Restoring with Flowables. Douglas A. Terry

market in Japan; Will Smith made his electrifying performance in the film Independence Day; David Bowie was inducted into the Rock and Roll Hall of Fame; and flowable resin composites were developed and introduced to the world as a revolutionary restorative biomaterial.¹ The average individual would probably rank this discovery as the least significant of these events, but this milestone dramatically affected the practice of adhesive dentistry.

The evolution of adhesive dentistry, with filled adhesives and sealants, led to the development and discovery of flowable resin composites. However, it was not until 1996 that these biomaterials had their own identity and became known as flowables. These first-generation flowable formulations were designed to simplify the placement technique and to expand the range of clinical applications for resin composites^1,2 They were configured by using filler particle sizes identical to those of conventional hybrid composites while reducing the filler load and/or increasing the diluent monomers.^3,4 Thus, a multitude of variations in viscosity, consistency, and handling characteristics were available to the discriminating clinician for addressing many of the restorative and esthetic challenges presented to them each day.

These biomaterials were marketed by manufacturers for a wide range of applications, which included all classifications of anterior and posterior composite restorations, amalgam margin repair, block-out materials, composite repair, core buildup, crown margin repair, cavity liners, pit and fissure sealants, porcelain repair, anterior incisal edge repair, preventive resin restorations, provisional repair, porcelain veneer cementation, composite veneer fabrication, tunnel preparation restorations, adhesive cementation, restoring enamel defects, air abrasion cavity preparations, and void repairs in conventional resin composite restorations.^1,5 Unfortunately, these early flowable formulations demonstrated poor clinical performance, with inferior mechanical properties such as flexural strength and wear resistance compared with the conventional hybrid composites.^1,2 In fact, the mechanical and physical properties of composite materials improve in proportion to the volume of filler added,⁶ and the filler content of these early flowable formulations was reported to be 20% to 25% by weight less than that of the universal composite materials.¹ Numerous mechanical properties depend on this filler phase, including compression strength and/or hardness, flexural strength, elastic modulus, coefficient of thermal expansion, water absorption, and wear resistance.⁶ Thus, a reduction in the filler content of these first-generation flowables substantiates the reports by Bayne et al,¹ which state that the mechanical properties of these low-viscosity materials were approximately 60% to 80% of those of conventional hybrid composites. One scientific study⁷ reported that a comparison of flowable light-cured resin composites and conventional resin composites of the same brand name had very different characteristics and mechanical properties. Early attempts to use these flowable formulations in a wide variety of applications resulted in shortcomings that led to confusion and uncertainty for clinical predictability and performance when using these biomaterials. These shortcomings resulted in limitations on the expanded applications previously suggested by the manufacturer. Clinicians realized that these first-generation flowable composites were neither the same nor adequate substitutes for the highly filled conventional composites.

Next-Generation Flowable Resin Composites

Since the inception of these initial formulations, a multitude of flowables have undergone continuous evaluation and improvement through scientific research and development. These “next-generation” flowable composites are being re-engineered as alternatives to conventional hybrid composites. The development of new technology continues to improve the ability of the scientist, manufacturer, and clinician to measure more effectively and therefore create a more ideal composite. However, the search continues for an ideal restorative material that is similar to tooth structure, is resistant to masticatory forces, has similar physical and mechanical properties to that of the natural tooth, and possesses an appearance akin to natural dentin and enamel. As the mechanical properties of a restorative material approximate those of enamel and dentin, the restoration’s longevity increases.⁸ An ideal restorative material should fulfill the three basic requirements of function, esthetics, and biocompatability.⁹ At present, no restorative material fulfills all of these requirements. However, nanotechnology used in dental applications may provide some of these solutions.

Restorative Material Selection

When selecting the proper material for a particular clinical situation, clinicians must consider two significant factors for the material’s anticipated use: the mechanical requirements and the esthetic requirements. In addition, other compounding variables that have the potential to influence the clinical behavior and material performance should be considered before restorative treatment. These variables include the placement technique, cavity configuration, anticipated margin placement, curing light intensity, tooth anatomy and position, occlusion, patients’ oral habits, and ability to isolate the operative field.^10–15 In view of these considerations, it is understandable that clinicians have uncertainties about the selection of biomaterials and the techniques needed to optimize the materials’ properties and achieve predictable, long-term results. A review of the mechanical and esthetic requirements for choosing a resin composite system for a specific clinical situation may provide insight into future selection and application.

       Mechanical and esthetic requirements

In resin composite technology, the amount and size of particles represent crucial information for determining how best to use the composite materials. Alteration of the filler component remains the most significant development in the evolution of resin composites,¹⁶ because the filler particle size, distribution, and quantity incorporated dramatically affect the mechanical properties and potential clinical success of resin composites.¹⁷ In general, mechanical and physical properties of composites improve in relation to the amount of filler added. Many of the mechanical properties depend on this filler phase, including compressive strength and/or hardness, flexural strength, elastic modulus, coefficient of thermal expansion, water absorption, and wear resistance.⁶

Fig 1-1 These scanning electron micrographs