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Dual-cure resin composites have been advocated for years for core build-up of teeth that will either receive a full-coverage crown or as part of the restoration of an endodontically treated tooth. These were introduced to overcome some of the clinical problems associated with self-cure resins that had come into use. The benefit of both the dual-cure and self-cure resins was the ability to bulk-fill the preparation without concern that setting of the deepest portions of the restorative material would not occur. These depth-of-cure issues were common with light-curable resins as the light was not able to penetrate deeper than 4 mm to 5 mm depending on the shade of the resin and its opacity. Additionally, access issues could make it difficult for the light to reach portions of the preparation even when thinner increments were placed. Thus, alternative materials were developed to overcome this clinical problem.
Today, dual-cure resins are being used for more than core build-ups and restoration of endodontically treated teeth. This article will address the advantages and disadvantages of self-cure resins and dual-cure resins, what clinical applications they may be used for, and how to improve the adhesion of these materials as well as improving treatment success with dual-cure materials.
Is Ferrule Really Needed with Today’s Adhesives?
Restorative ferrule seems to be a forgotten concept and, in recent years, not emphasized in dental education. Yet, it is important today in spite of the advances in adhesive dentistry. The purpose of a core build-up or post-and-core is to retain the crown placed on the tooth. These are placed to provide sufficient structure so that lateral loads placed on the crown do not cause dislodgment of the crown. When these are not used when restoring a tooth that has moderate to severe loss of coronal structure, there may be too little vertical height present to resist these “tipping” forces. But it is important that the stresses placed on the tooth be located away from the interface between the resin and dentin to minimize loads that may lead to debonding of the resin material.
Ferrule is a term used in the barrel-making industry that referred to the metal band that encircled the wooden barrel to hold the slates in place and resist the pressure from the contents of the barrel. In dentistry, a ferrule refers to the encirclement of the crown's margin on the tooth, assisting in retaining the crown on the tooth. Before the adhesive dentistry era, amalgam was the only core material available for vital teeth and cast post and cores were used for endodontically treated teeth as part of the restorative phase. As the majority of amalgam cores were not adhesively bonded to the underlying tooth, and many cast post-and-cores were luted with weak cements, it was critical that the crown margin not terminate on the juncture of the tooth and restorative material. Placement of the crown margin too close to this juncture frequently resulted in dislodgement of the crown and the need for periodic re-cementations. Practitioners were taught to extend the crown margin several millimeters beyond the juncture onto sound tooth structure as a routine part of the crown preparation.
The era of adhesive dentistry led practitioners to believe that the bond between the resin and tooth structure was strong enough that loads placed on the core from off-axis loading would not overcome this bond. But, every dentist has experienced patients presenting with crown in hand and the core well retained in the dislodged crown but no longer bonded to the tooth. Typically, these cases demonstrate crowns that have minimal, if any, ferrule. This becomes more critical in the maxillary anterior where all loads placed on the tooth are off-axis compared to the posterior teeth that the direction of mastication places loads predominantly in an axial direction.
When a crown with a restorative core is loaded in an off-axis direction, the side of the tooth where the force is being applied is under tension and the opposing side is under compression. The mechanism of failure in this situation occurs at the crown margin that is under tension. Under repetitive loading, the “seal” at the crown’s margin may microscopically open due to a debonding of the resin from the tooth at the core/tooth interface, leading to recurrent decay in the long term or dislodgement of the crown in the short term. But, how much ferrule is required? Evidence is available to support the need for at least a 1.5-mm to 2-mm ferrule length of a crown preparation.1 One study concluded that crown preparations with a 0.5-mm to 1-mm ferrule failed at a significantly lower number of cycles than the 1.5-mm and 2-mm ferrules.2 A study comparing test specimens with no ferrule (0 mm) to 2 mm, grouped in 0.5-mm increments, found that when ferrule was low or nonexistent stress concentrations were high in the cervical region of the tooth. Conversely, in specimens with a 2-mm ferrule, higher stress concentrations were not noted. The authors concluded, “that a ferrule increases the mechanical resistance of a post/core/crown restoration.”3
Therefore, it is recommended that these principles be applied to those teeth with resin-bonded cores that are being prepared for full-coverage crowns.
Self-Etch Adhesives
When bonding a core we have a choice of what technique to use and this has little dependency on the core material that we have chosen. Adhesives fall into two categories depending on how the tooth is treated: total-etch and self-etch. The early self-etch adhesives had clinical issues when combined with self-cure resins used for core build-ups, so total-etch adhesive products were recommended. Part of the problem in achieving adequate bond strengths between the self-etch adhesives and the self-cure resins was that the pH was too low for the adhesive to condition the dentin, which hampered the setting of the self-cure resin.4 To a lesser extent, this was also clinically problematic with dual-cure composite resins.5,6 We need to understand that a dual-cure resin composite is a combination of a self-cure resin and a light-cure resin, essentially two materials with different setting reactions mixed together. So, a low pH of the adhesive may also yield lower and less predictable bond strengths on the core material.7 It has been reported that higher bond strengths are achieved when the adhesive is cured prior to placement of the overlaying dual-cure resin.8 Concern has been raised in the literature that dual-cure initiators in the adhesives when combined with light-cure composite resins may result in low bond strength, but this has not been reported with the dual-cure resin composites.9-12 Research into this theory as to debonding of self-etch adhesives and self-cure resins is not conclusive.13 But, it has been suggested that certain self-etch adhesives that contain phosphoric acid in their formula have very low pH values of 0.7 to 1.2, and may possess an acidic residual pH at the adhesive interface, even after self-etching has been completed on the dentin.14 It is believed that this residual acidity inhibits the activity of the “tertiary amine” self-cure activators in the dual-cure and self-cure composites.15 If the tertiary amines are unable to activate the polymerization of the composite at the adhesive interface, then this zone does not polymerize and debonding of the restorative material occurs. This may occur during removal of the matrix, during the finishing processes or, in the case of a core build-up, upon removal of the provisional crown.
However, self-etch adhesives that are 4-META based, such as Brush & Bond (Parkell, www.parkell.com) react differently.16, 17 Their self-etching capability is derived from the acid groups attached to the 4-META molecule (a dicarboxylic acid) which promotes penetration into the dentin. No phosphoric acid is present in the adhesive and it possesses a higher initial pH of 2 to 2.2. This results in a higher likelihood of a neutral pH at the adhesive interface after the self-etching of the dentin has occurred.18 After light-curing, no residual acidity is present to interfere with the tertiary amine activators in the overlying dual-cure and self-cure composites. This allows the adhesive bond between adhesive and overlaying restorative material to fully cure, thus holding the composite tightly in place.
Clinical Use of Dual-Cure Resins
Traditionally, dual-cure resins were used for core build-ups for vital teeth19 that would receive a crown or when restoring an endodontically treated tooth, with or without posts.20 Yet, these materials have other clinical uses, including bases for deep restorations and pediatric/geriatric posterior restorations.
Pediatric and Geriatric Posterior Restorations
When treating young children, it can be challenging dealing with their limited ability to sit in the dental chair to allow etching and incremental layering of light-curable composite resins. These patients may not allow or tolerate rubber dam placement or even the use of alternate isolation techniques. This adds to the potential management issues when using total-etch techniques, as even with the best efforts the patient may taste the etchant and the management issues rapidly decline from there. Use of self-etch adhesives, such as Brush & Bond (other alternative materials include: Bond-1® SF, Pentron Clinical Technologies, www.pentron.com; XP BOND®, DENTSPLY Caulk, www.caulk.com; and Clearfil® SE Protect, Kuraray, www.kuraraydental.com) are very useful in this regard by allowing the practitioner to eliminate the etching gel and the necessary rinsing associated with this technique.
After preparation of the tooth for caries removal, the tooth is lightly air-dried to remove any pooled water, keeping the dentin moist. One to three drops of Brush & Bond liquid are dispensed into a dappen dish and the special activator brush is used to stir the material. The brush is then carried to the mouth. Ensure that all surfaces are contacted and keep the surfaces to be bonded wet for 20 seconds during this step. Air is used to evaporate the solvents and the preparation is light-cured for 10 seconds.
A dual-cure composite resin such as HyperFil (Parkell) (other alternative materials are: Breeze, Pentron Clinical Technologies; Clearfil® DC Core, Kuraray; MultiCore®, Ivoclar Vivadent, www.ivoclarvivadent.us) is then dispensed directly into the preparation using an automix syringe, bulk-filling it. It is recommended to slightly overfill the preparation to ensure that proper anatomy can be created with the cured material. A light is then applied to the restoration and curing is performed for 40 seconds per surface. The restoration is then allowed to complete self-curing for 3 minutes for those areas that did not receive sufficient light to facilitate a full cure. Anatomy is then created with carbides and diamonds and the occlusion is checked and adjusted as needed. Polishing can be performed with abrasive impregnated composite polishers (eg, Ensure or Pogo, DENTSPLY Caulk; ET Illustra, Brasseler USA, www.brasseler.com; or HiLuster Plus, Kerr Corporation, www.kerrdental.com).
Geriatric patients present with similar management issues but these are more related to an inability to handle long appointments or to maintain their mouths in an open position due to muscle weakness associated with age. These weaker muscles along with inherent temporomandibular joint issues can lead to postoperative discomfort after longer appointments. Use of self-etch adhesive and dual-cure resin allows for shorter appointments and can increase postoperative comfort for these individuals. As with pediatric restorations, after removal of any old restorative material and decay, the tooth is dried. A self-etch adhesive is applied and cured followed by a dual-cure composite resin.
Core Build-Ups
Frequently, teeth treatment planned for placement of a full-coverage crown have been previously restored with a variety of restorative materials.21 As the practitioner does not know the condition of the dentin under the old restorative material, it is common practice to remove the old material and replace it with a core build-up prior to crown preparation.
As initially discussed in this article, ferrule is important when preparing teeth for crowns. The crown margin should extend, ideally, 2 mm beyond the margin of the core build-up resin. This can be a visualization challenge in the posterior where use of tooth-colored resins for core build-ups can make identification of where the resin ends and the tooth structure begins difficult, especially interproximally. Contrasting color between the tooth and dual-cure resins aids in this, making visualization easier.
After removal of the old restorative material and any remaining decay, a matrix is placed to contain the core build-up resin, and a self-etch adhesive such as Brush & Bond is applied to the dentin and enamel surfaces and light-cured. A contrasting color dual-cure resin build-up material such as Absolute Dentin (Parkell) (other alternative materials are: Build-It® FR™, Pentron Clinical Technologies; Clearfil® Esthetic Core, Kuraray; and MultiCore) is injected into the preparation and light-cured. Because these contrasting-color, dual-cure resins are more opaque than tooth-colored resins, light penetration is typically not greater than 2 mm to 3 mm. It is necessary to allow these materials sufficient time to continue polymerization via a self-cure mode before removal of the matrix or initiation of crown preparation. Crown preparation is then performed and ferrule can be achieved using visualization to note the contrast between resin and tooth.
This technique can also be used when restoring an endodontically treated tooth. After post preparation, a self-etch adhesive is applied to the post preparation and all exposed dentin coronally with the activator brush, burnishing the adhesive into the dentin walls. Without light-curing the adhesive, a dual-cure resin, such as Absolute Dentin or one of the others mentioned above, is injected into the post preparation using an intracanal tip on the automix syringe, backfilling the space to eliminate voids. Immediately, the previously fitted post is inserted to depth and additional material is injected to completely fill the missing coronal tooth structure. A light is then used to cure each surface for 30 seconds and the material is allowed to complete self-curing for an additional 4 minutes.
Bases in Deep Restorations
Use of light-curable restorative materials may be a challenge when the preparation is very deep or portions of it are not readily accessible to the light. Even when incremental layering is used, the light may not be able to access the deepest portions of a proximal box on a second molar or reach the distal or buccal of these teeth. Dual-cure resins are a useful alternative to base materials, allowing the restoration to be built up to the dentin-enamel junction using a bulk-fill technique then overlaid with a hybrid or nano composite.
This technique is similar to one previously described with some variations after placement of the dual-cure base. Isolation is placed and the tooth is prepared for a direct resin restoration (Figure 1 and Figure 2). A sectional matrix is placed interproximally, followed by wedge placement (Figure 3 and Figure 4). A V3 ring (Triodent, www.triodent.com) is placed to conform the sectional matrix to the contours of the tooth being filled and to develop a tight contact with the adjacent tooth (Figure 5 and 6). Adhesive is dispensed into a dappen dish (Figure 7) and the activator brush is wetted with the adhesive (Figure 8). The activator brush is then used to apply adhesive to the dentin and enamel, keeping the surface wet and agitated for 20 seconds (Figure 9). After adhesive placement and curing, (Figure 10) the dual-cure material (eg, dentin shade HyperFil) is injected into the preparation to the level of the dentin–enamel junction (Figure 11) and light-cured (Figure 12). To maximize esthetics, care is taken not to place the material on the cavosurface margin of the restoration at the occlusal surface (Figure 13). Should any material be in contact with the enamel on the occlusal surface, it may be removed quickly with a diamond. As self-etch adhesives provide higher bond strengths to prepared enamel, the practitioner has the option of using an acid-etchant gel on the enamel (only) or this can be roughed with a diamond during the preparation when caries and old restorative material is removed. Additional Brush & Bond is applied to the enamel and HyperFil base surface and light-cured. A traditional composite may also be used on the occlusal surface in a single layer and anatomy may be created in the uncured resin, followed by light-curing (Figure 14). Because this final layer is typically less than 2 mm thick, it can be placed in a single increment without concern for shrinkage issues. Initial anatomy can be developed using a cone burnisher (Figure 15) and fine details using an explorer (Figure 16). Light-curing is accomplished, ensuring adequate curing from all sides (Figure 17) The matrix, wedge and ring are removed (Figure 18) and anatomy is refined with finishing carbides and diamonds and the occlusion is checked and adjusted before final polishing (Figure 19).
Discussion
Restorative materials are offered in either a dual-cure (curable in either self-cure or light-activated curing), light-cure only, or self-cure only modes. Selecting which type can affect the success of the restoration.
The primary advantage of light-curable materials is control over when the product starts to set. Composite resins initially were provided in self-cure only, with no light-cure option. This required mixing two components on a mixing pad and then loading a tube to place the material into the preparation. Air bubbles frequently resulted in the restoration and the practitioner was rushed to place the material before the final set occurred.
To overcome these limitations, changes in the chemistry of the composites by incorporation of light-activated catalysts allowed resins to be manufactured as single-component resins. This provided improved handling, reduction in air bubbles, and improved esthetics as a result. But, these light-curable materials were not suitable for core build-ups or restoration of endodontically treated teeth due to the deeper depth of the preparations and inaccessibility of the light. With this in mind and the adhesion issues associated with the self-cure resins, manufacturers developed dual-cure resins. Essentially, these were a self-cure resin mixed with a light-curable resin, which allowed the surface to be cured quickly and the deeper portions to complete curing on their own. With improvements in the adhesives, bond strengths could be reached that were similar to those achieved with total-etch adhesives in these clinical situations.
Conclusion
Dual-cure composite resins and self-etch adhesives have a useful purpose in our clinical armamentarium. These materials should not be judged on fewer steps or less clinical time required. But, consideration for their ability to set without regard to contact with light will assist the practitioner in providing easier, more predictable restorations in clinical situations that challenge total-etch adhesives and light-curable composites.
References
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2. Libman WJ, Nicholls JI. Load fatigue of teeth restored with cast posts and cores and complete crowns. Int J Prosthodontics. 1995;8:155-161.
3. Ichim I, Kuzmanovic DV, Love RM. A finite element analysis of ferrule design on restoration resistance and distribution of stress within a root. Int Endod J. 2006;39(6):443-452.
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About the Author
Gregori M. Kurtzman, DDS, Private Practice, Silver Spring, Maryland