Technical Challenges with Posterior Composites

The Importance of Material Selection

A primary consideration in the success of any restoration is the material with which it is created. For direct restorations in the anterior and posterior, dentists need a material that offers low shrinkage, radiopacity, handling that is sculptable without being sticky, low wear, high fracture toughness and other physical properties, and an ability to maintain surface polish.

There has been a recent trend toward using bulk-fill flowable products in large restorations to save time, but the lack of long-term data on this practice should warrant caution on the dentist’s part. The most popular bulk-fill flowable has low-shrinkage stress, but the actual shrinkage is still believed to be approximately double that of top nano-filled and microhybrid composites.

The hybrid/microhybrid/nanocomposite category contains many excellent materials, but they are not all the same. Manufacturer claims can make it difficult to differentiate these materials. Some products claim universality, but they do not hold a polish. Others deceptively claim that only one shade is needed (true for some cases but not others, unless the dentist’s standards are low). Some may feel too firm to some dentists, while others may feel too sticky.

Understanding the differences between various composites requires a review of the evolution of composite fillers. The category of a composite has a strong effect on its suitability for any given indication. As most dentists know, these materials are classified by the size of their filler particles. The earliest versions of composite, known as macrofills, contained particles with sizes ranging from 10 μm to 50 μm. While this relatively large particle size made these materials strong, it also contributed to difficulties in retaining surface smoothness and resulted in relatively low wear resistance. These materials were soon supplanted in the market by microfills, which offered outstanding long-term smoothness due to their particle size under 100 nm. The drawback with microfills, however, is in their strength. Microfills are created by combining filler and resin and then polymerizing the material, after which it is ground into particles and combined with additional filler and resin. The level of filler loading of these materials is too low for indications that require strength or wear resistance; they are, therefore, typically advised only for use in anterior areas, where they are not under stress.

The categories of hybrid, microhybrid, and nanohybrid have since been developed to occupy the middle ground between microfills and macrofills, attempting to achieve a balance between esthetics and strength. As with microfills, milling and grinding techniques are used to create the particles used in these composites. Hybrids are formulated with 10-μm to 50-μm particles as well as additional particles of approximately 40 nm in size. Microhybrids and nanohybrids range from over 1 μm to less than 100 nm. These formulations are designed to achieve both good strength and wear resistance through high filler loading. The disparity in particle sizes, however, can give rise to a problem in polish retention, as individual particles can be plucked out of the restoration during wear. As the effects of this abrasion mount, these composites lose reflectivity and polish. Most of today’s composites fall into this category.

However, a different class of material offers properties that address the drawbacks of each of these categories. A nanocomposite, distinct from a nanohybrid, is engineered via a chemical process that creates molecular-sized particles from the bottom up. These particles are then made into nanosized fillers. This technology has been patented by 3M ESPE, which applies it in its Filtek™ Supreme line of composites, the most recent generation of which is Filtek™ Supreme Ultra Universal Restorative. This composite is entirely composed of nanosized particles, with a primary size of 20 nm, but it differs from a microfill in that individual nanoparticles attach to one another to form nanoclusters. These nanoclusters allow for higher filler loading and increase the composite’s strength. Furthermore, as the material wears, only individual nanoparticles are worn away from the nanoclusters, leaving nanosized voids. This gives the material the ability to maintain an excellent level of polish over the long term.

It is important to understand these distinctions in order to avoid the problems associated with using any composite as a universal material. Most composites are not universal. In addition, the frequency with which the term “nano” is applied to composite materials makes it difficult for most clinicians to understand the distinctions between them. There are several good materials that offer reasonable strength, shrinkage, and handling characteristics. However, for long-term polish retention combined with strength, the properties of the nanocomposite discussed here set it apart as a genuinely universal material. The following technique description demonstrates the steps for ensuring long-term success with this type of treatment in a case that shows its use in a moderate-sized posterior restoration.

Technique

The patient seen here was going through a multi-year process of treating and repairing older dental work. Many teeth had failing amalgams without overt decay, but discoloration and small marginal discrepancies led the clinician to suspect decay for No. 29, the second to last tooth needing repair. The patient was also unhappy with the appearance of the amalgam in this tooth (Figure 1).