Root Reinforcement after Obturation with Calcium Silicate–based Sealer and Modified Gutta-percha Cone

Root Reinforcement after Obturation with Calcium Silicate–based Sealer and Modified Gutta-percha Cone

Basic Research—Technology Root Reinforcement after Obturation with Calcium Silicate–based Sealer and Modified Gutta-percha Cone Sittichoke Osiri, DDS...

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Basic Research—Technology

Root Reinforcement after Obturation with Calcium Silicate–based Sealer and Modified Gutta-percha Cone Sittichoke Osiri, DDS, Danuchit Banomyong, DDS, PhD, Vanthana Sattabanasuk, DDS, PhD, and Kallaya Yanpiset, DDS, MS Abstract Introduction: A root canal obturated with a calcium silicate–based sealer (bioceramic sealer [BCS]) and a modified gutta-percha cone (bioceramic cone [BCC]) might improve the fracture resistance of the root. The objective of this study was to evaluate root reinforcement of a bioceramic cone/sealer (TotalFill; FKG Dentaire SA, La Chaux-de-Fonds, Switzerland) by investigating the fracture resistance, push-out bond strength, sealer penetration, and modulus of elasticity (MOE) in comparison with gutta-percha/AH Plus (Dentsply Maillefer, Tulsa, OK) (GP/AH). Methods: Eighty-four roots from bilateral mandibular premolars were prepared. For fracture resistance, 40 teeth were randomly divided into 4 groups (n = 10 each): intact roots (negative control), prepared roots (positive control), and the roots obturated with either BCC/BCS or GP/AH. Root canals were obturated with the matched singlecone technique and vertically loaded with a spreaderlike tip until fracture. For push-out bond strength (n = 10 each), coronal, middle, and apical root slices of BCC/ BCS and GP/AH were loaded with a cylindrical plunger, and failure modes were determined. Sealer penetration of BCC/BCS and GP/AH (n = 12 each) was evaluated for the maximum depth and the circumferential and total area of penetration at the coronal, middle, and apical levels using confocal laser scanning microscopy. The MOE was investigated according to ISO 4049:2000. Results: The fracture load of BCC/BCS, GP/AH, and the intact roots was not significantly different but significantly higher than the prepared, nonobturated roots. BCC/BCS provided a higher bond strength, maximum depth, and circumferential penetration at the apical root level as well as a greater sealer penetration area at all levels compared with GP/AH. The MOE of all materials was much lower than dentin. Conclusions: BCC/BCS and GP/AH bonded and reinforced the prepared roots; their fracture resistances were similar to the intact roots. (J Endod 2018;44:1843–1848)

Key Words Bioceramic sealer, bond strength, calcium silicate sealer, fracture resistance

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ndodontically treated Significance teeth are susceptible Weakened roots after root canal preparation can to fracture (1). Excessive be reinforced by obturation with either a calcium root canal preparation silicate–based sealer/modified gutta-percha cone might weaken the root or AH Plus sealer/regular gutta-percha cone. The that induces root fracture reinforcement effect is derived from the ability of (2) and the loss of teeth sealer to bond to root dentin with good sealer (3). Rotary nickelpenetration into dentinal tubules regardless of the titanium instruments facillow MOE of the materials. itate root canal preparation with their elasticity and shape memory effect. If root dentin is more preserved, better resistance to root fracture is expected (4). Obturation of root canals with bonded materials might improve the strength of the roots. Root canal sealer bonds to root dentin by micromechanical interlocking from sealer penetration into dentinal tubules (5), and chemical adhesion between sealer and dentin is also possible (5). In addition, sealer should adhere to obturation material in a similar manner. According to the monoblock concept (5), the obturation materials should also have an elastic modulus close to dentin (5, 6). If the obturated root responds to functional force as a single unit, a higher fracture resistance is expected. Gutta-percha (GP), a conventional obturation core, lacks adhesion to either dentin or root canal sealer and has a much lower elastic modulus than dentin (7). Epoxy resin–based sealer (AH Plus [AH]; Dentsply Maillefer, Tulsa, OK) bonds to root dentin but is unable to bond to GP without a modification (8, 9). However, resin-coated GP does not improve a bond between the sealer and cone (10, 11). Currently, a calcium silicate–based root canal sealer (bioceramic sealer [BCS]) and a modified gutta-percha cone (bioceramic-coated/impregnated cone [BCC]) have been introduced. The compositions of BCS are calcium silicate, calcium phosphate monobasic, calcium hydroxide, zirconium oxide, fillers, and thickening agents (12). A bioceramic cone is a modification of the GP cone by impregnating inside and coating on the external surface with bioceramic calcium silicate nanoparticles. The manufacturer recommends obturating root canals using BCC/BCS with the matched single-cone technique (13). Thick BCS acts as a primary obturation unit, whereas BCC is used as a core carrier to introduce sealer into the root canal with hydraulic pressure. BCS is a prepared mixture in an injectable syringe and is set with moisture

From the Department of Operative Dentistry and Endodontics, Faculty of Dentistry, Mahidol University, Bangkok, Thailand. Address requests for reprints to Dr Kallaya Yanpiset, Department of Operative Dentistry and Endodontics, Faculty of Dentistry, Mahidol University, 6 Yothi Street, Ratchathewi, Bangkok 10400, Thailand. E-mail address: [email protected] or [email protected] 0099-2399/$ - see front matter Copyright ª 2018 American Association of Endodontists. https://doi.org/10.1016/j.joen.2018.08.011

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Basic Research—Technology in the root canal (13). BCS creates a bond to dentin by micromechanical sealer penetration (14) and chemical hydroxyapatite formation (13, 15). Moreover, adhesion between bioceramic particles in BCS and BCC is expected. The fracture resistance of roots obturated with BCC/BCS might increase. Therefore, the purpose of this study was to determine the fracture resistance, push-out bond strength, and sealer penetration of roots obturated with BCC/BCS; root canals obturated with GP/AH were used for comparison. In addition, the modulus of elasticity (MOE) of BCC, GP, BCS, and AH was evaluated.

Materials and Methods Under ethical approval (MU-DT/PY-IRB-2017/DT028), 84 singlecanal mandibular premolars with straight or slightly curved roots were used. To control for biological variations, bilateral lower premolars extracted for orthodontic reasons from the same donor were collected from patients 17–30 years old and used in the same experiment. The teeth were stored in 0.12% thymol solution and used within 3 months. Roots with an open apex or any defects were excluded. Periapical radiographs were taken in buccolingual (BL) and mesiodistal (MD) views to confirm the presence of a single root canal. The BL and MD widths of the root canals were measured at a level 5 mm from the root apex. Teeth with a BL/MD ratio between 1.5 and 2.0 were selected (16). The thickness of the roots was also measured in the BL and MD dimensions at the cementoenamel junction level and then averaged. Teeth with root thicknesses within 15% above or below the average were included. The roots were decoronated using a slow-speed diamond saw (Isomet 1000; Buhler, Lake Bluff, NY) to obtain a 12-mm root length. The working length was determined at 1 mm short from the root apex. Root canal preparation was performed using the RaCe rotary nickel-titanium system (FKG Dentaire SA, La Chaux-de-Fonds, Switzerland) up to size 40/.06. The canal was irrigated with 2.5% sodium hypochlorite, and the smear layer was removed with 3 mL 17% EDTA (Endo Clean; M Dent, Bangkok, Thailand) and 5 mL 2.5% sodium hypochlorite. The prepared canal was dried with 2 paper points size 40/.06 (FKG Dentaire SA). AH or BCS (TotalFill, FKG Dentaire SA) was loaded into the root canal using a size 35 Lentulo spiral. A size 40/.06 matched BCC (TotalFill bioceramic cone, FKG Dentaire SA) or GP (TotalFill gutta-percha cone, FKG Dentaire SA) was coated with the sealer, inserted into the canal, and cut at the coronal end by an electrical heat carrier. The specimens were stored at 37 C and 100% humidity for 1 week to allow the sealer to completely set (17, 18).

Fracture Resistance Forty roots were randomly allocated into 4 groups (n = 10 each) as follows: 1. 2. 3. 4.

not instrumented and not obturated (negative control), instrumented but not obturated (positive control), instrumented and obturated with BCC/BCS, and instrumented and obturated with GP/AH.

Each root was coated with a thin layer of light-body silicone (Silagum-Light; DMG, Hamburg, Germany) up to 2 mm below the coronal end to simulate the periodontal ligament and then mounted in a polyvinyl chloride ring using self-cured acrylic resin (Instant Tray Mix; Lang Dental Mfg, Wheeling, IL) at the 2-mm level. Under water-immersion and at 37 C, the root vertically loaded by a 0.8-mm diameter spreader-like tip, which was slightly smaller than the size of coronal one third of root canal diameter, at a speed of 0.5 mm/ 1844

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min into the root canal until fracture using a universal testing machine (Instron Corp., Norwood, MA). The fracture force was recorded in newtons (19).

Push-out Bond Strength Twenty obturated roots were randomly divided into 2 groups (n = 10 each): GP/AH and BCC/BCS. Three 1-mm-thick root slices at the apical, middle, and coronal levels were obtained for each root by perpendicular sectioning the obturated root using a diamond blade with water coolant. Each specimen was loaded in the apicocoronal direction with a 0.3-mm, 0.5-mm, or 0.8-mm-diameter cylindrical stainless steel plunger, which was slightly smaller than the diameter of the root canal at each level for the apical, middle, and coronal slices at a crosshead speed of 0.5 mm/min until failure. The load at bond failure (N) was recorded and calculated into MPa as follows: Push  out bond strength ðMPaÞ ¼

area ¼ p  ðr1 þ r2 Þ 

loading forceðNÞ area ðmm2 Þ

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ðr1  r2 Þ þ t 2

where r1 = the larger radius, r2 = the smaller radius, and t = the thickness of the root section. The samples were examined under light microscopy at 100 magnification to determine the modes of failure (20) as follows: adhesive failure (cone-sealer or sealer-dentin), cohesive failure within the sealer, and mixed failure (either adhesive or cohesive failure <70% of the debonded area).

Sealer Penetration Twenty-four obturated roots were randomly divided into 2 groups (n = 12 per group; 10 cross-sectional and 2 longitudinal sections): GP/AH and BCC/BCS. Each sealer was mixed with 0.1% rhodamine B isothiocyanate fluorescent dye (Chem-Supply, City of Port Adelaide Enfield, Port Adelaide, Australia). For the cross-sectional section, 3 root slices 1 mm in thickness were prepared by root sectioning as previously described and polished for 10 seconds each with wet silicon carbide abrasive papers (grit 800, 1000, and 1200). The specimens were imaged using a confocal laser scanning microscope (FV10i-DOC; Olympus, Tokyo, Japan) at 10 magnification. The 3-dimensional stack image was evaluated for maximum depth (mm) of sealer penetration with FV10-ASW 4.2 viewer software (Olympus FV10i-DOC). Circumferential penetration (%) and the total area (mm2) of sealer penetration were evaluated (21) using Image J software (National Institutes of Health, Bethesda, MD). For the longitudinal section, the root was sectioned vertically in the BL direction, and polished for 10 seconds with the silicon carbide abrasive papers. The specimens were imaged using a confocal laser scanning microscope at 10 magnification, and the 3-dimensional stack images of sealer penetration were qualitatively observed. MOE The 3-point bending test was performed according to ISO standard 4049:2000. A stainless steel split mold was used to create a bar-shaped specimen 25  2 mm long, 2  0.1 mm wide, and 2  0.1 mm thick; 5 specimens for each material (ie, GP, BCC, AH, and BCS) were created. For the sealers, BCS or AH was mixed, filled into the mold placed on a glass slide, and then pressed on top with another glass slide. Each mold was kept at 37 C and 100% humidity for 1 week. The specimen was removed and polished with 320-grit abrasive paper. For the obturation JOE — Volume 44, Number 12, December 2018

A, apical level; AH, AH Plus; BCC, bioceramic cone; BCS, bioceramic sealer; C, coronal level; GP, gutta-percha; M, middle level; P25, 25th percentile; P75, 75th percentile. Different uppercase letters indicate a significant difference in the columns (P < .05) between BCC/BCS and GP/AH at a root level. Different lowercase letters indicate a significant difference in the rows (P < .05) among the root levels of BCC/BCS or GP/AH. *Mean load to fracture of negative control group was 139.05  24.13 N that was not significantly different from GP/AH and BCC/BCC. Mean load to fracture of positive control group was 105.72  11.81 N that was significantly lower than the other groups.

100a,B (100, 100) 90.69b,A (73.90, 100) 100 (100, 100) 100a,A (90.59, 100) 100 (100, 100) 100a,A (98.36, 100)

3.85  2.36b,B 1.60  1.20c,A

M

a,A a,A

12.54  1.45a,B 8.60  3.44a,A 1,211  187b,B 968  258b,A 1,660  334a,A 1,378  347a,A 1,900  258a,A 1,666  305a,A 5.20  1.85b,B 2.7  1.22b,A GP/AH

3.55  2.58ab,A 2.49  0.87ab,A BCC/BCS

151.64  30.34A 145.87  35.32A

2.43  1.84a,A 1.47  0.74a,A

C A M Group

C

M

A

C

Maximum depth (mm, mean ± SD)

Load to fracture* (N, mean ± SD)

Push-out bond strength (MPa, mean ± SD)

9.53  4.32a,B 5.62  2.87b,A

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Results

Penetration area (mm2, mean ± SD)

The fracture resistance, push-out bond strength, and sealer penetration of BCC/BCS and GP/AH are presented in Table 1. The fracture resistance of the roots was significantly different among the groups (P < .01). The prepared, nonobturated roots showed the significantly lowest fracture load. No significant difference was found among intact roots, GP/AH, and BCC/BCS (P > .05). BCC/BCS showed significantly higher push-out bond strength than GP/AH only at the apical area (P < .01). Regardless of the material type, the bond strengths of GP/AH and BCC/BCS at the apical level were higher than those at the middle level (P < .01). Table 2 shows the failure modes from the push-out bond test. GP/AH mostly revealed adhesive failure between the cone and sealer, whereas BCC/BCS mostly showed cohesive failure in the sealer (Fig. 1A–D). BCC/BCS showed significantly higher circumferential penetration at the apical level (P < .01), deeper sealer penetration at the apical level (P = .027), and wider sealer penetration areas at all root levels (P < .01) than those of GP/AH. Circumferential sealer penetration of BCC/BCS was not significantly different among the root levels (P > .05), whereas that of GP/AH at the apical level was significantly decreased (P < .01). For both sealers, the penetration depth and area displayed a decreasing trend from the coronal to the apical root level (Fig. 2A–D); the depth at the apical level was statistically lower than the coronal and middle levels (P < .01). The MOE of the materials is reported in Table 3. BCS presented significantly higher MOE than GP, AH, and BCC (P < .01), with no significant difference detected among the others.

TABLE 1. The Fracture Resistance, Push-out Bond Strength, and Sealer Penetration of the Experimental Groups (n = 10 per Group in Each Experiment)

Statistical Analysis Statistical analyses were performed using SPSS version 18.0 (SPSS Inc, Chicago, IL); a P value <.05 was set as a statistically significant level. Normal distribution was determined using the Shapiro-Wilk test. Homogeneity of variance was checked using the Levene test. The fracture resistance, push-out bond strength, sealer penetration in depth and area at different root levels, and MOE were analyzed with 1-way analysis of variance, and pair-wise comparisons were performed using the Games-Howell test and the Tukey post hoc test. The independent t test was used to compare the 2 sealers at the same root level. Circumferential sealer penetration was analyzed using nonparametric tests. The modes of failure were described by descriptive statistics.

Sealer penetration

where Ef = MOE (MPa), F = the maximum load (N), L = the length between the supports (mm), b = the width at the center specimen (mm), D = the maximum deflection (mm), and d = the thickness at the center of the specimen (mm).

C

FL3 4bDd 3

A

Ef ¼

% Circumferential penetration (median [P25, P75])

materials, BCC pellets and GP pellets were heated in a hot air oven at 200 C for 5 minutes, filled into the mold placed on a glass slide, and then pressed on top with another glass slide. After cooling down for 10 minutes, the specimen was removed from the mold and polished with 320-grit abrasive paper. The load was applied to the middle of the specimen at a crosshead speed of 0.75 mm/min using a universal testing machine until failure. The MOE value was calculated using the following formula:

A

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Basic Research—Technology TABLE 2. The Mode of Failure of the Groups at Different Root Levels Mode of failure Level

Group

Adhesive failure (cone-sealer)

Cohesive failure within sealer

Mixed failure

Coronal

BCC/BCS GP/AH BCC/BCS GP/AH BCC/BCS GP/AH

0 8 0 10 1 7

6 0 7 0 8 0

4 2 3 0 1 3

Middle Apical

AH, AH Plus; BCC, bioceramic cone; BCS, bioceramic sealer; GP, gutta-percha.

Discussion Using nickel-titanium rotary instruments has a potential to preserve more root dentin and maintain better fracture resistance than the conventional technique (22, 23). However, the results of the current study indicated that the roots were still weak after preparation. Obturation of root canals with either BCC/BCS or GP/AH increased the fracture resistance of the prepared roots compared with the intact roots. The reinforcement effect of BCC/BCS

in this study is consistent with the results of Celikten et al (24), who reported root canal obturation with BCC/BCS strengthened the prepared root. With a much lower MOE than that of dentin, BCC/BCS and GP/AH could bond to enhance the fracture resistance of the prepared roots. These findings were not consistent with the monoblock concept that proposes the MOE of material is an important factor for increased fracture resistance of the roots (5, 6). Our results propose that

Figure 1. (A–D) Representative images of failure after the push-out bond test in roots obturated with GP/AH and BCC/BCS. Most of the GP/AH showed adhesive failure between cone and sealer, whereas most of the BCC/BCS showed cohesive failure in the sealer. (A and C) Adhesive failure of GP/AH at the canal side and the cone side. (B and D) Cohesive failure within the sealer of BCC/BCS at the canal side and the cone side. RD, root dentin; AHP, AH Plus.

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Figure 2. (A–D) Confocal laser scanning microscopic images of sealer penetration in root canals obturated with GP/AH and BCC/BCS. BCC/BCS showed a higher penetration depth, total area, and circumferential penetration than GP/AH. The sealer penetration depth was lower at the apical area than at the coronal and middle root levels. (A and C) GP/AH in the cross-sectional and longitudinal view. (B and D) BCC/BCS in the cross-sectional and longitudinal view.

adhesion to root dentin plays a major role in reinforcing the prepared roots rather than the MOE. BCS showed higher sealer penetration than AH (25). Higher sealer penetration was expected to improve bond strength by increasing micromechanical interlocking (26), but the bond strengths of BCC/BCS and GP/AH were similar at the coronal and middle root levels (27, 28). Thus, the effect of sealer penetration on bond strength might be limited. Bond strengths at the apical level were usually higher than at the middle level (29, 30). A matched master cone at the apical level TABLE 3. The Elastic Modulus of Gutta-percha (GP), a Bioceramic Cone (BCC), AH Plus, and a Bioceramic Sealer (BCS) Group

Modulus of elasticity (GPa)

BCC GP BCS AH Plus

0.20  0.03a 0.28  0.04a 2.54  0.13b 0.30  0.09a

Different lowercase letters indicate a statistically significant difference (P < .05). The modulus of elasticity of dentin was reported to be 8.60  0.86 GPa (21).

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might generate high hydraulic force that could enhance sealer adaptation to root canal. Moreover, the matched cone created a thin layer of sealer, which the bond strength at apical level increased. BCC/BCS showed superior push-out bond strength to GP/AH, particularly at the apical level (31). This may be explained by bioceramic containing nanoparticles, slightly expanding after setting (14, 32), and the hydrophilic property (33) of BCS. In addition, the different particle sizes between BCS and AH might also affect the result. BCS has a smaller particle size on average of 0.2 mm, which might enhance the distribution of particles into dentinal tubules (34), especially smaller tubules at the apical root area. This phenomenon could make the sealer tags stronger with a higher resistance to push-out force. In contrast, AH sealer contains larger calcium tungstate particles with an average size of 8 mm and zirconium oxide particles with a size of 1.5 mm (manufacturer data). The larger particle might not enter easily into the small tubules at the apical root level, and the sealer tags would be weaker than those of BCS. This could result in the lower resistance to the push-out force of AH sealer. Another possible reason is that sclerotic dentin in the apical root area may interact with BC and form a chemical bond (35) to improve the bond strength of BCS.

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Basic Research—Technology BCC/BCS showed better push-out bond strength at the apical root area, but root reinforcement of BCC/BCS was similar to GP/AH. A higher bond strength in the apical area of BCS was not sufficient to increase the strengthening effect on the root. Cohesive failure within BCS was predominantly found at all root levels. This result suggests the adhesion between BCS-BCC and BCS-dentin was higher than the cohesive strength of BCS; the true bond strength of BCS to dentin should be higher than what has been reported. In contrast, adhesive failure was mostly found at the interface between the sealer and cone in GP/AH, which indicated AH did not bond well to GP. A chemical bond might be created between either the sealer and dentin or the sealer and the cone. AH may produce limited chemical adhesion to dentin from a mild covalent bond between its open epoxide ring and amino group in collagen fiber (36). BCS may induce a chemical bond by hydroxyapatite crystal precipitation from a hydration reaction, which possibly adheres to BCC and dentin. It has been hypothesized that adhesion at the BCS-BCC interface is a chemical union between the calcium silicate nanoparticles in the cone and the sealer. Nevertheless, the mechanism of the chemical bond between BCC and BCS has been still unproved. In addition, the bond between BCS and regular GP should be further investigated and compared with that of BCC and BCS to confirm the benefit of using BCC. With the limited number of samples used in this study, obturation with the BC system tended to be better than GP/AH in these investigated parameters although the difference was not statistically significant for some factors.

Conclusion Weakened, prepared roots could be reinforced by BCC/BCS or GP/AH because of their ability to bond to root dentin regardless of their low MOE values. However, BCC/BCS provides better adhesion and sealer penetration, especially at the apical area.

Acknowledgments The authors would like to thank Drs Chulaluk Komoltri and Natchalee Srimaneekarn for their assistance with statistical analysis. This research was supported by the Faculty of Dentistry, Mahidol University, Salaya, Thailand (grant no. DTDT-001-0-MI-I). The authors deny any conflicts of interest related to this study.

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