Robert G. Hill

The effects of strontium-substituted bioactive glasses on osteoblasts and osteoclasts in vitro.

Bioactive glasses (BG) which contain strontium have the potential to combine the known bone regenerative properties of BG with the anabolic and anti-catabolic effects of strontium cations. Here we created a BG series (SiO(2)-P(2)O(5)-Na(2)O-CaO) in which 0-100% of the calcium was substituted by strontium and tested their effects on osteoblasts and osteoclasts in vitro. We show that ions released from strontium-substituted BG enhance metabolic activity in osteoblasts. They also inhibit osteoclast activity by both reducing tartrate resistant acid phosphatase activity and inhibiting resorption of calcium phosphate films in a dose-dependent manner. Additionally, osteoblasts cultured in contact with BG show increased proliferation and alkaline phosphatase activity with increasing strontium substitution, while osteoclasts adopt typical resorption morphologies. These results suggest that similarly to the osteoporosis drug strontium ranelate, strontium-substituted BG may promote an anabolic effect on osteoblasts and an anti-catabolic effect on osteoclasts. These effects, when combined with the advantages of BG such as controlled ion release and delivery versatility, may make strontium-substituted BG an effective biomaterial choice for a range of bone regeneration therapies.

An alternative view of the degradation of bioglass

The aim of this letter is to try to explain the reactivity and bioactivity of such glasses in terms of their composition

Fluoride-containing bioactive glasses: Effect of glass design and structure on degradation, pH and apatite formation in simulated body fluid

Bioactive glasses are able to bond to bone through formation of carbonated hydroxyapatite in body fluids, and fluoride-releasing bioactive glasses are of interest for both orthopaedic and, in particular, dental applications for caries inhibition. Melt-derived glasses in the system SiO(2)-P(2)O(5)-CaO-Na(2)O with increasing amounts of CaF(2) were prepared by keeping network connectivity and the ratio of all other components constant. pH change, ion release and apatite formation during immersion of glass powder in simulated body fluid at 37 degrees C over up to 2 weeks were investigated. Crystal phases formed in SBF were characterized using infrared spectroscopy, X-ray diffraction with Rietveld analysis and solid-state nuclear magnetic resonance spectroscopy ((19)F and (31)P MAS-NMR). Results show that incorporation of fluoride resulted in a reduced pH rise in aqueous solutions compared to fluoride-free glasses and in formation of fluorapatite (FAp), which is more chemically stable than hydroxyapatite or carbonated hydroxyapatite and therefore is of interest for dental applications. However, for increasing fluoride content in the glass, fluorite (CaF(2)) was formed at the expense of FAp. Apatite formation could be favoured by increasing the phosphate content in the glass, as the release of additional phosphate into the SBF would affect supersaturation in the solution and possibly favour formation of apatite.

Structural analysis of a series of strontium-substituted apatites

A series of Sr-substituted hydroxyapatites, (Sr(x)Ca(1-)(x))(5)(PO(4))(3)OH, where x=0.00, 0.25, 0.50, 0.75 and 1.00, were made by a standard wet chemical route and investigated using X-ray diffraction (XRD), Rietveld refinement and Raman spectroscopy. We report apatites manufactured by two synthesis routes under 90 degrees C, and only the fully Sr-substituted sample had a small amount of an impurity phase, which is believed to be strontium pyrophosphate. Lattice parameters (a and c), unit cell volume and density were shown to increase linearly with strontium addition and were consistent with the addition of a slightly larger and heavier ion (Sr) in place of Ca. XRD Lorentzian peak widths increased to a maximum at x=0.50, then decreased with increasing Sr content. This indicated an increase in crystallite size when moving away from the x=0.50 composition (d approximately 9.4nm). There was a slight preference for strontium to enter the Ca(II) site in the mixed apatites (6 to 12% depending on composition). The position of the Raman band attributed to v(1)PO(4)(3-) at around 963cm(-1) in hydroxyapatite decreased linearly to 949cm(-1) at full Sr-substitution. The full width at half maximum of this peak also correlated well and increased linearly with increasing crystallite size calculated from XRD.

Influence of strontium and the importance of glass chemistry and structure when designing bioactive glasses for bone regeneration

The purpose of this article is to highlight some recent in vitro and in vivo studies of bioactive glasses containing strontium and to review selected literature on the in vitro and in vivo behaviour of bioactive glasses to relate this to the structure of the glass. The strontium-glass studies were performed well scientifically, but the results and conclusions could be misleading in terms of the effect of strontium, or more broadly glass chemistry, on the bioactivity and in vivo behaviour of bioactive glasses due to substitutions made on a weight basis. When strontium is substituted by weight for a lighter element such as calcium this will have a significant effect on structure and properties in particular biological response.

Influence of sodium oxide content on bioactive glass properties

The rate of in vivo degradation and level of bioactivity of bioactive glasses are composition dependent [1]. By altering bioactive glass composition, the rate of resorption can be controlled. The network connectivity of a glass can be used to predict various physical properties of the glass including its solubility and, hence, its bioactivity [2]. Glass solubility increases as network connectivity is reduced. Glasses in the soda-lime phosphosilicate system were studied. The initial choice of composition was based on phosphate content and low network connectivity. A systematic substitution of calcium oxide for sodium oxide on a molar basis was made in order to examine the influence of sodium oxide content on the glass properties while keeping the network connectivity constant. The glass transition temperature and the peak crystallization temperature were seen to decrease linearly with increasing sodium oxide content. Thermal expansion coefficient and glass density were also seen to be related to sodium oxide content. Preliminary in vitro biocompatibility studies revealed that the glasses of higher sodium oxide content were associated with a cytotoxic response. The measurement of media pH indicated that this cytotoxic effect was due to ion exchange reactions at the glass surface.

Structure of fluoride-containing bioactive glasses

Fluoride prevents dental cavities, stimulates bone mineralisation and decreases the melting temperature of glasses and is therefore an interesting component of bioactive glasses for use as dental or orthopaedic biomaterials. However, when designing new glass compositions, the structural role of fluoride in the glass needs to be better understood. We have characterised a glass series in the system SiO2–P2O5–CaO–Na2O with increasing concentrations of CaF2. Network connectivity was fixed at 2.13 by adding CaF2 while the ratio of all other components was kept constant. 19F and 29Si MAS NMR spectra showed that addition of CaF2 does not cause disruption of the glass network by formation of Si–F bonds but forms mixed calcium sodium fluoride species. 31P MAS NMR showed phosphate being present as orthophosphate. Hence it does not form part of the actual glass network backbone and no Si–O–P bonds are present. 23Na MAS NMR showed the presence of multiple sodium sites with an increase in the mean coordination number of sodium with increasing CaF2 content. The glass transition temperature decreased with increasing amounts of CaF2. As no Si–F bonds were formed, this can be explained by formation of hypothetical CaF+ species. The results can be used for designing new fluoride-containing bioactive glass compositions for specific applications.

Comparison of two heat-pressed all-ceramic dental materials

OBJECTIVES The processing route for two heat-pressed all-ceramic materials (Empress and OPC) is virtually identical. The purpose of this study was to determine the mechanical properties of both materials and determine if significant differences exist between them. METHODS X-ray powder diffraction of the ceramics before and after processing was carried out to identify the crystal phases present. The mechanical properties of both materials were tested. Specimens were tested for hardness, fracture toughness (indentation method) and flexural strength (biaxial method). The results were statistically evaluated and tested for differences using a Mann-Whitney test. Secondary electron imaging of both materials was carried out before and after processing. RESULTS X-ray powder diffraction revealed that OPC changes as a result of heat-pressing from being a complex mixture of crystalline oxides to a glass-ceramic. In contrast Empress is a glass-ceramic before and after processing. X-ray diffraction identified leucite as the main crystalline phase in both ceramics. The biaxial flexural strength of OPC was 153.6 (17.8) MPa and for Empress was 134.4 (11.5) MPa. The hardness of OPC was 7.28 (0.62) GPa and for Empress was 6.94 (0.79) GPa. Indentation fracture toughness of OPC was 1.36 (0.29) MPam0.5 and for Empress was 1.33 (0.08) MPam0.5. Secondary electron images show Empress to be the same before and after processing while OPC is clearly very different. Empress also appears to have a higher glass content compared with OPC. SIGNIFICANCE The results of X-ray diffraction show that Empress is pre-cerammed whilst OPC is not. Statistical analysis revealed that no significant difference exists between the two materials for any of the mechanical properties tested at a 95% (p < 0.05) confidence level. It was concluded that no difference exists between the two materials on completion of processing.

Melt-derived bioactive glass scaffolds produced by a gel-cast foaming technique

Porous melt-derived bioactive glass scaffolds with interconnected pore networks suitable for bone regeneration were produced without the glass crystallizing. ICIE 16 (49.46% SiO(2), 36.27% CaO, 6.6% Na(2)O, 1.07% P(2)O(5) and 6.6% K(2)O, in mol.%) was used as it is a composition designed not to crystallize during sintering. Glass powder was made into porous scaffolds by using the gel-cast foaming technique. All variables in the process were investigated systematically to devise an optimal process. Interconnect size was quantified using mercury porosimetry and X-ray microtomography (μCT). The reagents, their relative quantities and thermal processing protocols were all critical to obtain a successful scaffold. Particularly important were particle size (a modal size of 8 μm was optimal); water and catalyst content; initiator vitality and content; as well as the thermal processing protocol. Once an optimal process was chosen, the scaffolds were tested in simulated body fluid (SBF) solution. Amorphous calcium phosphate formed in 8h and crystallized hydroxycarbonate apatite (HCA) formed in 3 days. The compressive strength was approximately 2 MPa for a mean interconnect size of 140 μm between the pores with a mean diameter of 379 μm, which is thought to be a suitable porous network for vascularized bone regeneration. This material has the potential to bond to bone more rapidly and stimulate more bone growth than current porous artificial bone grafts.

High phosphate content significantly increases apatite formation of fluoride-containing bioactive glasses

Bioactive glass-containing toothpastes for treating dentine hypersensitivity work by precipitating hydroxycarbonate apatite (HCA) onto the tooth surface, but concerns exist over the long-term durability of HCA in the mouth. Fluoride-containing bioactive glasses form fluorapatite (FAp) in physiological solutions, which is more chemically stable against acid attack. The influence of phosphate content on apatite formation was investigated by producing a low-phosphate (about 1 mol% P(2)O(5)) and a high-phosphate (about 6 mol%) series of melt-derived bioactive glasses in the system SiO(2)P(2)O(5)CaONa(2)O; increasing amounts of CaF(2) were added by keeping the ratio of all other components constant. pH change, ion release and apatite formation during immersion in Tris buffer at 37°C over up to 7 days were investigated. Crystal phases formed in Tris buffer were characterized using infrared spectroscopy, X-ray diffraction and solid-state nuclear magnetic resonance (NMR) spectroscopy. An increase in phosphate or fluoride content allowed for apatite formation at lower pH; fluoride enhanced apatite formation due to lower solubility of FAp compared to hydroxyapatite or HCA. High phosphate content glasses formed apatite significantly faster (within 6h) than low phosphate content glasses (within 3 days). In addition, an increase in phosphate content favoured apatite formation rather than fluorite (CaF(2)). (19)F magic angle spinning NMR showed the apatite formed by fluoride-containing glasses to be FAp, which makes these glasses of particular interest for dental applications. This study shows that by varying the phosphate content, the reactivity and apatite formation of bioactive glasses can be controlled successfully.