Written by Kathy L.O'Keefe, DDS, MS Monday, 01 December 2003 00:00
The restoration of vital and nonvital teeth that have significant loss of tooth structure has undergone considerable evolution in philosophy and clinical application. The important issues associated with this topic include use of a post, the type of cement/adhesive to be used, and the material used for core buildup. When considering the adhesive, there are a number of variables to consider, including mode of cure, compatibility of adhesives and core materials, etching technique, and the significance of contamination with sulcular fluid, saliva, or blood.
Materials science research has revolutionized restorative dentistry. This has occurred at an impressive rate, and it has become challenging for the practicing dentist to select the most appropriate and current approach to difficult clinical situations. This article will review current research and clinical techniques used to restore teeth with significant loss of tooth structure.
THE USE OF POSTS IN NONVITAL TEETH
The philosophy concerning the use of post and core restorations has changed during the past 20 years. Research has shown that the placement of a post does not strengthen teeth and should only be used to retain the core build-up.1,2 Because core buildup material can now be adhesively bonded to tooth structure, many clinicians choose not to use posts as often as in the past.3 However, in teeth with significant loss of tooth structure, posts are used to aid in core retention. Modern prefabricated posts comprise a variety of materials, including stainless steel, titanium, ceramic, and fiber-reinforced composite.4,5 Some newer posts have been designed to conform to the anatomy of the canal. Posts are generally parallel-sided and tapered at the apical end or are tapered through their entire length. Tapered posts have been shown to provide a more intimate fit in the canal and are safer at the apical end because the canal does not have to be shaped as wide at the narrow, apical end of the root. However, they have also been shown to create a wedge effect, which can result in vertical root fractures.2,3
The stiffness of a post (modulus of elasticity) is also an issue. The mechanical properties (tensile and compressive strength, modulus of elasticity, and flexural strength and modulus) of metal, ceramic, and resin posts are quite different. It has been shown that resin-reinforced posts have approximately the same stiffness as dentin. The other types of posts have greater stiffness than dentin, a factor that should be considered in terms of the potential to cause root fracture.6
ADHESIVE VERSUS TRADITIONAL CEMENTS
Practitioners have differing goals in mind when cementing a post. Retrievability is one concern. Some practitioners feel more comfortable if they are able to re-access the canal, if necessary, at a later time. However, studies have shown that endodontic success rates can be as high as 90% to 95%.7,8 With the improved quality of endodontic materials and the high success rate of endodontic therapy, it is rare that access to an obturated canal must be obtained. Furthermore, research shows that most restored nonvital teeth fail because of lack of post and/or core retention.1,9 However, if the practitioner's goal is post retrievability, zinc phosphate cement is the choice. This cement provides adequate working time and low film thickness. However, zinc phosphate does not bond to tooth structure.
Glass ionomer cements can also be used to cement posts. Conventional glass ionomer cements release fluoride but have a minimal bond to tooth structure. Resin-modified glass ionomer cements are now commonly used for post cementation. This cement has a fairly high bond strength to tooth structure and releases fluoride, although not to the degree of conventional glass ionomer cements. However, there has been a problem in the past with delayed expansion of these cements, which could be a problem when cementing a post in a canal space. Most companies state that this problem has been resolved by lowering the amount of delayed expansion.10
|Table 1. Mean (standard deviation) tensile bond strength (MPa) data (N = 5 for each combination).
Reprinted with permission from Quintessence Publishing Co.
If the goal of post cementation is strength, adhesive resin cements are the materials of choice. The concept that posts do not strengthen endodontically treated teeth is based on research, but most of those studies employed traditional cementation methods and materials.11-13 This concept needs to be reevaluated with adhesive cements. There are now adhesive cements that can bond to all types of post materials as well as to dentin. One study examined the bond strength of adhesive and conventional resin cements to various post materials.14 The results of this study are provided in Table 1. In general, tensile bond strengths of 18 MPa or higher are clinically stable. The adhesive cements display high bond strengths to most post materials.
Researchers at the University of Texas Dental Branch at Houston have tested many adhesive systems under different bonding conditions. In general, results indicate that many factors affect the successful, long-term bond to dentin, including the presence of moisture, dentin depth, curing mode, chemical formulation, and contamination with biological fluids (ie, blood and saliva) and astringents.15
The newest generation of adhesives are self-etching products and contain an acidic monomer that etches and primes dentin simultaneously, thus eliminating the need for phosphoric acid etching.16,17 These products are less technique-sensitive because of the fewer steps involved. They also have been associated with less postoperative sensitivity because of the simultaneous dentin demineralization and resin penetration of dentinal tubules, which prevents a layer of demineralized dentin from forming below the adhesive layer. Also, self-etching adhesives may form a stronger bond to deep dentin because they precipitate smear layers that keep the large diameter tubules of deep dentin from being exposed.18 Some self-etching products provide excellent bond strengths to dentin.19 Total-etch systems, in which phosphoric acid is used as a conditioner of enamel and dentin prior to bonding, are also excellent adhesives. It is important that the manufacturer’s instructions be followed when using any bonding material.
Two of the most important issues when deciding on a core material are retention and strength. The core build-up must be retentive enough in the short term to resist dislodging forces during the preparation and impression phases, and it must remain retentive in the long term, specifically after the final restoration is completed. Retention of core material to tooth structure can be obtained by creating mechanical undercuts in the preparation, by bonding to tooth structure, or by a combination of both. It is always best not to depend solely on bond strength because there are many factors that can affect the bond of composite to dentin, such as moisture tolerance, contamination, and adhesive/core material compatibility.20
|Table 2. Mean (SD) microtensile bond strength (MPa) of core materials to dentin using a self-etched, light-cured and a total-etched,
Reprinted with permission from Quintessence Publishing Co.
There has been much discussion in the literature relating to adhesive/core compatibility. Many core materials are dual- or self-cured composites and may not adhere adequately to a light-cured or a dual-cured adhesive.21 In this study, a self-etched, light-cured adhesive and a dual-cured, total-etch adhesive were bonded with light-, dual-, and self-cured core materials to superficial dentin. There were some incompatible combinations. However, although the self-etched, light-cured adhesive that was studied is indicated primarily for bonding to light-cured resins, in this study it was compatible with all types of core materials (see Table 2).
|Figure 1. Total-etch adhesive used to bond light-cured core material to dentin (26.7 MPa). Note extensive resin tags.|
|Figure 2. Self-etching adhesive used to bond dual-cured core material to dentin (41.0 MPa). Note relatively short resin tags. (Figures 1 and 2 reprinted by permission of Quintessence Publishing Co.)|
|Figure 3. Incompatibility between dual-cured adhesive and self-cured core material.|
Figures 1, 2, and 3 demonstrate the adhesive interface of several of the experimental combinations in this study. The resin tags seen in Figure 1 are longer and more numerous than those in Figure 2. However, the bond strength of the combination in Figure 1 was lower (26.7 MPa) than the combination in Figure 2 (41.0 MPa). Resin tag quantity and quality do not necessarily relate to bond strength. Figure 3 is an example of a bonding incompatibility.
Other recent reports have indicated that there is less incompatibility between adhesives and core materials now than in the past. In general, the highest bond strengths were obtained with light-cured core materials.22
Table 3. Mean (SD) tensile bond strength (MPa) of various thicknesses of core materials to dentin.
Brand names on file
Core material strength is the other critical issue in obtaining a successful, long-term restoration. Many studies have shown that amalgam and composite are the two strongest buildup materials available.23-25 Because of the ability to bond to tooth structure and immediate setting, composite is today’s material of choice for a core. There are many composite buildup materials currently available. Most composite core materials are either dual- or self-cured because the restorations are often thick, and chemical-curing is therefore an advantage. However, in general, dual- and self-cured composites have been shown to have lower bond strengths to dentin than light-cured products.26-28 One study compared a light-cured core material, a dual-cured core material, and a self-cured core material to determine bond strength to dentin at different depths of cure (2, 4, 6, and 10 mm).29 The results (see Table 3) indicated that bond strength did not decrease with the light-cured core material until 10 mm of curing depth, and even at 10 mm the bond strength was greater than 18 MPa. These results suggest that it is possible to use a light-cured core material without compromising strength.
TECHNIQUE SENSITIVITY IN CORE PLACEMENT
Often, when restoring teeth with significant loss of tooth structure, the preparation will extend subgingivally. This can result in trauma to the soft tissues. One disadvantage of composite core material is the effect of contamination with blood on the bonding reaction. When bonding, it is advisable to use rubber dam isolation whenever possible. Recent studies have reported on the effect of contamination on the bond strength of composite to dentin.30,31 Most studies used 1- or 2-bottle total-etch systems rather than self-etching systems. To summarize the results, saliva did not appear to affect bond strength of total-etch adhesives to tooth structure, but blood, handpiece oil, and eugenol reduced bond strengths. However, re-etching with phosphoric acid raised the bond strengths to nearly control levels. The results of one recent study evaluating saliva contamination on a self-etching adhesive and a composite bonded to human dentin are shown in Table 4.32 Saliva did not markedly affect the bonding process, whether contamination occurred before or during adhesive placement.
Table 4. Mean (SD) tensile bond strength (MPa) of composite to saliva-contaminated dentin.
Brand name on file
A similar study using the same adhesive and composite demonstrated that contamination with blood did not affect the bond strength to any significant degree,33 which differed from previous studies.30,34,35 However, in another contamination study that evaluated astringents, it was found that both ferric sulfate and aluminum chloride astringents adversely affected the bond strength of composite to dentin.36 Although it was found that rinsing with a water spray restored bond strengths to half or more of the original strength, both astringents had a negative affect on the bond to dentin. Therefore, it may be desirable to achieve control of blood contamination without the use of astringents. Dry retraction cord or other types of tissue retraction techniques should be considered when bonding composite core materials.
|Figure 4. Preoperative photo of tooth No. 4 with broken buccal cusp and large resin restoration.||Figure 5. Close-up preoperative view of tooth No. 4 after rubber dam placement.|
|Figure 6. Occlusal preoperative view of tooth No. 4 with extensive existing composite restoration in view.||Figure 7. Remaining tooth structure after caries and restoration were removed.|
|Figure 8. Try-in of core former.||Figure 9. Verification of fit of core former, occlusal view.|
|Figure 10. Cord placed, Clearfil SE Bond primer applied with a brush for 20 seconds.||Figure 11. Gentle air-drying of primer and gentle air-drying of bond.|
|Figure 12. Clearfil SE Bond bond placed with a brush.||Figure 13. Preparation is light-cured for 10 seconds.|
|Figure 14. Clearfil Photo Core placed in matrix and then fit to preparation.||Figure 15. Core material light-cured for 40 seconds.|
|Figure 16. Core former is cut, then removed with hemostats.||Figure 17. Preparation is refined using diamond burs.|
|Figure 18. Immediate postoperative occlusal view of completed preparation with rubber dam in place.||Figure 19. Postoperative view of completed preparation prior to impression.|
The case presented is of a vital tooth requiring a core buildup. The preoperative view (Figure 4) illustrates that the buccal cusp of tooth No. 4 is fractured to the level of the gingival margin. After the rubber dam was placed (Figures 5 and 6), the remainder of the previous restoration was removed. Figure 7 demonstrates the remaining tooth structure after all caries and previous composite restoration were removed. Next, a core former was trimmed and fit to the remaining tooth structure (Figures 8 and 9). Once a correct fit was verified, dry retraction cord was placed and Clearfil SE Bond Primer and Adhesive (Kuraray) were applied to the rinsed and gently dried tooth according to the manufacturer’s instructions, then the adhesive was light-cured for 10 seconds (Figures 10 through 13). Clearfil Photo Core (Kuraray) was placed inside the core former, inserted onto the preparation, and light-cured in one increment for 40 seconds (Figures 14 and 15). In Figure 16, the core former matrix is removed with hemostats. The preparation was then refined with diamond burs (Figures 17 and 18). Figure 19 illustrates the completed core buildup, which is ready for provisionalization or the final impression.
This article has discussed the evolution and current status of materials and techniques for restoring vital and nonvital teeth that have lost a significant amount of tooth structure. A case was presented illustrating the clinical technique for creating a bonded core buildup for a vital tooth.
The author thanks Dr. Lilliam Pinzon for research and Dr. Rose-Marie Fay for photography.
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