JJOD 2370 1–9 journal of dentistry xxx (2014) xxx–xxx
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The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth
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Y.Y. Zhang, M.D. Peng, Y.N. Wang *, Q. Li ** Key Laboratory for Oral Biomedical Engineering of Ministry of Education, School and Hospital of Stomatology, Wuhan University, 237# Luo Yu Road, Wuhan, China
article info
abstract
Article history:
Objectives: To evaluate the fracture resistance of fibre post-restored treated teeth with
Received 11 March 2014
various ferrule configurations by using fracture failure tests and extended finite element
Received in revised form
analysis (XFEM).
15 September 2014
Methods: 60 Maxillary central incisors were collected and divided into six groups (n = 10)
Accepted 9 October 2014
according to various ferrule configurations with different ferrule heights in the labial or
Available online xxx
palatal region. All of the teeth were endodontically treated and restored by using fibre posts,
Keywords:
retained restorations until fracture occurred. The ultimate load was recorded and analyzed
Extended finite element method
by one way analysis of variance (ANOVA). The fractured specimens were longitudinal
Fracture failure
sectioned and investigated by micro-stereomicroscope and scanning electronic microscope.
composite cores and metal crowns. Fracture failure tests were performed on the post
Post-restored teeth
XFEM was used to model the fracture of the post-restored teeth and exhibit crack initiation
Ferrule
and propagation in the cement layers. Results: Fracture failure tests indicated that the palatal ferrule significantly enhanced the fracture resistance of the post-restored teeth, regardless the height of the labial ferrule. The fractography investigation exhibited that the crack initiated at the palatal margin of Q2 the cement layer and propagated to the cervical region of the root. XFEM confirmed these findings and demonstrated that increasing of the palatal ferrule could effectively enhance the anti-fracture ability of the adhesive cement and protected the integrity of adhesive cement. Conclusion: Adhesive interface was the susceptible structure of the post retained restorations. Increasing palatal ferrule height could effectively reduce the stress concentrated within the palatal adhesive cement. # 2014 Elsevier Ltd. All rights reserved.
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1.
Introduction
The restoration of endodontically treated teeth still represents a challenging task for clinicians. The task is complicated by
substantial loss of coronal tooth structure and the ability to predict restorative long-term success. It has been disputed whether the mechanical properties of dental hard tissues, such as modulus of elasticity, compressive strength, or brittleness, would change after endodontic treatment.1 However,
* Corresponding author at: Department of Prosthodontics, School and Hospital of Stomatology, Wuhan University, 237# Luo Yu Road, Wuhan, China. Tel.: +86 27 87646696; fax: +86 27 87873260. ** Corresponding author at: Department of Prosthodontics, School and Hospital of Stomatology, Wuhan University, 237# Luo Yu Road, Wuhan, China. Tel.: +86 27 87686220; fax: +86 27 87873260. E-mail addresses: [email protected] (Y.N. Wang), [email protected] (Q. Li). http://dx.doi.org/10.1016/j.jdent.2014.10.003 0300-5712/# 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Zhang YY, et al. The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.003
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endodontically treated teeth were claimed to be weaker and more prone to fracture than vital teeth, according to the clinical observations.2 The loss of structural integrity might contribute primarily to the vulnerability of endodontically treated teeth and their reduced resistance to fracture.1 The likelihood of survival of a post-retained restoration thus directly related to the quantity and quality of the remaining tooth structure and the efficiency of the restorative procedures used to replace lost structural integrity.3 Although restorative methods affect the fracture resistance and fracture mode of post-restored teeth at a certain extent,4 preserving intact coronal tooth structure and maintaining cervical tissue are still considered to be more crucial to optimize the biomechanical behaviour of the endodontically treated teeth.2,5–8 Parallel walls of dentine extending coronally from the crown margin provide a ferrule, which after being encircled by a crown provides a protective effect by reducing stresses within a tooth called the ‘‘ferrule effect’’.9 The result is an elevation in resistance form of the crown from the fracture of tooth structure. It has been proved both in laboratorial investigation and clinical observations that the less coronal dentine remains, the weaker the fracture resistance of endodontically treated teeth.10,11 It has been proposed that 1.5 mm coronal dentine is necessary for the long-term survival of restoration.12–14 In the clinical situation, however, the ideal status of circumferential ferrule with adequate and uniform height could not be always achieved. A tooth with varying ferrule configurations, especially absence of the labial or palatal wall, really plagued dentists in the clinical practice. Ng et al.15 have declared that the coronal dentine on the palatal aspect plays more important roles in the anti-fracture ability of endodontically treated teeth than those on the labial ones. Nevertheless, the effects of the various ferrule configurations on the fracture resistance of post-retained restorations have not been comprehensively investigated. For the restorations of endodontically treated teeth, clinical observations and experimental investigations have shown that the fracture of residual dentine or loosening of the postcore assembly reflects a major mode of failure.16,17 Unexpected damage accumulation and interfacial debonding decrease the mechanical properties of luting agents, then lead to entire failure of the post-restored crown. Thus, a full understanding of stress fields developed in the restoration and tooth becomes particularly important, especially for the weaker structures such as the cement layers. Finite element analysis (FEA) is a promising approach for investigating the biomechanics in dentistry, which allows bioengineering virtual simulations of oral environments.18 However, the reliability of these results deducing in the clinical practice is still under suspicion. A hypothesis that the structure would be intact during the whole loading period was often adopted in the FEA studies, which is obviously not consistent with the complexity of clinical process.19 The present study attempts to use isotropic damage initiation criteria in conjunction with the extended finite element method (XFEM) to model the fracture of the post-restored teeth and exhibit crack initiation and propagation in cement layers. XFEM is a method developed for computationally predicting crack initiation and propagation, which has recently been successfully applied in solving medical problems, simulating dynamic fracture, fatigue,
and various crack patterns in brittle materials.20–23 Compared with conventional discontinuous analysis, one of the main advantages of XFEM is that it not only allows automatic initiation and propagation of cracksmodelled without predefining in isotropic elastic materials, but also avoids the requirement of elements remeshing during crack extension.21,24 Another major advantage of the method is that more accurate results can be acquired by increasing the degrees of freedom in the failure element only.25 The objective of the present study was to investigate the influence of ferrule configurations on the fracture process of fibre post-retained teeth by using a combined method of fracture failure test and XFEM. The null hypothesis was that fracture resistance does not vary as a function of the ferrule configuration.
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Materials and methods
A combined experimental and numerical method was used to evaluate the mechanical performance of fibre post-restored teeth with various ferrule configurations under external loads.
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In vitro tests
The study protocol was approved by the Ethics Committee of the School and Hospital of Stomatology, Wuhan University. Patients who donated their teeth were informed of the purposes of the study. Written informed consents were obtained prior to teeth extraction.
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2.2.
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Fabrication of post retained restorations
Sixty intact human maxillary incisors were collected directly after extraction and stored in 0.1% thymol solution less than 3 months. The teeth were randomly assigned to six groups and embedded in acrylic resin at the level of 2 mm below the cemento-enamel junction (CEJ). A thin coating of silicone on the roots, approximate 0.2–0.3 mm in thickness, were made to imitate a human periodontal ligament periodontium. To confirm the even size distribution within groups, the root lengths (from the root apex to the proximal CEJ), the mesiodistal and labial-palatal dimensions of the teeth were measured (at the level of the CEJ) using acalliper (LA-7, Peacock Inc., Tokyo, Japan). Overall, the mean of root length was 13.5 1.5 mm, while the means of mesio-distal and labial-palatal dimensions were 6.1 0.5 mm and 7.5 0.7 mm. The specimens were endodontically prepared using a crown-down technique by ProTaper (Dentsply-Maillefer, Konstanz, Swizerland) and then filled by gutta-percha (Lexicon Gutta Percha Points, Dentsply, Tulsa, OK, USA) and sealer (AH plus, Dentsply/De Trey, Constance, Germany). The teeth were decoronated at six levels to the most labial and palatal points of CEJ, based on the ferrule configurations (Fig. 1B1–B6): Group 1 (as negative control): 0 mm labial ferrule and 0 mm palatal ferrule. Group 2: 1 mm labial ferrule and 0 mm palatal ferrule. Group 3: 2 mm labial ferrule and 0 mm palatal ferrule. Group 4: 0 mm labial ferrule and 1 mm palatal ferrule. Group 5: 0 mm labial ferrule and 2 mm palatal ferrule.
Please cite this article in press as: Zhang YY, et al. The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.003
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Fig. 1 – Schematic diagram of the specimens and FE models. (A) Specimen with a metal crown; (C) representative schematic diagram of FE model; (B1–B6) specimens with different ferrule height; (D1–D6) FE models with different ferrule height. 0 mm labial ferrule and 0 mm palatal ferrule (B1, D1); 1 mm labial ferrule and 0 mm palatal ferrule (B2, D2); 2 mm labial ferrule and 0 mm palatal ferrule (B3, D3); 0 mm labial ferrule and 1 mm palatal ferrule(B4, D4); 0 mm labial ferrule and 2 mm palatal ferrule (B5, D5); 2 mm labial ferrule and 2 mm palatal ferrule (B6, D6).
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Group 6 (as positive control): 2 mm labial ferrule and 2 mm palatal ferrule. The root canals were enlarged using tapered drills (MacroLock Kit, RTD Inc., St. Egreve, France) in the recommended sequence, leaving at least 4 mm of the root filling in the apical portion. A post space with the depth of 8 mm below labial CEJ was achieved. The root canals were treated using 37% phosphoric acid (Ultra-Etch, Ultradent, South Jordan, UT, USA), primer and adhesive (Adper Single Bond 2, 3M ESPE, St. Paul, MN, USA). A quartz fibre post (Macro-Lock 4#, RTD Inc., St. Egreve, France) was luted into the post space using an adhesive resin cement (PermaCem, DMG Inc., Hamburg, Germany) and light-cured for 40 s from the top of the post with a halogen light unit (ESPE Elipar Trilight, 3M ESPE, St. Paul, MN, USA). Subsequently, a standardized core was built up with a composite (LuxaCore, DMG Inc., Hamburg, Germany) using CoreForms (kerr) for anterior teeth with layers of maximum 2 mm. Each layer was polymerized from each side for 40 s with a halogen light unit. The coronal portion of the resin core and/or dentine was
prepared followed the criteria for the full-coverage crown with 1 mm width of shoulder at the CEJ level (Fig. 1A.). Metal crowns were fabricated and cemented on the prepared teeth using glass ionomer cement (Fuji, GC Inc., Tokyo, Japan).
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2.3.
Fracture failure tests and fractography investigation
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A universal testing machine (Model 8841, Instron, Norwood, MA, USA) was used to perform the fracture failure tests. Each test was performed at a crosshead speed of 0.5 mm/min until fracture occurred. A compressive load was applied on the lingual surface (2 mm below the incisal edge) with an angle of 458 to the long axis of the root. The ultimate loads that the specimen could withstand were recorded. The fractured specimens were facial-lingually sectioned using a low speed diamond saw (Isomet, Buehler Inc., Lake Bluff, IL, USA) under water cooling. The section surfaces were investigated using a micro-stereomicroscope (Stemi SV11 Apo, Carl Zeiss Micro Imaging Inc., Thornwood, NY, USA) at magnifications of 1 and scanning electronic microscope
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Please cite this article in press as: Zhang YY, et al. The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.003
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(SEM, JSM-5510LV, JEOL, Tokyo, Japan) for the micro-structures at magnifications of 35. Micrographs of representative areas of the fractured interfaces were recorded.
Table 1 – Material properties used in the finite element analysis. Material
Young’s modulus (GPa)
3.
Finite element analysis
3.1.
Geometry acquisition
Dentin28 Periodontal liagament29 Cortical bone30 Gutta-percha31 Ni–Cr alloy32 Composite resin corea Adhesive resin cementa Glass ionomer cementa Fibre posta
18.6 0.31 – 68.9e3 0.45 – 13.7 0.30 – 0.14 0.45 – 200.0 0.33 – 7.0 0.30 – 2.8 0.33 13.0 22.0 0.35 6.5 EL = 39.0, ET = ET0 = 13.5; GLT = GLT0 = 6.5, GTT = 5.0; nLT = nLT0 = 0.285, nTL = nT0 L = 0.285, nTT0 = 0.35b
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A standardized model of a maxillary right central incisor (Nissin Dental Products Inc., Kyoto, Japan) was adopted as the prototype of the tooth, which has the similar dimension as the tested natural teeth. The geometric morphological data of the tooth, the tooth after preparation and the simulated alveolar bone were acquired by using a laser-based 3D digitizing system (D800, Wieland Dental & Technik GmbH & Co. KG, Schwenninger, German) and files with ‘‘.stl’’ extension (stereolithography) were consequently generated. Based on these files, finite element (FE) models with different ferrule configurations were obtained using ScanIP1 module (Simpleware Ltd, UK). Appropriate modifications for some surface points were introduced from the average dimensions of the tooth structures. The structure of periodontal ligament was generated by creating a uniform 0.3 mm shell around the root or the bone.26 The fibre post was virtually modelled according to the drawing sheet supplied by the manufacturer with some simplifications, and placed into the FE models. The glass ionomer cement layer within the crown–core interface and adhesive resin cement layer within the post-dentine interface were modelled. Clinically, the thickness of cement layer varies from 0.02 mm to 0.1 mm, which appears fairly difficult to be accurately modelled unless an unrealistically fine mesh is used. Considering the appropriateness of geometrical aspect ratio, the cement layers were created with a constant thickness of 0.2 mm.27 Overall, the FE models consist of eight fundamental parts: (1) alveolar bone, (2) gingiva, (3) periodontal ligament, (4) residual roots (with different ferrule configurations), (5) fibre post, (6) resin core, (7) metal crown, (8) adhesive resin cement layer, and (9) glass ionomer cement layer (Fig. 1C and D1–D6).
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A static structural analysis was performed to calculate the stress distribution in different ferrule configurations. All numerical simulations were performed using ABAQUS/CAE (SIMULIA, Version 6.10, Providence, RI, USA). The finite element analysis was based on following assumptions: (1) all solids were assumed to be homogeneous, isotropic, and linear elastic; (2) perfect bonding between components; (3) no flaw in any component in the initial model; (4) the adhesive resin cement layer and glass ionomer cement layer were of uniform thickness and allowed crack growth when the maximum principal stress reached a certain value; (5) rigid constrains at the base and mesio-distal surfaces of the bone. The number of tetrahedral elements for the models range from 509,618 to 549,438. The properties of the materials (Young’s modulus and Poisson’s ratio) were determined from literature and the manufacturers (Table 1). A load of 350 N (45 angle degree with respect to the longitudinal axis of the tooth), was applied on the lingual surface (2 mm below the incisal edge) of the crown.
Poisson’s ratio
Tensile strength (MPa)
a
Information provided by the manufacturers. EL, ET, and ET’: elastic modulus in longitudinal (parallel to fibres) and 2 perpendicular directions in transverse plane of fibre post. GLT and GLT0 : longitudinal in plane shear modulus. GTT: transverse shear modulus. nTL, nT’L, and nTT0 : first subscript in Poisson’s ratio refers to direction of load, and second to direction of displacement. b
In the present study, XFEM was used to simulate initiation and propagation of a discrete crack along an arbitrary path without the requirement of remeshing in the bulk materials. The cement layers of adhesive resin and glass ionomer were considered as the quasi-brittle biomaterials, which could resist significant stronger compression stress than the tension stress. The maximum principal stress criterion was chosen as the fracture criterion. The maximum principal strain criterion > could be represented as: f ¼ < ssmax s 0max represented the 0
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maximum allowable principal stress. And smax represented the maximum principal stress on the FE model. Damage was
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max
assumed to be initialized when the value of s max =s 0max reached one. The minimum load leaded to adhesive resin cement layer and glass ionomer cement layer damage and the damage location would be analyzed and discussed.
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3.3.
Statistical analysis
The means of fracture failure load were calculated and analyzed using one-way analysis of variance (ANOVA) with the ferrule configuration setting as the variable. Tukey’s post hoc test was performed to evaluate the differences among the groups. All analyses were performed using the Statistical Package for the Social Sciences statistical package (SPSS 13.0 for Windows, SPSS Inc, Chicago, IL, USA). The level of significance was set at 0.05.
4.
Results
4.1.
Fracture failure tests and fractography investigations
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Fracture failure loads (N) and standard deviations are presented in Table 2. Group 6 presented the highest fracture
Please cite this article in press as: Zhang YY, et al. The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.003
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Table 2 – Means of fracture failure strength values (N) and the results of Tukey’s post hoc analysis. Group 1 2 3 4 5 6 a
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Labial ferrule height (mm)
Palatal ferrule height (mm)
n
Mean (N)
SD
0 1 2 0 0 2
0 0 0 1 2 2
10 10 10 10 10 10
130.01 167.32 203.56 280.24 380.17 532.82
30.31 46.20 67.93 59.26 87.35 126.43
Tukey’s intervalsa a a a b c d
For Tukey’s intervals, the same letter means no significant difference within the groups.
failure values, whereas, the lowest fracture resistance appeared in the Group 1. Tukey’s multiple comparisons showed that there were no significant differences among Group 1, 2 or 3, in which the teeth have no palatal ferrule. The ultimate loads were increased with the increase of palatal ferrule height in Groups 4 and 5 (Table 2). Typical fracture patterns are represented in Fig. 2. In all of the teeth, cracks were detected at the palatal margin of the crown and the cervical region of the root in the labial side. In the palatal margin of the crowns, damage was found in the glass ionomer cement in all the groups. However, crack of resin adhesive interface was only detected in lower palatal ferrule groups. The crack initiated at the location of the palatal
core–dentine interface, and then extended to the interface of dentine-post in lower palatal ferrule groups (1, 2, and 3). Whereas, the crack was observed occurring within the palatal ferrule in the higher palatal ferrule groups (4, 5, and 6).
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XFEM results
The maximum principal stress distribution on glass ionomer cement layer and adhesive resin cement layer are exhibited in Fig. 3. An obvious stress concentrations were detected around the palatal margin of cement layer in groups 1, 2 and 3. The fracture locations and the estimated loads were listed in Table 3. The loads, which caused the initial damage in the
Fig. 2 – Schematic diagram of the fracture modes using the representative fractographics. Schematic diagram of fracture modes in groups without palatal ferrule (A1) and groups with palatal ferrule (B1): the red dotted line represents fracture line. The representative fractographics at 1T magnification in groups without palatal ferrule (A2) and groups with palatal ferrule (B2). Red arrows point to the crack locations. (A3, B3) Display the SEM results of palatal region of restorations at 35T magnification. (A3) Exhibits the debonding at the interface of core and dentine. (B3) shows integrated interface of core– dentine and fracture occurred within dentine tissue. AR, adhesive resin cement layer; C, crown; D, dentine; GI, glass ionomer cement layer; P, fibre post; RC, resin core. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Zhang YY, et al. The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.003
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Fig. 3 – The maximum principal stress distributions in glass ionomer cement layer and adhesive cement layer. The maximum principal stress contours of glass ionomer cement layer (A1–A6). The maximum principal stress contours of adhesive resin cement layer (B1–B6). Contour details exhibited obvious maximum principal stress concentration in palatal margin of cement layers in groups 1, 2 and 3.
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glass ionomer cement layer, were similar in all of the FE models. However, the ultimate load leading to crack initiation in adhesive resin cement layer and crack location were various in different ferrule configurations. Increasing of the palatal ferrule was more effective in enhancing the anti-fracture ability of adhesive cement than the labial ferrule. In lower palatal ferrule height groups, fractures in adhesive resin cement layer were firstly detected in the palatal margin, then extended along the interface between core and root (Fig. 4A1– A3). However, in higher ferrule model, adhesive cement cracks were firstly detected in root canal at bone level (Fig. 4B1–B4.). The results of this XFEM were consistent with the in vitro tests.
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The complex relationship between ferrule height and the fracture resistance ability of post-restored crowns in anterior region was important to predict the clinical prognosis in response to biomechanical loading. In clinical practice, a uniform ferrule with height of 1.5 mm was proposed as the basic requirement for achieving good prognosis.33 However, the prognosis of the post-restored tooth with varying ferrule
Discussion
configurations has not been fully investigated. In this study, fracture failure tests and XFEM analysis were used to investigate the fracture resistance of teeth with different ferrule configurations, which focused on the damage of the susceptible structure within the restorations. The null hypothesis that fracture resistance does not vary as a function of the ferrule configuration was rejected for the significant differences among the groups with various ferrule configurations. It has been confirmed that a uniform 2 mm ferrule is superior to the defect or lack of a ferrule in the fracture resistance under static load. The lowest ultimate load that induced restoration failure was detected in the Group 1, tooth without a ferrule (130.01 30.31 N). The means of fracture failure load were slightly enhanced while the labial ferrule height increased. However, there was no significant difference of fracture failure load among the Groups 1, 2 or 3. The reported maximal occluding force for males exerted by a maxillary incisor was 146 44 N.34 Allowing for SD of variation, teeth with no palatal ferrule, no matter with or without labial ferrule, are at risk of fracture when subjected to maximal clenching force. Concerned about the position of ferrule, palatal ferrule obviously plays more important roles in fracture resistance of endodontically treated teeth.
Table 3 – The predicted minimal failure loads (N) of glass ionomer cement layer and adhesive resin cement layer and cracks location (from the FEA results). Group
Glass ionomer cement layer Failure load (N)
1 2 3 4 5 6
57.4 59.7 56.8 58.4 57.6 59.2
Adhesive resin cement layer
Crack location Palatal Palatal Palatal Palatal Palatal Palatal
cervical cervical cervical cervical cervical cervical
region region region region region region
Failure load (N)
Crack location
100.9 142.7 146.6 225.5 238.2 235.6
Dentine–core interface Dentine–core interface Dentine–core interface Upper third of root Upper third of root Upper third of root
Please cite this article in press as: Zhang YY, et al. The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.003
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Fig. 4 – Representative XFEM results in Model 1 and Model 6. XFEM results show the crack loacations. (A1–A3) Crack initiated in glass ionomer cement layer and extended to adhesive cement layer directly with the increasement of load. The failure mode was detected in Models 1, 2, and 3. (B1–B4) Crack originated from glass ionomer cement layer as limited damage. Crack in adhesive resin cement layer exhibits in the dentine-post interface at bone level. This failure mode was detected in Models 4, 5, and, 6.
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In the fractography investigation, all of the specimens suffered from the fracture of dentine, crack of glass ionomer cement, and/or debonding of adhesive resin cement, which were reported in several previous studies.35–37 It could be concluded that the failure of post retained restorations is a typical damage driven continuum-to-discrete process. The adhesive interfaces were proposed as the most brittle structures within the post-core restorations. Investigating the mechanical responses of post-restored teeth, therefore, it is crucially required to model the fracture phenomena. XFEM was used to model the fracture, exhibit crack initiation, and propagation in adhesive interfaces. In this study, glass ionomer cement and resin adhesive cement were considered as typical quasi-brittle materials, and prone to be tensile fracture, which represent significant higher compressive resistance than the tensile strength. Once these two materials were loaded beyond their maximum tensile strength, inherent damage were accumulated before macroscopic fracture occurs. Consequently, crack bands appeared orthogonal to the principle tensile stress direction.18 Therefore, the max principle stress criterion was introduced in the XFEM analysis to determine the crack initiation and simulate the damage propagation. XFEM investigations revealed that the initialized crack occurred around the palatal margin. It indicated that the
potential debonding firstly occurred between the crown and dentine interface, while the tensile stress approached or exceeded the tolerable bond strength. The estimated fracture loads, which caused the damage within glass ionomer cement, were similar among all the groups. However, a localized damage occurred within the glass ionomer cement did not mean the immediate failure of the restorations unless the cracks extended to the adhesive resin cement layer and leaded more serious damage. With respect to the resin adhesive cement, the estimated fracture loads were obviously different among the various ferrule configurations. Moreover, the location of the failure occurrence also has been influenced by the palatal ferrule height. In the lower palatal height models (Models 1, 2 and 3), no matter the labial ferrule increased, the initialized fracture loads of the resin cement were almost not change, and the fractures were located at the level of dentine-core interface. In the Models 4, 5 and 6, the initialized fracture location changed to the dentine-post interface, and the estimated damage load was much higher. The changes of fracture location were consistent with the fractography investigations. It indicated that the increasing of palatal ferrule height efficiently protects the dentine–core interface by forbidding the damage propagation from glass ionomer cement layer to adhesive resin cement layer.
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From the XFEM simulation of the fracture process, it could be assumed that the fractures originated from the weakest structure of glass ionomer cement at the palatal crown margin, and then propagated down towards the labial direction though the interface of adhesive resin cement layer. Therefore, placing the interface of adhesive resin cement layer away from palatal cervical region by increasing palatal ferrule might contribute primarily in enhancing the anti-fracture load of post-restored teeth. In the present study, good agreement between the numerical predictions and the experimental results were achieved concerned about the ultimate fracture failure load and the fracture locations. It illustrated that XFEM could be used as a practicable approach to complement the experimental tests and clinical observations for assessing the mechanical response of restorations under occlusal loads. The pattern and magnitude of load in the fracture failure tests cannot accurately mimic the clinical situations. It is an inevitable limitation in these kinds of in vitro experiments. Therefore, the most conventional loading protocols from literature were adopted in the present study.38,39 The cracking model presented herein is based on static fracture mechanics, where crack development in the quasi-brittle biomaterials is not only considered dependent on a quasi-static loading process, but also dependent on pre-existing cyclic loadinginduced damage.40,41 The fatigue-induced failure has not been taken into account in these models. Clinical experience indicates that most fractures in prosthodontics restorations occur after several years. Generally, such failures are unrelated to episodes of acute overload, but result from fatigue failure. The absence of fatigue loading is the limitation of the study. However, static loading is usually the first step in the evaluation of biomaterials and is commonly used in order to obtain basic knowledge regarding the fracture behaviour and load capacity. The fracture failure tests and XFEM analysis could not fully exhibit the damage process of post-core restorations, however, the results were still helpful to clarify the susceptible area of restorations of endodontically treated teeth and analyze the possible process of fatigue cracking formation and development. Another limitation of this study was the hypothesis of prefect bonding within all of the interfaces. Theoretically, there are complex contact relationships within the interfaces. If the nonlinear contact situations and the fracture were both investigated in FEA, the convergence could be hardly achieved. The results of XFEM only provide the susceptible areas of the endodontically treated teeth. The specific crack locations of post-restored teeth need to be analyzed by further studies. From fracture failure test and XFEM results, a possible damage expanding way was proposed that the fracture originated from glass ionomer cement interface of core–crown, propagated down towards post through the interior adhesive area, and finally leaded to stress concentration in root. The protecting of the palatal cervical region of the teeth was crucially important for long-term survival of post retained restorations.
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Within the limitations of this study the following conclusions can be drawn.
Conclusion
Acknowledgements
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Supported by the National Natural Science Foundation of Q3 China (No. 81100784). The authors thank Prof. Qing Li (School of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney) for his technical and software supports and advices with the modelling process.
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references
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1. Peroz I, Blankenstein F, Lange KP, Naumann M. Restoring endodontically treated teeth with posts and cores – a review. Quintessence International 2005;36:737–46. 2. Schwartz RS, Robbins JW. Post placement and restoration of endodontically treated teeth: a literature review. Journal of Endodontics 2004;30:289–301. 3. Buttel L, Krastl G, Lorch H, Naumann M, Zitzmann NU, Weiger R. Influence of post fit and post length on fracture resistance. International Endodontic Journal 2009;42:47–53. 4. Dietschi D, Duc O, Krejci I, Sadan A. Biomechanical considerations for the restoration of endodontically treated teeth: a systematic review of the literature – Part 1. Composition and micro- and macrostructure alterations. Quintessence International 2007;38:733–43. 5. Dietschi D, Duc O, Krejci I, Sadan A. Biomechanical considerations for the restoration of endodontically treated teeth: a systematic review of the literature. Part II (Evaluation of fatigue behavior, interfaces, and in vivo studies). Quintessence International 2008;39:117–29. 6. Zicari F, Van Meerbeek B, Scotti R, Naert I. Effect of ferrule and post placement on fracture resistance of endodontically treated teeth after fatigue loading. Journal of Dentistry 2013;41:207–15. 7. Ferrari M, Cagidiaco MC, Grandini S, De Sanctis M, Goracci C. Post placement affects survival of endodontically treated premolars. Journal of Dental Research 2007;86:729–34. 8. Roscoe MG, Noritomi PY, Novais VR, Soares CJ. Influence of alveolar bone loss, post type, and ferrule presence on the biomechanical behavior of endodontically treated maxillary canines: strain measurement and stress distribution. Journal of Prosthetic Dentistry 2013;110:116–26. 9. Stankiewicz N, Wilson P. The ferrule effect. Dental Update 2008;35:222–4. 10. Ma PS, Nicholls JI, Junge T, Phillips KM. Load fatigue of teeth with different ferrule lengths, restored with fiber posts, composite resin cores, and all-ceramic crowns. Journal of Prosthetic Dentistry 2009;102:229–34. 11. Schmitter M, Rammelsberg P, Lenz J, Scheuber S, Schweizerhof K, Rues S. Teeth restored using fiberreinforced posts: in vitro fracture tests and finite element analysis. Acta Biomaterialia 2010;6:3747–54.
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The cement layers were the most susceptible structures within the post-core restorations. Fractures originated from the weakest structure of glass ionomer cement at the palatal crown margin, and then propagated down towards the labial direction though the interface of adhesive resin cement layer. Palatal ferrule height could efficiently protect the dentine– core interface by forbidding the damage extension from glass ionomer cement layer to adhesive resin cement layer.
Please cite this article in press as: Zhang YY, et al. The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.003
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12. Sorensen JA, Engelman MJ. Ferrule design and fracture resistance of endodontically treated teeth. Journal of Prosthetic Dentistry 1990;63:529–36. 13. Libman WJ, Nicholls JI. Load fatigue of teeth restored with cast posts and cores and complete crowns. International Journal of Prosthodontics 1995;8:155–61. 14. Da Silva NR, Raposo LHA, Versluis A, Fernandes-Neto AJ, Soares CJ. The effect of post, core, crown type, and ferrule presence on the biomechanical behavior of endodontically treated bovine anterior teeth. Journal of Prosthetic Dentistry 2010;104:306–17. 15. Ng CC, Dumbrigue HB, Al-Bayat MI, Griggs JA, Wakefield CW. Influence of remaining coronal tooth structure location on the fracture resistance of restored endodontically treated anterior teeth. Journal of Prosthetic Dentistry 2006;95:290–6. 16. Naumann M, Blankenstein F, Dietrich T. Survival of glass fibre reinforced composite post restorations after 2 years-an observational clinical study. Journal of Dentistry 2005;33:305–12. 17. Monticelli F, Osorio R, Albaladejo A, Aguilera FS, Ferrari M, Tay FR, et al. Effects of adhesive systems and luting agents on bonding of fiber posts to root canal dentin. Journal of Biomedical Materials Research Part B Applied Biomaterials 2006;77:195–200. 18. Ichim I, Li Q, Li W, Swain MV, Kieser J. Modelling of fracture behaviour in biomaterials. Biomaterials 2007;28:1317–26. 19. Li W, Swain MV, Li Q, Ironside J, Steven GP. Fibre reinforced composite dental bridge. Part II: Numerical investigation. Biomaterials 2004;25:4995–5001. 20. Melenk JM, Babusˇka I. The partition of unity finite element method: basic theory and applications. Computer Methods in Applied Mechanics and Engineering 1996;139:289–314. 21. Feerick EM, Liu XC, McGarry P. Anisotropic mode-dependent damage of cortical bone using the extended finite element method (XFEM). Journal of the Mechanical Behavior of Biomedical Materials 2013;20:77–89. 22. Barani A, Keown AJ, Bush MB, Lee JJ, Chai H, Lawn BR. Mechanics of longitudinal cracks in tooth enamel. Acta Biomaterialia 2011;7:2285–92. 23. Barani A, Bush MB, Lawn BR. Effect of property gradients on enamel fracture in human molar teeth. Journal of the Mechanical Behavior of Biomedical Materials 2012;15:121–30. 24. Giner E, Sukumar N, Denia FD, Fuenmayor FJ. Extended finite element method for fretting fatigue crack propagation. International Journal of Solids and Structures 2008;45:5675–87. 25. Zhang Z, Guazzato M, Sornsuwan T, Scherrer SS, Rungsiyakull C, Li W, et al. Thermally induced fracture for core-veneered dental ceramic structures. Acta Biomaterialia 2013;9:8394–402. 26. Ichim I, Schmidlin PR, Kieser JA, Swain MV. Mechanical evaluation of cervical glass-ionomer restorations: 3D finite element study. Journal of Dentistry 2007;35:28–35.
27. Li W, Swain MV, Li Q, Steven GP. Towards automated 3D finite element modeling of direct fiber reinforced composite dental bridge. Journal of Biomedical Materials Research Part B: Applied Biomaterials 2005;74:520–8. 28. Peyton FA, Mahler DB, Hershenov B. Physical properties of dentin. Journal of Dental Research 1952;31:366–70. 29. Yettram AL, Wright KW, Houston WJ. Centre of rotation of a maxillary central incisor under orthodontic loading. British Journal of Orthodontics 1977;4:23–7. 30. Borchers L, Reichart P. Three-dimensional stress distribution around a dental implant at different stages of interface development. Journal of Dental Research 1983;62:155–9. 31. Friedman CM, Sandrik JL, Heuer MA, Rapp GW. Composition and mechanical properties of gutta-percha endodontic points. Journal of Dental Research 1975;54:921–5. 32. Williams KR, Edmundson JT, Rees JS. Finite element stress analysis of restored teeth. Dental Materials 1987;3:200–6. 33. Sahafi A, Peutzfeldt A, Ravnholt G, Asmussen E, Gotfredsen K. Resistance to cyclic loading of teeth restored with posts. Clinical Oral Investigations 2005;9:84–90. 34. Goodacre CJ, Bernal G, Rungcharassaeng K, Kan JY. Clinical complications in fixed prosthodontics. Journal of Prosthetic Dentistry 2003;90:31–41. 35. Li Q, Xu B, Wang Y, Cai Y. Effects of auxiliary fiber posts on endodontically treated teeth with flared canals. Operative Dentistry 2011;36:380–9. 36. Hayashi M, Takahashi Y, Imazato S, Ebisu S. Fracture resistance of pulpless teeth restored with post-cores and crowns. Dental Materials 2006;22:477–85. 37. Sorrentino R, Aversa R, Ferro V, Auriemma T, Zarone F, Ferrari M, et al. Three-dimensional finite element analysis of strain and stress distributions in endodontically treated maxillary central incisors restored with different post, core and crown materials. Dental Materials 2007;23:983–93. 38. Ramı´rez-Sebastia` A, Bortolotto T, Cattani-Lorente M, Giner L, Roig M, Krejci I. Adhesive restoration of anterior endodontically treated teeth: influence of post length on fracture strength. Clinical Oral Investigations 2014;18:545–54. 39. Santos-Filho PC, Verı´ssimo C, Soares PV, Saltarelo RC, Soares CJ, Marcondes Martins LR. Influence of ferrule, post system, and length on biomechanical behavior of endodontically treated anterior teeth. Journal of Endodontics 2014;40:119–23. 40. Chuang SF, Yaman P, Herrero A, Dennison JB, Chang CH. Influence of post material and length on endodontically treated incisors: an in vitro and finite element study. Journal of Prosthetic Dentistry 2010;104:379–88. 41. Valdivia AD, Raposo LH, Simamoto-Ju´nior PC, Novais VR, Soares CJ. The effect of fiber post presence and restorative technique on the biomechanical behavior of endodontically treated maxillary incisors: an in vitro study. Journal of Prosthetic Dentistry 2012;108:147–57.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/jden 1 2 3
The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth
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Y.Y. Zhang, M.D. Peng, Y.N. Wang *, Q. Li ** Key Laboratory for Oral Biomedical Engineering of Ministry of Education, School and Hospital of Stomatology, Wuhan University, 237# Luo Yu Road, Wuhan, China
article info
abstract
Article history:
Objectives: To evaluate the fracture resistance of fibre post-restored treated teeth with
Received 11 March 2014
various ferrule configurations by using fracture failure tests and extended finite element
Received in revised form
analysis (XFEM).
15 September 2014
Methods: 60 Maxillary central incisors were collected and divided into six groups (n = 10)
Accepted 9 October 2014
according to various ferrule configurations with different ferrule heights in the labial or
Available online xxx
palatal region. All of the teeth were endodontically treated and restored by using fibre posts,
Keywords:
retained restorations until fracture occurred. The ultimate load was recorded and analyzed
Extended finite element method
by one way analysis of variance (ANOVA). The fractured specimens were longitudinal
Fracture failure
sectioned and investigated by micro-stereomicroscope and scanning electronic microscope.
composite cores and metal crowns. Fracture failure tests were performed on the post
Post-restored teeth
XFEM was used to model the fracture of the post-restored teeth and exhibit crack initiation
Ferrule
and propagation in the cement layers. Results: Fracture failure tests indicated that the palatal ferrule significantly enhanced the fracture resistance of the post-restored teeth, regardless the height of the labial ferrule. The fractography investigation exhibited that the crack initiated at the palatal margin of Q2 the cement layer and propagated to the cervical region of the root. XFEM confirmed these findings and demonstrated that increasing of the palatal ferrule could effectively enhance the anti-fracture ability of the adhesive cement and protected the integrity of adhesive cement. Conclusion: Adhesive interface was the susceptible structure of the post retained restorations. Increasing palatal ferrule height could effectively reduce the stress concentrated within the palatal adhesive cement. # 2014 Elsevier Ltd. All rights reserved.
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1.
Introduction
The restoration of endodontically treated teeth still represents a challenging task for clinicians. The task is complicated by
substantial loss of coronal tooth structure and the ability to predict restorative long-term success. It has been disputed whether the mechanical properties of dental hard tissues, such as modulus of elasticity, compressive strength, or brittleness, would change after endodontic treatment.1 However,
* Corresponding author at: Department of Prosthodontics, School and Hospital of Stomatology, Wuhan University, 237# Luo Yu Road, Wuhan, China. Tel.: +86 27 87646696; fax: +86 27 87873260. ** Corresponding author at: Department of Prosthodontics, School and Hospital of Stomatology, Wuhan University, 237# Luo Yu Road, Wuhan, China. Tel.: +86 27 87686220; fax: +86 27 87873260. E-mail addresses: [email protected] (Y.N. Wang), [email protected] (Q. Li). http://dx.doi.org/10.1016/j.jdent.2014.10.003 0300-5712/# 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Zhang YY, et al. The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.003
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endodontically treated teeth were claimed to be weaker and more prone to fracture than vital teeth, according to the clinical observations.2 The loss of structural integrity might contribute primarily to the vulnerability of endodontically treated teeth and their reduced resistance to fracture.1 The likelihood of survival of a post-retained restoration thus directly related to the quantity and quality of the remaining tooth structure and the efficiency of the restorative procedures used to replace lost structural integrity.3 Although restorative methods affect the fracture resistance and fracture mode of post-restored teeth at a certain extent,4 preserving intact coronal tooth structure and maintaining cervical tissue are still considered to be more crucial to optimize the biomechanical behaviour of the endodontically treated teeth.2,5–8 Parallel walls of dentine extending coronally from the crown margin provide a ferrule, which after being encircled by a crown provides a protective effect by reducing stresses within a tooth called the ‘‘ferrule effect’’.9 The result is an elevation in resistance form of the crown from the fracture of tooth structure. It has been proved both in laboratorial investigation and clinical observations that the less coronal dentine remains, the weaker the fracture resistance of endodontically treated teeth.10,11 It has been proposed that 1.5 mm coronal dentine is necessary for the long-term survival of restoration.12–14 In the clinical situation, however, the ideal status of circumferential ferrule with adequate and uniform height could not be always achieved. A tooth with varying ferrule configurations, especially absence of the labial or palatal wall, really plagued dentists in the clinical practice. Ng et al.15 have declared that the coronal dentine on the palatal aspect plays more important roles in the anti-fracture ability of endodontically treated teeth than those on the labial ones. Nevertheless, the effects of the various ferrule configurations on the fracture resistance of post-retained restorations have not been comprehensively investigated. For the restorations of endodontically treated teeth, clinical observations and experimental investigations have shown that the fracture of residual dentine or loosening of the postcore assembly reflects a major mode of failure.16,17 Unexpected damage accumulation and interfacial debonding decrease the mechanical properties of luting agents, then lead to entire failure of the post-restored crown. Thus, a full understanding of stress fields developed in the restoration and tooth becomes particularly important, especially for the weaker structures such as the cement layers. Finite element analysis (FEA) is a promising approach for investigating the biomechanics in dentistry, which allows bioengineering virtual simulations of oral environments.18 However, the reliability of these results deducing in the clinical practice is still under suspicion. A hypothesis that the structure would be intact during the whole loading period was often adopted in the FEA studies, which is obviously not consistent with the complexity of clinical process.19 The present study attempts to use isotropic damage initiation criteria in conjunction with the extended finite element method (XFEM) to model the fracture of the post-restored teeth and exhibit crack initiation and propagation in cement layers. XFEM is a method developed for computationally predicting crack initiation and propagation, which has recently been successfully applied in solving medical problems, simulating dynamic fracture, fatigue,
and various crack patterns in brittle materials.20–23 Compared with conventional discontinuous analysis, one of the main advantages of XFEM is that it not only allows automatic initiation and propagation of cracksmodelled without predefining in isotropic elastic materials, but also avoids the requirement of elements remeshing during crack extension.21,24 Another major advantage of the method is that more accurate results can be acquired by increasing the degrees of freedom in the failure element only.25 The objective of the present study was to investigate the influence of ferrule configurations on the fracture process of fibre post-retained teeth by using a combined method of fracture failure test and XFEM. The null hypothesis was that fracture resistance does not vary as a function of the ferrule configuration.
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2.
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Materials and methods
A combined experimental and numerical method was used to evaluate the mechanical performance of fibre post-restored teeth with various ferrule configurations under external loads.
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2.1.
100
In vitro tests
The study protocol was approved by the Ethics Committee of the School and Hospital of Stomatology, Wuhan University. Patients who donated their teeth were informed of the purposes of the study. Written informed consents were obtained prior to teeth extraction.
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2.2.
106
Fabrication of post retained restorations
Sixty intact human maxillary incisors were collected directly after extraction and stored in 0.1% thymol solution less than 3 months. The teeth were randomly assigned to six groups and embedded in acrylic resin at the level of 2 mm below the cemento-enamel junction (CEJ). A thin coating of silicone on the roots, approximate 0.2–0.3 mm in thickness, were made to imitate a human periodontal ligament periodontium. To confirm the even size distribution within groups, the root lengths (from the root apex to the proximal CEJ), the mesiodistal and labial-palatal dimensions of the teeth were measured (at the level of the CEJ) using acalliper (LA-7, Peacock Inc., Tokyo, Japan). Overall, the mean of root length was 13.5 1.5 mm, while the means of mesio-distal and labial-palatal dimensions were 6.1 0.5 mm and 7.5 0.7 mm. The specimens were endodontically prepared using a crown-down technique by ProTaper (Dentsply-Maillefer, Konstanz, Swizerland) and then filled by gutta-percha (Lexicon Gutta Percha Points, Dentsply, Tulsa, OK, USA) and sealer (AH plus, Dentsply/De Trey, Constance, Germany). The teeth were decoronated at six levels to the most labial and palatal points of CEJ, based on the ferrule configurations (Fig. 1B1–B6): Group 1 (as negative control): 0 mm labial ferrule and 0 mm palatal ferrule. Group 2: 1 mm labial ferrule and 0 mm palatal ferrule. Group 3: 2 mm labial ferrule and 0 mm palatal ferrule. Group 4: 0 mm labial ferrule and 1 mm palatal ferrule. Group 5: 0 mm labial ferrule and 2 mm palatal ferrule.
Please cite this article in press as: Zhang YY, et al. The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.003
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Fig. 1 – Schematic diagram of the specimens and FE models. (A) Specimen with a metal crown; (C) representative schematic diagram of FE model; (B1–B6) specimens with different ferrule height; (D1–D6) FE models with different ferrule height. 0 mm labial ferrule and 0 mm palatal ferrule (B1, D1); 1 mm labial ferrule and 0 mm palatal ferrule (B2, D2); 2 mm labial ferrule and 0 mm palatal ferrule (B3, D3); 0 mm labial ferrule and 1 mm palatal ferrule(B4, D4); 0 mm labial ferrule and 2 mm palatal ferrule (B5, D5); 2 mm labial ferrule and 2 mm palatal ferrule (B6, D6).
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Group 6 (as positive control): 2 mm labial ferrule and 2 mm palatal ferrule. The root canals were enlarged using tapered drills (MacroLock Kit, RTD Inc., St. Egreve, France) in the recommended sequence, leaving at least 4 mm of the root filling in the apical portion. A post space with the depth of 8 mm below labial CEJ was achieved. The root canals were treated using 37% phosphoric acid (Ultra-Etch, Ultradent, South Jordan, UT, USA), primer and adhesive (Adper Single Bond 2, 3M ESPE, St. Paul, MN, USA). A quartz fibre post (Macro-Lock 4#, RTD Inc., St. Egreve, France) was luted into the post space using an adhesive resin cement (PermaCem, DMG Inc., Hamburg, Germany) and light-cured for 40 s from the top of the post with a halogen light unit (ESPE Elipar Trilight, 3M ESPE, St. Paul, MN, USA). Subsequently, a standardized core was built up with a composite (LuxaCore, DMG Inc., Hamburg, Germany) using CoreForms (kerr) for anterior teeth with layers of maximum 2 mm. Each layer was polymerized from each side for 40 s with a halogen light unit. The coronal portion of the resin core and/or dentine was
prepared followed the criteria for the full-coverage crown with 1 mm width of shoulder at the CEJ level (Fig. 1A.). Metal crowns were fabricated and cemented on the prepared teeth using glass ionomer cement (Fuji, GC Inc., Tokyo, Japan).
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2.3.
Fracture failure tests and fractography investigation
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A universal testing machine (Model 8841, Instron, Norwood, MA, USA) was used to perform the fracture failure tests. Each test was performed at a crosshead speed of 0.5 mm/min until fracture occurred. A compressive load was applied on the lingual surface (2 mm below the incisal edge) with an angle of 458 to the long axis of the root. The ultimate loads that the specimen could withstand were recorded. The fractured specimens were facial-lingually sectioned using a low speed diamond saw (Isomet, Buehler Inc., Lake Bluff, IL, USA) under water cooling. The section surfaces were investigated using a micro-stereomicroscope (Stemi SV11 Apo, Carl Zeiss Micro Imaging Inc., Thornwood, NY, USA) at magnifications of 1 and scanning electronic microscope
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Please cite this article in press as: Zhang YY, et al. The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.003
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(SEM, JSM-5510LV, JEOL, Tokyo, Japan) for the micro-structures at magnifications of 35. Micrographs of representative areas of the fractured interfaces were recorded.
Table 1 – Material properties used in the finite element analysis. Material
Young’s modulus (GPa)
3.
Finite element analysis
3.1.
Geometry acquisition
Dentin28 Periodontal liagament29 Cortical bone30 Gutta-percha31 Ni–Cr alloy32 Composite resin corea Adhesive resin cementa Glass ionomer cementa Fibre posta
18.6 0.31 – 68.9e3 0.45 – 13.7 0.30 – 0.14 0.45 – 200.0 0.33 – 7.0 0.30 – 2.8 0.33 13.0 22.0 0.35 6.5 EL = 39.0, ET = ET0 = 13.5; GLT = GLT0 = 6.5, GTT = 5.0; nLT = nLT0 = 0.285, nTL = nT0 L = 0.285, nTT0 = 0.35b
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A standardized model of a maxillary right central incisor (Nissin Dental Products Inc., Kyoto, Japan) was adopted as the prototype of the tooth, which has the similar dimension as the tested natural teeth. The geometric morphological data of the tooth, the tooth after preparation and the simulated alveolar bone were acquired by using a laser-based 3D digitizing system (D800, Wieland Dental & Technik GmbH & Co. KG, Schwenninger, German) and files with ‘‘.stl’’ extension (stereolithography) were consequently generated. Based on these files, finite element (FE) models with different ferrule configurations were obtained using ScanIP1 module (Simpleware Ltd, UK). Appropriate modifications for some surface points were introduced from the average dimensions of the tooth structures. The structure of periodontal ligament was generated by creating a uniform 0.3 mm shell around the root or the bone.26 The fibre post was virtually modelled according to the drawing sheet supplied by the manufacturer with some simplifications, and placed into the FE models. The glass ionomer cement layer within the crown–core interface and adhesive resin cement layer within the post-dentine interface were modelled. Clinically, the thickness of cement layer varies from 0.02 mm to 0.1 mm, which appears fairly difficult to be accurately modelled unless an unrealistically fine mesh is used. Considering the appropriateness of geometrical aspect ratio, the cement layers were created with a constant thickness of 0.2 mm.27 Overall, the FE models consist of eight fundamental parts: (1) alveolar bone, (2) gingiva, (3) periodontal ligament, (4) residual roots (with different ferrule configurations), (5) fibre post, (6) resin core, (7) metal crown, (8) adhesive resin cement layer, and (9) glass ionomer cement layer (Fig. 1C and D1–D6).
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3.2.
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A static structural analysis was performed to calculate the stress distribution in different ferrule configurations. All numerical simulations were performed using ABAQUS/CAE (SIMULIA, Version 6.10, Providence, RI, USA). The finite element analysis was based on following assumptions: (1) all solids were assumed to be homogeneous, isotropic, and linear elastic; (2) perfect bonding between components; (3) no flaw in any component in the initial model; (4) the adhesive resin cement layer and glass ionomer cement layer were of uniform thickness and allowed crack growth when the maximum principal stress reached a certain value; (5) rigid constrains at the base and mesio-distal surfaces of the bone. The number of tetrahedral elements for the models range from 509,618 to 549,438. The properties of the materials (Young’s modulus and Poisson’s ratio) were determined from literature and the manufacturers (Table 1). A load of 350 N (45 angle degree with respect to the longitudinal axis of the tooth), was applied on the lingual surface (2 mm below the incisal edge) of the crown.
Poisson’s ratio
Tensile strength (MPa)
a
Information provided by the manufacturers. EL, ET, and ET’: elastic modulus in longitudinal (parallel to fibres) and 2 perpendicular directions in transverse plane of fibre post. GLT and GLT0 : longitudinal in plane shear modulus. GTT: transverse shear modulus. nTL, nT’L, and nTT0 : first subscript in Poisson’s ratio refers to direction of load, and second to direction of displacement. b
In the present study, XFEM was used to simulate initiation and propagation of a discrete crack along an arbitrary path without the requirement of remeshing in the bulk materials. The cement layers of adhesive resin and glass ionomer were considered as the quasi-brittle biomaterials, which could resist significant stronger compression stress than the tension stress. The maximum principal stress criterion was chosen as the fracture criterion. The maximum principal strain criterion > could be represented as: f ¼ < ssmax s 0max represented the 0
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maximum allowable principal stress. And smax represented the maximum principal stress on the FE model. Damage was
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max
assumed to be initialized when the value of s max =s 0max reached one. The minimum load leaded to adhesive resin cement layer and glass ionomer cement layer damage and the damage location would be analyzed and discussed.
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XFEM modelling 241
3.3.
Statistical analysis
The means of fracture failure load were calculated and analyzed using one-way analysis of variance (ANOVA) with the ferrule configuration setting as the variable. Tukey’s post hoc test was performed to evaluate the differences among the groups. All analyses were performed using the Statistical Package for the Social Sciences statistical package (SPSS 13.0 for Windows, SPSS Inc, Chicago, IL, USA). The level of significance was set at 0.05.
4.
Results
4.1.
Fracture failure tests and fractography investigations
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Fracture failure loads (N) and standard deviations are presented in Table 2. Group 6 presented the highest fracture
Please cite this article in press as: Zhang YY, et al. The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.003
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Table 2 – Means of fracture failure strength values (N) and the results of Tukey’s post hoc analysis. Group 1 2 3 4 5 6 a
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Labial ferrule height (mm)
Palatal ferrule height (mm)
n
Mean (N)
SD
0 1 2 0 0 2
0 0 0 1 2 2
10 10 10 10 10 10
130.01 167.32 203.56 280.24 380.17 532.82
30.31 46.20 67.93 59.26 87.35 126.43
Tukey’s intervalsa a a a b c d
For Tukey’s intervals, the same letter means no significant difference within the groups.
failure values, whereas, the lowest fracture resistance appeared in the Group 1. Tukey’s multiple comparisons showed that there were no significant differences among Group 1, 2 or 3, in which the teeth have no palatal ferrule. The ultimate loads were increased with the increase of palatal ferrule height in Groups 4 and 5 (Table 2). Typical fracture patterns are represented in Fig. 2. In all of the teeth, cracks were detected at the palatal margin of the crown and the cervical region of the root in the labial side. In the palatal margin of the crowns, damage was found in the glass ionomer cement in all the groups. However, crack of resin adhesive interface was only detected in lower palatal ferrule groups. The crack initiated at the location of the palatal
core–dentine interface, and then extended to the interface of dentine-post in lower palatal ferrule groups (1, 2, and 3). Whereas, the crack was observed occurring within the palatal ferrule in the higher palatal ferrule groups (4, 5, and 6).
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XFEM results
The maximum principal stress distribution on glass ionomer cement layer and adhesive resin cement layer are exhibited in Fig. 3. An obvious stress concentrations were detected around the palatal margin of cement layer in groups 1, 2 and 3. The fracture locations and the estimated loads were listed in Table 3. The loads, which caused the initial damage in the
Fig. 2 – Schematic diagram of the fracture modes using the representative fractographics. Schematic diagram of fracture modes in groups without palatal ferrule (A1) and groups with palatal ferrule (B1): the red dotted line represents fracture line. The representative fractographics at 1T magnification in groups without palatal ferrule (A2) and groups with palatal ferrule (B2). Red arrows point to the crack locations. (A3, B3) Display the SEM results of palatal region of restorations at 35T magnification. (A3) Exhibits the debonding at the interface of core and dentine. (B3) shows integrated interface of core– dentine and fracture occurred within dentine tissue. AR, adhesive resin cement layer; C, crown; D, dentine; GI, glass ionomer cement layer; P, fibre post; RC, resin core. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Zhang YY, et al. The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.003
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Fig. 3 – The maximum principal stress distributions in glass ionomer cement layer and adhesive cement layer. The maximum principal stress contours of glass ionomer cement layer (A1–A6). The maximum principal stress contours of adhesive resin cement layer (B1–B6). Contour details exhibited obvious maximum principal stress concentration in palatal margin of cement layers in groups 1, 2 and 3.
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glass ionomer cement layer, were similar in all of the FE models. However, the ultimate load leading to crack initiation in adhesive resin cement layer and crack location were various in different ferrule configurations. Increasing of the palatal ferrule was more effective in enhancing the anti-fracture ability of adhesive cement than the labial ferrule. In lower palatal ferrule height groups, fractures in adhesive resin cement layer were firstly detected in the palatal margin, then extended along the interface between core and root (Fig. 4A1– A3). However, in higher ferrule model, adhesive cement cracks were firstly detected in root canal at bone level (Fig. 4B1–B4.). The results of this XFEM were consistent with the in vitro tests.
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The complex relationship between ferrule height and the fracture resistance ability of post-restored crowns in anterior region was important to predict the clinical prognosis in response to biomechanical loading. In clinical practice, a uniform ferrule with height of 1.5 mm was proposed as the basic requirement for achieving good prognosis.33 However, the prognosis of the post-restored tooth with varying ferrule
Discussion
configurations has not been fully investigated. In this study, fracture failure tests and XFEM analysis were used to investigate the fracture resistance of teeth with different ferrule configurations, which focused on the damage of the susceptible structure within the restorations. The null hypothesis that fracture resistance does not vary as a function of the ferrule configuration was rejected for the significant differences among the groups with various ferrule configurations. It has been confirmed that a uniform 2 mm ferrule is superior to the defect or lack of a ferrule in the fracture resistance under static load. The lowest ultimate load that induced restoration failure was detected in the Group 1, tooth without a ferrule (130.01 30.31 N). The means of fracture failure load were slightly enhanced while the labial ferrule height increased. However, there was no significant difference of fracture failure load among the Groups 1, 2 or 3. The reported maximal occluding force for males exerted by a maxillary incisor was 146 44 N.34 Allowing for SD of variation, teeth with no palatal ferrule, no matter with or without labial ferrule, are at risk of fracture when subjected to maximal clenching force. Concerned about the position of ferrule, palatal ferrule obviously plays more important roles in fracture resistance of endodontically treated teeth.
Table 3 – The predicted minimal failure loads (N) of glass ionomer cement layer and adhesive resin cement layer and cracks location (from the FEA results). Group
Glass ionomer cement layer Failure load (N)
1 2 3 4 5 6
57.4 59.7 56.8 58.4 57.6 59.2
Adhesive resin cement layer
Crack location Palatal Palatal Palatal Palatal Palatal Palatal
cervical cervical cervical cervical cervical cervical
region region region region region region
Failure load (N)
Crack location
100.9 142.7 146.6 225.5 238.2 235.6
Dentine–core interface Dentine–core interface Dentine–core interface Upper third of root Upper third of root Upper third of root
Please cite this article in press as: Zhang YY, et al. The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.003
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Fig. 4 – Representative XFEM results in Model 1 and Model 6. XFEM results show the crack loacations. (A1–A3) Crack initiated in glass ionomer cement layer and extended to adhesive cement layer directly with the increasement of load. The failure mode was detected in Models 1, 2, and 3. (B1–B4) Crack originated from glass ionomer cement layer as limited damage. Crack in adhesive resin cement layer exhibits in the dentine-post interface at bone level. This failure mode was detected in Models 4, 5, and, 6.
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In the fractography investigation, all of the specimens suffered from the fracture of dentine, crack of glass ionomer cement, and/or debonding of adhesive resin cement, which were reported in several previous studies.35–37 It could be concluded that the failure of post retained restorations is a typical damage driven continuum-to-discrete process. The adhesive interfaces were proposed as the most brittle structures within the post-core restorations. Investigating the mechanical responses of post-restored teeth, therefore, it is crucially required to model the fracture phenomena. XFEM was used to model the fracture, exhibit crack initiation, and propagation in adhesive interfaces. In this study, glass ionomer cement and resin adhesive cement were considered as typical quasi-brittle materials, and prone to be tensile fracture, which represent significant higher compressive resistance than the tensile strength. Once these two materials were loaded beyond their maximum tensile strength, inherent damage were accumulated before macroscopic fracture occurs. Consequently, crack bands appeared orthogonal to the principle tensile stress direction.18 Therefore, the max principle stress criterion was introduced in the XFEM analysis to determine the crack initiation and simulate the damage propagation. XFEM investigations revealed that the initialized crack occurred around the palatal margin. It indicated that the
potential debonding firstly occurred between the crown and dentine interface, while the tensile stress approached or exceeded the tolerable bond strength. The estimated fracture loads, which caused the damage within glass ionomer cement, were similar among all the groups. However, a localized damage occurred within the glass ionomer cement did not mean the immediate failure of the restorations unless the cracks extended to the adhesive resin cement layer and leaded more serious damage. With respect to the resin adhesive cement, the estimated fracture loads were obviously different among the various ferrule configurations. Moreover, the location of the failure occurrence also has been influenced by the palatal ferrule height. In the lower palatal height models (Models 1, 2 and 3), no matter the labial ferrule increased, the initialized fracture loads of the resin cement were almost not change, and the fractures were located at the level of dentine-core interface. In the Models 4, 5 and 6, the initialized fracture location changed to the dentine-post interface, and the estimated damage load was much higher. The changes of fracture location were consistent with the fractography investigations. It indicated that the increasing of palatal ferrule height efficiently protects the dentine–core interface by forbidding the damage propagation from glass ionomer cement layer to adhesive resin cement layer.
Please cite this article in press as: Zhang YY, et al. The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.003
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From the XFEM simulation of the fracture process, it could be assumed that the fractures originated from the weakest structure of glass ionomer cement at the palatal crown margin, and then propagated down towards the labial direction though the interface of adhesive resin cement layer. Therefore, placing the interface of adhesive resin cement layer away from palatal cervical region by increasing palatal ferrule might contribute primarily in enhancing the anti-fracture load of post-restored teeth. In the present study, good agreement between the numerical predictions and the experimental results were achieved concerned about the ultimate fracture failure load and the fracture locations. It illustrated that XFEM could be used as a practicable approach to complement the experimental tests and clinical observations for assessing the mechanical response of restorations under occlusal loads. The pattern and magnitude of load in the fracture failure tests cannot accurately mimic the clinical situations. It is an inevitable limitation in these kinds of in vitro experiments. Therefore, the most conventional loading protocols from literature were adopted in the present study.38,39 The cracking model presented herein is based on static fracture mechanics, where crack development in the quasi-brittle biomaterials is not only considered dependent on a quasi-static loading process, but also dependent on pre-existing cyclic loadinginduced damage.40,41 The fatigue-induced failure has not been taken into account in these models. Clinical experience indicates that most fractures in prosthodontics restorations occur after several years. Generally, such failures are unrelated to episodes of acute overload, but result from fatigue failure. The absence of fatigue loading is the limitation of the study. However, static loading is usually the first step in the evaluation of biomaterials and is commonly used in order to obtain basic knowledge regarding the fracture behaviour and load capacity. The fracture failure tests and XFEM analysis could not fully exhibit the damage process of post-core restorations, however, the results were still helpful to clarify the susceptible area of restorations of endodontically treated teeth and analyze the possible process of fatigue cracking formation and development. Another limitation of this study was the hypothesis of prefect bonding within all of the interfaces. Theoretically, there are complex contact relationships within the interfaces. If the nonlinear contact situations and the fracture were both investigated in FEA, the convergence could be hardly achieved. The results of XFEM only provide the susceptible areas of the endodontically treated teeth. The specific crack locations of post-restored teeth need to be analyzed by further studies. From fracture failure test and XFEM results, a possible damage expanding way was proposed that the fracture originated from glass ionomer cement interface of core–crown, propagated down towards post through the interior adhesive area, and finally leaded to stress concentration in root. The protecting of the palatal cervical region of the teeth was crucially important for long-term survival of post retained restorations.
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Within the limitations of this study the following conclusions can be drawn.
Conclusion
Acknowledgements
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Supported by the National Natural Science Foundation of Q3 China (No. 81100784). The authors thank Prof. Qing Li (School of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney) for his technical and software supports and advices with the modelling process.
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The cement layers were the most susceptible structures within the post-core restorations. Fractures originated from the weakest structure of glass ionomer cement at the palatal crown margin, and then propagated down towards the labial direction though the interface of adhesive resin cement layer. Palatal ferrule height could efficiently protect the dentine– core interface by forbidding the damage extension from glass ionomer cement layer to adhesive resin cement layer.
Please cite this article in press as: Zhang YY, et al. The effects of ferrule configuration on the anti-fracture ability of fibre post-restored teeth. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.10.003
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