Glass Ionomer Cements, have a very specific way of behaving. This module will focus on how the constituents of the material behave clinically and when the material can be useful in our practice.
A generalised overview: GIC contains an acid e.g. poly acrylic acid and a glass base that when they react form a salt matrix. The glass surface is where the acid-base reaction occurs, therefore, the unreacted core of the particles are now encased in the matrix.
Encapsulated:
Hand Mix:
1/ The poly acid attacks the surface of the glass. This raises the pH and ionisation propagates, releasing glass constituents.
2/ Calcium, Alumina and fluoride ions are liberated (released) into the surrounding solution creating a silica gel.
3/ Polymer chains open (from the polyacrylic acid) and condensation of the matrix occurs. As the material hardens the Aluminium is slower to cross-link with the polymers, compared to the Calcium ions. This can take hours to be fully set. This is now the insoluble salt matrix that incorporates the unreacted glass particles.
4/ The maturation of the GIC can take weeks and this system is very dynamic in the oral environment.
GIC adhere chemically to the tooth. They do this by chelating calcium. Ion exchange displaces calcium and phosphate in the hydroxyapatite in enamel and form hydrogen bonds to the collagen fibres in dentine. This bond is dynamic, as if disrupted, it shows the capabilities to reform. Therefore failure of GIC usually occurs cohesively within the material itself, rather than at the tooth-restorative interface.
Enamel:
Freshly cut and clean enamel will suffice. The argument against etching the enamel is that it decreases the crystal content of enamel and therefore the GIC has less to react with. Etching is primarily for increasing surface area for micro-mechanical bonding (as seen for resin composites).
Dentine:
However, dentine on the other hand is a more complex structure and is a living tissue. When we prepare the cavity, debris clogs the dentine tubules and is incorporated between the collagen fibres, known as the smear layer. If this layer wasn't modified in some way the bond strength would be considerably weaker because the wettability of the GIC is decreased; not being able to infiltrate between the fibres and tubules.
Dentine conditioners aim to remove the smear layer, and enhance the collagen scaffold. The main constituent of the conditioner is primarily polyacrylic acids (25%).
GIC is vulnerable to water. This differs in the way resin composites are vulnerable. The resin reaction is directly inhibited by the contamination of water, whereas, when using GIC, it is vulnerable to water incorporation particularly during its setting reaction and the week or so to follow. Therefore, with an increase in the absorption of water, the cements properties are weakened. Under occlusal load this can exacerbate wear and a process known as washout occurs (loss of the glass from matrix).
GIC protection varnishes are recommend to limit the amount of water sorption occurring. This should try and offer protection for at least an hour but 24hrs is recommended in some texts.
GIC must not be desiccated. If the material is over dried, crazing can form and propagate into cohesive loss forming cracks. Rehydration can occur but the crack doesn't fill back in. This scenario could occur if carrying out multiple restorations adjacent to one another and the first placed in the series is succumbed to numerous drying events.
This property is seen to be advantageous due to its perceived anti-cariogenic, remineralisation effects, and anti-biofilm properties. Fluoride can migrate between the matrix and wash out into the surrounding tissues. If using a fluoridated toothpaste, the GIC does have the ability to restock as it incorporates the fluoride ions.
An awareness of the setting time should be obtained as finishing the restoration surface too soon as the material matures, can produce a loss in surface integrity, and therefore, an increased risk of further restorative damage. This can be up to 6mins. Ideally place and leave. Some texts reference leaving for 24hrs before finishing.
Due to the strong marginal seal and ability to bond to both enamel and dentine, GIC is seen to be useful for:
Bases: There is some contradiction in the text and probably between supervisor opinion. GIC can be used as a base/ indirect pulp cap (sandwich technique), but it is also argued that when etched, the matrix can be damaged, therefore weakened, and the resin has a high affinity to the glass which call pull it out from the salt matrix. All of which could undermine the restoration. Leaving for 24hrs and allowing greater setting is advised in some text if the sandwich method is to be used.
The aim when creating RMGIC was to keep the fluoride releasing qualities alongside the chemical adhesion of GIC and incorporate the strength of the resin composite.
Note: Some text may refer to RMGIC as Hybrid ionomers.
RMGIC still go through an acid-base reaction but with the additional polymerisation of the resin. The aim is to provide early strength in comparison to their GIC counterpart by the resin creating a mesh for the slower GIC reaction to occur. The polymerisation can be light activated; however, the light is unable to penetrate though the matrix as well as it does in resin composites. Therefore the max width light can pass through is 0.5mm. RMGIC usually have a dark curing system built in, which uses a radox reaction pathway to combat this limitation; however this isn't as strong.
There are some limitations to this method of theory.
DO NOT use with all ceramic non supported indirect restorations, in particular veneers. The material swells too much and is likely to crack and damage the ceramic. It wouldn't be ideal to send the patient away with a beautiful new veneer for it so come back broken and failed pretty quick.
Avoid placing onto radicular tissue.
As you can see RMGIC don't really offer too much more compared to GIC. They do have their place, but it appears limited.
These resources were used throughout to help produce this lesson:
Bonsor SJ, Pearson G. A clinical guide to applied dental materials. Elsevier Health Sciences; 2012 Dec 5.
Von Fraunhofer JA. Dental materials at a glance. John Wiley & Sons; 2013 May 31.
Dental Update:
Burke FT. Dental materials: what goes where? class V restorations. Dental update. 2015 Nov 2;42(9):829-39.
Dental Update:
Frencken JE. The state-of-the-art of ART restorations. Dental update. 2014 Apr 2;41(3):218-24.
