doi:10.1111/iej.12241
Fracture resistance and stress distribution of simulated immature teeth after apexification with mineral trioxide aggregate
nior1,2, R. D. Pereira3, C. Verıssimo4, C. J. Soares4, A. L. Faria-e-Silva5, M. Brito-Ju C. C. Camilo1,2 & M. D. Sousa-Neto2 1
Interinstitutional PhD program, State University of Montes Claros, Montes Claros; 2Department of Restorative Dentistry, Faculty of Dentistry, University of S~ao Paulo, Ribeir~ ao Preto; 3Department of Dentistry, United Universities of the North of Minas, Montes Claros; 4Department of Operative Dentistry and Dental Materials, School of Dentistry, Federal University of Uberl^ andia, Uberl^ andia; and 5Department of Dentistry, Federal University of Sergipe, Aracaju, Brazil
Abstract nior M, Pereira RD, Verıssimo C, Soares CJ, Brito-Ju Faria-e-Silva AL, Camilo CC, Sousa-Neto MD. Fracture resistance and stress distribution of simulated immature teeth after apexification with mineral trioxide aggregate. International Endodontic Journal, 47, 958–966, 2014.
Aim To evaluate the effect of adhesive restorations on fracture resistance and stress distribution in teeth with simulated immature apices and apical plugs of mineral trioxide aggregate (MTA). Methodology Sixty bovine incisors were sectioned 8 mm above and 12 mm below the cemento-enamel junction (CEJ). The root canal was enlarged using a diamond bur, resulting in remaining root canal walls with 0.1–0.2 mm of thickness. A 5-mm apical plug of MTA was placed and the teeth were restored according to the following groups: GP – the root canal was filled with gutta-percha and endodontic sealer; CR – the root canal was filled with light-cured composite resin inserted incrementally; FP – a fibre post was cemented into the root canal; and RFP – the fibre post was relined with composite resin prior to the cementation into the
Introduction Immature teeth with diseased or necrotic pulps generally exhibit arrested root development and open apices.
Correspondence: Manoel Brito-J unior, Departamento de Odontologia, Unimontes, Av. Rui Braga s/n, Vila Mauriceia, 39401-089 Montes Claros, MG, Brazil (Tel.: +55 38 3229 8284; e-mail: [email protected]).
958
International Endodontic Journal, 47, 958–966, 2014
root canal. A load was applied on the crown of all teeth at 135° to their long axis until fracture. Data was analysed by one-way ANOVA and SNK tests (a = 0.05), whilst the fracture pattern was evaluated according to the position of the fracture. Stress distributions in the restored teeth were verified by finite element analysis. Results Teeth restored with fibre posts and relined fibre posts were associated with the highest fracture resistance, whilst the GP group had the lowest values. GP and RC groups had similar fracture resistance values (P = 0.109). All fractures types involved the cervical and middle thirds of roots. The GP model had high levels of stress concentration in the cervical and middle thirds of roots. No difference was found amongst the stress concentration in the RC, FP and RFP models. Conclusion Restorative protocols alter the fracture resistance and stress distribution of immature teeth after placement of MTA apical plugs. Keywords: apexification, dental restoration, permanent, mineral trioxide aggregate, post and core technique, tooth fracture. Received 20 July 2013; accepted 1 January 2014
Partial pulpotomy, revascularization, apexogenesis and apexification are therapeutic approaches indicated to treat these cases (Garcia-Godoy & Murray 2012). Successful apexification or revascularization procedures with the formation of a calcified apical barrier or continued root development in immature teeth have been reported (Banchs & Trope 2004, Cotti et al. 2008, Chala et al. 2011). The use of mineral trioxide aggregate (MTA) to achieve root-end closure
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Brito-J unior et al. Mechanical behaviour of restored immature teeth
has been recommended in several clinical studies (Simon et al. 2007, Holden et al. 2008, Sarris et al. 2008, Witherspoon et al. 2008, Mente et al. 2009). However, the fragility of roots due to the presence of thin dentine walls can compromise tooth restoration after MTA placement. Root filling of immature teeth with gutta-percha does not reinforce the remaining root, and a higher risk of fracture is observed (Bortoluzzi et al. 2007, Hemalatha et al. 2009). Composite resin has the ability to bond to root dentine walls, increasing the strength of the roots (Wilkinson et al. 2007). However, this restorative material when used in the root canal has relatively low strength under tensile stress (Cabrera & Macorra 2011). An alternative is the use of fibre posts (Schmoldt et al. 2011), which have an elastic modulus similar to dentine and may bond adhesively to dentine, reducing the failure of the restorative complex and supporting tensile stress more effectively (Prisco et al. 2003, Santos-Filho et al. 2008, Soares et al. 2008a,b). Despite the advantages of using a fibre post, the mismatch between the diameter of the post space and the fibre post is a problem, particularly in immature teeth with large root canals. The use of an auxiliary fibre post (Li et al. 2011) or fibre post relining (Fariae-Silva et al. 2009, Macedo et al. 2010, 2013), as demonstrated in over-flared root canals, could overcome this difficulty. The increased contact between the relined fibre post and the canal walls increases the bond strength and can improve the fracture resistance of teeth (Silva et al. 2011). However, limited information about fracture resistance and stress patterns is available when comparing composite resin, fibre posts and relined fibre posts to restore immature teeth. This study aimed to evaluate the fracture resistance and stress distribution of simulated immature teeth root filled with an apical barrier of MTA and restored with composite resin, fibre post or relined fibre post. The null tested hypothesis was that these restorative protocols do not affect fracture resistance and stress distribution.
Materials and methods Fracture resistance Sixty bovine incisors with similar root dimensions were selected based on the measurement of their buccolingual and mesiodistal dimensions in millime-
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
tres, allowing a maximum deviation of 10% from the determined mean. Teeth with carious lesions, root fractures and/or cracks (under 409 magnification) were discarded. The teeth were sectioned 8 mm above and 12 mm below the cemento-enamel junction using a low-speed diamond saw (Bortoluzzi et al. 2007). Pulp tissue was removed using a size 60 Hedstr€ om file (Dentsply Maillefer, Ballaigues, Switzerland), and the root canal was irrigated with 1% sodium hypochlorite (NaOCl). The root canals were enlarged coronal-apically using a size 3017 HL diamond bur (KG Sorensen, Barueri, SP, Brazil) to create a standard hole of 2.5 mm in diameter. Use of the bur was followed by irrigation with 1% NaOCl. The remaining thickness of the root canal walls (buccal, lingual, mesial and distal) near the root apex was measured with a digital calliper and was standardized at 0.1–0.2 mm. After removing the smear layer with buffered 14.3% EDTA solution (Biodin^ amica Quımica Ltda., Ibipor~ a, PR, Brazil) for 3 min, the root canals were irrigated again with 1% NaOCl and dried with paper points (Dentsply Maillefer). A 5-mm apical plug with white MTA (Angelus, Londrina, PR, Brazil) was placed. MTA was mixed at a powder to liquid ratio of 3 : 1, inserted into the root canal with an MTA carrier and condensed with hand pluggers under ultrasonic vibration (Lawley et al. 2004). Placement of MTA increments was verified with radiographs to control the apical plug thickness. During condensation procedures, the teeth were placed on the condensation-cured silicon impression material (Zetaplus dense, Zhermack, Badia Polsine, Italy) to simulate periapical tissue, thus preventing MTA extrusion. The teeth were radiographed again to confirm the height and homogeneity of apical plug, followed by storage of samples at 37 °C and 100% humidity for 1 week. After this period, the teeth were inserted into resin cylinders with simulated periodontal ligament as described previously by Soares et al. (2005). Then, the external surfaces of the root were covered in wax until 2 mm below the CEJ. For this, the roots were dipped into molten wax, resulting in a wax layer 0.2– 0.3 mm thick. Afterwards, the wax-covered roots were embedded in acrylic resin (Cl assico, Rio de Janeiro, RJ, Brazil) cylinders. After resin polymerization, the roots were removed from the cylinder and the wax removed from the root surface, creating a space in the resin cylinder. The polyether impression material Impregum F (3M ESPE, St. Paul, MN, USA) was mixed and placed in the space created in the resin cylinder. The tooth was re-inserted into the
International Endodontic Journal, 47, 958–966, 2014
959
Mechanical behaviour of restored immature teeth Brito-J unior et al.
cylinder and the excess material removed with a scalpel blade. The samples were randomly allocated to one of following groups: GP: The root canal was only filled with nonstandardized gutta-percha cones (Odous, Belo Horizonte, MG, Brazil) and endodontic sealer (Sealer 26, Dentsply, Petropolis, RJ, Brazil), using a thermo-mechanical technique (Tagger et al. 1984). A radiograph was taken to confirm that the canal was filled. The guttapercha was removed from the coronal region to create a space of 7 mm. The light-cured composite resin Filtek Z-350 (3M ESPE) was inserted in 2 mm increments until the entire space was filled. Each increment was light-cured for 40s. CR: The root canal walls were acid-etched for 15s, rinsed with distilled water and dried with absorbent paper cones. The adhesive system Adper Single Bond 2 (3M ESPE) was applied, air-dried and light-cured for 40s. The composite resin Filtek Z-350 was incrementally inserted until it filled the root canal with each increment being light-cured for 40s. FP: The glass-fibre post Exacto size 3 (Angelus) was cemented into the root canal with a self-adhesive resin cement, RelyX U100 (3M ESPE, Seefeld, Germany). The mixed cement was inserted into the canal with a size 80 K-file (Dentsply Maillefer), and the post was inserted, followed by light activation of the cement for 40s. The post was sectioned 3 mm below the incisal edge, and the remaining space was filled with composite resin as in the GP and CR groups. RFP: Prior to cementation, the fibre post was relined with composite resin. The canal walls were lubricated with hydro-soluble gel before the fibre post covered with resin composite Filtek Z-350 was inserted into the canal. The resin composite was light-cured for 20s, the relined fibre post was removed, and the resin composite was light-cured for another 40s. Copious rinsing removed the lubricant gel from the root canal, and the relined fibre post was cemented as in the FP group before the coronal space was filled with composite resin. All light-curing procedures were performed using a light-emitting diode curing unit (Radii Cal; SDI, Bayswater, Victoria, Australia) with 600-mW cm 2 irradiance. The fracture resistance tests were performed using a universal testing machine (EZ-L, Shimadzu, Kyoto, Japan). Each sample was placed in a custom apparatus that allowed application of compressive loading at 135° to the long axis of the tooth in a buc-
960
International Endodontic Journal, 47, 958–966, 2014
cal/lingual direction. The compressive loading was applied at a speed of 0.5 mm min 1, and the force required to cause fracture (N) was recorded by a 5KN load cell hardwired to software. Statistical analysis was performed using SigmaStat statistical software version 3.5 (Systat Software, Point Richmond, CA, USA). The fracture data showed normality and equal variance. Thus, data were submitted to one-way ANOVA followed by the Student–Newman–Keuls (SNK) test (a = 0.05). The fracture patterns were evaluated using a classification described by Santos-Filho et al. (2008): (i) resin core or post fracture, (ii) cervical root fracture, (iii) mid root fracture, (iv) apical root fracture and (v) vertical root fracture.
Two-dimensional finite element analysis Four two-dimensional (2-D) models were created following the experimental groups: GP model, CR model, FP model and RFP model. The geometrical model was based on digital photographs of buccolingual crosssections of the samples used in the experimental tests. The images were then exported to software for image manipulation (Image J software, public domain, Javabased image processing and analysis software developed at the National Institutes of Health, Bethesda, MD, USA). The coordinates obtained were exported to the finite element analysis software Marc/Mentat (MSC software, Santa Ana, CA, USA). The crosssectional model was divided into elements to facilitate the calculation of stress distribution (meshing process using plane-strain quadrilateral isoparametric elements for the structures linear, with four nodes – Fig. 1a). Friction contact was inserted between the dentine and gutta-percha/MTA; these structures could slide along or separate from the dentine but could not penetrate. The friction coefficient was 0.3 (Coulomb friction). Boundary condition was defined with a 100 N as that force applied to the enamel surface with a simulated cylindrical tip that was the same used in laboratory tests at 45° to the buccolingual long axis. A nodal displacement constraint (model fixation) was applied at the bottom and lateral surfaces of the support cylinder on the X and Y axes (Fig. 1b). A nonlinear structural analysis was performed, and all materials were considered linear, isotropic and homogeneous; therefore, only two mechanical properties were necessary to perform finite element analysis: elastic modulus and Poisson’s ratio (Table 1), with the exception of the glass-fibre post, which was considered orthotropic, as this material has different
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Brito-J unior et al. Mechanical behaviour of restored immature teeth
(a)
(b)
Figure 1 (a) Mesh generation and (b) load application with the simulated cylindrical tip and fixed displacements on X and Y
axes.
Table 1 Mechanical properties of isotropic structures (MPa)
Table 2 Mechanical properties of the glass-fibre post
Material Bovine enamel Bovine dentine Gutta-percha Composite resin Polyether Polystyrene resin MTA Resin cement
Elastic modulus (MPa)
Poisson’s ratio
Properties
Glass-fibre post
84 000 18 600 0.69 15 800 50 13 500 15 700 8130
0.30 0.31 0.45 0.24 0.45 0.31 0.23 0.30
EX (MPa) EY (MPa) EZ (MPa) gXY gXZ gYZ Gxy (MPa) Gxz (MPa) Gxz (MPa)
37 000 9500 9500 0.34 0.34 0.27 3544.8 3544.8 1456.7
mechanical performance when the load is apply axially or perpendicular to the direction of the fibres. For this condition, nine mechanical properties are necessary to characterize this material in the finite element analysis: elastic modulus in each three orthogonal planes; Poisson’s ratio in each combination of three orthogonal planes; and the resultant shear modulus in each three orthogonal planes (Table 2). The finite element stress analysis was performed using Mentat (pre- and post-processor) and Marc (solver) (MSC software, Newport Beach, CA, USA). The stress
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
E – elastic modulus; g – Poison’s ratio; G – shear modulus; and x, y and z – specific orthogonal plane directions.
distributions in the restored teeth were analysed using von Mises equivalent stresses and maximum principal stress.
Results Table 3 shows the mean fracture strength values, standard deviations and confidence intervals for each
International Endodontic Journal, 47, 958–966, 2014
961
Mechanical behaviour of restored immature teeth Brito-J unior et al.
Table 3 The force required (N) to cause fracture in experimental groups
Group
Mean
SD
CI 95%
Student– Newman– Keuls rankinga
RFP (relined fibre post) FP (fibre post) CR (composite resin) GP (guttapercha + sealer)
2230.1
265.7
2040.0–2420.2
A
2175.1
292.5
1965.9–2384.3
A
1856.1
314.2
1631.3–2080.9
B
1646.9
262.2
1459.3–1834.5
(a)
(b)
(c)
(d)
B
Distinct letters indicate statistical difference (a = 0.05). SD, standard deviation; and CI, confidence interval.
a
group. One-way ANOVA revealed significant effects of treatment (P < 0.001), whilst the power of the performed test was 0.99. The Student–Newman–Keuls’s test revealed that the relined fibre post group had similar fracture resistance to the fibre post group (P = 0.668) and, significantly, higher fracture resistance than the resin composite group (P = 0.015). The gutta-percha group had similar fracture resistance to the resin composite group (P = 0.109) and significantly lower fracture resistance than the relined fibre post (P < 0.001) and fibre post (P < 0.001) groups. With regard to fracture patterns, only fractures type II and III were observed in all experimental conditions, without significant differences between groups. Figure 2 shows examples of specimens fractured horizontally or obliquely at the cervical (type II) or middle (type III) thirds of roots. The results of finite element analysis using the von Mises and maximum principal stress are shown in Figs 3 and 4, respectively. The GP model had high stress concentration in the root dentine in the cervical and middle thirds (Fig. 3a). The GP model also exhibited a high concentration of tensile stress in the lingual root surface (Fig. 4a). The stress distributions by von Mises criteria and maximum principal stress in the roots restored with resin composite (RC), fibre post (FP) and relined fibre post (RFP) were similar, with high levels of compression stress at the buccal root surface (Fig. 4b–d). The RC, FP and RFP models also had stress concentration in the cervical thirds of the root and crown, but at lower levels than were verified for the GP model.
962
International Endodontic Journal, 47, 958–966, 2014
Figure 2 Examples of specimens fractured at cervical (a, b) and middle (c, d) thirds of roots.
Discussion Despite the high rate of success of apexification procedures using MTA, the tooth remains weakened due to the presence of thin root canal walls. Thus, restorative protocols able to reinforce the remaining tooth are important (Bortoluzzi et al. 2007, Wilkinson et al. 2007, Hemalatha et al. 2009, Dikbas et al. 2014 ). In the present study, immature apices were simulated in bovine incisors by diamond burs using a previously described methodology (Bortoluzzi et al. 2007). Furthermore, this methodology resulted in a remaining crown of around 8 mm for application of loads during the tests of fracture resistance. Destructive mechanical tests, such as fracture resistance, are important means of analysing tooth behaviour in situations of concentrated and high-intensity load application (Santos-Filho et al. 2008). However, they provide limited biomechanical information that does not allow the analysis of stress distribution, which can explain the failure patterns. Thus, the association of nondestructive analysis such as a finite element is essential for the biomechanical analysis of tooth structure. The combination of these methods provides a visualization and quantification of the stresses and strains generated in the structure when
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Brito-J unior et al. Mechanical behaviour of restored immature teeth
(a)
(b)
(c)
(d)
Figure 3 The von Mises stress distribution (MPa). (a) GP group (gutta-percha); (b) CR group (composite resin); (c) FP group (fibre post); and (d) RFP group (relined fibre post).
(a)
(b)
(c)
(d)
Figure 4 Stress distribution by maximum principal stress (MPa); compressive stress (negative/blue); and tensile stress (positive/ yellow). (a) GP group (gutta-percha); (b) CR group (composite resin); (c) FP group (fibre post); and (d) RFP group (relined fibre post).
low-intensity loads are applied, predicting the possible sites of failure initiation (Soares et al. 2008a,b). The experimental model used in this study resulted in four intact coronal walls. Usually, root filling with
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
gutta-percha and sealer and direct placement of a restoration such as composite resin is used clinically in this situation (Cvek 1992). However, the presence of flared root canals or weakened roots may compromise
International Endodontic Journal, 47, 958–966, 2014
963
Mechanical behaviour of restored immature teeth Brito-J unior et al.
the long-term outcome. It was hypothesized that filling root canals with an adhesive material with a similar elastic modulus to dentine could improve the fracture resistance and stress distribution of immature teeth (Lawley et al. 2004, Wilkinson et al. 2007). The composite resin root filling (CR group) gave similar results to those observed with the GP group in the fracture resistance test. However, in the finite element analysis, the GP group revealed worse stress distribution. Figure 3 shows more ‘red colour’ on the cervical region, which means more stress concentration; Fig. 4 shows high levels of tensile stress in the root canal at the cervical and middle thirds. Such findings may explain the lower fracture resistance caused by higher levels of stress concentration; thus, the hypothesis of the study was accepted. Composite resin and fibre glass posts have a similar elastic modulus to dentine and are able to distribute applied loads, reducing stress concentrations (Cabrera & Macorra 2011), as shown in this study. To determine this mechanical behaviour, a proper bonding at the dentine/composite interface is needed for dissipation of stresses generated by loading (Santos et al. 2010). However, adhesive procedures in the root canal have several limitations and commonly result in low bond-strength values. This occurs mainly due to the difficulty of moisture control, the complexity of dentine and issues relating to light activation of the adhesive (Chersoni et al. 2005, Wu et al. 2009). Furthermore, the polymerization shrinkage of composite inserted into a constricted root canal space results in high stresses that can disrupt bonding at the composite/dentine interface (Aksornmuang et al. 2011). It is important to emphasize that the composite was inserted incrementally into the root canals to reduce polymerization stress and to permit proper light activation. Clinically, the use of dual-cured resin composite to fill the root canal can be a viable alternative; however, the present study used a light-cured composite as this material is readily available. The von Mises stress is a failure criterion that demonstrates energy transmission in a structure, that is, where there is a higher concentration of energy, whilst the maximum principal stress discriminates the tensile and compressive stress fields. Some dental materials (i.e. composite resin) and structures (i.e. enamel) have high compressive strength but are brittle when subjected to tensile stresses (Chai et al. 2009). Thus, the maximum principal stress shows the locations within the structure that are more prone to failure by tensile stress. In this case, teeth restored
964
International Endodontic Journal, 47, 958–966, 2014
without fibre posts or resin composite had high levels of stress concentration in the root at the cervical and middle thirds, which corroborate the results of fracture resistance and modes of failure. The present study revealed that the use of fibre posts was able to improve fracture resistance and stress distribution. However, no significant difference was found between the RC, FP and RFP groups in the finite element analysis. This may be explained by the limitation of the 2-D finite element simulation and also because the interface between composite and dentine was simulated as a perfect bond during all load application processes, which is not realistic. Another limitation of finite element analysis relates to the assumed properties for materials and tissues, namely, that all materials used are linear, isotropic and homogeneous. In reality, most dental materials and tooth tissues are anisotropic and nonhomogeneous. In addition, the loading scenarios investigated lack the complexity that occurs during functional load in the patient, which results in nonlinearity of the load application and its effects. However, in general as with composite resin, fibre posts have elastic moduli similar to dentine (Prisco et al. 2003, Santos-Filho et al. 2008, Soares et al. 2008a,b). Thus, the bond strength between different components was preserved in the presence of the fibre post, which can explain the results. Additionally, the cementation procedures using self-adhesive resin cements used in this study were simpler than conventional bonding procedures used in root canals. Furthermore, self-adhesive resin cements appear to have low shrinkage due to their viscoelastic properties (Amaral et al. 2011). The effect of polymerization shrinkage is additionally reduced by lower volume of material as the fibre post occupies most of the space in the root canal. This additional advantage is more pronounced when the fibre post is relined (D’Arcangelo et al. 2007, Faria-e-Silva et al. 2009, Macedo et al. 2010). These factors contribute to improve the bonding of restorative material to dentine. An unfavourable situation for post-restored teeth has been related mainly to debonding, which results in stress within the adhesive interfaces favouring fracture of the post (Santos et al. 2010). Thus, bond strengths of resin cements to fibre posts have been challenged by thermal or mechanical cycling protocols simulating clinical function in different experimental designs (Amaral et al. 2011, Bitter et al. 2012, Mazzitelli et al. 2012, Macedo et al. 2013). In the present study, these protocols were not performed
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Brito-J unior et al. Mechanical behaviour of restored immature teeth
before the test of fracture resistance, which could represent a methodological limitation. However, it has been demonstrated that the bonding ability of the self-adhesive cement RelyX U100, as used in this study, is less affected by mechanical or thermocycling procedures (Amaral et al. 2011, Mazzitelli et al. 2012, Macedo et al. 2013). Moreover, simulation of ageing is a controversial topic (Bitter et al. 2012). In the present study, teeth filled with gutta-percha and sealer were included to provide a comparative assessment with bonding techniques. The absence of a group with instrumented canals without restoration could represent a limitation of this study. However, the findings of previous studies (Bortoluzzi et al. 2007, Schmoldt et al. 2011) showed that canals filled with gutta-percha and sealer have root fracture strengths similar to those of weakened roots that did not receive reinforcement. In addition, several studies have already demonstrated that the adhesive restorations significantly strengthen the roots when compared to positive controls (Carvalho et al. 2005, Alsamadani et al. 2012, Seto et al. 2013).
Conclusion Within the limits of this study, it can be concluded that simulated immature teeth with open apices filled with an MTA apical plug and restored with fibre posts were a better adhesive strategy for restoring immature teeth, from the standpoint of stress and fracture resistance, than restoration with gutta-percha and sealer.
Acknowledgements The authors gratefully acknowledge Prof. Maria Helena Santos for the use of the BioMat laboratory (UFVJM, Diamantina, Brazil) and to FAPEMIG for financial support.
References Aksornmuang J, Nakajima M, Senawongse P, Tagami J (2011) Effects of C-factor and resin volume on the bonding to root canal with and without fibre post insertion. Journal of Dentistry 39, 422–9. Alsamadani KH, Abdaziz el-SM, Gad el-S (2012) Influence of different restorative techniques on the strength of endodontically treated weakened roots. International Journal of Dentistry 2012, 343712. Amaral M, Rippe MP, Bergoli CD, Monaco C, Valandro LF (2011) Multi-step adhesive cementation versus one-step
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
adhesive cementation: push-out bond strength between fiber post and root dentin before and after mechanical cycling. General Dentistry 59, e185–91. Banchs F, Trope M (2004) Revascularization of immature permanent teeth with apical periodontitis: new treatment protocol? Journal of Endodontics 30, 196–200. Bitter K, Perdigao J, Exner M, Neumann K, Kielbassa A, Sterzenbach G (2012) Reliability of fiber post bonding to root canal dentin after simulated clinical function in vitro. Operative Dentistry 37, 397–405. Bortoluzzi EA, Souza EM, Reis JM, Esberard RM, TanomaruFilho M (2007) Fracture strength of bovine incisors after intra-radicular treatment with MTA in an experimental immature tooth model. International Endodontic Journal 40, 684–91. Cabrera E, Macorra JC (2011) Microtensile bond strength distributions of three composite materials with different polymerization shrinkages bonded to dentin. The Journal of Adhesive Dentistry 13, 39–48. Carvalho CA, Valera MC, Olivera LD, Camargo CH (2005) Structural resistance in immature teeth using root reinforcements in vitro. Dental Traumatology 21, 155–9. Chai H, Lee JJ, Kwon JY, Lucas PW, Lawn BR (2009) A simple model for enamel fracture from margin cracks. Acta Biomaterialia 5, 1663–7. Chala S, Abouqal R, Rida S (2011) Apexification of immature teeth with calcium hydroxide or mineral trioxide aggregate: systematic review and meta-analysis. Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology and Endodontics 112, e36–42. Chersoni S, Acquaviva GL, Prati C et al. (2005) In vivo fluid movement through dentin adhesives in endodontically treated teeth. Journal of Dental Research 84, 223–7. Cotti E, Mereu M, Lusso D (2008) Regenerative treatment of an immature, traumatized tooth with apical periodontitis: report of a case. Journal of Endodontics 34, 611–6. Cvek M (1992) Prognosis of luxated non-vital maxillary incisors treated withc alciumhydroxide and filled with guttapercha. A retrospective clinical study. Endodontic & Dental Traumatology 18, 45–55. D’Arcangelo C, Cinelli M, De Angelis F, D’Amario M (2007) The effect of resin cement film thickness on the pullout strength of a fiber-reinforced post. The Journal of Prosthetic Dentistry 98, 193–8. Dikbas I, Tanalp J, Koksal T, Yalniz A, G€ ung€ or T (2014) Investigation of the effect of different prefabricated intracanal posts on fracture resistance of simulated immature teeth. Dental Traumatology 30, 49–54. Faria-e-Silva AL, Pedrosa-Filho CF, Menezes MS, Silveira DM, Martins LR (2009) Effect of relining on fiber post retention to root canal. Journal Applied of Oral Science 17, 600–4. Garcia-Godoy F, Murray PE (2012) Recommendations for using regenerative endodontic procedures in permanent immature traumatized teeth. Dental Traumatology 28, 33–41.
International Endodontic Journal, 47, 958–966, 2014
965
Mechanical behaviour of restored immature teeth Brito-J unior et al.
Hemalatha H, Sandeep M, Kulkarni S, Yakub SS (2009) Evaluation of fracture resistance in simulated immature teeth using Resilon and Ribbond as root reinforcements-an in vitro study. Dental Traumatology 25, 433–8. Holden DT, Schwartz SA, Kirkpatrick TC, Schindler WG (2008) Clinical outcomes of artificial root-end barriers with mineral trioxide aggregate in teeth with immature apices. Journal of Endodontics 34, 812–7. Lawley GR, Schindler WG, Walker WA 3rd, Kolodrubetz D (2004) Evaluation of ultrasonically placed MTA and fracture resistance with intracanal composite resin in a model of apexification. Journal of Endodontics 30, 167–72. Li Q, Xu B, Wang Y, Cai Y (2011) Effects of auxiliary fiber posts on endodontically treated teeth with flared canals. Operative Dentistry 36, 380–9. Macedo VC, Faria e Silva AL, Martins LR (2010) Effect of cement type, relining procedure, and length of cementation on pull-out bond strength of fiber posts. Journal of Endodontics 36, 1543–6. Macedo V, Souza N, Faria-e-Silva AL et al. (2013) Pullout bond strength of fiber posts luted to different depths and submitted to artificial aging. Operative Dentistry 38, E1–6. Mazzitelli C, Monticelli F, Toledano M, Ferrari M, Osorio R (2012) Effect of thermal cycling on the bond strength of self-adhesive cements to fiber posts. Clinical Oral Investigations 16, 909–15. Mente J, Hage N, Pfefferle T et al. (2009) Mineral trioxide aggregate apical plugs in teeth with open apical foramina: a retrospective analysis of treatment outcome. Journal of Endodontics 35, 1354–8. Prisco D, De Santis R, Mollica F, Ambrosio L, Rengo S, Nicolais L (2003) Fiber post adhesion to resin luting cements in the restoration of endodontically-treated teeth. Operative Dentistry 28, 515–21. Santos AF, Meira JB, Tanaka CB et al. (2010) Can fiber posts increase root stresses and reduce fracture? Journal of Dental Research 89, 587–91. Santos-Filho PC, Castro CG, Silva GR, Campos RE, Soares CJ (2008) Effects of post system and length on the strain and fracture resistance of root filled bovine teeth. International Endodontic Journal 41, 493–501. Sarris S, Tahmassebi JF, Duggal MS, Cross IA (2008) A clinical evaluation of mineral trioxide aggregate for root-end
966
International Endodontic Journal, 47, 958–966, 2014
closure of non-vital immature permanent incisors in children-a pilot study. Dental Traumatology 24, 79–85. Schmoldt SJ, Kirkpatrick TC, Rutledge RE, Yaccino JM (2011) Reinforcement of simulated immature roots restored with composite resin, mineral trioxide aggregate, gutta-percha, or a fiber post after thermocycling. Journal of Endodontics 37, 1390–3. Silva GR, Santos-Filho PC, Simamoto-Junior PC, Martins LR, Mota AS, Soares CJ (2011) Effect of post type and restorative techniques on the strain and fracture resistance of flared incisor roots. Brazilian Dental Journal 22, 230–7. Simon S, Rilliard F, Berdal A, Machtou P (2007) The use of mineral trioxide aggregate in one-visit apexification treatment: a prospective study. International Endodontic Journal 40, 186–97. Soares CJ, Pizi EC, Fonseca RB, Martins LR (2005) Influence of root embedment material and periodontal ligament simulation on fracture resistance tests. Brazilian Oral Research 19, 11–6. Soares CJ, Santana FR, Castro CG et al. (2008a) Finite element analysis and bond strength of a glass post to intraradicular dentin: comparison between microtensile and push-out tests. Dental Materials 24, 1405–11. Soares CJ, Soares PV, de Freitas Santos-Filho PC, Castro CG, Magalhaes D, Versluis A (2008b) The influence of cavity design and glass fiber posts on biomechanical behavior of endodontically treated premolars. Journal of Endodontics 34, 1015–9. Tagger M, Tamse A, Katz A, Korzen BH (1984) Evaluation of the apical seal produced by a hybrid root canal filling method, combining lateral condensation and thermatic compaction. Jounal of Endodontics 10, 299–303. Wilkinson KL, Beeson TJ, Kirkpatrick TC (2007) Fracture resistance of simulated immature teeth filled with resilon, gutta-percha, or composite. Journal of Endodontics 33, 480–3. Witherspoon DE, Small JC, Regan JD, Nunn M (2008) Retrospective analysis of open apex teeth obturated with mineral trioxide aggregate. Journal of Endodontics 34, 1171–6. Wu H, Hayashi M, Okamura K et al. (2009) Effects of light penetration and smear layer removal on adhesion of postcores to root canal dentin by self-etching adhesives. Dental Materials 25, 1484–92.
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Fracture resistance and stress distribution of simulated immature teeth after apexification with mineral trioxide aggregate
nior1,2, R. D. Pereira3, C. Verıssimo4, C. J. Soares4, A. L. Faria-e-Silva5, M. Brito-Ju C. C. Camilo1,2 & M. D. Sousa-Neto2 1
Interinstitutional PhD program, State University of Montes Claros, Montes Claros; 2Department of Restorative Dentistry, Faculty of Dentistry, University of S~ao Paulo, Ribeir~ ao Preto; 3Department of Dentistry, United Universities of the North of Minas, Montes Claros; 4Department of Operative Dentistry and Dental Materials, School of Dentistry, Federal University of Uberl^ andia, Uberl^ andia; and 5Department of Dentistry, Federal University of Sergipe, Aracaju, Brazil
Abstract nior M, Pereira RD, Verıssimo C, Soares CJ, Brito-Ju Faria-e-Silva AL, Camilo CC, Sousa-Neto MD. Fracture resistance and stress distribution of simulated immature teeth after apexification with mineral trioxide aggregate. International Endodontic Journal, 47, 958–966, 2014.
Aim To evaluate the effect of adhesive restorations on fracture resistance and stress distribution in teeth with simulated immature apices and apical plugs of mineral trioxide aggregate (MTA). Methodology Sixty bovine incisors were sectioned 8 mm above and 12 mm below the cemento-enamel junction (CEJ). The root canal was enlarged using a diamond bur, resulting in remaining root canal walls with 0.1–0.2 mm of thickness. A 5-mm apical plug of MTA was placed and the teeth were restored according to the following groups: GP – the root canal was filled with gutta-percha and endodontic sealer; CR – the root canal was filled with light-cured composite resin inserted incrementally; FP – a fibre post was cemented into the root canal; and RFP – the fibre post was relined with composite resin prior to the cementation into the
Introduction Immature teeth with diseased or necrotic pulps generally exhibit arrested root development and open apices.
Correspondence: Manoel Brito-J unior, Departamento de Odontologia, Unimontes, Av. Rui Braga s/n, Vila Mauriceia, 39401-089 Montes Claros, MG, Brazil (Tel.: +55 38 3229 8284; e-mail: [email protected]).
958
International Endodontic Journal, 47, 958–966, 2014
root canal. A load was applied on the crown of all teeth at 135° to their long axis until fracture. Data was analysed by one-way ANOVA and SNK tests (a = 0.05), whilst the fracture pattern was evaluated according to the position of the fracture. Stress distributions in the restored teeth were verified by finite element analysis. Results Teeth restored with fibre posts and relined fibre posts were associated with the highest fracture resistance, whilst the GP group had the lowest values. GP and RC groups had similar fracture resistance values (P = 0.109). All fractures types involved the cervical and middle thirds of roots. The GP model had high levels of stress concentration in the cervical and middle thirds of roots. No difference was found amongst the stress concentration in the RC, FP and RFP models. Conclusion Restorative protocols alter the fracture resistance and stress distribution of immature teeth after placement of MTA apical plugs. Keywords: apexification, dental restoration, permanent, mineral trioxide aggregate, post and core technique, tooth fracture. Received 20 July 2013; accepted 1 January 2014
Partial pulpotomy, revascularization, apexogenesis and apexification are therapeutic approaches indicated to treat these cases (Garcia-Godoy & Murray 2012). Successful apexification or revascularization procedures with the formation of a calcified apical barrier or continued root development in immature teeth have been reported (Banchs & Trope 2004, Cotti et al. 2008, Chala et al. 2011). The use of mineral trioxide aggregate (MTA) to achieve root-end closure
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Brito-J unior et al. Mechanical behaviour of restored immature teeth
has been recommended in several clinical studies (Simon et al. 2007, Holden et al. 2008, Sarris et al. 2008, Witherspoon et al. 2008, Mente et al. 2009). However, the fragility of roots due to the presence of thin dentine walls can compromise tooth restoration after MTA placement. Root filling of immature teeth with gutta-percha does not reinforce the remaining root, and a higher risk of fracture is observed (Bortoluzzi et al. 2007, Hemalatha et al. 2009). Composite resin has the ability to bond to root dentine walls, increasing the strength of the roots (Wilkinson et al. 2007). However, this restorative material when used in the root canal has relatively low strength under tensile stress (Cabrera & Macorra 2011). An alternative is the use of fibre posts (Schmoldt et al. 2011), which have an elastic modulus similar to dentine and may bond adhesively to dentine, reducing the failure of the restorative complex and supporting tensile stress more effectively (Prisco et al. 2003, Santos-Filho et al. 2008, Soares et al. 2008a,b). Despite the advantages of using a fibre post, the mismatch between the diameter of the post space and the fibre post is a problem, particularly in immature teeth with large root canals. The use of an auxiliary fibre post (Li et al. 2011) or fibre post relining (Fariae-Silva et al. 2009, Macedo et al. 2010, 2013), as demonstrated in over-flared root canals, could overcome this difficulty. The increased contact between the relined fibre post and the canal walls increases the bond strength and can improve the fracture resistance of teeth (Silva et al. 2011). However, limited information about fracture resistance and stress patterns is available when comparing composite resin, fibre posts and relined fibre posts to restore immature teeth. This study aimed to evaluate the fracture resistance and stress distribution of simulated immature teeth root filled with an apical barrier of MTA and restored with composite resin, fibre post or relined fibre post. The null tested hypothesis was that these restorative protocols do not affect fracture resistance and stress distribution.
Materials and methods Fracture resistance Sixty bovine incisors with similar root dimensions were selected based on the measurement of their buccolingual and mesiodistal dimensions in millime-
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
tres, allowing a maximum deviation of 10% from the determined mean. Teeth with carious lesions, root fractures and/or cracks (under 409 magnification) were discarded. The teeth were sectioned 8 mm above and 12 mm below the cemento-enamel junction using a low-speed diamond saw (Bortoluzzi et al. 2007). Pulp tissue was removed using a size 60 Hedstr€ om file (Dentsply Maillefer, Ballaigues, Switzerland), and the root canal was irrigated with 1% sodium hypochlorite (NaOCl). The root canals were enlarged coronal-apically using a size 3017 HL diamond bur (KG Sorensen, Barueri, SP, Brazil) to create a standard hole of 2.5 mm in diameter. Use of the bur was followed by irrigation with 1% NaOCl. The remaining thickness of the root canal walls (buccal, lingual, mesial and distal) near the root apex was measured with a digital calliper and was standardized at 0.1–0.2 mm. After removing the smear layer with buffered 14.3% EDTA solution (Biodin^ amica Quımica Ltda., Ibipor~ a, PR, Brazil) for 3 min, the root canals were irrigated again with 1% NaOCl and dried with paper points (Dentsply Maillefer). A 5-mm apical plug with white MTA (Angelus, Londrina, PR, Brazil) was placed. MTA was mixed at a powder to liquid ratio of 3 : 1, inserted into the root canal with an MTA carrier and condensed with hand pluggers under ultrasonic vibration (Lawley et al. 2004). Placement of MTA increments was verified with radiographs to control the apical plug thickness. During condensation procedures, the teeth were placed on the condensation-cured silicon impression material (Zetaplus dense, Zhermack, Badia Polsine, Italy) to simulate periapical tissue, thus preventing MTA extrusion. The teeth were radiographed again to confirm the height and homogeneity of apical plug, followed by storage of samples at 37 °C and 100% humidity for 1 week. After this period, the teeth were inserted into resin cylinders with simulated periodontal ligament as described previously by Soares et al. (2005). Then, the external surfaces of the root were covered in wax until 2 mm below the CEJ. For this, the roots were dipped into molten wax, resulting in a wax layer 0.2– 0.3 mm thick. Afterwards, the wax-covered roots were embedded in acrylic resin (Cl assico, Rio de Janeiro, RJ, Brazil) cylinders. After resin polymerization, the roots were removed from the cylinder and the wax removed from the root surface, creating a space in the resin cylinder. The polyether impression material Impregum F (3M ESPE, St. Paul, MN, USA) was mixed and placed in the space created in the resin cylinder. The tooth was re-inserted into the
International Endodontic Journal, 47, 958–966, 2014
959
Mechanical behaviour of restored immature teeth Brito-J unior et al.
cylinder and the excess material removed with a scalpel blade. The samples were randomly allocated to one of following groups: GP: The root canal was only filled with nonstandardized gutta-percha cones (Odous, Belo Horizonte, MG, Brazil) and endodontic sealer (Sealer 26, Dentsply, Petropolis, RJ, Brazil), using a thermo-mechanical technique (Tagger et al. 1984). A radiograph was taken to confirm that the canal was filled. The guttapercha was removed from the coronal region to create a space of 7 mm. The light-cured composite resin Filtek Z-350 (3M ESPE) was inserted in 2 mm increments until the entire space was filled. Each increment was light-cured for 40s. CR: The root canal walls were acid-etched for 15s, rinsed with distilled water and dried with absorbent paper cones. The adhesive system Adper Single Bond 2 (3M ESPE) was applied, air-dried and light-cured for 40s. The composite resin Filtek Z-350 was incrementally inserted until it filled the root canal with each increment being light-cured for 40s. FP: The glass-fibre post Exacto size 3 (Angelus) was cemented into the root canal with a self-adhesive resin cement, RelyX U100 (3M ESPE, Seefeld, Germany). The mixed cement was inserted into the canal with a size 80 K-file (Dentsply Maillefer), and the post was inserted, followed by light activation of the cement for 40s. The post was sectioned 3 mm below the incisal edge, and the remaining space was filled with composite resin as in the GP and CR groups. RFP: Prior to cementation, the fibre post was relined with composite resin. The canal walls were lubricated with hydro-soluble gel before the fibre post covered with resin composite Filtek Z-350 was inserted into the canal. The resin composite was light-cured for 20s, the relined fibre post was removed, and the resin composite was light-cured for another 40s. Copious rinsing removed the lubricant gel from the root canal, and the relined fibre post was cemented as in the FP group before the coronal space was filled with composite resin. All light-curing procedures were performed using a light-emitting diode curing unit (Radii Cal; SDI, Bayswater, Victoria, Australia) with 600-mW cm 2 irradiance. The fracture resistance tests were performed using a universal testing machine (EZ-L, Shimadzu, Kyoto, Japan). Each sample was placed in a custom apparatus that allowed application of compressive loading at 135° to the long axis of the tooth in a buc-
960
International Endodontic Journal, 47, 958–966, 2014
cal/lingual direction. The compressive loading was applied at a speed of 0.5 mm min 1, and the force required to cause fracture (N) was recorded by a 5KN load cell hardwired to software. Statistical analysis was performed using SigmaStat statistical software version 3.5 (Systat Software, Point Richmond, CA, USA). The fracture data showed normality and equal variance. Thus, data were submitted to one-way ANOVA followed by the Student–Newman–Keuls (SNK) test (a = 0.05). The fracture patterns were evaluated using a classification described by Santos-Filho et al. (2008): (i) resin core or post fracture, (ii) cervical root fracture, (iii) mid root fracture, (iv) apical root fracture and (v) vertical root fracture.
Two-dimensional finite element analysis Four two-dimensional (2-D) models were created following the experimental groups: GP model, CR model, FP model and RFP model. The geometrical model was based on digital photographs of buccolingual crosssections of the samples used in the experimental tests. The images were then exported to software for image manipulation (Image J software, public domain, Javabased image processing and analysis software developed at the National Institutes of Health, Bethesda, MD, USA). The coordinates obtained were exported to the finite element analysis software Marc/Mentat (MSC software, Santa Ana, CA, USA). The crosssectional model was divided into elements to facilitate the calculation of stress distribution (meshing process using plane-strain quadrilateral isoparametric elements for the structures linear, with four nodes – Fig. 1a). Friction contact was inserted between the dentine and gutta-percha/MTA; these structures could slide along or separate from the dentine but could not penetrate. The friction coefficient was 0.3 (Coulomb friction). Boundary condition was defined with a 100 N as that force applied to the enamel surface with a simulated cylindrical tip that was the same used in laboratory tests at 45° to the buccolingual long axis. A nodal displacement constraint (model fixation) was applied at the bottom and lateral surfaces of the support cylinder on the X and Y axes (Fig. 1b). A nonlinear structural analysis was performed, and all materials were considered linear, isotropic and homogeneous; therefore, only two mechanical properties were necessary to perform finite element analysis: elastic modulus and Poisson’s ratio (Table 1), with the exception of the glass-fibre post, which was considered orthotropic, as this material has different
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Brito-J unior et al. Mechanical behaviour of restored immature teeth
(a)
(b)
Figure 1 (a) Mesh generation and (b) load application with the simulated cylindrical tip and fixed displacements on X and Y
axes.
Table 1 Mechanical properties of isotropic structures (MPa)
Table 2 Mechanical properties of the glass-fibre post
Material Bovine enamel Bovine dentine Gutta-percha Composite resin Polyether Polystyrene resin MTA Resin cement
Elastic modulus (MPa)
Poisson’s ratio
Properties
Glass-fibre post
84 000 18 600 0.69 15 800 50 13 500 15 700 8130
0.30 0.31 0.45 0.24 0.45 0.31 0.23 0.30
EX (MPa) EY (MPa) EZ (MPa) gXY gXZ gYZ Gxy (MPa) Gxz (MPa) Gxz (MPa)
37 000 9500 9500 0.34 0.34 0.27 3544.8 3544.8 1456.7
mechanical performance when the load is apply axially or perpendicular to the direction of the fibres. For this condition, nine mechanical properties are necessary to characterize this material in the finite element analysis: elastic modulus in each three orthogonal planes; Poisson’s ratio in each combination of three orthogonal planes; and the resultant shear modulus in each three orthogonal planes (Table 2). The finite element stress analysis was performed using Mentat (pre- and post-processor) and Marc (solver) (MSC software, Newport Beach, CA, USA). The stress
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
E – elastic modulus; g – Poison’s ratio; G – shear modulus; and x, y and z – specific orthogonal plane directions.
distributions in the restored teeth were analysed using von Mises equivalent stresses and maximum principal stress.
Results Table 3 shows the mean fracture strength values, standard deviations and confidence intervals for each
International Endodontic Journal, 47, 958–966, 2014
961
Mechanical behaviour of restored immature teeth Brito-J unior et al.
Table 3 The force required (N) to cause fracture in experimental groups
Group
Mean
SD
CI 95%
Student– Newman– Keuls rankinga
RFP (relined fibre post) FP (fibre post) CR (composite resin) GP (guttapercha + sealer)
2230.1
265.7
2040.0–2420.2
A
2175.1
292.5
1965.9–2384.3
A
1856.1
314.2
1631.3–2080.9
B
1646.9
262.2
1459.3–1834.5
(a)
(b)
(c)
(d)
B
Distinct letters indicate statistical difference (a = 0.05). SD, standard deviation; and CI, confidence interval.
a
group. One-way ANOVA revealed significant effects of treatment (P < 0.001), whilst the power of the performed test was 0.99. The Student–Newman–Keuls’s test revealed that the relined fibre post group had similar fracture resistance to the fibre post group (P = 0.668) and, significantly, higher fracture resistance than the resin composite group (P = 0.015). The gutta-percha group had similar fracture resistance to the resin composite group (P = 0.109) and significantly lower fracture resistance than the relined fibre post (P < 0.001) and fibre post (P < 0.001) groups. With regard to fracture patterns, only fractures type II and III were observed in all experimental conditions, without significant differences between groups. Figure 2 shows examples of specimens fractured horizontally or obliquely at the cervical (type II) or middle (type III) thirds of roots. The results of finite element analysis using the von Mises and maximum principal stress are shown in Figs 3 and 4, respectively. The GP model had high stress concentration in the root dentine in the cervical and middle thirds (Fig. 3a). The GP model also exhibited a high concentration of tensile stress in the lingual root surface (Fig. 4a). The stress distributions by von Mises criteria and maximum principal stress in the roots restored with resin composite (RC), fibre post (FP) and relined fibre post (RFP) were similar, with high levels of compression stress at the buccal root surface (Fig. 4b–d). The RC, FP and RFP models also had stress concentration in the cervical thirds of the root and crown, but at lower levels than were verified for the GP model.
962
International Endodontic Journal, 47, 958–966, 2014
Figure 2 Examples of specimens fractured at cervical (a, b) and middle (c, d) thirds of roots.
Discussion Despite the high rate of success of apexification procedures using MTA, the tooth remains weakened due to the presence of thin root canal walls. Thus, restorative protocols able to reinforce the remaining tooth are important (Bortoluzzi et al. 2007, Wilkinson et al. 2007, Hemalatha et al. 2009, Dikbas et al. 2014 ). In the present study, immature apices were simulated in bovine incisors by diamond burs using a previously described methodology (Bortoluzzi et al. 2007). Furthermore, this methodology resulted in a remaining crown of around 8 mm for application of loads during the tests of fracture resistance. Destructive mechanical tests, such as fracture resistance, are important means of analysing tooth behaviour in situations of concentrated and high-intensity load application (Santos-Filho et al. 2008). However, they provide limited biomechanical information that does not allow the analysis of stress distribution, which can explain the failure patterns. Thus, the association of nondestructive analysis such as a finite element is essential for the biomechanical analysis of tooth structure. The combination of these methods provides a visualization and quantification of the stresses and strains generated in the structure when
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Brito-J unior et al. Mechanical behaviour of restored immature teeth
(a)
(b)
(c)
(d)
Figure 3 The von Mises stress distribution (MPa). (a) GP group (gutta-percha); (b) CR group (composite resin); (c) FP group (fibre post); and (d) RFP group (relined fibre post).
(a)
(b)
(c)
(d)
Figure 4 Stress distribution by maximum principal stress (MPa); compressive stress (negative/blue); and tensile stress (positive/ yellow). (a) GP group (gutta-percha); (b) CR group (composite resin); (c) FP group (fibre post); and (d) RFP group (relined fibre post).
low-intensity loads are applied, predicting the possible sites of failure initiation (Soares et al. 2008a,b). The experimental model used in this study resulted in four intact coronal walls. Usually, root filling with
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
gutta-percha and sealer and direct placement of a restoration such as composite resin is used clinically in this situation (Cvek 1992). However, the presence of flared root canals or weakened roots may compromise
International Endodontic Journal, 47, 958–966, 2014
963
Mechanical behaviour of restored immature teeth Brito-J unior et al.
the long-term outcome. It was hypothesized that filling root canals with an adhesive material with a similar elastic modulus to dentine could improve the fracture resistance and stress distribution of immature teeth (Lawley et al. 2004, Wilkinson et al. 2007). The composite resin root filling (CR group) gave similar results to those observed with the GP group in the fracture resistance test. However, in the finite element analysis, the GP group revealed worse stress distribution. Figure 3 shows more ‘red colour’ on the cervical region, which means more stress concentration; Fig. 4 shows high levels of tensile stress in the root canal at the cervical and middle thirds. Such findings may explain the lower fracture resistance caused by higher levels of stress concentration; thus, the hypothesis of the study was accepted. Composite resin and fibre glass posts have a similar elastic modulus to dentine and are able to distribute applied loads, reducing stress concentrations (Cabrera & Macorra 2011), as shown in this study. To determine this mechanical behaviour, a proper bonding at the dentine/composite interface is needed for dissipation of stresses generated by loading (Santos et al. 2010). However, adhesive procedures in the root canal have several limitations and commonly result in low bond-strength values. This occurs mainly due to the difficulty of moisture control, the complexity of dentine and issues relating to light activation of the adhesive (Chersoni et al. 2005, Wu et al. 2009). Furthermore, the polymerization shrinkage of composite inserted into a constricted root canal space results in high stresses that can disrupt bonding at the composite/dentine interface (Aksornmuang et al. 2011). It is important to emphasize that the composite was inserted incrementally into the root canals to reduce polymerization stress and to permit proper light activation. Clinically, the use of dual-cured resin composite to fill the root canal can be a viable alternative; however, the present study used a light-cured composite as this material is readily available. The von Mises stress is a failure criterion that demonstrates energy transmission in a structure, that is, where there is a higher concentration of energy, whilst the maximum principal stress discriminates the tensile and compressive stress fields. Some dental materials (i.e. composite resin) and structures (i.e. enamel) have high compressive strength but are brittle when subjected to tensile stresses (Chai et al. 2009). Thus, the maximum principal stress shows the locations within the structure that are more prone to failure by tensile stress. In this case, teeth restored
964
International Endodontic Journal, 47, 958–966, 2014
without fibre posts or resin composite had high levels of stress concentration in the root at the cervical and middle thirds, which corroborate the results of fracture resistance and modes of failure. The present study revealed that the use of fibre posts was able to improve fracture resistance and stress distribution. However, no significant difference was found between the RC, FP and RFP groups in the finite element analysis. This may be explained by the limitation of the 2-D finite element simulation and also because the interface between composite and dentine was simulated as a perfect bond during all load application processes, which is not realistic. Another limitation of finite element analysis relates to the assumed properties for materials and tissues, namely, that all materials used are linear, isotropic and homogeneous. In reality, most dental materials and tooth tissues are anisotropic and nonhomogeneous. In addition, the loading scenarios investigated lack the complexity that occurs during functional load in the patient, which results in nonlinearity of the load application and its effects. However, in general as with composite resin, fibre posts have elastic moduli similar to dentine (Prisco et al. 2003, Santos-Filho et al. 2008, Soares et al. 2008a,b). Thus, the bond strength between different components was preserved in the presence of the fibre post, which can explain the results. Additionally, the cementation procedures using self-adhesive resin cements used in this study were simpler than conventional bonding procedures used in root canals. Furthermore, self-adhesive resin cements appear to have low shrinkage due to their viscoelastic properties (Amaral et al. 2011). The effect of polymerization shrinkage is additionally reduced by lower volume of material as the fibre post occupies most of the space in the root canal. This additional advantage is more pronounced when the fibre post is relined (D’Arcangelo et al. 2007, Faria-e-Silva et al. 2009, Macedo et al. 2010). These factors contribute to improve the bonding of restorative material to dentine. An unfavourable situation for post-restored teeth has been related mainly to debonding, which results in stress within the adhesive interfaces favouring fracture of the post (Santos et al. 2010). Thus, bond strengths of resin cements to fibre posts have been challenged by thermal or mechanical cycling protocols simulating clinical function in different experimental designs (Amaral et al. 2011, Bitter et al. 2012, Mazzitelli et al. 2012, Macedo et al. 2013). In the present study, these protocols were not performed
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Brito-J unior et al. Mechanical behaviour of restored immature teeth
before the test of fracture resistance, which could represent a methodological limitation. However, it has been demonstrated that the bonding ability of the self-adhesive cement RelyX U100, as used in this study, is less affected by mechanical or thermocycling procedures (Amaral et al. 2011, Mazzitelli et al. 2012, Macedo et al. 2013). Moreover, simulation of ageing is a controversial topic (Bitter et al. 2012). In the present study, teeth filled with gutta-percha and sealer were included to provide a comparative assessment with bonding techniques. The absence of a group with instrumented canals without restoration could represent a limitation of this study. However, the findings of previous studies (Bortoluzzi et al. 2007, Schmoldt et al. 2011) showed that canals filled with gutta-percha and sealer have root fracture strengths similar to those of weakened roots that did not receive reinforcement. In addition, several studies have already demonstrated that the adhesive restorations significantly strengthen the roots when compared to positive controls (Carvalho et al. 2005, Alsamadani et al. 2012, Seto et al. 2013).
Conclusion Within the limits of this study, it can be concluded that simulated immature teeth with open apices filled with an MTA apical plug and restored with fibre posts were a better adhesive strategy for restoring immature teeth, from the standpoint of stress and fracture resistance, than restoration with gutta-percha and sealer.
Acknowledgements The authors gratefully acknowledge Prof. Maria Helena Santos for the use of the BioMat laboratory (UFVJM, Diamantina, Brazil) and to FAPEMIG for financial support.
References Aksornmuang J, Nakajima M, Senawongse P, Tagami J (2011) Effects of C-factor and resin volume on the bonding to root canal with and without fibre post insertion. Journal of Dentistry 39, 422–9. Alsamadani KH, Abdaziz el-SM, Gad el-S (2012) Influence of different restorative techniques on the strength of endodontically treated weakened roots. International Journal of Dentistry 2012, 343712. Amaral M, Rippe MP, Bergoli CD, Monaco C, Valandro LF (2011) Multi-step adhesive cementation versus one-step
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
adhesive cementation: push-out bond strength between fiber post and root dentin before and after mechanical cycling. General Dentistry 59, e185–91. Banchs F, Trope M (2004) Revascularization of immature permanent teeth with apical periodontitis: new treatment protocol? Journal of Endodontics 30, 196–200. Bitter K, Perdigao J, Exner M, Neumann K, Kielbassa A, Sterzenbach G (2012) Reliability of fiber post bonding to root canal dentin after simulated clinical function in vitro. Operative Dentistry 37, 397–405. Bortoluzzi EA, Souza EM, Reis JM, Esberard RM, TanomaruFilho M (2007) Fracture strength of bovine incisors after intra-radicular treatment with MTA in an experimental immature tooth model. International Endodontic Journal 40, 684–91. Cabrera E, Macorra JC (2011) Microtensile bond strength distributions of three composite materials with different polymerization shrinkages bonded to dentin. The Journal of Adhesive Dentistry 13, 39–48. Carvalho CA, Valera MC, Olivera LD, Camargo CH (2005) Structural resistance in immature teeth using root reinforcements in vitro. Dental Traumatology 21, 155–9. Chai H, Lee JJ, Kwon JY, Lucas PW, Lawn BR (2009) A simple model for enamel fracture from margin cracks. Acta Biomaterialia 5, 1663–7. Chala S, Abouqal R, Rida S (2011) Apexification of immature teeth with calcium hydroxide or mineral trioxide aggregate: systematic review and meta-analysis. Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology and Endodontics 112, e36–42. Chersoni S, Acquaviva GL, Prati C et al. (2005) In vivo fluid movement through dentin adhesives in endodontically treated teeth. Journal of Dental Research 84, 223–7. Cotti E, Mereu M, Lusso D (2008) Regenerative treatment of an immature, traumatized tooth with apical periodontitis: report of a case. Journal of Endodontics 34, 611–6. Cvek M (1992) Prognosis of luxated non-vital maxillary incisors treated withc alciumhydroxide and filled with guttapercha. A retrospective clinical study. Endodontic & Dental Traumatology 18, 45–55. D’Arcangelo C, Cinelli M, De Angelis F, D’Amario M (2007) The effect of resin cement film thickness on the pullout strength of a fiber-reinforced post. The Journal of Prosthetic Dentistry 98, 193–8. Dikbas I, Tanalp J, Koksal T, Yalniz A, G€ ung€ or T (2014) Investigation of the effect of different prefabricated intracanal posts on fracture resistance of simulated immature teeth. Dental Traumatology 30, 49–54. Faria-e-Silva AL, Pedrosa-Filho CF, Menezes MS, Silveira DM, Martins LR (2009) Effect of relining on fiber post retention to root canal. Journal Applied of Oral Science 17, 600–4. Garcia-Godoy F, Murray PE (2012) Recommendations for using regenerative endodontic procedures in permanent immature traumatized teeth. Dental Traumatology 28, 33–41.
International Endodontic Journal, 47, 958–966, 2014
965
Mechanical behaviour of restored immature teeth Brito-J unior et al.
Hemalatha H, Sandeep M, Kulkarni S, Yakub SS (2009) Evaluation of fracture resistance in simulated immature teeth using Resilon and Ribbond as root reinforcements-an in vitro study. Dental Traumatology 25, 433–8. Holden DT, Schwartz SA, Kirkpatrick TC, Schindler WG (2008) Clinical outcomes of artificial root-end barriers with mineral trioxide aggregate in teeth with immature apices. Journal of Endodontics 34, 812–7. Lawley GR, Schindler WG, Walker WA 3rd, Kolodrubetz D (2004) Evaluation of ultrasonically placed MTA and fracture resistance with intracanal composite resin in a model of apexification. Journal of Endodontics 30, 167–72. Li Q, Xu B, Wang Y, Cai Y (2011) Effects of auxiliary fiber posts on endodontically treated teeth with flared canals. Operative Dentistry 36, 380–9. Macedo VC, Faria e Silva AL, Martins LR (2010) Effect of cement type, relining procedure, and length of cementation on pull-out bond strength of fiber posts. Journal of Endodontics 36, 1543–6. Macedo V, Souza N, Faria-e-Silva AL et al. (2013) Pullout bond strength of fiber posts luted to different depths and submitted to artificial aging. Operative Dentistry 38, E1–6. Mazzitelli C, Monticelli F, Toledano M, Ferrari M, Osorio R (2012) Effect of thermal cycling on the bond strength of self-adhesive cements to fiber posts. Clinical Oral Investigations 16, 909–15. Mente J, Hage N, Pfefferle T et al. (2009) Mineral trioxide aggregate apical plugs in teeth with open apical foramina: a retrospective analysis of treatment outcome. Journal of Endodontics 35, 1354–8. Prisco D, De Santis R, Mollica F, Ambrosio L, Rengo S, Nicolais L (2003) Fiber post adhesion to resin luting cements in the restoration of endodontically-treated teeth. Operative Dentistry 28, 515–21. Santos AF, Meira JB, Tanaka CB et al. (2010) Can fiber posts increase root stresses and reduce fracture? Journal of Dental Research 89, 587–91. Santos-Filho PC, Castro CG, Silva GR, Campos RE, Soares CJ (2008) Effects of post system and length on the strain and fracture resistance of root filled bovine teeth. International Endodontic Journal 41, 493–501. Sarris S, Tahmassebi JF, Duggal MS, Cross IA (2008) A clinical evaluation of mineral trioxide aggregate for root-end
966
International Endodontic Journal, 47, 958–966, 2014
closure of non-vital immature permanent incisors in children-a pilot study. Dental Traumatology 24, 79–85. Schmoldt SJ, Kirkpatrick TC, Rutledge RE, Yaccino JM (2011) Reinforcement of simulated immature roots restored with composite resin, mineral trioxide aggregate, gutta-percha, or a fiber post after thermocycling. Journal of Endodontics 37, 1390–3. Silva GR, Santos-Filho PC, Simamoto-Junior PC, Martins LR, Mota AS, Soares CJ (2011) Effect of post type and restorative techniques on the strain and fracture resistance of flared incisor roots. Brazilian Dental Journal 22, 230–7. Simon S, Rilliard F, Berdal A, Machtou P (2007) The use of mineral trioxide aggregate in one-visit apexification treatment: a prospective study. International Endodontic Journal 40, 186–97. Soares CJ, Pizi EC, Fonseca RB, Martins LR (2005) Influence of root embedment material and periodontal ligament simulation on fracture resistance tests. Brazilian Oral Research 19, 11–6. Soares CJ, Santana FR, Castro CG et al. (2008a) Finite element analysis and bond strength of a glass post to intraradicular dentin: comparison between microtensile and push-out tests. Dental Materials 24, 1405–11. Soares CJ, Soares PV, de Freitas Santos-Filho PC, Castro CG, Magalhaes D, Versluis A (2008b) The influence of cavity design and glass fiber posts on biomechanical behavior of endodontically treated premolars. Journal of Endodontics 34, 1015–9. Tagger M, Tamse A, Katz A, Korzen BH (1984) Evaluation of the apical seal produced by a hybrid root canal filling method, combining lateral condensation and thermatic compaction. Jounal of Endodontics 10, 299–303. Wilkinson KL, Beeson TJ, Kirkpatrick TC (2007) Fracture resistance of simulated immature teeth filled with resilon, gutta-percha, or composite. Journal of Endodontics 33, 480–3. Witherspoon DE, Small JC, Regan JD, Nunn M (2008) Retrospective analysis of open apex teeth obturated with mineral trioxide aggregate. Journal of Endodontics 34, 1171–6. Wu H, Hayashi M, Okamura K et al. (2009) Effects of light penetration and smear layer removal on adhesion of postcores to root canal dentin by self-etching adhesives. Dental Materials 25, 1484–92.
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
