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Resistance against Fracture in Teeth Managed by Root Canal Treatment on Restoring with Onlays, Inlays, and Endocrowns: A Comparative Analysis.

AIM AND OBJECTIVE
To compare the fracture resistance in teeth managed by root canal treatment after restoring with different types of onlays, inlays, and endocrowns prepared with hybrid ceramics and pulp chambers restored with fiber-reinforced composite and resin composite that were radiopaque, light-cured, and flowable.


MATERIALS AND METHODS
The present study was carried out on 252 extracted mandibular molars. All the specimens were divided into six groups randomly. Each group consisted of 42 specimens. Group 1 consisted of intact teeth without any access cavity. It was the control group. Group 2 consisted of teeth with endocrown and empty pulp chamber. Group 3 consisted of teeth with mesio-occlusal-distal (MOD) onlay prepared with hybrid ceramics and pulp chamber filled with flowable, light-cured, radiopaque resin composite. Group 4 consisted of teeth with MOD onlay and pulp chamber filled with fiber-reinforced composite. Group 5 consisted of teeth with MOD inlay and pulp chamber filled with flowable, light-cured, radiopaque resin composite. Group 6 consisted of teeth with MOD inlay and pulp chamber filled with fiber-reinforced composite. Inlay, onlay, and endocrowns were prepared with computer-aided design (CAD) and computer-aided machine (CAM) using hybrid ceramics. Universal testing machine was used for the measurement of the fracture resistance of each specimen. Inferential statistics were performed by applying Fisher's exact test and chi-square test.


RESULTS
Fracture strength was found to be maximum in the intact teeth group followed by the endocrown. The fracture strength was minimum in the inlay group. The fracture strength was intermediate in the onlay groups.


CONCLUSION
Endocrown showed maximum fracture resistance as compared to the inlay and onlay restorations.


CLINICAL SIGNIFICANCE
Proper management of root canal-treated teeth is one of the greatest challenges for endodontists. It has been observed that tooth preparation design and the material used for the restoration of root canal-treated teeth play a vital role in the resistance against fracture in the teeth.


IntroductIon
The most common negative outcome of root canal-treated teeth is the highest possibility of getting fractured, as compared to vital teeth. This is due to the loss of significant tooth structure and other changes in the structure of the endodontically treated teeth. 1,2 One of the important steps after endodontic therapy is to restore the resistance of teeth against fracture, which gets affected adversely as a result of loss of tooth structure during cavity preparation during root canal treatment. 3,4 It should be taken care of during the restoration of root canal-treated teeth that biomechanical properties of the treated teeth should be similar to that of a tooth when intact. The resistance against fracture is achieved through adequate retention and maintaining the integrity of adhesiveness between dentin of the root, reconstruction of core, and the final restoration that should form a complex, which should be integrated and unique. 5,6 The restoration of the tooth after root canal treatment involves the direct restoration as well as indirect posterior restoration of teeth. Various types of indirect posterior restorations after root canal treatment are classified on the basis of the amount of the coverage of the cusp. 7,8 The most common indirect restorations are onlays, inlays, overlays, and endocrowns. In inlays, there is no coverage of cusp, whereas in onlays, there is at least coverage of one cusp, and in overlays, all the cusps are covered. 9,10 Coverage of cusps leads Root canal treatment was carried out in the same manner in all extracted teeth by the same operator. After completion of root canal treatment in all teeth, ethylene alcohol was used for cleaning the access cavity of every tooth to eliminate the debris and residual sealer from the walls of the access cavity. It was taken care that the size of the access cavity in each tooth was almost the same.

Division of All Specimens in Six Groups
All the teeth were divided randomly into six groups (groups 1-6). Each group consisted of 42 specimens. Group 1 consisted of intact teeth without any access cavity. It was the control group. Group 2 consisted of teeth with endocrown and an empty pulp chamber. Group 3 consisted of teeth with mesio-occlusal-distal (MOD) onlay prepared with Cerasmart and pulp chamber filled with G-aenial Universal Flo. Group 4 consisted of teeth with MOD onlay prepared with Cerasmart and pulp chamber filled with EverX posterior. Group 5 consisted of teeth with MOD inlay prepared with Cerasmart and pulp chamber filled with G-aenial Universal Flo. Group 6 consisted of teeth with MOD inlay prepared with Cerasmart and pulp chamber filled with EverX posterior ( Table 1).

Filling of the Pulp Chamber with Fiber-reinforced Composite and Flowable Composite
Access cavities were prepared in teeth of groups 3, 4, 5, and 6 and were etched with 37% phosphoric acid (AM dental etching gel). After that, the bonding agent (Ivoclar bonding agent) was applied followed by light curing for the duration of 20 seconds (Waldent Smart). Then, there was a filling of access cavities present in the teeth included in groups 3 and 5 with light-cured and flowable composite resin, while access cavities of the teeth included in groups 4 and 6 were filled with EverX posterior, and they were cured for 40 seconds. The restoration of composite resin material was shaped flat at the roof of the pulp chamber.

Preparation of the Cavity Design for Indirect Posterior Restorations
Then, standard MOD was prepared on all teeth with the help of high-speed tapered diamond bur (6° taper) by the same operator along with copious irrigation. The cavity was prepared in such a manner that the minimum thickness of the remaining buccal and lingual walls was compulsorily made up of 2 mm. The depth of the horizontal pulp wall was 3 mm. There was the preparation of proximal boxes such that there was the preparation of isthmus of 2 mm depth. The lingual and buccal axial walls were made divergent. The gingival margin was prepared such that it was placed 1 mm above the cementoenamel junction. During the onlay preparation, there was a reduction in nonfunctional and the crown in one assembly causing the formation of a monoblock restoration. 11,12 It is taken care that preparation of cavity for posterior restoration should be the least invasive, which has the advantage of tooth structure conservation as well as proper distribution of stress. 13 It has been reported in the literature that indirect posterior restorations have better resistance to fracture in comparison to the direct restorative techniques. The selection of the restorative material for endodontically treated teeth should be in such a manner that it must maintain a balance between two components; first is for improved strength of the restoration, and the second is for the preservation of the tooth structure. 14,15 For the components mentioned here, three designs of tooth preparation were selected, namely inlay, onlay, and endocrowns. Newly introduced computeraided design and computer-aided machine (CAD/CAM)-based hybrid nanoceramics, namely Cerasmart, were selected for this study, because it has been found that Cerasmart has a higher fracture resistance load when compared with other contemporary materials. 16,17 In recent times, several new varieties of composites have been introduced with the purpose to simulate dentin in order to absorb maximum stress for reducing the occurrence of fracture.
Composite reinstated with fibers (EverX posterior) and composite that are flowable, radiopaque, and light-cured (G-aenial Universal Flo) are few of these composites. 18 These materials are observed to have a low modulus of elasticity, so they are able to absorb stress in such a way that it gets concentrated inside of the restoration without transferring it to the tooth structure. 19 As per the best of our knowledge, no such research has been conducted to compare the impact of different patterns of indirect posterior restorations like inlays, onlays, and endocrowns made up of CAD or CAM hybrid ceramics and different new composites (EverX posterior and G-aenial Universal Flo) used for the restoration of pulp chamber in teeth that underwent root canal treatment.
Therefore, for carrying out such analysis, five different techniques were compared, which were endocrowns prepared from Cerasmart, inlay made up of Cerasmart with fiber-reinforced composite occupying pulp chamber, inlay made up of Cerasmart with light-cured flowable composite occupying pulp chamber, onlay made up of Cerasmart with fiber-reinforced composite occupying pulp chamber, and onlay made up of Cerasmart with light-cured flowable composite occupying pulp chamber.

MAterIAls A n d Methods
It was an in vitro study conducted in the Patna Dental College and Hospital. This research was approved by the ethical committee of the institution. The study was conducted on 252 specimens. The calculation of the sample size was carried out in accordance with the findings of Kadam and Bhalerao. 20 The inclusion criteria for specimens in the study were extracted mandibular molars with similar dimensions for all the teeth. The exclusion criteria were the presence of caries and cracks in the teeth, abnormal tooth anatomy, and fractured tooth. Mandibular molars were selected for the study because they are the most common teeth that undergo root canal treatment.

Cleaning and Storage of Study Specimens
All the extracted teeth were cleaned and stored in thymol solution. The concentration of the thymol solution was 0.2%. Then, all the extracted molars were rooted in a self-polymerizing resin 2 mm below the cementoenamel junction. The molars were placed perpendicular to the self-polymerizing resin.

results
There was a statistically significant difference in the resistance against fracture among the groups with p-value ≤0.05. Fracture strength was found to be maximum in the intact teeth group. It was followed by the endocrown group in terms of fracture resistance. The fracture strength was minimum in the inlay groups (groups 5 and 6). The fracture strength was intermediate in the onlay groups (groups 3 and 4). However, the difference was not statistically significant between the specimens of groups 5 and 6. It showed that the fracture strength was not statistically different in the specimens consisting of inlay prepared with Cerasmart along with pulp chamber filled with flowable, radiopaque, and light-cured composite and the specimens consisting of inlay prepared with Cerasmart along with pulp chamber filled with EverX posterior. However, the resistance against fracture was slightly more in the specimens consisting of inlay prepared with Cerasmart having flowable, radiopaque, and light-cured composite occupying the pulp chamber.
The difference in fracture resistance was not statistically significant between the specimens of groups 3 and 4. It showed that the fracture strength was not statistically different in the specimens consisting of onlay prepared with Cerasmart having flowable, radiopaque, and light-cured composite occupying the pulp chamber and specimens consisting onlay prepared using Cerasmart having pulp chamber filled with EverX posterior. However, the resistance against fracture was slightly more in the specimens consisting of onlay prepared with Cerasmart and flowable, radiopaque, and light-cured composite occupying the pulp chamber (Table 3).
When the pattern of fracture was analyzed, then it was found that type 2 fracture was most common in the group 1 specimen. functional cusps by 2 mm, and also a 90° butt-joint margin can be created (Fig. 1).

Preparation of Indirect Posterior Restorations with Cerasmart
The indirect restorations like inlay, onlay, and endocrowns were prepared with resin nanoceramic CAD or CAM Cerasmart blocks utilizing the CAD or CAM machine. Designing of the restorations was carried out with the help of Exocad software so that the anatomy and contour of all the restorations were similar. The milling of the Cerasmart blocks was carried out with the help of a milling machine according to the instructions prescribed by the manufacturer. After the formation of all the restorations, 9% hydrofluoric acid was used for treating the surfaces of the teeth followed by rinsing with water. Then, the restorations were air-dried and coated with a silane agent according to recommendations by the manufacturer.

Cementation of the Posterior Indirect Restorations like Inlay, Onlay, and Endocrowns
Now 37% phosphoric acid was used for etching of all teeth surfaces for 15 seconds, and then water was used for rinsing. Then, they were air-dried for the duration of 10 seconds. Then, there was the placement of a bonding agent that was polymerized for the duration of 20 seconds. After that, cementation of all restorations was done. There was the use of dual-cure composite resin cement that was dual-cured (Prevest Fusion). Finally, a universal testing machine was used for the measurement of the fracture resistance of all teeth.

Statistical Analysis
After that, the fracture pattern in all teeth was evaluated on the basis of classification for the type of fracture and pattern of fracture ( Table 2). Statistical analysis was carried out and Kolmogorov-Smirnov test was utilized for the evaluation of the distribution of continuous variables. On the contrary, analysis of variance was used for comparison of the fracture strength among groups. Chi-square

Fracture type
Patterns of fracture Type 1 fracture There is no sign of visible fracture Type 2 fracture Fracture is limited to the tooth Type 3 fracture Fracture is limited to the restoration Type 4 fracture Fracture found in both the tooth and restoration located above the cementoenamel junction Type 5 fracture Fracture found in both the tooth and restoration located below the cementoenamel junction There are several etiological factors for the fracture of tooth, which have undergone root canal treatment and most of them are not under the direct control of the clinician. 31 Proper tooth preparation and proper material used for the restoration of the teeth managed by root canal treatment are one of the important factors affecting the fracture resistance. Hence, continuous efforts are being made regarding the search for the ideal preparation of the tooth and adequate restorative material for the root canaltreated teeth to decrease their negative effect on the resistance against fracture. 32 This study was conducted in this direction to find ideal cavity design and restorative material for teeth that underwent root canal treatment.
There are several recent materials that can be used for the restoration of lost tooth structure. During recent times, hybrid ceramics have been introduced, which are based on CAD and CAM. These hybrid ceramics have characteristics of both polymer and ceramic. They are formed as a result of the infiltration of ceramic materials in the polymer matrix. 33 One such materials is Cerasmart (GC India Dental). 34 They are blocks of nanoceramic composite resin that is formed when the matrix of polymer gets reinforced with nanohybrid fillers of ceramic. Another recent material for the posterior restoration of root canal-treated teeth is EverX posterior (GC India Dental). It is a glass and polyethylene fibers reinstated composite. 35 Some of the important properties of this material are its contribution to increasing the strength of the tooth and ability to absorb stress like dentin. Therefore, it is advised to use in the form of a bulk base in those areas where high stress is present.
Another recent restorative material that can be used in root canal-treated teeth is G-aenial Universal Flo (GC India Dental). It is a radiopaque composite that is light-cured and flowable. 35 Both G-aenial Universal Flo and EverX posterior are used for restoring the pulp chamber of the root canal-treated tooth, while Cerasmart (hybrid ceramics) is used for preparing indirect posterior restorations (inlay, onlay, and endocrown) in root canal-treated teeth. No study has been conducted to compare the effect of these indirect posterior restorations and other restorative materials in the pulp chamber on resistance against fracture in root canal-treated teeth. It was found that endocrown prepared with Cerasmart is the Type 5 was most common in group 2, while type 3 and type 5 were most common in group 3; type 3 was most common in groups 4, 5, and 6. It can be observed that the type 3 pattern of fracture was most common in the inlays made of Cerasmart (Table 4).

dIscussIon
In the current study, it was found that maximum fracture resistance was found in the intact teeth. Several studies in the past have concluded that fracture resistance was low in root canal-treated teeth. 21,22 It was found in our study that the fracture resistance was maximum in the endocrowns as compared with inlays and onlays. The fracture resistance was lowest in the inlays. These results were similar to several studies conducted in the past in which it was found that fracture resistance was maximum in endocrowns. 23,24 However, Gré et al. conducted a study and found that fracture resistance was similar in endocrowns as compared to other conventional crowns. 25 It was also found in the present study that fracture resistance was greater in onlays as compared with inlays. Concordant results were shown by Yoon et al.; they conducted a study and concluded that protection provided by the onlay restoration was greater as compared with the inlays in the endodontically treated teeth. 26 In our study, it was observed that fracture resistance was greater in specimens with flowable, radiopaque, and light-cured composite occupying the pulp chamber as compared to the EverX posterior occupying the pulp chamber with onlays as indirect posterior restorations. These results are similar to the results obtained in a study conducted by Atalay et al. in which it was found flowable composite showed high fracture resistance. 27 Özkır conducted a study to compare the effect of cavity design on the resistance against fracture in root canal-treated teeth and observed that composite reinforced with fibers showed better resistance against fracture. 28 Goracci et al. also conducted a similar study and found that the resistance against fracture was maximum when composite with reinforced fibers was used as compared with other conventional composite resin materials. But in the present study, the resistance against fracture was not affected due to reinforcement of composite with fibers. 29 Rocca et al. conducted a similar study to evaluate the resistance against fracture in root canal-treated teeth in which fiber-reinforced composite was placed in the pulp chamber and the overlying indirect posterior restorations were prepared with hybrid ceramics. It was found that reinforcement of composite with fibers did not improve the resistance against fracture. 30 In the present study also, similar results were obtained because the resistance against fracture was lesser in composite reinforced with fibers as compared to the flowable, radiopaque, and light-cured composite.
The management of teeth that have undergone root canal treatment is one of the greatest challenges for endodontists.  better restoration in terms of fracture resistance for the teeth that underwent root canal treatment. It may be due to the fact that there is the utilization of the space of pulp chamber in endocrowns that helped in increasing the stability of restoration.
The clinical implication drawn from this study was that endocrown prepared with Cerasmart can be a better option as indirect posterior restoration in root canal-treated teeth to avoid fracture.
The limitations of this study were that it was an in vitro study due to which clinical conditions were not simulated. Another limitation was the high expenses during the study due to which the size of the sample was limited. Another limitation was that in this study, selfcured adhesive cement was used instead of light-cured adhesive resin cement, which is used normally for Cerasmart because this Cerasmart is a variant of hybrid ceramics. This can reduce the integrity of the tooth restoration complex.
In order to achieve adequate integrity, there was pretreatment of enamel before carrying out luting. This helped in increasing the strength of bonding of self-cured adhesive resin cement with enamel. 36 conclusIon It can be concluded from this study that endocrown showed maximum resistance against fracture in teeth that underwent root canal treatment in comparison to other types of restorations and flowable, radiopaque, and light-cured composite used for restoring the pulp chamber presented more resistance against fracture in teeth that underwent root canal treatment. More studies should be carried out simulating the clinical conditions. references