Dicalcium phosphate cements: brushite and monetite

New depowdering-friendly designs for three-dimensional printing of calcium phosphate bone substitutes

Powder-based three-dimensional printing (3DP) is a versatile method that allows creating synthetic calcium phosphate (CaP) scaffolds of complex shapes and structures. However, one major drawback is the difficulty of removing all remnants of loose powder from the printed scaffolds, the so-called depowdering step. In this study, a new design approach was proposed to solve this problem. Specifically, the design of the printed scaffolds consisted of a cage with windows large enough to enable depowdering while still trapping loose fillers placed inside the cage. To demonstrate the potential of this new approach, two filler geometries were used: sandglass and cheese segment. The distance between the fillers was varied and they were either glued to the cage or free to move after successful depowdering. Depowdering efficiency was quantified by microstructural morphometry. The results showed that the use of mobile fillers significantly improved depowdering. Based on this study, large 3DP scaffolds can be realized, which might be a step towards a broader clinical use of 3D printed CaP scaffolds.

Size matters: effects of PLGA-microsphere size in injectable CPC/PLGA on bone formation

The aim of this study was to evaluate the effect of PLGA microsphere dimensions on bone formation after injection of calcium phosphate cement (CPC)/PLGA in a guinea pig tibial intramedullarly model. To this end, injectable CPC/PLGA formulations were prepared using PLGA microspheres with either a small (~25 µm) or large (~100 µm) diameter, which were incorporated at a 20:80 ratio (wt%) within apatite CPC. Both CPC/PLGA formulations were injected into a marrow-ablated tibial intramedullary cavity and, after an implantation period of 12 weeks, histology and histomorphometry were used to address bone formation. The results demonstrated bone ingrowth throughout the entire scaffold material for both CPC/PLGA formulations upon PLGA microsphere degradation. More importantly, bone formation within the CPC matrix was > two-fold higher for CPC-PLGA with 25 µm PLGA microspheres. Additionally, the pattern of bone and marrow formation showed distinct differences related to PLGA microsphere dimension. In general, this study demonstrates that PLGA microsphere dimensions of ~25 µm, leading to pores of ~25 µm within CPC, are sufficient for bone ingrowth and allow substantial bone formation. Further, the results demonstrate that PLGA microsphere dimensions provide a tool to control bone formation for injectable CPC/PLGA bone substitutes. Copyright © 2013 John Wiley & Sons, Ltd.

Calcium Orthophosphate-Based Bioceramics

Various types of grafts have been traditionally used to restore damaged bones. In the late 1960s, a strong interest was raised in studying ceramics as potential bone grafts due to their biomechanical properties. A bit later, such synthetic biomaterials were called bioceramics. In principle, bioceramics can be prepared from diverse materials but this review is limited to calcium orthophosphate-based formulations only, which possess the specific advantages due to the chemical similarity to mammalian bones and teeth. During the past 40 years, there have been a number of important achievements in this field. Namely, after the initial development of bioceramics that was just tolerated in the physiological environment, an emphasis was shifted towards the formulations able to form direct chemical bonds with the adjacent bones. Afterwards, by the structural and compositional controls, it became possible to choose whether the calcium orthophosphate-based implants remain biologically stable once incorporated into the skeletal structure or whether they were resorbed over time. At the turn of the millennium, a new concept of regenerative bioceramics was developed and such formulations became an integrated part of the tissue engineering approach. Now calcium orthophosphate scaffolds are designed to induce bone formation and vascularization. These scaffolds are often porous and harbor different biomolecules and/or cells. Therefore, current biomedical applications of calcium orthophosphate bioceramics include bone augmentations, artificial bone grafts, maxillofacial reconstruction, spinal fusion, periodontal disease repairs and bone fillers after tumor surgery. Perspective future applications comprise drug delivery and tissue engineering purposes because calcium orthophosphates appear to be promising carriers of growth factors, bioactive peptides and various types of cells.

Calcium phosphate cements with strontium halides as radiopacifiers

High radiopacity is required to monitor the delivery and positioning of injectable implants. Inorganic nonsoluble radiopacifiers are typically used in nondegradable bone cements; however, their usefulness in resorbable cements is limited due to their low solubility. Strontium halides, except strontium fluoride, are ionic water-soluble compounds that possess potential as radiopacifiers. In this study, we compare the radiopacity, mechanical properties, composition, and cytotoxicity of radiopaque brushite cements prepared with strontium fluoride (SrF2 ), strontium chloride (SrCl2 ·6H2 O), strontium bromide (SrBr2 ), or strontium iodide (SrI2 ). Brushite cements containing 10 wt % SrCl2 ·6H2 O, SrBr2 , or SrI2 exhibited equal to or higher radiopacity than commercial radiopaque cements. Furthermore, the brushite crystal lattice in cements that contained the ionic radiopacifiers was larger than in unmodified cements and in cements that contained SrF2 , indicating strontium substitution. Despite the fact that the strontium halides increased the solubility of the cements and affected their mechanical properties, calcium phosphate cements containing SrCl2 ·6H2 O, SrBr2 , and SrI2 showed no significant differences in Saos-2 cell viability and proliferation with respect to the control. Strontium halides: SrCl2 ·6H2 O, SrBr2 , and SrI2 may be potential candidates as radiopacifiers in resorbable biomaterials although their in vivo biocompatibility, when incorporated into injectable implants, is yet to be assessed.

Polarized infrared reflectance spectra of brushite (CaHPO4·2H2O) crystal investigation of the phosphate stretching modes

Polarized infrared (IR) reflectance measurements at near-normal incidence were recorded from the ac-plane of a monoclinic brushite (CaHPO4·2H2O) crystal in the 800-1200 cm(-1) spectral range (P-O stretching modes). The adjustment of these data, on the basis of a dispersion analysis (DA) model for monoclinic case, allowed the determination of oscillators parameters for the four P-O stretching observed modes of the phosphate group.

Effects of DCPD cement chemistry on degradation properties and cytocompatibility: comparison of MCPM/β-TCP and MCPM/HA formulations

Dicalcium phosphate dihydrate (DCPD) cements are attractive biomaterials for bone repair, and a number of different DCPD cement formulations have been proposed in the literature. In this study, we have specifically compared monocalcium phosphate monohydrate (MCPM)/hydroxyapatite (HA) and MCPM/β-tricalcium phosphate (β-TCP) formulations to test the hypothesis that DCPD cement chemistry affects the degradation properties and cytocompatibility of the cement. Using simple in vitro models we found that MCPM/β-TCP formulations degraded primarily by DCPD dissolution, which was associated with a slight pH drop and relatively low mass loss. Cytocompatibility testing of cement conditioned culture media revealed no significant change in cell viability relative to the negative control for all of the MCPM/β-TCP formulations. In contrast, the MCPM/HA formulations were prone to undergo rapid conversion of DCPD to HA, resulting in a sharp pH drop and extensive mass loss. A stoichiometric excess of HA in the cement was found to accelerate the conversion process, and significant cytotoxicity was observed for the MCPM/HA formulations containing excess HA. Collectively, these results show that, although the product of the setting reaction is the same, DCPD cements produced with MCPM/HA and MCPM/β-TCP formulations differ significantly in their degradation properties and cytocompatibility. These differences may have important implications for the selection of a DCPD cement formulation for clinical application.

Brushite cement additives inhibit attachment to cell culture beads

Brushite-forming calcium phosphate cements are of great interest as bone replacement materials because they are resorbable in physiological conditions. Cell-attached culture beads formed from this material could be of great use for cell therapy. Despite a significant amount of work on optimizing the physicochemical properties of these materials, there are very few studies that have evaluated the capacity of the materials to facilitate cell adhesion. In this study, we have formed resorbable calcium phosphate (brushite) culture beads and for the first time we showed that cell attachment to the surface of the brushite cement (BC) could be inhibited by the presence of an intermediate dicalcium phosphate-citrate complex, formed in the cement as a result of using citric acid, a retardant and viscosity modifier used in many cement formulations. The BC beads formed from the mixture of β-TCP/orthophosphoric acid using citric acid did not allow cell attachment without further treatment. Ageing of BC beads in serum-free Dulbecco's Modified Eagle's Medium (DMEM) solution at 37°C for 1 week greatly enhanced the cell adhesion capacity of the material. Scanning electron microscopy, X-ray diffraction (XRD), and confocal Raman microspectrometry indicated the increased capacity for cell adhesion was due to the changes in phase composition of BC. XRD patterns collected before and after ageing in aqueous solution and a high initial mass loss, suggest the formation of a dicalcium phosphate-citrate complex within the matrix. Since compacts formed from brushite powder supported cell attachment, it was hypothesized that the dicalcium phosphate-citrate complex prevented attachment to the cement surface.

Mechanical and In Vitro Biocompatibility of Brushite Cement Modified by Polyethylene Glycol

Brushite (dicalcium phosphate dihydrate, DCPD) cement, owing to its high solubility in physiological condition and ability to guide new bone formation, is widely used to treat bone defects. In the present study, we have evaluated the effects of poly ethylene glycol (PEG) addition on the setting time, compressive strength and in vitro biocompatibility of brushite cement. The brushite cements were prepared by mixing β-tricalcium phosphate [β-TCP, Ca(3)(PO(4))(2)] and monocalcium phosphate monohydrate [MCPM, Ca(H(2)PO(4))(2). H(2)O]. PEG was introduced at 2.0 and 5.0 wt% with the liquid. Introduction of PEG resulted in marginal increase in both initial and final setting time; however, significantly affected the compressive strength. Effects of PEG incorporation on in vitro biocompatibility of brushite cements were studied by using human fetal osteoblast cells (hFOB) cells. Field emission scanning electron microscope (FESEM) images and immunohistochemical analysis indicated that pure and PEG incorporated brushite cement facilitates cell adhesion, proliferation and differentiation. Fewer cells expressed vinculin protein with increased PEG content in the cement. Cell proliferation was found to decrease with increased PEG concentration while the cell differentiation increased with PEG content. Our results provide a better understanding of in vitro biocompatibility of PEG added brushite cements that can be used to customize the cement compositions based on application need.

Low temperature fabrication of spherical brushite granules by cement paste emulsion

Secondary protonated calcium phosphates such as brushite (CaHPO(4)·2H(2)O) or monetite (CaHPO(4)) have a higher resorption potential in bone defects than sintered ceramics, e.g. tricalcium phosphate or hydroxyapatite. However, processing of these phosphates to monolithic blocks or granules is not possible by sintering due to thermal decomposition of protonated phosphates at higher temperatures. In this study a low temperature technique for the preparation of spherical brushite granules in a cement setting reaction is presented. These granules were synthesized by dispersing a calcium phosphate cement paste composed of β-tricalcium phosphate and monocalcium phosphate together with a surfactant to an oil/water emulsion. The reaction products were characterized regarding their size distribution, morphology, and phase composition. Clinically relevant granule sizes ranging from 200 μm to 1 mm were obtained, whereas generally smaller granules were received with higher oil viscosity, increasing temperature or higher powder to liquid ratios of the cement paste. The hardened granules were microporous with a specific surface area of 0.7 m(2)/g and consisted of plate-like brushite (>95 % according to XRD) crystals of 0.5-7 μm size. Furthermore it was shown that the granules may be also used for drug delivery applications. This was demonstrated by adsorption of vancomycin from an aqueous solution, where a load of 1.45-1.88 mg drug per g granules and an almost complete release within 2 h was obtained.

Influence of water content on hardening and handling of a premixed calcium phosphate cement

Handling of calcium phosphate cements is difficult, where problems often arise during mixing, transferring to syringes, and subsequent injection. Via the use of premixed cements the risk of handling complications is reduced. However, for premixed cements to work in a clinical situation the setting time needs to be improved. The objective of this study is to investigate the influence of the addition of water on the properties of premixed cement. Monetite-forming premixed cements with small amounts of added water (less than 6.8 wt.%) were prepared and the influence on injectability, working time, setting time and mechanical strength was evaluated. The results showed that the addition of small amounts of water had significant influence on the properties of the premixed cement. With the addition of just 1.7 wt.% water, the force needed to extrude the cement from a syringe was reduced from 107 (±15) N to 39 (±9) N, the compression strength was almost doubled, and the setting time decreased from 29 (±4) min to 19 (±2) min, while the working time remained 5 to 6h. This study demonstrates the importance of controlling the water content in premixed cement pastes and how water can be used to improve the properties of premixed cements.

The effect of autoclaving on the physical and biological properties of dicalcium phosphate dihydrate bioceramics: brushite vs. monetite

Dicalcium phosphate dihydrate (brushite) is an osteoconductive biomaterial with great potential as a bioresorbable cement for bone regeneration. Preset brushite cement can be dehydrated into dicalcium phosphate anhydrous (monetite) bioceramics by autoclaving. This heat treatment results in changes in the physical characteristics of the material, improving in vivo bioresorption. This property is a great advantage in bone regeneration; however, it is not known how autoclaving brushite preset cement might improve its capacity to regenerate bone. This study was designed to compare brushite bioceramics with monetite bioceramics in terms of physical characteristics in vitro, and in vivo performance upon bone implantation. In this study we observed that monetite bioceramics prepared by autoclaving preset brushite cements had higher porosity, interconnected porosity and specific surface area than their brushite precursors. In vitro cell culture experiments revealed that bone marrow cells expressed higher levels of osteogenic genes Runx2, Opn, and Alp when the cells were cultured on monetite ceramics rather than on brushite ones. In vivo experiments revealed that monetite bioceramics resorbed faster than brushite ones and were more infiltrated with newly formed bone. In summary, autoclaving preset brushite cements results in a material with improved properties for bone regeneration procedures.