Fracture strength and survival rate of endodontically treated maxillary incisors with approximal cavities after restoration with different post and core systems: an in-vitro study
Introduction
Approximal caries with involvement of the dentin is prevalent in about 25–40% of adolescents and for incisor teeth it is the most common form of caries [1], [2], [3]. In most cases, conservative treatment is successful in restoring approximal class III defects. Root canal therapy is required in such teeth when pulpal infection occurs as a result of extensive decay, resulting in a potential further loss of structural integrity in addition to existing cavities. Endodontic treatment is often followed by internal discoloration of the now pulpless tooth. If the aesthetic results are unsatisfactory or the loss of structure requires restoration of the tooth, full coverage crowns are indicated for the aesthetic and structural restoration of such teeth with multiple defects [4].
Several factors have been discussed that influence the strength of endodontically treated teeth. The loss of moisture from dentin after endodontic treatment would appear to be insignificant [5]. According to Reeh et al. [6] cavity preparation and endodontic treatment are associated with a loss of tooth structure. Cavity preparation seems to account for most of the loss in strength. Dentin collagen content is variable depending on location and may contribute to a lower fracture resistance of incisors when compared to posterior teeth [7].
While early publications have called for the stabilization of endodontically treated teeth with intra-canal posts [8], it has been demonstrated that post reinforcement is not beneficial in completely intact teeth [9], [10], [11]. In addition, endodontically treated but otherwise intact teeth without posts perform comparable or better than those restored with post and cores if used as crown abutments [12], [13], [14]. These results have been corroborated by findings reported in clinical studies [15], [16].
The choice of an appropriate restoration for endodontically treated teeth is guided by strength and esthetics. The use of composite resins as a core material has enhanced the ability to reproduce the shade and translucency of natural teeth. This is especially important when all-ceramic crowns are chosen as the final restoration. The available all-ceramic post systems offer improved esthetics and are a promising alternative to conventional metal posts [17], [18]. The restoration of teeth with adhesively cemented internal restorations also provides the opportunity for improved mechanical stability over conventional cemented restorations [11], [19].
As previously reported, the fracture resistance of endodontically treated teeth is largely dependent on the amount of remaining dentin [6], [20]. However, it is unclear at which degree of substance loss endodontic reinforcement with posts becomes necessary in incisors with typical class III defects. The aim of this study was to compare crowned endodontically treated maxillary incisors restored with either titanium posts, zirconia posts or composite resin in the root canal and teeth without any endodontic restoration. A near clinical model was created to compare the different restorative approaches under thermo-mechanical fatigue conditions simulating clinical parameters. The null hypotheses to be tested were that there is no difference in fracture strengths and fracture patterns between the four different types of endodontic restorations after 1.2 million cycles of thermomechanical fatigue loading.
Section snippets
Materials and methods
Human maxillary central incisors were obtained directly after extraction and were stored in 0.1% thymol solution during the course of the study. Teeth with cracks, caries and fillings or roots shorter than 10 mm were discarded. Teeth were selected for the study if the variation in length was within 1 mm when compared to the mean value of 23 mm [21], [22]. The 64 selected teeth were randomly divided into four blocks of 16 teeth each (Table 1).
Endodontic treatment was performed by stepwise filing
Dynamic loading
All samples except one in group 3 survived the 1.2 million cycles of dynamic loading and thermocycling in the artificial oral environment. The fracture, which occurred during the chewing simulation, only became apparent upon removal of the sample from the chewing simulator at the end of the experiment. For this reason, no accurate information was available on the exact time at which the specimen broke. The results of the chewing simulation are presented as survival rates for each group (Table 1
Discussion
A variety of endodontic posts are available to the dental practitioner for the build-up and stabilization of non-vital teeth. In this study, cylindro-conical posts were used. Some studies have reported a wedging effect attributed to tapered posts [31], while others could not demonstrate differences between parallel-sided and tapered posts [12]. To exclude any variations resulting from different post geometries, all posts in this experiment were industrially machined to identical shape and
Acknowledgements
The authors gratefully acknowledge Komet (Gebr. Brasseler GmbH & Co. KG, Lemgo, Germany) for donating the posts used in this study. We also wish to acknowledge the expertise of Mr Thomas Gerds, Department of Medical Biometry, Albert-Ludwigs University, Freiburg, Germany for performing the statistical analysis and Peter Yaman, Clinical Professor, University of Michigan for his help with the English manuscript.
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