While universal composites can certainly be used for anterior restorations, microfill composites are still recognized as the class of composites that brought cosmetic dentistry into prominence almost 30 years ago. They have allowed us to replace enamel of anterior teeth with a substitute that mimics it almost as well as porcelain, but also to keep the control of the final restoration in our hands, rather than abdicating that artistry to the laboratory. Microfills are primarily used as an enamel replacement in Class III, IV, and V restorations, diastema closures, and of course, hand-sculpted veneers.
Besides their ability to be polished easily to a high shine, microfills tend to resist wear due to abrasion very well. Microfills are still the most esthetic enamel replacement material, but polishable microhybrids and nanohybrids have definitely pushed the use of these materials into the hands of relatively few clinicians. Since they are more elastic than hybrids, microfills have also been suggested as an ideal material for abfraction lesions due to their presumed ability to bend with a tooth. Hybrids, being stiffer, would presumably be more likely to dislodge from a Class V preparation if the tooth is under occlusal stress. However, this concept has yet to be demonstrated clinically.
Finishing and Polishing
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.
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 model's head (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.
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. While the way we handle a material can certainly cause some of these voids, many of them are caused by incomplete air extraction by the manufacturer during the fabrication of the material.
This test shows how well the material has been vacuum mixed by the manufacturer to remove air from the final product. Discs of composite 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.
If the material comes with a shade guide, we compared the A2 tab to the actual material and to the Vita A2.
Shade Shift after Curing
This test shows how much color change there is after light curing the material. This is only important if you want to do a shade check without bothering to light cure the mockup. If the material does not have a shade shift, you could do a mockup without light curing. However, removing uncured material from a tooth can be messy, so light curing is a good idea in any event. This shift, if any, was measured using the spectrophotometer.
When you select a shade, you are typically using some type of shade guide based on Vita shades. These shade tabs allow you to determine which hue or basic color you should use to match the tooth to be restored as closely as possible. However, the contrast ratio, better known as the translucency or opacity of the material may place an equally important role as the hue. If you choose a material that has too much translucency, the restoration may appear gray or dark (low in value). Conversely, a material that is very opaque may appear too light (high in value).
To test materials for their relative translucency/opacity, we fabricate discs of selected enamel, body, and opaque shades in 1.0mm thicknesses and incisal shades in 0.5mm thicknesses. These discs are placed in a digital spectrophotometer, which measures this property on a scale from 0-100, with 0 being totally clear and 100 being totally opaque. Based on our clinical comparisons, here is a rough guide to the relative scores:
Note that translucency/opacity is sometimes confused with the so-called "chameleon effect", which is that elusive quality of a material that presumably allows it to blend into tooth structure, taking up the color of its surroundings. This characteristic is virtually impossible to quantify and may depend as much on the refractive properties of the restorative material as it does on any other factor, such as the translucency/opacity.
Conventional wisdom hypothesizes that low shrinkage materials should put less stress on the bond to tooth, which in turn means restorations should leak less and be more resistant to secondary caries. However, there are probably other properties of composites that may figure into this overall formula for success. Nevertheless, the low shrinkage trend has to be considered a positive one. Measurements were made using a computerized imaging device and software developed by Bisco called AccuVol. These shrinkage measurements were recorded after 5 and 10 seconds to ascertain how much of the total shrinkage occurred in the initial stages of curing and then at five minutes after each specimen was cured for 40 seconds.
Working Time under Dental Unit Light
This test will tell you when the material will start to cure under the dental unit light if the light is at high power and positioned at a typical working distance (30.0in/76.2cm) from the preparation.