Crystalline Micro-Sized Carbonated Apatites: Chemical Anisotropy of the Crystallite Surfaces, Biocompatibility, Osteoconductivity, and Osteoinductive Effect Enhanced by Poly(ethylene phosphoric acid)

Carbonated hydroxyapatites (CAp) are very close to natural bone apatite in chemical composition and are regarded as a prospective bone mineral substitute for bone surgery and orthopedics. However, until now, the studies and applications of CAp were limited because of the amorphous nature of the synthetic CAp. In the present work, microsized highly crystalline carbonated apatites with uniform hexagonal (hCAp) or platelike (pCAp) morphology have been studied for the first time in vitro and in vivo, comparing against commercial hydroxyapatite (HAp) and β-tricalcuim phosphate (βTCP). In vitro experiments on dissolution of those calcium phosphate ceramics (CPCs) in acetate (pH 5.5) and Tris (pH 7.3) buffer solutions showed the following rank order of the dissolution rates: βTCP > hCAp > pCAp > HAp. The higher dissolution rate of hCAp in comparison with pCAp is explained by chemical anisotropy of the crystallite surfaces, which was proven by SEM studies of the changes in the morphology of hCAp and pCAp crystallites during hydrolysis. A 5-week experiment on subcutaneous implantation of CPC species showed the following rank order of bioresorption rates: βTCP > pCAp > hCAp > HAp. pCAp matrixes exhibited the highest biocompatibility, confirmed by histomorphological analysis. Three-month bone regeneration experiments involving a rat tibial defect model were conducted with 250-500 μm granules of pCAp and pCAp-PEPA [pCAp, pretreated with 2 wt % poly(ethylene phosphoric acid)]. Notably, pCAp-PEPA implants were resorbed at higher rates and induced the formation of more mature osseous tissue, a compact bone with Haversian systems.

Fibroblasts Mediate Ectopic Bone Formation of Calcium Phosphate Ceramics

Heterogeneity of fibroblasts directly affects the outcome of tissue regeneration; however, whether bioactive ceramics regulate bone regeneration through fibroblasts is unclear. Ectopic bone formation model with biphasic calcium phosphate (BCP) implantation was used to investigate the temporal and spatial distribution of fibroblasts around ceramics. The effect of BCP on L929 fibroblasts was evaluated by EdU assay, transwell assay, and qRT-PCR. Further, the effect of its conditioned medium on osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) was confirmed by ALP staining. SEM and XRD results showed that BCP contained abundant micro- and macro-pores and consisted of hydrogen-apatite (HA) and β-tricalcium phosphate (β-TCP) phases. Subsequently, BCP implanted into mice muscle successfully induced osteoblasts and bone formation. Fibroblasts labelled by vimentin gathered around BCP at 7 days and peaked at 14 days post implantation. In vitro, BCP inhibited proliferation of L929 fibroblast but promoted its migration. Moreover, expression of Col1a1, Bmp2, and Igf1 in L929 treated by BCP increased significantly while expression of Tgfb1 and Acta did not change. ALP staining further showed conditioned media from L929 fibroblasts treated by BCP could enhance osteogenic differentiation of BMSCs. In conclusion, fibroblasts mediate ectopic bone formation of calcium phosphate ceramics.

Strontium Functionalization of Biomaterials for Bone Tissue Engineering Purposes: A Biological Point of View

Strontium (Sr) is a trace element taken with nutrition and found in bone in close connection to native hydroxyapatite. Sr is involved in a dual mechanism of coupling the stimulation of bone formation with the inhibition of bone resorption, as reported in the literature. Interest in studying Sr has increased in the last decades due to the development of strontium ranelate (SrRan), an orally active agent acting as an anti-osteoporosis drug. However, the use of SrRan was subjected to some limitations starting from 2014 due to its negative side effects on the cardiac safety of patients. In this scenario, an interesting perspective for the administration of Sr is the introduction of Sr ions in biomaterials for bone tissue engineering (BTE) applications. This strategy has attracted attention thanks to its positive effects on bone formation, alongside the reduction of osteoclast activity, proven by in vitro and in vivo studies. The purpose of this review is to go through the classes of biomaterials most commonly used in BTE and functionalized with Sr, i.e., calcium phosphate ceramics, bioactive glasses, metal-based materials, and polymers. The works discussed in this review were selected as representative for each type of the above-mentioned categories, and the biological evaluation in vitro and/or in vivo was the main criterion for selection. The encouraging results collected from the in vitro and in vivo biological evaluations are outlined to highlight the potential applications of materials’ functionalization with Sr as an osteopromoting dopant in BTE.

eIF2α-ATF4 Pathway Activated by a Change in the Calcium Environment Participates in BCP-Mediated Bone Regeneration

Biphasic calcium phosphate (BCP) ceramic is a classic bone void filler and a common basis of new materials for bone defect repair. However, the specific mechanism of BCP in osteogenesis has not been fully elucidated. Endoplasmic reticulum stress (ERs) and the subsequent PERK-eIF2α-ATF4 pathway can be activated by various factors, including trauma and intracellular calcium changes, and therefore worth exploring as a potential mechanism in BCP-mediated bone repair. Herein, a rat lateral femoral epicondyle defect model in vivo and a simulated BCP-mediated calcium environment in vitro were constructed for the analysis of BCP-related osteogenesis and the activation of ERs and the eIF2α-ATF4 pathway. An inhibitor of eIF2α dephosphorylation (salubrinal) was also used to explore the effect of the eIF2α-ATF4 pathway on BCP-mediated bone regeneration. The results showed that the ERs and eIF2α-ATF4 pathway activation were observed during 4 weeks of bone repair, with a rapid but brief increase immediately after artificial defect surgery and a re-increase after 4 weeks with the resorption of BCP materials. Mild ERs and the activated eIF2α induced by the calcium changes mediated by BCP regulated the expression of osteogenic-related proteins and had an important role during the defect repair. In conclusion, the eIF2α-ATF4 pathway activated by a change in the calcium environment participates in BCP-mediated bone regeneration. eIF2α-ATF4 and ERs could provide new directions for further studies on new materials in bone tissue engineering.

Building Osteogenic Microenvironments With Strontium-Substituted Calcium Phosphate Ceramics

Bioceramics have experienced great development over the past 50 years. Modern bioceramics are designed to integrate bioactive ions within ceramic granules to trigger living tissue regeneration. Preclinical and clinical studies have shown that strontium is a safe and effective divalent metal ion for preventing osteoporosis, which has led to its incorporation in calcium phosphate-based ceramics. The local release of strontium ions during degradation results in moderate concentrations that trigger osteogenesis with few systemic side effects. Moreover, strontium has been proven to generate a favorable immune environment and promote early angiogenesis at the implantation site. Herein, the important aspects of strontium-enriched calcium phosphate bioceramics (Sr-CaPs), and how Sr-CaPs affect the osteogenic microenvironment, are described.

Nano-Hydroxyapatite Coating Promotes Porous Calcium Phosphate Ceramic-Induced Osteogenesis Via BMP/Smad Signaling Pathway

BACKGROUND

The hierarchical porous structure and surface topography of calcium phosphate (CaP) bioceramics have a crucial impact on their osteoinductivity.

PURPOSE

To fabricate a biomimetic bone graft with an interconnected porous structure analogous to that of trabecular bone and a bioactive nanostructured surface with excellent osteoinductive potential.

MATERIALS AND METHODS

A biphasic CaP (BCP) substrate with highly porous structure was fabricated by an improved sponge replication method. Surface modification was performed by uniformly depositing a hydroxyapatite (HA) nanoparticle layer to create nHA-coated BCP scaffolds. The effects of these scaffolds on osteogenic differentiation of murine bone marrow-derived stem cells (BMSCs) were investigated in vitro, and their osteoinductivity was further assessed in vivo.

RESULTS

The BCP and nHA-coated BCP scaffolds had similar trabecular bone-like architectures but different surface structures, with mean grain sizes of ~55 nm and ~1 μm, respectively. Compared with the BCP substrate, the nHA-coated BCP scaffolds favored cell adhesion and promoted osteogenic differentiation of BMSCs, as evidenced by upregulated expression of osteogenic genes, enhanced alkaline phosphatase activity, and increased osteocalcin production. This could be attributed to activation of the BMP/Smad signaling pathway, as significantly higher expression levels of BMPRI, Smad1, Smad4, and Smad5 were observed in the nHA-coated BCP group. The nHA-coated BCP scaffold not only maintained scaffold integrity but also induced ectopic bone formation when implanted into rabbit dorsal muscle in vivo for 90 days, whereas the BCP substrate underwent marked biodegradation that led to severe inflammation with no sign of osteogenesis.

CONCLUSION

The present study demonstrates the potential of this biomimetic bone graft with a trabecular framework and nanotopography for use in orthopedic applications.

The material and biological characteristics of osteoinductive calcium phosphate ceramics

The discovery of osteoinductivity of calcium phosphate (Ca-P) ceramics has set an enduring paradigm of conferring biological regenerative activity to materials with carefully designed structural characteristics. The unique phase composition and porous structural features of osteoinductive Ca-P ceramics allow it to interact with signaling molecules and extracellular matrices in the host system, creating a local environment conducive to new bone formation. Mounting evidence now indicate that the osteoinductive activity of Ca-P ceramics is linked to their physicochemical and three-dimensional structural properties. Inspired by this conceptual breakthrough, many laboratories have shown that other materials can be also enticed to join the rank of tissue-inducing biomaterials, and besides the bones, other tissues such as cartilage, nerves and blood vessels were also regenerated with the assistance of biomaterials. Here, we give a brief historical recount about the discovery of the osteoinductivity of Ca-P ceramics, summarize the underlying material factors and biological characteristics, and discuss the mechanism of osteoinduction concerning protein adsorption, and the interaction with different types of cells, and the involvement of the vascular and immune systems.

Silicon bioceramic loaded with vancomycin stimulates bone tissue regeneration

Porous ceramics doped with silicon and pure β-TCP were analyzed in terms of internal microstructure, cell behavior, and the percentage of newly formed bone. Additionally the materials were tested to determine which of the two had better properties to load and release vancomycin hydrochloride. Internal pore distribution and porosity were determined through high pressure mercury porosimetry and the specific surface area was measured by the Brunauer Emmet-Teller method. The proliferation and viability of the human osteoblast-like cell line MG-63 was studied to validate both materials. The materials were tested on eight New Zealand rabbits which created defects, 10 mm in diameter, in the calvaria bone. After 8 and 12 weeks a histological and histomorphometric analysis was performed. Si-β-TCP showed a higher porosity and specific surface area. The cytocompatibility test revealed acceptable results in terms of proliferation and viability whereas the percentage of new bone was higher in Si-β-TCP with a two-time study being statistically significant with 12 weeks of healing (p < 0.05).The vancomycin loaded within the ceramic scaffolds were burst released and the material had the ability to inhibit bacterial growth. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2307-2315, 2018.

[Finite element study on calcium phosphate ceramic screw implanting after removing dynamic hip screw]

Objective

To investigate the validity of improving the femur's mechanical characteristics by implanting calcium phosphate ceramic screws after removing dynamic hip screw (DHS).

Methods

The three dimensional finite element model of the femur was built based on the CT scanning of a normal male volunteer. Then the models of the femur with and without DHS were established. According to calcium phosphate ceramic screws with porosity and apparent elastic modulus, 80% and 0.1 GPa were set as group A, 50% and 1.0 GPa as group B, and 30% and 1.5 GPa as group C. Von Mises stress distribution and maximum stress were recorded when the joint was maximally loaded in a gait cycle.

Results

The Von Mises in normal femoral shaft was uniform; no phenomena of stress concentration was observed and the maximum stress located at the joint load-bearing site of the proximal femur. The stress concentration was observed in the femur without DHS, and the maximum stress located at the distal femur around the screw hole. By comparing several different calcium phosphate ceramic screws, the stress distribution of group B was similar to normal femur model, and the maximum stress located at the joint load-bearing site. The other screws of groups A and C showed varying degrees of stress concentration.

Conclusion

Implanting calcium phosphate ceramic screw can improve the mechanical characteristics of the femur after removing dynamic hip screw, and the calcium phosphate ceramic screw with 50% porosity and 1.0 GPa apparent elastic modulus is suitable for implanting.

Submicron-surface structured tricalcium phosphate ceramic enhances the bone regeneration in canine spine environment

Calcium phosphate ceramics with submicron-scaled surface structure can trigger bone formation in non-osseous sites and are expected to enhance bone formation in spine environment. In this study, two tricalcium phosphate ceramics having either a submicron-scaled surface structure (TCP-S) or a micron-scaled one (TCP-B) were prepared and characterized regarding their physicochemical properties. Granules (size 1-2 mm) of both materials were implanted on either left or right side of spinous process, between the two lumbar vertebrae (L3-L4), and in paraspinal muscle of eight beagles. After 12 weeks of implantation, ectopic bone was observed in muscle in TCP-S explants (7.7 ± 3.7%), confirming their ability to inductively form bone in non-osseous sites. In contrast, TCP-B implants did not lead to bone formation in muscle. Abundant bone (34.1 ± 6.6%) was formed within TCP-S implants beside the two spinous processes, while limited bone (5.1 ± 4.5%) was seen in TCP-B. Furthermore, the material resorption of TCP-S was more pronounced than that of TCP-B in both the muscle and spine environments. The results herein indicate that the submicron-scaled surface structured tricalcium phosphate ceramic could enhance bone regeneration as compared to the micron-scaled one in spine environment. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1865-1873, 2016.

Influence of different calcium phosphate ceramics on growth and differentiation of cells in osteoblast-endothelial co-cultures

Strategies for improvement of angiogenesis and vasculogenesis using different cells and materials are paramount aims in the field of bone tissue engineering. Thereby, the interaction between different cell types and scaffold materials is crucial for growth, differentiation, and long-term outcomes of tissue-engineered constructs. In this study, we evaluated the interaction of osteoblasts and endothelial cells in three-dimensional tissue-engineered constructs using beta tricalciumphosphate (β-TCP, [ß-Ca₃ (PO₄ )₂ ]) and calcium-deficient hydroxyapatite (CDHA, [Ca₉ (PO₄ )₅ (HPO₄ )OH]) ceramics as scaffolds. We focused on initial cell organization, cell proliferation, and differential expression of osteoblastic and endothelial markers employing monocultures and co-cultures of endothelial cells of two different origins [human umbilical vein endothelial cells (HUVECs) and outgrowth endothelial cells (OECs)] with primary human osteoblasts (hOBs). Despite different chemical and physical characteristics of CDHA and β-TCP ceramics, similar patterns in cell growth, differentiation, and gene expression were detected in tissue-engineered constructs consisting of hOB, HUVEC, and HUVEC/hOB-co-cultures. Under dynamic cell culture conditions we found proliferation of these cells with stable endothelial and osteoblastic differentiation patterns. Both material types are highly biocompatible with these cells providing a promising perspective for the future research. In this study, both materials did not support growth and differentiation of OEC. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1950-1962, 2017.

Microporous calcium phosphate ceramics driving osteogenesis through surface architecture

The presence of micropores in calcium phosphate (CaP) ceramics has shown its important role in initiating inductive bone formation in ectopic sites. To investigate how microporous CaP ceramics trigger osteoinduction, we optimized two biphasic CaP ceramics (i.e., BCP-R and BCP-S) to have the same chemical composition, equivalent surface area per volume, comparable protein adsorption, similar ion (i.e., calcium and phosphate) exchange and the same surface mineralization potential, but different surface architecture. In particular, BCP-R had a surface roughness (Ra) of 325.4 ± 58.9 nm while for BCP-S it was 231.6 ± 35.7 nm. Ceramic blocks with crossing or noncrossing channels of 250, 500, 1000, and 2000 µm were implanted in paraspinal muscle of dogs for 12 weeks. The percentage of bone volume in the channels was not affected by the type of pores (i.e., crossing vs. closed) or their size, but it was greatly influenced by the ceramic type (i.e., BCP-R vs. BCP-S). Significantly, more bone was formed in the channels of BCP-R than in those of BCP-S. Since the two CaP ceramics differed only in their surface architecture, the results hereby demonstrate that microporous CaP ceramics may induce ectopic osteogenesis through surface architecture.

The stability mechanisms of an injectable calcium phosphate ceramic suspension

Calcium phosphate ceramics are widely used as bone substitutes in dentistry and orthopedic applications. For minimally invasive surgery an injectable calcium phosphate ceramic suspension (ICPCS) was developed. It consists in a biopolymer (hydroxypropylmethylcellulose: HPMC) as matrix and bioactive calcium phosphate ceramics (biphasic calcium phosphate: BCP) as fillers. The stability of the suspension is essential to this generation of "ready to use" injectable biomaterial. But, during storage, the particles settle down. The engineering sciences have long been interested in models describing the settling (or sedimentation) of particles in viscous fluids. Our work is dedicated to the comprehension of the effect of the formulation on the stability of calcium phosphate suspension before and after steam sterilization. The rheological characterization revealed the macromolecular behavior of the suspending medium. The investigations of settling kinetics showed the influence of the BCP particle size and the HPMC concentration on the settling velocity and sediment compactness before and after sterilization. To decrease the sedimentation process, the granule size has to be smaller and the polymer concentration has to increase. A much lower sedimentation velocity, as compared to Stokes law, is observed and interpreted in terms of interactions between the polymer network in solution and the particles. This experimentation highlights the granules spacer property of hydrophilic macromolecules that is a key issue for interconnection control, one of the better ways to improve osteoconduction and bioactivity.

Porous hydroxyapatite and biphasic calcium phosphate ceramics promote ectopic osteoblast differentiation from mesenchymal stem cells

Because calcium phosphate (Ca-P) ceramics have been used as bone substitutes, it is necessary to investigate what effects the ceramics have on osteoblast maturation. We prepared three types of Ca-P ceramics with different Ca-P ratios, i.e. hydroxyapatite (HA), beta-tricalcium phosphate (β-TCP), and biphasic calcium phosphate (BCP) ceramics with dense-smooth and porous structures. Comprehensive gene expression microarray analysis of mouse osteoblast-like cells cultured on these ceramics revealed that porous Ca-P ceramics considerably affected the gene expression profiles, having a higher potential for osteoblast maturation. In the in vivo study that followed, porous Ca-P ceramics were implanted into rat skeletal muscle. Sixteen weeks after the implantation, more alkaline-phosphatase-positive cells were observed in the pores of hydroxyapatite and BCP, and the expression of the osteocalcin gene (an osteoblast-specific marker) in tissue grown in pores was also higher in hydroxyapatite and BCP than in β-TCP. In the pores of any Ca-P ceramics, 16 weeks after the implantation, we detected the expressions of marker genes of the early differentiation stage of chondrocytes and the complete differentiation stage of adipocytes, which originate from mesenchymal stem cells, as well as osteoblasts. These marker gene expressions were not observed in the muscle tissue surrounding the implanted Ca-P ceramics. These observations indicate that porous hydroxyapatite and BCP had a greater potential for promoting the differentiation of mesenchymal stem cells into osteoblasts than β-TCP.