What Is A Composite?

Resin composites consist of four parts:

• An organic polymer matrix
• Inorganic filler particles
• A coupling agent
• An initiator-accelerator system.

The organic polymer matrix usually consists of dimethacrylate. The inorganic filler particle can be finely ground quartz or glass, or other materials. These provide a number of characteristics, including reinforcing the matrix, allowing for translucency, and controlling the amount of shrinkage when cured. The coupling agent is known as silane, and its responsibility is to form a bond between the organic and inorganic phases of the composite. The initiator-accelerator system is what allows for the hardening of the composite when a light is placed on it – this is due to a chemical known as camphorquinone that absorbs light (ideally at 465nm) which triggers the reaction.

As the composite is subjected to light, a reaction known as polymerization occurs. This is important – during polymerization, a reduction of volume of the composite occurs, and shrinkage results. Shrinkage is a big deal when understanding and respecting our dental materials, and it can allow for small leaks into the preparation if the materials are not used properly. There are a few ways manufactures have limited this polymerization shrinkage in their materials, including using pre-polymerized resins, maximizing the amount of filler, and using methacrylate that has a high molecular mass. These are important details about the characteristics regarding composites that will help us understand the differing properties of bulk flow and traditional composites¹.

How Do We Measure The Effectiveness Of Composites?

There are numerous ways to measure the physical properties of composites, and although we touched on polymerization shrinkage above, we can dive into a few of those properties now.

• Depth of cure: The depth of cure measures how large of increments the composite can be placed when cured. Ideally, we’d prefer to be able to have a large depth of cure, but this can undermine the restoration.
• Working and setting times: Most composites share the same setting times due to the initiator-accelerator system. Be sure to watch out for ambient and operatory light when placing your composite. The normal range for setting time is 3 to 5 minutes, but this should be confirmed on the manufacture’s directions. Ideally, we want to have enough time to shape the composite before it sets completely.
• Polymerization shrinkage: We’ve already discussed this, but let’s revisit – composites will undergo shrinkage as they harden. This can leave a small gap between the restoration and the preparation. We want the polymerization shrinkage to be low.
• Thermal properties: The linear coefficient of thermal expansion is a rate at which a material changes size as the temperature changes. When it comes to composites, their value is HIGHER than that of dentin, meaning composite changes in size more readily than dentin when eating colder/ warmer foods. This can result in bond failure over time. We want the thermal properties to be as close to dentin and enamel as possible.
• Mechanical properties include flexural strength, compressive strength, and tensile strength, and these all differ in measurement between various composites types.
• Esthetics properties: Like mentioned earlier, filler content of a composite can add translucency and make a restoration look more natural. Of course, we want the restoration to look as natural as possible, and all composites have this advantage over traditional amalgams.

What’s The Big Deal About Bulk-Fill, Anyway?

Now that we understand a bit about how composites are characterized, let’s talk a bit about the background of bulk-fill composites. Bulk-fill composites are a type of resin-based composite that claim to be light-curable in increments of 4-10 mm. Normally, traditional composites only allow a depth of cure maximum of 2 mm thickness. The reason traditional composites can only be cured at depths of 2 mm is due to polymerization shrinkage – the shrinkage increases as the thickness increases (remember, we want polymerization shrinkage to be low so there is a tight seal between our restoration and preparation). Bulk-flow composites have claimed to not adversely affect polymerization shrinkage while allowing for larger depths of curing². In this post, we are going to explore the studies that compare polymerization shrinkages, as well as strengths and other properties, between bulk-fill composites and traditional composites.

So What Does The Research Say?

If you’ve made it this far into the post, you know that dental materials have a lot of jargon and can be a bit confusing. Let’s try and sift through some of the details and find out which material has the advantage over the other.

• Depth of cure: ‘It is widely accepted that traditional RBC restorations should be cured in increments of 2 mm… the majority of light-cured bulk-fill materials are limited to 4-5 mm increments³’
  • Winner: Bulk-Fill Composites* The study goes on to to mention that not every manufacturer is the same – some materials did not light cure at the depths they were advertised to cure at, and some studies show that direct, high-intensity light is required to achieve proper curing. It’s for this reason that light application must be ideal when using bulk-fill composites³. It’s also important to note that bulk-fill composites tend to be more translucent in order to achieve this depth of cure which will result in a grayer appearing tooth.
• Working and setting time: In a study evaluating the effect of resin thickness, and curing time on the micro-hardness of two bulk -fill resin composites, it was found that bulk-fill composites could be cured properly within different time intervals (10, 20, 30, and 60 seconds)⁴.
  • Winner: Bulk-Fill Composites
• Polymerization shrinkage: A study determined that some bulk-fill materials, depending on the brand, resulted in less polymerization shrinkage and shrinkage stress. The study concluded that ‘a significant reduction in polymerization shrinkage stress while maintaining comparable curing efficiency at 4 mm for some bulk-fill composites’ when compared to a traditional composite⁵.
  • Winner: Bulk-Fill Composites
• Thermal properties: In a study comparing the physiomechanical and thermal properties of bulk-fill and conventional composites, there was no statistical difference between the coefficient of thermal expansion between the 11 composites included in the experiment⁶.
  • Winner: TIE
• Mechanical properties: There is a concern with the strength of bulk-fill materials when comparing them to conventional resin-based composites. A study compared the flexural strength, elastic modulus, and surface hardness between bulk-fill and traditional composites. It was found that these values were generally lower for bulk-fill composites. Other studies show that, once the bulk-fill is placed, it is recommended that a conventional RBC is placed on top of the bulk-fill in order to make up for the inferior properties⁷.
  • Winner: Conventional RBC’s
• Esthetic properties: In all RBC materials, esthetics are improved from traditional amalgams. However, bulk-fill composites traditionally have limited shades and translucencies. In areas where esthetics are a concern, traditional composites should be used³.
  • Winner: Conventional RBC’s

The mechanical properties of the bulk-fill composites were mostly lower compared with the conventional high viscosity material, and, at best, comparable to the conventional flowable composite.

Physico-mechanical characteristics of commercially available bulk-fill composites, Leprince et al, 2014

To Summarize:

• Composites consist of four chemical parts – changing the composition of these allows for different properties
• There’s multiple ways to measure the effectiveness of composites
• Polymerization shrinkage is a concept that describes when composites shrink during curing
• Bulk-fill composites were shown to have less polymerization shrinkage at a higher depth of cure, but require strict light-cure management
• Conventional composites were shown to have better mechanical properties than bulk-fill composites
• Conventional composites have more esthetic choices available
• More studies need to be performed to confirm the long-term success of bulk-fill restorations – best use of the material would be in situations where time of the procedure is short

References:

¹Sakaguchi, R., Powers, J. (2012). Craig’s Restorative Dental Materials, 13th Edition. [VitalSource Bookshelf 9.4.0]. Retrieved from vbk://978-0-323-08108-5

²Balkaya, H., Arslan, S., & Pala, K. (2019). A randomized, prospective clinical study evaluating effectiveness of a bulk-fill composite resin, a conventional composite resin and a reinforced glass ionomer in Class II cavities: one-year results. Journal of applied oral science : revista FOB, 27, e20180678. https://doi.org/10.1590/1678-7757-2018-0678

³Chesterman, J., Jowett, A., Gallacher, A. et al. Bulk-fill resin-based composite restorative materials: a review. Br Dent J 222, 337–344 (2017). https://doi.org/10.1038/sj.bdj.2017.214

⁴Nagi, S. M., Moharam, L. M., & Zaazou, M. H. (2015). Effect of resin thickness, and curing time on the micro-hardness of bulk-fill resin composites. Journal of clinical and experimental dentistry, 7(5), e600–e604. https://doi.org/10.4317/jced.52536

⁵El-Damanhoury, H., & Platt, J. (2014). Polymerization shrinkage stress kinetics and related properties of bulk-fill resin composites. Operative dentistry, 39(4), 374–382. https://doi.org/10.2341/13-017-L

⁶Nascimento, A. S., Rodrigues, J., Torres, R., Santos, K. O., Fook, M., Albuquerque, M. S., Lima, E. A., Filgueira, P., Santos, J., Oliveira, L., & Braz, R. (2019). Physicomechanical and thermal analysis of bulk-fill and conventional composites. Brazilian oral research, 33, e008. https://doi.org/10.1590/1807-3107bor-2019.vol33.0008

⁷Leprince, J. G., Palin, W. M., Vanacker, J., Sabbagh, J., Devaux, J., & Leloup, G. (2014). Physico-mechanical characteristics of commercially available bulk-fill composites. Journal of dentistry, 42(8), 993–1000. https://doi.org/10.1016/j.jdent.2014.05.009

Liability Statement:

The medical advice given in this blog should only be utilized by a medical professional who has received a medical degree. I am not responsible for the medical advice given in this blog and each case should be reviewed extensively with sources outside my blog. This blog is for education purposes only and is not meant as a substitute for an academic institution.