. Macroscopically white tissue, indicative of bone, was present at both superior and inferior surfaces of the explanted specimen. Histological evaluation revealed complete osseointegration of the 5 mm high EIT Cellular Titanium^® cervical cage, displaying mature lamellar bone in combination with bone marrow throughout the cage. Furthermore, a pattern of trabecular bone apposition (without fibrous tissue interface) and physiological remodeling activity was observed directly on the cellular titanium scaffold.

Conclusion: This histological retrieval study of a radiologically fused cervical EIT cage clearly demonstrates complete osseointegration within a 2-year time frame. The scaffold exhibits a bone in growth pattern and maturation of bone tissue similar of what has been demonstrated in animal studies evaluating similar porous titanium implants. The complete osseointegration throughout the cage indicates physiological loading conditions even in the central part of the cage. This pattern suggests the absence, or at least the minimization, of stress-shielding in this type of porous titanium cage.

3D-printed Patient-specific Spine Implants: A Systematic Review

STUDY DESIGN

Systematic review.

OBJECTIVE

To review the current clinical use of 3-dimensional printed (3DP) patient-specific implants in the spine.

SUMMARY OF BACKGROUND DATA

Additive manufacturing is a transformative manufacturing method now being applied to spinal implants. Recent innovations in technology have allowed the production of medical-grade implants with unprecedented structure and customization, and the complex anatomy of the spine is ideally suited for patient-specific devices. Improvement in implant design through the process of 3DP may lead to improved osseointegration, lower subsidence rates, and faster operative times.

METHODS

A comprehensive search of the literature was conducted using Ovid MEDLINE, EMBASE, Scopus, and other sources that resulted in 1842 unique articles. All manuscripts describing the use of 3DP spinal implants in humans were included. Two independent reviewers (N.W. and N.E.S.) assessed eligibility for inclusion. The following outcomes were collected: pain score, Japanese Orthopedic Association (JOA) score, subsidence, fusion, Cobb angle, vertebral height, and complications. No conflicts of interest existed. No funding was received for this work.

RESULTS

A total of 17 studies met inclusion criteria with a total of 35 patients. Only case series and case reports were identified. Follow-up times ranged from 3 to 36 months. Implant types included vertebral body replacement cages, interbody cages, sacral reconstruction prostheses, iliolumbar rods, and a posterior cervical plate. All studies reported improvement in both clinical and radiographic outcomes. 11 of 35 cases showed subsidence >3 mm, but only 1 case required a revision procedure. No migration, loosening, or pseudarthrosis occurred in any patient on the basis of computed tomography or flexion-extension radiographs.

CONCLUSIONS

Results of the systematic review indicate that 3DP technology is a viable means to fabricate patient-matched spinal implants. The effects on clinical and radiographic outcome measures are still in question, but these devices may produce favorable subsidence and pseudoarthrosis rates. Currently, the technology is ideally suited for complex tumor pathology and atypical bone defects. Future randomized controlled trials and cost analyses are still needed.

LEVEL OF EVIDENCE

IV-systematic review.

The Use of Simulated Body Fluid (SBF) for Assessing Materials Bioactivity in the Context of Tissue Engineering: Review and Challenges

Some special implantable materials are defined as “bioactive” if they can bond to living bone, forming a tight and chemically-stable interface. This property, which is inherent to some glass compositions, or can be induced by applying appropriate surface treatments on otherwise bio-inert metals, can be evaluated in vitro by immersion studies in simulated body fluid (SBF), mimicking the composition of human plasma. As a result, apatite coating may form on the material surface, and the presence of this bone-like “biomimetic skin” is considered predictive of bone-bonding ability in vivo. This review article summarizes the story and evolution of in vitro bioactivity testing methods using SBF, highlighting the influence of testing parameters (e.g., formulation and circulation of the solution) and material-related parameters (e.g., composition, geometry, texture). Suggestions for future methodological refinements are also provided at the end of the paper.

Exploring Macroporosity of Additively Manufactured Titanium Metamaterials for Bone Regeneration with Quality by Design: A Systematic Literature Review

Additive manufacturing facilitates the design of porous metal implants with detailed internal architecture. A rationally designed porous structure can provide to biocompatible titanium alloys biomimetic mechanical and biological properties for bone regeneration. However, increased porosity results in decreased material strength. The porosity and pore sizes that are ideal for porous implants are still controversial in the literature, complicating the justification of a design decision. Recently, metallic porous biomaterials have been proposed for load-bearing applications beyond surface coatings. This recent science lacks standards, but the Quality by Design (QbD) system can assist the design process in a systematic way. This study used the QbD system to explore the Quality Target Product Profile and Ideal Quality Attributes of additively manufactured titanium porous scaffolds for bone regeneration with a biomimetic approach. For this purpose, a total of 807 experimental results extracted from 50 different studies were benchmarked against proposed target values based on bone properties, governmental regulations, and scientific research relevant to bone implants. The scaffold properties such as unit cell geometry, pore size, porosity, compressive strength, and fatigue strength were studied. The results of this study may help future research to effectively direct the design process under the QbD system.

Additively manufactured biodegradable porous metals

Partially due to the unavailability of ideal bone substitutes, the treatment of large bony defects remains one of the most important challenges of orthopedic surgery. Additively manufactured (AM) biodegradable porous metals that have emerged since 2018 provide unprecedented opportunities for fulfilling the requirements of an ideal bone implant. First, the multi-scale geometry of these implants can be customized to mimic the human bone in terms of both micro-architecture and mechanical properties. Second, a porous structure with interconnected pores possesses a large surface area, which is favorable for the adhesion and proliferation of cells and, thus, bony ingrowth. Finally, the freeform geometrical design of such biomaterials could be exploited to adjust their biodegradation behavior so as to maintain the structural integrity of the implant during the healing process while ensuring that the implant disappears afterwards, paving the way for full bone regeneration. While the AM biodegradable porous metals that have been studied so far have shown many unique properties as compared to their solid counterparts, the unprecedented degree of flexibility in their geometrical design has not yet been fully exploited to optimize their properties and performance. In order to develop the ideal bone implants, it is important to take advantage of the full potential of AM biodegradable porous metals through detailed and systematic study on their biodegradation behavior, mechanical properties, biocompatibility, and bone regeneration performance. This review paper presents the state of the art in AM biodegradable porous metals and is focused on the effects of material type, processing, geometrical design, and post-AM treatments on the mechanical properties, biodegradation behavior, in vitro biocompatibility, and in vivo bone regeneration performance of AM porous Mg, Fe, and Zn as well as their alloys. We also identify a number of knowledge gaps and the challenges encountered in adopting AM biodegradable porous metals for orthopedic applications and suggest some promising areas for future research.

Tissue Integration and Biological Cellular Response of SLM-Manufactured Titanium Scaffolds

Background: SLM (Selective Laser Melting)–manufactured Titanium (Ti) scaffolds have a significant value for bone reconstructions in the oral and maxillofacial surgery field. While their mechanical properties and biocompatibility have been analysed, there is still no adequate information regarding tissue integration. Therefore, the aim of this study is a comprehensive systematic assessment of the essential parameters (porosity, pore dimension, surface treatment, shape) required to provide the long-term performance of Ti SLM medical implants. Materials and methods: A systematic literature search was conducted via electronic databases PubMed, Medline and Cochrane, using a selection of relevant search MeSH terms. The literature review was conducted using the preferred reporting items for systematic reviews and meta-analysis (PRISMA). Results: Within the total of 11 in vitro design studies, 9 in vivo studies, and 4 that had both in vitro and in vivo designs, the results indicated that SLM-generated Ti scaffolds presented no cytotoxicity, their tissue integration being assured by pore dimensions of 400 to 600 µm, high porosity (75–88%), hydroxyapatite or SiO2–TiO2 coating, and bioactive treatment. The shape of the scaffold did not seem to have significant importance. Conclusions: The SLM technique used to fabricate the implants offers exceptional control over the structure of the base. It is anticipated that with this technique, and a better understanding of the physical interaction between the scaffold and bone tissue, porous bases can be tailored to optimize the graft’s integrative and mechanical properties in order to obtain structures able to sustain osseous tissue on Ti.

Promotion of Osseointegration between Implant and Bone Interface by Titanium Alloy Porous Scaffolds Prepared by 3D Printing

Titanium alloy prostheses have been widely used for the treatment of orthopedic diseases, in which the interconnected porosity and appropriate pore size are crucial for the osseointegration capacity. Three-dimensional (3D) printing technology provides an efficient method to construct prosthesis scaffolds with controllable internal and surface structure, but printing high-porosity (>60%) scaffolds with pore diameters below 300 μm as implants structures has not yet been studied. In this work, four types of titanium alloy scaffolds with interconnected porosity more than 70% were successfully prepared by selective laser melting (SLM). The actual mean pore sizes of cylindrical scaffolds are 542, 366, 202, and 134 μm. Through the in vitro characterization of the scaffolds, in vivo experiments, and mechanical experiments, it is concluded that as the scaffold pore diameter decreases, the titanium alloy scaffold with diameter of 202 μm has the strongest osseointegration ability and is also the most stable one with the surrounding bone. These findings provide a reference for the clinical pore-size design of porous scaffolds with optimal bone growth stability on the surface of the titanium alloy implant.