Flowable Composites A Versatile Restorative Material
Flowable Composites A Versatile Restorative Material
The use of composite resins in dentistry has grown exponentially since their introduction over 30 years ago. Traditional composites are heavily filled with reinforcing particles to improve mechanical properties and wear resistance.1,2 Their viscosity increases with the filler load content, giving them a putty-like consistency that may not be desirable in certain situations. Flowable (or low-viscosity) composites were introduced in late 1996 to address requests for a direct esthetic restorative material with fluid injectability that could be placed rapidly and easily.3 This article reviews the composition, properties, applications, and latest innovations of this popular material.
Composition and Properties
With a filler content by weight between 50% and 70%,4 the first generation of flowable composites (ie, traditional flowable composites) has a 20% to 25% lower filler content than hybrid composites, but has similar particle sizes.5 In addition, a greater proportion of diluent monomers can be added to the composition, 6 resulting in an increase in the ratio of resin to filler and a reduction in viscosity. Improved flowability allows these resins to be packaged in syringes with small-gauge dispensing needles or in unidose tubes that are inserted in a dispensing gun, facilitating and simplifying placement.
For direct resin-based materials, the greater the filler content, the greater the mechanical properties.1 In general, traditional flowable resins have significantly lower mechanical properties than hybrid composites.1,3 Filler content is a major factor in preventing polymerization shrinkage,7 which is why traditional flowable composites, with their lower filler content, will shrink more upon polymerization and are less rigid (lower elastic modulus) than conventional composites.8 The low elastic modulus could help reduce polymerization shrinkage stresses, improving marginal seal and decreasing microleakage.9,10 However, research shows inconsistent results regarding the latter. While some authors found that using flowable composites may help reduce microleakage,11 others found no differences. 12 These confounding results could be explained by the significant compositional variations among brands.5 In other words, there is no ?standard? flowable composite, so clinicians should carefully review the information available before selecting a specific material.
Another important clinical property of flowable composites is radiopacity. Some low-viscosity resins can be less radiopaque than the adjacent enamel or hybrid composite, giving the radiographic appearance of recurrent caries under a conventional resin. Clinicians should select a highly radiopaque material to facilitate secondary caries detection under posterior composite restorations.13
The most commonly cited use for flowable composites is small Class I preparations in nonstress?bearing surfaces. 14 Preventive resin restorations, where minimal wear is expected, fulfill these criteria.15 The increased flow, wettability, and needle tip dispensing facilitate resin placement into narrow, deep pits and fissures. Heavily filled resins may be harder to place in these preparations, making traditional flowable composites a convenient choice. A recent study recommends limiting the use of flowables to occlusal restorations with an isthmus width of one-quarter or less of the intercuspal distance.16
Another common application of lowviscosity resins is to seal the gingival floor of the proximal box in Class II restorations. Some authors found this application improves the marginal seal,17,18 while others failed to show any differences.19 Once again, this discrepancy can likely be explained by variations in the composition between different brands.
Several practitioners like to use flowable composites as a liner under a higher- modulus resin. The low stiffness of the flowable resin is believed to act as a stress-absorbing layer that offsets the polymerization shrinkage of restorative composites, which may relieve stress on the adhesive interface and enhance the durability of the marginal seal.20 However, this technique may not improve the interfacial adaptation19 or the clinical performance of hybrid composite occlusal restorations. 21,22 The same seems to be true when flowable resins are used to restore noncarious cervical lesions.23,24 Anecdotal reports claim reduced postoperative sensitivity when using a flowable composite liner,25 but clinical research shows no differences.26 Despite these findings, the effortless adaptation to the undercuts of cavity preparations and the ease of application make the selection of flowable composites appealing for these situations.
Because ease of use is one of the main reasons for the popularity of flowable composites, additional applications have been suggested, including repairs of composites28 and bis-acryl provisional restorations, 29 reattachment of tooth segments,30 and customization of nonanatomical implant impression copings.31
Recently, a new generation of highly filled (about 80% by weight), lowshrinkage flowable composites has been introduced.28 Manufacturers claim that their filler content and polymerization shrinkage are similar to those of hybrid composites, while maintaining the same flowability. The rationale behind this new generation is to expand the application range of flowable composites to include larger, stress-bearing restorations. The higher filler content should improve the mechanical properties and the lower shrinkage should address concerns regarding interfacial adaptation.
In a recent study, Ikeda and colleagues found that a highly filled flowable composite may be adequate for relatively large restorations while maintaining excellent marginal integrity.28 The authors recommend an incremental filling technique to reduce the effect of polymerization stress. Laboratory testing also corroborates the enhanced properties of these new flowable resins.27
Flowable composites are versatile restorative materials suitable for a variety of applications. The popularity of the first generation seems to be based more on its ease of use than research-supported clinical benefits. Recently introduced highly filled, low-shrinkage flowable composites may expand the range of applications and provide important benefits for the practitioner. While initial in vitro testing has been encouraging, additional clinical research is needed to confirm these results.
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