The effect of ferrule on the fracture mode of endodontically treated canines restored with fibre posts and metal-ceramic or all-ceramic crowns

Abstract

Objectives

The aim of this study was to evaluate the fracture mode of endodontically treated canines (ETC) restored with glass-fibre posts (GFP) and either metal-ceramic (MC) or all-ceramic (AC) crowns, in the presence or absence of 2 mm ferrule height.

Methods

Fifty recently extracted human maxillary canines were endodontically treated and randomly divided into 5 groups of 10 specimens each. Four groups (with or without ferrule) were restored with GFP, composite cores and either MC or AC crowns. The fifth group with intact ETC served as control. Teeth were loaded to fracture at a 135° angle to their long axis, at a cross-head speed 0.5 mm/min. The fracture modes were divided through optical microscopy to repairable and irreparable. Statistical analysis was performed by means of the Fisher's Exact test.

Results

All groups presented brittle fracture following bending deformation. Irreparable root fractures were the primary fracture mode independently of ferrule. The percentages (%) of repairable fractures were not statistically significant different amongst the MC and AC restored teeth independently of ferrule. However the 2 mm ferrule was associated to a higher percentage of repairable fractures, especially in teeth restored with AC crowns.

Conclusions

The percentage (%) of repairable fractures was higher in all-ceramic compared to metal-ceramic ETC and in restorations with ferrule, although the differences were not statistically significant.

Clinical significance

This in vitro study supports evidence that neither the type of crown restoration nor the presence of ferrule affects statistically significantly the mode of fracture of ETC restored with GFP.

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:

• 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.

• 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.