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Although resin-based composites have existed in dentistry for more than 50 years, the category of flowable composite resins is relatively new. In late 1996, the first generation of flowable composite was introduced and had been designed to be less viscous than composite resin, but not as fluid as dental sealant. Bayne and colleagues1 wrote of these early materials, “The success of the early flowable composite products was more a result of marketing than of any special properties beyond flow.”
Composite resins are tooth-colored restoratives that are comprised of organic resin matrices and inorganic fillers consisting of proprietary combinations of silica, quartz, zirconia, and prepolymerized resins. Recently, composite categories have evolved based on resin type and filler size and now include hybrid, nanofill, microfill, packable, and flowable. What defines a flowable composite is low-viscosity material capable of being dispensed into small preparations through needles or cannulas (usually 20-gauge) and low filler content typically 20% to 25% less than nonflowable materials.2,3 Some materials are more flowable (ie, less viscous) than others, and, in general, this is directly related to the filler content and particle-size distribution. When 11 flowable composites were compared using a modified flow test technique, PermaFlo® (Ultradent) and Wave (SDI) were more fluid than FZ250 (3M ESPE). The more fluid materials had lower flexural strengths than the more viscous materials when tested at 24 hours and again at 1 month.4
It is commonly believed that filler content is an important characteristic influencing volumetric shrinkage and wear resistance. Shrinkage of flowable composites is estimated at approximately 3.8% to 6.4% and 1% to 3% higher than nonflowable resins.5 Practitioners have limited the range of application and filling techniques to address these shortcomings. In doing so, flowable composites are currently indicated for purposes such as lining Class II proximal boxes, cavity adaptation, repair of bis-acryl provisional, amalgam repair, and restoration of small nonocclusal-bearing lesions (Table 1).6-8 In addition, by placing the flowable composite material in increments not exceeding 2 mm, the challenge of polymerization stress is believed to be minimized.1,9,10
With more than 50 flowable composites on the US market covering a wide range of physical properties among them, it may be difficult to determine which material is most appropriate for each clinical situation. For that reason, this article examines two controversial subjects in regard to flowable composites. First, will the use of flowable composites to seal the gingival cavosurface in the proximal box of a Class II preparation lead to improved outcomes and less microleakage? Second, do recently introduced bulk-fill flowable composites perform as claimed and reduce shrinkage stress?
Volumetric Shrinkage and Microleakage
Shrinkage of flowable composites, despite material advances, remains a principal challenge to restorative dentists. In a worst-case scenario, shrinkage can compromise the success of the restoration and contribute to a poor marginal seal, microleakage, microfracture, and recurrent caries.11
After the flowable composite is expressed into the cavity preparation and before polymerization, the monomer constituents are held together loosely with minimal potential energy.12 At some point during the polymerization reaction, a gel phase is reached and an elastic modulus is created.13 Elastic modulus refers to the rigidity of a material and its ability to resist deformation. If the flowable resin is placed into a confined space and then shrinks during polymerization, stress will develop. According to Hooke’s law, stress is determined by the stiffness of a material when subject to a given strain.14 In this case, the shrinkage stresses are transferred to the surrounding tooth structure because the elastic modulus of tooth is far greater than the restorative material.15
As a result of shrinkage stresses being transferred to the tooth, deformation of the tooth occurs. This may result in postoperative sensitivity, opening of microfractures, bond failure, microleakage leading to recurrent caries, and deterioration of the restorative margin.12,13,16 Several factors have been identified as influencing the shrinkage stress of a restoration: the size and geometry of the restoration (ie, C-factor, depth and diameter), materials used, and curing protocol.17-19
Incremental filling techniques have been proposed as a means to reduce shrinkage stress of composite restorations. There has been disagreement among authors recently on this issue; Versluis et al13 and Abbas et al20 showed that deformation and cuspal deflection could be minimized when bulk-fill techniques were used. Lee et al21 and Park et al22 demonstrated that incremental filling techniques should be used to mitigate polymerization shrinkage and cuspal deformation. Despite differing conclusions, incremental filling techniques are generally recommended and dentists may choose to restore composite restorations in this manner on the basis of additional factors such as acceptable depth of cure, proper adaptation, and adequate bond formation.13,23
The use of flowable composites as a liner to seal the gingival margin and provide an intermediate stress-absorbing layer under Class II restorations is controversial. Some studies have shown decreased microleakage when flowable composite is used as a liner at the gingival margin,24-34 while others have discouraged the practice.35-41 Some research has concluded that location of the gingival margin either above or below the cemento-enamel junction is the most important factor in determining the best restorative material to use. In these studies, glass-ionomer cement has been suggested because of its adhesion properties and lack of shrinkage over flowable composite to prevent microleakage.42-45 When synthesizing the results of these studies, it appears that the use of flowable composite as a liner in the proximal box is recommended to increase cavity adaptation and reduce microleakage. It is important to note, however, that because of high volumetric shrinkage and the possible resulting stress generated, 1-mm increments were most often used in the studies reviewed.
Recent advances in monomer technology have ushered in a new category of bulk-fill flowable composites that are designed to address material shortcomings of earlier products. The new category of bulk-fill flowable composites promotes the effective use of 4-mm increments while decreasing shrinkage stresses generated during polymerization.46,47 In 2009, the first bulk-fill flowable resin, SureFil® SDR® flow (DENTSPLY Caulk), was introduced. Currently, four bulk-fill flowable composites are on the US market: SureFil SDR flow, Filtek™ Bulk Flow (3M ESPE), Venus® Bulk Flow (Heraeus Kulzer), and x-tra base (Voco). A comparison of the four currently available bulk-fill flowables is presented in Table 2.
The information provided by the manufacturer of SureFil SDR flow regarding the chemical composition indicates that the organic resin matrix consists of a patent-registered urethane dimethacrylate with incorporated photoactive groups able to control polymerization kinetics. From DENTSPLY’s website: “Through the use of the ‘Polymerization Modulator’, the resin forms a more relaxed network and provides significantly lower polymerization stress.”48 Ilie and Hickel examined SureFil SDR flow compared with other composites and found that the contraction stress generated by SureFil SDR flow was 1.1 mPa compared with 5.3 mPa and 6.5 mPa of Esthet-X® (DENTSPLY Caulk) and Filtek Supreme Plus Flow (3M ESPE), respectively.49 The authors theorized that the stress-relieving properties of SureFil SDR flow, in part, were the result of a delayed gel phase and slower polymerization allowing for increased flow. Compared with the other composites, SureFil SDR flow showed the lowest result on the Vickers hardness test, which may be due to the fact it has the lowest filler content by volume (44%). For this reason, the SureFil SDR flow directions for use recommend that a 2-mm occlusal cap be placed using a traditional composite restorative such as TPH Spectra™ (DENTSPLY Caulk).50
C-factor (configuration factor) is an estimation of the stresses generated through a given cavity configuration by a ratio of bonded to unbonded surfaces. According to Feilzer et al,51 the higher the C-factor (ie, the higher the number of bonded surfaces), the higher the stress generated (eg, Class I, Class II). Conversely, a cavity with a higher ratio of unbonded surfaces should result in lower shrinkage stress (eg, Class III, Class IV). Findings from two recent studies have also suggested that cavity depth and diameter may impact shrinkage stress and resulting microleakage.18,52 Examining the effect of bulk-fill high C-factor cavities with a low-shrinkage flowable composite (SureFil SDR flow), Van Ende and colleagues showed that 4-mm increments placed in high C-factor preparations (mimicking Class I and Class II preparations) did not compromise bond strength secondary to shrinkage stress.53,54 The authors concluded that if bulk-fill techniques are desired for restoration of high C-factor cavities, the dentist should consider low-stress materials to avoid adhesive de-bonding and microleakage. These conclusions appear to align with the conclusions of Rogendorf et al,55 which said that bulk-fill low-shrinkage flowable resin can be used in an open-sandwich technique without a negative impact on marginal integrity.
Cuspal deflection and deformation were studied in Class II preparations by Moorthy et al.56 The authors compared a conventional resin-based composite with two low-stress bulk-fill flowable composites, SureFil SDR flow and x-tra base (Voco). After restoration, cuspal deflection was measured and found to be reduced by greater than 50% when the flowable resins were used; no significant difference was noted between the flowable resins tested. The authors suggested that bulk filling to within 2 mm of the occlusal cavosurface can reduce operator time because of reduced incremental layers without additional shrinkage stress or loss of marginal quality.
To this point, bulk-fill flowable composite resins have been discussed; however, more viscous bulk-fill materials also exist on the US market and deserve mention. These materials can be used to restore occlusal surfaces and have a published depth of cure of 4 mm to 6 mm; examples include Alert® (Pentron), Tetric EvoCeram® Bulk Fill (Ivoclar Vivodent), and x-tra fil (Voco). Christensen’s Clinician’s Report from January 2012 observed that a potential negative of bulk-fill composites such as the above-named materials was that they were found to have a frequent occurrence of voids.46 The bulk-fill flowables tested showed voids occurred infrequently or occasionally.
An interesting hybrid between the bulk-fill flowable composites and bulk-fill composites is SonicFill™ (Kerr). The technique involves the use of a proprietary handpiece and composite. Shear stress is applied to the composite via sonic vibration, thereby lowering the viscosity of the material by a claimed 87%. In this way, the material can flow into the cavity preparation up to a depth of 5 mm. The manufacturer claims a more gradual buildup of viscosity once the shear stress is removed, allowing a lessening of contraction stresses and no increase in cuspal deflection. To date, few published studies examine the effects of sonic vibration on composite resin or the long-term effects of SonicFill other than on the manufacturer’s website.57-59
Flowable composites are components used in everyday restorative practice. Research has described their shortcomings with respect to physical properties and applications. When used properly for the right indications, flowable composites can provide clinical advantages. The introduction of bulk-fill materials, as either flowable or more viscous composites, is a desired enhancement to composite technology, and these materials are capable of increasing procedural efficiency without increasing negative outcomes. Based on this article, the following clinical suggestions can be made:
Flowable composite materials are a less-filled, less viscous material that can be syringed into small preparations. Because of high shrinkage and low wear resistance, the choice of applications for early flowables should be limited. Each incremental depth should not exceed 2 mm.
Volumetric shrinkage of composite resins remains a clinical concern and should be carefully considered when choosing materials or restorative techniques.
The utilization of incremental filling techniques is recommended to minimize shrinkage stress during composite resin placement.
The use of flowable composite resin is recommended to seal the gingival margin of the proximal box in Class II preparations. If the gingival margin extends below the cemento-enamel junction, dentists should consider the use of glass-ionomer cement.
Low-stress bulk-fill flowables are capable of being placed in 4-mm increments without increased cuspal deflection or loss of marginal integrity. It is important to note that bulk-fill flowable resins do not provide improvements in marginal quality or cavity adaptation over earlier flowable composites; however, they do not negatively affect it either.60
The authors are DENTSPLY Caulk employees.
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