Replacing a Posterior Amalgam Restoration With Composite
Laser light curing unit ensures complete polymerization in deep preparations
Alireza Sadr, DDS, PhD
In both research and practice, resin composites are widely considered to be reliable materials—a fact that can largely be attributed to the development of their mechanical properties and adhesive technologies. Most composites and bonding agents are light-cure materials, and years ago, an incremental filling technique was recommended to ensure complete polymerization. Later, bulk-fill resin composites were introduced, which permitted the placement of single increments of up to 4-mm deep.1
During this paradigm shift, dental light curing units have been evolving to improve the degree of polymerization that can be achieved in such deep preparations. For deep preparations and thick increments, it was initially suggested that prolonged light curing times could be used to compensate for the attenuation of the light through the material; however, some studies reported that the degree of conversion and the kinetics of the composite polymerization achieved using this method did not conform to the laws of radiant exposure. Therefore, to improve depth of cure, high irradiance light curing units that were capable of delivering energy in excess of 2000 mw/cm2 were developed.2
Regarding LED light curing units, whichever one a dentist uses, there is a significant attenuation of light intensity as the distance from the source increases. This is due to the nature of light dispersion, which in many situations, may partly be addressed by using special optical lens setups and increasing the light intensity of the source. Although standard LED light curing units function perfectly for routine placement of composite, in some cases encountered by dentists, it may be more challenging to get these units close enough to ensure complete polymerization. For example, it may be difficult to to get adequate light to reach all of the material in a case involving a deep and difficult to access proximal margin that needs to be adhesively sealed or in a case involving an adhesive procedure that takes place inside the root canal.
For these and other challenging situations, a more recent innovation in light curing—the laser dental light curing unit—can offer a solution to help ensure complete polymerization. Unlike light from other sources, laser light is collimated, which means that all of the rays are parallel to each other. This is an important characteristic of laser light because the light beam doesn't diverge over longer distances in the way that light from an LED source does.
A patient presented to the practice with an old and failing mesial-occlusal-lingual amalgam restoration on a first molar that was showing signs of dentin cracking. After the patient was anesthetized and rubber dam isolation was achieved, the amalgam was removed along with any secondary decay, and the dentin cracks were prepared (Figure 1). It should be noted that the complete elimination of such cracks can be difficult. When a crack extends over the pulpal floor in a vital tooth without irreversible pulpitis, the clinical technique described for restoration of an at-risk tooth should be followed.3
A sectional matrix was placed, and a deep gingival proximal margin was formed. Because an attempt was made to remove the crack as deeply as possible supragingivally, in the mesial box, the gingival margin was 6-mm deep or greater from the top of the matrix. The proximal margin was bonded and sealed using a multistep adhesive system (CLEARFIL™ SE BOND 2, Kuraray Noritake), and circumferential walls and a marginal ridge were formed using a universal hybrid composite (CLEARFIL MAJESTY™ ES-2 Universal, Kuraray Noritake) in a single step after bonding. Because one of the major challenges in dealing with bonding to tooth structure this deep is light curing,4 the light curing in this case was accomplished with a laser light curing unit (Monet®, CAO Group, Inc.) (Figure 2). This laser curing light was selected because its head design fits into tight molar spaces and, according to the manufacturer, its beam facilitates the light curing process in 3 seconds.5 In addition, in this author's unpublished optical coherence tomography experiments with this laser light curing unit at the University of Washington, it was demonstrated that it could effectively cure a 4-mm deep composite restoration from as far away from the surface as 8 mm in 3 seconds, and the restorations exhibited less debonding than those cured by a standard LED light curing unit for 20 seconds.6 Although this conclusion was dependent on several factors, including the correct angulation of the light curing unit head and the type of composite used, this laser light curing unit demonstrated that it had the capability to cure composite restorations to their depth, regardless of its distance from them.
Due to the depth of the preparation and the presence of cracked dentin in this case, an ultra-high molecular weight polyethylene continuous fiber material was used to bridge the pulpal floor cracks, mitigate stresses, and reinforce the composite.7 After the creation of the proximal and buccolingual walls with composite was complete, a polyethylene fiber mesh (Ribbond® Ultra THM, Ribbond) was cut to the desired length, wetted with a solvent-free bonding agent, and placed over the tooth with a thin layer of the hybrid composite (Figure 3). Close adaptation (ie, lamination) of the fiber against the cracked dentin was achieved, and then the fiber-reinforced composite was light cured using the laser curing light (Figure 4). To complete the restoration, a final occlusal layer of the hybrid composite was placed and then light cured at once (Figure 5).
In this case, a laser light curing unit was used to ensure a predictable bond and complete polymerization of the adhesive and the composite that were placed to create a proximal wall and marginal ridge as well as the fiber-reinforced composite that was placed in the deepest part of the preparation. The uniquely consistent energy delivered by this unit is a game changer not only for thick increments of newer bulk-fill composites but also for thinner increments of gold standard light-cure resins and base materials used in deeper preparations. In addition, the accelerated curing time provided by this laser curing light also significantly saves chair time, particularly for multistep procedures, such as the one presented in this case, that involve meticulous bonding and fiber lamination to complete the restoration.
About the Author
Alireza Sadr, DDS, PhD
Associate Professor and Vice Chair
Department of Restorative Dentistry
School of Dentistry
University of Washington
1. Hayashi J, Espigares J, Takagaki T, et al. Real-time in-depth imaging of gap formation in bulk-fill resin composites. Dent Mater. 2019;35(4):585-596.
2. Hayashi J, Tagami J, Chan D, Sadr A. New bulk-fill composite system with high irradiance light polymerization: integrity and degree of conversion. Dent Mater. 2020;36(12):1615-1623.
3. Chyz GT. Restoration of an "at risk" tooth. Inside Dentistry. 2010;6(7):58-66.
4. Al-Zain AO, Platt JA. Effect of light-curing distance and curing time on composite microflexural strength. Dent Mater J. 2021;40(1):202-208.
5. Rocha MG, Maucoski C, Roulet J-F, Price RB. Depth of cure of 10 resin-based composites light-activated using a laser diode, multi-peak, and single-peak light-emitting diode curing lights. J Dent. 2022;122:104141. doi: 10.1016/j.jdent.2022.104141.
6. Haghighi S, Sadr A, Chan DC. Laser and LED light units comparison for bulk composite replacement. Virtual interactive talk presented at: 99th General Session of the IADR; July 21, 2021.
7. Sadr A, Bakhtiari B, Hayashi J, et al. Effects of fiber reinforcement on adaptation and bond strength of a bulk-fill composite in deep preparations. Dent Mater. 2020;36(4):527-534.
FOR MORE INFORMATION, CONTACT:
CAO Group, Inc.