Universal composites include sculptable microhybrid and nanohybrid materials. They typically contain more than one type of filler particle, usually some type of glass and silica. When these two types of fillers are combined, we supposedly get the best combination (or compromise) of strength and esthetics. It is this superior strength characteristic, in addition to radiopacity and wear resistance, which makes a universal composite a good choice for a posterior composite, while the esthetic properties are especially important for anterior use.
Even though there are subcategories of materials such as ormocers, nanomers, and giomers, all of which have some unique qualities, we have included them in this category since they are all used for the same purposes. Nanohybrids, also known as nanomers, in particular, have garnered a substantial amount of publicity due to their extremely small particles and the purported advantages of even more polishability without sacrificing strength. As a matter of fact, virtually all universal composites introduced in the last few years claim to have at least some nanofillers. However, their long-term benefits over more conventional microhybrids still need to be demonstrated and not all nanomers are created equal.
Curing Time for Gingival Wall Increment
This test will tell you if the increment of restorative material you place on the gingival wall of the proximal box has achieved the benchmark 80% hardness value of the occlusal surface. The test is done in simulated Class II preparations with the depth of the gingival wall 6mm from the tip of the light. The gingival increment is placed no thicker than 2mm.
Depth of Cure (mm)
Although some manufacturers of composites and curing lights continue to promote that their products will cure beyond the time-tested 2mm increment in 40 seconds, our tests indicate these claims are largely the figments of the marketing departments imagination. This test will tell you the maximum thickness of each increment of restorative material you can place, using hardness measurements in 1.0mm increments from the occlusal surface in the proximal box.
The depth of cure limit is reached when the hardness value in the proximal box falls below the benchmark 80% of the occlusal surface hardness. This 80% hardness value is generally recognized as the standard to judge whether a composite has been adequately cured, although we are unaware of any clinical correlation with this value. Nevertheless, it is indisputable that the physical properties of a restorative material will not be maximized if a restoration is undercured. The test is done in a simulated Class II preparation with the depth of the gingival wall being 6mm from the tip of the light.
NOTE: If this test found that a material did not cure to the 80% mark even at 2mm, we still assigned a depth of cure of 2mm, since clinical experience has shown curing virtually any material at that thickness for 40 seconds with a reasonably effective curing light will be successful.
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 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:
70-80 If you are looking for a material to block out shine-through from the back of the mouth, a product scoring in this range, usually called a dentin shade, would be a good choice. Just remember to keep a material with this much opacity at least 1mm lingual to the expected surface of your restoration. If you goof and end up with an opaque material too close to the surface, the restoration may appear too light.
60-70 If you are using a single shade, sometimes designated as body, to restore a preparation, a product scoring in this range would probably work reasonably well, although using one shade to restore a lesion is usually always a compromise.
50-60 If you have placed a dentin shade to block out shine-through, a material in this range, usually called an enamel shade, would be a good choice to restore the final 1mm of the restoration, except at the incisal edge.
25-40 If you want to simulate incisal translucency, a product in this range would probably work.
15-25 If you want to fill small defects and you do not want to affect the basic background shade, this range should suffice.
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.