Glass ionomer cement
[5] Glass-ionomer based hybrids incorporate another dental material, for example resin-modified glass ionomer cements (RMGIC) and compomers (or modified composites).
Glass ionomer cements act as sealants when pits and fissures in the tooth occur and release fluoride to prevent further enamel demineralisation and promote remineralisation.
[9] However, it is recommended that the use of fluoride varnish alongside glass ionomer sealants should be applied in practice to further reduce the risk of secondary dental caries.
[5] Dental sealants were first introduced as part of the preventative programme, in the late 1960s, in response to increasing cases of pits and fissures on occlusal surfaces due to caries.
For example, the addition of metal or resin particles into the sealant is favoured due to the longer working time and the material being less sensitive to moisture during setting.
One of the early commercially successful GICs, employing G338 glass and developed by Wilson and Kent, served purpose as non-load bearing restorative materials.
The properties of G338 being shown to be related to its phase-composition, specifically the interplay between its three amorphous phases Ca/Na-Al-Si-O, Ca-Al-F and Ca-P-O-F, as characterised by mechanical testing, differential scanning calorimetry (DSC) and X-ray diffraction (XRD),[11] as well as quantum chemical modelling and ab initio molecular dynamics simulations.
This makes them ideal in many situations when tooth preservation is foremost and with minimally invasive techniques, particularly Class V fillings where there is a larger area of exposed dentin with only a thin ring of enamel.
[9] The main disadvantage of glass ionomer sealants or cements has been inadequate retention or simply lack of strength, toughness, and limited wear resistance.
At the stage of mixing, a spatula should be used to rapidly incorporate the powder into the liquid for a duration of 45–60 seconds depending on manufacture instructions and the individual products.
[24] Once mixed together to form a paste, an acid-base reaction occurs which allows the glass ionomer complex to set over a certain period of time and this reaction involves four overlapping stages: It is important to note that glass ionomers have a long setting time and need protection from the oral environment in order to minimize interference with dissolution and prevent contamination.
The acid begins to attack the surface of the glass particles, as well as the adjacent tooth substrate, thus precipitating their outer layers but also neutralising itself.
As the pH of the aqueous solution rises, the polyacrylic acid begins to ionise, and becoming negatively charged it sets up a diffusion gradient and helps draw cations out of the glass and dentine.
[citation needed] The second phase is gelation, where as the pH continues to rise and the concentration of the ions in solution to increase, a critical point is reached and insoluble polyacrylates begin to precipitate.
These polyanions have carboxylate groups whereby cations bind them, especially Ca2+ in this early phase, as it is the most readily available ion, crosslinking into calcium polyacrylate chains that begin to form a gel matrix, resulting in the initial hard set, within five minutes.
[citation needed] GICs have good adhesive relations with tooth substrates, uniquely chemically bonding to dentine and, to a lesser extend, to enamel.
During initial dissolution, both the glass particles and the hydroxyapatite structure are affected, and thus as the acid is buffered the matrix reforms, chemically welded together at the interface into a calcium phosphate polyalkenoate bond.
A substantial amount of both strontium and fluoride ions was found to cross the interface into the partially demineralised dentine affected by caries.
Findings of a systematic review and meta-analysis suggested that conventional glass ionomers were not recommended for Class II restorations in primary molars.
[36] This material showed poor anatomical form and marginal integrity, and composite restorations were shown to be more successful than GIC when good moisture control could be achieved.
A systematic review supports the use of RMGIC in small to moderate sized class II cavities, as they are able to withstand the occlusal forces on primary molars for at least one year.
[37] In addition, adhesive ability and longevity of GIC from a clinical standpoint can be best studied with restoration of non- carious cervical lesions.
[38] Unfortunately, reviews for Class II restorations in permanent teeth with glass ionomer cement are scarce with high bias or short study periods.
