Bonding mechanism

The bonding mechanism of the GICs to dental hard tissues is very complex, and may be different for RM-GICs compared to conventional GICs. Simplistically, an ionic bond occurs between the carboxyl (COO-) ions in the cement acid and the calcium (Ca++) ions in enamel and dentine.

When freshly mixed conventional GIC is placed on enamel or dentine, dissolution of any smear layer occurs but demineralization is minimal since the tooth hydroxyapatite buffers the acid, and polyalkenoic is quite weak [83]. Phosphate ions (negatively charged) and calcium ions (positively charged) are displaced from the hydroxyapatite, and are absorbed into the unset cement. This results in an intermediate layer between the 'pure' GIC and the 'pure' hydroxyapatite; the so called 'ion-exchange' layer [45]. Problems of specimen preparation of a water-based material have hindered investigation of this layer, although better techniques are now becoming available [49].

The ion-exchange layer appears to consist of calcium and phosphate ions from the GIC, and aluminium, silicic, fluoride and calcium and/ or strontium ions (depending on glass composition) from the GIC [67]. The thickness of the ion-exchange layer appears to be in the order of a few micrometres, and merges into the GIC on one side and into the enamel/dentine on the other. Unfortunately there is some confusion in the literature [24, 31, 49, 76] regarding the ion-exchange layer. Other terms have been proposed such as 'zone of interaction', 'interdiffusion zone', 'hybrid layer', 'interphase', and 'intermediate layer'. In particular, the notation 'hybrid layer' causes confusion with the 'hybrid layer' formed between resin composite and dentine (see below). The term 'ion-exchange layer' should be used, since it accurately describes its nature. It has been shown that this layer is resistant to acid and base treatment, and has thus also been referred to as the 'acid-base resistant layer' [79].

Measurement of the bond strength of GIC to enamel and dentine is complicated by the brittle nature of the GIC. Laboratory bond strength tests invariably result in cohesive failure of the GIC, rather than failure within the ion exchange layer. Consequently, the true strength of the ion-exchange layer is not known; values in the range 3-10 MPa are commonly reported, i.e., approximately the cohesive strength of GIC [76, 79].

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