Conventional glass-ionomer cement setting

Studies have been made of the inclusion of two possible substances to enhance the bioactivity of conventional glass-ionomer cements, hydroxyapatite [7] and bioglass powder [8]. Both were found to reduce the speed of the setting reaction of the cement, with the reduction being greater with larger proportions of additive. In all cases, the set cements were consistently weaker than those with no additive (Table 8.1). This result has also been found with completely inert powders, such as borax [9]. 

Table 8.1 Effect on mechanical properties and setting behaviour of hydroxyapatite incorporated into a conventional glass-ionomer cement [7]...

From the and T MAS-NMR spectra, it was apparent that the adjunct filler was substantially hydroxyapatite rather than fluorapatite. The diminution in the P peak due to orthophosphate was taken to indicate that this apatite filler was undergoing a reaction with the polymeric acid, and contributing to setting. This reaction has not been observed previously when synthetic hydroxyapatite was included in a conventional glass-ionomer cement [7], though reaction of poly(acryUc acid) with other calcium phosphate minerals has been observed previously [25]. 

Polyacid-modified composite resins were developed in an attempt to make a composite resin with the sort of ion-release capability of glass-ionomer cements, especially of fluoride [38]. They are similar to conventional composites in that they are mainly based on the hydrophobic monomers bis-GMA or urethane dimethaaylate, and their setting is typically initiated by light. In addition, they contain inert fillers of appropriate particle size. 

The glass-ionomer cement is an acid-base cement based on weak polymeric acids and powdered glasses that are basic in character [1], Their setting takes place within water and resnlts in a complex polysalt matrix, formed by chemical reaction of the acidic polymer solntion with the basic glass. The structure also contains a snbstantial amount of nnreacted glass that acts as reinforcing filler [2], In terms of materials classification, this makes them composites, but convention in dentistry is to consider them distinct from composites, and to classify them as cements. 

The bonding of resin-modified glass-ionomer cements is associated with the formation of a gel phase at the interface between the material and the tooth surface [82,88]. This phase seems to originate from the acid-base part of the formulation, as it consists substantially of calcium polyacrylate, a substance that forms as the cement sets. However, the gel phase is more substantial in these materials than in conventional glass-ionomers, so that its occurrence owes something to the overall composition of resin-modified glass-ionomers [89]. 

Resin-modified glass-ionomers are much less biocompatible than conventional glass-ionomers. This is due to the HEM A that they contain. This monomer may cause sensitization to dental personnel, and can be leached from set cements and pass through thin layers of dentine to damage the pulp of the patient. They should be used with care. 

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