The Effect of Glass Fiber-Reinforced Epoxy Resin Dowel Diameter on the Fracture Resistance of Endodontically Treated Teeth Kuan Chuan Tey, BDS, MDSc & Joo Loon Lui, BDS, MSc, FADM Department of Conservative Dentistry, University of Malaya, Kuala Lumpur, Malaysia

Keywords Glass fiber dowel; diameter; fracture resistance; endodontically treated teeth; reinforcing resin composite; dowel and core. Correspondence Kuan Chuan Tey, University of Malaya Department of Conservative Dentistry, Jalan Lembah Pantai, Kuala Lumpur 50603, Malaysia. E-mail: [email protected] This study was carried out with financial support from the University of Malaya Postgraduate Research Grant (PPP) Account No. PS380/2010A. The authors deny any conflicts of interest. Accepted September 24, 2013 doi: 10.1111/jopr.12146

Abstract Purpose: To determine the effect of glass fiber-reinforced epoxy resin (FRC) dowels of different diameters on the failure load of endodontically treated teeth with different remaining dentine and reinforcing resin composite (RRC) thicknesses and the mode of failure in each group. Materials and Methods: Fifty extracted intact human maxillary central incisors were decoronated 2 mm incisal to the buccal cementoenamel junction and endodontically treated. The teeth were randomly assigned to one of five groups (n = 10): group B, dowel space prepared with size 0 dowel drill/size 0 FRC dowel/no RRC; group W, size 1 dowel space/size 1 FRC dowel/no RRC; group R, size 3 dowel space/size 3 FRC dowel/no RRC; group WR, size 3 dowel space/size 1 FRC dowel/RRC; group BR, size 3 dowel space/size 0 FRC dowel/RRC. Ferrules of 2 and 0.5 mm were prepared at the facio-lingual and proximal margin respectively. All specimens were restored with a Ni-Cr crown, thermocycled and loaded at 135° from the long axis in a universal testing machine at a 0.5 mm/min crosshead speed until fracture. Data were analyzed using ANOVA followed by post hoc comparisons (Bonferroni) with α = 0.05. Results: Mean failure loads (N) for groups B, W, R, WR, and BR were as follows: 1406 (SD = 376), 1259 (379), 1085 (528), 959 (200), and 816 (298). Significant differences were found between groups B and BR. Group B had the highest favorable failure mode. Conclusion: Within the limitations of this study, the use of a smaller FRC dowel and RRC is recommended rather than enlargement of dowel spaces to accurately fit larger FRC dowels, as the enlargement of dowel space may increase the risk of unfavorable failure.

Endodontically treated teeth are structurally compromised. Whether because of decay, previous restoration, fractures, or wear of sound dentin, these teeth require careful and immediate attention in reconstruction to ensure their maintenance as functioning and esthetic members of the dental arch. Due to their natural translucency, fiber dowels can now be used to achieve the esthetic demands of full ceramic restorations in the anterior region. With an elastic property similar to dentin,1 fiber dowels can significantly reduce the risk of root fractures compared with metal dowels.2-5 Another added advantage of the use of a fiber dowel is the purported ease of removal in the case of endodontic retreatment.2 Endodontically treated anterior teeth restored with fiber dowels exhibited higher failure loads than teeth restored with zirconia and titanium dowels.6 Fracture patterns favoring retreatment7,8 have been observed in restored teeth. Hence, fiber dowels are now preferred to restore endodontically treated anterior teeth.6

A dowel system should satisfy the requirements of both the tooth and the restoration. Dowels cemented into the canal space provide core retention and should not be used with the intention of reinforcing the tooth.9 Hence, minimum dowel space should be prepared consistent with the core retention. Dowels and cores significantly reduce the fracture resistance of the teeth and should be used only to secure retention and resistance form for full coverage crowns. Teeth with a larger diameter cast dowel have a reduced fracture resistance compared to teeth with a smaller diameter.10 Lloyd and Palik reviewed the literature regarding dowel diameter and identified three distinct philosophies of dowel space preparation. They concluded that a combination of the one-third and 1.0 mm minimal philosophies yielded a practical guideline for dowel space preparation.11 One of the disadvantages of a fiber dowel is that it is prefabricated and only available in manufacturer-predetermined sizes, which do not fit the entire canal. In daily clinical practice, the

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clinician needs to decide on the diameter of the dowel to be used, either to use a larger diameter dowel and to enlarge the dowel space with a corresponding drill, which will result in an accurate fitting of dowel to the dowel space; or to use a smaller diameter but loose fitting dowel to the dowel space, necessitating filling the void with luting cement or reinforcing resin composites. Therefore, the purpose of this study was to determine the effect of glass fiber-reinforced epoxy resin dowels of different diameters on the failure load of endodontically treated teeth with (i) different remaining dentine and (ii) different thicknesses of reinforcing resin composite. The mode of fracture in each group was also determined. Glass fiber-reinforced epoxy resin dowel and resin composite have moduli of elasticity close to dentin and are bonded adhesively into the canal space, creating a monoblock within the root/dowel/core assembly. When all of the components have a similar elastic modulus, a more uniform stress distribution throughout the restored tooth with lowered interfacial stress and chance for failure are observed.12 Therefore, the null hypothesis (Ho) states that there is no difference in the failure load and mode of fracture regardless of (1) the amount of remaining dentine, (2) fiber dowel diameter, and (3) the thickness of reinforcing resin composite in the root.

Materials and methods Ninety-seven recently extracted, intact human maxillary central incisors were collected and were stored in 0.9% physiologic normal saline. Prior for use, the teeth were disinfected in 0.5% chloramine T trihydrate solution for 7 days. The teeth were placed in normal saline and stored in a refrigerator at 4°C when not in use. Fifty intact maxillary central incisor teeth with comparable coronal and root dimensions were selected from the number collected. A digital caliper (Mitutoyo, Kawasaki, Japan) was used to measure the coronal and root length as well as faciopalatal and mesiodistal root width at the cementoenamel junction (CEJ). The coronal height was limited to 10 ± 1 mm, and the root length was limited to 12.5 ± 1 mm. The faciopalatal and mesiodistal dimensions at the CEJ were limited to 6.75 ± 0.25 mm and 6.25 ± 0.25 mm, respectively. The selected specimens were examined stereoscopically at 10× magnification with a stereoscopic microscope (Kyowa Optical, Sagamhara, Japan) to verify the absence of cracks. Periapical radiographs of each specimen were taken to ensure uniformity of the canal and absence of internal resorption or calcification of the canal. The crowns of the specimen were decoronated perpendicular to their long axes 2.0 mm coronal to the buccal CEJ with a diamond disc (Giflax, Senden, Germany) under water coolant. Standardized root canal preparation using the step-back technique was performed on the specimens by one operator, who was a general dental practitioner, using K-files (SybronEndo, Orange, CA) with master apical file size 45 and working length 1 mm from the apices. A size 50 K-file was inserted into the canal, and if it could reach the working length, the specimen was excluded due to its large canal. The canals were further prepared using the step-back technique until size 60. Sodium 2

Table 1 Groups tested Group (size)

Dowel space diameter (mm)

Dowel diameter (mm)

B (0) W (1) R (3) BR (0 in 3) WR (1 in 3)

A = 0.60; C = 1.30 A = 0.80; C = 1.50 A = 1.00; C = 2.00 A = 1.00; C = 2.00 A = 1.00; C = 2.00

A = 0.60; C = 1.30 A = 0.80; C = 1.50 A = 1.00; C = 2.00 A = 0.6; C = 1.30 A = 0.8; C = 1.50

A = apical section diameter; C = coronal section diameter. Group B—Glass fiber-reinforced epoxy resin dowel size 0 in dowel space prepared with size 0 drill. Group W—Glass fiber-reinforced epoxy resin dowel size 1 in dowel space prepared with size 1 drill. Group R—Glass fiber-reinforced epoxy resin dowel size 3 in dowel space prepared with size 3 drill. Group WR—Glass fiber-reinforced epoxy resin dowel size 1 and reinforcing resin cement in dowel space prepared with size 3 drill. Group BR—Glass fiber-reinforced epoxy resin dowel size 0 and reinforcing resin cement in dowel space prepared with size 3 drill.

hypochlorite solution 2.6% (Clorox, Kuala Lumpur, Malaysia) was used to irrigate the canal throughout instrumentation. When the canals were fully instrumented, paper points were used to dry the canals, which were then filled with 17% ethylenediaminetetraacetic acid (EDTA) (Smear Clear; SybronEndo) and left for 60 seconds. Final irrigation was achieved using distilled water to remove all of the remaining irrigant. The instrumented teeth were obturated by lateral condensation technique with gutta-percha cones (SybronEndo) and a resin-based sealer (AH 26; Dentsply, Konstanz, Germany) Excess gutta-percha was removed using a flame-heated endodontic condenser (Dentsply), and vertical condensation was perfomed. The canal orifices were filled with Cavit (3M ESPE, St. Paul, MN), and the obturated teeth then stored in distilled water at 37°C for 24 hours for the full setting of the sealer. The 50 selected root-treated teeth were randomly assigned to five groups of 10 teeth each, which were groups B (control), W, R, BR, and WR according to the size of the glass dowel used. For groups B (Blue), W (White), and R (Red), the dowel spaces were prepared up to the corresponding size of the dowel used (FRC Prostec Plus; Ivoclar Vivadent, Schaan, Liechtenstein). For group BR the dowel space was prepared up to size 3 (Red), and the glass fiber-reinforced epoxy resin dowel used was size 0 (Blue). For group WR the dowel space was prepared up to size 3 (Red), and the glass fiber-reinforced epoxy resin dowel used was size 1 (White). The groups are illustrated in Table 1 and Figure 1. The gutta-percha was first removed using Gates Glidden burs size 3 and 4 (Dentsply Maillefer, Ballaigues, Switzerland) 4.0 mm from the apices. The required preparation depth was determined and marked on the corresponding drill by using a silicone stopper. The dowel spaces of the teeth were prepared using the low-speed drill provided in the dowel system with 4.0 mm apical gutta-percha remaining. For teeth in group B, the size 0 (blue color-coded) drill was used to prepare the dowel space. For teeth in group W, the size 1 (white color-coded) drill was used to prepare the dowel space. For teeth in groups R, BR, and WR the size 3 (red color-coded)

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Figure 1 Specimen groups. Group B—glass fiber dowel size 0 in dowel space prepared with size 0 drill; group W—glass fiber dowel size 1 in dowel space prepared with size 1 drill; group R—glass fiber dowel size 3 in dowel space prepared with size 3 drill; group WR—glass fiber dowel size 1 and reinforcing resin cement in dowel space prepared with size 3 drill; group BR—glass fiber dowel size 0 and reinforcing resin cement in dowel space prepared with size 3 drill.

drill was used to prepare the dowel space. Periapical radiographs of each specimen were taken to ensure correct dowel space preparation without weakening of roots and absence of gutta-percha. In groups B, W, and R, the fiber dowels were cemented with a self-curing resin luting cement (Multilink N Primers A/B and Multilink N; Ivoclar Vivadent). Subsequently, the cemented dowel was further light cured for 40 seconds with a light-curing unit (Spectrum 800; Dentsply, York, PA) with an 800 mW/cm2 light intensity by positioning the light guide tip at a distance of 1 to 2 mm from the dowel space openings. In groups WR and BR, the dowel space preparation was the same, except the dowel space was filled with Tetric N-Ceram (Ivoclar Vivadent) in Cavifil form before the dowel was placed. For these two groups, the resin composite was injected into the dowel space until the dowel space was fully filled with resin composite. Immediately, the dowel was inserted in a clockwise rotation until the full length marked by silicone stopper. The excess resin was briefly light cured for 5 seconds, and excess was removed with a probe. Subsequently, the cemented dowel was further light cured for 40 seconds with a light intensity of 800 mW/cm2 . Multilink N Primer A/B was applied to the dowel-cemented teeth, left for 15 seconds, and dried with dry, oil-free air. The composite cores were built up using Tetric N-Ceram with 6.5 mm core heights measured from the facial tooth margin according to the morphological shape of maxillary central incisors. The composite cores were light cured for 20 seconds with 800 mW/cm2 light intensity. Tooth reduction for crown preparation was performed to standardized specification (Fig 2). The crown margin was designed to follow the simulated contours of the free gingival tissue with the facial and lingual extent of the margin 1.5 mm more apical than the proximal margins. The margin was 1 mm wide with a 12° total occlusal convergence as well as 1.0 mm of facial reduction with a round shoulder diamond bur. The lingual reduction was 0.5 mm. Ferrules of 2.0 mm were created at the

Figure 2 Standardized specification of crown preparation. Facial—1 mm reduction; lingual—1 mm reduction at margin, 0.5 mm reduction at lingual surface; incisal—2 mm reduction; ferrule—2 mm height.

facio-lingual margin, and a 0.5 mm ferrule was created at the proximal margin. The core height of 6 mm was prepared with a facio-lingual thickness of 1 mm at the incisal edge. A plastic crown former PD 171 (Produits Dentaires, Vevey, Switzerland) was used as reduction guide. Crowns were cast with Ni-Cr Alloy (System KN; Adentatec, K¨oln Germany) and seated to the prepared tooth to ensure good marginal fit. The inner surface of the crown was cleaned with airborne particle abrasion and water, followed with dry, oil-free air before being coated with Metal/Zircornia Primer (Ivoclar Vivadent). The primer was left for 180 seconds before drying with dry, oil-free air. The prepared teeth were coated with mixed Multilink Primer A/B and left for 15 seconds before drying with dry, oil-free air. Multilink N was dispensed fully into the inner surface of the cast crown and fitted to the tooth. Excess cement was briefly cured for 2 seconds with the light curing unit. The excess cement was removed with a probe, and the fitted crown was left for 360 seconds to allow the cement to set. The roots were then cleaned and marked 3.0 mm from the crown margin. The roots were coated with a 0.1 to 0.2 mm thin layer of vinylpolysiloxane silicone (Aquasil; Dentsply), to simulate the periodontal ligament. The teeth were embedded 3.0 mm apical to the crown margin in a block of self-cure resin epoxy resin (Mirapox 950; Miracon, Kuala Lumpur, Malaysia) in a custom-made cubic mold (23 mm width × 23 mm length × 25 mm height). The specimens were left for 24 hours to allow complete setting of the epoxy resin.

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Table 2 Mean failure load and standard deviation for each group

Table 3 Multiple comparison Bonferroni test between the different groups for mean failure load

Group B W R WR BR

Mean failure load (N) ± (SD)

P

1406 (376) 1259 (379) 1085 (528) 959 (200) 816 (298)

B W R WR BR *

The mounted teeth were stored in distilled water at 37°C and 100% humidity for 24 hours prior to thermocycling in a thermocycling machine (fabricated by Faculty of Engineering, University of Malaya, Malaysia). The teeth were placed in a wire mesh attached to the machine arm and were thermocycled 500 times between 5 and 55°C with a 30-second dwell time and 2- second transfer interval (ISO/TS 11405:2003). After that, the specimens were fixed in a customized jig fabricated to align the long axis of the tooth at an angle of 45° to the horizontal plane and 135° to the loading rod tip. This jig was secured to the lower compartment of a high precision universal testing machine (Shimadzu, Kyoto, Japan). A unidirectional static load was applied to the center of the palatal crown surface 5.0 mm from its incisal edge using a flat end rod (2.0 mm × 10.0 mm) at a 0.5 mm/min crosshead speed. The load was applied until failure occurred as measured by a sudden drop of the stress-strain curve, which was displayed on the computer monitor connected to the machine. The mode of failure for each of the specimens was noted by visual inspection. The failure mode was classified into either favorable or unfavorable.5 The favorable failure modes were complete or partial dowel and core debonding or dowel/core/tooth complex fracture above the epoxy resin level. Unfavorable failure modes were fracture of the dowel/core/tooth complex below the epoxy resin, vertical root fracture, or tooth cracks below the epoxy resin level. All data were analyzed using Statistical Program for Social Science (SPSS) for Windows version 12.0 (SPSS Inc., Chicago, IL). One-way ANOVA was used to detect the presence of group differences. The post hoc (Bonferroni) test was used for multiple comparisons. The probability level for statistical significance was set at α = 0.05.

Results The mean and standard deviation values for the load at failure of each group are as shown in Table 2. Group B (control) had the highest mean failure load. Group BR had the lowest mean failure load. The ANOVA test was significant (p < 0.05), suggesting that at least one pair of the groups was significantly different. Post hoc (Bonferroni) pairwise comparisons were conducted to test the differences between each pair of means. Pairwise comparisons showed a statistically significant difference in failure loads between groups B and BR (p < 0.05) (Table 3), whereas no statistically significant difference was found between other groups. The null hypothesis was therefore rejected (p < 0.05). 4

B

W

R

WR

BR

– 1.000 0.599 0.101 0.009∗

– 1.000 0.784 0.108

– 1.000 1.000

– 1.000



Significant difference between the groups (p < 0.05).

Figure 3 Failure modes of specimens. Favorable failure (left)—fracture of the cervical region above epoxy resin; unfavorable fracture (right)— fracture of the root below epoxy resin.

Figure 3 illustrates both favorable and unfavorable failure modes. Figure 4 shows the number and percentage of both favorable and unfavorable failure modes in each group. Group BR had the highest percentage of unfavorable failure. Group B had the lowest.

Discussion On the basis of the statistical analysis of the data, the null hypothesis was rejected. There was a significant difference between the dowel systems studied. The results show that the failure load for group B (most dentine thickness) was significantly higher than those in group BR (least dentine thickness; with reinforcing resin composite). The selection of intact human central incisors represents the best possible option to simulate the clinical situation for endodontically treated teeth. Previous studies had reported their use as an acceptable way to research dowel-restored teeth.13,14 The main disadvantage of using human teeth was the relatively large variation in size and mechanical properties,15 often resulting in large standard deviations as shown in the results of this study. In this study, the teeth were carefully selected for standardized size, as this was reported as an important variation in the resistance to fracture of the specimen.16 Root length and faciopalatal and mesiodistal dimensions were standardized to minimize the dimensional variation in the specimens. This was also observed for fabrication of standardized core build-ups and full coverage crowns.

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Influence of FRC Dowel Diameter on ETT Fracture Resistance

Figure 4 Percentage of failure modes.

The teeth were randomly assigned to five groups of 10 teeth each. For groups B, W, and R, dowel spaces were carefully prepared using the corresponding reamer, which resulted in a close fit of dowels to the canal walls. For groups BR and WR, the dowel spaces were prepared using the largest reamer with diameters of 1.00 mm and 2.00 mm at the apical and cervical, respectively. The fiber dowels used in groups WR and BR had a 20% and 40% reduction in apical diameter, respectively. Therefore, a flared canal was stimulated in these groups. For groups B, W, and R, fiber dowels were cemented with a dual-cure resin cement. This adhesive system was applied together with the self-etching, self-curing primer. The use of self-etching adhesives were previously recommended by Kivanc and Gorgul.6 Using a self-activating, dual-curing adhesive system in combination with a dual-curing cement has been reported to enable effective luting of fiber dowels, regardless of the amount of light transmitted through the dowel.17 The cemented dowels were light cured for 40 seconds with an 800 mW/cm light intensity, which exceeded the acceptable light density of 600 mW/cm2 .18 With this, the mechanical properties of the teeth would be enhanced, as the dual-polymerized resin luting agents used had higher or equal flexural strength compared to the autopolymerized mode.19 Photo-initiated polymerization of the adhesive resin and dual-cure resin composite was also necessary to achieve good bonding to root canal dentin.20 For groups BR and WR, flared canals were simulated. The same primer was applied to the canal walls, but the resin luting cement was replaced by light-curing resin composite. The void between the fiber dowel and canal wall was thus filled with resin composite to serve as reinforcement for the canal. The use of resin composite to strengthen weakened roots was first recommended by Lui in 1987.21 Boschian et al studied the bond strength between luting materials, root dentin and fiber dowels and concluded that the bond strength tests and SEM observations showed that in vitro, composite resins perform better than resin cements. They recommended the use of these ma-

terials, which may significantly reinforce residual tooth structure; therefore reducing the risk for fracture and debonding.22 However, although hybrid composites showed superior bond strengths in oversized canals, they were still not as high as those of dowels in precisely fitting dowel spaces using common resin cements.23 In this study, light curing resin composites were used even though the dowel length was about 8.0 mm. A study had shown that complete cure of resin composites with light transmitting dowels can be achieved at this length.24 For this study, the core was built up with hybrid composite, with a modulus of elasticity of 11 GPa, very similar to the modulus of elasticity of dentine (15 GPa) and was thus able to realize a tooth/dowel/core monoblock that could homogeneously distribute masticatory loads and reduce stress.25 The core build-up was done without any matrix, as a study showed that when hybrid composites were used to build up a core onto a fiber dowel, a higher homogeneity of the abutment and a better dowel/core integration were achieved if the build-up was done in the absence of any matrix.26 Although cores built up with flowable composites showed the highest integrity and the best adaptation onto the dowel,27 bond strength to fiber dowel remains relatively weak. Therefore, hybrid composites for core build-up were better alternatives to flowable composites as core build-up materials.28 The teeth in this study were restored with Ni-Cr cast crowns. When the teeth were decoronated perpendicular to their long axes 2.0 mm coronal to the buccal CEJ, it was possible to design the prepared margins to follow the simulated contours of the free gingival tissues with the facial and lingual extended 1.5 mm more apical than the proximal margin. Therefore, the ferrule was created with 2 mm at the facio-palatal margin and 0.5 mm at the proximal margin. It has been a common practice that when there is extensive tooth loss with remaining structure 2 mm above the CEJ, a crown restoration will be indicated. Thus, the study design resembled the in vivo condition, allowing extrapolation of results to the clinical situation.16,29-32

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However, some studies have shown that different ferrule designs did not have any significant influence on the fracture resistance of teeth restored with fiber dowels.33-36 This contrasted with other studies that showed that increased ferrule length improved the mechanical behavior of teeth restored with metal crowns irrespective of dowel or core type.37-39 Hu et al concluded that using an FRC dowel might lead to a long fatigue life in restoring pulpless teeth with flared canals, and that dentin ferrule preparation was necessary to enhance the restorations’ resistance to cyclic fatigue.40 Besides, the insertion of a fiber dowel could reduce the percentage of catastrophic failure of these restorations under function.33,41 Schmitter et al found that increased ferrule height and resin bonding of the crown resulted in higher fracture loads. They concluded that crowns, especially those with a small ferrule height, should be resin bonded. It was also reported that centrally positioned fiberreinforced dowels did not contribute to load transfer as long as the bond between the tooth and composite core was intact.42 Therefore, fiber dowels can safely be used for their reinforcing properties regardless of height; such a property of these types of dowels is thus an additional advantage in clinical practice.36 In this study, the biological width of the periodontium was simulated by embedding the teeth 3.0 mm apical to the crown margin in a block of self-cure epoxy resin.2 It was believed that the use of a rigid material to embed extracted teeth may lead to distorted load values and possibly affect the mode of failure of the specimens.16 Therefore, the roots were coated with a 0.1 to 0.2 mm thin layer of VPS silicone to simulate the periodontal ligament.16 The moduli of elasticity of these materials therefore approximated those of the viscoelastic periodontal ligament and the alveolar bone, respectively.43 On the other hand, however, this simulation of periodontal ligament might have a limitation due to its complex viscoelastic nature in vivo14 and thus might cause uncontrolled tooth movement under load.44 That said, the design of this study was similar to other studies that simulated the periodontal ligament of a natural tooth.45-48 The specimens were thermocycled according to ISO/TS 11405(2003). Specimens that underwent thermocycling gave more meaningful results, as the moisture and temperature changes in the oral environment were simulated. In relation to bond strength of resin cement, various studies have shown a significant increase in bond strengths of resin cement after thermocycling.49-52 However, in relation to fracture resistance, no such studies had been reported in the literature. In this study, the loading angle was set at 135° to the long axis of the teeth. The average inter-incisal angle between maxillary and mandibular incisors in class I occlusion was thus simulated.53 The teeth were loaded with an increasing, unidirectional static load applied at a standardized fixed point of the metal crowns until failure. Most in vitro studies comparing fracture resistance of dowel-restored teeth used static loading.2,48,54-60 However, this might not be representative of the actual in vivo situation, as the masticatory forces are multidirectional and repeatedly applied on a larger area. Therefore, the cyclic loading test could give a more realistic simulation of the oral environment. Nonetheless, the design of this study provided a baseline on the maximum clenching or parafunctional masticatory force a dowel-restored tooth could withstand either as a single tooth or a fixed prosthesis. 6

The descending order of fracture strength for the tested group was B > W > R > WR > BR. These results would suggest that the lower load value obtained for the BR group was due in part to the decrease in remaining dentine, and instead of a fully fitting glass fiber-reinforced epoxy resin dowel, reinforcing resin composite was used as replacement. On the other hand, although numerically different, there were no significant differences between the other groups. The results of the mean failure load is consistent with the results of a study by Bonfante et al, with fracture loads ranging from 745 to 920 N.61 The fiber dowel system used in this study (FRC Postec Plus) probably contributed to the greater strength as similarly displayed in this study.62 Comparison between groups B, W, and R, which had no reinforcing resin composite, showed no significant differences in mean failure load. Group B, with the largest amount of remaining dentin, provided the highest fracture resistance and more favorable failure mode. The enlargement of dowel space therefore decreased the fracture resistance; however, this decrease was not statistically significant. The effect of the intraradicular reinforcement with composite on the fracture resistance of the teeth was one of the interests of this study. Under the conditions of this study, the increased thickness of reinforcing composite had no effect on the fracture resistance of enlarged canals. Though the results of this study differed somewhat from those previously reported,63 it should be noted that the teeth were restored with metallic dowels after intra-radicular reconstruction with composite rather than fiber dowels as used in this study, where the monoblock effect was in play. Comparison between groups R, BR, and WR, which had the same dowel space but different glass fiber-reinforced epoxy resin dowel diameters and reinforcing resin composite thicknesses in groups BR and WR, showed no significant differences in mean failure load. Group BR, with the largest amount of reinforcing resin composite, displayed the lowest fracture resistance. The increase of the thickness of reinforcing resin composite decreased the fracture resistance, but this difference was not statistically significant. In comparing groups W and WR, which had the same fiber dowel diameter but different dentine thicknesses, there were no significant differences in mean failure load. The loss of remaining dentine was replaced with reinforcing resin composite, resulting in a decreased in fracture resistance but this difference was also not statistically significant. Similarly, comparison between groups B and BR with the same glass fiber-reinforced epoxy resin dowel diameter and different dentine thicknesses showed significant differences in mean failure load. The loss of remaining dentine was replaced with reinforcing resin composite, resulting in a significant decrease in fracture resistance; however, the dentin in group B was the thickest, while that in group BR was the thinnest in this study. One of the possible explanations was the generation of large lacunae, or bubbles, in the reinforcing resin due to insertion of a large volume of resin composite in the dowel spaces in a single increment in this particular resin reinforcement group. There could be reduced cohesive resistance of cement and even reduced bond strength to dentine, although a study showed that

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the increased cement thickness surrounding the FRC dowel did not impair the bond strength.64 Insertion of a large volume of resin composite in the dowel space in a single increment in this group might induce a high stress at the adhesive interface due to the high polymerization shrinkage and impaired control of formation of lacunae in the resin. Therefore, a reduction in cohesive strength of the resin and bond strength to dentin could result. Another possible explanation for the decreased fracture resistance was the curing of the reinforcing resin composite at the most apical part of the dowel space. It was reported that the larger the dowel diameter, the greater the depth of cure.65 This could explain why the fracture load was significantly different between groups B and BR but not between groups B and WR, since the dowel diameter in WR was larger than BR. The maximum occluding force exerted by a maxillary incisor tooth (in men) has been reported as 146 ± 44 N.66 The load required to cause failure in this study was higher than that observed in normal functional activity at the anterior region, ranging from 166.1 to 264.77 N.29,67 Despite group BR being the weakest in fracture resistance, the failure load was still higher than the failure load of a similar study using cast dowel and core with a mean failure load of 600 N.68 This was consistent with studies that showed higher fracture resistance in teeth restored with glass fiber-reinforced epoxy resin dowels than those restored with metal dowels.60,69 Also, it had been shown that cast dowel and core showed lower strain values than groups with glass fiber-reinforced epoxy resin dowels when restored with metal crowns.37 The results from this study showed that the strength of teeth was directly proportional to the amount of remaining dentine. The diameter of glass fiber-reinforced epoxy resin dowel and reinforcing resin composite thicknesses should not have an effect on the fracture resistance when the remaining dentine thicknesses were similar. The result was thus consistent with Rodriguez-Cervantes et al’s study.70 They concluded that the dowel diameter had a significant effect on the biomechanical performance of teeth restored with stainless steel dowels. Lower failure loads were found as dowel diameter increased; however, the dowel diameters of those teeth restored with glass fiberreinforced epoxy resin dowels had no significant effect. They therefore proposed the use of glass fiber-reinforced epoxy resin dowels to achieve a restorative technique that was less sensitive to dowel dimensions.70 The results in this study contrasted with other studies that showed low fracture strength values in groups with narrower diameter.71,72 These studies, however, did not include a crown, and the force was applied directly to the core or dowel. Therefore, in such studies, they found that the narrower dowels had lower fracture resistance than the wider dowels. One of the advantages of the use of glass fiber-reinforced epoxy resin dowels was the reported similar modulus of elasticity to dentin.12,13 It was believed that the creation of a monoblock dentine/dowel/core system would transmit and distribute functional stresses across the bonding interface to the tooth more properly, with the potential to reinforce weakened tooth structure. Therefore, if an excessive load were applied to the tooth, the dowel would be able to absorb the stresses, thus reducing the possibility of root fracture. The modulus of elas-

Influence of FRC Dowel Diameter on ETT Fracture Resistance

ticity of the glass fiber-reinforced epoxy resin dowels used in this study was 48 MPa, three times the modulus of elasticity of dentin (15 MPa). The vast differences of moduli of elasticity of the materials would yield a high stress concentration in the root. This explains why more unfavorable failures were observed in this study than those in similar studies. Differences were observed among groups in relation to the mode of failure. Evaluation of the root after testing revealed 30% unfavorable root fracture in group B, and the incidence of such root fracture increased with the enlargement of dowel space. When the dowel space was enlarged to size 3, in groups R, WR, and BR, the percentage of unfavorable root fracture increased to 70%. This might be attributed to the fact that more tooth structure was removed during dowel space enlargement, the resistance to loading force was diminished, and the possibility of fracture increased. In addition, endodontic treatment and preparing the root canal to receive a dowel might lead to cracks and defects that could also concentrate stresses and increase the possibility of root fracture.73 This study somewhat differed from those reported by Newman et al,48 who reported no root fracture for all the specimens restored with glass fiber-reinforced epoxy resin dowels. These inconsistencies might be attributed to the design of their study, where the force was directly applied on the glass fiberreinforced epoxy resin dowels, whereas in this study, the teeth were prepared with ferrule and restored with core and metal crown. The force was thus distributed via ferrule and transmitted to the root rather directly to the dowel. For teeth restored with reinforcing resin composite, the stresses from polymerization shrinkage of the resin composite could affect the failure mode.74 This could mean that although using reinforcing resin composite might not directly increase fracture resistance, it might instead create a higher incidence of more favorable and restorable failure mode compared to enlargement of dowel space to accommodate a larger glass fiberreinforced epoxy resin dowel. The efficacy of these techniques should be evaluated in longitudinal clinical studies. Adhesively luted resin/fiber dowels with composite cores appear to be the best currently available option in terms of tooth fracture and biomechanical behavior.75 However, controlled clinical studies showing their performance in a long-term situation are required. Study limitations

Only maxillary central incisors were used; therefore, these results could only be applied to this group of teeth. On the basis of the conditions of the study, it remained uncertain whether complete light polymerization of the reinforcing resin composite at the most apical portion was achieved; however, the manufacturer’s recommended technique was strictly followed. Because the static laboratory study did not always simulate dynamic conditions in which the forces were constantly changing their rate, magnitude, and direction, long-term clinical evaluation of this correlation should be carried out. Recommendation for further studies

Further studies are needed to investigate the effect of fiber dowel diameter on the fracture resistance of other endodontically treated teeth (e.g., premolars and molars). A comparison

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between the static load and fatigue load could also be conducted to investigate any correlation between the results obtained. Full ceramic crowns instead of metal crowns can be used, as they are more relevant, especially in the esthetic zone. Clinical relevance

The use of smaller glass fiber-reinforced epoxy resin dowels and reinforcing resin composite is recommended rather than enlargement of dowel spaces to accurately fit a larger glass fiber-reinforced epoxy resin dowel as the enlargement of dowel spaces increases the risk of unfavorable failure.

Conclusions Within the limitations of this study, the following conclusions were made: 1. Endodontically treated maxillary central incisors restored with crowns in group B were significantly more fracture resistant than those in group BR (p = 0.05). 2. There was no statistically significant effect of glass fiberreinforced epoxy resin dowel diameter on fracture resistance of endodontically treated central incisors restored with crowns with the same remaining dentinal wall thicknesses (p = 0.05). 3. Enlarged dowel spaces caused more unfavorable failure regardless of the diameters of the glass fiber-reinforced epoxy resin dowels and reinforcing resin composite thicknesses.

Acknowledgments Special thanks to Ivoclar Vivadent Marketing Ltd., Singapore for supplying FRC Postec Plus fiber dowels.

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The effect of glass fiber-reinforced epoxy resin dowel diameter on the fracture resistance of endodontically treated teeth.

To determine the effect of glass fiber-reinforced epoxy resin (FRC) dowels of different diameters on the failure load of endodontically treated teeth ...
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