The Use of Germanium to Control the Properties of Glass Ionomer Cements
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Vertebral body fractures (VBFs) are the most common type of osteoporotic fracture, affecting over one million individuals worldwide each year. VBFs can be severely debilitating and may lead to life-threatening complications, associated with intractable pain in the most at-risk patients. Fortunately for these patients, minimally invasive vertebral body augmentation (VBA) procedures provide immediate and lasting pain relief, by stabilizing the fracture via transpedicular injection of a bone cement. However, the state-of-the-art bone cements are prevented from optimizing patient outcome due to deficiencies in biocompatibility, handling characteristics, and/or mechanical durability. The excellent biocompatibility and appropriate mechanical properties of aluminum-free glass ionomer cements (GICs) have motivated the research and development of GICs based on zinc silicate glass chemistries for use in VBA. However, the clinical potential of zinc silicate GICs is inhibited by the inability to achieve practical handling characteristics, without diminishing the strength of the cement. The research presented herein examines a novel approach of controlling GIC properties, whereby the silicon content in the glass chemistry is replaced by germanium. Three broad investigations explore the impact of this substitution on the performance of zinc silicate GICs. An initial screening demonstrated that the complete replacement of silicon by germanium significantly improved the handling characteristics of zinc silicate GICs, whilst maintaining the strength necessary for VBA. To determine the mechanistic basis of these behaviors, thorough investigations of the influence of germanium on the GIC setting reaction and mechanical properties were conducted. From these investigations, it was revealed that germanium delays, but does not hinder the GIC setting reaction. This allows the handling characteristics of the cements to be decoupled from their mechanical properties. However, in order for the extended GIC setting reaction to yield mechanically durable cements, the zinc silicate glasses must contain both silicon and germanium in equal quantities. This research demonstrates that replacing silicon with germanium in the glass chemistry successfully controls GIC properties. Ultimately, this approach identified the necessary compositional changes to produce aluminum-free GICs that balance appropriate handling characteristics with sufficient mechanical properties. In conclusion, Ge-containing GICs are clinically viable as injectable bone cements for use in VBA.