Optimizing Core Buildups and Post and Core Restorations: Improving Clinical Techniques With New Resin Materials

INTRODUCTION
Core buildups and post/core restorations can be a challenge when adhesives are utilized to lute the base due to inadequate light contact to polymerize the resins. This issue becomes increasingly problematic when deeper posterior preparations are being restored. In these cases, unpolymerized adhesive may contribute to marginal leakage, leading to bond failure throughout time and subsequent caries at the margins. This has been addressed by the manufacturers with the development of self-cure promoters that can be added to light-cured adhesives. Yet these can lead to adhesive pooling in the apical of post preparations, hampering insertion of the post to the desired depth within the tooth.
Kuraray America, in researching these problems, has addressed these clinical issues with an advanced and improved single component self-etch adhesive (Clearfil S3 Bond Plus), pairing their new dual-cure resin (Clearfil DC Core Plus) to this improved adhesive to create a total core system. As an alternative to using a self-cure promoter, they added new catalysts to the adhesive and core resin that initiates the Clearfil S3 Bond Plus to set when Clearfil DC Core Plus contacts the adhesive without direct light contact to the resin adhesive. Additionally, working time was improved for the adhesive without compromising the bond strength to dentin.

Post and Core Clinical Technique
The flow of Clearfil DC Core Plus allows for its use as a luting resin for fiber or metal posts, thus eliminating the need for a separate luting cement for posts. However, it needs to be utilized with a dentin adhesive to ensure adequate bonding to the dentin walls of the post preparation. When combined with Clearfil S3 Bond Plus, concerns related to adequate light penetration to cure the adhesive at the apical extent of the post preparation are eliminated. Additionally, since the adhesive is cured after contact with the Clearfil DC Core Plus resin, the potential for pooling of any cured adhesive, thus preventing full seating of the post, is avoided.
Following post preparation, Clearfil S3 Bond Plus is applied to the apical depth of the post preparation with a microapplicator, scrubbing the self-etch adhesive into the post preparation and any exposed dentin; this is not light-cured at this time. The post to be placed is also lightly coated with additional adhesive; this may be light-cured or not, depending on the practitioner’s preference. Utilizing the thin tip on the Clearfil DC Core Plus automix syringe, the tip is placed at the apical extent of the post preparation. Then, resin is expressed from the automix syringe, backfilling the post preparation to the canal orifice and coating all dentin surfaces. Next, the prefitted post is inserted to depth in the post preparation. A light-curing tip is then placed on the top of the post and activated for 20 seconds, followed by bulk-filling the remainder of the preparation with additional core resin. The material is then light-cured from the buccal and then the lingual to ensure full polymerization of the core resin. Modification of the accelerators in the core resin decreases light-curing times compared to the previous version of this material (Clearfil DC Core), with an increase in bond strength as well as flexural strength being reported (Table).

Core Build-Up Clinical Technique
Although most practitioners would ideally like to be able to bulk fill preparations when a core is to be placed, an improvement in bond strength has been shown when the initial layer of the dual-cure core material is 1.5 mm compared to bulk filling of the preparation. Following placement of the Clearfil S3 Bond Plus to the dentin and prepared enamel margins with a microapplicator, a thin layer of Clearfil DC Core Plus is placed to just cover the preparation surface. Then light-cure the adhesive and initial core material layer for 10 seconds; the remainder of the preparation is bulk filled with additional core resin and light-cured from the occlusal, buccal and lingual, to ensure complete polymerization of the resin.

CASE 1: POST AND CORE WITH CROWN

Diagnosis and Treatment Planning
An 84-year-old female presented with the loss of a crown from her maxillary right first molar (tooth No. 3) (Figure 1). She was in the process of treatment at our office for an implant placed several months previously by a periodontist at the second premolar site. She indicated the crown on tooth No. 3 was approximately 20 years old. A radiograph demonstrated prior endodontic treatment had been performed; however, no post and core was present. Examination noted sufficient tooth structure present to place a ferrule for a new crown (Figure 2). It was decided that a direct post and core and a new crown would be done.

Figure 1. Patient presented with existing crown off the maxillary right first molar that had prior endodontic treatment.	Figure 2. Occlusal view demonstrating an absence of a post and core, and tooth had been restored by placing composite into the pulp chamber after endodontic treatment.
Figure 3. Old composite core has been removed, demonstrating sufficient tooth to achieve ferrule with a new crown. Orifices for 3 canals are identified obturated with gutta-percha.	Figure 4. Clearfil DC Core Plus (Kuraray America), an improved dual-core resin material, and the advanced Clearfil S3 Bond Plus self-etch adhesive (Kuraray America).

Clinical Protocol
The old composite was removed with a carbide bur (No. 1557MX FG XTREMECUT [Brasseler USA]) in a high-speed handpiece, and orifices for 3 canals were identified, each obturated with gutta-percha (Figure 3). Post preparations were created in each of the 3 canals to allow placement of a fiber post into each, thus locking the new core to the remaining tooth structure. Clearfil S3 Bond Plus was scrubbed with a microbrush into the dentin in the post spaces and all other exposed dentin (Figures 4 and 5). Next, Clearfil DC Core Plus (Figure 6a) was injected into the post spaces to act as a luting material for the fiber posts. The fiber posts, after a coating of Clearfil S3 Bond Plus was applied and light-cured, were inserted to depth and light-cured for 40 seconds; this was done by touching the light to the top of the posts to transmit light down the posts and accelerate setting of the dual-cure resin (Figure 6b). Additional Clearfil DC Core Plus resin was placed to form a core. Next, this was light-cured for 40 seconds, then allowed to self-cure for 4 minutes. The crown preparation, designed with a proper circumferential ferrule placed onto natural tooth structure, was done (Figures 7 and 8). An impression was then made using a vinyl polysiloxane impression material (Correct Plus [PENTRON]), and a provisional restoration was placed. The case was then sent to our dental laboratory team to have the new crown (monolithic zirconia) fabricated (BruxZir [Glidewell Laboratories]).

Figure 5. Following post preparation into each of the 3 canals, Clearfil S3 Bond Plus was applied to the post spaces and exposed dentin with a microbrush.

Figure 7. Additional Clearfil DC Core Plus was injected in the coronal aspect around the post heads to form a core and was allowed to fully set, then the tooth was prepared for a new crown restoration. Note ferrule of natural tooth structure present to retain the new crown.	Figure 8. Occlusal view following crown preparation illustrating the coronal aspects of the 3 fiber posts locking the core to the remaining tooth structure.

CASE 2: CORE BUILDUP

Diagnosis and Treatment Planning
A 56-year-old male presented with a defective amalgam restoration on the lower right second molar (tooth No. 31). Treatment recommendation was a core buildup followed by a full-coverage crown to restore function.

Clinical Protocol
The tooth was isolated and the old amalgam and decay removed with a high-speed handpiece. Based on remaining tooth structure, the tooth clearly demonstrated the need for a full-coverage crown (Figure 9). An Automatrix (DENTSPLY Caulk) was placed to contain the core build-up during core fabrication. Clearfil S3 Bond Plus was applied to all dentin and enamel surfaces with a microbrush (scrubbing it into the dentin), and then light-cured for 20 seconds (Figure 10). Utilizing the intraoral tip on the automix syringe, a thin layer of Clearfil DC Core Plus (just covering the dentin) was placed and then light-cured for 20 seconds (Figure 11). Placement of a thin layer followed by light-curing has been shown to improve bond strength of the resin to the core as compared to bulk filling the preparation; this ensures that the material in contact with the dentin surface has been cured with the light, not relying on the self-cure mode only (Figure 12). The preparation was then bulk filled with additional Clearfil DC Core Plus, then light-cured for 40 seconds from the occlusal (Figure 13). Next, the restoration was allowed to self-cure for 4 minutes to ensure complete setting. Following the setting of the core resin, the matrix was removed and the core was then contoured and shaped (Figure 14). The patient was dismissed and given another appointment for the crown preparation.

Figure 9. Following removal of the defective amalgam and decay, a matrix was placed to contain the core buildup to be placed.	Figure 10. Clearfil S3 Bond Plus was applied to the exposed dentin and enamel with a microbrush, then light-cured for 20 seconds.
Figure 11. A thin layer of Clearfil DC Core plus was applied, using the intraoral tip on the automix syringe just covering the dentin, then light-cured for 20 seconds.	Figure 12. To improve the bond of the core to the tooth it is recommended that the first layer be thin, just covering the dentin, then light-cured before additional material is placed.
Figure 13. The core area is then filled with additional resin, completely filling the preparation.	Figure 14. The matrix has been removed, the core shaped, and is ready for crown preparation at the next appointment.

Figure 15. Comparison of bond strength to enamel and dentin of both one day and aged samples of various self-etch adhesives (data: Kuraray America).

DISCUSSION
Initially introduced in 2005, Clearfil S3 Bond is a single component self-etch adhesive. Most single component self-etch adhesives, unlike Clearfil S3 Bond, undergo phase separation of the hydrophilic and hydrophobic components. But the use of a unique proprietary technology creates a homogeneous bond layer, preventing phase separation in the bottle between uses. Phase separation between the hydrophilic and hydrophobic components in an adhesive leads to blisters (microbubbles) within the adhesive layer, decreasing the bond strength with the dentin. Therefore, a lack of phase separation in an adhesive provides improved bond strength and marginal integrity compared to similar products available where phase separation may occur.^1,2
As part of continued product development, Kuraray America improved the adhesive (Clearfil S3 Bond Plus) (Figure 15) by modification of the catalysts, improving the curing properties with a subsequent increase in bond strength^3-9 with a decrease in time needed to self-etch the tooth surface and light-cure the adhesive. With the addition of fluoride release to the adhesive, this provides a long-term durable bond to dentin with less technique sensitivity than with other single component self-etch adhesives (Table). Postoperative sensitivity has been a challenge with resin bonding. But elimination of this postoperative sensitivity has been well documented in the literature with the use of self-etch adhesives compared to total-etch adhesives. Clearfil S3 Bond Plus, a universal self-etch adhesive, may be used for bonding anterior and posterior direct restorative materials and is also well suited when utilized with Clearfil DC Core Plus for core buildups and post/core restorations.
With the improvement in the Clearfil S3 Bond Plus adhesive, Kuraray America also improved its dual-cure core resin, Clearfil DC Core Plus, with the incorporation of a new accelerator (catalyst) to the core resin. This allows, when mated with the Clearfil S3 Bond Plus, self-curing of the adhesive within the tooth upon contact of the core resin. Additionally, the new Clearfil DC Core Plus has added fluoride release, improving the resin tooth interface and decreasing the potential for recurrent decay at this interface. Improvement also was made in the flow and body of the uncured resin core material, allowing placement without slumping prior to light-curing, allowing the practitioner to forgo use of a matrix in some clinical applications. This also prevents the slump of uncured material typically observed after placement in a maxillary preparation due to gravity’s effect on the material following placement. Improved flow also permits its usage in combination with Clearfil S3 Bond Plus for cementation of posts, allowing the practitioner to eliminate a separate post luting material when placing a post/core restoration and eliminate any interfaces between a luting material and the core material.

IN SUMMARY
Material selection is as important as technique and preparation. What may work well in one clinical situation (such as a direct Class II resin restoration) may not work in others. With improvements in dental material chemistry, we now have adhesives that are better mated to the resins with which they are paired. Designing adhesives that have an optimal self-cure mode when combined with a dual-cure resin for cores/buildups ensures full setting of the adhesive without the need to use a self-cure promoter mixed into the adhesive. This also improves working time, yields better bond strength, and decreases the chance of adhesive pooling.

References

1. Kubo S, Yokota H, Yokota H, et al. Three-year clinical evaluation of a flowable and a hybrid resin composite in non-carious cervical lesions. J Dent. 2010;38:191-200.
2. Brackett MG, Dib A, Franco G, et al. Two-year clinical performance of Clearfil SE and Clearfil S3 in restoration of unabraded non-carious class V lesions. Oper Dent. 2010;35:273-278.
3. Hanabusa M, Akimoto N, Ohmori K, et al. Bond-durability of an experimental 1-bottle self-adhesive after 3-months water storage. J Dent Res. 2011;90(special issue A). Abstract 1090.
4. Hinamoto A, Nishigaki N, Takei M. Adhesive property of a new self-etching bond system “MTB-200.” J Dent Res. 2011;90(special issue A). Abstract 1088.
5. Hosaka K, Nakajima M, Takahashi M, et al. Microtensile bond strength of the newly developed one-step adhesive. J Dent Res. 2011;90(special issue A). Abstract 1902.
6. Akimoto N, Hanabusa M, Miyauchi T, et al. Influence of dentin surface conditions on bond-strength with 1-bottle systems. J Dent Res. 2011;90(special issue A). Abstract 2462.
7. Kakuda S, Hinamoto A, Fu J, et al. Long term bonding performance of contemporary and experimental self-etching adhesives. J Dent Res. 2011;90(special issue A). Abstract 1557.
8. Suzuki K, Kawana M, Takei M, et al. Characteristics of a new core build-up system “NDC-100” and “MTB-200.” J Dent Res. 2011;90(special issue A). Abstract 1110.
9. Hinamoto A, Nojiri Y, Takei M, et al. Bonding performance of a new adhesive “Clearfil Tri-S Bond Plus.” J Dent Res. 2012;91(special issue A). Abstract 794.

Dr. Kurtzman is in private general practice in Silver Spring, Md, and a former assistant clinical professor at the University of Maryland. He has earned Fellowships in the AGD, American Academy of Implant Prosthodontics, American College of Dentists, International Congress of Oral Implantologists (ICOI), Pierre Fauchard Academy, Association of Dental Implantology; Masterships in the AGD and ICOI; and a Diplomate in the ICOI and American Dental Implant Association. He has lectured internationally on the topics of restorative dentistry, endodontics and implant surgery and prosthetics, removable and fixed prosthetics, periodontics, and has more than 320 published articles. Dr. Kurtzman has been honored to be included in Dentistry Today’s Leaders in Continuing Education annually since 2006. He can be reached at dr_kurtzman@maryland-implants.com.

Disclosure: Dr. Kurtzman receives honoraria for speaking from Kuraray America.