Ionomers – Glass/Resin

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 and form a chemical bond with overlaid composite when used as a liner or base. Etching a resin ionomer to increase its bond strength to resin is neither necessary nor advantageous. On the other hand, inadvertent etching will not interfere with the bond of a resin ionomer to composite.

Indications

The products covered in this category are used for bases, core buildups, and/or definitive restorations.

Bases If the definition of a base is a thick liner, then there is virtually no reason to use an ionomer for this purpose. However, they still can be used for blocking out undercuts in inlay/onlay preparations. 

Buildups and Cores When small to medium sections of a crown preparation need to be rebuilt, a glass or resin ionomer works well due to its flowable consistency (in most cases) and tenacious bond that is augmented by using them in combination with the recommended primer or conditioner. There is also the speculation that fluoride release will render the tooth preparation more caries-resistant. However, none of these products match the hardness of dentin. This means that when you are prepping them along with adjacent tooth structure, you must adjust the pressure applied to the handpiece or you will have the tendency to ditch the ionomer due to its relative softness. 

Restorations Mainly used in Class V in permanent teeth and Class I/II in primary teeth. Even though composites are more sculptable and polishable, restoring frank carious lesions in highly susceptible patients seems to be the main indication for these materials due to their fluoride release. They can also be very helpful when used for fast, virtually self-adhesive provisional restorations in fractured teeth.

Delivery Options

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. They are also the only automix option for restorations and buildups at this time. 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.

Tests

Flow If a material is being used for restorative or buildup purposes, it may require different flow properties depending on how you are going to use it. 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.

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 37ºC/95% humidity, but they should be close enough for apples to apples comparisons.

Translucency/Opacity Discs in 1.0mm thicknesses 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 product commentary. More translucent materials tend to blend better into tooth structure.

Finishing and Polishing Materials destined for definitive restorations were polished with three standardized instruments for 30 seconds and then were compared to a natural extracted tooth with a typical enamel glossy surface.

Porosity When you are close to finishing and polishing a restoration, the last thing you need is to find small voids in the surface. These voids will collect everything from lipstick to coffee and will basically doom the success of your work of art. And repairing them is a real nuisance and not easily accomplished. Since all the materials destined for definitive restorations must be mixed, this process can incorporate air that can certainly cause some of these voids, but for the encapsulated materials, this is out of our control. Discs of material are fabricated and then transilluminated on a light box. The discs are then photographed using a special Nikon camera attached to a stereomicroscope. The images are imported into a graphical software program and the porosity is quantified. The higher the percentage of porosity, the higher the probability you will have voids present on the surface when the restoration is finished and polished.

Fluorescence Many venues use black light for its special effects. If a restorative material does not exhibit fluorescence similar to tooth structure, it could create an embarrassing situation for a patient. This test shows how the material appears in vivo under black light. Veneers are fabricated 1mm thick, placed on the maxillary right central incisor of a human model, and viewed/photographed with the head of a model (still attached to her body!) placed in a custom-made, black light box. You cannot test fluorescence on a tabletop or on extracted teeth, the latter of which fluoresce at a different level than vital teeth. Materials exhibiting fluorescence that do not match that of natural teeth would not be a good choice in the mouth of patients with high esthetic needs. Note that, even though we use a veneer for this test, we are not suggesting these materials are suitable for this type of restoration.

Hardness Discs of material are fabricated and then placed in a digital hardness tester to determine the Knoop hardness (KHN). You would have less of tendency to ditch a material during the preparation of a crown if the material was close to the hardness of dentin (45.2). For liners, harder materials may resist occlusal forces better than softer ones.