Fracture Resistance of Primary Canines Restorations Reinforced with a Flowable or a Packable Composite Resin Posts; In- Vitro


 Aim and Background:

Restoration of severely damaged primary anterior teeth are deemed as serious challenges for pediatric dentists. The aim of this study was to evaluate the fracture resistance of primary canines restored with flowable or packable composite resin posts.
Material and Methods

This study was conducted on a group of extracted primary canines. After cutting the coronal section, standard pulpectomy was performed and root canals were filled with ZOE paste leaving 3 mm coronal root space. A liner was placed above the root canal filling as a barrier. Samples were then randomly divided into two groups of fifteen. Both groups, received etch, bond, then in group I Core Flo Composite resin placement into canal, in group II, post shaping using designated packable composite resin and inserted into the canal. Followed by Core buildup and Crown reconstruction using Z250 composite resin. Samples were mounted at 148 ° angle on the acrylic blocks and stored in de-ionized water for 72 hours. Fracture resistance was measured using Universal Testing Machine. Data analysis were performed using independent t-test. Chi- square test were also employed to assess the type of fracture within groups.
Results

The mean fracture resistance in the group I was 830.31(+/-328.69N) and 803.09(+/-326.45N) in group II. Moreover, statistical analyses of the results showed no significant difference between the mean values of the fracture resistance (p-value=0.822) of groups as well as the type of fracture (p-value=0.79) between groups.
Conclusion

The ease of use and proper adhesion of the Core Flo composite resin, seems to help its use in the reconstruction of severely destructed anterior primary teeth.


Introduction
Dental caries is considered as the most common chronic and destructive dental disease during childhood which along with dental trauma, are the main causes for child referral for tooth restoration 1 Nowadays, the prevalence of Early Childhood Caries (ECC) cases is growing remarkably, so that, children might face with several issues including, speech di culties, reduction in facial vertical height and lack of con dence due to aesthetic concerns. Therefore, to avoid such problems, destructed teeth need to be restored properly 2 .
Restoration and maintenance of damaged primary teeth are challenging for pediatric dentists, as managing children behavior during the procedure is di cult 2 . In recent decades, using new generation of restorative materials introduced to the market such as strip crown, polycarbonate crown, veneered stainless steel crowns, and ART glass, it is feasible to restore decayed teeth with an inadequate structure 3 However, the application of these materials could lead to failure due to their intolerance toward intensive occlusal forces 4,5 Despite of numerous issues existing in the usage of composite resin, these substances recommended as the materials of choice for restoration of anterior teeth because of aesthetic demands 6 .
If the anterior tooth is signi cantly destructed, only a small fraction of tooth and its enamel remains for bonding. This is due to the fact that it has a narrow crown. In addition, the residual enamel of the cervical area of tooth has a restricted capability for bonding to restorative material 1 To overcome this issue, it has been suggested to use intracanal post or similar retainers following pulpectomy of tooth in order to enforce retention of coronal restoration 5 Presently, numerous types of posts are used in pediatric dentistry like, Nickle -Titanium posts, orthodontic wires, biologic posts, composite posts, ber reinforced composite, ber-based posts and ceramic posts. [7][8][9] One of the simplest and most-effective approaches recommended for the restoration of signi cantly destroyed primary anterior teeth is to pack the composite resin into the canal and construct a short composite resin post 10 Composite resin posts have a similar modulus of elasticity with intracanal dentin and will easily bond to the tooth structure. As a result of an adequate mechanical retention, a better distribution of occlusal forces exists with these posts. Simple application, no necessity to laboratory fabrication, low price, and optimal adaptation are among the other advantages of composite resin posts. [9][10][11] Composite resin posts are e cient as there is adequate homogeneity between them and covering composite resin. In addition, contrary to metallic post, it is not necessary to apply an opaque layer of composite resin to obscure metal. On the other hand, regarding polymerization shrinkage and reduction in retention, short composite posts are more prone to fracture 9 . Having adequate fracture resistance is important while using this method to restore primary teeth. This means an inadequate fracture resistance can lead to breakage of material under the pressure. The type of restorative material and the amount of tooth structure that has been lost are two important factors in this phenomenon. 8 The aim of this investigation was to evaluate the fracture resistance of primary anterior canine restorations reinforced with either a owable or a packable composite resin post.

Method And Materials
Study protocol was approved by the Ethics Committee of the Dental School at Shahid Beheshti University of Medical Sciences. Thirty healthy freshly extracted canine teeth were obtained with at least 2/3 root length remaining. The teeth with any degrees of caries, previous restorations and visible fractures or cracks were excluded from this study. Parents of patients were informed of the purpose of this study too. Dental calculus and residual soft tissues were removed from tooth surface by dental scalers and curettes. All teeth were placed in 0.5% chloramine solution for a week and then stored in distilled water at 4 ° C before test course. The teeth were then cut from the coronal section and 1 mm above the CEJ by a diamond bur on a high speed hand piece with air and water spray. The pulp was then removed through a standard pulpectomy procedure. Subsequently, root canals were lled with ZOE paste (PD, Switzerland).A 48 hours period was taken to complete the pulpectomy process after which 4 mm of ZOE paste was removed from the coronal part of the canal using a slow-speed hand piece with a round carbide bur. A 1 mm thick of liner (Pulpdent, USA) was then placed on top of the ZOE paste and before placement of composites resin posts.
Samples were then randomly divided into two groups of 15.
In the group I, owable composite (Core Flo DC LITE, USA) being used to make the post while the restoration of the crown was performed with a conventional packable composite (composite 3M EPSE Z250, USA).In the group II, packable composite(3M EPSE Z250, USA) was used to make the post along with the nal restoration. Root canals and residual enamel were etched with 37% Phosphoric Acid gel (Scothbond™ Etchant, 3M ESPE, MN, USA) for 15 seconds, rinsed for 30 seconds and dried for 10 seconds, whereas the dentin area remained slightly moist. Thereafter, two consecutive coats of bonding agent (Tetric-N Bond, Ivocolar Vivadent, Switzerland) were applied on the etched surfaces, uniformly dispersed by a compressed air blast for 2-5 seconds and cured by an LED light cure (Woodpecker, China) for 20 seconds. Finally, the composite resin was placed in layers with a thickness of 1.5 mm in the root canal space with each layer being subjected to curing for 40 seconds. Posts were built to 2 mm above the ori ce and the crowns were built up with an approximate height of 4 mm (normal size).
Each tooth was then placed in acrylic blocks from the apical portion (2 mm below CEJ) and mounted at 148 ° angle (Acrylic acrylic, Acropars, Iran).
All sample teeth were oated in de-ionized water for about 72 hours, at room temperature. In order to evaluate fracture resistance under regulated pressure, they have been loaded by Universal Testing Machine (Zwick Role, Denmark).
Samples were xed in a special xture and received a progressively increasing load with a crosshead speed of 0.5 mm/min along the long axis of the primary canines on the mid-palatal surface. The increase in force was continued until the specimen was fractured. The pattern of fracture was then looked at using a 10x magni cation stereomicroscope. Pattern of failure on samples were as follows: Independent t-test was used to compare the fracture resistance in two methods of tooth reconstruction. Pvalue of <0.05 was considered as a signi cant limit. Chi-square test was used to determine the differences between the types of failure of the two groups.

Results
Minimum, maximum and the mean of fracture resistance in two groups were recorded and compared (Table 1). Independent t-test was conducted to compare fracture resistance between the two groups. The mean value of fracture resistance in group I (Reinforced with Core Flo composite resin post and core) was higher than group II (reinforced with packable composite resin post and core), however this difference was not statistically signi cant (p-value=0.82).

Table1
Fracture resistance in two groups in Newton (N) The type of fracture was evaluated under stereo microscope (Table 2).
Chi-square test showed no signi cant difference between fracture resistance of the two groups (p-value=0.79).

Discussion
During daily routine activities, teeth are being impacted by various mechanical forces in the oral cavity. These forces directly affect the durability of teeth restorations. Different tests have been conducted to analyze the mechanical forces applied on teeth including shear bond strength and fracture resistance. Each of these tests has been used to assess those forces applied on teeth. Enhancement of physical characteristics of restorative materials is considered as a critical priority when applying any restorative material on teeth. 12 Fracture resistance is considered as one of the main features of a suitable restorative material and one of the critical factors in attaining durable restorations 13 .
Findings of the current investigation revealed that the mean of forces leading to fracture in two groups as Earlier studies have argued that the materials applied as a short post and core in anterior primary Maxillary teeth are more functional as these materials have closer elasticity to that of dentine. ideal intracanal post provides adequate retention while are with appropriate facture resistance proof too. Similarity of the post shape and shape of the canal provide a higher level of distribution of forces along the coronal restoration while reducing probability of tooth fracture [13]. Among different types of introduced approaches, compacting composite into the canal space in order to construct a short post has been propounded as the best option 18 .Using composite posts are preferred due to their ease of use, low cost, no laboratory step, and good versatility. Wide use of composites in dental clinics is an indication of the need to carry more speci city studies on them 9 . Pithan et al. concluded that no signi cant difference could be seen between composite resin posts, orthodontic wires and ber glass posts. The application of orthodontic wire as an intra-canal retention was reported as a more technique sensitive in comparison to the posts made of composite resin. In addition, orthodontic wires could lead to root fracture while tooth is exposed to intensive mastication forces 23 .
Sadek et al. stated that bond strength of various types of composite resin used as core material adjacent to ber posts was highest in Core-Flo (bisco). Although, the difference between groups was not statistically signi cant 24 .Clinical application of owable composite resin for reconstruction of severely destroyed teeth because of its inherent weakness, low modulus of elasticity and inadequate strength toward occlusal forces is questionable. It seems that concurrent utilization of adaptive properties of owable composite resin used as core material could serve as an acceptable restoration for root treated teeth 24,25 .

Conclusion
According to the results of this investigation with the high adhesion potential of the Core Flo composite, it appears that restored teeth with this material have a higher resistance rate to fracture of the restored anterior primary teeth.

Declarations
Con ict of Interest: No con ict of interest to declare.
Ethical Approval and Consent to participate None to Declare.

Consent for publication
We have full consent for free publication.

Availability of data and materials
The data that support the ndings of this study are available from Deparment of pedodontics of shahid beheshti university of medical sciences, but restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available. Data are however available from the authors upon reasonable request and with permission of Deparment of pedodontics of shahid beheshti university of medical sciences.

Competing interests
None to Declare.
Funding None to declare.

Authors Contributions
Dr.Hanjani collected the data and wrote the manuscript. Dr.Roodgarian collected the data and helped writing the manuscript. Dr.Naja performed the statistical analysis. Dr.Gashtasb supervised the data collection and manuscript writing. Dr.Ghasemi supervised the whole study from data collection, statistical analysis, and manuscript writing. The author(s) read and approved the nal manuscript.