The Journal of the American Dental Association
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J Am Dent Assoc, Vol 135, No 5, 646-652.
© 2004 American Dental Association
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ADVANCES IN DENTAL PRODUCTS

Fracture resistance of roots endodontically treated with a new resin filling material



FABRICIO B. TEIXEIRA, D.D.S., M.Sc., Ph.D., ERICA C. N. TEIXEIRA, D.D.S., M.Sc., JEFFREY Y. THOMPSON, B.S., Ph.D. and MARTIN TROPE, D.D.S., D.M.D.


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
Background. The authors evaluated the fracture resistance of endodontically treated teeth filled with either gutta-percha or a new resin-based obturation material.

Methods. The authors prepared and randomly divided 80 single-canal extracted teeth into five groups: lateral and vertical condensation with gutta-percha, lateral and vertical condensation with the new resin-based obturation material, and a control group with no filling material. The specimens were stored in 100 percent humidity for two weeks, mounted in polyester resin and loaded to failure.

Results. The authors found statistically significant differences among the experimental groups (P < .05). The groups with the new material displayed higher mean fracture loads and the gutta-percha groups lower mean fracture load values than the control unfilled group. However, the differences were not significant. The groups with the new material displayed significantly higher mean fracture loads than gutta-percha groups independent of the filling technique used.

Conclusions. Filling the canals with the new resin-based obturation material increased the in vitro resistance to fracture of endodontically treated single-canal extracted teeth when compared with standard gutta-percha techniques.

Clinical Implications. If other properties of the new resin-based obturation material compare favorably with those of gutta-percha for filling the root canal, it should be considered as a replacement for gutta-percha, as the results of this study indicate that it could provide enhanced resistance to tooth fracture.

Endodontically treated teeth are widely considered to be more susceptible to fracture than are vital teeth. The reasons most often reported have been the dehydration of dentin after endodontic therapy, excessive pressure during obturation and the removal of tooth structure during endodontic treatment.1–3 The strength of an endodontically treated tooth is related directly to the method of canal preparation and to the amount of remaining sound tooth structure. It commonly is believed that the loss of dentin creates an increased susceptibility to fracture.3,4 Some studies have reported strong evidence that endodontically treated teeth, with or without posts, are susceptible to root fracture.4,5 According to Bender and Freedland, 6 the greatest incidence of vertical root fracture occurs in teeth that have undergone endodontic therapy.

A new resin-based obturation material could be considered as a replacement for gutta-percha.

Resin-based dental materials have been proposed as a means to reinforce an endodontically treated tooth through the use of adhesive sealers in the root canal system.7 However, for a dental material to reinforce the tooth, the material must bond to dentin. Therefore, an essential attribute of a good dentin adhesive system is the adhesive’s ability to wet and infiltrate dentin.

Some investigators8,9 have suggested the use of glass ionomer cements in endodontics, and such use has been shown to have long-term adhesive effects bonding to the hydroxyapatite component of enamel and dentin.10,11 A glass ionomer–based sealer, Ketac-Endo Aplicap (3M-ESPE, Maplewood, Minn.), was introduced in the early 1990s for use as an endodontic sealer with the potential to increase resistance to root fracture. Trope and Ray12 reported that Ketac-Endo Aplicap has the potential for root reinforcement. Canals obturated in conjunction with a glass ionomer sealer exhibited a higher resistance to fracture than canals instrumented but not obturated or those obturated with gutta-percha and Roth 801 sealer (Roth International, Chicago).

In restorative dentistry, numerous studies have demonstrated coronal reinforcement of the tooth through bonded restorations. Bonded amalgams, composites and glass ionomers all have been shown to reinforce remaining tooth structure by bonding to dentin and enamel.13–17 Similarly, bonding endodontic obturation materials could enhance the ability of endodontically treated teeth to resist fracture. Dentin-bonding agents and resins used in restorative dentistry have been examined for use as root canal filling materials. However, bonding agents and resins studied to date as root filling materials had problems in working properties, radiopacity and lack of re-treatability when used for endodontic purposes.18,19 Imai and Komabayashi20 tested a new type of root canal filling resin for its ability to adhere to dentin. The authors found that the resin material had properties desirable for root canal filling, such as adhesion to dentin, good sealing ability and removability.

Bonding endodontic obturation materials could enhance the ability of endodontically treated teeth to resist fracture.

In recent years, an endodontic obturation material based on polyester chemistry and containing bioactive and radiopaque fillers has been developed and tested (Resilon, Resilon Research, North Branford, Conn.). It performs, handles and looks like gutta-percha. In addition, when used in conjunction with a resin-based sealant or bonding agent it forms a monoblock within the canals that bonds to the dentinal walls. Because the resin core, sealant and dentinal wall all are "attached," it appears logical that they have the potential to strengthen the walls against fracture.

We conducted a study to compare in vitro root fracture resistance after root canals were filled with either gutta-percha or Resilon by two different techniques.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 
Instrumentation and obturation. We collected 80 single-canal extracted teeth and stored them in saline containing 0.2 percent sodium azide at room temperature. We examined all teeth under x25 magnification with a dental operating microscope to rule out any teeth with pre-existing root fractures. We sectioned acceptable teeth at the cementoenamel junction with a diamond disk (Brasseler Dental Products, Savannah, Ga.). We used root specimens that were at least 14 millimeters in length. We established the working length by placing a size 15 FlexoFile (Dentsply Maillefer, Tulsa, Okla.) into the canal until we observed it at the apical foramen, then decreasing the file length by 1 mm. We instrumented all groups to a size 40 master apical file using a "balanced force" technique (as described by Roane and colleagues21) and flared them using a step-back technique in 1-mm increments to a size 55 file. We accomplished the final flare in the cervical 10 mm of the canals with FibreFill P-type reamers nos. 090 and 130 (Pentron Clinical Technologies, Wallingford, Conn.) to establish a standard preparation of all roots.

Along with this instrumentation, we irrigated between use of each succeeding file, delivering 10 milliliters of 5.25 percent sodium hypochlorite, or NaClO, with a 27-gauge needle. We performed recapitulation with a size 15 FlexoFile to keep the apical foramen patent. After we completed instrumentation, all specimens received a final flush with 10 mL of neutralized 17 percent ethylenediaminetetraacetic acid, or EDTA (PulpDent, Watertown, Mass.), followed by 10 mL of NaClO to remove the smear layer.22 After that, we used 10 mL of sterile water to remove any remaining NaClO residue. We dried the canals with sterile paper points (Hygienic, Akron, Ohio). Then we randomly assigned the specimens into five experimental groups (n = 16 per group) as follows.

Group 1: control. This group received no obturation; the root canal opening was sealed with a temporary filling material (Cavit, Premier Dental Products, Plymouth Meeting, Pa.).

Group 2: lateral condensation with gutta-percha. We accomplished lateral condensation using a size 40 gutta-percha master point (Kerr, Orange, Calif.) dipped in AH26 sealer (Dentsply Maillefer). We mixed the sealer according to the manufacturer’s instructions and placed it with a lentulo (Dentsply Caulk, Milford, Del.). After placement of the master cone (Kerr) at the appropriate working length, we used a spreader and medium-fine gutta-percha accessory cones (Kerr) dipped in AH26 sealer for the lateral compaction. We seared off excess gutta-percha and condensed it with a plugger (Premier Dental Products) 1 mm below the canal opening. We sealed the canal opening with Cavit.

Group 3: vertical condensation with softened gutta-percha. We accomplished vertical condensation using the System-B technique (EIE/Analytic, Orange, Calif.) and the Obtura II Warm Gutta-Percha System (Obtura Spartan, Fenton, Mo.) using a size 40 gutta-percha master point (Kerr) dipped in AH26 sealer. We mixed the sealer according to the manufacturer’s instructions and placed it with a lentulo. After we placed the master cone to the working length, we used the System-B plugger for the vertical compression and performed the backfill using soft gutta-percha with the Obtura II system. We seared off the excess gutta-percha and condensed it with a plugger (Premier Dental Products) 1 mm below the canal opening. We sealed the canal opening with Cavit.

The vertical and lateral condensation groups were not significantly different from each other regardless of the filling materials used.

Group 4: lateral condensation with Resilon. We accomplished lateral condensation using a size 40 Resilon master point dipped in resin sealer. The sealer is a dual-curable resin-based composite sealer. We inserted the self-etching primer into the canals and removed the excess with paper points. We placed the resin sealer with a lentulo.

After placing the master cone to the working length, we used a spreader and medium-fine Resilon accessory cones dipped in resin sealer for the lateral condensation. We seared off excess material and condensed it with a plugger (Premier Dental Products) 1 mm below the canal opening. After this procedure, we cured the material in the root canal with visible light for 30 seconds. We sealed the canal opening with Cavit.

Group 5: vertical condensation with softened Resilon. We accomplished vertical condensation using the System-B technique and Obtura II Warm Gutta-Percha System using a size 40 Resilon master point dipped in resin sealer. We inserted the self-etch primer into the canals and removed the excess with paper points. We placed the sealer with a lentulo. After placing the master cone to the working length, we used the System-B plugger to sear off the master point about 4 mm from the apex and for the vertical compression. We accomplished the backfilling using soft Resilon through the Obtura II system. We seared off excess material and condensed it with a plugger (Premier Dental Products) 1 mm below the canal opening. After this procedure, we cured the material in the root canal with visible light for 30 seconds. We sealed the canal opening with Cavit.

We stored all root specimens in 100 percent humidity for two weeks to allow the sealer to set completely.

Preparation for mechanical testing. After two weeks, we prepared the root specimens for mechanical testing (Model 4411, Instron, Canton, Mass.). We embedded the apical root ends individually in phenolic rings with polyester resin (Castoglas Resin, Buehler, Lake Bluff, Ill.), leaving 9 mm of each root exposed. We used a carbide bur to remove the temporary material and to shape the root canal access to accept the loading fixture. We mounted the cylinders with the vertically aligned roots in the testing machine one at a time (Figure 1Go).



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  		Figure 1. Specimen prepared for the mechanical test at the Instron machine (Model 4411, Instron, Canton, Mass.).

 
The application of a vertical loading force to fracture was similar to the technique used in the study by Sedgley and Messer23 to test the brittleness of endodontically treated teeth. We mounted and aligned a loading fixture with a spherical tip (r = 2 mm) with the center of the canal opening of each specimen (Figure 2Go). We subjected each specimen to load at a crosshead speed of 1.0 mm per minute until the root fractured. For this study, we defined "fracture" as the point at which a sharp and instantaneous drop greater than 25 percent of the applied load was observed. For most specimens, an audible crack also was observed. We terminated the test at this point and recorded the force, measured in newtons. We subjected the data to analysis of variance, or ANOVA, and Fisher protected least significance difference tests at 95 percent level of confidence using statistical software (SPSS Release 9.0 for Windows, SPSS, Chicago). We performed a comparison among groups.



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  		Figure 2. The loading fixture aligned with the center of the canal opening of the specimen post fracture.

 

   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSION
 REFERENCES
 
The tableGo shows the means and standard deviations for each experimental group. ANOVA revealed significant difference between treatments (P = .037). Resilon groups displayed higher and gutta-percha groups displayed lower mean fracture load values than did the unfilled control group. However, the differences from the controls were not significant. The vertical and lateral condensation groups were not significantly different from each other regardless of whether we used Resilon or gutta-percha filling materials. Resilon groups displayed significantly higher mean fracture load values than the gutta-percha groups, independent of the filling technique used.


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  		TABLE FORCE MEASURED IN NEWTONS REQUIRED TO CAUSE VERTICAL ROOT FRACTURE (N = 16).

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSION
 REFERENCES
 
Many studies have suggested that as removal of tooth structure increases, fracture resistance of the tooth decreases.13–17 Root canal instrumentation is an unavoidable step in endodontic treatment. However, it is understood that as dentin is removed during the instrumentation phase, a weakening effect on the root is inevitable. If we add the wedging forces of the spreader during lateral condensation, or perform excessive dentin removal to facilitate pluggers for vertical condensation, the potential for root fracture is very real. Any material that can compensate for this weakening effect would be useful.

When extracted human teeth are used for this type of study, the potential for large uncontrollable variations in strength exists. Therefore, all controllable factors should be standardized as much as possible. Each group of root specimens that we used consisted of randomly selected teeth from a collection of maxillary and mandibular single-rooted canines, premolars and maxillary incisors. As was done in previous studies,7,12 we controlled some dimensions of the specimens, such as root length or buccolingual diameter. We measured these and chose teeth to eliminate these variations in dimension. In addition, we instrumented the roots with the same technique and a uniform-sized P-type low-speed bur, producing a uniform canal to 9 mm below the cervical area of the root. Thus, we selected teeth that were as similar as possible and assigned them into groups randomly.

Lateral and vertical gutta-percha techniques have ardent advocates in the endodontic community. As the techniques are so different, and the potential for weakening the roots by different mechanisms so real, we felt that we needed to test both techniques as well as the new material’s potential for strengthening the roots.

We used a final rinse with EDTA followed by NaClO to enhance the bonding of the materials tested to the dentinal surface of the root. Weiger and colleagues11 recommended using EDTA followed by NaClO to optimize adhesion of sealers to the root canal walls. However, not all investigators agree that this combination is the best. Powis and colleagues10 advised against the use of citric acid and EDTA solutions to condition the dentin. They recommended high molecular weight, multi-functional substances such as polyacrylic acid, tannic acid or dodicin to ensure effective cleaning and wetting of the tooth surfaces.

The concept of dentin bonding in restorative dentistry has been introduced in endodontic treatment, and promising results have been reported in methylmethacrylate-tributyl borane, or MMA/TBB, -based resin sealer.19,24 The resin composed of 4-methacryloxyethyl trimellitate anhydride, or 4-META, and MMA-TBB—which is known commercially as Super-Bond C&B (Sun Medical, Shiga, Japan) or C&B Metabond (Parkell, Farmingdale, N.Y.)—has been reported to produce consistently high bond strengths, and has been successfully used clinically for 15 years.25,26

Resins may have the potential to enhance the endodontic seal by reducing microleakage from both apical and coronal directions.

Few studies18,27,28 have evaluated the potential of using dentin-bonding agents and resins as obturation materials in nonsurgical root canal treatment. Reasons for not using resins have centered on questionable results, difficult and unpredictable methods of delivery into the root system and the inability to re-treat the canal if necessary.29 However, these materials may have the potential to enhance the endodontic seal by reducing microleakage from both apical and coronal directions, thereby contributing to the success of orthograde endodontic treatment.

In this study, we evaluated a new thermoplastic synthetic polymer based on polyester, which contains bioactive and radiopaque fillers. Resilon performs in every way like gutta-percha except that it allows the bonding agent (sealer) to attach to the resin core and the dentin wall, thus forming a monoblock. The Resilon sealer (Epiphany) is a dual-curable dental resin-based composite sealer whose resin matrix is a mixture of bisphenol-A glycidyl methacrylate, or Bis-GMA; ethoxylated Bis-GMA; urethane dimethacrylate resin; and hydrophilic difunctional methacrylates.

In our study, as in other mechanical studies,29,30 the force was applied along the long axis of the root with a rounded punch, which produced root fracture when contact was made between the punch and the walls of the canal opening. The roots used were narrower in a mesiodistal direction, and the majority fractured in a buccolingual direction (Figure 3Go).



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  		Figure 3. Buccolingual direction of fracture commonly encountered in the roots.

 
As can be seen from the tableGo, the standard deviations within the groups were rather high. This is not unusual in these types of studies, and it reflects the differences found in extracted teeth. Thus, it is very difficult to find statistical differences between groups. When we found differences, however, we assumed that the results were important. We saw no differences between the lateral condensation and vertical condensation groups using the same material.

Because the canal dimensions were the same for all teeth in this study, the theoretical increased weakening effect of the wedging effect of the spreader for lateral condensation was not borne out. Neither the gutta-percha nor the Resilon groups were significantly different from the unfilled control group. However, clinically, one never leaves an instrumented empty canal unfilled. Therefore, the clinically relevant comparison is between the Resilon and gutta-percha groups. In this case, the Resilon groups were significantly more resistant to fracture than were the gutta-percha groups, indicating that the monoblock concept is important not only to resist bacterial penetration through the material but also to hold the root together, thereby increasing the resistance to fracture.

Some studies have suggested that the root canal sealer, especially glass ionomer cement (Ketac-Endo Aplicap), might strengthen root dentin. Trope and Ray,12 using maxillary and mandibular canine roots, showed that Ketac-Endo Aplicap strengthened roots weakened by canal instrumentation. Lertchirakarn and colleagues30 suggested that Ketac-Endo Aplicap strengthened endodontically treated roots and may be used for weak roots, which are likely to be susceptible to vertical root fracture. Imai and Komabayashi20 tested an injectable type of adhesive filling resin for root canal walls and found it to have a bond strength of 7.3 megapascals. Under the conditions of this in vitro study, use of Resilon to increase root fracture resistance in endodontically treated roots is appropriate. Additional studies are needed to assess whether or not Resilon reduces the incidence of vertical root fractures clinically.


   CONCLUSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
REFERENCES
 
On the basis of our findings, it can be concluded that filling the root canals with Resilon increased the in vitro resistance to fracture of single-canal extracted teeth. Resilon demonstrated high fracture-resistance values and could be an alternative to the conventional gutta-percha. Further studies should be performed to support our results and this new material’s clinical use.


   FOOTNOTES
 

DISCLOSURE
Author Martin Trope, D.D.S., D.M.D., is a paid consultant for Pentron Clinical Technologies, Wallingford, Conn. He also is a paid consultant for Resilon Research, North Branford, Conn., manufacturer of the material discussed in this article.


Dr. Fabricio Teixeira is an assistant professor, Endodontic Department at Piracicaba School of Dentistry, State University of Campinas, São Paulo, Brazil, and a visiting scholar, Department of Endodontics, School of Dentistry, University of North Carolina at Chapel Hill. Address reprint requests to Dr. Teixeira at the University of North Carolina at Chapel Hill, School of Dentistry, Department of Endodontics, 05 Brauer Hall, CB 7450, Chapel Hill, N.C. 27599-7450, e-mail "teixeirf{at}dentistry.unc.edu".


Dr. Erica Teixeira is a doctoral student, Materials Science Program, University of North Carolina at Chapel Hill.


Dr. Thompson is an associate professor, Department of Operative Dentistry and Department of Biomedical Engineering, University of North Carolina at Chapel Hill.


Dr. Trope is the J.B. Freedland Professor, Department of Endodontics, School of Dentistry, University of North Carolina at Chapel Hill.


   REFERENCES
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSION
 REFERENCES
 

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