Bone regeneration capacity of magnesium phosphate cements in a large animal model

Nanofiber-induced hierarchically-porous magnesium phosphate bone cements accelerate bone regeneration by inhibiting Notch signaling

Magnesium phosphate bone cements (MPC) have been recognized as a viable alternative for bone defect repair due to their high mechanical strength and biodegradability. However, their poor porosity and permeability limit osteogenic cell ingrowth and vascularization, which is critical for bone regeneration. In the current study, we constructed a novel hierarchically-porous magnesium phosphate bone cement by incorporating extracellular matrix (ECM)-mimicking electrospun silk fibroin (SF) nanofibers. The SF-embedded MPC (SM) exhibited a heterogeneous and hierarchical structure, which effectively facilitated the rapid infiltration of oxygen and nutrients as well as cell ingrowth. Besides, the SF fibers improved the mechanical properties of MPC and neutralized the highly alkaline environment caused by excess magnesium oxide. Bone marrow stem cells (BMSCs) adhered excellently on SM, as illustrated by formation of more pseudopodia. CCK8 assay showed that SM promoted early proliferation of BMSCs. Our study also verified that SM increased the expression of OPN, RUNX2 and BMP2, suggesting enhanced osteogenic differentiation of BMSCs. We screened for osteogenesis-related pathways, including FAK signaing, Wnt signaling and Notch signaling, and found that SM aided in the process of bone regeneration by suppressing the Notch signaling pathway, proved by the downregulation of NICD1, Hes1 and Hey2. In addition, using a bone defect model of rat calvaria, the study revealed that SM exhibited enhanced osteogenesis, bone ingrowth and vascularization compared with MPC alone. No adverse effect was found after implantation of SM in vivo. Overall, our novel SM exhibited promising prospects for the treatment of critical-sized bone defects.

Polyetheretherketone bioactivity induced by farringtonite

Polyetheretherketone (PEEK) is considered as an excellent biomaterial for bone grafting and connective tissue replacement. The clinical potential is, however, limited by its bioinertness, poor osteoconduction, and weak antibacterial activity. These disadvantages can be overcome by introducing suitable additives to produce mineral-polymer composites or coatings. In this work, a PEEK-based bioactive composite has been obtained by blending the polymer with magnesium phosphate (Mg3(PO4)2) particles in amounts ranging from 1 to 10 wt.% using the hot press technique. The obtained composite exhibited improved mechanical and physical properties, above the lower limits set for bone engineering applications. The tested grafts were found to not induce cytotoxicity. The presence of magnesium phosphate induced the mineralisation process with no adverse effects on the expression of the marker crucial for osteoblastic differentiation. The most promising results were observed in the grafts containing 1 wt.% of magnesium phosphate embedded within the PEEK matrix. The improved bioactivity of grafts, together with suitable physical-chemical and mechanical properties, indicate this composite as a promising orthopaedic implant material.

In Vivo Investigation of 3D-Printed Calcium Magnesium Phosphate Wedges in Partial Load Defects

Bone substitutes are ideally biocompatible, osteoconductive, degradable and defect-specific and provide mechanical stability. Magnesium phosphate cements (MPCs) offer high initial stability and faster degradation compared to the well-researched calcium phosphate cements (CPCs). Calcium magnesium phosphate cements (CMPCs) should combine the properties of both and have so far shown promising results. The present study aimed to investigate and compare the degradation and osseointegration behavior of 3D powder-printed wedges of CMPC and MPC in vivo. The wedges were post-treated with phosphoric acid (CMPC) and diammonium hydrogen phosphate (MPC) and implanted in a partially loaded defect model in the proximal rabbit tibia. The evaluation included clinical, in vivo µ-CT and X-ray examinations, histology, energy dispersive X-ray analysis (EDX) and scanning electron microscopy (SEM) for up to 30 weeks. SEM analysis revealed a zone of unreacted material in the MPC, indicating the need to optimize the manufacturing and post-treatment process. However, all materials showed excellent biocompatibility and mechanical stability. After 24 weeks, they were almost completely degraded. The slower degradation rate of the CMPC corresponded more favorably to the bone growth rate compared to the MPC. Due to the promising results of the CMPC in this study, it should be further investigated, for example in defect models with higher load.

Improving bone defect healing using magnesium phosphate granules with tailored degradation characteristics

OBJECTIVES

Dental implant placement frequently requires preceding bone augmentation, for example, with hydroxyapatite (HA) or β-tricalcium phosphate (β-TCP) granules. However, HA is degraded very slowly in vivo and for β-TCP inconsistent degradation profiles from too rapid to rather slow are reported. To shorten the healing time before implant placement, rapidly resorbing synthetic materials are of great interest. In this study, we investigated the potential of magnesium phosphates in granular form as bone replacement materials.

METHODS

Spherical granules of four different materials were prepared via an emulsion process and investigated in trabecular bone defects in sheep: struvite (MgNH4PO4·6H2O), K-struvite (MgKPO4·6H2O), farringtonite (Mg3(PO4)2) and β-TCP.

RESULTS

All materials except K-struvite exhibited promising support of bone regeneration, biomechanical properties and degradation. Struvite and β-TCP granules degraded at a similar rate, with a relative granules area of 29% and 30% of the defect area 4 months after implantation, respectively, whereas 18% was found for farringtonite. Only the K-struvite granules degraded too rapidly, with a relative granules area of 2% remaining, resulting in initial fibrous tissue formation and intermediate impairment of biomechanical properties.

SIGNIFICANCE

We demonstrated that the magnesium phosphates struvite and farringtonite have a comparable or even improved degradation behavior in vivo compared to β-TCP. This emphasizes that magnesium phosphates may be a promising alternative to established calcium phosphate bone substitute materials.

A Bioactive Gelatin-Methacrylate Incorporating Magnesium Phosphate Cement for Bone Regeneration

Maintaining proper mechanical strength and tissue volume is important for bone growth at the site of a bone defect. In this study, potassium magnesium phosphate hexahydrate (KMgPO4·6H2O, MPC) was applied to gelma-methacrylate hydrogel (GelMA) to prepare GelMA/MPC composites (GMPCs). Among these, 5 GMPC showed the best performance in vivo and in vitro. These combinations significantly enhanced the mechanical strength of GelMA and regulated the degradation and absorption rate of MPC. Considerably better mechanical properties were noted in 5 GMPC compared with other concentrations. Better bioactivity and osteogenic ability were also found in 5 GMPC. Magnesium ions (Mg2+) are bioactive and proven to promote bone tissue regeneration, in which the enhancement efficiency is closely related to Mg2+ concentrations. These findings indicated that GMPCs that can release Mg2+ are effective in the treatment of bone defects and hold promise for future in vivo applications.

Effect of Atmospheric Pressure Plasma Jet Treatments on Magnesium Phosphate Cements: Performance, Characterization, and Applications

Atmospheric pressure plasma treatments are nowadays gaining importance to improve the performance of biomaterials in the orthopedic field. Among those, magnesium phosphate-based cements (MPCs) have recently shown attractive features as bone repair materials. The effect of plasma treatments on such cements, which has not been investigated so far, could represent an innovative strategy to modify MPCs' physicochemical properties and to tune their interaction with cells. MPCs were prepared and treated for 5, 7.5, and 10 min with a cold atmospheric pressure plasma jet. The reactive nitrogen and oxygen species formed during the treatment were characterized. The surfaces of MPCs were studied in terms of the phase composition, morphology, and topography. After a preliminary test in simulated body fluid, the proliferation, adhesion, and osteogenic differentiation of human mesenchymal cells on MPCs were assessed. Plasma treatments induce modifications in the relative amounts of struvite, newberyite, and farringtonite on the surfaces on MPCs in a time-dependent fashion. Nonetheless, all investigated scaffolds show a good biocompatibility and cell adhesion, also supporting osteogenic differentiation of mesenchymal cells.

Dual-Setting Bone Cement Based On Magnesium Phosphate Modified with Glycol Methacrylate Designed for Biomedical Applications

Magnesium phosphate cement (MPC) is a suitable alternative for the currently used calcium phosphates, owing to beneficial properties like favorable resorption rate, fast hardening, and higher compressive strength. However, due to insufficient mechanical properties and high brittleness, further improvement is still expected. In this paper, we reported the preparation of a novel type of dual-setting cement based on MPC with poly(2-hydroxyethyl methacrylate) (pHEMA). The aim of our study was to evaluate the effect of HEMA addition, especially its concentration and premix time, on the selected properties of the composite. Several beneficial effects were found: better formability, shortened setting time, and improvement of mechanical strengths. The developed cements were hardening in ∼16-21 min, consisted of well-crystallized phases and polymerized HEMA, had porosity between ∼2-11%, degraded slowly by ∼0.1-4%/18 days, their wettability was ∼20-30°, they showed compressive and bending strength between ∼45-73 and 13-20 MPa, respectively, and, finally, their Young's Modulus was close to ∼2.5-3.0 GPa. The results showed that the optimal cement composition is MPC+15%HEMA and 4 min of polymer premixing time. Overall, our research suggested that this developed cement may be used in various biomedical applications.

Ready-To-Use and Rapidly Biodegradable Magnesium Phosphate Bone Cement: In Vivo Evaluation in Sheep

In clinical practice, hydroxyapatite (HA) cements for bone defect treatment are frequently prepared by mixing a powder component and a liquid component shortly before implantation in the operation theater, which is time-consuming and error-prone. In addition, HA cements are only slightly resorbed, that is, cement residues can still be found in the bone years after implantation. Here, these challenges are addressed by a prefabricated magnesium phosphate cement paste based on glycerol, which is ready-to-use and can be directly applied during surgery. By using a trimodal particle size distribution (PSD), the paste is readily injectable and exhibits a compressive strength of 9-14 MPa after setting. Struvite (MgNH4 PO4 ·6H2 O), dittmarite (MgNH4 PO4 ·H2 O), farringtonite (Mg3 (PO4 )2 ), and newberyite (MgHPO4 ·3H2 O) are the mineral phases present in the set cement. The paste developed here features a promising degradation of 37% after four months in an ovine implantation model, with 25% of the implant area being newly formed bone. It is concluded that the novel prefabricated paste improves application during surgery, has a suitable degradation rate, and supports bone regeneration.

Experimental magnesium phosphate cement paste increases torque of trochanteric fixation nail advanced™ blades in human femoral heads

BACKGROUND

The use of polymethylmethacrylate cement for in-situ implant augmentation has considerable disadvantages: it is potentially cytotoxic, exothermic and non-degradable. Therefore, the primary aim of this study was to develop a magnesium phosphate cement which meets the requirements for in-situ implant augmentation as an alternative. Secondly, this experimental cement was compared to commercial bone cements in a biomechanical test set-up using augmented femoral head blades.

METHODS

A total of 40 human femoral heads were obtained from patients who underwent total hip arthroplasty. After bone mineral density was quantified, specimens were assigned to four treatment groups. A blade of the Trochanteric Fixation Nail Advanced™ was inserted into each specimen and augmented with either Traumacem™ V+, Paste-CPC, the experimental magnesium phosphate cement or no cement. A rotational load-to-failure-test (0° to 90°) was performed.

FINDINGS

A conventional two-component magnesium phosphate cement failed in-situ implant augmentation consistently due to filter pressing. Only a glycerol-based magnesium phosphate paste was suitable for the augmentation of femoral head blades. While the blades augmented with Traumacem™ V+ yielded the highest maximum torque overall (22.1 Nm), the blades augmented with Paste-CPC and the magnesium phosphate paste also showed higher maximum torque values (15.8 and 12.8 Nm) than the control group (10.8 Nm).

INTERPRETATION

This study shows for the first time the development of a degradable magnesium phosphate cement paste which fulfills the requirements for in-situ implant augmentation. Simultaneously, a 48% increase in stability is demonstrated for a scenario where implant anchorage is difficult in osteoporotic bone.

Preparation of lithium-containing magnesium phosphate-based composite ceramics having high compressive strength, osteostimulation and proangiogenic effects

Fairly high concentrations of magnesium and lithium are conducive to improving the osteogenic and angiogenic capacities. In the current study, lithium-containing magnesium phosphate-based ceramics (AMP/LMPGs) were prepared from amorphous magnesium phosphate (AMP) at a low sintering temperature (650 °C), and the lithium/magnesium-containing phosphate glasses (LMPGs) were utilized as sintering additives. During the sintering procedure of AMP/LMPGs, the AMP reacted with LMPGs, producing new compounds. The AMP/LMPGs displayed nano-size grains and plentiful micropores. The addition of LMPGs noticeably increased the porosity as well as compressive strength of the AMP/LMPGs ceramics. The AMP/LMPGs sustainedly released Mg, P and Li ions, forming Mg-rich ionic microenvironment, which ameliorated cellular proliferation, osteogenic differentiation and proangiogenic capacities. The AMP/LMPGs ceramics with considerably high compressive strength, osteostimulation and proangiogenic effects were expected to efficiently regenerate the bone defects.

Is it possible to 3D bioprint load-bearing bone implants? A critical review

Rehabilitative capabilities of any tissue engineered scaffold rely primarily on the triad of (i) biomechanical properties such as mechanical properties and architecture, (ii) chemical behavior such as regulation of cytokine expression, and (iii) cellular response modulation (including their recruitment and differentiation). The closer the implant can mimic the native tissue, the better it can rehabilitate the damage therein. Among the available fabrication techniques, only 3D bioprinting (3DBP) can satisfactorily replicate the inherent heterogeneity of the host tissue. However, 3DBP scaffolds typically suffer from poor mechanical properties, thereby, driving the increased research interest in development of load-bearing 3DBP orthopedic scaffolds in recent years. Typically, these scaffolds involve multi-material 3D printing, comprising of at-least one bioink and a load-bearing ink; such that mechanical and biological requirements of the biomaterials are decoupled. Ensuring high cellular survivability and good mechanical properties are of key concerns in all these studies. 3DBP of such scaffolds is in early developmental stages, and research data from only a handful of preliminary animal studies are available, owing to limitations in print-capabilities and restrictive materials library. This article presents a topically focused review of the state-of-the-art, while highlighting aspects like available 3DBP techniques; biomaterials' printability; mechanical and degradation behavior; and their overall bone-tissue rehabilitative efficacy. This collection amalgamates and critically analyses the research aimed at 3DBP of load-bearing scaffolds for fulfilling demands of personalized-medicine. We highlight the recent-advances in 3DBP techniques employing thermoplastics and phosphate-cements for load-bearing applications. Finally, we provide an outlook for possible future perspectives of 3DBP for load-bearing orthopedic applications. Overall, the article creates ample foundation for future research, as it gathers the latest and ongoing research that scientists could utilize.

Exploring the potential of magnesium oxychloride, an amorphous magnesium phosphate, and newberyite as possible bone cement candidates

Magnesium phosphate-based bone cements, particularly struvite (MgNH4PO4∙6H2O)-forming cements, have attracted increased scientific interest in recent years because they exhibit similar biocompatibility to hydroxyapatite while degrading much more rapidly in vivo. However, other magnesium-based minerals which might be promising are, to date, little studied. Therefore, in this study, we investigated three magnesium-based bone cements: a magnesium oxychloride cement (Mg3(OH)5Cl∙4H2O), an amorphous magnesium phosphate cement based on Mg3(PO4)2, MgO, and NaH2PO4, and a newberyite cement (MgHPO4·3H2O). Because it is not sufficiently clear from the literature to what extent these cements are suitable for clinical use, all of them were characterized and optimized regarding setting time, setting temperature, compressive strength and passive degradation in phosphate-buffered saline. Because the in vitro properties of the newberyite cement were most promising, it was orthotopically implanted into a partially weight-bearing tibial bone defect in sheep. The cement exhibited excellent biocompatibility and degraded more rapidly compared to a hydroxyapatite reference cement; after 4 months, 18% of the cement was degraded. We conclude that the newberyite cement was the most promising candidate of the investigated cements and has clear advantages over calcium phosphate cements, especially in terms of setting time and degradation behavior.

An Overview of Magnesium-Phosphate-Based Cements as Bone Repair Materials

In the search for effective biomaterials for bone repair, magnesium phosphate cements (MPCs) are nowadays gaining importance as bone void fillers thanks to their many attractive features that overcome some of the limitations of the well-investigated calcium-phosphate-based cements. The goal of this review was to highlight the main properties and applications of MPCs in the orthopedic field, focusing on the different types of formulations that have been described in the literature, their main features, and the in vivo and in vitro response towards them. The presented results will be useful to showcase the potential of MPCs in the orthopedic field and will suggest novel strategies to further boost their clinical application.

Degradation and Bone-Contact Biocompatibility of Two Drillable Magnesium Phosphate Bone Cements in an In Vivo Rabbit Bone Defect Model

The use of bone-cement-enforced osteosynthesis is a growing topic in trauma surgery. In this context, drillability is a desirable feature for cements that can improve fracture stability, which most of the available cement systems lack. Therefore, in this study, we evaluated a resorbable and drillable magnesium-phosphate (MgP)-based cement paste considering degradation behavior and biocompatibility in vivo. Two different magnesium-phosphate-based cement (MPC) pastes with different amounts of phytic acid (IP 6) as setting retarder (MPC 22.5 and MPC 25) were implanted in an orthotopic defect model of the lateral femoral condyle of New Zealand white rabbits for 6 weeks. After explantation, their resorption behavior and material characteristics were evaluated by means of X-ray diffraction (XRD), porosimetry measurement, histological staining, peripheral quantitative computed tomography (pQCT), cone-beam computed tomography (CBCT) and biomechanical load-to-failure tests. Both cement pastes displayed comparable results in mechanical strength and resorption kinetics. Bone-contact biocompatibility was excellent without any signs of inflammation. Initial resorption and bone remodeling could be observed. MPC pastes with IP 6 as setting retardant have the potential to be a valuable alternative in distinct fracture patterns. Drillability, promising resorption potential and high mechanical strength confirm their suitability for use in clinical routine.

Calcium Phosphate Cements as Carriers of Functional Substances for the Treatment of Bone Tissue

Interest in calcium phosphate cements as materials for the restoration and treatment of bone tissue defects is still high. Despite commercialization and use in the clinic, the calcium phosphate cements have great potential for development. Existing approaches to the production of calcium phosphate cements as drugs are analyzed. A description of the pathogenesis of the main diseases of bone tissue (trauma, osteomyelitis, osteoporosis and tumor) and effective common treatment strategies are presented in the review. An analysis of the modern understanding of the complex action of the cement matrix and the additives and drugs distributed in it in relation to the successful treatment of bone defects is given. The mechanisms of biological action of functional substances determine the effectiveness of use in certain clinical cases. An important direction of using calcium phosphate cements as a carrier of functional substances is the volumetric incorporation of anti-inflammatory, antitumor, antiresorptive and osteogenic functional substances. The main functionalization requirement for carrier materials is prolonged elution. Various release factors related to the matrix, functional substances and elution conditions are considered in the work. It is shown that cements are a complex system. Changing one of the many initial parameters in a wide range changes the final characteristics of the matrix and, accordingly, the kinetics. The main approaches to the effective functionalization of calcium phosphate cements are considered in the review.

An injectable porous bioactive magnesium phosphate bone-cement foamed with calcium carbonate and citric acid for periodontal bone regeneration

Magnesium phosphate cement (MPC) has been evaluated as a novel bone substitute owing to its favorable biocompatibility, plasticity, and osteogenic potential. However, the low porosity of MPC prevents growth factors and osteoblasts from fully growing into the material, thereby limiting its clinical use. In this study, different concentrations (0-5%) of calcium carbonate and citric acid (CA) were used as foaming agents to prepare porous MPC. The MPC containing 3% CaCO₃/CA exhibited the best physicochemical properties, including greater porosity, improved injectability, extended setting time, and decreased hydration temperature. The proliferation and adhesion of cells on 3%CaCO₃/CA-MPC were higher than those on MPC alone. To explore its osteogenesis in vivo, 3% CaCO₃/CA-MPC and Bio-Oss® bone powder were implanted into periodontal bone defects in rats for 4 weeks and 12 weeks, respectively. Micro-CT and histological analysis demonstrated the improved bone regeneration of 3%CaCO₃/CA-MPC compared to the blank group (P < 0.05); it had slightly lower bone regeneration than the Bio-Oss® group but no statistical difference. The results indicated that porous MPC foamed with calcium carbonate and CA improved its physicochemical properties and enhanced its biocompatibility, making it a promising material for bone regeneration.

Injectable magnesium oxychloride cement foam-derived scaffold for augmenting osteoporotic defect repair

HYPOTHESIS

Cement augmentation has been widely applied to promote osteoporotic fracture healing, whereas the existing calcium-based products suffer from the excessively slow degradation, which may impede bone regeneration. Magnesium oxychloride cement (MOC) shows promising biodegradation tendency and bioactivity, which is expected to be a potential alternative to the classic calcium-based cement for hard-tissue-engineering applications.

EXPERIMENTS

Here, a hierarchical porous MOC foam (MOCF)-derived scaffold with favorable bio-resorption kinetic and superior bioactivity is fabricated through Pickering foaming technique. Then, a systematic characterization in terms of material properties and in vitro biological performance have been conducted to evaluate the feasibility of the as-prepared MOCF scaffold to be a bone-augmenting material for treating osteoporotic defects.

FINDINGS

The developed MOCF shows excellent handling performance in the paste state, while exhibiting sufficient load-bearing capacity after solidification. In comparison with the traditional bone cement, calcium deficient hydroxyapatite (CDHA), our porous MOCF scaffold demonstrates a much higher biodegradation tendency and better cell recruitment ability. Additionally, the eluted bioactive ions by MOCF commits to a biologically inductive microenvironment, where the in vitro osteogenesis is significantly enhanced. It is anticipated that this advanced MOCF scaffold will be competitive for clinical therapies to augment osteoporotic bone regeneration.

Bioinspired strontium magnesium phosphate cement prepared utilizing the precursor method for bone tissue engineering

Bioinspired strontium magnesium phosphate cements for bone tissue engineering were prepared using a new, facile, environmentally friendly and high yielding (98.5%) precursor method. The bioinspired SMPCs have uniform particle distributions, excellent mechanical strengths and high biocompatibilities. The in vitro responses of bone marrow stromal cells to the SMPCs, including viability, osteogenic differentiation and alkaline phosphatase activity, were evaluated. The results show that the SMPC containing 0.5 mol of strontium (referred to as SMPC-2) has a higher degradation rate and biological activity than magnesium phosphate cements and the other SMPCs. In addition, the synergistic effect of strontium and magnesium ion release from SMPC-2 creates a conducive environment for cell proliferation, mineralized calcium deposition and new bone formation. These observations demonstrate the feasibility of using the new precursor method to generate SMPCs and the utility of these biologically compatible and highly effective cements for bone tissue engineering.

Carboxymethyl chitosan-alginate enhances bone repair effects of magnesium phosphate bone cement by activating the FAK-Wnt pathway

There is a continuing need for artificial bone substitutes for bone repair and reconstruction, Magnesium phosphate bone cement (MPC) has exceptional degradable properties and exhibits promising biocompatibility. However, its mechanical strength needs improved and its low osteo-inductive potential limits its therapeutic application in bone regeneration. We functionally modified MPC by using a polymeric carboxymethyl chitosan-sodium alginate (CMCS/SA) gel network. This had the advantages of: improved compressive strength, ease of handling, and an optimized interface for bioactive bone in-growth. The new composites with 2% CMCS/SA showed the most favorable physicochemical properties, including mechanical strength, wash-out resistance, setting time, injectable time and heat release. Biologically, the composite promoted the attachment and proliferation of osteoblast cells. It was also found to induce osteogenic differentiation in vitro, as verified by expression of osteogenic markers. In terms of molecular mechanisms, data showed that new bone cement activated the Wnt pathway through inhibition of the phosphorylation of β-catenin, which is dependent on focal adhesion kinase. Through micro-computed tomography and histological analysis, we found that the MPC-CMCS/SA scaffolds, compared with MPC alone, showed increased bone regeneration in a rat calvarial defect model. Overall, our study suggested that the novel composite had potential to help repair critical bone defects in clinical practice.

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CMCS/SA improves the mechanical strength of MPC while minimizing tissue damage.

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The MPC-CMCS/SA composite is easily manipulable for clinical application.

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MPC-CMCS/SA has good biocompatibility, and is easier for cell adhesion and proliferation.

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The MPC-CMCS/SA composite enhances osteogenic differentiation in vitro through the integrin-FAK-Wnt axis.

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The MPC-CMCS/SA composite enhances critical bone defect repair in vivo.

Evaluation of Fillers for Magnesium Potassium Phosphate Cement (MKPC) for the Encapsulation of Low and Intermediate Level Metallic Radioactive Wastes

This study investigates the effect of coal fly ash (FA), wollastonite (WO), pumice (PM), and metakaolin (MK) as filler materials in the rheological, mechanical, chemical, and mineralogical properties of a magnesium potassium phosphate cement (MKPC), designed for the encapsulation of low and intermediate level radioactive wastes containing reactive metals. Workability, compression strength, dimensional stability, pH, chemical composition, and mineralogical properties were studied in different pastes and mortars of MKPC with a fixed molar ratio of MgO/KH2PO4 = 1. No new mineral phases were found with the addition of the fillers, denoting their low chemical impact on the MKPC system. Moreover, all formulations with a water/cement mass ratio of <0.65 presented compressive strengths higher than 30 MPa after 90 days, and pH values lower than 8.5, corresponding to the passivation zone of aluminum corrosion.

3D bioprinting of emulating homeostasis regulation for regenerative medicine applications

Homeostasis is the most fundamental mechanism of physiological processes, occurring simultaneously as the production and outcomes of pathological procedures. Accompanied by manufacture and maturation of intricate and highly hierarchical architecture obtained from 3D bioprinting (three-dimension bioprinting), homeostasis has substantially determined the quality of printed tissues and organs. Instead of only shape imitation that has been the remarkable advances, fabrication for functionality to make artificial tissues and organs that act as real ones in vivo has been accepted as the optimized strategy in 3D bioprinting for the next several years. Herein, this review aims to provide not only an overview of 3D bioprinting, but also the main strategies used for homeostasis bioprinting. This paper briefly introduces the principles of 3D bioprinting system applied in homeostasis regulations firstly, and then summarizes the specific strategies and potential trend of homeostasis regulations using multiple types of stimuli-response biomaterials to maintain auto regulation, specifically displaying a brilliant prospect in hormone regulation of homeostasis with the most recently outbreak of vasculature fabrication. Finally, we discuss challenges and future prospects of homeostasis fabrication based on 3D bioprinting in regenerative medicine, hoping to further inspire the development of functional fabrication in 3D bioprinting.

Degradation of 3D-printed magnesium phosphate ceramics in vitro and a prognosis on their bone regeneration potential

Regenerative bone implants promote new bone formation and ideally degrade simultaneously to osteogenesis. Although clinically established calcium phosphate bone grafts provide excellent osseointegration and osteoconductive efficacy, they are limited in terms of bioresorption. Magnesium phosphate (MP) based ceramics are a promising alternative, because they are biocompatible, mechanically extremely stable, and degrade much faster than calcium phosphates under physiological conditions. Bioresorption of an implant material can include both chemical dissolution as well as cellular resorption. We investigated the bioresorption of 3D powder printed struvite and newberyite based MP ceramics in vitro by a direct human osteoclast culture approach. The osteoclast response and cellular resorption was evaluated by means of fluorescence and TRAP staining, determination of osteoclast activities (CA II and TRAP), SEM imaging as well as by quantification of the ion release during cell culture. Furthermore, the bioactivity of the materials was investigated via SBF immersion, whereas hydroxyapatite precipitates were analyzed by SEM and EDX measurements. This bioactive coating was resorbed by osteoclasts. In contrast, only chemical dissolution contributed to bioresorption of MP, while no cellular resorption of the materials was observed. Based on our results, we expect an increased bone regeneration effect of MP compared to calcium phosphate based bone grafts and complete chemical degradation within a maximum of 1.5-3.1 years.

Physicochemical degradation of calcium magnesium phosphate (stanfieldite) based bone replacement materials and the effect on their cytocompatibility

Regenerative bone implants should be completely replaced by new bone within a period of time corresponding to the growth rate of native bone. To meet this requirement, suitable biomaterials must be biodegradable and promote osteogenesis. The combination of slowly degrading but osteoconductive calcium phosphates (CPs) with rapidly degrading and mechanically more resilient magnesium phosphates represents a promising material class for this purpose. In order to create the best possible conditions for optimal implant integration, microporous calcium magnesium phosphate (CMP) cements were processed using 3D powder printing. This technique enables the production of a defect-adapted implant with an optimal fit and a high degree of open porosity to promote bone ingrowth. Four different compositions of 3D printed CMP ceramics were investigated with regard to essential properties of bone implants, including chemical composition, porosity, microstructure, mechanical strength, and cytocompatibility. The ceramics consisted of farringtonite (Mg₃(PO₄)₂) and stanfieldite (Ca₄Mg₅(PO₄)₆), with either struvite (NH₄MgPO₄·6H₂O) or newberyite (MgHPO₄·3H₂O) and brushite (CaHPO₄·2H₂O) as additional phases. The CMP materials showed open porosities between 13 and 28% and compressive strengths between 11 and 17 MPa, which was significantly higher, as compared with clinically established CP. The cytocompatibility was evaluated with the human fetal osteoblast cell line hFOB 1.19 and was proven to be equal or to even exceed that of tricalcium phosphate. Furthermore, a release of 4-8 mg magnesium and phosphate ions per mg scaffold material could be determined for CMPs over a period of 21 d. In the case of struvite containing CMPs the chemical dissolution of the cement matrix was combined with a physical degradation, which resulted in a mass loss of up to 3.1 wt%. In addition to its beneficial physical and biological properties, the proven continuous chemical degradation and bioactivity in the form of CP precipitation indicate an enhanced bone regeneration potential of CMPs.

Comparison of degradation behavior and osseointegration of 3D powder-printed calcium magnesium phosphate cement scaffolds with alkaline or acid post-treatment

Due to the positive effects of magnesium substitution on the mechanical properties and the degradation rate of the clinically well-established calcium phosphate cements (CPCs), calcium magnesium phosphate cements (CMPCs) are increasingly being researched as bone substitutes. A post-treatment alters the materials’ physical properties and chemical composition, reinforcing the structure and modifying the degradation rate. By alkaline post-treatment with diammonium hydrogen phosphate (DAHP, (NH4)2HPO4), the precipitation product struvite is formed, while post-treatment with an acidic phosphate solution [e.g., phosphoric acid (PA, H3PO4)] results in precipitation of newberyite and brushite. However, little research has yet been conducted on newberyite as a bone substitute and PA post-treatment of CMPCs has not been described in the accessible literature so far. Therefore, in the present study, the influence of an alkaline (DAHP) or acid (PA) post-treatment on the biocompatibility, degradation behavior, and osseointegration of cylindrical scaffolds (h = 5.1 mm, Ø = 4.2 mm) produced from the ceramic cement powder Ca0.75Mg2.25(PO4)2 by the advantageous manufacturing technique of three-dimensional (3D) powder printing was investigated in vivo. Scaffolds of the material groups Mg225d (DAHP post-treatment) and Mg225p (PA post-treatment) were implanted into the cancellous part of the lateral femoral condyles in rabbits. They were evaluated up to 24 weeks by regular clinical, X-ray, micro-computed tomographic (µCT), and histological examinations as well as scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) analysis and compared with tricalcium phosphate (TCP). All materials showed excellent biocompatibility and rapid osseointegration. While TCP degraded only slightly, the CMPCs showed almost complete degradation. Mg225d demonstrated significantly faster loss of form and demarcability from surrounding bone, scaffold volume reduction, and significantly greater degradation on the side towards the bone marrow than to the cortex than Mg225p. Simultaneously, numerous bone trabeculae have grown into the implantation site. While these were mostly located on the side towards the cortex in Mg225d, they were more evenly distributed in Mg225p and showed almost the same structural characteristics as physiological bone after 24 weeks in Mg225p. Based on these results, the acid post-treated 3D powder-printed Mg225p is a promising degradable bone substitute that should be further investigated.

In Vitro Degradation and In Vivo Biocompatibility of Strontium-Doped Magnesium Phosphate-Reinforced Magnesium Composites

Magnesium is projected for use as a degradable orthopedic biomaterial. However, its fast degradation in physiological media is considered as a significant challenge for its successful clinical applications. Bioactive reinforcements containing Mg-based composites constitute one of the promising approaches for developing degradable metallic implants because of their adjustable mechanical behaviors, corrosion resistance, and biological response. Strontium is a trace element known for its role in enhancing osteoblast activity. In this study, bioactive SrO-doped magnesium phosphate (MgP)-reinforced Mg composites containing 1, 3, and 5 wt % MgP were developed through the casting route. The influence of the SrO-doped MgP reinforcement on degradation behaviors of the composites along with its cell-material interactions and in vivo biocompatibility was investigated. The wt % and distribution of MgP particles significantly improved the mechanical properties of the composite. HBSS immersion study indicated the least corrosion rate (0.56 ± 0.038 mmpy) for the Mg-3MgP composite. The higher corrosion resistance of Mg-3MgP leads to a controlled release of Sr-containing bioactive reinforcement, which eventually enhanced the cytotoxicity as measured using MG-63 cell-material interactions. The in vivo biocompatibility of the composite was evaluated using the rabbit femur defect model. Micro-computed tomography (μ-CT) and histological analysis supported the fact that Mg-3MgP maintained its structural integrity and enhanced osteogenesis (50.36 ± 2.03%) after 2 months of implantation. The results indicated that the Mg-MgP composite could be used as a degradable internal fracture fixation device material.

A Bilayer Membrane Doped with Struvite Nanowires for Guided Bone Regeneration

Guided bone regeneration (GBR) therapy demonstrates a prominent curative effect on the management of craniomaxillofacial (CMF) bone defects. In this study, a GBR membrane consisting of a microporous layer and a struvite-nanowire-doped fibrous layer is constructed via non-solvent induced phase separation, followed by an electrospinning procedure to treat critical-sized calvarial defects. The microporous layer shows selective permeability for excluding the rapid-growing non-osteogenic tissues and potential wound stabilization. The nanowire-like struvite is synthesized as the deliverable therapeutic agent within the fibrous layer to facilitate bone regeneration. Such a membrane displays a well-developed heterogeneous architecture, satisfactory mechanical performance, and long-lasting characteristics. The in vitro biological evaluation reveals that apart from being a strong barrier, the bilayer struvite-laden membrane can actively promote cellular adhesion, proliferation, and osteogenic differentiation. Consequently, the multifunctional struvite-doped membranes are applied to treat 5 mm-sized bilateral calvarial defects in rats, resulting in overall improved healing outcomes compared with the untreated or the struvite-free membrane-treated group, which is characterized by enhanced osteogenesis and significantly increased new bone formation. The encouraging preclinical results reveal the great potential of the bilayer struvite-doped membrane as a clinical GBR device for augmenting large-area CMF bone reconstruction.

Magnesium for Implants: A Review on the Effect of Alloying Elements on Biocompatibility and Properties

An attempt is made to cover the whole of the topic of biodegradable magnesium (Mg) alloys with a focus on the biocompatibility of the individual alloying elements, as well as shed light on the degradation characteristics, microstructure, and mechanical properties of most binary alloys. Some of the various work processes carried out by researchers to achieve the alloys and their surface modifications have been highlighted. Additionally, a brief look into the literature on magnesium composites as also been included towards the end, to provide a more complete picture of the topic. In most cases, the chronological order of events has not been particularly followed, and instead, this work is concentrated on compiling and presenting an update of the work carried out on the topic of biodegradable magnesium alloys from the recent literature available to us.

Magnesium Phosphate Bioceramics for Bone Tissue Engineering

Magnesium phosphate (MgP) is a family of newly developed resorbable bioceramics for bone tissue engineering. Although calcium phosphates (CaP) are the most commonly used bioceramics, low solubility, and slow degradation, when implanted in vivo, are their main drawbacks. Magnesium (Mg) is an essential element in the human body as it plays important role in bone metabolism, DNA stabilization, and skeletal development. Recent research on magnesium phosphates has established their higher degradability, in vitro, and in vivo biocompatibility. Compared to CaP, very limited research work has been found in the area of MgP. The prime goal of this review is to bring out the importance of magnesium phosphate ceramics for biomedical applications. In this review, we have discussed the synthesis methods, mechanical properties, in vitro and in vivo biocompatibility of MgP bioceramics. Moreover, we have highlighted the recent developments in metal ion-doped MgPs and MgP scaffolds for bone tissue engineering.

Characterization and Biomechanical Study of a Novel Magnesium Potassium Phosphate Cement

Magnesium potassium phosphate cement (MKPC) has attracted considerable attention as a bone regeneration material. However, there are only a few reports on its biomechanical properties. To evaluate the biomechanical properties of MKPC, we compared the mechanical parameters of pedicle screws enhanced with either MKPC or polymethyl methacrylate (PMMA) bone cement. The results show that the maximum pull-out force of the pedicle screws was 417.86 ± 55.57 and 444.43 ± 19.89 N after MKPC cement setting for 30 min and 12 h, respectively, which was better than that of the PMMA cement. In fatigue tests, the maximum pull-out force of the MKPC cement group was 435.20 ± 7.96 N, whereas that of the PMMA cement in the control group was 346.80 ± 7.66 N. Furthermore, the structural characterization analysis of the MKPC cement revealed that its microstructure after solidification was an irregular tightly packed crystal, which improved the mechanical strength of the cement. The maximum exothermic temperature of the MKPC reaction was 45.55 ± 1.35 °C, the coagulation time was 7.89 ± 0.37 min, and the compressive strength was 48.29 ± 4.76 MPa, all of which meet the requirements of clinical application. In addition, the MKPC cement did not significantly inhibit cell proliferation or increase apoptosis, thus indicating good biocompatibility. In summary, MKPC exhibited good biomechanical properties, high initial strength, good biocompatibility, and low exothermic reaction temperature, demonstrating an excellent application potential in the field of orthopedics.

Accelerated bone regeneration through rational design of magnesium phosphate cements

Results of several studies during past years suggested that magnesium phosphate cements (MPCs) not only show excellent biocompatibility and osteoconductivity, but they also provide improved regeneration capacity due to higher solubility compared to calcium phosphates. These findings also highlighted that chemical similarity of bone substitutes to the natural bone tissue is not a determinant factor in the success of regenerative strategies. The aim of this study was to further improve the degradation speed of MPCs for a fast bone ingrowth within a few months. We confirmed our hypothesis, that decreasing the powder-liquid ratio (PLR) of cement results in an increased content of highly soluble phases such as struvite (MgNH₄PO₄⋅6H₂O) as well as K-struvite (MgKPO₄⋅6H₂O). Promising compositions with a low PLR of 1 g ml^-1 were implanted in partially-loaded tibia defects in sheep. Both cements were partially degraded and replaced by bone tissue after 4 months. The degradation speed of the K-struvite cement was significantly higher compared to the struvite cement, initially resulting in the formation of a cell-rich resorption zone at the surface of some implants, as determined by histology. Overall, both MPCs investigated in this study seem to be promising as an alternative to the clinically well-established, but slowly degrading calcium phosphate cements, depending on defect size and desired degradation rate. Whereas the K-struvite cement might require further modification towards a slower resorption and reduced inflammatory response in vivo, the struvite cement appears promising for the treatment of bone defects due to its continuous degradation with simultaneous new bone formation. STATEMENT OF SIGNIFICANCE: Cold setting bone cements are used for the treatment of bone defects that exceed a critical size and cannot heal on their own. They are applied pasty into the bone defect and harden afterwards so that the shape adapts to the individual defect. Magnesium phosphates such as magnesium ammonium phosphate hexahydrate (struvite) belong to a new class of these cold setting bone cements. They degrade much faster than the clinically established calcium phosphates. In this study, a magnesium phosphate that has hardly been investigated so far was implanted into partially-loaded defects in sheeps: Potassium magnesium phosphate hexahydrate. This showed even faster resorption compared to the struvite cement: after 4 months, 63% of the cement was already degraded.

Preparation and characterization of novel lithium magnesium phosphate bioceramic scaffolds facilitating bone generation

Both magnesium and lithium are able to stimulate osteogenic and angiogenic activities. In this study, lithium magnesium phosphate (Li_0.5Mg_2.75(PO₄)₂, Li₁Mg_2.5(PO₄)₂ and Li₂Mg₂(PO₄)₂) biomaterials were synthesized by a solid-state reaction method, and their bioceramic blocks and scaffolds were fabricated by compression molding and 3D printing, respectively. The results indicated that the lithium magnesium phosphates consisted of the Mg₃(PO₄)₂ phase and/or LiMgPO₄ phase. Compared with the lithium-free Mg₃(PO₄)₂ bioceramics, the lithium magnesium phosphate bioceramics showed a lower porosity and consequently a higher compressive strength, and stimulated in vitro cellular proliferation, osteogenic differentiation and proangiogenic activity. In vivo results manifested that the Li₂Mg₂(PO₄)₂ bioceramic scaffolds efficiently promoted bone regeneration of critical-size calvarial defects in rats. Benefiting from the high compressive strength and capacity of stimulating osteogenesis and angiogenesis, the Li₂Mg₂(PO₄)₂ bioceramic scaffolds are considered promising for efficiently repairing the bone defects.

Biodegradable magnesium phosphates in biomedical applications

As an essential element, magnesium is involved in a variety of physiological processes. Magnesium is the second most abundant cation in cells and the fourth most abundant cation in living...

Bone regeneration capacity of newly developed spherical magnesium phosphate cement granules

OBJECTIVES

Magnesium phosphate-based cements begin to catch more attention as bone substitute materials and especially as alternatives for the more commonly used calcium phosphates. In bone substitutes for augmentation purposes, atraumatic materials with good biocompatibility and resorbability are favorable. In the current study, we describe the in vivo testing of novel bone augmentation materials in form of spherical granules based on a calcium-doped magnesium phosphate (CaMgP) cement.

MATERIALS AND METHODS

Granules with diameters between 500 and 710 μm were fabricated via the emulsification of CaMgP cement pastes in a lipophilic liquid. As basic material, two different CaMgP formulations were used. The obtained granules were implanted into drill hole defects at the distal femoral condyle of 27 New Zealand white rabbits for 6 and 12 weeks. After explantation, the femora were examined via X-ray diffraction analysis, histological staining, radiological examination, and EDX measurement.

RESULTS

Both granule types display excellent biocompatibility without any signs of inflammation and allow for proper bone healing without the interposition of connective tissue. CaMgP granules show a fast and continuous degradation and enable fully adequate bone regeneration.

CONCLUSIONS

Due to their biocompatibility, their degradation behavior, and their completely spherical morphology, these CaMgP granules present a promising bone substitute material for bone augmentation procedures, especially in sensitive areas.

CLINICAL RELEVANCE

The mostly insufficient local bone supply after tooth extractions complicates prosthetic dental restoration or makes it even impossible. Therefore, bone augmentation procedures are oftentimes inevitable. Spherical CaMgP granules may represent a valuable bone replacement material in many situations.

A rat model of ischemic osteonecrosis for investigating local therapeutics using biomaterials

Osteonecrosis is one of the most disabling diseases affecting pediatric and adult populations. Local application of biomaterials is a promising therapeutic strategy for osteonecrosis. Currently, there is a lack of low-cost animal models of osteonecrosis for testing and developing biomaterials-driven therapeutics. To develop a rat model of ischemic osteonecrosis (IO), the distal femoral epiphysis was selected due to its size 7.7 folds larger than the proximal femoral epiphysis (p<0.0001). The feasibility of intraosseous drillings and the local application of biomaterials were determined. Four model biomaterials were successfully applied: injectable hydrogel, microsphere, bone cement, and implant. The IO was induced by surgically cauterizing the blood vessels supplying the distal femoral epiphysis. Osteonecrosis of the whole epiphysis was achieved with a complete absence of blood flow and near 100% of apoptotic osteocytes. At eight weeks after IO, severe bone deformity and osteoarthritis developed in the affected epiphysis. The histological analysis showed 50% lacunae empty in the IO group compared to 2% in the control group (p<0.0001). The μCT analysis showed the epiphyseal quotient decreased to 0.46 in the IO group compared to 0.53 in the control group (p<0.0001), and the distal femoral epiphysis in the IO group was 19% smaller than the control group (p<0.01). The Safranin O stained sections showed articular cartilage erosions and subchondral bone fractures in the IO group. In summary, we established a clinically relevant IO model on rats that is compatible with the application of biomaterials for treatment. STATEMENT OF SIGNIFICANCE: Osteonecrosis is one of the most serious orthopedic conditions, leading to permanent joint deformity and end-stage osteoarthritis. An efficient and low-cost animal model is essential for development and testing of new treatment strategies for osteonecrosis. This is the first study to develop a clinically relevant model of osteonecrosis on the distal femoral epiphysis of rats. The model is highly efficient in developing osteonecrosis with relatively low cost and it provides suitable skeletal size to apply various forms of biomaterials. More importantly, it mimicked the pathological features and progression of osteonecrosis in humans. The study is expected to have an important impact on the development and testing of innovative biological therapeutics for osteonecrosis.

Extrusion-Based 3D Printing of Calcium Magnesium Phosphate Cement Pastes for Degradable Bone Implants

This study aimed to develop printable calcium magnesium phosphate pastes that harden by immersion in ammonium phosphate solution post-printing. Besides the main mineral compound, biocompatible ceramic, magnesium oxide and hydroxypropylmethylcellulose (HPMC) were the crucial components. Two pastes with different powder to liquid ratios of 1.35 g/mL and 1.93 g/mL were characterized regarding their rheological properties. Here, ageing over the course of 24 h showed an increase in viscosity and extrusion force, which was attributed to structural changes in HPMC as well as the formation of magnesium hydroxide by hydration of MgO. The pastes enabled printing of porous scaffolds with good dimensional stability and enabled a setting reaction to struvite when immersed in ammonium phosphate solution. Mechanical performance under compression was approx. 8-20 MPa as a monolithic structure and 1.6-3.0 MPa for printed macroporous scaffolds, depending on parameters such as powder to liquid ratio, ageing time, strand thickness and distance.

Resorbable Mg2+-Containing Phosphates for Bone Tissue Repair

Materials based on Mg2+-containing phosphates are gaining great relevance in the field of bone tissue repair via regenerative medicine methods. Magnesium ions, together with condensed phosphate ions, play substantial roles in the process of bone remodeling, affecting the early stage of bone regeneration through active participation in the process of osteosynthesis. In this paper we provide a comprehensive overview of the usage of biomaterials based on magnesium phosphate and magnesium calcium phosphate in bone reconstruction. We consider the role of magnesium ions in angiogenesis, which is an important process associated with osteogenesis. Finally, we summarize the biological properties of calcium magnesium phosphates for regeneration of bone.

A bioactive magnesium phosphate cement incorporating chondroitin sulfate for bone regeneration

Magnesium phosphate cement (MPC) has been evaluated as an inorganic bone filler due to its favorable biocompatibility, biodegradability, rapid setting, high initial strength, and osteogenic potential. However, the setting time of MPC is so rapid that it makes it difficult to use in practice, and the clinical properties of MPC could be further be improved by adding bioactive materials. Here we developed novel bioactive chondroitin sulfate (CS)-MPC composites (CS-MPCs) by incorporating different amounts of CS into MPC. The compositions, microstructures, and physiochemical properties of CS-MPCs and their inducedin vitrocellular responses andin vivobone regeneration properties were evaluated. CS-MPCs had a longer setting time, lower hydration temperature, higher compressive strength, and more neural pH than MPC. CS-MPCs demonstrated similar degradation ratios relative to MPC in Tris-HCl solution. CS-MPCs promoted pre-osteoblast cell proliferation, attachment, and differentiationin vitroand enhanced bone formation surrounding implantsin vivo. In conclusion, through CS modification, our novel CS-MPCs have improved physiochemical properties that enhance compatibilityin vitroand bone regenerationin vivo, making them attractive materials for bone regeneration.

Magnesium Ammonium Phosphate Composite Cell-Laden Hydrogel Promotes Osteogenesis and Angiogenesis In Vitro

Injectable hydrogels provide an effective strategy for minimally invasive treatment on irregular bony defects in the maxillofacial region. To improve the osteoinduction of gelatin methacrylate (GelMA), we fabricated a three-dimensional (3D) culture system based on the incorporation of magnesium ammonium phosphate hexahydrate (struvite) into GelMA. The optimal concentration of struvite was investigated using the struvite extracts, and 500 μg mL^-1 was found to be the most suitable concentration for the osteogenesis of dental pulp stem cells (DPSCs) and angiogenesis of human umbilical vein endothelial cells (HUVECs). We prepared the GelMA composite (MgP) with 500 μg mL^-1 struvite. Struvite did not affect the cross-linking of GelMA and released Mg²⁺ during degradation. The cell delivery system using MgP improved the laden-cell viability, upregulated the expression of osteogenic and angiogenic-differentiation-related genes, and promoted cell migration. Overall, the modifications made to the GelMA in this study improved osteoinduction and demonstrated great potential for application in vascularized bone tissue regeneration.

Experimental Drillable Magnesium Phosphate Cement Is a Promising Alternative to Conventional Bone Cements

Clinically used mineral bone cements lack high strength values, absorbability and drillability. Therefore, magnesium phosphate cements have recently received increasing attention as they unify a high mechanical performance with presumed degradation in vivo. To obtain a drillable cement formulation, farringtonite (Mg₃(PO₄)₂) and magnesium oxide (MgO) were modified with the setting retardant phytic acid (C₆H₁₈O₂₄P₆). In a pre-testing series, 13 different compositions of magnesium phosphate cements were analyzed concentrating on the clinical demands for application. Of these 13 composites, two cement formulations with different phytic acid content (22.5 wt% and 25 wt%) were identified to meet clinical demands. Both formulations were evaluated in terms of setting time, injectability, compressive strength, screw pullout tests and biomechanical tests in a clinically relevant fracture model. The cements were used as bone filler of a metaphyseal bone defect alone, and in combination with screws drilled through the cement. Both formulations achieved a setting time of 5 min 30 s and an injectability of 100%. Compressive strength was shown to be ~12–13 MPa and the overall displacement of the reduced fracture was <2 mm with and without screws. Maximum load until reduced fracture failure was ~2600 N for the cements only and ~3800 N for the combination with screws. Two new compositions of magnesium phosphate cements revealed high strength in clinically relevant biomechanical test set-ups and add clinically desired characteristics to its strength such as injectability and drillability.

In-Vivo Degradation Behavior and Osseointegration of 3D Powder-Printed Calcium Magnesium Phosphate Cement Scaffolds

Calcium magnesium phosphate cements (CMPCs) are promising bone substitutes and experience great interest in research. Therefore, in-vivo degradation behavior, osseointegration and biocompatibility of three-dimensional (3D) powder-printed CMPC scaffolds were investigated in the present study. The materials Mg225 (Ca_0.75Mg_2.25(PO₄)₂) and Mg225d (Mg225 treated with diammonium hydrogen phosphate (DAHP)) were implanted as cylindrical scaffolds (h = 5 mm, Ø = 3.8 mm) in both lateral femoral condyles in rabbits and compared with tricalcium phosphate (TCP). Treatment with DAHP results in the precipitation of struvite, thus reducing pore size and overall porosity and increasing pressure stability. Over 6 weeks, the scaffolds were evaluated clinically, radiologically, with Micro-Computed Tomography (µCT) and histological examinations. All scaffolds showed excellent biocompatibility. X-ray and in-vivo µCT examinations showed a volume decrease and increasing osseointegration over time. Structure loss and volume decrease were most evident in Mg225. Histologically, all scaffolds degraded centripetally and were completely traversed by new bone, in which the remaining scaffold material was embedded. While after 6 weeks, Mg225d and TCP were still visible as a network, only individual particles of Mg225 were present. Based on these results, Mg225 and Mg225d appear to be promising bone substitutes for various loading situations that should be investigated further.

Characterization and In Vitro Evaluations of Injectable Calcium Phosphate Cement Doped with Magnesium and Strontium

Injectable calcium phosphate cement is a promising biomaterial for hard tissue repair due to its osteoinductivity, biocompatibility properties, and its use to correct defect areas involving narrow cavities with limited accessibility by the minimally invasive technique. Microwave-synthesized hydroxyapatite (HA) was used for the preparation of cement. In recent years, both magnesium and strontium calcium phosphate cements have exhibited rapid setting, improved mechanical strength, and a good resorption rate. A big step still remains to develop injectable magnesium and strontium phosphate cements with ideal self-setting properties, adequate mechanical strength, and good biocompatibility for clinical applications. In this study, both magnesium and strontium were doped with synthesized semiamorphous and crystalline hydroxyapatite (HA). The powder mixture was mixed with Na₂HPO₄, NaH₂PO₄, and a carboxymethyl cellulose (CMC) solution to develop the novel magnesium and strontium calcium phosphate cement. The setting time, physiochemical properties of hardened cement, microstructure, mechanical strength, and injectability of the prepared cement were studied. The toxicity evaluation and cell adhesion, which are necessary to identify the suitability of the material for different applications, were quantified and investigated using fibroblast cells. The setting time of cement was reduced substantially for magnesium- or strontium-doped cement by 2 min. The phase composition of the hardened cement expresses the semiamorphous or crystalline phase of HA with additives. Smooth and complete injection of cement paste was observed in semiamorphous HA-based cement. The intercellular reactive oxygen stress (ROS) of the Sr²⁺-doped cement sample showed varied degrees of toxicity to cells in terms of different concentrations. The Mg²⁺-doped cement showed significant attachment of cells after treatment at varying incubation times.

Osteoclast and osteoblast response to strontium-doped struvite coatings on titanium for improved bone integration

To develop implants with improved bone ingrowth, titanium substrates were coated with homogeneous and dense struvite (MgNH4PO4·6H2O) layers by means of electrochemically assisted deposition. Strontium nitrate was added to the coating electrolyte in various concentrations, in order to fabricate Sr-doped struvite coatings with Sr loading ranging from 10.6 to 115 μg/cm2. It was expected and observed that osteoclast activity surrounding the implant was inhibited. The cytocompatibility of the coatings and the effect of Sr-ions in different concentrations on osteoclast formation were analyzed in vitro. Osteoclast differentiation was elucidated on morphological, biochemical as well as on gene expression level. It could be shown that moderate concentrations of Sr2+ had an inhibitory effect on osteoclast formation, while the growth of osteoblastic cells was not negatively influenced compared to pure struvite surfaces. In summary, the electrochemically deposited Sr-doped struvite coatings are a promising approach to improve bone implant ingrowth.

Unravelling the Effect of Citrate on the Features and Biocompatibility of Magnesium Phosphate-Based Bone Cements

In the framework of new materials for orthopedic applications, Magnesium Phosphate-based Cements (MPCs) are currently the focus of active research in biomedicine, given their promising features; in this field, the loading of MPCs with active molecules to be released in the proximity of newly forming bone could represent an innovative approach to enhance the in vivo performances of the biomaterial. In this work, we describe the preparation and characterization of MPCs containing citrate, an ion naturally present in bone which presents beneficial effects when released in the proximity of newly forming bone tissue. The cements were characterized in terms of handling properties, setting time, mechanical properties, crystallinity, and microstructure, so as to unravel the effect of citrate concentration on the features of the material. Upon incubation in aqueous media, we demonstrated that citrate could be successfully released from the cements, while contributing to the alkalinization of the surroundings. The cytotoxicity of the materials toward human fibroblasts was also tested, revealing the importance of a fine modulation of released citrate to guarantee the biocompatibility of the material.

Tough magnesium phosphate-based 3D-printed implants induce bone regeneration in an equine defect model

One of the important challenges in bone tissue engineering is the development of biodegradable bone substitutes with appropriate mechanical and biological properties for the treatment of larger defects and those with complex shapes. Recently, magnesium phosphate (MgP) doped with biologically active ions like strontium (Sr²⁺) have shown to significantly enhance bone formation when compared with the standard calcium phosphate-based ceramics. However, such materials can hardly be shaped into large and complex geometries and more importantly lack the adequate mechanical properties for the treatment of load-bearing bone defects. In this study, we have fabricated bone implants through extrusion assisted three-dimensional (3D) printing of MgP ceramics modified with Sr²⁺ ions (MgPSr) and a medical-grade polycaprolactone (PCL) polymer phase. MgPSr with 30 wt% PCL (MgPSr-PCL30) allowed the printability of relevant size structures (>780 mm³) at room temperature with an interconnected macroporosity of approximately 40%. The printing resulted in implants with a compressive strength of 4.3 MPa, which were able to support up to 50 cycles of loading without plastic deformation. Notably, MgPSr-PCL30 scaffolds were able to promote in vitro bone formation in medium without the supplementation with osteo-inducing components. In addition, long-term in vivo performance of the 3D printed scaffolds was investigated in an equine tuber coxae model over 6 months. The micro-CT and histological analysis showed that implantation of MgPSr-PCL30 induced bone regeneration, while no bone formation was observed in the empty defects. Overall, the novel polymer-modified MgP ceramic material and extrusion-based 3D printing process presented here greatly improved the shape ability and load-bearing properties of MgP-based ceramics with simultaneous induction of new bone formation.

Analysis of the osteogenic and mechanical characteristics of iron (Fe2+/Fe3+)-doped β‑calcium pyrophosphate

The calcium phosphate is the main mineral constituent of bone. Although there has been significant amount of research on finding ideal synthetic bone, no suitable scaffold material has yet been found. In this investigation, the iron doped brushite (CaHPO₄·2H₂O) has been investigated for osteogenic potential and mechanical properties. The synthesis of iron-oxide doping in the form of Fe^2+,3+-ions were carried out using the solution based method in which the ammonium hydrogen phosphate and calcium nitrate solutions were used in stoichiometric ratio for synthesizing CaHPO₄·2H₂O, with doping concentrations of Fe^2+,3+-ions between 5 mol% and 30 mol%. The synthesized powders were analysed using X-ray powder diffraction, FTIR, SEM and Raman spectroscopic techniques. The heat treatment of synthesized powder was carried out at 1000 °C in air for 5 h, and it was found that the dominant crystalline phase in samples with <20 mol% was β-CPP, which also formed an iron-rich solid solution phase. Increasing the concentrations of Fe^2+,3+-ions enhances the phase fraction of FePO₄ and amorphous phase. Amongst the Fe^2+,3+-doped β-CPP minerals, it was found that the 10 mol% Fe^2+,3+-doped β-CPP offers the best combination of bio-mechanical and osteogenic properties as a scaffold for bone tissue regenerative engineering.

Long-Term in Vivo Performance of Low-Temperature 3D-Printed Bioceramics in an Equine Model

Bone has great self-healing capacity, but above a certain critical size, bone defects will not heal spontaneously, requiring intervention to achieve full healing. Among the synthetic calcium phosphate (CaP) bone replacement materials, brushite (CaHPO₄·2H₂O)-based materials are of particular interest because of their degree of solubility and the related high potential to promote bone regeneration after dissolution. They can be produced tailor-made using modern three-dimensional (3D) printing technology. Although this type of implant has been widely tested in vitro, there are only limited in vivo data and less so in a relevant large animal model. In this study, material properties of a 3D-printed brushite-based scaffold are characterized, after which the material is tested by in vivo orthotopic implantation in the equine tuber coxae for 6 months. The implantation procedure was easy to perform and was well tolerated by the animals, which showed no detectable signs of discomfort. In vitro tests showed that compressive strength along the vertical axis of densely printed material was around 13 MPa, which was reduced to approximately 8 MPa in the cylindrical porous implant. In vivo, approximately 40% of the visible volume of the implants was degraded after 6 months and replaced by bone, showing the capacity to stimulate new bone formation. Histologically, ample bone ingrowth was observed. In contrast, empty defects were filled with fibrous tissue only, confirming the material's osteoconductive capacity. It is concluded that this study provides proof that the 3D-printed brushite implants were able to promote new bone growth after 6 months' implantation in a large animal model and that the new equine tuber coxae bone model that was used is a promising tool for bone regeneration studies.