In vitro studies of human and rat osteoclast activity on hydroxyapatite, beta-tricalcium phosphate, calcium carbonate

Development, physicochemical characterization and in-vitro biocompatibility study of dromedary camel dentine derived hydroxyapatite for bone repair

This study aimed to produce hydroxyapatite from the dentine portion of camel teeth using a defatting and deproteinizing procedure and characterize its physicochemical and biocompatibility properties. Biowaste such as waste camel teeth is a valuable source of hydroxyapatite, the main inorganic constituent of human bone and teeth which is frequently used as bone grafts in the biomedical field. Fourier Transform infrared (FTIR), and micro-Raman spectroscopy confirmed the functional groups as-sociated with hydroxyapatite. X-ray diffraction (XRD) studies showed camel dentine-derived hydroxyapatite (CDHA) corresponded with hydroxyapatite spectra. Scanning electron micros-copy (SEM) demonstrated the presence of dentinal tubules measuring from 1.69-2.91 µm. The inorganic phases of CDHA were primarily constituted of calcium and phosphorus, with trace levels of sodium, magnesium, potassium, and strontium, according to energy dispersive X-ray analysis (EDX) and inductively coupled plasma mass spectrometry (ICP-MS). After 28 days of incubation in simulated body fluid (SBF), the pH of the CDHA scaffold elevated to 9.2. in-vitro biocompatibility studies showed that the CDHA enabled Saos-2 cells to proliferate and express the bone marker osteonectin after 14 days of culture. For applications such as bone augmentation and filling bone gaps, CDHA offers a promising material. However, to evaluate the clinical feasibility of the CDHA, further in-vivo studies are required.

Monocyte Differentiation on Atomic Layer-Deposited (ALD) Hydroxyapatite Coating on Titanium Substrate

Hydroxyapatite (HA; Ca₁₀(PO₄)₆(OH)₂) coating of bone implants has many beneficial properties as it improves osseointegration and eventually becomes degraded and replaced with new bone. We prepared HA coating on a titanium substrate with atomic layer deposition (ALD) and compared monocyte differentiation and material resorption between ALD-HA and bone. After stimulation with macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor kappa-B ligand (RANKL), human peripheral blood monocytes differentiated into resorbing osteoclasts on bovine bone, but non-resorbing foreign body cells were observed on ALD-HA. The analysis of the topography of ALD-HA and bone showed no differences in wettability (water contact angle on ALD-HA 86.2° vs. 86.7° on the bone), but the surface roughness of ALD-HA (Ra 0.713 µm) was significantly lower compared to bone (Ra 2.30 µm). The cellular reaction observed on ALD-HA might be a consequence of the topographical properties of the coating. The absence of resorptive osteoclasts on ALD-HA might indicate inhibition of their differentiation or the need to modify the coating to induce osteoclast differentiation.

Ag/Ag3PO4 Nanoparticle-Decorated Hydroxyapatite Functionalized Calcium Carbonate: Ultrasound-Assisted Sustainable Synthesis, Characterization, and Antimicrobial Activity

Silver nanoparticles are usually prepared by the reduction of silver cations through chemical and non-sustainable procedures that involve the use of reducing chemical agents. Therefore, many efforts have been made in the search for sustainable alternative methods. Among them, an ultrasound-assisted procedure could be a suitable and sustainable method to afford well-dispersed and nanometric silver particles. This paper describes a sustainable, ultrasound-assisted method using citrate as a reducing agent to prepare silver@hydroxyapatite functionalized calcium carbonate composites. For comparison, an ultrasound-assisted reduction was performed in the presence of NaBH₄. The composites obtained in the presence of these two different reducing agents were compared in terms of nanoparticle nature, antimicrobial activity, and cytotoxic activity. The nanoparticle nature was investigated by several techniques, including X-ray powder diffraction, field-emission scanning electron microscopy, transmission electron microscopy, UV-Vis spectroscopic measurements, and X-ray photoemission spectroscopy. Nanoparticles with a predominance of Ag or Ag₃PO₄ were obtained according to the type of reducing agent used. All composites were tested for antimicrobial and antibiofilm activities against Gram-positive and Gram-negative (Staphylococcus aureus and Pseudomonas aeruginosa, respectively) bacteria and for cytotoxicity towards human skin keratinocytes and human fibroblasts. The nature of the nanoparticles, Ag or Ag₃PO₄, and their predominance seemed to affect the in vitro silver release and the antimicrobial and antibiofilm activities. The composites obtained by the citrate-assisted reduction gave rise to the best results.

In Vivo Analysis of the Regeneration Capacity and Immune Response to Xenogeneic and Synthetic Bone Substitute Materials

Although various studies have investigated differences in the tissue reaction pattern to synthetic and xenogeneic bone substitute materials (BSMs), a lack of knowledge exists regarding the classification of both materials based on the DIN ISO 10993-6 scoring system, as well as the histomorphometrical measurement of macrophage subtypes within their implantation beds. Thus, the present study was conducted to analyze in vivo responses to both xenogeneic and synthetic bone substitute granules. A standardized calvaria implantation model in Wistar rats, in combination with established scoring, histological, histopathological, and histomorphometrical methods, was conducted to analyze the influence of both biomaterials on bone regeneration and the immune response. The results showed that the application of the synthetic BSM maxresorb^® induced a higher pro-inflammatory tissue response, while the xenogeneic BSM cerabone^® induced a higher anti-inflammatory reaction. Additionally, comparable bone regeneration amounts were found in both study groups. Histopathological scoring revealed that the synthetic BSM exhibited non-irritant scores at all timepoints using the xenogeneic BSM as control. Overall, the results demonstrated the biocompatibility of synthetic BSM maxresorb^® and support the conclusion that this material class is a suitable alternative to natural BSM, such as the analyzed xenogeneic material cerabone^®, for a broad range of indications.

Surface Electric Fields Increase Human Osteoclast Resorption through Improved Wettability on Carbonate-Incorporated Apatite

Osteoclast-mediated bioresorption can be an efficient means of incorporating the dissolution of biomaterials in the bone remodeling process. Because of the compositionally and structurally close resemblance of biomaterials with the natural mineral phases of the bone matrix, synthetic carbonate-substituted apatite (CA) is considered as an ideal biomaterial for clinical use. The present study therefore investigated the effects of electrical polarization on the surface characteristics and interactions with human osteoclasts of hydroxyapatite (HA) and CA. Electrical polarization was found to improve the surface wettability of these materials by increasing the surface free energy, and this effect was maintained for 1 month. Analyses of human osteoclast cultures established that CA subjected to a polarization treatment enhanced osteoclast resorption but did not affect the early differentiation phase or the adherent morphology of the osteoclasts as evaluated by staining. These data suggest that the surface characteristics of the CA promoted osteoclast resorption. The results of this work are expected to contribute to the future design of cell-mediated bioresorbable biomaterials capable of resorption by osteoclasts and of serving as a scaffold for bone regeneration.

The Granule Size Mediates the In Vivo Foreign Body Response and the Integration Behavior of Bone Substitutes

The physicochemical properties of synthetically produced bone substitute materials (BSM) have a major impact on biocompatibility. This affects bony tissue integration, osteoconduction, as well as the degradation pattern and the correlated inflammatory tissue responses including macrophages and multinucleated giant cells (MNGCs). Thus, influencing factors such as size, special surface morphologies, porosity, and interconnectivity have been the subject of extensive research. In the present publication, the influence of the granule size of three identically manufactured bone substitute granules based on the technology of hydroxyapatite (HA)-forming calcium phosphate cements were investigated, which includes the inflammatory response in the surrounding tissue and especially the induction of MNGCs (as a parameter of the material degradation). For the in vivo study, granules of three different size ranges (small = 0.355-0.5 mm; medium = 0.5-1 mm; big = 1-2 mm) were implanted in the subcutaneous connective tissue of 45 male BALB/c mice. At 10, 30, and 60 days post implantationem, the materials were explanted and histologically processed. The defect areas were initially examined histopathologically. Furthermore, pro- and anti-inflammatory macrophages were quantified histomorphometrically after their immunohistochemical detection. The number of MNGCs was quantified as well using a histomorphometrical approach. The results showed a granule size-dependent integration behavior. The surrounding granulation tissue has passivated in the groups of the two bigger granules at 60 days post implantationem including a fibrotic encapsulation, while a granulation tissue was still present in the group of the small granules indicating an ongoing cell-based degradation process. The histomorphometrical analysis showed that the number of proinflammatory macrophages was significantly increased in the small granules at 60 days post implantationem. Similarly, a significant increase of MNGCs was detected in this group at 30 and 60 days post implantationem. Based on these data, it can be concluded that the integration and/or degradation behavior of synthetic bone substitutes can be influenced by granule size.

In vivo bioresorbability and bone formation ability of sintered highly pure calcium carbonate granules

Calcium carbonate-based bone substitutes derived from natural coral exoskeleton (aragonite) are resorbed and remodeled faster than calcium phosphate-based substitutes. However, coral species with structures appropriate for use as bone substitutes are very limited. Therefore, it is important to evaluate potential of artificial calcium carbonate ceramics as a bone substitute. In this study, calcium carbonate granules with various porosities and pore sizes were prepared by sintering a highly pure (>99.98%) calcium carbonate powder (calcite), and their resorption properties and bone formation abilities were examined in vivo for the first time. The sintered calcium carbonate was resorbed faster than β-tricalcium phosphate, which has a similar structure. However, sintered calcium carbonate did not promote new bone formation during long-term implantation. Furthermore, both resorption and new bone formation were affected by the pore structure. The optimal structures of the artificially sintered calcium carbonate bone substitute were also discussed.

CircRNAs and LncRNAs in Osteoporosis

Osteoporosis is a systemic bone disease with bone fragility and increased fracture risk. The non-coding RNAs (ncRNAs) have appeared as important regulators of cellular signaling and pertinent human diseases. Studies have demonstrated that circular RNAs (circRNAs) and long non-coding RNAs (lncRNAs) are involved in the progression of osteoporosis through a variety of pathways, and are considered as targets for the prophylaxis and treatment of osteoporosis. Based on an in-depth understanding of their roles and mechanisms in osteoporosis, we summarize the functions and molecular mechanisms of circRNAs and lncRNAs involved in the progression of osteoporosis and provide some new insights for the prognosis, diagnosis and treatment of osteoporosis.

β-tricalcium phosphate for bone substitution: Synthesis and properties

β-tricalcium phosphate (β-TCP) is one the most used and potent synthetic bone graft substitute. It is not only osteoconductive, but also osteoinductive. These properties, combined with its cell-mediated resorption, allow full bone defects regeneration. Its clinical outcome is sometimes considered to be "unpredictable", possibly due to a poor understanding of β-TCP physico-chemical properties: β-TCP crystallographic structure is not fully uncovered; recent results suggest that sintered β-TCP is coated with a Ca-rich alkaline phase; β-TCP apatite-forming ability and osteoinductivity may be enhanced by a hydrothermal treatment; β-TCP grain size and porosity are strongly modified by the presence of minute amounts of β-calcium pyrophosphate or hydroxyapatite impurities. The aim of the present article is to provide a critical, but still rather comprehensive review of the current state of knowledge on β-TCP, with a strong focus on its synthesis and physico-chemical properties, and their link to the in vivo response. STATEMENT OF SIGNIFICANCE: The present review documents the richness, breadth, and interest of the research devoted to β-tricalcium phosphate (β-TCP). β-TCP is synthetic, osteoconductive, osteoinductive, and its resorption is cell-mediated, thus making it one of the most potent bone graft substitutes. This comprehensive review reveals that there are a number of aspects, such as surface chemistry, crystallography, or stoichiometry deviations, that are still poorly understood. As such, β-TCP is still an exciting scientific playground despite a 50 year long history and > 200 yearly publications.

The role of antimiR-26a-5p/biphasic calcium phosphate in repairing rat femoral defects

Although miRNAs have been implicated in the osteogenic differentiation of stem cells, their role in bone repair and reconstruction in tissue‑engineered bone grafts remains unclear. We previously reported that microRNA (miR)‑26a‑5p inhibited the osteogenic differentiation of adipose‑derived mesenchymal stem cells (ADSCs), and that antimiR‑26a‑5p exerted the opposite effect. In the present study, the role of miR‑26a‑5p‑ and antimiR‑26a‑5p‑modified ADSCs combined with biphasic calcium phosphate (BCP) scaffolds was evaluated in a rat femur defect model. The aim of the present study was to improve the understanding of the role of miR‑26a‑5p in bone regeneration in vivo, as well as to provide a new method to optimize the osteogenic ability of BCPs. ADSCs were infected with Lv‑miR‑26a‑5p, Lv‑miR‑NC, Lv‑antimiR‑26a‑5p or Lv‑antimiR‑NC respectively, and then combined with BCP scaffolds to repair rat femoral defects. Using X‑rays, micro‑computed tomography and histology at 2, 4, and 8 weeks postoperatively, the quantity and rate of bone regeneration were analyzed, revealing that they were the highest in animals treated with antimiR‑26a‑5p and the lowest in the miR‑26a‑5p treatment group. The expression levels of osteocalcin, collagen I, Runt‑related transcription factor 2, Wnt family member 5A and calmodulin‑dependent protein kinase II proteins were positively correlated with the bone formation rate. Taken together, the present results demonstrated that miR‑26a‑5p inhibited bone formation while antimiR‑26a‑5p accelerated bone formation via the Wnt/Ca2+ signaling pathway. Therefore, antimiR‑26a‑5p‑modified ADSCs combined with BCP scaffolds may be used to construct an effective tissue‑engineering bone graft for bone repair and reconstruction.

Effect of calcium phosphate phases affecting the crosstalk between osteoblasts and osteoclasts in vitro

Previous studies have reported that octacalcium phosphate (OCP) enhances osteoblast differentiation and osteoclast formation during the hydrolysis process to hydroxyapatite (HA). However, the crystal phases that affect the crosstalk between osteoclasts and osteoblasts are unknown, which should determine the bone substitute material's property of OCP. The present study was designed to investigate whether the chemical composition and crystal structure of calcium phosphates affect osteoclast formation and the osteoclast-osteoblast crosstalk. Biodegradable β-tricalcium phosphate (β-TCP) was used as the control material. Osteoclasts were cultured on HA/OCP or HA/TCP disks and their cellular responses were assessed. Both OCP and β-TCP had a similar ability to create multinucleated osteoclasts. However, OCP promoted the expression of complement component 3a (C3a), a positive coupling factor, in osteoclasts, whereas β-TCP enhanced that of EphrinB2 (EfnB2) and collagen triple helix repeat containing 1 (Cthrc1). During osteoclast culture, phosphate ions were released from the crystals, and OCP-HA conversion was advanced in HA/OCP mixtures and OCP. X-ray diffraction analysis revealed no remarkable changes in the crystal structures of HA/TCP mixtures and β-TCP before and after osteoclast culture. These results indicate that the distinct chemical environment induced by the calcium phosphate phases affects the crosstalk between osteoclasts and osteoblasts. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1001-1013, 2019.

Cemented injectable multi-phased porous bone grafts for the treatment of femoral head necrosis

Cemented injectable multi-phased porous bone grafts for the treatment of femoral head necrosis.

Manipulation of osteoclastogenesis: Bioactive multiphasic silica/collagen composites and their effects of surface and degradation products

The intent of the present study was to demonstrate that multiphasic silica/collagen xerogels are able to manipulate cellular processes. These xerogels were prepared by a sol-gel approach allowing the incorporation of mineral phases. The resulting nanocomposites are designed as biomaterial for bone regeneration. Human osteoclasts derived from peripheral blood mononuclear cells were cultured both indirectly and directly, either in presence of different xerogel types or on their surface, to investigate the factor with the main influence on osteoclastogenesis. To this end, the incorporation of a third phase to silica/collagen xerogels was used to affect osteoclastogenesis. In cell culture, ambient ion conditions controlled by both the degradation products of the xerogel and the bioactivity-dependent ion release and reprecipitation were shown to have the main effect on osteoclast specific enzyme tartrate-resistant acid phosphatase (TRAP) 5b. Late stage of osteoclastogenesis characterized by resorption was strongly dependent on the xerogels composition. Surface chemistry of the xerogels was displayed to play an important role in osteoclast resorption. Biphasic silica/collagen xerogels and triphasic xerogels with calcium carbonate offered widespread resorbed areas, whereas hydroxyapatite containing xerogels showed distinctly reduced resorption. The incorporation of strontium carbonate and phosphate, respectively, as third phase changed TRAP 5b activity dose-dependently and inhibited resorption within 21 days. Quantitative evaluation on osteoclast differentiation was carried out using biochemical methods (TRAP 5b, cathepsin K) and was supported by confocal laser scanning microscopy and scanning electron microscopy (SEM). Qualitative estimation of resorption was carried out by SEM.

Engineering in-vitro stem cell-based vascularized bone models for drug screening and predictive toxicology

The production of veritable in-vitro models of bone tissue is essential to understand the biology of bone and its surrounding environment, to analyze the pathogenesis of bone diseases (e.g., osteoporosis, osteoarthritis, osteomyelitis, etc.), to develop effective therapeutic drug screening, and to test potential therapeutic strategies. Dysregulated interactions between vasculature and bone cells are often related to the aforementioned pathologies, underscoring the need for a bone model that contains engineered vasculature. Due to ethical restraints and limited prediction power of animal models, human stem cell-based tissue engineering has gained increasing relevance as a candidate approach to overcome the limitations of animals and to serve as preclinical models for drug testing. Since bone is a highly vascularized tissue, the concomitant development of vasculature and mineralized matrix requires a synergistic interaction between osteogenic and endothelial precursors. A number of experimental approaches have been used to achieve this goal, such as the combination of angiogenic factors and three-dimensional scaffolds, prevascularization strategies, and coculture systems. In this review, we present an overview of the current models and approaches to generate in-vitro stem cell-based vascularized bone, with emphasis on the main challenges of vasculature engineering. These challenges are related to the choice of biomaterials, scaffold fabrication techniques, and cells, as well as the type of culturing conditions required, and specifically the application of dynamic culture systems using bioreactors.

New bioactive glass scaffolds with exceptional qualities for bone tissue regeneration: response of osteoblasts and osteoclasts

Tissue regeneration is a significantly improved alternative to tissue replacement by implants. It requires porous bioscaffolds for the restoration of natural tissue rather than relying on bio-inactive, often metallic implants. Recently, we developed technology for fabricating novel, nano-macroporous bioactive 'tailored amorphous multi-porous (TAMP)' hard tissue scaffolds using a 70 mol% SiO₂-30 mol% CaO model composition. The TAMP silicate scaffolds, fabricated by a modified sol-gel process, have shown excellent biocompatibility via the rapid formation of hydroxyapatite in biological fluids as well as in early tests with bone forming cells. Here we report an in depth investigation of the response of MC3T3-E1 pre-osteoblast cells and bone marrow derived (BMD) osteoclasts to these TAMP scaffolds. Light and electron microscopic imaging, gene and protein expression, and enzyme activity analyses demonstrate that MC3T3-E1 pre-osteoblasts adhere, proliferate, colonize, and differentiate on and inside the bioactive TAMP scaffolds. Additionally, BMD precursor cells mature into active osteoclasts and remodel the scaffold, highlighting the exceptional qualities of this novel scaffold material for bone tissue regeneration.

Introducing an attractive method for total biomimetic creation of a synthetic biodegradable bioactive bone scaffold based on statistical experimental design

A new total biomimetic technique based on both the water uptake and degradation processes is introduced in this study to provide an interesting procedure to fabricate a bioactive and biodegradable synthetic scaffold, which has a good mechanical and structural properties. The optimization of effective parameters to scaffold fabrication was done by response surface methodology/central composite design (CCD). With this method, a synthetic scaffold was fabricated which has a uniform and open-interconnected porous structure with the largest pore size of 100-200μm. The obtained compressive ultimate strength of ~35MPa and compression modulus of 58MPa are similar to some of the trabecular bone. The pore morphology, size, and distribution of the scaffold were characterized using a scanning electron microscope and mercury porosimeter. Fourier transform infrared spectroscopy, EDAX and X-ray diffraction analyses were used to determine the chemical composition, Ca/P element ratio of mineralized microparticles, and the crystal structure of the scaffolds, respectively. The optimum biodegradable synthetic scaffold based on its raw materials of polypropylene fumarate, hydroxyethyl methacrylate and nano bioactive glass (PPF/HEMA/nanoBG) as 70/30wt/wt%, 20wt%, and 1.5wt/wt% (PHB.732/1.5) with desired porosity, pore size, and geometry were created by 4weeks immersion in SBF. This scaffold showed considerable biocompatibility in the ranging from 86 to 101% for the indirect and direct contact tests and good osteoblast cell attachment when studied with the bone-like cells.

Dicalcium Phosphate Anhydrous: An Appropriate Bioceramic in Regeneration of Critical-Sized Radial Bone Defects in Rats

The present study aimed to evaluate and compare the effectiveness of composites of calcium phosphates including β-tri calcium phosphate (β-TCP), dicalcium phosphate anhydrous (DCPA, monetite), mono-calcium phosphate monohydrate (MCPM), and hydroxyapatite (HA) with the chitosan-gelatin-platelet gel (CGP) on the healing of experimentally induced critical size radial bone defects in rats after 8 weeks of injury. Eighty bilateral bone defects were created in the radial bones of 40 adult male Sprague-Dawley rats. The defects were either left empty (untreated or defect group), or treated with autograft, CGP, CGP-DCP, CGP-TCP, CGP/β-TCP/DCPA (CGP-TD), CGP-TD/MCPM (CGP-TDM), and CGP-TDM/HA (CGP-TDMH) scaffolds. The injured forelimbs were evaluated by radiography, gross morphology, three-dimensional computed tomography scanning, histopathology, histomorphometry, scanning electron microscopy, and biomechanical testing. The materials were analyzed using X-ray diffraction to verify the crystalline nature of their structures, and their crystallinity was revealed based on the diffraction peaks achieved from the XRD analysis. The best results were achieved by the CGP-DCP scaffold and the autograft. The CGP-TCP and CGP-TDMH scaffolds were not degraded, while the CGP-DCP, CGP-TDM, CGP-TD, and CGP scaffolds were biodegraded and enhanced bone formation compared with the CGP-TCP and CGP-TDMH groups (P < 0.05). Overall, the CGP-DCP treated defects showed significant improvement in bone formation and union, bone volume, maximum load, and stiffness compared to the CGP group (P < 0.05). It could be concluded that the CGP-DCP scaffold can be considered as a suitable substitute to autograft. In fact, this study demonstrated that DCPA or monetite has high healing potential due to its biocompatibility, biodegradability and biomechanical, osteoconductive and osteoinductive properties of this bioceramic.

Maxillary Sinus Lift with Beta-Tricalcium Phosphate (β-TCP) in Edentulous Patients: A Nanotomographic and Raman Study

Sinus lift elevation restores bone mass at the maxilla in edentulate patients before the placement of dental implants. It consists of opening the lateral side of the sinus and grafting beta-tricalcium phosphate granules (β-TCP) under the olfactory membrane. Bone biopsies were obtained in five patients after 60 weeks. They were embedded undecalcified in poly(methyl methacrylate) (pMMA); blocks were analyzed by nanocomputed tomography (nanoCT); specific areas were studied by Raman microspectroscopy. Remnants of β-TCP were osseointegrated and covered with mineralized bone; osteoid tissue was also filling the inner porosity. Macrophages having engulfed numerous β-TCP grains were observed in marrow spaces. β-TCP was identified by nanoCT as osseointegrated particles and as granules in the cytoplasm of macrophages. Raman microspectroscopy permitted to compare the spectra of β-TCP and bone in different areas. The ratio of the ~820 cm^-1 band of pMMA (-CH₂ groups) on the ν1 phosphate band at 960 cm^-1 reflected tissue hydration because water was substituted by MMA during histological processing. In bone, the ratio of the ~960 cm^-1 phosphate to the amide 1 band and the ratio ν2 phosphate band by the 1240-1250 amide III band reflect the mineralization degree. Specific bands of β-TCP were found in osseointegrated β-TCP granules and in the grains phagocytized by the macrophages. The hydration degree was maximal for β-TCP phagocytized by macrophages. Raman microspectroscopy associated with nanoCT is a powerful tool in the analysis of the biomaterial degradation and osseointegration.

The biocompatibility of bone cements: progress in methodological approach

The ideal bone graft substitute should have certain properties and there are many studies dealing with mixture of polymethylmetacrilate (PMMA) and ß-tricalciumphospate (ß-TCP) presenting the best characteristics of both. Scanning Electron Microscopy (SEM), for ultra-structural data, resulted a very reliable in vivo model to better understand the bioactivity of a cement and to properly evaluate its suitability for a particular purpose. The present study aims to further improve the knowledge on osteointegration development, using both parameters obtained with the Environmental Scanning Electron Microscopy (ESEM) and focused histological examination. Two hybrid bone graft substitute were designed among ceramic and polymer-based bone graft substitutes. Based on ß-TCP granules sizes, they were created with theoretical different osteoconductive properties. An acrylic standard cement was chosen as control. Cements were implanted in twelve New Zealand White (NZW) rabbits, which were sacrificed at 1, 2, 3, 6, 9 and 12 months after cement implantation. Histological samples were prepared with an infiltration process of LR white resin and then specimens were studied by X-rays, histology and Environmental Scanning Electron Microscopy (ESEM). Comparing the resulting data, it was possible to follow osteointegration’s various developments resulting from different sizes of ß-TCP granules. In this paper, we show that this evaluation process, together with ESEM, provides further important information that allows to follow any osteointegration at every stage of develop.

Biphasic calcium phosphate ceramics for bone reconstruction: A review of biological response

Autologous bone graft is considered as the gold standard in bone reconstructive surgery. However, the quantity of bone available is limited and the harvesting procedure requires a second surgical site resulting in severe complications. Due to these limits, scientists and clinicians have considered alternatives to autologous bone graft. Calcium phosphates (CaPs) biomaterials including biphasic calcium phosphate (BCP) ceramics have proven efficacy in numerous clinical indications. Their specific physico-chemical properties (HA/TCP ratio, dual porosity and subsequent interconnected architecture) control (regulate/condition) the progressive resorption and the bone substitution process. By describing the most significant biological responses reported in the last 30years, we review the main events that made their clinical success. We also discuss about their exciting future applications as osteoconductive scaffold for delivering various bioactive molecules or bone cells in bone tissue engineering and regenerative medicine.

STATEMENT OF SIGNIFICANCE

Nowadays, BCPs are definitely considered as the gold standard of bone substitutes in bone reconstructive surgery. Among the numerous clinical studies in literature demonstrating the performance of BCP, Passuti et al. and Randsford et al. studies largely contributed to the emergence of the BCPs. It could be interesting to come back to the main events that made their success and could explain their large adhesion from scientists to clinicians. This paper aims to review the most significant biological responses reported in the last 30years, of these BCP-based materials. We also discuss about their exciting future applications as osteoconductive scaffold for delivering various bioactive molecules or bone cells in bone tissue engineering and regenerative medicine.

Lightweight Open-Cell Scaffolds from Sea Urchin Spines with Superior Material Properties for Bone Defect Repair

Sea urchin spines (Heterocentrotus mammillatus), with a hierarchical open-cell structure similar to that of human trabecular bone and superior mechanical property (compressive strength ∼43.4 MPa) suitable for machining to shape, were explored for potential applications of bone defect repair. Finite element analyses reveal that the compressive stress concentrates along the dense growth rings and dissipates through strut structures of the stereoms, indicating that the exquisite mesostructures play an important role in high strength-to-weight ratios. The fracture strength of magnesium-substituted tricalcium phosphate (β-TCMP) scaffolds produced by hydrothermal conversion of urchin spines is about 9.3 MPa, comparable to that of human trabecular bone. New bone forms along outer surfaces of β-TCMP scaffolds after implantation in rabbit femoral defects for one month and grows into the majority of the inner open-cell spaces postoperation in three months, showing tight interface between the scaffold and regenerative bone tissue. Fusion of beagle lumbar facet joints using a Ti-6Al-4V cage and β-TCMP scaffold can be completed within seven months with obvious biodegradation of the β-TCMP scaffold, which is nearly completely degraded and replaced by newly formed bone ten months after implantation. Thus, sea urchin spines suitable for machining to shape have advantages for production of biodegradable artificial grafts for bone defect repair.

Surrogate Outcome Measures of In Vitro Osteoclast Resorption of β Tricalcium Phosphate

Introduction of porosity to calcium phosphate scaffolds for bone repair has created a new challenge when measuring bioresorption in vitro, rendering traditional outcome measures redundant. The aim of this study is to identify a surrogate endpoint for use with 3D scaffolds. Murine RAW 264.7 cells are cultured on dense discs of β-tricalcium phosphate in conditions to stimulate osteoclast (OC) formation. Multinucleated OCs are visible from day 6 with increases at days 8 and 10. Resorption pits are first observed at day 6 with much larger pits visible at days 8, 10, and 12. The concentration of calcium ions in the presence of cells is significantly higher than cell-free cultures at days 3 and 9. Using linear regression analysis, Ca ion release could account for 35.9% of any subsequent change in resorption area. The results suggest that Ca ion release is suitable to measure resorption of a beta-tricalcium phosphate ceramic substrate in vitro. This model could replace the more accepted resorption pit assay in circumstances where quantification of pits is not possible, e.g., when characterizing 3D tissue engineered bone scaffolds.

Comparison between mandibular and femur derived bone marrow stromal cells: osteogenic and angiogenic potentials in vitro and bone repairing ability in vivo

M-BMSCs contains stronger osteogenic and angiogenic potentials, and better bone repairing ability.

The Crosstalk between Osteoclasts and Osteoblasts Is Dependent upon the Composition and Structure of Biphasic Calcium Phosphates

Biphasic calcium phosphates (BCPs), consisting of hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP), exhibit good biocompatibility and osteoconductivity, maintaining a balance between resorption of the biomaterial and formation of new bone. We tested whether the chemical composition and/or the microstructure of BCPs affect osteoclasts (OCs) differentiation and/or their ability to crosstalk with osteoblasts (OBs). To this aim, OCs were cultured on BCPs with HA content of 5, 20 or 60% and their differentiation and activity were assessed. We found that OC differentiation is partially impaired by increased HA content, but not by the presence of micropores within BCP scaffolds, as indicated by TRAP staining and gene profile expression. We then investigated whether the biomaterial-induced changes in OC differentiation also affect their ability to crosstalk with OBs and regulate OB function. We found that BCPs with low percentage of HA favored the expression of positive coupling factors, including sphingosine-kinase 1 (SPHK1) and collagen triple helix repeat containing 1 (Cthrc1). In turn, the increase of these secreted coupling factors promotes OB differentiation and function. All together our studies suggest that the chemical composition of biomaterials affects not only the differentiation and activity of OCs but also their potential to locally regulate bone formation.

Osteoclastogenesis/osteoblastogenesis using human bone marrow-derived cocultures on nanotopographical polymer surfaces

BACKGROUND

Optimised nanotopography with controlled disorder (NSQ50) has been shown to stimulate osteogenesis and new bone formation in vitro. Following osteointegration the implant interface must undergo constant remodeling without inducing immune response.

AIM

We aimed to assess the effect of nanotopography on bone remodelling using osteoclast and osteoblast cocultures.

MATERIALS & METHODS

We developed a novel osteoblast/osteoclast coculture using solely human bone marrow derived mesenchymal and hematopeotic progenitor cells without extraneous supplementation. The coculture was been applied to NSQ50 or flat control polycarbonate substrates and assessed using immunohistochemical and immunofluorescent microscopy, scanning electron microscopy and quantitative reverse-transcription PCR methods.

RESULTS

These confirm the presence of mature osteoclasts, osteoblasts and bone formation in coculture. Osteoblast differentiation increased on NSQ50, with no significant difference in osteoclast differentiation.

CONCLUSION

Controlled disorder nanotopography appears to be selectively bioactive. We recommend this coculture method to be a better in vitro approximation of the osseous environment encountered by implants.

Osteogenic and osteoclastogenic differentiation of co-cultured cells in polylactic acid-nanohydroxyapatite fiber scaffolds

The design of bone substitutes involves the creation of a microenvironment supporting molecular cross-talk between cells and scaffolds during tissue formation and remodelling. Bone remodelling process includes the cooperation of bone-building cells and bone-resorbing cells. In this paper we developed polylactic acid (PLA) and composite PLA-nanohydroxyapatite (nHA) scaffolds with 20 and 50wt.% of nHA by electrospinning technique to be used in bone tissue engineering. The developed scaffolds have different fiber diameter, porosity with interconnected pores and mechanical properties. Taking cues from the bone environment features we investigated the differentiation of human mesenchymal stem cells (hMSCs) from bone marrow in osteoblasts and the osteoclastogenesis in the developed scaffolds in homotypic and in co-culture up to 46 days. PLA and composite PLA-nHA scaffolds induced osteogenic and osteoclastogenic differentiation. Both osteoblasts and osteoclasts displayed high expression of specific markers (osteopontin, osteocalcin, RANK, RANKL) and functions such as secretion of ALP, cathepsin K and TRAP activity on composite scaffolds especially on PLA-nHA containing 20wt.% of nHA. The heterotypic interactions between osteoblasts and osteoclasts co-cultured in the developed scaffolds triggered their functional differentiation and activation.

An integrated computational model of the bone microenvironment in bone-metastatic prostate cancer

Bone metastasis will impact most men with advanced prostate cancer. The vicious cycle of bone degradation and formation driven by metastatic prostate cells in bone yields factors that drive cancer growth. Mechanistic insights into this vicious cycle have suggested new therapeutic opportunities, but complex temporal and cellular interactions in the bone microenvironment make drug development challenging. We have integrated biologic and computational approaches to generate a hybrid cellular automata model of normal bone matrix homeostasis and the prostate cancer-bone microenvironment. The model accurately reproduces the basic multicellular unit bone coupling process, such that introduction of a single prostate cancer cell yields a vicious cycle similar in cellular composition and pathophysiology to models of prostate-to-bone metastasis. Notably, the model revealed distinct phases of osteolytic and osteogenic activity, a critical role for mesenchymal stromal cells in osteogenesis, and temporal changes in cellular composition. To evaluate the robustness of the model, we assessed the effect of established bisphosphonate and anti-RANKL therapies on bone metastases. At approximately 100% efficacy, bisphosphonates inhibited cancer progression while, in contrast with clinical observations in humans, anti-RANKL therapy fully eradicated metastases. Reducing anti-RANKL yielded clinically similar results, suggesting that better targeting or dosing could improve patient survival. Our work establishes a computational model that can be tailored for rapid assessment of experimental therapies and delivery of precision medicine to patients with prostate cancer with bone metastases.

The role of osteoclasts in bone tissue engineering

The success of scaffold-based bone regeneration approaches strongly depends on the performance of the biomaterial utilized. Within the efforts of regenerative medicine towards a restitutio ad integrum (i.e. complete reconstruction of a diseased tissue), scaffolds should be completely degraded within an adequate period of time. The degradation of synthetic bone substitute materials involves both chemical dissolution (physicochemical degradation) and resorption (cellular degradation by osteoclasts). Responsible for bone resorption are osteoclasts, cells of haematopoietic origin. Osteoclasts play also a crucial role in bone remodelling, which is essential for the regeneration of bone defects. There is, however, surprisingly limited knowledge about the detailed effects of osteoclasts on biomaterials degradation behaviour. This review covers the relevant fundamental knowledge and progress made in the field of osteoclast activity related to biomaterials used for bone regeneration. In vitro studies with osteoclastic precursor cells on synthetic bone substitute materials show that there are specific parameters that inhibit or enhance resorption. Moreover, analyses of the bone-material interface reveal that biomaterials composition has a significant influence on their degradation in contact with osteoclasts. Crystallinity, grain size, surface bioactivity and density of the surface seem to have a less significant effect on osteoclastic activity. In addition, the topography of the scaffold surface can be tailored to affect the development and spreading of osteoclast cells. The present review also highlights possible areas on which future research is needed and which are relevant to enhance our understanding of the complex role of osteoclasts in bone tissue engineering.

Local microarchitecture affects mechanical properties of deposited extracellular matrix for osteonal regeneration

Multiple biomimetic approaches have been attempted to accelerate the regeneration of functional bone tissue. While most synthetic scaffolds are designed to mimic the architecture of trabecular bone, in the current study, cortical bone-like extracellular matrix was regenerated in vitro within organized structures. Biphasic calcium phosphate (BCaP) and hydroxyapatite (HAp) scaffolds were developed with longitudinal microchannels (250 μm diameter) that resembled native osteons in cortical bone. BCaP and HAp scaffolds had a compressive strength of 7.61±1.42 and 9.98±0.61 MPa respectively. The constructs were investigated in vitro to evaluate the organization and stiffness of the extracellular matrix (ECM) formed by human fetal osteoblasts (HFObs) cultured inside the microchannels. The ECM deposited on the BCaP scaffolds was found to have a higher micro-hardness (h) (1.93±0.40 GPa) than the ECM formed within the HAp microchannels (h=0.80±0.20 GPa) (p<0.05) or native bone (h=0.47-0.74 GPa). ECM deposition within the microchannels resembled osteoid organization and showed a significant increase in both osteoid area and thickness after 24 days (p<0.001). These observations indicate that controlled microarchitecture, specifically cylindrical microchannels, plays a fundamental role in stimulating the appropriate cellular response aimed at recreating organized, cortical bone-like matrix. These findings open the door for researchers to develop a new generation of cortical bone scaffolds that can restore strong, organized bone.

Titanium-enriched hydroxyapatite-gelatin scaffolds with osteogenically differentiated progenitor cell aggregates for calvaria bone regeneration

Adequate bony support is the key to re-establish both function and esthetics in the craniofacial region. Autologous bone grafting has been the gold standard for regeneration of problematic large bone defects. However, poor graft availability and donor-site complications have led to alternative bone tissue-engineering approaches combining osteoinductive biomaterials and three-dimensional cell aggregates in scaffolds or constructs. The goal of the present study was to generate novel cell aggregate-loaded macroporous scaffolds combining the osteoinductive properties of titanium dioxide (TiO2) with hydroxyapatite-gelatin nanocomposites (HAP-GEL) for regeneration of craniofacial defects. Here we investigated the in vivo applicability of macroporous (TiO2)-enriched HAP-GEL scaffolds with undifferentiated and osteogenically differentiated multipotent adult progenitor cell (MAPC and OD-MAPC, respectively) aggregates for calvaria bone regeneration. The silane-coated HAP-GEL with and without TiO2 additives were polymerized and molded to produce macroporous scaffolds. Aggregates of the rat MAPC were precultured, loaded into each scaffold, and implanted to rat calvaria critical-size defects to study bone regeneration. Bone autografts were used as positive controls and a poly(lactic-co-glycolic acid) (PLGA) scaffold for comparison purposes. Preimplanted scaffolds and calvaria bone from pig were tested for ultimate compressive strength with an Instron 4411(®) and for porosity with microcomputerized tomography (μCT). Osteointegration and newly formed bone (NFB) were assessed by μCT and nondecalcified histology, and quantified by calcium fluorescence labeling. Results showed that the macroporous TiO2-HAP-GEL scaffold had a comparable strength relative to the natural calvaria bone (13.8±4.5 MPa and 24.5±8.3 MPa, respectively). Porosity was 1.52±0.8 mm and 0.64±0.4 mm for TiO2-HAP-GEL and calvaria bone, respectively. At 8 and 12 weeks postimplantation into rat calvaria defects, greater osteointegration and NFB were significantly present in the TiO2-enriched HAP-GEL constructs with OD-MAPCs, compared to the undifferentiated MAPC-loaded constructs, cell-free HAP-GEL with and without titanium, and PLGA scaffolds. The tissue-engineered TiO2-enriched HAP-GEL constructs with OD-MAPC aggregates present a potential useful therapeutic approach for calvaria bone regeneration.

Characterization of bone mineral-resembling biomaterials for optimizing human osteoclast differentiation and resorption

Bioresorption is a biological mechanism by which biomaterials are resorbed and thereby disappear from implantation sites partially or completely over a period of time. Osteoclast-medicated bioresorption is a possible new advantage to incorporate material degradation into remodeling in bone metabolism process. The purpose of this study was to investigate the osteoclastogenesis and bioresorption of synthesized calcium phosphate materials. Differentiation into mature human osteoclasts on carbonated hydroxyapatite (CA) was significantly enhanced compared to hydroxyapatite (HA) and β-tricalcium phosphate, based on the quantitative gene expressions of molecular markers for osteoclast differentiation. Osteoclasts adhered and differentiated into giant multinuclear TRAP-positive cells on every type of synthesized sample based on the histological analysis. Morphological observations using fluorescence and quantitative analysis revealed that the actin rings of osteoclasts on CA were thick, small in diameter and co-localized with vinculin, similar to the rings found on bone slices. In contrast, the actin rings of osteoclasts on HA and culture dishes were thin and large in diameter. Scanning electron microscopic images and quantitative analysis indicated that the resorption pits on CA were significantly deeper than those on HA due to the enhanced tight sealing ability between osteoclasts and their substrate.

Bioactive glass foam scaffolds are remodelled by osteoclasts and support the formation of mineralized matrix and vascular networks in vitro

Remodelling of scaffolds and new bone formation is critical for effective bone regeneration. Herein is reported the first demonstration of resorption pits due to osteoclast activity on the surface of sol-gel bioactive glass foam scaffolds. Bioactive glass foam scaffolds are known to have osteogenic potential and suitable pore networks for bone regeneration. Degradation of the scaffolds is known to be initially solution mediated, but for effective bone regeneration, remodelling of the scaffold by osteoclasts and vascularisation of the scaffold is necessary. The culture of C7 macrophages on a bioactive glass scaffold induces the cells to differentiate into (TRAP(+ve) ) osteoclasts. They then form distinctive resorption pits within 3 weeks, while MC3T3-E1 pre-osteoblasts deposit mineralized osteoid on their surfaces in co-culture. The scaffolds are of the 70S30C (70 mol% SiO2 , 30 mol% CaO) composition, with modal pore and interconnect diameters of 373 μm and 172 μm respectively (quantified by X-ray micro-tomography and 3D image analysis). The release of soluble silica and calcium ions from 70S30C scaffolds induces an increase in osteoblast numbers as determined via the MTT assay. Scaffolds also support growth of endothelial cells on their surface and tube formation (characteristic of functional microvasculature) following 4 days in culture. This data supports the hypothesis that 70S30C bioactive glass scaffolds promote the differentiation of the 3 main cell types involved in vascularized bone regeneration.

Substituted hydroxyapatites for bone repair

Calcium phosphates such as hydroxyapatite have a wide range of applications both in bone grafts and for the coating of metallic implants, largely as a result of their chemical similarity to the mineral component of bone. However, to more accurately mirror the chemistry, various substitutions, both cationic (substituting for the calcium) and anionic (substituting for the phosphate or hydroxyl groups) have been produced. Significant research has been carried out in the field of substituted apatites and this paper aims to summarise some of the key effect of substitutions including magnesium, zinc, strontium, silicon and carbonate on physical and biological characteristics. Even small substitutions have been shown to have very significant effects on thermal stability, solubility, osteoclastic and osteoblastic response in vitro and degradation and bone regeneration in vivo.

Recent advances in research applications of nanophase hydroxyapatite

Hydroxyapatite, the main inorganic material in natural bone, has been used widely for orthopaedic applications. Due to size effects and surface phenomena at the nanoscale, nanophase hydroxyapatite possesses unique properties compared to its bulk-phase counterpart. The high surface-to-volume ratio, reactivities, and biomimetic morphologies make nano-hydroxyapatite more favourable in applications such as orthopaedic implant coating or bone substitute filler. Recently, more efforts have been focused on the possibility of combining hydroxyapatite with other drugs and materials for multipurpose applications, such as antimicrobial treatments, osteoporosis treatments and magnetic manipulation. To build more effective nano-hydroxyapatite and composite systems, the particle synthesis processes, chemistry, and toxicity have to be thoroughly investigated. In this Minireview, we report the recent advances in research regarding nano-hydroxyapatite. Synthesis routes and a wide range of applications of hydroxyapatite nanoparticles will be discussed. The Minireview also addresses several challenges concerning the biosafety of the nanoparticles.

Cell-based resorption assays for bone graft substitutes

The clinical utilization of resorbable bone substitutes has been growing rapidly during the last decade, creating a rising demand for new resorbable biomaterials. An ideal resorbable bone substitute should not only function as a load-bearing material but also integrate into the local bone remodeling process. This means that these bone substitutes need to undergo controlled resorption and then be replaced by newly formed bone structures. Thus the assessment of resorbability is an important first step in predicting the in vivo clinical function of bone substitute biomaterials. Compared with in vivo assays, cell-based assays are relatively easy, reproducible, inexpensive and do not involve the suffering of animals. Moreover, the discovery of RANKL and M-CSF for osteoclastic differentiation has made the differentiation and cultivation of human osteoclasts possible and, as a result, human cell-based bone substitute resorption assays have been developed. In addition, the evolution of microscopy technology allows advanced analyses of the resorption pits on biomaterials. The aim of the current review is to give a concise update on in vitro cell-based resorption assays for analyzing bone substitute resorption. For this purpose models using different cells from different species are compared. Several popular two-dimensional and three-dimensional optical methods used for resorption assays are described. The limitations and advantages of the current ISO degradation assay in comparison with cell-based assays are discussed.

Effects of surface microtopography on the assembly of the osteoclast resorption apparatus

Bone degradation by osteoclasts depends on the formation of a sealing zone, composed of an interlinked network of podosomes, which delimits the degradation lacuna into which osteoclasts secrete acid and proteolytic enzymes. For resorption to occur, the sealing zone must be coherent and stable for extended periods of time. Using titanium roughness gradients ranging from 1 to 4.5 µm R(a) as substrates for osteoclast adhesion, we show that microtopographic obstacles of a length scale well beyond the range of the 'footprint' of an individual podosome can slow down sealing-zone expansion. A clear inverse correlation was found between ring stability, structural integrity and sealing-zone translocation rate. Direct live-cell microscopy indicated that the expansion of the sealing zone is locally arrested by steep, three-dimensional 'ridge-like barriers', running parallel to its perimeter. It was, however, also evident that the sealing zone can bypass such obstacles, if pulled by neighbouring regions, extending through flanking, obstacle-free areas. We propose that sealing-zone dynamics, while being locally regulated by surface roughness, are globally integrated via the associated actin cytoskeleton. The effect of substrate roughness on osteoclast behaviour is significant in relation to osteoclast function under physiological and pathological conditions, and may constitute an important consideration in the design of advanced bone replacements.