The layering technique has been advocated for posterior composites to reduce polymerization shrinkage and allow adequate depth of cure. Recently, bulk-fill composites have been introduced that allow the practitioner to place composites in increments of 4 mm. Initial testing of these materials has determined that bulk-fill composites can obtain a depth of cure and shrinkage stress at 4 mm comparable with a conventional composite with a 2-mm increment. Bulk-fill composites were first introduced in a flowable consistency and have since been formulated with a high-viscosity universal consistency. Some flowable materials do not exhibit sufficient wear resistance and must be covered with a more highly filled composite. The technique for placing these materials is slightly different than for conventional composites, as the layers of composites are not condensed against the matrix, and therefore, a contoured matrix, ring, and sufficient wedging are recommended. Flowable bulk-fill materials are well suited for Class II boxes and build-ups around posts.
The impetus for bulk-filling composites is a desire to reduce the time required for composite placement and to eliminate the possibility of voids between composite layers. The time savings allowed by placing a single layer of composite can be appreciated by both the dentist and patient. In addition, an unwanted side effect of the incremental placement technique is the introduction of voids between composite layers. For universal handling composites, the stickiness of the composite to the placement instrument can make it difficult to completely adapt the new layer of composite to the previous layer. When placing additional layers of flowable composite, air can become trapped between layers. Internal voids may be innocuous in some clinical cases; however, in stress-bearing areas, a void may act as a site of stress concentration and eventual fracture.4 Voids or gaps present on the external surface of a composite restoration may be more prone to staining.
Manufacturers have achieved this depth of cure by modifying the translucency of the composite and including more effective light initiators. When a composite is light cured from the occlusal direction, light energy from the curing light must be transmitted through the bulk of the composite without being absorbed or deflected before reaching the bottom of the restoration. By adjusting the translucency of the composite or matching the refractive index of the filler and the resin, light can travel through an increased depth of a bulk-filled composite.18,19 A side effect of this modification is that several of these materials appear more visually translucent than tooth structure.
The other method that manufacturers have used to increase depth of cure is by incorporating more efficient and robust light initiators. Most dental composites polymerize by activating the photoinitiator camphorquinone (CQ). When the energy from a curing light excites CQ, it stimulates an amine co-initiator to release a free radical and initiate resin polymerization. Ivocerin®, a new germanium-based photoinitiator patented by Ivoclar Vivadent (www.ivoclarvivadent.us), does not require a co-initiator and requires less energy from a curing light to produce crosslinking of the resin composite.8,20 Therefore, composite at the bottom of a 4-mm layer requires less light to achieve sufficient mechanical and physical properties.
In addition, manufacturers have added blends of photoinitiators that are activated at different wavelengths of light to optimize the energy from a curing light. CQ is maximally stimulated at 468 nm. Ivocerin has a maximum absorption of 410 nm. The photoinitiators phenyl-propanedione (PPD) and Lucirin® TPO (BASF, www.basf.com) absorb a lower wavelength of light (385-400 nm) and have been used in light and translucent shades of composite because they do not exhibit the yellowing associated with CQ.
The range of maximum of absorption of different photoinitiators has clinicians concerned that their curing lights do not have the spectral output to effectively polymerize these composites. For reference, a typical LED curing light has a spectral output of utilizable wavelength ranging from 430 to 480 nm. New polywave or multi-wavelength LEDs have expanded spectral outputs. These new lights include VALO® by Ultradent (www.ultradent.co) with a range of 395 to 480 nm and the Bluephase® 20i by Ivoclar Vivadent with a range of 385 to 515 nm. Testing by the author, however, has revealed that Tetric EvoCeram® Bulk Fill with Ivocerin (Ivoclar Vivadent) could be effectively cured to 4 mm with both a traditional and polyphase LED curing light (Figure 1). Because Tetric EvoCeram Bulk Fill is the only bulk-fill composite marketed with a new photoinitiator, the concern of composite and light compatibility may not be of clinical relevance with current materials.
Ivoclar Vivadent released its highly filled material Tetric EvoCeram® Bulk Fill and later a flowable version, Tetric EvoFlow Bulk Fill. These materials contain the photoinitiator Ivocerin as well as stress-relieving "flexible" filler particles.6 Kerr's SonicFill™ (www.kerrdental.com) utilizes sonic energy (vibrations within the audible frequency) to reduce the viscosity of a highly filled material and allow it to flow into a preparation. The sonic energy is supplied by a specific handpiece that attaches to an air hookup and is dispensed by pressing on a rheostat. After dispensing, the material returns to a sculptable state, allowing the clinician to contour the restoration. The high concentration of filler particles (83.5 wt%) reduces polymerization shrinkage of the composite as the resin, not the fillers, contract upon polymerization. The manufacturer does not explain how the increased depth of cure is obtained. Other bulk-fill composite materials include QuiXX® (DENTSPLY), Beautifil® (Shofu,www.shofu.com), Venus® Bulk Fill (Heraeus Kulzer, www.heraeus-kulzer.com), HyperFIL™ (Parkell Inc., www.parkell.com), X-tra Base and X-tra Fil (Voco, www.voco.com).
The placement techniques for bulk-fill composites vary for flowable and higher viscosity materials. Bulk-fill flowable composites are primarily used for deep bases and interproximal boxes under conventional composites. Interproximal boxes can easily reach depths beyond 4 mm, and therefore at least two increments of composite are needed. To reinforce this point, it is important to note that bulk-fill does not mean one increment up to the occlusal surface. Instead, it implies increments up to 4 mm in depth. Even for shallow preparations of 4 mm or less, flowable bulk-fill composites are not indicated for the final occlusal layer. The two main reasons these materials are not placed up to the occlusal surface of a restoration are their low wear resistance and high translucency.
A study of several flowable bulk-fill composites showed that all flowables had significantly more wear than the control (Z100™, 3M ESPE); more wear was seen for SureFil SDR (0.04 mm3) than Venus Bulk Fill (0.024 mm3) or Filtek Bulk Fill (0.012 mm3). In addition, flowable bulk-fill composites tend to have a very high translucency that prevents them from blending with natural tooth structure. For this reason, manufacturers recommend covering or "capping" these materials with a conventional posterior composite.
Higher viscosity bulk-fill composites can be used up to the occlusal surface, however. Two clinical cases using the bulk-fill composites Tetric EvoCeram Bulk Fill and SonicFill are presented in Figure 2 through Figure 6. Limitations when placing a high-viscosity bulk-fill composite include difficulty of condensing and esthetic restraints. Condensing a high-viscosity resin composite ensures adequate adaptation to the tooth preparation and sufficient interproximal contacts. Condensing 4 mm of composite is more difficult than condensing 2 mm. To ensure adequate interproximal contacts, a sectional or circumferential matrix with an inciso-gingival curvature can be used to obtain a tight area of contact gingival to the marginal ridge. A ring should be used to help separate the teeth and improve the tightness of the contact area.
An esthetic limitation of bulk-fill composites is that most materials are available in a small selection of shades. In addition, some of these materials have slightly higher translucency than conventional posterior composites. The esthetics achieved by these composites should be sufficient for posterior restorations for most patients.
Another possible clinical application of bulk-fill composites is for use as a core build-up material. A clinical case demonstrating this technique is presented in Figure 7 through Figure 9. Build-ups are often placed into large, deep preparations with pins or a post. Therefore, important characteristics to consider for these materials are their shrinkage stress, depth of cure, flow, and toughness/strength. Flowable bulk-fill composites are desirable materials to use for build-ups because they have appropriate flow to adapt to a post or pin, low shrinkage stress, and high depth of cure, allowing them to be placed in bulk with the strength and toughness of a resin composite.
Bulk-fill composites are a new class of material with scientific evidence for claims of low polymerization shrinkage and 4-mm depths of cure. The appearance, handling, and mechanical properties of bulk-fill composites vary between flowable and high-viscosity materials. In general, flowable materials have a deeper depth of cure; however, high-viscosity materials have better wear resistance and less transparency. The handling and esthetic limitations of these materials may not be sufficient for use in anterior restorations; however, bulk-fill materials are a good solution for efficient posterior restorations and possibly core build-ups.
Nathaniel Lawson, DMD, PhD, is an assistant professor in the division of biomaterials at the University of Alabama at Birmingham (UAB). He started his education as a biomedical engineering student at Tulane University. He completed his DMD at the UAB School of Dentistry and his MS and PhD in the department of biomedical engineering at UAB. Following graduation, he worked in private practice in Alabama and Illinois. Dr. Lawson is passionate about testing new materials and techniques in the laboratory. His main research focus involves testing the strength, color, wear, roughness and bond strength of ceramics, composites, cements, impression materials, and adhesives.
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Explain the importance of shrinkage stress and depth of cure in the bulk-fill technique.
Understand how bulk-fill composites reduce shrinkage stress and increase depth of cure.
Describe the types, clinical applications, and limitations of bulk-fill composites.