Classification

The most practical classification of the GICs is on their clinical usage [45, 87]. Type I GICs are the luting cements, characterized by low film thickness and rapid set; when available as an RM-GIC, the photopolymerization reaction will be absent. Type II GICs are restorative cements, with sub-types 1 and 2. Type II-1 GICs are aesthetic cements (available in both conventional and resin-modified presentations) and Type II-2 GlCs are 'reinforced' (however, despite their description, are not necessarily stronger than Type II-1 products). However, they are more wear-resistant. Type III GICs are the lining cements and fissure sealants, characterized by low viscosity and rapid set.

In the mid- to late-1990s, high powder:liquid ratio conventional GlCs were introduced, alternatively termed 'packable' or 'high viscosity' GlCs [62]. These products (e.g., Ketac Molar, 3M/Espe, Seefeld, Bavaria, Germany; Chemflex, Dentsply, York, Pennsylvania, USA; Fuji IX and Fuji IX GP, GC International) are promoted principally for small cavities in deciduous teeth, temporary restorations, liner/base applications, and in the 'Atraumatic Restorative Treatment' (ART) technique [26, 79]. The most recently accepted uses of GICs have been as a liner and base under deep composite restorations, which was described in 1984 [38] and has been referred to as the sandwich technique, Deep cervical lesions and proximal boxes of class II cavities whose gingival floor is on root surfaces are areas where there is increased diameter of dentinal tubules that will affect the bond strength because of increased chances of hydrolytic degradation (The 'open sandwich' technique, also known as the 'cervical lining'). Because of their chemical bonding capabilities, glass ionomer adhere to these surface better then dental adhesive-bonding agents. Based on evidence-based dentistry protocols, the recommendation is to treat the surfaces with a polyacrylic acid conditioner, which is rinsed before glass ionomers are applied. This weak acid modifies the smear layer by leaving the smear plugs behind, improving the seal and eliminating postoperative sensitivity. A new self-conditioner for resin-modified glass ionomers, recently developed by Fuji (GC America, IL, USA), does not require rinsing before applying the glass ionomer material [9]. Both Fuji II and Fuji IX (GC America, IL, USA) have unique automix dispensing capsules, simplifying placement of these materials. Resin-modified ionomers, such as Fuji II LC, are routinely used as liners at 1 mm or less, and a material such as Fuji IX or Riva (SDI, Bensenville, IL, USA) is preferred for larger areas of dentin replacement.

Based on abundant evidence, conventional and metal-modified glass ionomers are not recommended in class 2 restorations in both primary and permanent molars. To compensate for this, RM-GICs were developed to produce better mechanical properties than the conventional ones. The resin hydroxyethyl methacrylate (HEMA) or bis-glycerol methacrylate was added to the liquid. The resin modification of these cements allowed the base curing reaction to be supplemented by a light or chemical curing process, allowing for a command set. The obvious advantages were better fracture toughness, increased tensile strength, and a decrease in desiccation and hydration problems [20]. The limiting factors were the setting shrinkage, which was found to be greater than with conventional cements, and the limited depth of cure with more opaque lining cements [5]. The mean age of these failed glass ionomer restorations at replacement in permanent teeth in general practice was found to be 5.5 years for patients older than 30 years [43]. Secondary caries, bulk fracture (1.4%-14%), and marginal fracture (from poor anatomic form) constituted the main reasons for failure. In developing countries, highly viscous glass ionomer materials have became popular in atraumatic restorative treatment techniques for class 1 restorations in posterior teeth. In class 2 restorations, these high viscous glass ionomers are still considered satisfactory after 3 years of clinical service, despite large percentages of failed restorations. However, a recently concluded retrospective study showed that the failure of class 2 restorations with these materials rose to 60% at 72 months. It was hypothesized that carieslike loss of material was seen on radiographs and that the presence of proximal contacts promoted disintegration of these materials [66, 80].

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