The versatility of flowable composites. Part 1: theory and new classification

Conventional FCs

Conventional FCs cannot be applied in bulk or be used as a core build-up material. Furthermore, they have no bioactive repairing properties or self-adhesive features. These FCs need to be applied with minimal thickness, using a conventional bonding technique. They are indicated for various situations, including as a fissure sealant or a liner under a restoration, and in the restoration of a class V cavity. These situations will be addressed in depth with evidence in Part 2 after discussing other types of flowable composites. Table 2 provides a list of the available commercial products as well as their composition.

Self-adhering FCs

A more recent development in the field is the introduction of self-adhering FCs that contain functional monomers to promote adhesion to tooth structures without etching, rinsing or drying.¹²

These products are generally indicated for use only as a liner, porcelain repair and pit-and-fissure sealant. However, incidents of microleakage are more prevalent with this type of FC in comparison to conventional FCs. While a self-adhering flowable would be a dentist's dream, more studies are needed to look at their long-term clinical success.

Bulk-fill FCs

Manufacturers have developed a variety of resin-based composites to simplify the application protocol for posterior restorations deeper and wider than 4 mm to a single step. Known as bulk-fill materials, they can be classified as light or dual-cured, high- or low-viscosity resin composites.¹³Table 3 provides a list of some of the commercial products available at present, as well as their specific requirements and features.^13,14

With regard to clinical performance, in vitro and in vivo studies have shown that bulk-fill FCs are appropriate for use in restorative dentistry alongside other restorative materials.^15–17

In a randomized clinical trial, a bulk-fill (SureFil SDR flow, Dentsply Sirona) was capped with a nanocomposite and compared to a conventional FC applied using the incremental technique. The authors found comparable success over 5 years. The annual failure rate (AFR) was 1.4% and 2.1%, respectively, for the SDR and conventional FC in Class II restorations and zero AFR in Class I restorations.¹⁵ Another randomized clinical trial compared 3M Filtek Bulk Fill Flowable Restorative capped with Filtek P60 Posterior Restorative, Tetric EvoCeram Bulk Fill, SonicFill 3 SingleFill Composite System and a nanohybrid composite. These products were used on 172 Class II restorations in 43 patients. At 1-year follow up, all restorations were found to be in acceptable condition with no observable differences between the products.¹⁷ Karaman et al conducted a randomized clinical trial of Class II restorations in non-vital teeth using VOCO x-tra base bulk-fill FC capped with nano-hybrid VOCO Grandio and compared with restorations made from BISCO Dental AELITEFLO conventional FC capped with nano-hybrid VOCO Grandio applied using the incremental technique. Both methods of restoration were found to be acceptable and comparable in their clinical performance throughout the 3-year evaluation period.¹⁶

Bulk-fill FCs offer efficient, less time-consuming and less technique sensitive tooth-coloured restorations compared to conventional flowable and packable composites. These materials can be especially beneficial for restoring cavities when procedural time is particularly critical, for example in children and nervous patients. Although these results were encouraging, further clinical studies, with evaluation periods of longer than 5 years, are required to establish long-term clinical performance and longevity, as well as possible flowability improvements to withstand occlusal forces without a cap.

Core build-up low and medium viscosity FCs

These materials can be applied to badly broken vital or non-vital teeth, as well as to restore and fortify weak spots. The core material can be a cast or plastic core acting as a foundation for the tooth. This allows the practitioner to establish a favourable form to enhance the resistance and retention of the overlying prosthesis. Resin composite is considered one of the plastic core materials.

Core build-up composite materials come in different viscosities: low, medium and high. Table 4 highlights some core build-up materials with different flow properties.

Three studies have tested and compared the properties of these materials. One study investigated the shear bond strength (SBS) of three different build-up materials. One of the materials, MultiCore dual-cure resin (Ivoclar Vivadent, Liechenstein) had the best mean SBS relative to the other tested materials.¹⁸ Another two studies concluded that all build-up composites were found to have acceptable water sorption and solubility, and could last 1 year in service without compromising on compressive strength.^19,20

Tauböck et al detailed that exposing dual-cured materials to light imparted no additional microhardness benefits over self-cured materials. Self-curing materials allow them to be placed in areas with little to no light exposure.²¹ Significant differences were found in the physical and mechanical properties of various core composite materials. Flowable or injectable core build-up materials are considered crucial for endodontically treated, badly broken teeth because they can be used to fill the prepared canal as a luting material, followed by build up of the coronal part of the tooth. These materials help to ensure an adequate seal and flow into all irregularities of the preparation and reduce the chairside time. An example of this situation is when a clinician has a widened canal that cannot be fitted with any of the prefabricated posts available. In one report, instead of prescribing the indirect cast post core, a fibre-reinforced composite post (everStick, Stick Tech Ltd, Turku, Finland) was cemented with core build-up material.²² This is made possible due to the flowability and injectability of the material to ease the adaptability to the tooth structure. Once again, flowable composite in the form of core build-up proved to be an essential material to the restorative dentist.

Bioactive repair flowable products

Some active materials have both a flowable consistency and chemical effects on pulpal tissues. Such products include Pulpdent Corporation's (USA) ACTIVA BioACTIVE – RESTORATIVE and ACTIVA BioACTIVE – BASE/LINER that combine the benefits of composites with glass ionomers.²³ These materials can chemically bond to teeth, release and recharge with ions, have a good seal against microleakage and are strong and resistant to wear and fracture.²⁴ One short-term study of two cases reported no complaints of post-operative tooth sensitivity. While the authors of the study lauded the materials as a significant development in the field of adhesive restorative dentistry, they recommended that longer-duration studies should be conducted in the future.²⁴

Bioactive FCs, such as Beautifil Flow Plus and Plus X (Shofu, Japan), which contain Giomer, a proprietary filler particle, can release fluoride ions and reduce the rate of secondary caries. One clinical evaluation of Class I and II restorations with a 13-year recall, showed good retention rates and few secondary caries. Table 5 displays the available bioactive Giomer products.²⁵

BISCO Dental TheraCal LC²⁶ is a light-cured resin-modified calcium silicate product indicated for use in direct and indirect pulp capping procedures that has favourable effects in comparison to calcium hydroxide. It also has acceptable sealing capabilities and tolerated immortalized odontoblast cells well.²⁶ Although Gandolfi et al mentioned that TheraCal can release calcium and hydroxyl ions and alkalinize the surrounding fluids to increase the pH,²⁶ Camilleri et al showed that the calcium-releasing ability is limited, and less than that of Biodentine, which may affect calcium hydroxide formation and limit the reparative properties of TheraCal.²⁷ However, the same author claimed the possibility that the released calcium ions were not in hydroxide form.²⁷ Compared to calcium hydroxide-based products, the ability of TheraCal to be light cured is a potential advantage to dental practitioners to avoid setting failure and potential microleakage.

However, despite the unique features of these products and their encouraging studies, additional in vitro and in vivo studies of both materials are required to further evaluate long-term clinical outcomes.