Although dismissed as having low bond strength, possessing poor esthetics, and being old-fashioned, the ionomer family of materials continues to be used by many dentists, especially for luting purposes. This group of materials basically consists of glass ionomers, which have been with us for three decades, and resin ionomers, the more recent addition to the clan. Both types presumably form a chemical bond to tooth structure and release more fluoride than any other class of materials. While all glass ionomers are self-cured, their resin cousins are available in both self-cured and dual-cured versions. This category covers products used for luting definitive restorations.
These products are mainly used for cementing metal or ceramometal restorations, in addition to ceramics with high strength cores such as zirconia. They have also been suggested to cement posts and orthodontic appliances. However, we believe resin cements are more applicable for posts, especially prefabricated ones. On the other hand, the automix, paste-paste products are among the easiest of all luting products to use, especially for full coverage restorations.
Powder/Liquid (hand-mixed) This is the old standby. It allows you to vary the viscosity, does not require any special equipment, any amount can be mixed at the same time, and it is typically still the less expensive alternative. However, it can be very messy, the consistency of the mixes depends on the expertise of the person doing the mixing, and the cleanup can be a nuisance unless you are using disposable spatulas.
Paste/Paste (hand-mixed) Much less messy than powder/liquid, dispensing is typically calibrated and mixing is fairly easy, taking the human factor somewhat out of the equation. And you can still mix any volume of material that is needed. However, cleanup is still a nuisance, some type of instrument is usually needed to extrude the material, and the cost is higher. Nevertheless, this option is a step up from powder/liquid.
Paste/Paste (automix) The best option for those who do not want to keep a triturator in the treatment room. Although this type of dispensing is now commonplace with resin cements, provisional cements, and core materials, this is a relatively new innovation for ionomers. Other than the limited selection available, the only other real disadvantage is the need to clean and sterilize the dispensers of some of the products. This option still allows you to dispense as much material as you need and it can even save you from overdispensing since there is no need to guess how much material to dispense and mix.
Capsules The original automix option (before syringes and cartridges). For the most part, they give you consistent mixes and allow you to inject the cement directly into restorations and preparations. However, the volume of material is predetermined. If you need more, you must mix another capsule, which is a nuisance and can be very expensive. Activating the capsules, while not difficult, can be a challenge for inexperienced staff members. If a capsule is not activated properly and it becomes unusable, the expense can add up. And, of course, you need a triturator in the treatment room.
If a material is too thick, it may prevent positive seating of the restoration as well as possible fracture from the force needed to seat it. A material that is too thin may not have enough body to fill the gap between the restoration and the tooth. We performed flow tests on all the products in this category. Flow is graded on a grid going from 0-5, with 0 being very runny and 5 having virtually no flow.
Extraoral Working Time
If you have an assistant and are using an automix material, you will rarely if ever have to worry about the cement prematurely setting up in the restoration before you can seat it. However, if you are working alone, seating multiple restorations at one time, and/or using a hand-mixed version, you may find yourself not being able to fully seat the restoration. If this occurs and you do not discover it before the cement goes through its final setting curve, you may find yourself sectioning the restorations to remove them and then you will be faced with the embarrassing and expensive process of remaking them. Therefore, having adequate working time is very important in saving you from the aforementioned scenario.
Extraoral working time could be especially critical when you are working alone. In this scenario, you (the dentist) would most likely mix and load the restorations with cement and then place them on the bracket table or cart. You would then redirect your attention to the patient, hoping he or she has not closed his or her mouth in the interim, which would require you to repeat most if not all of the intraoral steps on the preparations once again. You then retrieve and seat the restorations, hoping the cement has not prematurely set. Therefore, the time between mixing, loading, and seating the restorations could be critical.
To test this working time, we used a test restoration and standardized preparation. For the control, we loaded cement into the test restoration (crown) and immediately seated it on the preparation and visually assessed the marginal integrity. We then repeated the test, but instead of seating the restoration immediately after mixing and loading the cement, the crown was allowed to sit on the countertop for a duration consistent with the stated extraoral working time of the manufacturer stated (if the manufacturer gave a range, we tested the shortest and longest times).
Then the crown was seated and the marginal integrity was again assessed. If there was no difference between the control and test specimens, we concluded that the stated working time was accurate. However, if the working time was not correct and a marginal gap was created by the partially set cement, we made note of that fact. In this instance, you would be well advised to seat your restorations before the working time expired. The results can be found in each commentary.
Cement Removal Time
Length of time each luting cement requires to cure sufficiently hard so that it could be removed cleanly from the margins of a restoration, but before it gets too hard. These times may not exactly match the times in the mouth, although the materials were allowed to cure in our temperature/humidity chamber set at 37°C/95% humidity, but they should be close enough for apples to apples comparisons.
Rock-Hard Extraoral Set Time
Length of time each material requires to cure sufficiently hard so that it could not be penetrated with a sharp explorer using hand pressure. Note that we did not light cure at all since the idea behind a dual-cured material (if applicable) is that it will still polymerize if no light reaches it. These times may not match the setting times in the mouth, although the materials were allowed to cure in our temperature/humidity chamber set at 37°C/95% humidity, but they should be close enough for apples to apples comparisons.
Discs in 100 microns thicknesses (to simulate the thickness of the cement under a restoration) were measured for relative degrees of translucency/opacity in a spectrophotometer. The scale was 0-100, with 0 being totally clear and 100 being totally opaque. Our findings are in each commentary. More translucent materials will have less of a tendency to show up like a white line at the margin.