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

Biological and mechanical performance of calcium phosphate cements modified with phytic acid

Calcium phosphate cements, primarily brushite cements, require the addition of setting retarders to ensure adequate processing time and processability. So far, citric acid has been the primary setting retarder used in this context. Due to the poor biocompatibility, it is crucial to explore alternative options for better processing. In recent years, the setting retarder phytic acid (IP6) has been increasingly investigated. This study investigates the biological behaviour of calcium phosphate cements with varying concentrations of IP6, in addition to their physical properties. Therefore cytocompatibility in vitro testing was performed using osteoblastic (MG-63) and osteoclastic (RAW 264.7 differentiated with RANKL) cells. We could demonstrate that the physical properties like the compressive strength of specimens formed with IP6 (brushite_IP6_5 = 11.2 MPa) were improved compared to the reference (brushite = 9.8 MPa). In osteoblast and osteoclast assays, IP6 exhibited significantly better cytocompatibility in terms of cell activity and cell number for brushite cements up to 11 times compared to the brushite reference. In contrast, the calcium-deficient hydroxyapatite (CDHA) cements produced similar results for IP6 (CDHA_IP6_0.25 = 27.0 MPa) when compared to their reference (CDHA = 21.2 MPa). Interestingly, lower doses of IP6 were found to be more effective than higher doses with up to 3 times higher. Additionally, IP6 significantly increased degradation in both passive and active resorption. For these reasons, IP6 is emerging as a strong new competitor to established setting retarders such as citric acid. These cements have potential applications in bone augmentation, the stabilisation of non-load bearing fractures (craniofacial), or the cementation of metal implants.

Influence of the sintering atmosphere on the physico-chemical properties and the osteoclastic resorption of β-tricalcium phosphate cylinders

One of the most widely used materials for bone graft substitution is β-Tricalcium phosphate (β-TCP; β-Ca3(PO4)2). β-TCP is typically produced by sintering in air or vacuum. During this process, evaporation of phosphorus (P) species occurs, leading to the formation of a calcium-rich alkaline layer. It was recently shown that the evaporation of P species could be prevented by co-sintering β-TCP with dicalcium phosphate (DCPA; CaHPO4; mineral name: monetite). The aim of this study was to see how a change of sintering atmosphere could affect the physico-chemical and biological properties of β-TCP. For this purpose, three experimental groups were considered: β-TCP cylinders sintered in air and subsequently polished to remove the surface layer (control group); the same polished cylinders after subsequent annealing at 500 °C in air to generate a calcium-rich alkaline layer (annealed group); and finally, β-TCP cylinders sintered in a monetite-rich atmosphere and subsequently polished (monetite group). XPS analysis confirmed that cylinders from the annealed group had a significantly higher Ca/P molar ratio at their surface than that of the control group while this ratio was significantly lower for the cylinders from the monetite group. Sintering β-TCP in the monetite-rich atmosphere significantly reduced the grain size and increased the density. Changes of surface composition affected the activity of osteoclasts seeded onto the surfaces, since annealed β-TCP cylinders were significantly less resorbed than β-TCP cylinders sintered in the monetite-rich atmosphere. This suggests that an increase of the surface Ca/P molar ratio leads to a decrease of osteoclastic resorption. STATEMENT OF SIGNIFICANCE: Minimal changes of surface and bulk (< 1%) composition have major effects on the ability of osteoclasts to resorb β-tricalcium phosphate (β-TCP), one of the most widely used ceramics for bone substitution. The results presented in this study are thus important for the calcium phosphate community because (i) β-TCP may have up to 5% impurities according to ISO and ASTM standards and still be considered to be "pure β-TCP", (ii) β-TCP surface properties are generally not considered during biocompatibility assessment and (iii) a rationale can be proposed to explain the various inconsistencies reported in the literature on the biological properties of β-TCP.

Effect of minor amounts of β-calcium pyrophosphate and hydroxyapatite on the physico-chemical properties and osteoclastic resorption of β-tricalcium phosphate cylinders

β-Tricalcium Phosphate (β-TCP), one of the most used bone graft substitutes, may contain up to 5 wt% foreign phase according to standards. Typical foreign phases include β-calcium pyrophosphate (β-CPP) and hydroxyapatite (HA). Currently, the effect of small amounts of impurities on β-TCP resorption is unknown. This is surprising since pyrophosphate is a very potent osteoclast inhibitor. The main aim of this study was to assess the effect of small β-CPP fractions (<1 wt%) on the in vitro osteoclastic resorption of β-TCP. A minor aim was to examine the effect of β-CPP and HA impurities on the physico-chemical properties of β-TCP powders and sintered cylinders. Twenty-six batches of β-TCP powder were produced with a Ca/P molar ratio varying between 1.440 and 1.550. Fifteen were further processed to obtain dense and polished β-TCP cylinders. Finally, six of them, with a Ca/P molar ratio varying between 1.496 (1 wt% β-CPP) and 1.502 (1 wt% HA), were incubated in the presence of osteoclasts. Resorption was quantified by white-light interferometry. Osteoclastic resorption was significantly inhibited by β-CPP fraction in a linear manner. The presence of 1% β-CPP reduced β-TCP resorption by 40%, which underlines the importance of controlling β-CPP content when assessing β-TCP biological performance.

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.

In vitro measurement of the chemical changes occurring within β-tricalcium phosphate bone graft substitutes

Several mechanisms proposed to explain the osteoinductive potential of calcium phosphates involve surface mineralization ("bioactivity") and mention the occurrence of concentration gradients between the inner and the outer part of the implanted material. Determining the evolution of the local chemical environment occurring inside the pores of an implanted bone graft substitute (BGS) is therefore highly relevant. A quantitative and fast method was developed to measure the chemical changes occurring within the pores of β-Tricalcium Phosphate (β-TCP) granules incubated in a simulated body fluid. A factorial design of experiment was used to test the effect of particle size, specific surface area, microporosity, and purity of the β-TCP granules. Large pH, calcium and phosphate concentration changes were observed inside the BGS and lasted for several days. The kinetics and magnitude of these changes (up to 2 pH units) largely depended on the processing and properties of the granules. Interestingly, processing parameters that increased the kinetics and magnitude of the local chemical changes are parameters considered to favor calcium phosphate osteoinduction, suggesting that the model might be useful to predict the osteoinductive potential of BGSs. STATEMENT OF SIGNIFICANCE: Recent results suggest that in situ mineralization of biomaterials (polymers, ceramics, metals) might be key in their ability to trigger ectopic bone formation. This is the reason why the effect on in situ mineralization of various synthesis parameters of β-tricalcium phosphate granules was studied (size, microporosity, specific surface area, and Ca/P molar ratio). To the best of our knowledge, this is the first article devoted to the chemical changes occurring within the pores of a bone graft substitute. We believe that the manuscript will prove to be highly important in the design and mechanistic understanding of drug-free osteoinductive biomaterials.

Three-Dimensional Co-culture of Primary Human Osteocytes and Mature Human Osteoclasts in Collagen Gels

Osteoclasts are pivotal cells for bone remodeling and their activity is coordinated by osteocytes that reside inside the bone matrix. In vitro co-cultures of osteocytes and osteoclasts are therefore advantageous to analyze the crosstalk between these cell species. In this study, primary osteocytes were isolated from human bone in a multistep isolation process and embedded into three-dimensional collagen gels. Mature human osteoclasts were generated by differentiation of human peripheral blood mononuclear cells (PBMCs). Different surfaces were tested for osteoclast formation: suspension dishes, collagen gels, and normal tissue culture polystyrene. After detachment from the surfaces, osteoclasts showed typical morphology and gene expression of osteoclast markers. Osteoclasts that were differentiated on collagen exhibited the highest osteoclast marker expression. Cocultivation of mature osteoclasts with osteocytes was performed in a transwell system, with osteocytes, embedded in collagen gels at the apical side and osteoclasts on the basal side of a porous polyethylen terephtalate membrane, which allowed the separate gene expression analysis for osteocytes and osteoclasts. After 7 days of co-culture both cell species showed their typical morphology, which is multinucleated giant cells for osteoclasts and star-shaped cells with dendritic extensions for osteocytes. Furthermore, osteoclast markers tartrate-resistant acid phosphatase, carbonic anhydrase II, and cathepsin K were detected both on gene expression and protein level in single and co-cultures. Osteocytes showed gene expression of typical osteocyte markers E11, sclerostin, dentin matrix protein 1, osteocalcin, and receptor activator of nuclear factor-κ ligand both in single and co-culture. Impact statement This study is the first to establish an in vitro bone model that contains both primary human osteocytes and primary human osteoclasts. Previous studies applied rodent osteocyte cell lines to examine the influence of osteocytes on osteoclast function. This model mimics the clinical situation better since osteocytes are postmitotic cells whose function might be different in primary state compared with a proliferating cell line. Furthermore, the co-culture model can be the basis for in vitro triple culture models involving osteoblasts as the third bone cell species.

Spatiotemporally Controlled Release of Rho-Inhibiting C3 Toxin from a Protein-DNA Hybrid Hydrogel for Targeted Inhibition of Osteoclast Formation and Activity

In osteoporosis, bone structure can be improved by the introduction of therapeutic molecules inhibiting bone resorption by osteoclasts. Here, biocompatible hydrogels represent an excellent option for the delivery of pharmacologically active molecules to the bone tissue because of their biodegradability, injectability, and manifold functionalization capacity. The present study reports the preparation of a multifunctional hybrid hydrogel from chemically modified human serum albumin and rationally designed DNA building blocks. The hybrid hydrogel combines advantageous characteristics, including rapid gelation through DNA hybridization under physiological conditions and a self-healing and injectable nature with the possibility of specific loading and spatiotemporally controlled release of active proteins, making it an advanced biomaterial for the local treatment of bone diseases, for example, osteoporosis. The hydrogels are loaded with a recombinant Rho-inhibiting C3 toxin, C2IN-C3lim-G205C. This toxin selectively targets osteoclasts and inhibits Rho-signaling and, thereby, actin-dependent processes in these cells. Application of C2IN-C3lim-G205C toxin-loaded hydrogels effectively reduces osteoclast formation and resorption activity in vitro, as demonstrated by tartrate-resistant acid phosphatase staining and the pit resorption assay. Simultaneously, osteoblast activity, viability, and proliferation are unaffected, thus making C2IN-C3lim-G205C toxin-loaded hybrid hydrogels an attractive pharmacological system for spatial and selective modulation of osteoclast functions to reduce bone resorption.