Investigation into the ultrasonic setting of glass ionomer cements Part I Postulated modalities

Abstract

Glass ionomer cements (GICs) are formed by the reaction of an ion leachable alumino-silicate glass with an aqueous solution of poly (alkenoic acid). Water is used as the reaction medium [1]. An acid-base reaction occurs, whereby the acid attacks and degrades the glass structure, releasing metal cations which are then chelated by the carboxylate groups and serve to crosslink the polyacid chains. The cement consists of residual glass particles embedded in a hydrogel polysalt matrix [2]. The setting reaction in GICs is a continuous process evident by the increase in compressive strength of the cement with ageing time [3–5]. GICs undergo a two-step setting reaction. During the first step the material is susceptible to water uptake and during the second it is susceptible to dehydration. For example, when GICs are stored in water after an initial set of 15 min, a surface softening occurs, which may be caused by an inhibition of the setting reaction in a superficial layer of the cement [6]. This short-term relationship with water restricts the full potential of GICs for healthcare applications. It is for this reason that resin modified GICs (RMGICs) were developed. These materials, which are conventional GICs to which an organic, photo-polymerizable monomer has been incorporated [7], can be command set by the application of an intense light source. However, RMGICs have recognized drawbacks, related to the presence of both non-polymerized monomer and the resin itself [8, 9]. The authors have already shown that conventional GICs can be command set by ultrasonic excitation [10]. Ultrasound not only imparts an instant set to a GIC, but also imparts superior mechanical properties when compared to its chemically cured counterpart, particularly within the first 24 h after setting. The ultrasonically cured GIC does not require the incorporation of additional chemicals and therefore avoids the drawbacks associated with RMGICs. In the previous study [10], nano-indentation testing was used to show that ultrasonically cured Fuji IX had hardness an order of magnitude greater than its chemically cured counterpart (Table I), while also