Do bulk fills perform better than conventional composites in terms of shrinkage stress?

Bulk fill composites have a reputation for an increased risk of presenting shrinkage stress, which can compromise your direct restorations. However, a study published in January this year in the Journal of Applied Oral Science challenges these concerns about this versatile material-and what they discovered might surprise you.

Resin composites have challenges over time.¹ They fail in extensive posterior restorations, especially in Class II restoration with margins in dentin or cementum because of²:

• Wear

• Adhesive border weakening

• Errors in the use of the material

• Incomplete polymerization

• Polymerization shrinkage

Shrinkage stress is caused by polymerization shrinkage that occurs when curing resin composites. Polymerization shrinkage within a range of one to six percent, is common with resin composites.³ In direct restorations, polymerization shrinkage causes debonding along the restoration and tooth border or at the margins of the restoration, which can cause gaps (both internally and at the margin), micro-cracking of the restoration or the tooth, and cuspal movement and marginal staining.⁴

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If the material wasn’t bonded to the surrounding walls of the preparation, the composite would shrink and flow into place and not cause any additional problems (i.e., no stress on the restoration). However, when placed in a preparation where the material is meant to be bonded to the surrounding walls, the shrinkage that occurs creates tension on the adhesive bond and can create gaps at the margin.

Once you have marginal gaps, saliva and bacteria can leak into the area and degrade the bond with the natural tooth. This situation could lead to secondary caries, changes in the pulp and, ultimately, restoration failure.

One of the ways to combat the problems associated with polymerization shrinkage in direct restorations is to lay the composite in the preparation in thin layers. The layers will decrease the chances of polymerization shrinkage, which cause the problems at the margins. However, incremental layering is time-consuming and technically challenging.

Bulk fill composites are a resin composite allowing for larger incremental loads for curing, which makes them faster and less technique sensitive to achieve proper outcomes. However, based on what clinicians know about polymerization shrinkage caused by curing resin composites, many clinicians think bulk fills in the thicker increments will have much higher levels of risk for shrinkage stress.

Researchers from Ohio, Brazil and Michigan decided to find out if what many clinicians think they know about shrinkage stress in bulk fill composites is true.

The following is a summary of what they discovered in their research.

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Who did the research?

• Fabio Antonio Piola Rizzante, Case Western Reserve University, School of Dental Medicine, department of comprehensive care, Cleveland, Ohio

• Rafael Francisco Lia Mondelli, University of São Paulo, Faculdade de Odontologia de Bauru, Departamento de Dentística, Endodontia e Materiais Odontológicos, Bauru, São Paulo, Brazil

• Adilson Yoshio Furuse, University of São Paulo, Faculdade de Odontologia de Bauru, Departamento de Dentística, Endodontia e Materiais Odontológicos, Bauru, São Paulo, Brazil

• Ana Flávia Sanches Borges, University of São Paulo, Faculdade de Odontologia de Bauru, Departamento de Dentística, Endodontia e Materiais Odontológicos, Bauru, São Paulo, Brazil

• Gustavo Mendonca, University of Michigan, School of Dentistry, department of biologic and material sciences – division of prosthodontics, Ann Arbor, Michigan

• Sérgio Kiyoshi Ishikiriama, University of São Paulo, Faculdade de Odontologia de Bauru, Departamento de Dentística, Endodontia e Materiais Odontológicos, Bauru, São Paulo, Brazil

Why did they test shrinkage stress?

Since their introduction, bulk fills have raised concerns that by using larger increments, the polymerization shrinkage and related shrinkage stress could make gaps on the adhesive surface and lead to poor outcomes and success rates. The study assessed the polymerization stress of nine different bulk fill composites as well as their elastic modulus. The researchers hypothesized that the properties of bulk fill and regular composites will differ based on their materials mix between the monomer and the filler content. Their stated objective was “to evaluate the polymerization shrinkage stress and the elastic modulus of different bulk-fill resin composites.”

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What did they test?

The nine composites tested in this study included:

• Admira Fusion x-tra – VOCO

• Filtek™ Bulk Fill Posterior – 3M

• Filtek™ Bulk Fill Flowable – 3M

• Surefil SDR^® flow – Dentsply Sirona

• Tetric EvoCeram Bulk Fill – Ivoclar Vivadent

• x-tra base – VOCO

• x-tra fil – VOCO

• Filtek™ Z350 flow – 3M

• Filtek™ Z350XT – 3M  

How did the researchers test them?

• Fourteen specimens were made for the linear shrinkage test; seven at 12mm3 and seven at 24 mm³. Seven additional specimens measuring 2x2x25 mm were used for the Young’s modulus testing.

• They evaluated linear shrinkage for 300 seconds by putting the specimens (both the 12 mm and 24 mm) between two metallic bases on a universal testing machine (Instron model 3342). The articulated arm was connected to the 50 kgf (kilogram-force) load cell to the upper base and using it with a BENCOR Multi-testing device as the lower base. The results were analyzed by the software and then recorded as force (Newtons) multiplied by time (seconds) in the graphs, and then converted to MPa (megapascal pressure unit) by dividing the force results by the 12mm² (the area of the specimen’s transversal section).

• They tested Young’s modulus by taking prepared specimens and positioning with a three-point bending device and applying force with the articulated arm of the universal testing machine (using 0.5 millimeters per minute cross speed) until it broke. The elasticity was determined though the onboard Instron software.

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What were the results?

• Surefil SDR flow and Tetric EvoCeram Bulk Fill were the only two bulk fill composites that generated lower stress after 20 seconds in the 12 mm³ specimens per three-way ANOVA (analysis of variance)

• etric EvoCeram Bulk Fill, Surefil SDR flow, and x-tra fil created the lowest stress at 300 seconds, and next was Admira Fusion x-tra, Filtek Bulk Fill Posterior, x-tra base and Filtek Bulk Fill Flowable, which had similar results to Filtek Z350XT

• The highest stress values for all the times tested were made by Filtek Z350 flow

• Most of the bulk fill composites produced lower stress than Filtek Z350XT at 20 seconds using 24 mm³ samples, with the only exception being x-tra base

• Once the time increased to 300 seconds, the bulk fill composites with the lowest stress values were Surefil SDR flow, Filtek Bulk Fill Posterior and Admira Fusion x-tra, followed by Tetric EvoCeram Bulk Fill and x-tra fil

• One-way ANOVA showed that Filtek Bulk Fill Flowable, Surefil SDR flow, Filtek Z350 flow and Admira Fusion x-tra presented the lowest values for elastic modulus, meaning they had the lowest resistance to external force, and next lowest were x-tra base and Tetric EvoCeram Bulk Fill

• Among the analyzed materials, Filtek Bulk Fill Posterior, Filtek Z350XT and x-tra fil showed the greatest elastic modulus, meaning they had the highest resistance to external force

What did they learn?

The researchers accepted their hypothesis. The bulk fill and regular composites exhibited different properties as discussed and expected by the team related to their material makeup (e.g., monomer types and filler content ratios).

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They also concluded that bulk fill resin composites either had the same or less shrinkage stress compared to the regular composites, chiefly with heftier increments. When it came to Young’s modulus values, bulk fills presented a wide range of performance, but those were typically like a regular composite. Moreover, the culprit for polymerization stress occurrence appeared to be volumetric shrinkage rather than elastic modulus.

What additional research is needed on bulk fills?

The researchers were satisfied with the data they could confirm from their work on this study. However, many questions remain about bulk fill composites. The researchers felt additional studies were needed on the following areas:

• How bulk fill composites’ properties influence the long-term maintenance of internal and marginal adaptation

• The relationship between the patient’s tooth structure and the bulk fill composites, specifically regarding adaptation and cusp deflection

• Indications for each composite for clinicians, as they all behaved differently in the test not only by the viscosity classification but also between brands

Bulk fills have a reputation for increasing risk for the shrinkage stress in direct restoration caused by the polymerization process. However, research shows that bulk fills perform as well as or better than conventional composites that are applied in an incremental layering technique. Although more research is required to explore all the indications and performance records, bulk fill composites are proving to be a reliable option for direct resin composite restorations. 

Click here to read the entire study in the Journal of Applied Oral Science.

References

1. Rizzante, Fabio Antonio Piola, Mondelli, Rafael Francisco Lia, Furuse, Adilson Yoshio, Borges, Ana Flávia Sanches, Mendonça, Gustavo, & Ishikiriama, Sérgio Kiyoshi. (2019). Shrinkage stress and elastic modulus assessment of bulk-fill composites. Journal of Applied Oral Science, 27, e20180132. Epub January 07, 2019.https://dx.doi.org/10.1590/1678-7757-2018-0132.

2. Ibid.

3. SOARES, Carlos José, FARIA-E-SILVA, André Luis, RODRIGUES, Monise de Paula, VILELA, Andomar Bruno Fernandes, PFEIFER, Carmem Silvia, TANTBIROJN, Daranee, & VERSLUIS, Antheunis. (2017). Polymerization shrinkage stress of composite resins and resin cements – What do we need to know?. Brazilian Oral Research, 31(Suppl. 1), e62. Epub August 28, 2017.https://dx.doi.org/10.1590/1807-3107bor-2017.vol31.0062.

4. Ibid.