Читать книгу Restoring with Flowables - Douglas A. Terry - Страница 9
ОглавлениеThe year 1996 was an exciting one all over the world. The Dow Jones Industrial Average reached a record high of 6,000; the Nobel Prize in Chemistry was awarded to Robert F. Curl Jr, Harold W. Kroto, and Richard E. Smalley for their discovery of fullerene, a molecule composed entirely of carbon; General Motors launched the first electric car of the modern era; divers discovered the ancient port of Alexandria; eBay opened its doors for business; DVDs hit the 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 composites1,2 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 provide a comparison of the size, shape, orientation, and concentration of filler components for specific conventional hybrid and flowable resin composite systems: (a) G-aenial Universal Flo (GC America); (b) G-aenial Flo (GC America); (c) G-aenial Sculpt (GC America); (d) Clearfil Majesty ES Flow Low (Kuraray); (e) Filtek Supreme Ultra Low (3M ESPE).
The esthetic appearance of the surface of a resin composite restoration is also a direct reflection of the particle size. Esthetic restorations require biomaterials to have optical properties similar to those of tooth structure. Because resin composite does not have hydroxyapatite crystals, enamel rods, and dentinal tubules, the composite restoration must create an illusion based on the way light is reflected, refracted, transmitted, and absorbed by dentin and enamel microstructures. Recreating a natural anatomical surface requires a similar orientation of enamel and dentin. Newer formulations of resin composites possess optical properties that render the tooth polychromatic. In addition, the filler particle sizes and distribution can influence the color and esthetics of a restoration through a phenomenon called the double-layer effect, also known as the chameleon effect or blending effect.18–20 This mechanism applies to the relationship between natural tooth structure and esthetic materials. It occurs when a composite material is placed as a restoration and diffused light enters from the surrounding hard dental tissues; when emitted from the restoration, the shade is altered by absorbing color from the tooth and the adjacent teeth. This color alteration depends on the scattering and absorption coefficients of the surrounding hard dental tissues and restorative material, which can produce an undetectable color match by blending with tooth color.21 Furthermore, the surface quality of the composite restoration is influenced by the composition and the filler characteristics of the composite.22,23 Newer formulations of nanocomposites have altered filler components with finer filler size, shape, orientation, and concentration, improving not only their physical and mechanical properties but also their optical characteristics (Fig 1-1). These universal resin composite systems allow the composite to be polished to a higher degree, which can influence color integration between the material and the tooth structure.
Current Developments in Nanotechnology with Resin Composite
Nanotechnology, or nanoscience,24 refers to the research and development of an applied science at the atomic, molecular, or macromolecular level, also known as molecular engineering/manufacturing. The prefix nano- is defined as a unit of measurement in which the characteristic dimension is one-billionth of a unit.25 Although the nanoscale is small in size, its potential is vast. There has been significant advancement in the world of small. Small has become a common research theme for building nanomotors, nanorobots, nanocircuits, and nanoparticles. Recent advances by scientists and engineers in manipulating matter at this small magnitude indicate potential applications of this nanoscience in every arena of our economy, including telecommunications, aerospace, computers, textiles, homeland security, microelectronics, biomedicine, and dentistry.25
In dentistry, nanotechnology24 may provide resin composites with filler particles that are dramatically smaller in size and that can be formulated in higher concentrations and polymerized into the resin system with molecules designed to be compatible with polymers and provide unique characteristics (physical, mechanical, and optical). In addition, optimizing the adhesion of restorative biomaterials to the mineralized hard tissues of the tooth is a decisive factor for enhancing the mechanical strength, marginal adaptation, and seal of the adhesive restoration, as well as improving its reliability and longevity. Currently, the particle sizes of many of the conventional composites are so dissimilar to the structural sizes of the hydroxyapatite crystal, dentinal tubule, and enamel rod that a potential exists for compromises in adhesion between the macroscopic (40 nm to 0.7 μm) restorative material and the nanoscopic (1 nm to 10 nm) tooth structure.26 However, nanotechnology has the potential to improve this continuity between the tooth structure and the nanosized filler particle and to provide a more stable and natural interface between the mineralized hard tissues of the tooth and these advanced restorative biomaterials.
Flowable composite materials have been evaluated in numerous studies1–5,7,27–45 since their inception. Some of the more recent studies39,42,43 indicate that the clinical performance of specifically tested flowable resin composites is similar to or better than that of specifically tested universal resin composites. Attar et al28 showed that different flowable composites possessed a wide range of mechanical and physical properties. Earlier studies by Gallo et al29 on specific flowable resin composites suggested that these materials should be limited to small- and moderate-sized restorations with isthmus widths of one-fourth or less of the intercuspal distance.36 However, Torres et al43 reported that, after 2 years of clinical service, no significant differences were found between Class II restorations restored with GrandioSO (VOCO) conventional nanocomposites and those restored with GrandioSO Heavy Flow (VOCO) flowable hybrid nanocomposites. A study by Karaman et al39 showed similar clinical performances over 24 months in restorations of noncarious cervical lesions restored with conventional nanocomposites (Grandio, VOCO) and those restored with flowable material (Grandio Flow, VOCO). A more recent study by Sumino et al42 indicated that the flowable materials G-aenial Universal Flo, G-aenial Flo, and Clearfil Majesty Flow (Kuraray) had significantly greater flexural strength and a higher elastic modulus than the corresponding conventional nanocomposite materials, Kalore (GC America) and Clearfil Majesty Esthetic (Kuraray). The wear and mechanical properties of these specific flowable materials suggested an improved clinical performance compared with that of the universal composites. Several in vitro studies conducted at GC Research and Development comparing specific flowable material properties of several conventional composites found results similar to those of Sumino et al. Of the next-generation flowable systems studied, G-aenial Universal Flo and Clearfil Majesty ES Flow (Kuraray) showed superior gloss retention and similar wear resistance to the conventional nanocomposites tested, which included Filtek Supreme Ultra (3M ESPE), Herculite Ultra (Kerr), Clearfil Majesty ES-2, and G-aenial Sculpt (Table 1-1).
According to these studies, the recently developed specific nanohybrid flowable resin (or universal injectable) composite systems (ie, Clearfil Majesty ES Flow and G-aenial Universal Flo) may possess properties that meet the aforementioned mechanical, physical, and optical requisites. These properties and the clinical behavior of the biomaterial formulations are contingent on their structure. New resin filler technology allows higher filler loading because of the surface treatment of the particles and the increase in the distribution of particle sizes. The unique resin filler matrix allows the particles to be situated very closely to each other, and this reduced interparticle spacing and homogenous dispersion of the particles in the resin matrix increases the reinforcement and protects the matrix.46–48 In addition, the proprietary chemical treatment of the filler particles allows proper wettability of the filler surface by the monomer and thus an improved dispersion and a stable and stronger bond between the filler and resin. Research studies48–52 clearly indicate the importance that filler content and coupling agents represent in determining characteristics such as strength and wear resistance. Recent studies4,36,53 report that flowable composites have comparable shrinkage stress to conventional composites. According to the manufacturers, these next-generation flowable composite formulations are purported to offer mechanical, physical, and esthetic properties similar to or better than those of many universal composites.54 The clinical attributes of universal flowable composites include easier insertion and manipulation, improved adaptation to the internal cavity wall55 (Fig 1-2), increased wear resistance, greater elasticity, color stability, enhanced polishability and retention of polish, and radiopacity similar to enamel. Furthermore, the clinical indications for these next-generation flowable resin composites are increasing as the properties of the materials and the bond strength of adhesives to dental tissues improve. With improved mechanical properties reported,42 these highly filled formulations are indicated for use in anterior and posterior restorative applications.56 The clinical applications of these specific next-generation composites include sealants and preventive resin restorations; emergency repair of fractured teeth and restorations; fabrication, modification, and repair of composite prototypes and provisional restorations57; anterior and posterior composite restorations; composite tooth splinting58; and intraoral repair of fractured ceramic and composite restorations.58 In addition, these composites can be used to repair denture teeth,58 establish vertical dimension, alter occlusal schemes before definitive restoration,56 manage spatial parameters during orthodontic treatment, eliminate cervical sensitivity,58 resurface occlusal wear on posterior composite restorations,58 establish incisal edge length before esthetic crown lengthening,56 develop composite prototypes for copy milling,56 and place pediatric composite crowns.59
Fig 1-2 (a) Hybrid resin composite (Estelite Sigma Quick, Tokuyama) placed without a flowable resin composite liner. Note the gap at the interface between the resin composite (RC) and the all-in-one bonding agent (B). (b) Same hybrid resin composite placed with a flowable resin (FR) composite liner. There is no gap between the flowable resin composite and the bonding agent. D, dentin. (Confocal laser scanning microscope images courtesy of Alireza Sadr, DDS, PhD.)
Fig 1-3 (a) Preoperative occlusal view of defective amalgam restorations with recurrent decay on the maxillary right second premolar and first molar. (b) After the preexisting amalgam restorations were removed, the occlusal outline was extended to include carious enamel, provide access to the carious dentin, and remove any residual amalgam staining. (c and d) A selective enamel etch procedure was performed on the prepared and unprepared enamel. A 37.5% phosphoric acid gel (Gel Etchant, Kerr) was applied for 15 seconds, and then the surfaces were rinsed for 5 seconds.
Fig 1-3 (cont) (e to g) A self-etch adhesive (G-aenial Bond, GC America) was placed on the enamel and dentin surfaces with an applicator tip for 10 seconds, air dried for 5 seconds using an Adec warm air tooth dryer, and light cured for 10 seconds. (h and i) An opacious A2-shaded flowable resin composite (G-aenial Universal Flo) was applied to the first molar as a cavity liner with a syringe applicator, uniformly distributed with a ball-tipped instrument (M-1 Ball Burnisher XP, American Eagle), and light cured for 20 seconds.
The lack of evidence-based research and clinical trial data on flowable biomaterials requires clinicians to evaluate the individual mechanical properties of these materials to determine whether their properties are equal or superior to those of existing materials. As the clinical performance of these next-generation flowable materials has improved over time, the research data have concurred. Although no direct correlation has been found between a material’s mechanical and physical properties and its clinical performance, such a correlation might suggest the potential success of a restorative biomaterial for a specific clinical situation.1 However, clinical longevity for restorations developed with these biomaterials remain to be determined through clinical studies for each specific clinical application (Figs 1-3 and 1-4).
Fig 1-3 (cont) (j to l) An opacious A2-shaded hybrid resin composite (Kalore) was applied to the first molar in increments using an oblique layering build-up technique. Successive increments of hybrid composite were applied and light cured for 40 seconds each. (m and n) An opacious A2-shaded flowable resin composite (G-aenial Universal Flo) was applied to the second premolar in increments using an oblique layering build-up technique. Successive increments of flowable composite were applied and light cured for 20 seconds each. (o) Anatomical stratification buildup using two different resin composite systems.
Fig 1-3 (cont) (p and q) The final occlusal layer, a translucent A1-shaded hybrid resin composite (Kalore), was applied to the first molar with a long-bladed interproximal instrument and invaginated with an endodontic file while the material was still soft. This same procedure was performed on the second premolar using a translucent A1-shaded flowable resin composite (G-aenial Universal Flo). A diluted ochre tint (Kolor + Plus, Kerr) was applied into specific regions of the invaginations with an endodontic file and polymerized for 40 seconds. (r) The completed restorations reveal the harmonious integration of two different resin composite systems with existing tooth structure. (s) A 2-year clinical follow-up. Note the cavosurface wear at the incline of the distolingual cusp of the molar with the conventional hybrid composite, whereas the premolar shows no clinical evidence of wear.
Fig 1-4 (a to f) A 5-year review of posterior resin composite restorations using a flowable resin composite system (G-aenial Universal Flo) with an incremental layering technique. Note the minimal wear.
Conclusion
Only time can provide the answers of knowledge, wisdom, and truth. Knowledge of the past and a desire to create are limited by the materials clinicians have available to them for restorative procedures. Advancements in resin composite technology continue to improve the practice of dentistry. Continuing technologic breakthroughs allow the clinician not only to comprehend the building blocks of the ideal composite restoration but also to implement and maximize the potential of new materials to attain more predictable and esthetic results. Although new ideas and concepts continually flood the marketplace, one should not discount the power a new biomaterial may have on planning, design, or procedure. These developments promise to simplify the clinical applications for esthetic and restorative techniques and ultimately improve the level of health care provided to the contemporary dental patient. Because only the passage of time can determine the success of a material, future clinical trials will be required to determine the long-term benefits of these new flowable resin formulations. The clinical applications provided in the following chapters demonstrate the potential of these flowable nanoparticle composite formulations to expand treatment options for a wider range of clinical situations.
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