Development and Characterisation of Strontium-containing Bioactive Glasses and Aluminium-free Glass Polyalkenoate Cements
Creators: name: Fredholm, Yann Christophe
Publisher: name: Imperial College London
Year: 2009

description: Bioactive glass (BG) reactivity is the result of a special silicate structure, disrupted by the presence of alkali and earth-alkali metal ions. These glasses dissolve in body fluids and form a hydroxy-carbonated apatite (HCA) on their surface, which mimics the composition and structure of the mineral phase of bone. This feature, coupled with the release of biologically active ions, explains their excellent osseointegration. In the first part of this thesis, we investigated the effect of substituting strontium for calcium in a BG system based on SiO2-P2O5-Na2O-CaO. The reason for introducing strontium in a BG was driven by its strengthening effect on bone in the treatment of osteoporosis. The glass structure and physical properties were investigated by X-ray diffraction (XRD), solid state nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) spectroscopy and thermal analysis. Dissolution profiles and in vitro bioactivity of Sr-BG were investigated in Tris buffer and simulated body fluid by inductively coupled plasma (ICP) spectroscopy, XRD and FTIR. Glass cytotoxicity was assessed by culturing osteoblast-like cells (SaOs-2) on Sr-BG discs for 14 days. It was found that substitution of Sr for Ca does not modify the Q2 silicate structure of the glass, nor its orthophosphate environment. However, it expands and weakens the glass network, thus the glasses dissolved more rapidly with higher strontium content and formed more HCA. A live/dead cellular assay showed an increased cell proliferation for higher strontium containing glasses. Glass polyalkenoate cements (GPCs) also use the potential of reactive glasses to release multivalent cations, which cross-link the carboxylic groups of polyacid chains in aqueous solution to form a hard cement. GPCs do not shrink, do not have significant exotherm and have good adhesive properties. However, the use of commercial GPCs, based on fluoro-alumino-silicate glasses, has been limited to dentistry and minor surgery due to biocompatibility issues. Aluminium is known to have neurotoxic effect and inhibits remineralisation. The aim of the second part of this study was to investigate new glass compositions for the development of aluminium-free GPCs for orthopaedic applications. Sr-BG compositions were adapted for cement forming purpose by removing soda and including Mg2+, Zn2+ and Fe3+ to provide an alternative to Al3+. The selected cations are all naturally present in the body and can potentially enter into the glass network, mimicking the formation of Al- O-Si bonds present in conventional GPCs. The mechanical properties of the cements were assessed by performing compression and bonding strength tests, whereas their working (WT) and setting times were measured with an oscillating rheometer. Finally, cement cytotoxicity was evaluated by culturing SaOs-2 cells on cement discs. It was established that only high zinc containing glasses lead to cements with suitable mechanical properties to be used as bone cements. WT was short but increased up to two minutes by using citric acid as a chelating agent. The high Zn release from the cements induced cytotoxicity and predominated over the potential positive effect of Sr on cells.