Excellent Results with Today’s Resin-Based Composites
A review of the advances making placement more predictable
Over the past 2 decades, dentistry has seen the increased use of resin-based composites, as their reliability, predictability, quality, and characteristics have improved. Over time, clinicians have found amalgam-restored teeth to be more susceptible to internal and external fractures than teeth restored with composite alone. In addition, composite restorations allow for a minimally invasive approach, in which only the carious dentin is removed. This type of preparation design diminishes the need for macro-mechanical retention that is required for amalgam restorations.
In the past 30 years, patients have become more concerned with the esthetics of restorations, which has led to the widespread, gradual replacement of amalgam in posterior teeth. Additionally, the mercury found in amalgam has potential adverse health effects that are worrisome for many patients. Together, these factors have led to the increased use of composite over amalgam in general practice.
In the early 1990s, Bowen questioned composite adhesion to cavity walls and stated that polymerization shrinkage and pulp irritation were problems of resin-based composite restorations.1 The lifespan of posterior composites was reported to be 3 to 10 years at that time. Nevertheless, there was still a shift in the late 1990s away from use of amalgam restorations in posterior teeth in favor of composite. Improvements in posterior composite technology and adhesion may have been largely driven by patient demand for esthetic, tooth-colored restorations.
Changes in composite composition, including increased filler content, smaller particle size, improving adhesion of filler to organic matrix, and the introduction of low-molecular-weight monomers, greatly improved qualities such as esthetics and handling.2 The five different types of resin-based composites on the market today are:
• Microfills—the advantages of microfills include a low modulus of elasticity and high polishability. Disadvantages include low fracture toughness and higher marginal breakdown.
• Hybrids—these materials have better clinical properties than microfills, but they do not maintain a high polish.
• Microhybrids—due to a smaller particle size, these have high polishability similar to microfills, higher strength, and improved wear resistance.
• Packables—these are indicated for stress-bearing areas and ideal for Class II restorations. However, they are not superior to hybrids.
• Flowables—these improve the marginal adaptation of posterior composites, but they are more prone to polymerization shrinkage.
Although all composites demonstrate some degree of polymerization shrinkage from light curing, proper technique can make a big difference in the ultimate success or failure of a restoration. There are varying techniques, but the most common is careful wedge-shaped incremental application in oblique layers, which helps to minimize contraction of each layer from opposing walls.2
Just as resin-based composites have evolved and improved over the years, so has adhesive dentistry. These advances have helped make composite resin more reliable. Although a complete review of adhesive systems is beyond the scope of this article, an overview is presented here.
Bonding systems are categorized based on their components and grouped into generations as new ones emerge. The primary categories are fourth and fifth generations, also known as “total-etch” or “etch and rinse,” and sixth and seventh generations, or “self-etch.” The fifth and sixth generations are most widely used because they are effective and relatively simple, with each containing two steps.2
Total-etch systems etch the enamel and dentin with 30% to 40% phosphoric acid. This removes the smear layer and demineralizes the dentin tubules. The primer and adhesive, contained in one bottle, are applied and penetrate the micropores of etched enamel, forming the hybrid layer.
Self-etch systems are less technique sensitive and save time. The acid and primer are combined in one step, and the adhesive is the second step.2 The smear layer is not removed. The self-etching primer, invented by Watanabe and Nakabayashi, consists of an aqueous solution of 20% phenyl-P in 30% HEMA for bonding to enamel and dentin at the same time.3