Biomechanical characteristics of rib fracture fixation systems.

BACKGROUND: The primary aim of this study was to determine and compare the biomechanical properties of a fractured or intact rib after implant fixation on an embalmed thorax. METHODS: Five systems were fixated on the bilateral fractured or intact (randomly allocated) 6th to 10th rib of five post-mortem embalmed human specimens. Each rib underwent a four-point bending test to determine the bending structural stiffness (Newton per m2), load to failure (Newton), failure mode, and the relative difference in bending structural stiffness and load to failure as compared to a non-fixated intact rib. FINDINGS: As compared to a non-fixated intact rib, the relative difference in stiffness of a fixated intact rib ranged from -0.14 (standard deviation [SD], 0.10) to 0.53 (SD 0.35) and for a fixated fractured rib from -0.88 (SD 0.08) to 0.17 (SD 0.50). The most common failure mode was a new fracture at the most anterior drill hole for the plate and screw systems and a new fracture within the anterior portion of the implant for the clamping systems. INTERPRETATION: The current fixation systems differ in their design, mode of action, and biomechanical properties. Differences in biomechanical properties such as stiffness and load to failure especially apply to fractured ribs. Insight in the differences between the systems might guide more specific implant selection and increase the surgeon's awareness for localizing hardware complaints or failure.

Nuclear factor erythroid 2-related factor 2 (Nrf2) deficiency causes age-dependent progression of female osteoporosis.

BACKGROUND: Nuclear factor erythroid 2-related factor 2 (Nrf2) is a crucial transcription factor for cellular redox homeostasis. The association of Nrf2 with elderly female osteoporotic has yet to be fully described. The aim was to elucidate a potential age-dependent Nrf2 contribution to female osteoporosis in mice. METHODS: Eighteen female wild type (WT) and 16 Nrf2-knockout (KO) mice were sacrificed at different ages (12 weeks = young mature adult and 90 weeks = old) to analyze their femurs. The morphological properties (trabecular and cortical) were evaluated by micro-computed tomography (µCT) and compared to gold standard histochemistry analysis. The quasi-static compression tests were performed to calculate the mechanical properties of bones. Additionally, the population of bone resorbing cells and aromatase expression by osteocytes was immunohistochemically evaluated and empty osteocyte lacunae was counted in cortical bone. RESULTS: Old Nrf2-KO mice revealed a significantly reduced trabecular bone mineral density (BMD), cortical thickness, cortical area, and bone fraction compared to old WT mice, regardless of no significant difference in skeletally mature young adult mice between WT and KO. Specifically, while all old WT mice showed thin metaphyseal trabeculae, trabecular bone was completely absent in 60% of old KO mice. Additionally, old KO mice showed significantly more osteoclast-like cells and fewer aromatase-positive osteocytes than WT mice, whereas the occurrence of empty osteocyte lacunae did not differ between both groups. Nrf2-KO mice further showed an age-dependently reduced fracture resilience compared to age-matched WT mice. CONCLUSION: Our results suggest that chronic Nrf2 loss can lead to age-dependent progression of female osteoporosis.

Surface-treated 3D printed Ti-6Al-4V scaffolds with enhanced bone regeneration performance: an in vivo study.

BACKGROUND: Given their highly adjustable and predictable properties, three-dimensional(3D) printed geometrically ordered porous biomaterials offer unique opportunities as orthopedic implants. The performance of such biomaterials is, however, as much a result of the surface properties of the struts as it is of the 3D porous structure. In our previous study, we have investigated the in vitro performances of selective laser melted (SLM) Ti-6Al-4V scaffolds which are surface modified by the bioactive glass (BG) and mesoporous bioactive glass (MBG), respectively. The results demonstrated that such modification enhanced the attachment, proliferation, and differentiation of human bone marrow stromal cells (hBMSC). Here, we take the next step by assessing the therapeutic potential of 3D printed Ti-6Al-4V scaffolds with BG and MBG surface modifications for bone regeneration in a rabbit bone defect model. METHODS: 3D printed Ti-6Al-4V scaffolds with BG and MBG surface modifications were implanted into the femoral condyle of the rabbits, the Ti-6Al-4V scaffolds without surface modification were used as the control. At week 3, 6, and 9 after the implantation, micro-computed tomography (micro-CT) imaging, fluorescence double-labeling to determine the mineral apposition rate (MAR), and histological analysis of non-decalcified sections were performed. RESULTS: We found significantly higher volumes of regenerated bone, significantly higher values of the relevant bone morphometric parameters, clear signs of bone matrix apposition and maturation, and the evidence of progressed angiogenesis and blood vessel formation in the groups where the bioactive glass was added as a coating, particularly the MGB group. CONCLUSIONS: The MBG coating resulted in enhanced osteoconduction and vascularization in bone defect healing, which was attributed to the release of silicon and calcium ions and the presence of a nano-mesoporous structure on the surface of the MBG specimens.

Degradation behaviors and in-vivo biocompatibility of a rare earth- and aluminum-free magnesium-based stent.

Biodegradable stents can provide scaffolding and anti-restenosis benefits in the short term and then gradually disappear over time to free the vessel, among which the Mg-based biodegradable metal stents have been prosperously developed. In the present study, a Mg-8.5Li (wt.%) alloy (RE- and Al-free) with high ductility (> 40%) was processed into mini-tubes, and further fabricated into finished stent through laser cutting and electropolishing. In-vitro degradation test was performed to evaluate the durability of this stent before and after balloon dilation. The influence of plastic deformation and residual stress (derived from the dilation process) on the degradation was checked with the assistance of finite element analysis. In addition, in-vivo degradation behaviors and biocompatibility of the stent were evaluated by performing implantation in iliac artery of minipigs. The balloon dilation process did not lead to deteriorated degradation, and this stent exhibited a decent degradation rate (0.15 mm/y) in vitro, but divergent result (> 0.6 mm/y) was found in vivo. The stent was almost completely degraded in 3 months, revealing an insufficient scaffolding time. Meanwhile, it did not induce possible thrombus, and it was tolerable by surrounding tissues in pigs. Besides, endothelial coverage in 1 month was achieved even under the severe degradation condition. In the end, the feasibility of this stent for treatment of benign vascular stenosis was generally discussed, and perspectives on future improvement of Mg-Li-based stents were proposed.

Topological design, permeability and mechanical behavior of additively manufactured functionally graded porous metallic biomaterials.

Recent advances in additive manufacturing (AM) have enabled the fabrication of functionally graded porous biomaterials (FGPBs) for application as orthopedic implants and bone substitutes. Here, we present a step-wise topological design of FGPB based on diamond unit cells to mimic the structure of the femoral diaphysis. The FGPB was manufactured from Ti-6Al-4V powder using the selective laser melting (SLM) technique. The morphological parameters, permeability and mechanical properties of FGPB samples were measured and compared with those of the biomaterials with uniform porous structures based on the same type of the unit cell. The FGPB exhibited a low density (1.9 g/cm3), a moderate Young's modulus (10.44 GPa), a high yield stress (170.6 MPa), a high maximum stress (201 MPa) and favorable ductility, being superior to the biomaterials with uniform porous structures in comprehensive mechanical properties. In addition, digital image correlation (DIC) and finite element (FE) simulation were used to unravel the mechanisms governing the deformation and yielding behavior of these biomaterials particularly at the strut junctions. Both DIC and FE simulations confirmed that the deformation and yielding of the FGPB occurred largely in the load-bearing layers but not at the interfaces between layers. Defect-coupled FE models based on solid elements provided further insights into the mechanical responses of the FGPB to compressive loads at both macro- and micro-scales. With the defect-coupled representative volume element model for the FGPB, the Young's modulus and yield stress of the FGPBs were predicted with less than 2% deviations from the experimental data. The study clearly demonstrated the capabilities of combined experimental and computational methods to resolve the uncertainties of the mechanical behavior of FGPBs, which would open up the possibilities of applying various porosity variation strategies for the design of biomimetic AM porous biomaterials. STATEMENT OF SIGNIFICANCE: Functionally graded bone scaffolds significantly promote the recovery of segmental bone defect. In the present study, we present a step-wise topological design of functionally graded porous biomaterial (FGPB) to mimic the structure of the femoral diaphysis. The Ti-6Al-4V FGPB exhibited a superior combination of low density, moderate Young's modulus, high yield stress and maximum stress as well as favorable ductility. The biomechanical performance of FGPB was studied in both macro and micro perspectives. The defect-coupled model revealed the significant yielding in the load-bearing parts and the Young's modulus and yield stress of the FGPBs were predicted with less than 2% deviations from the experimental data. The superiority of combined experimental and computational methods has been confirmed.

Mesoporous Bioactive Glass Functionalized 3D Ti-6Al-4V Scaffolds with Improved Surface Bioactivity.

Porous Ti-6Al-4V scaffolds fabricated by means of selective laser melting (SLM), having controllable geometrical features and preferable mechanical properties, have been developed as a class of biomaterials that hold promising potential for bone repair. However, the inherent bio-inertness of the Ti-6Al-4V alloy as the matrix of the scaffolds results in a lack in the ability to stimulate bone ingrowth and regeneration. The aim of the present study was to develop a bioactive coating on the struts of SLM Ti-6Al-4V scaffolds in order to add the desired surface osteogenesis ability. Mesoporous bioactive glasses (MBGs) coating was applied on the strut surfaces of the SLM Ti-6Al-4V scaffolds through spin coating, followed by a heat treatment. It was found that the coating could maintain the characteristic mesoporous structure and chemical composition of MBG, and establish good interfacial adhesion to the Ti-6Al-4V substrate. The compressive strength and pore interconnectivity of the scaffolds were not affected by the coating. Moreover, the results obtained from in vitro cell culture experiments demonstrated that the attachment, proliferation, and differentiation of human bone marrow stromal cells (hBMSCs) on the MBG-coated Ti-6Al-4V scaffolds were improved as compared with those on the conventional bioactive glass (BG)-coated Ti-6Al-4V scaffolds and bare-metal Ti-6Al-4V scaffolds. Our results demonstrated that the MBG coating by using the spinning coating method could be an effective approach to achieving enhanced surface biofunctionalization for SLM Ti-6Al-4V scaffolds.

Study on the Mg-Li-Zn ternary alloy system with improved mechanical properties, good degradation performance and different responses to cells.

Novel Mg-(3.5, 6.5wt%)Li-(0.5, 2, 4wt%)Zn ternary alloys were developed as new kinds of biodegradable metallic materials with potential for stent application. Their mechanical properties, degradation behavior, cytocompatibility and hemocompatibility were studied. These potential biomaterials showed higher ultimate tensile strength than previously reported binary Mg-Li alloys and ternary Mg-Li-X (X=Al, Y, Ce, Sc, Mn and Ag) alloys. Among the alloys studied, the Mg-3.5Li-2Zn and Mg-6.5Li-2Zn alloys exhibited comparable corrosion resistance in Hank's solution to pure magnesium and better corrosion resistance in a cell culture medium than pure magnesium. Corrosion products observed on the corroded surface were composed of Mg(OH)2, MgCO3 and Ca-free Mg/P inorganics and Ca/P inorganics. In vitro cytotoxicity assay revealed different behaviors of Human Umbilical Vein Endothelial Cells (HUVECs) and Human Aorta Vascular Smooth Muscle Cells (VSMCs) to material extracts. HUVECs showed increasing nitric oxide (NO) release and tolerable toxicity, whereas VSMCs exhibited limited decreasing viability with time. Platelet adhesion, hemolysis and coagulation tests of these Mg-Li-Zn alloys showed different degrees of activation behavior, in which the hemolysis of the Mg-3.5Li-2Zn alloy was lower than 5%. These results indicated the potential of the Mg-Li-Zn alloys as good candidate materials for cardiovascular stent applications. STATEMENT OF SIGNIFICANCE: Mg-Li alloys are promising as absorbable metallic biomaterials, which however have not received significant attention since the low strength, controversial corrosion performance and the doubts in Li toxicity. The Mg-Li-Zn alloy in the present study revealed much improved mechanical properties higher than most reported binary Mg-Li and ternary Mg-Li-X alloys, with superior corrosion resistance in cell culture media. Surprisingly, the addition of Li and Zn showed increased nitric oxide release. The present study indicates good potential of Mg-Li-Zn alloy as absorbable cardiovascular stent material.

Advanced bredigite-containing magnesium-matrix composites for biodegradable bone implant applications.

The present research was aimed at developing magnesium-matrix composites that could allow effective control over their physiochemical and mechanical responses when in contact with physiological solutions. A biodegradable, bioactive ceramic - bredigite was chosen as the reinforcing phase in the composites, based on the hypothesis that the silicon- and magnesium-containing ceramic could protect magnesium from fast corrosion and at the same time stimulate cell proliferation. Methods to prepare composites with integrated microstructures - a prerequisite to achieve controlled biodegradation were developed. A systematic experimental approach was taken in order to elucidate the in vitro biodegradation mechanisms and kinetics of the composites. It was found that the composites with 20-40% homogenously dispersed bredigite particles, prepared from powders, could indeed significantly decrease the degradation rate of magnesium by up to 24 times. Slow degradation of the composites resulted in the retention of the mechanical integrity of the composites within the strength range of cortical bone after 12days of immersion in a cell culture medium. Cell attachment, cytotoxicity and bioactivity tests confirmed the stimulatory effects of bredigite embedded in the composites on the attachment, viability and differentiation of bone marrow stromal cells. Thus, the multiple benefits of adding bredigite to magnesium in enhancing degradation behavior, mechanical properties, biocompatibility and bioactivity were obtained. The results from this research showed the excellent potential of the bredigite-containing composites for bone implant applications, thus warranting further in vitro and in vivo research.

Microscopic full-field three-dimensional strain measurement during the mechanical testing of additively manufactured porous biomaterials.

A custom-designed micro-digital image correlation system was used to track the evolution of the full-surface three-dimensional strain field of Ti6Al4V additively manufactured lattice samples under mechanical loading. The high-magnification capabilities of the method allowed to resolve the strain distribution down to the strut level and disclosed a highly heterogeneous mechanical response of the lattice structure with local strain concentrations well above the nominal global strain level. In particular, we quantified that strain heterogeneity appears at a very early stage of the deformation process and increases with load, showing a strain accumulation pattern with a clear correlation to the later onset of the fracture. The obtained results suggest that the unique opportunities offered by the proposed experimental method, in conjunction with analytical and computational models, could serve to provide novel important information for the rational design of additively manufactured porous biomaterials.

Fabrication of novel magnesium-matrix composites and their mechanical properties prior to and during in vitro degradation.

In our previous study, we developed Mg-matrix composites with bredigite as the reinforcing phase and achieved improved degradation resistance in comparison with Mg. However, the effects of materials processing method and process parameters on the mechanical behavior of the composites before and during degradation were still unknown. This research was aimed at determining the mechanical properties of Mg-bredigite composites prior to and during degradation. It was found that by optimizing the process parameters of Pressure Assisted Sintering (PAS), low-porosity Mg-bredigite composites with strong interfaces between homogeneously distributed bredigite particles and the Mg matrix could be fabricated. By reinforcing Mg with 20vol% bredigite particles, the ultimate compressive strength and ductility of Mg increased by 67% and 111%, respectively. The in vitro degradation rate of the Mg-20% bredigite composite in a cell culture medium was 24 times lower than that of monolithic Mg. As a result of retarded degradation, the mechanical properties of the composite after 12 days of immersion in the cell culture medium were comparable to those of cortical bone. The encouraging results of this research warrant further investigations on the in vivo degradation behavior and mechanical properties of the composites.

Investigation into the hot workability of the as-extruded WE43 magnesium alloy using processing map.

The research concerned the characterization of the hot-working behavior of the as-extruded WE43 magnesium alloy potentially for biomedical applications and the construction of processing maps to guide the choice of forming process parameters. Isothermal uniaxial compression tests were performed over a temperature range of 350-480°C and strain rate range of 0.001-10s(-1). Flow stresses obtained were used to construct processing maps. Domains in processing maps corresponding to relevant deformation mechanisms, i.e., dynamic recrystallization (DRX), dynamic recovery (DRV) and flow instability, were identified, according to power dissipation efficiency and flow instability parameter values. Microstructures of compression-tested specimens were examined to validate these deformation mechanisms. Two mechanisms of DRX nucleation, i.e., particle-stimulated nucleation (PSN) and grain boundary bulging, were found to be operative at the low-temperature and high-temperature DRX domains, respectively. Flow instability was related to adiabatic shear bands and abnormal grain growth. An optimum condition for the hot working of this alloy was determined to be at a temperature of 475°C and a strain rate of 0.1s(-1).

Multipass cold drawing of magnesium alloy minitubes for biodegradable vascular stents.

Magnesium alloys possess highly limited room-temperature formabilities. This presents a technological barrier to the fabrication of minitubes for biodegradable vascular stents. The research was aimed at developing precision forming technology to fabricate ZM21 magnesium alloy minitubes with a refined microstructure. A multipass cold drawing process with a moving mandrel was successfully developed to convert seamless hollow billets through five passes of cold drawing and an interpass annealing treatment into minitubes with an outside diameter of 2.9 mm and a wall thickness of 0.217 mm, ready for laser cutting into vascular stents. It was found that a cumulative reduction in cross-section area as much as 32% could be applied to the material without causing fracture. However, a further reduction in cross-section area required annealing at 300°C for 1h to change a twinned microstructure into a recrystallized grain structure and to regain formability. The interpass annealing treatment after the fourth pass led to a reduction in drawing force by 22%, in comparison with the drawing force at the fourth pass of drawing. The variations in the outside diameter and wall thickness of the minitubes could be kept within 5 and 12 µm, respectively. Further research is directed toward improvements in dimensional precisions.

In vitro degradation behavior and bioactivity of magnesium-Bioglass(®) composites for orthopedic applications.

To improve the bioactivity and degradation behavior of biodegradable magnesium, biodegradable metal matrix composites with the ZK30 magnesium alloy as the matrix and bioactive glass (BG, 45S5) as the reinforcement were prepared. The microstructures of the ZK30-BG composites showed homogeneous dispersion of BG particles throughout the matrix. XRD and EDX analyses confirmed the retention of the morphological characteristics and composition of BG particles in the composites. Immersion tests in the minimum essential medium with Earle's balanced salts at 37°C showed that the composites with 5 and 10% BG had lower rates of degradation and hydrogen evolution than the matrix alloy. In addition, the tests confirmed that the composites possessed an enhanced ability to induce calcium and phosphate ion deposition on sample surfaces during degradation, suggesting accelerated surface mineralization that would lead to improved bioactivity when compared with the matrix alloy. In vitro cytotoxicity tests showed that the ionic products of the composites formed during degradation possessed a superior ability to support the survival, proliferation, and osteoblastic differentiation of bone marrow stromal cells to those of the ZK30 alloy. The ZK30-BG composites with enhanced bioactivity and reduced degradation rate could be promising biodegradable materials for orthopedic implants.

Magnesium-based composites with improved in vitro surface biocompatibility.

In this study, bioactive glass (BG, 45S5) particles were added to a biodegradable magnesium alloy (ZK30) through a semi-solid high-pressure casting process in order to improve the surface biocompatibility of the biomaterial and potentially its bioactivity. The observation of the as-cast microstructures of ZK30-BG composites indicated homogeneous dispersion of BG particles in the matrix. SEM, EDX and EPMA showed the retention of the morphological characteristics and composition of BG particles in the as-cast composite materials. In vitro tests in a cell culture medium confirmed that the composites indeed possessed an enhanced ability to induce the deposition of a bone-like apatite layer on the surface, indicating an improved surface biocompatibility as compared with the matrix alloy.