Novel Bioactive Cements to Promote Bone Regeneration

Currently available cements and granules for bone repair include devices based on glass ionomer cement (GICs) technology. These cements are based on the setting reaction between an aluminium containing fluorosilicate glass, poly (acrylic acid) (PAA) and a setting modifier. The glass powder is acid degradable, which crosslinks with the ionised acid, resulting in a matrix of polyacrylates salts with reacted glass particles. However, bone demineralisation, as well as neurotoxicity in craniofacial applications are drawbacks associated with aluminium. These disadvantages have created a scientific interest on developing aluminium free compositions with the potential to be used as cements and bone grafts. Therefore, new glass compositions have been researched to substitute the alumina (Al2O3) with oxides such as ZnO, GeO, MgO, and TiO. However, no previous detailed study has investigated variations of the classic Hench Bioglass® composition (45SiO2-24.5Na2O-24.5CaO-6P2O5 wt. %) for preparation of cements, with Mirvakily studying this system, but focusing solely on one cement composition (Mirvakily, 2009). In the present work, eight glasses were prepared and characterised by X-ray diffraction (XRD), differential thermal analysis (DTA), scanning electron microscopy (SEM), and Fourier transformed infrared spectroscopy (FTIR-ATR). These glasses were mixed with varying quantities of poly (acrylic acid), and a phosphoric acid solution (H3PO4 (sol)), to evaluate their cement forming properties and setting times. The most suitable glasses were chosen along with the optimised cement combination, this being a powder/liquid (P/L) ratio of two, 10.7 % PAA powder and 25% H3PO4 (sol), to further investigate their mass change, ion release and conductivity when immersed in distilled water, along with their setting chemistry. Results showed that glasses based on a SiO2-Na2O-CaO-SrO-P2O5 system could be used to produce setting pastes that were stable in distilled water, with net setting times varying between 34 min and 115 min. The setting mechanism was found to have similarities to GICs, by release of Ca2+, Sr2+, and Na+ after PAA ionisation and formation of the respective polyacrylate salts. The use of phosphoric acid was found to be essential to prevent the gelation in water of aged cements (set for one day, 37 °C), suggesting that this addition aided glass dissolution and precipitation of phosphate containing salts. Their mass loss in distilled water, reached its maximum after one day for the cements prepared with the 45S5 and 45S5Sr10 glasses (with lower SiO2 content), while this peak was observed after one week for the cements prepared with the 49P9 and 53P4 glasses (with higher SiO2 content). The cement dissolution was further confirmed by the release of Si, Ca, Na, and P; with the direct in vitro short-term test showing that the cement based on the 45S5 glass was not cytotoxic. The study carried out with the commercial cement Serenocem™, showed that its dissolution and ion release was very low, which suggested the limited solubility of the Al containing glass. Therefore, the findings of this research provide an alternative system for the preparation of aluminium free cements for craniofacial applications, showing that no additional ionic substitution was required to produce a setting cement with ion release comparable to that of bioactive glasses.