We take a closer look at the benefits of bulk fills over conventional composite resins for direct restorations, what the literature says about their performance, and areas identified in the literature to bear in mind as differences when placing bulk fills.
Bulk-fill composites help clinicians simplify the restoration process for direct restorations by allowing for placement up to 4 to 5 mm. However, by reducing the layers, bulk fills require extra attention to other areas of the restorative technique process. Let’s take a closer look at the benefits of bulk fills over conventional composite resins for direct restorations, what the literature says about their performance, and areas identified in the literature to keep in mind as differences when placing bulk fills.
The Benefits of Bulk Fills
As composite resin materials improved over the past few decades, the prevailing wisdom determined that a layering technique in 2mm increments was the best way to overcome the complications caused by polymerization shrinkage in restorations with composite resin. However, it was also time-consuming and increased the risk of bubbles between the layers, which created tension on the cured material. To counteract this complication and simplify the placement process, manufacturers developed a new composite type for direct restorations, known as bulk-fill resin composites.
Bulk-fill composites have a few benefits over conventional incremental composites, including the following:1
How Do Bulk Fills Perform vs. Conventional?
In terms of performance comparison, research suggests that bulk-fills seem to perform as well as conventional composite clinically. Based on the review for Polymers, the authors suggest that the clinical performance of bulk-fill composites for direct restorations is similar to that of conventional composites, with the caveat that more unbiased studies were necessary.2 Also, in a case report in Odovtos International Journal of Dental Sciences, these sentiments were echoed by the contributing authors. They said that various studies reported satisfactory bulk-fill restorations in posterior teeth with results similar to conventionally layered composites.3 Authors in The Journal of Conservative Dentistry went even further to say that when it came to adaptability and gap formations on the pulpal floor, bulk-fills did even better than conventional composites.4
On the other hand, Operative Dentistry suggests that bulk-fill performance depends on the cavity configuration. In the 2018 study, incremental composites had higher bond strength than bulk-fills in high C-factor cavities, aka cavities with more bond surface. In low C-factor cavities, there was not a difference in performance between the 2 types of composite.5
However, there also exists a feeling that more study of bulk-fills is needed. Like the authors in Polymers, the authors in the Journal of Adhesive Dentistry also suggest more clinical studies that focus on using the bulk-fill technique on deep and large restorations to understand the clinical benefits of the materials.6
Differences in Technique When Placing Bulk Fills for Direct Restorations
There are differences in the technique necessary when placing bulk-fills. The most obvious difference is in the ability to place in up to 4-5 mm increments. However, there are a few more nuanced differences that we found when using them for direct restorations.
2-Step Bulk Technique
Per the journal Polymers (Basel), bulk fills are categorized as base (flowable) or full-body resin composites.The flowable bulk fills have a lower viscosity that makes them excellent for adaptation to deep preparations. However, flowable bulk-fill composites have fewer fillers which decreases their wearability.7
The 2-step bulk technique can address this drawback. Clinicians should use a flowable bulk-fill composite to fill the cavity preparation for all but the last 2mm (provided it isn’t deeper than 4-5 mm). Then, on top of the cured flowable bulk-fill material, clinicians should place a 2mm layer of conventional composite, cure, and then proceed with the finishing process. By contrast, full-body composites have a higher filler content than the base types and do not need the extra layer of conventional composite.8
Place the bulk fill in bulk.
One of the significant benefits of using bulk-fill resin composites is that they do not require layering, which runs a higher risk for voids. However, the risk of voids is not nonexistent with bulk-fill composites; there could be voids hiding in the mass of the material. A 2017 study published in the South African Dental Journal revealed that randomly chosen samples of different bulk-fill composite ampules had voids. The authors suggested materials manufacturers fill the syringes of bulk-fill flowable composites under vacuum to eliminate these voids' risk.9
However, it is not just the manufacturers that can cause voids in bulk-fill materials. Clinicians can, too. To reduce the risk of clinician-originating gaps, it is essential to exercise care in placement techniques. The Journal of the Dental Association of Thailand published a study in 2018 suggesting that injectable resin composites reduced the voids present in the material.10 Moreover, with flowable composites, the tip should remain embedded in the material during placement. Also, clinicians can use the tip to get back in the corners of the preparation.11
Don’t feel the burn.
As the layers of the bulk-fill composite are thicker, achieving the proper depth of cure might require more curing time of the material, depending on the output of the light, the light’s angle, the light’s tip size, and curing area, among other factors. However, longer curing times can generate heat, which increases the risk of thermal damage to the tooth’s pulp.12
To counteract the heat generated from the light curing units, clinicians can try a couple of different things. Reducing the thickness of the composite layer would also reduce the need for longer curing time.13 However, that does negate the main benefit of the bulk-fill composite. You can also use flowable composite as a “pulpal insulator” for the light curing unit’s irradiance.14 From a technique standpoint, another way to reduce the heat is to break the curing time into shorter increments to give the tooth’s pulp a chance to cool. In addition, you can reduce the heat by dissipating it with an air stream during curing.15
Bulk-fill composites have many benefits for direct restorative dentistry, not the least of which is the ability to fill a preparation with a thicker layer of material than conventional composites. With performance metrics that mimic their conventional counterparts, these composites can produce excellent-looking and long-lasting restorations.
However, like all things in composite restorations, they have technique-specific nuances that make them produce better outcomes. Using a 2-step bulk technique, avoiding voids during placement of the material by using injectables when you can and keeping the tip in the material during extrusion, and protecting the pulp from thermal damage caused by the curing light, you provide bulk-fill materials the best possible chance to produce successful outcomes