The influence of heating and storage medium on the setting of glass-ionomer cements expressed in flexural strength.
Prof Domagoj Glavina
The chemical setting reaction of GIC materials involves two different phases: The initial setting based on cross linking and later a further maturation phase. The initial setting starts directly when mixing the powder with the liquid components of the material. The liquid (acid) etches the surface of the glass particles, releasing calcium, aluminum, sodium and fluorine ions. Initially the released calcium anions cross-link the poly-acrylic chains exothermically. With time, these calcium ions replaced by aluminum ions. Sodium and fluorine ions do not participate in the cross-linking and are dispersed uniformly all over the set material.
During the following maturation phase, the cross linked poly-acrylic network gets hydrated by water from the environment. The remaining, unreacted glass particles are embedded by silica gel that has been formed during the cation removal from the glass particles. So, the hardened material consists of an agglomeration of by silica gel surrounded unreacted glass particles in an amorphous matrix of hydrated calcium and aluminum poly-salts. In situ, the carboxyl groups may have reacted with the calcium ions from the adjacent mineralized tissue during the adhesion phase. At the same time, the fluorine ions diffuse from the GIC material into the surrounding mineralized tissues.
In short, the setting reaction of GIC is a sensitive and complicated one, where water household, ionization and the temperature are significant for the progress and the efficacy of the chemical reaction and consequently of influence on the mechanical properties and thus it’s clinical performance.
Flexural Strength Development after 1 week
Because of potential water uptake and/or extrusion throughout the whole setting process, the storage media for the initially set material is most important. A sensitive means to express the mechanical quality of the set brittle material is the flexural strength, usually determined in a 3-point bending test. It has been demonstrated experimentally that, after 1 week storage in distilled water, the GIC flexural strength values are in the order of magnitude of 20 MPa, for the samples stored in artificial saliva about 35 MPa, and samples stored in Vaseline showed to reach values over 40 MPa.
When the samples were thermo-cured (or heated) with a polymerization unit 1200-1600 mW/cm2 for 40s , that is, when additional energy was delivered during setting, the flexural strength values significantly increases for all storage media. In distilled water flexural strength increased to around 35 MPa, in artificial saliva to 45 MPa and in the Vaseline up to more than 55 MPa.
When the samples were coated with what on the outer surface in order to prevent water exchange between the sample and the surroundings, the flexural strength values in water remained the same as those without coating, about 20 MPa. But, in the artificial saliva storage, the flexural strength values enlarged up to 50 MPa and in Vaseline even more, up to 75 MPa.
From the above stated experimental results on flexural strength tests it can be concluded that results are dissimilar for different storage media. Flexural strength was best after storage in Vaseline, suggesting that the ion leakage is here substantially lower than in artificial saliva or distilled water. Distilled water storage showed worst results for all cases. When no additional heat cure was involved and no coating was applied, the results were dependent on storage media, meaning that diffusion of ions into and outwards from the material are significant in distilled water. In the coated group, the influence of storage media was particularly accentuated when additional heat was supplied during polymerization. Thes results were in line with the results obtained when only heating was studied as the variable parameter. Coating served to obstruct ion exchange in water and in artificial saliva (comparable results), while storage in Vaseline showed significant increase in flexural strength. Here, not only the coating limited ion diffusion but also the absence of water in the Vaseline was responsible for keeping the water household inside the samples intact .
Flexural Strength Development after 1 month
After 1 month, when material is fully matured, for the samples set with only acid-base reaction it could be observed almost similar results in flexural strength , comparing to values after 1 week, in the distilled water and artificial saliva (around 22 and 36 MPA respectively). But, for the samples stored in the Vaseline significant increase in flexural strength can be noted (around 65 MPa). Same pattern was observed for the thermo-cured (heated) samples, values for flexural strength in distilled water were around 35 MPa, but there were significant increase in flexural strength for the samples stored in artificial saliva, around 50 MPa and even more for the samples stored in Vaseline, more than 75 MPa. When the coat was applied the flexural strength values are even higher. Samples stored in distilled water showed the worst values, around 50 MPa. Interestingly, in the artificial saliva, flexural strength values were almost the same like in distilled water, around 50 MPa. Yet, samples stored in Vaseline showed significantly higher values, mounting to about 80 MPa.
From the latter findings it can be concluded that similarly the 1 week strengthening process, maturation of the initially set GIC still continues for at least 1 month. Also now, flexural strength was highest for storage in Vaseline, suggesting that also in a much later stage of the developing material control of the water household an ion migration is important.
Furthermore it has to be kept in mind that GIC is very sensitive to acid erosion due to it’s inorganic nature and consequently easily erodes in low pH environment. The pH of to open to air exposed distilled water gradually decreases from 7 to 5,65, due to CO2 uptake. Moreover H3O+ ions (that are essential for attacking the glass particles and ion release during the initial chemical reaction) diffuse from the GIC material towards the surrounding water making that water even more acidic. Artificial saliva is also a water based fluid. So, similar acidification from CO2 and H3O+ uptake may occur. Due to it’s special composition that resembles human saliva, some buffering can be expected. This will partially obstruct ion diffusion from the GIC explaining flexural strength values in between those from samples stored in water or in Vaseline.
Thermo-curing (additional heating) lead to changes in molecular kinetic energy due to an elevated temperature. This will result in rearrangement of the molecules in the material during setting and formation of more cross-links between the molecular chains. Temperature at the tip of the polymerization unit is between 50-60 oC. This will increase the temperature in the material at a depth of 2 mm rises 5-10°C depending on the polymerization unit power. Molecular rearrangement occurring within the material facilitates a better cohesion of the material, responsible for less ionic exchange with the environment. Moreover, heat induces a change in the coordination of the aluminum in the matrix of the GIC. Aluminum present in the glasses is predominantly in the 4-coordination, but also in a 5- and a 6-coordination. During setting a proportion of the 4-coordination aluminum is converted to 6-coordination types within the matrix of the cement. This conversion may be responsible for the experienced improved chemical reaction.
The procedure of supplying external energy to setting GIC, easily carried out by using a high energy irradiating light-curing unit, has shown to significantly improve the GIC mechanical stability expressed in flexural strength and surface hardness.
Also storage media for the samples is shown to be an extremely important factor in testing procedure of GIC. Using different storage media can significantly influence the results on mechanical properties. Since the ISO standard proposes distilled water as storage medium, and that same medium acts aggressively on the material, the standard requires a serious reconsideration.
The clinical implications of above presented laboratory results are that fresh GIC restorations require coating as a protection against premature deterioration by oral fluids and that application of a heat-cure technique substantially improves the mechanical properties of the restorative material and thus the longevity of the restoration.