The effect of ferrule on the fracture mode of endodontically treated canines restored with fibre posts and metal-ceramic or all-ceramic crowns
Introduction
Post and core restorations are widely used to build up endodontically treated teeth (ETT) with moderate to severe loss of coronal tooth structure and to retain definitive restorations.1, 2, 3
A number of in vitro studies4, 5, 6 have investigated factors that may affect fracture resistance and fracture mode of ETT restored with different post systems, such as post and core material, crown design, type of abutment and amount of lost hard tissue. Ideally, a post system should exhibit fracture resistance higher than the average masticatory forces and it should minimize the stress on a tooth by distributing occlusal loads evenly to avoid unfavourable root fracture. Fractures above cementoenamel junction (CEJ) and the alveolar bone can be considered favourable as the remaining tooth structure allows a new restorative treatment. The fracture of the post and the root of the tooth below the alveolar bone are considered unfavourable, as retreatment would not be applicable.
The importance of preserving a minimum amount (2 mm) of coronal dentine height after preparation, known as “ferrule effect”, on the fracture resistance and prevention of root fracture of ETT has been reported in various studies7, 8, 9, 10, 11 although other studies refute this statement.12, 13, 14
It has been reported that when the ferrule effect is present, stresses are redistributed in the outer surface regions of the coronal third of the root15 thus a possible fracture in this area can be repairable. When the ferrule is absent, occlusal forces must be resisted exclusively by a post that may eventually fracture, otherwise vertical root fracture may occur.16 It has been reported that an increased ferrule length significantly increases the fracture resistance of endodontically treated canines (ETC),8 even though ferrule does not prevent catastrophic oblique or vertical root fractures.9, 10 In contrast to the above studies, Eraslan et al.14 reported that the use of a ferrule in ETT restored with an all-ceramic post-and-core reduced the values of von Mises stresses on tooth-restoration complex. However, the stress values were equally reduced for both the glass fibre-reinforced post system and zirconium oxide ceramic post, which had a greater elastic modulus. The lack, therefore, of sufficient studies deems necessary the further investigation of this factor.
Various post materials and designs have been suggested in an attempt to increase the resistance to fracture of crowned ETT. It has been suggested that stiffer posts and cores may improve the strength of the coronal restoration5 and provide a more uniform distribution of stress throughout the tooth structure.17 However, at high loads stiffer posts usually present with catastrophic fracture modes of teeth, such as vertical or oblique root fractures below the cementoenamel junction. A more flexible post may bend under high loads, which may cause fracture of the post or the core or even decementation and loss of the restoration, however the fractures are repairable and do not require tooth extraction.9, 18 According to some authors the polymeric posts’ root fractures are rare compared with metallic posts.5, 19, 20, 21 This is explained by the fact that polymeric posts present a Young's modulus similar to that of dentine (dentine = 14–18 GPa, polymeric = 9–50 GPa, metal = 180–380 GPa) thus reducing the risk of root fracture whilst simultaneously providing the possibility to restore failures.5, 22, 23 Furthermore, glass fibre endodontic posts are fabricated from longitudinal glass fibres embedded in a resin composite matrix. This makes them suitable for adhesive bonding, so the post, core, and restoration may behave as a single unit, thereby transmitting the loads in a more favourable manner.23 In contrast to the above, there are some studies that find no differences in the type of fracture for fibre posts or report more catastrophic fractures. The explanation given for these findings has to do with bending of the posts due to their low modulus of elasticity, resulting in more stress exerted on the tooth structures.24, 25, 26 As any conclusion about the advantage of using glass fibre posts over stiffer posts is still precarious and as the need for more aesthetic post materials is widely increasing, further investigation is needed. However a particular focus on the fracture mode instead of the fracture resistance of the teeth has to be considered, as vertical or oblique fracture corresponds to tooth extraction, whilst on the contrary a lower strength is commonly associated with decementation or fracture of the post or core, complications easily restored with necessitating the tooth removal.
Different types of crowns have been proposed in an attempt to increase the fracture resistance and to decrease the fractures of endodontically treated anterior teeth restored with fibre-reinforced posts/composite cores. Studies have confirmed that the difference in fracture rate between various posts disappeared when the crowns were placed.27, 28 This was confirmed by a retrospective clinical study that showed that the presence of a post had little effect on the fracture rate of a crowned tooth.29 Sherfudhin et al.30 reported that the use of fibre posts changed the catastrophic failure type of the control group (no post) to almost complete favourable fracture for the other groups that consisted of various ferrule designs and all-ceramic crowns.
The majority of the aforementioned studies have tested the fracture mode of endodontically treated human or even bovine incisors.10, 15, 16, 31 Only a few studies32, 33, 34 have used canines although they are very important teeth especially in cases where canine guidance if planned as a final occlusal scheme for a prosthetic rehabilitation. In such cases canines are expected to withstand the eccentric forces occurring during lateral movements, and therefore it is important to identify the restorative factors that may affect their fracture mode in relation to reparability.
The aim of this study was to evaluate the mode of fracture of ETC restored with fibre posts and either metal-ceramic (MC) or all-ceramic (AC) crowns in the presence or absence of ferrule. The research hypotheses of this investigation were the following:
- 1.
The presence of 2 mm coronal structure (ferrule) above the CEJ would affect the mode of fracture of ET canines restored with fibre posts and either MC or AC restorations.
- 2.
The type of restoration (MC or AC) would affect the mode of fracture of ET canines restored with fibre posts irrespectively of the presence or absence of 2 mm coronal structure (ferrule) above the CE.
Section snippets
Materials and methods
Freshly extracted maxillary human canines of comparable root width and length, as measured with a digital calliper from the apex to the cementoenamel (CEJ), were selected for this study. Each tooth was examined under a stereo microscope to ensure the absence of cracks and microfractures. Radiographs were also made to ensure that there was no carious lesions and internal root resorption. Fifty teeth that matched the above criteria were selected from an initial collection of 62.
The overall mean
Results
The distribution of the fracture modes (repairable and irreparable) per experimental group (C, AC, AC-f, MC, MC-f) is presented in Table 2, Table 3. A slight trend towards less irreparable fractures was recorded for all ceramic restorations and teeth prepared with ferrule. The percentages (%) of repairable fractures were not statistically significant different amongst the metal-ceramic and all-ceramic experimental groups with ferrule (exact P = 0.348) as shown in Table 2(a).
The distribution of
Discussion
The research hypotheses of this study have to be rejected as statistically significant differences in the mode of fracture were not related to either the type of restoration or the presence of 2 mm of remaining coronal tooth structure. However the 2 mm of tooth structure was weakly associated to a higher percentage of repairable fractures, especially in teeth restored with all ceramic crowns (30%), whilst the lowest amount of repairable fractures was recorded for MC restorations without ferrule
Conclusions
Within the limitations of the present study, the following conclusions can be drawn:
- (1)
The type of restoration (MC or AC) did not significantly affect the mode of fracture of ET canines restored with fibre posts irrespectively of the presence or absence of 2 mm ferrule above the CE. Although there was not a statistically significant difference, the percentage (%) of repairable fractures was higher in all-ceramic ETC than in metal-ceramic ETC irrespectively of the presence or not of ferrule.
- (2)
The
Acknowledgements
The authors wish to acknowledge Assoc. Professor D. Bikiaris for providing his laboratory infrastructure for the fracture test and Lecturer G. Menexes for the statistical analysis.
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2022, Journal of the Mechanical Behavior of Biomedical MaterialsCitation Excerpt :After the coronal preparation of an endodontically treated tooth, it has been recommended that an extension of at least 2 mm apically to the junction of the core and the remaining tooth structure, namely “ferrule effect” is present (Morgano, 1996). The purpose of the ferrule is to redistribute the occurring stress on the outer coronal third of the root, possibly shifting the failure mode to a repairable fracture (Fragou et al., 2012; Magne et al., 2017b). Irrespective of post-core material, a failure rate of only 3% has been observed after a 10-year follow-up for teeth with preservation of 2-mm ferrule (Ellner et al., 2003).
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2021, Journal of Materials Research and TechnologyCitation Excerpt :A post and core approach is suggested to build endodontically treated teeth (ETT) that will preserve the masticatory role and esthetic appearance of the tooth, which cannot be achieved by using conventional restorations. The type of materials and methods used in the fabrication of posts may affect the resistance of ETTs to fracture when the crown is the final restoration [4]. To obtain an optimal result after restoring the ETT with a post, the material that is used for post fabrication must be biocompatible, bonded to the tooth and have mechanical properties similar to those of dentin [5,6].
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2020, Journal of Prosthetic DentistryCitation Excerpt :Loaded bodies could concentrate stress in the structure with higher elastic modulus, transferring the stress with more intensity to adjacent structures.12 Besides the composition of the post itself, its adhesion to the dentin root also affects stress concentration9-19 because nonadhered posts tend to concentrate more intense loads in specific regions.10,12,25-29 Using a post with material that is more rigid than dentin and does not adhere adhesively to the post may increase the risk of root fracture.10,25-27
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2020, Journal of Prosthetic DentistryEffects of design parameters on fracture resistance of glass simulated dental crowns
2016, Dental MaterialsCitation Excerpt :Of these, margin design appears to be most important, being of a profound effect on fracture resistance of all-ceramic crowns, which can largely determine the extent of removal of sound tooth tissue during preparation of taper abutment for achieving adequate esthetic outcomes and structural integrity of tooth-crown system. On the other hand, not only does minimal preparation, by preserving as much sound tooth tissue surrounding the cervical area (ferrule effect) as possible, increase long term prognosis of the tooth, but also helps preserve pulp vitality, thereby maintaining health status of the host abutment tooth [12]. The concern remains on how these design parameters affect the structural strength of the crown against mechanical loading.