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Nano-Dimer Conversion Technology
New chemistry addresses old concerns of composite resin.
Over 40 years ago, Bowen patented composite resin using Bis-GMA resin and started the evolution of tooth-colored restorations.1,2 For much of this time, dental material manufacturers have looked to optimize this original concept to create a more ideal dental restorative material. While it has been a huge step for esthetic-based restorative dentistry, Bis-GMA chemistry has had its challenges. The biggest issues that still exist even after 40 years of steady improvement are polymerization shrinkage and the associated polymerization stress.3-8
Polymerization shrinkage and stress are associated with multiple clinical adverse outcomes.9-13 Microleakage associated with polymerization shrinkage can lead to postoperative sensitivity, recurrent decay, and marginal staining. Polymerization stress is often associated with postoperative sensitivity either to temperature or pressure. Throughout the years, manufacturers have worked diligently to minimize polymerization shrinkage. Clinically, dentists have developed a multitude of techniques to overcome polymerization shrinkage.
Dr. Jeffery Stansbury and his research team at the University of Colorado have recently developed a non Bis-GMA composite resin formula that uses a nano-dimer conversion technology.14-18 Septodont (http://www.septodontusa.com) is now incorporating this new chemistry into its newest composite resin, N’Durance®. Dimer technology has many benefits, but its most significant is the reduction in polymerization shrinkage. Volumetric shrinkage of Bis-GMA nanohybrid-based composites have a range of 1.85% to 3%, while N’Durance has a reported polymerization shrinkage of 1.27%.19,20 In addition to this new chemistry having minimized polymerization shrinkage, it also demonstrates a dramatically lower polymerization shrinkage stress value.21 While most Bis-GMA composites show a value of 1.5 MPa to 2 MPa, N’Durance demonstrates a value of 1.2 MPa. A decrease in the amount of polymerization stress clinically means that at the adhesive interface there is less stress trying to pull apart the bond, which would lead to microgap formation.
Another unique characteristic of this new chemistry is that there is a dramatic increase in the monomer conversion.22,23 A high-monomer conversion means that there is less free monomer present after polymerization, making the material more biocompatible. Other benefits of this new chemistry are a very low water sorption rate and solubility. Clinically this will be seen as excellent color stability and stain resistance of the material. The manufacturer has optimized its filler technology with a combination of ytterbium fluoride, barium glass, and silica to create a highly radio-opaque material that is easily distinguishable on dental radiographs. Lastly, N’Durance can be used with any total-etch or self-etch system. Clinically, this means that the dentist can use whatever bonding system he/she prefers with this composite resin system.
Composite resins are certainly an important part of anterior esthetic/restorative dentistry, allowing us to provide patients with esthetics and function when a tooth has been compromised by disease, trauma, or is malformed/malpositioned. The less glamorous application of composite resins is in the posterior region. It is, however, the posterior region where composites are used most often. It is also the posterior region where dentists face significant technical challenges. Posterior teeth take on a heavier occlusal load. Posterior teeth are more often the teeth that exhibit postoperative sensitivity. For many dentists, a Class II composite is one of the more clinically challenging procedures. For all of these reasons dentists rely on a predictable resin material that will meet the functional requirements of the posterior dentition, work with existing techniques to get optimal results, and, lastly, meet the dentist’s and the patient’s esthetic requirements.
A 35-year-old patient presented with a 20-year-old sealant on tooth No. 19 that exhibited signs of recurrent decay (Figure 1). As the area of decay appeared to be small, the preparation was started without the use of local anesthetic. Instead of preparing the tooth conventionally with a handpiece and rotary bur, air abrasion was used to perform the entire preparation (Figure 2). Figure 3 shows the final preparation of the restoration with the removal of the old sealant material and the areas of decay. Before the preparation was filled, a caries detector (SEEK®, Ultradent Products, Inc, http://www.ultradent.com) was used to ensure that all the decay had in fact been eliminated.
Using a total-etch technique, the preparation was etched, rinsed, and a fifth-generation bonding agent was applied (Prime & Bond® NT™, DENTSPLY Caulk, http://www.caulk.com). The preparation was then filled with an incisal shade of N’Durance and light-cured for 40 seconds. An amalgam burnisher lightly moistened with unfilled resin was used to condense, adapt, and form the resin to the cavity preparation. The occlusion was verified and the restoration was polished using a series of rubber points. Figure 4 shows the final completed restoration.
One of the most common clinical procedures in today’s dental office is the replacement of an old occlusal amalgam with a tooth-colored resin restoration. While it is a common procedure, the Class I composite is not without its challenges. Many clinicians have experienced the frustration of postoperative sensitivity with what should be one of the easier dental restorations. The Class I preparation has the challenge of having a very high C-factor, meaning that the nature of the preparation inherently has very high stresses associated with restoration on the tooth. This high C-factor makes it highly sensitive to shrinkage/stress-related issues. A composite material that exhibits minimal polymerization shrinkage and very low polymerization stress would be ideal for the Class I resin restoration.
A 42-year-old patient presented with an aging amalgam restoration that exhibited leaking margins and recurrent decay (Figure 5). A rubber dam was placed and the old amalgam restoration was removed. A caries detector (SEEK) was used to identify areas of recurrent decay. These areas were removed with a round bur (#4) in a slow-speed handpiece. The process was continued until all decay was removed.
During the restoration process, there are many concepts and theories that exist to decrease the amount of postoperative sensitivity experienced. In 2001, Christensen published a widely read opinion that “self-etching primers are here.” 24 He expressed a widely held opinion that self-etching systems were now ready for mainstream use and that there was clinical experience and scientific evidence that self-etching systems were inherently less often associated with postoperative sensitivity. In an effort to practically reduce the potential for postoperative sensitivity this restoration was treated with a self-etching bonding system (OptiBond® All-in-One, Sybron Dental Specialties, http://www.kerrdental.com).
Next, an initial layer of a flowable composite was placed on the pulpal floor. Septodont has used the same dimer chemistry to create a very low-shrinkage flowable composite, N’Durance Dimer Flow. While Bis-GMA flowable composites demonstrate polymerization shrinkages of 3.5% to 6%, Dimer Flow has a 3% rate of polymerization shrinkage. Dimer Flow also demonstrates a 75% rate of monomer conversion compared to a range of 45% to 67% among Bis-GMA flowable composites.
The use of flowable composites as an initial layer is based on the concept that it will act as a stress-absorbing lining material between the adhesive system and the restorative composite resin.25 The combination of flowables and viscous composite ensures a more intimate contact with the dentin bonding agent because of the lower viscosity and has resulted in enhanced internal adaptation.26
Figure 6 shows the placement of a highly radio-opaque, low-shrinkage, A1 shade of flowable composite (N’Durance Dimer Flow) on the pulpal floor. This increment was placed as a thin layer and light-cured for 20 seconds.
The bulk of the remaining restoration was filled with an incisal shade of N’Durance. The use of an incisal shade creates the opportunity for the restoration to pick up the shade of the flowable composite below as well as the color of the natural tooth along the preparation walls to allow for the development of a restoration that will virtually disappear. Lastly, the occlusion was refined and the restoration polished with series of composite polishing points. The final restoration demonstrates a functional and esthetic success (Figure 7).
One of the more clinically challenging restorations is the Class II composite. Many dentists struggle to create a clinically acceptable restoration. Additionally, Class II resin restorations often have issues with postoperative sensitivity and/or recurrent decay. Proper technique and state-of-the-art composite resins can make the Class II resin restoration more predictable.
A 34-year-old patient presented with a pair of adjacent Class II amalgams, each of which demonstrated recurrent decay radiographically (Figure 8). The patient expressed a desire to have his old amalgams replaced with a tooth-colored restoration.
With Class II resins, rubber dam isolation is required to maintain optimum control of the field. The old restorations were removed. Caries was removed with a #4 round bur, and a caries detection agent (SEEK) is used to ensure that all decay has been removed. Figure 9 shows the final preparations.
One of the keys to successful Class II restorations is the selection of an appropriate matrix system and its correct placement for ideal adaptation and proximal contact development. For this case, a sectional matrix system was selected (Composi-Tight® 3D™, Garrison Dental Solutions, http://www.garrisondental.com). The sectional matrix was anatomically contoured, and with proper matrix and wedge placement, excellent adaptation and anatomical contours could be achieved (Figure 10).
With the matrix in place, the restorations were completed one at a time starting with the second premolar. A total-etch bonding technique was selected using a fifth-generation bonding system (OptiBond® Solo Plus, Sybron Dental Specialties). The restoration was initiated with the placement of a thin layer of a low-shrinkage, low-stress flowable A1 shade composite (N’Durance Dimer Flow) in the most apical aspect of the proximal box (Figure 11). The use of a flowable composite in the part of the preparation ensures the complete seal of the area of the restoration that could be most adversely effected by marginal leakage. Flowable composites by their low contact angles and viscosity are ideal for “sealing” this most critical area of the preparation. The flowable composite was light-cured for 20 seconds.
The rest of the restoration was completed using a low-shrinkage, low-stress composite resin (N’Durance). The extremely low polymerization shrinkage and stress characteristics of this composite will help to minimize the potential for marginal microleakage and help to prevent postoperative sensitivity and recurrent decay. The initial layer of the composite was placed in a 2-mm increment. Care was taken to condense and sculpt the interproximal area. An A1 shade was used with this first 2-mm layer. This layer was cured for 20 seconds.
The final layer of the composite was created using an incisal shade of the composite. The reason for this is that a more translucent composite will pick up the shade of the initial layer of the composite as well as blend into the color of the natural tooth along the axial walls. The final optical effect is a restoration that has an excellent shade match and the appearance of more lifelike vitality. This final layer was light-cured for 20 seconds.
The rubber dam was then removed and the occlusion was created using a 12-fluted football-shaped carbide bur. Further anatomy was created using a RAPTOR bur (Axis Dental Corp, http://www.axisdental.com). Lastly, the composite was polished using a series of rubber polishing points. Figure 12 shows the next-day postoperative photograph of the two Class II resin restorations. The final result is a pair of Class II restorations that are esthetically pleasing, have excellent proximal contact, demonstrate appropriate anatomical form, and, after more than 6 months, there has been no postoperative sensitivity.
As dentistry continues to search for the perfect direct restorative material, we have seen many improvements. Dimer-acid chemistry has provided a significant breakthrough in dental materials. Its physical properties and its easy and practical implementation bring us closer and closer to the ideal tooth-colored restoration material.
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About the Author
Gary M. Radz, DDS
Associate Clinical Professor
University of Colorado School of Dentistry
Cosmetic Dentistry of Colorado