- Email: [email protected]
Bond strength between root dentin and three glass-fiber post systems
Bond strength between root dentin and three glass-fiber post systems Mustafa Kalkan, DDS, PhD,a Aslihan Usumez, DDS, PhD,b A. Nilgun Ozturk, DDS, PhD,c Sema Belli, DDS, PhD,d and Gurcan Eskitascioglu, DDS, PhDe Faculty of Dentistry, Selcuk University, Konya, Turkey; Faculty of Dentistry, Gazi University, Ankara, Turkey Statement of problem. Glass-fiber posts were introduced for use after endodontic therapy instead of metal alloy and ceramic posts. There are several new types of glass-fiber post systems available, but little is known about how well these posts bond to the root surface. Purpose. The purpose of this in vitro study was to compare the bond strengths of 3 different types of glassfiber post systems—opaque, translucent, and electrical glass—in 3 different locations of prepared post spaces. Material and methods. Sixty human intact single-rooted extracted teeth were used. The root canals were prepared using a step-back technique and obturated with gutta-percha using lateral condensation. The roots were divided into 3 experimental groups and further divided into 2 subgroups according to testing time (n=10). Roots were restored with 1 of the following post systems according to the manufacturer’s instructions: opaque glass–fiber posts (Snowpost), translucent glass–fiber posts (FiberMaster), and electrical glass–fiber posts (Everstick). A self-etching primer (Clearfil Liner Bond) was applied to the walls of the post spaces, allowed to etch for 30 seconds, and gently air dried. A dual-polymerized bonding agent (Clearfil Liner Bond, Bond A and B) was then applied to the same walls. A dual-polymerizing resin luting agent (Panavia F) was mixed for 20 seconds and then placed in the post spaces using a lentulo spiral instrument. The roots were placed in light-protected cylinders; then the light source was placed directly on the flat cervical tooth surfaces and the cement was polymerized. Specimens were stored in light-proof boxes for 24 hours or 1 week after the polymerization procedure. Each root was cut horizontally, and six 1-mm-thick root segments (2 apical, 2 middle, and 2 cervical) were prepared.Using a push-out test, the bond strength between post and dentin was measured after 24 hours or 1 week using a universal testing machine. Statistical analysis was performed with 3-way ANOVA followed by independent t tests (a=.05) to detect differences between groups defined by the specific interacting variables. The different combinations of posts and luting material from the cervical segments were analyzed with SEM. Results. The 3-way ANOVA indicated that push-out test values varied significantly according to the post system used (opaque, electrical, and translucent) (P,.01); the root segments (cervical, middle, and apical) (P,.01), however, did not vary statistically according to the time of testing (24 hours and 1 week). Opaque and electrical glass– fiber posts showed higher bond strength values than translucent posts (P,.01). Push-out bond strength values of cervical segments were significantly higher than the middle and apical segments in translucent and electrical glass– fiber post groups (P,.01). In the opaque glass–fiber post group, there were no significant differences between cervical and middle segments. In SEM analysis, a distinct hybrid zone with long, numerous resin tags located between luting material and dentin was exhibited in all post systems. Conclusion. The opaque and electrical glass–fiber posts exhibited similar bond strengths, and translucent posts exhibited the lowest bond strength. The highest bond strength was observed in the cervical third of the post spaces for translucent and electrical glass–fiber post groups. (J Prosthet Dent 2006;96:41-6.)
CLINICAL IMPLICATIONS As the opaque and electrical glass–fiber post systems tested showed higher push-out bond strength values than translucent post systems, these may be preferred whenever there is a need for better retention of the post system, particularly for teeth with biomechanically compromised root canal preparation.
a
Private practice, Antalya, Turkey. Associate Professor, Department of Prosthodontics, Faculty of Dentistry, Selcuk University. c Associate Professor, Department of Prosthodontics, Faculty of Dentistry, Selcuk University. d Professor, Head, Department of Endodontics, Faculty of Dentistry, Selcuk University. e Professor, Department of Prosthodontics, Faculty of Dentistry, Gazi University. b
JULY 2006
E
ndodontically treated teeth are known to present a higher risk of biomechanical failure than vital teeth.1-3 Posts are generally indicated to restore missing tooth structure and pulpless teeth.3 The choice of an appropriate restoration for endodontically treated teeth is guided by strength and esthetics. Available prefabricated posts THE JOURNAL OF PROSTHETIC DENTISTRY 41
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were traditionally made of metal, and their use resulted in complex combinations of materials (dentin, metal posts, cements, and core materials) with different degrees of stiffness.4,5 Newer tooth-colored posts have improved the esthetics of teeth restored with posts and cores.6,7 Based on the current literature, placement of a post should be considered only when the remaining cervical tooth tissue can no longer provide adequate support and retention for a restoration.8,9 Although the most common complication in post-core–retained treatment is crown debonding,10 root fracture is still the complication that results in the greatest damage.9 When using posts, factors such as the length, design, and material of the post should be considered.3,8,9 Glass fiber–reinforced resin post systems were introduced in 1992.11 These posts are composed of unidirectional glass fibers embedded in a resin matrix. Matrix polymers are commonly epoxy polymers with a high degree of monomer conversion and a highly cross-linked structure.11 An advantage of glass fibers is that they distribute stress over a broad surface area, increasing the load threshold at which the post begins to show evidence of micro-fractures.12 Consequently, fiber-reinforced posts are reported to reduce the risk of tooth fractures and display higher survival rates than teeth restored with rigid zirconia posts.7 Glass-fiber posts can be made of different types of glass. Electrical glass, so termed because its chemical composition makes it an excellent electrical insulator, is made from a mixture of SiO2, CaO, B2O3, Al2O3, and some other oxides of alkali metals.13,14 It is the most commonly used glass type and is also the most economical glass fiber for composite resins, offering sufficient strength in most applications at a low cost.13 A choice must be made between different types of posts based on their light-transmitting capacities.12 Nontranslucent posts block light passage; therefore, light-polymerizing luting materials must be substituted with autopolymerizing resin composites. These materials are typically fluid and have a long polymerization time.12 They should be applied with a thin disposable metal tip to minimize the formation of air bubbles.12 Selecting an appropriate adhesive and luting procedure for bonding posts to root dentin is another challenge. Sealing is expected to be strong due to recent improvements in the sealing ability of adhesive resin luting agents.12,15 Moreover, various types of bonding systems can be used in combination with different luting resins.16 In a recent investigation, carbon fiber post and core foundations cemented with dentin bonding and resin luting agents showed less microleakage than those luted with glass-ionomer and zinc-phosphate cements.17 Resin luting agents may be polymerized through a chemical reaction, a light-polymerization process, or a combination of both mechanisms.16 Most 42
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current resin luting agents polymerize using a dual-polymerizing process that requires light exposure to initiate the reaction.16,18 However, it has been reported that light-polymerized composite resin generates more polymerization shrinkage stress and exhibits less flow than autopolymerized composite resin.19 The contraction stress produced may exceed 20 MPa.20 Bond strength between post and tooth has been measured through conventional tensile testing on external root dentin21 or on the post space surface with pullout22,23 and push-out methods.12,24 The latter has the benefit of being more clinically relevant.25 However, it has been suggested that a highly nonuniform stress may be developed at the adhesive interface when the push-out test is performed on the entire post26 or on thick root segments.24,25 Goracci et al27 compared a microtensile technique with a ‘‘micro’’ push-out test for the ability to accurately measure the bond strength of fiber posts luted inside post spaces. In measuring the bond strength of luted fiber posts, the authors concluded that a push-out test was more dependable than a microtensile technique; each prepared specimen provided a useful measurement, and data variability was small. With the microtensile technique, a high number of premature failures and a large spread for the data distribution occurred.27 There is little information in the literature on the bonding capability of esthetic endodontic fiber posts cemented with a resin luting agent. The purpose of this in vitro study was to compare the bond strengths of 3 adhesively luted fiber post systems—opaque glass–fiber post (OGFP), translucent glass–fiber post (TGFP), and electrical glass–fiber post (EGFP)—in 3 segments of teeth (cervical, middle, and apical third). Bond strength tests were performed after 24 hours and 1 week. The first research hypothesis was that bond strengths of TGFP and EGFP systems were higher than OGFP, and the second hypothesis was that bond strengths after 1 week were higher than after 24 hours.
MATERIAL AND METHODS To evaluate whether the type of post system and testing time influenced bond strength, 3 different post systems—OGFP: Snowpost (Carbotech, Ganges, France), TGFP: FiberMaster, (NTI, Kahla, Germany), and EGFP: EverStick (StickTech Ltd, Turku, Finland)— and 2 time periods (24 hours and 1 week) were studied. Sixty maxillary central incisors freshly extracted for periodontal reasons, with straight root canals, anatomically similar root segments, and fully developed apices, were selected. The teeth were cleaned of soft tissue and calculus and cut perpendicular to the long axis at the cementoenamel junction with a slow-speed diamond saw (Isomet; Buehler, Lake Bluff, Ill). To standardize root canal lengths for the experiment, the roots were cut VOLUME 96 NUMBER 1
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Fig. 1. Schematic view of preparation of specimen preparation for push-out test.
Table I. Mean push-out bond strength values (SDs) after 24 hours and 1 week (MPa)
Fiber Master Cervical Middle Apical Snowpost Cervical Middle Apical Everstick Cervical Middle Apical
24 hours
1 week
0.62 6 0.14 0.34 6 0.16 0.37 6 0.11
0.63 6 0.19 0.36 6 0.15 0.43 6 0.27
0.68 6 0.13 0.65 6 0.09 0.41 6 0.09
0.82 6 0.19 0.65 6 0.25 0.43 6 0.15
1.01 6 0.54 0.48 6 0.13 0.45 6 0.10
1.02 6 0.05 0.51 6 0.25 0.45 6 0.12
to a uniform length of 14 mm. The pulp tissue was removed with a barbed broach (Dentsply-Maillefer, Ballaigues, Switzerland). Canal patency was determined by passing a file (size 10 K-file; Dentsply-Maillefer) through the apical foramen. Canal working lengths were established 1.0 mm short of the apical foramina. A step-back technique was used for canal instrumentation. The same operator instrumented all root canals to the same size (#55 file; Dentsply-Maillefer). During instrumentation, canals were irrigated with 1 mL of 5.25% NaOCl. Upon completion of the instrumentation, the roots were divided into 3 groups of 20 each. Before obturation, the root canals were dried with paper points (Dentsply-Maillefer) and obturated with lateral condensation of gutta-percha (Dentsply-Maillefer) and a resin sealer (AH Plus; Dentsply DeTrey GmbH, Konstanz, Germany). The sealer was introduced into root canals using a lentulo spiral instrument (DentsplyMaillefer). Gutta-percha points were coated with the sealer and placed in the root canals to the working length. A finger spreader (Dentsply-Maillefer) was JULY 2006
Fig. 2. Push-out test device.
Table II. Three-way ANOVA
Post system Time of testing Root region Post system 3 Time of testing Post system 3 Root region Time of testing 3 Root region Post system 3 Time of testing 3 Root region
df
Mean square
F
P
2 1 2 2 4 2 4
233 .009 2.43 .001 .27 .002 .007
5.72 2.2 59.8 .27 6.53 .43 .18
.004 .140 ,.001 .762 ,.001 .650 .947
inserted into the root canals to a level approximately 1 mm short of the working length. Cervical root canal openings were then filled with a provisional restorative material (Cavit-G; 3M ESPE, Seefeld, Germany), and the gutta-percha filled root canals were placed in a humidor (100% relative humidity) for 1 week at 37°C. The most similar sizes available among the post systems were chosen and used in this study. All posts for all systems were 1.2 mm in diameter. Each was marked at a point 10 mm from its apical end. A line was drawn around the post at this point, and all posts were cut to a 10-mm length with a water-cooled diamond-fissure rotary cutting instrument (Komet-Brasseler GmbH, Lemgo, Germany). This procedure standardized the post lengths and established diameter similarity between posts with tapered designs. Gutta-percha was removed from the cervical aspect of the root canal with reamers (Peeso; Dentsply-Maillefer). The post spaces were all prepared to a depth of 10 mm with special preparation drills supplied from the manufacturer of the OGFP (Snowpost, Lot no: H 040; Carbotech). The post taper was the same for the OGFP and TGFP systems. The EGFP is an adjustable system that takes the shape of the post space. A self-etching primer (Clearfil Liner Bond; Kuraray, Osaka, Japan) was applied to the walls of the post spaces, 43
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Fig. 3. SEM view of interface between dentin (right side) and resin cement (left side) in cervical root segment of translucent post specimen (original magnification 31500), which was etched in preparation for examination.
Fig. 5. SEM view of interface between dentin (right side) and resin cement (left side) in cervical root segment of electrical post specimen (original magnification 31500), which was etched in preparation for examination.
allowed to etch for 30 seconds, and gently air dried. A dual-polymerized bonding agent (Clearfil Liner Bond, Bond A and B; Kuraray) was then applied to the same walls. A dual-polymerizing resin luting agent (Panavia F; Kuraray) was mixed for 20 seconds and placed in the post spaces using a lentulo spiral instrument (Dentsply-Maillefer). Posts were coated with cement and slowly seated by finger pressure. Excess cement was removed with an explorer. Cement was polymerized for 40 seconds with the same light-polymerizing unit (550 mW/cm2, Hilux 550; Hilux, Ankara, Turkey). Silicone (Speedex; Coltene/Whaledent Inc, Cuyahoga Falls, Ohio) molds were used as a supporting structure 44
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Fig. 4. SEM view of interface between dentin (left side) and resin cement (right side) in cervical root segment of opaque post specimen (original magnification 31500), which was etched in preparation for examination.
for the roots, and the light source was placed directly on the flat cervical tooth surfaces in all post groups. Specimens were stored in light-proof boxes after the polymerization procedure for 24 hours or 1 week. Each root (n=20) was cut horizontally with a slow-speed diamond saw (Buehler) to produce six 1-mm-thick segments (2 apical, 2 middle, and 2 cervical) (Fig. 1). The post was loaded with a 1-mm diameter cylindrical plunger. The plunger tip was sized and positioned to touch only the post, without stressing the surrounding post space walls. The load was applied on the apical aspect of the root slice and in an apical-coronal direction, so as to push the post toward the larger part of the root slice, thus avoiding any limitation to the post movement owing to the post space taper (Fig. 2). Loading was performed on a testing machine (Model 500; Testometric, Lancashire, UK) at a cross-head speed of 1.0 mm/min until bond failure occurred (Fig. 2). The force (N) required to debond the post from the dentin disc was recorded for all posts. To express the bond strength in MPa, the load at failure (N) was divided by the area of the bonded interface, which was calculated with the following formula: A ¼ 2prh
where p is the constant 3.14, r is the post radius, and h is the thickness of the slice in mm. The data were statistically analyzed (SPSS/PC 10.0; SPSS Inc, Chicago, Ill) using a 3-way analysis of variance (ANOVA) (post type, testing time, and root segments), making pairwise comparisons among groups (a=.05). Independent t tests were used to detect differences between groups defined by the specific interacting variables. The different combinations of posts and luting material from the cervical segments were analyzed using VOLUME 96 NUMBER 1
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scanning electron microscope (SEM) (Testometric) observation. After endodontic treatment, application of the adhesive system and post were performed following the manufacturers’ instructions. One specimen was prepared from each group for SEM analysis. The specimens were then horizontally cut with a low-speed diamond saw (Buehler). Specimens from the cervical segments were polished with 240-, 400-, and 600-grit silicon carbide abrasive paper (Atlas Zimpara, Istanbul, Turkey). The surface of the root slice was etched with phosphoric acid (3M Scotchbond Etching Gel; 3M ESPE, St Paul, Minn) for 10 seconds to remove the organic and mineral components of the dentin so that the hybrid layer and resin tag formation could be better analyzed, and then washed and gently air dried for 3 seconds. Specimens were sputter coated (SCD 005; BAL-TEC AG, ˚ gold-palladium alloy Balzers, Germany) with 200 A (Foil Target AU; BAL-TEC AG). Each specimen was examined by SEM (435 VP; Leo SEM Products, Cambridge, UK) at a 15-kV accelerating voltage. The images were achieved with a computerized program (Analysis 2.1; Olympus Soft Imaging Solutions GmbH, Munster, Germany) at 31500 magnification.
RESULTS Push-out test results are shown in Table I. The 3-way ANOVA indicated that push-out test values varied statistically according to root segment (P,.01), but that the post system used (P,.01) did not vary statistically according to time of testing (Table II). The statistical analysis demonstrated a significant interaction between the post system and root region. The OGFP and EGFP groups showed higher bond strength values than the TGFP group (P,.01). Push-out bond strength values of cervical segments were significantly higher than the middle and apical segments in the EGFP and TGFP groups (P,.01). In the OGFP group there were not significant differences between cervical and middle segments (P=.73). In SEM analysis, the distribution of the luting material in the canal and the interface between luting material and post were also evaluated. All post systems exhibited a distinct hybrid zone with long, numerous resin tags between luting material and dentin (Figs. 3 through 5).
DISCUSSION In this study, the push-out bond strengths of 3 different post systems after light-initiated polymerization were measured. The results obtained do not support the first research hypothesis that the bond strengths of TGFP and EGFP systems are higher than OGFP, and also do not support the second hypothesis that bond strengths after 1 week are higher than after 24 hours. The OGFP and EGFP groups exhibited higher bond strengths than the TGFP group. Push-out bond JULY 2006
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strength values of the cervical segments were significantly higher than the middle and apical segments in EGFP and TGFP groups, but in the OGFP group there were not significant differences between the cervical and middle segments. These results are in agreement with those of Le Bell et al,14 who evaluated the depth of polymerization in post spaces. Specimens for testing were prepared using human teeth. The manufacturer’s instructions were followed carefully when posts were cemented to ensure that in vitro procedures were the same as those used clinically. In the current study, test specimens were not completely restored, and neither thermal cycling nor mechanical stressing was applied. These factors may limit the direct application of study results to clinical situations. Factors possibly interfering with the development of high bond strengths to root dentin are the nonuniform adaptation of the bonding material or its incomplete polymerization, both related to the difficult access of post space walls during handling.12 These factors may account for the lower bond strengths achieved by the adhesive cements in the middle and apical root segments. A resin luting agent may create polymerization shrinkage stresses within the post space.16 These shrinkage stresses contribute to what has been defined as the C factor, the ratio of bonded to unbonded surface areas in root canal dentin.19,20 It has been shown that a C factor in post spaces may be as high as 200.16 Especially with light-polymerizing materials, the polymerization stress generated in the geometrically adverse configuration of a post space may be so intense that resin composites may detach from the dentin walls, thus creating interfacial gaps.12 To attain proper polymerization in such situations, maximizing strength and adhesion of the cement, a chemically activated component of a dual-catalyst system should be effective.18 To reach maximum physical properties of luting cements between a post and dentin, the conversion rate should be as high as possible.12 Le Bell et al14 investigated the possibility of polymerizing glass fiber– reinforced resin composite material (FRC) into a post space by determining the depth of light-initiated polymerization of the FRC. The authors concluded that in the longest cylinders, FRC showed a slightly higher degree of conversion compared to resin only, and added that this might be due to the fibers’ ability to conduct light. In the current study, 3 different types of fiber systems (opaque, translucent, and electrical glass) were used, and opaque and electrical glass–fiber systems exhibited higher bond strength values than the translucent post system.
CONCLUSION Within the limitations of this study, the following conclusions were drawn: 45
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1. Push-out test values differed significantly according to the post system. The EGFP (Everstick) and OGFP (Snowpost) showed higher bond strength values than the TGFP (FiberMaster). 2. Push-out bond strength values of the cervical segments were significantly higher than the middle and apical segments in the EGFP and TGFP groups, but in the OGFP group, there were no significant differences between cervical and middle segments. 3. There were no significant differences in push-out test values among testing times (24 hours and 1 week) for all groups.
REFERENCES 1. Caputo AA, Standlee JP. Biomechanics in clinical dentistry. Chicago: Quintessence; 1987. p. 185-203. 2. Caputo AA, Standlee JP. Pins and posts—why, when and how. Dent Clin North Am 1976;20:299-311. 3. Sorensen JA, Martinoff JT. Intracoronal reinforcement and coronal coverage: a study of endodontically treated teeth. J Prosthet Dent 1984;51: 780-4. 4. Akkayan B, Gu¨lmez T. Resistance to fracture of endodontically treated teeth restored with different post systems. J Prosthet Dent 2002;87:431-7. 5. Fredriksson M, Astback J, Pamenius M, Arvidson K. A retrospective study of 236 patients with teeth restored by carbon fiber-reinforced epoxy resin posts. J Prosthet Dent 1998;80:151-7. 6. Sidoli GE, King PA, Setchell DJ. An in vitro evaluation of a carbon fiber based post and core system. J Prosthet Dent 1997;78:5-9. 7. Manocci F, Ferrari M, Watson TF. Intermittent loading of teeth restored using quartz fiber, carbon-quartz fiber, and zirconium dioxide ceramic root canal posts. J Adhes Dent 1999;1:153-8. 8. Hunter AJ, Feiglin B, Williams JF. Effects of post placement on endodontically treated teeth. J Prosthet Dent 1989;62:166-72. 9. Sorensen JA, Martinoff JT. Clinically significant factors in dowel design. J Prosthet Dent 1984;52:28-35. 10. Torbjorner A, Karlsson S, Odman PA. Survival rate and failure characteristics for two post designs. J Prosthet Dent 1995;73:439-44. 11. Goldberg AJ, Burstone CJ. The use of continuous fiber reinforcement in dentistry. Dent Mater 1992;8:197-202. 12. Pest LB, Cavalli G, Bertani P, Gagliani M. Adhesive post-endodontic restorations with fiber posts: push-out tests and SEM observations. Dent Mater 2002;18:596-602. 13. Murphy J. Reinforced plastics handbook. St Louis: Elsevier; 1998. p. 63-106. 14. Le Bell AM, Tanner J, Lassila LV, Kangasniemi I, Vallittu PK. Depth of light-initiated polymerization of glass fiber–reinforced composite in a simulated root canal. Int J Prosthodont 2003;16:403-8.
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15. Usumez A, Cobankara FK, Ozturk N, Eskitascioglu G, Belli S. Microleakage of endodontically treated teeth with different post systems. J Prosthet Dent 2004;92:163-9. 16. Bouillaguet S, Troesch S, Wataha JC, Krejci I, Meyer JM, Pashley DH. Microtensile bond strength between adhesive cements and root canal dentin. Dent Mater 2003;19:199-205. 17. Bachicha WS, DiFiore PM, Miller DA, Lauthenschlager EP, Pashley DH. Microleakage of endodontically treated teeth restored with posts. J Endodon 1998;24:703-8. 18. Peutzfeldt A. Dual-cure resin cements: in vitro wear and effect of quantity of remaining double bonds, filler volume, and light curing. Acta Odontol Scand 1995;53:29-34. 19. Feilzer AJ, De Gee AJ, Davidson CL. Setting stress in composite resin in relation to configuration of the restoration. J Dent Res 1987;66:1636-9. 20. Alster D, Feilzer AJ, de Gee AJ, Davidson CL. Polymerization contraction stress in thin resin composite layers as a function of layer thickness. Dent Mater 1997;13:146-50. 21. Nikaido T, Takano Y, Sasafuchi Y, Burrow MF, Tagami J. Bond strengths to endodontically treated teeth. Am J Dent 1999;12:177-80. 22. Drummond JL. In vitro evaluation of endodontic posts. Am J Dent 2000; 13:5-8. 23. Mitghell CA, Orr JF, Connor KN, Magill JPG, Maguire GR. Comparative study of four glass ionomer luting cements during post pull-out tests. Dent Mater 1994;10:88-9. 24. Patioerno JM, Rueggeberg FA, Anderson RW, Weller RN, Pashley DH. Push-out strength and SEM evaluation of resin composite bonded to internal cervical dentin. Endod Dent Traumatol 1996;12:227-36. 25. Sudsangiam S, Van Noort R. Do dentin bond strength tests serve a useful purpose? J Adhes Dent 1999;1:57-67. 26. Gallo JR, Miller T, Xu X, Burgess JO. In vitro evaluation of the retention of composite fiber and stainless steel posts. J Prosthodont 2002;11:25-9. 27. Goracci C, Tavares AU, Fabianelli A, Monticelli F, Raffaelli O, Cardoso PC, et al. The adhesion between fiber posts and root canal walls: comparison between microtensile and push-out bond strength measurements. Eur J Oral Sci 2004;112:353-61. Reprint requests to: DR ASLIHAN USUMEZ SELCUK UNIVERSITY DISHEKIMLIGI FAKULTESI KAMPUS, KONYA TURKEY FAX: 90-332-241-0062 E-MAIL: [email protected] 0022-3913/$32.00 Copyright Ó 2006 by The Editorial Council of The Journal of Prosthetic Dentistry.
doi:10.1016/j.prosdent.2006.05.005
VOLUME 96 NUMBER 1
CLINICAL IMPLICATIONS As the opaque and electrical glass–fiber post systems tested showed higher push-out bond strength values than translucent post systems, these may be preferred whenever there is a need for better retention of the post system, particularly for teeth with biomechanically compromised root canal preparation.
a
Private practice, Antalya, Turkey. Associate Professor, Department of Prosthodontics, Faculty of Dentistry, Selcuk University. c Associate Professor, Department of Prosthodontics, Faculty of Dentistry, Selcuk University. d Professor, Head, Department of Endodontics, Faculty of Dentistry, Selcuk University. e Professor, Department of Prosthodontics, Faculty of Dentistry, Gazi University. b
JULY 2006
E
ndodontically treated teeth are known to present a higher risk of biomechanical failure than vital teeth.1-3 Posts are generally indicated to restore missing tooth structure and pulpless teeth.3 The choice of an appropriate restoration for endodontically treated teeth is guided by strength and esthetics. Available prefabricated posts THE JOURNAL OF PROSTHETIC DENTISTRY 41
THE JOURNAL OF PROSTHETIC DENTISTRY
were traditionally made of metal, and their use resulted in complex combinations of materials (dentin, metal posts, cements, and core materials) with different degrees of stiffness.4,5 Newer tooth-colored posts have improved the esthetics of teeth restored with posts and cores.6,7 Based on the current literature, placement of a post should be considered only when the remaining cervical tooth tissue can no longer provide adequate support and retention for a restoration.8,9 Although the most common complication in post-core–retained treatment is crown debonding,10 root fracture is still the complication that results in the greatest damage.9 When using posts, factors such as the length, design, and material of the post should be considered.3,8,9 Glass fiber–reinforced resin post systems were introduced in 1992.11 These posts are composed of unidirectional glass fibers embedded in a resin matrix. Matrix polymers are commonly epoxy polymers with a high degree of monomer conversion and a highly cross-linked structure.11 An advantage of glass fibers is that they distribute stress over a broad surface area, increasing the load threshold at which the post begins to show evidence of micro-fractures.12 Consequently, fiber-reinforced posts are reported to reduce the risk of tooth fractures and display higher survival rates than teeth restored with rigid zirconia posts.7 Glass-fiber posts can be made of different types of glass. Electrical glass, so termed because its chemical composition makes it an excellent electrical insulator, is made from a mixture of SiO2, CaO, B2O3, Al2O3, and some other oxides of alkali metals.13,14 It is the most commonly used glass type and is also the most economical glass fiber for composite resins, offering sufficient strength in most applications at a low cost.13 A choice must be made between different types of posts based on their light-transmitting capacities.12 Nontranslucent posts block light passage; therefore, light-polymerizing luting materials must be substituted with autopolymerizing resin composites. These materials are typically fluid and have a long polymerization time.12 They should be applied with a thin disposable metal tip to minimize the formation of air bubbles.12 Selecting an appropriate adhesive and luting procedure for bonding posts to root dentin is another challenge. Sealing is expected to be strong due to recent improvements in the sealing ability of adhesive resin luting agents.12,15 Moreover, various types of bonding systems can be used in combination with different luting resins.16 In a recent investigation, carbon fiber post and core foundations cemented with dentin bonding and resin luting agents showed less microleakage than those luted with glass-ionomer and zinc-phosphate cements.17 Resin luting agents may be polymerized through a chemical reaction, a light-polymerization process, or a combination of both mechanisms.16 Most 42
KALKAN ET AL
current resin luting agents polymerize using a dual-polymerizing process that requires light exposure to initiate the reaction.16,18 However, it has been reported that light-polymerized composite resin generates more polymerization shrinkage stress and exhibits less flow than autopolymerized composite resin.19 The contraction stress produced may exceed 20 MPa.20 Bond strength between post and tooth has been measured through conventional tensile testing on external root dentin21 or on the post space surface with pullout22,23 and push-out methods.12,24 The latter has the benefit of being more clinically relevant.25 However, it has been suggested that a highly nonuniform stress may be developed at the adhesive interface when the push-out test is performed on the entire post26 or on thick root segments.24,25 Goracci et al27 compared a microtensile technique with a ‘‘micro’’ push-out test for the ability to accurately measure the bond strength of fiber posts luted inside post spaces. In measuring the bond strength of luted fiber posts, the authors concluded that a push-out test was more dependable than a microtensile technique; each prepared specimen provided a useful measurement, and data variability was small. With the microtensile technique, a high number of premature failures and a large spread for the data distribution occurred.27 There is little information in the literature on the bonding capability of esthetic endodontic fiber posts cemented with a resin luting agent. The purpose of this in vitro study was to compare the bond strengths of 3 adhesively luted fiber post systems—opaque glass–fiber post (OGFP), translucent glass–fiber post (TGFP), and electrical glass–fiber post (EGFP)—in 3 segments of teeth (cervical, middle, and apical third). Bond strength tests were performed after 24 hours and 1 week. The first research hypothesis was that bond strengths of TGFP and EGFP systems were higher than OGFP, and the second hypothesis was that bond strengths after 1 week were higher than after 24 hours.
MATERIAL AND METHODS To evaluate whether the type of post system and testing time influenced bond strength, 3 different post systems—OGFP: Snowpost (Carbotech, Ganges, France), TGFP: FiberMaster, (NTI, Kahla, Germany), and EGFP: EverStick (StickTech Ltd, Turku, Finland)— and 2 time periods (24 hours and 1 week) were studied. Sixty maxillary central incisors freshly extracted for periodontal reasons, with straight root canals, anatomically similar root segments, and fully developed apices, were selected. The teeth were cleaned of soft tissue and calculus and cut perpendicular to the long axis at the cementoenamel junction with a slow-speed diamond saw (Isomet; Buehler, Lake Bluff, Ill). To standardize root canal lengths for the experiment, the roots were cut VOLUME 96 NUMBER 1
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Fig. 1. Schematic view of preparation of specimen preparation for push-out test.
Table I. Mean push-out bond strength values (SDs) after 24 hours and 1 week (MPa)
Fiber Master Cervical Middle Apical Snowpost Cervical Middle Apical Everstick Cervical Middle Apical
24 hours
1 week
0.62 6 0.14 0.34 6 0.16 0.37 6 0.11
0.63 6 0.19 0.36 6 0.15 0.43 6 0.27
0.68 6 0.13 0.65 6 0.09 0.41 6 0.09
0.82 6 0.19 0.65 6 0.25 0.43 6 0.15
1.01 6 0.54 0.48 6 0.13 0.45 6 0.10
1.02 6 0.05 0.51 6 0.25 0.45 6 0.12
to a uniform length of 14 mm. The pulp tissue was removed with a barbed broach (Dentsply-Maillefer, Ballaigues, Switzerland). Canal patency was determined by passing a file (size 10 K-file; Dentsply-Maillefer) through the apical foramen. Canal working lengths were established 1.0 mm short of the apical foramina. A step-back technique was used for canal instrumentation. The same operator instrumented all root canals to the same size (#55 file; Dentsply-Maillefer). During instrumentation, canals were irrigated with 1 mL of 5.25% NaOCl. Upon completion of the instrumentation, the roots were divided into 3 groups of 20 each. Before obturation, the root canals were dried with paper points (Dentsply-Maillefer) and obturated with lateral condensation of gutta-percha (Dentsply-Maillefer) and a resin sealer (AH Plus; Dentsply DeTrey GmbH, Konstanz, Germany). The sealer was introduced into root canals using a lentulo spiral instrument (DentsplyMaillefer). Gutta-percha points were coated with the sealer and placed in the root canals to the working length. A finger spreader (Dentsply-Maillefer) was JULY 2006
Fig. 2. Push-out test device.
Table II. Three-way ANOVA
Post system Time of testing Root region Post system 3 Time of testing Post system 3 Root region Time of testing 3 Root region Post system 3 Time of testing 3 Root region
df
Mean square
F
P
2 1 2 2 4 2 4
233 .009 2.43 .001 .27 .002 .007
5.72 2.2 59.8 .27 6.53 .43 .18
.004 .140 ,.001 .762 ,.001 .650 .947
inserted into the root canals to a level approximately 1 mm short of the working length. Cervical root canal openings were then filled with a provisional restorative material (Cavit-G; 3M ESPE, Seefeld, Germany), and the gutta-percha filled root canals were placed in a humidor (100% relative humidity) for 1 week at 37°C. The most similar sizes available among the post systems were chosen and used in this study. All posts for all systems were 1.2 mm in diameter. Each was marked at a point 10 mm from its apical end. A line was drawn around the post at this point, and all posts were cut to a 10-mm length with a water-cooled diamond-fissure rotary cutting instrument (Komet-Brasseler GmbH, Lemgo, Germany). This procedure standardized the post lengths and established diameter similarity between posts with tapered designs. Gutta-percha was removed from the cervical aspect of the root canal with reamers (Peeso; Dentsply-Maillefer). The post spaces were all prepared to a depth of 10 mm with special preparation drills supplied from the manufacturer of the OGFP (Snowpost, Lot no: H 040; Carbotech). The post taper was the same for the OGFP and TGFP systems. The EGFP is an adjustable system that takes the shape of the post space. A self-etching primer (Clearfil Liner Bond; Kuraray, Osaka, Japan) was applied to the walls of the post spaces, 43
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Fig. 3. SEM view of interface between dentin (right side) and resin cement (left side) in cervical root segment of translucent post specimen (original magnification 31500), which was etched in preparation for examination.
Fig. 5. SEM view of interface between dentin (right side) and resin cement (left side) in cervical root segment of electrical post specimen (original magnification 31500), which was etched in preparation for examination.
allowed to etch for 30 seconds, and gently air dried. A dual-polymerized bonding agent (Clearfil Liner Bond, Bond A and B; Kuraray) was then applied to the same walls. A dual-polymerizing resin luting agent (Panavia F; Kuraray) was mixed for 20 seconds and placed in the post spaces using a lentulo spiral instrument (Dentsply-Maillefer). Posts were coated with cement and slowly seated by finger pressure. Excess cement was removed with an explorer. Cement was polymerized for 40 seconds with the same light-polymerizing unit (550 mW/cm2, Hilux 550; Hilux, Ankara, Turkey). Silicone (Speedex; Coltene/Whaledent Inc, Cuyahoga Falls, Ohio) molds were used as a supporting structure 44
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Fig. 4. SEM view of interface between dentin (left side) and resin cement (right side) in cervical root segment of opaque post specimen (original magnification 31500), which was etched in preparation for examination.
for the roots, and the light source was placed directly on the flat cervical tooth surfaces in all post groups. Specimens were stored in light-proof boxes after the polymerization procedure for 24 hours or 1 week. Each root (n=20) was cut horizontally with a slow-speed diamond saw (Buehler) to produce six 1-mm-thick segments (2 apical, 2 middle, and 2 cervical) (Fig. 1). The post was loaded with a 1-mm diameter cylindrical plunger. The plunger tip was sized and positioned to touch only the post, without stressing the surrounding post space walls. The load was applied on the apical aspect of the root slice and in an apical-coronal direction, so as to push the post toward the larger part of the root slice, thus avoiding any limitation to the post movement owing to the post space taper (Fig. 2). Loading was performed on a testing machine (Model 500; Testometric, Lancashire, UK) at a cross-head speed of 1.0 mm/min until bond failure occurred (Fig. 2). The force (N) required to debond the post from the dentin disc was recorded for all posts. To express the bond strength in MPa, the load at failure (N) was divided by the area of the bonded interface, which was calculated with the following formula: A ¼ 2prh
where p is the constant 3.14, r is the post radius, and h is the thickness of the slice in mm. The data were statistically analyzed (SPSS/PC 10.0; SPSS Inc, Chicago, Ill) using a 3-way analysis of variance (ANOVA) (post type, testing time, and root segments), making pairwise comparisons among groups (a=.05). Independent t tests were used to detect differences between groups defined by the specific interacting variables. The different combinations of posts and luting material from the cervical segments were analyzed using VOLUME 96 NUMBER 1
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scanning electron microscope (SEM) (Testometric) observation. After endodontic treatment, application of the adhesive system and post were performed following the manufacturers’ instructions. One specimen was prepared from each group for SEM analysis. The specimens were then horizontally cut with a low-speed diamond saw (Buehler). Specimens from the cervical segments were polished with 240-, 400-, and 600-grit silicon carbide abrasive paper (Atlas Zimpara, Istanbul, Turkey). The surface of the root slice was etched with phosphoric acid (3M Scotchbond Etching Gel; 3M ESPE, St Paul, Minn) for 10 seconds to remove the organic and mineral components of the dentin so that the hybrid layer and resin tag formation could be better analyzed, and then washed and gently air dried for 3 seconds. Specimens were sputter coated (SCD 005; BAL-TEC AG, ˚ gold-palladium alloy Balzers, Germany) with 200 A (Foil Target AU; BAL-TEC AG). Each specimen was examined by SEM (435 VP; Leo SEM Products, Cambridge, UK) at a 15-kV accelerating voltage. The images were achieved with a computerized program (Analysis 2.1; Olympus Soft Imaging Solutions GmbH, Munster, Germany) at 31500 magnification.
RESULTS Push-out test results are shown in Table I. The 3-way ANOVA indicated that push-out test values varied statistically according to root segment (P,.01), but that the post system used (P,.01) did not vary statistically according to time of testing (Table II). The statistical analysis demonstrated a significant interaction between the post system and root region. The OGFP and EGFP groups showed higher bond strength values than the TGFP group (P,.01). Push-out bond strength values of cervical segments were significantly higher than the middle and apical segments in the EGFP and TGFP groups (P,.01). In the OGFP group there were not significant differences between cervical and middle segments (P=.73). In SEM analysis, the distribution of the luting material in the canal and the interface between luting material and post were also evaluated. All post systems exhibited a distinct hybrid zone with long, numerous resin tags between luting material and dentin (Figs. 3 through 5).
DISCUSSION In this study, the push-out bond strengths of 3 different post systems after light-initiated polymerization were measured. The results obtained do not support the first research hypothesis that the bond strengths of TGFP and EGFP systems are higher than OGFP, and also do not support the second hypothesis that bond strengths after 1 week are higher than after 24 hours. The OGFP and EGFP groups exhibited higher bond strengths than the TGFP group. Push-out bond JULY 2006
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strength values of the cervical segments were significantly higher than the middle and apical segments in EGFP and TGFP groups, but in the OGFP group there were not significant differences between the cervical and middle segments. These results are in agreement with those of Le Bell et al,14 who evaluated the depth of polymerization in post spaces. Specimens for testing were prepared using human teeth. The manufacturer’s instructions were followed carefully when posts were cemented to ensure that in vitro procedures were the same as those used clinically. In the current study, test specimens were not completely restored, and neither thermal cycling nor mechanical stressing was applied. These factors may limit the direct application of study results to clinical situations. Factors possibly interfering with the development of high bond strengths to root dentin are the nonuniform adaptation of the bonding material or its incomplete polymerization, both related to the difficult access of post space walls during handling.12 These factors may account for the lower bond strengths achieved by the adhesive cements in the middle and apical root segments. A resin luting agent may create polymerization shrinkage stresses within the post space.16 These shrinkage stresses contribute to what has been defined as the C factor, the ratio of bonded to unbonded surface areas in root canal dentin.19,20 It has been shown that a C factor in post spaces may be as high as 200.16 Especially with light-polymerizing materials, the polymerization stress generated in the geometrically adverse configuration of a post space may be so intense that resin composites may detach from the dentin walls, thus creating interfacial gaps.12 To attain proper polymerization in such situations, maximizing strength and adhesion of the cement, a chemically activated component of a dual-catalyst system should be effective.18 To reach maximum physical properties of luting cements between a post and dentin, the conversion rate should be as high as possible.12 Le Bell et al14 investigated the possibility of polymerizing glass fiber– reinforced resin composite material (FRC) into a post space by determining the depth of light-initiated polymerization of the FRC. The authors concluded that in the longest cylinders, FRC showed a slightly higher degree of conversion compared to resin only, and added that this might be due to the fibers’ ability to conduct light. In the current study, 3 different types of fiber systems (opaque, translucent, and electrical glass) were used, and opaque and electrical glass–fiber systems exhibited higher bond strength values than the translucent post system.
CONCLUSION Within the limitations of this study, the following conclusions were drawn: 45
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1. Push-out test values differed significantly according to the post system. The EGFP (Everstick) and OGFP (Snowpost) showed higher bond strength values than the TGFP (FiberMaster). 2. Push-out bond strength values of the cervical segments were significantly higher than the middle and apical segments in the EGFP and TGFP groups, but in the OGFP group, there were no significant differences between cervical and middle segments. 3. There were no significant differences in push-out test values among testing times (24 hours and 1 week) for all groups.
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15. Usumez A, Cobankara FK, Ozturk N, Eskitascioglu G, Belli S. Microleakage of endodontically treated teeth with different post systems. J Prosthet Dent 2004;92:163-9. 16. Bouillaguet S, Troesch S, Wataha JC, Krejci I, Meyer JM, Pashley DH. Microtensile bond strength between adhesive cements and root canal dentin. Dent Mater 2003;19:199-205. 17. Bachicha WS, DiFiore PM, Miller DA, Lauthenschlager EP, Pashley DH. Microleakage of endodontically treated teeth restored with posts. J Endodon 1998;24:703-8. 18. Peutzfeldt A. Dual-cure resin cements: in vitro wear and effect of quantity of remaining double bonds, filler volume, and light curing. Acta Odontol Scand 1995;53:29-34. 19. Feilzer AJ, De Gee AJ, Davidson CL. Setting stress in composite resin in relation to configuration of the restoration. J Dent Res 1987;66:1636-9. 20. Alster D, Feilzer AJ, de Gee AJ, Davidson CL. Polymerization contraction stress in thin resin composite layers as a function of layer thickness. Dent Mater 1997;13:146-50. 21. Nikaido T, Takano Y, Sasafuchi Y, Burrow MF, Tagami J. Bond strengths to endodontically treated teeth. Am J Dent 1999;12:177-80. 22. Drummond JL. In vitro evaluation of endodontic posts. Am J Dent 2000; 13:5-8. 23. Mitghell CA, Orr JF, Connor KN, Magill JPG, Maguire GR. Comparative study of four glass ionomer luting cements during post pull-out tests. Dent Mater 1994;10:88-9. 24. Patioerno JM, Rueggeberg FA, Anderson RW, Weller RN, Pashley DH. Push-out strength and SEM evaluation of resin composite bonded to internal cervical dentin. Endod Dent Traumatol 1996;12:227-36. 25. Sudsangiam S, Van Noort R. Do dentin bond strength tests serve a useful purpose? J Adhes Dent 1999;1:57-67. 26. Gallo JR, Miller T, Xu X, Burgess JO. In vitro evaluation of the retention of composite fiber and stainless steel posts. J Prosthodont 2002;11:25-9. 27. Goracci C, Tavares AU, Fabianelli A, Monticelli F, Raffaelli O, Cardoso PC, et al. The adhesion between fiber posts and root canal walls: comparison between microtensile and push-out bond strength measurements. Eur J Oral Sci 2004;112:353-61. Reprint requests to: DR ASLIHAN USUMEZ SELCUK UNIVERSITY DISHEKIMLIGI FAKULTESI KAMPUS, KONYA TURKEY FAX: 90-332-241-0062 E-MAIL: [email protected] 0022-3913/$32.00 Copyright Ó 2006 by The Editorial Council of The Journal of Prosthetic Dentistry.
doi:10.1016/j.prosdent.2006.05.005
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