Introduction: To expand the use of glass ionomer cement (GIC) in dental and or orthopaedic applications requires materials with improved handling and mechanical characteristics, i.e. lower viscosities and higher strengths. However, the nature of the GIC setting reaction is such that these properties are coupled together, where reducing viscosity diminishes strength. Unexpectedly, the addition of germaninum (Ge) to zinc silicate GICs has been observed to decouple this handling-mechanical relationship by delaying setting whilst maintaining strength[1]. The objective of this investigation is to explore the effect of Ge on the GIC setting reaction to identify potential mechanisms responsible for this behavior.
Methods: Five <45 μm glass powder compositions (48 – x SiO2, x GeO2, 36 ZnO, 16 CaO; where x = 12, 24, 36, 48 mol%) were synthesized. Glass degradation was assessed under simulated setting conditions using acetic acid from 0.5 to 60 min, monitoring the concentrations of ions released using ICP-OES. Subsequently, GICs were prepared by mixing fresh glass powders with polyacrylic acid (PAA, Mw = 12,500 g/mol, 50 wt% aq. solution) at a 4:3 ratio. Cement structure and properties were evaluated using ATR-FTIR and rheology (for 60 min). Double torsion fracture toughness (KIC) for each cement was also evaluated[2].
Results and Discussions: Unusually, it was observed that increased Ge content yielded faster degrading glasses, behavior typical of fast setting, high viscosity GICs. However, rheology results (Fig. 1) showed initial GIC viscosity reduced as x increased, and setting rate reduced from 0< x <24 mol%, but reversed as x > 24 mol%. IR peak positions were consistent amongst all five compositions indicating that Ge does not influence which cations from the glass bond to carboxylate groups of the poly acid, but Ge was found to influence the rate at which these bonds formed. Interestingly, the lower viscosity generally improved KIC (Fig. 2), and the slowest setting composition (x = 24 mol%) had the highest resistance to fracture (0.27 MPa m-1/2), atypical behavior in conventional GIC systems.
Conclusion: This counter-intuitive combination of behaviors is attributed to the presence of a complex species specific to Ge-containing glasses that delays, but does not hinder, the formation of the GIC matrix. These findings indicate a mechanism that decouples glass reactivity from cement setting rate allowing materials to combine a low viscosity with high resistance to fracture. These findings have the potential to enhance the utility of dental GICs used in atraumatic restorative techniques, where the treatment of carries requires delivery of material through ever-reducing injection tips. These results are also relevant for the development of GICs for minimally invasive orthopaedic procedures such as vertebroplasty, a procedure for the stabilization of spinal fractures.
Figure 1: Gelation profiles of the experimental GICs, presented as change in complex viscosity over time.
Figure 2: Double torsion fracture toughness (KIC) for experimental GICs (mean + SD). * denotes statistically significant differences (p < 0.05). No KIC value could be obtained for x = 0 mol% as the material set too quickly, preventing sample production.
The authors acknowledge Dr. Richard Price of Dalhousie's Faculty of Dentistry for his technical assistance
References:
[1] Dickey et al. 2013 JMBBM 23
[2] De Barra and Hill, 1998 Biomaterials 19