Fracture Resistance of Endodontically Treated Mandibular Permanent First Molars Reinforced with Diagonal Horizontal Postdesign: A In vitro Study

Abstract

Aim:

This study aims to compare fracture resistance of mandibular first molars reinforced with diagonally placed horizontal posts after endodontic therapy.

Materials and Methods:

Thirty-two mandibular first molar teeth were selected, standardized, divided into four groups (GP) – GP I (endodontically treated-no post)), GP II (horizontal post-buccolingual), GP III (horizontal postdiagonal), and GP IV (natural teeth). The individual teeth were mounted on bases, coded, and suitably prepared for the facture testing procedure.

Results:

Buccolingual type of fracture was the most common among the experimental groups with a percentage of incidence of 37.50%. The cuspal type of fracture was the next most common with a percentage of incidence of 25.0%. The incidence of mesiodistal, comminuted, and proximal type of fractures was also observed and had a percentage of incidence of 9.38%, 9.38%, and 9.38%, respectively. Group IV had the highest fracture resistance with a mean value of 1279.82 (N). Group I, Group II, and Group III had mean values of 684.16(N), 756.84(N), and 758.17(N), respectively. There was a statistically significant difference between Gp IV and Gp (I–III) in load values of root fracture (P < 0.05).

Conclusion:

Within the limitations of this study, it can be concluded that diagonally placed horizontal postdesign has a higher resistance to facture when compared to buccolingually place horizontal postdesign and root canal treated non postgroup. These designs have the potential to be a conservative postendodontic management option. Further evaluation of different horizontal postdesigns and assessment in a clinical setting is recommended.

INTRODUCTION

Excessive loss or removal of tooth structure affects fracture resistance of restored teeth after completion of endodontic therapy.[1] In teeth with curved canals, to achieve sufficient cleansing, a considerable amount of radicular dentin removal is done, eventually leading to structural weakening and decreased fracture resistance.[2,3] Coronal and radicular dentin should be preserved as much as possible during endodontic therapy. Resistance to fracture depends on the thickness of remaining dentin, in a buccolingual direction.[4] Endodontic therapy should ideally aim to reinforce the residual structure of the tooth contributing to long-term success.[5] The amount of remaining dentin in a tooth after endodontic procedures is in a directly proportional relationship to resistance of the tooth to fracture.[6] Failure rates of endodontically treated teeth were almost double in cases where the process of adequate postendodontic restoration had not been followed.[7] Full coverage restoration when delayed after endodontic therapy invariably leads to fracture. Analysis of endodontic treatment outcomes suggests that tooth fracture may be a greater problem than reinfection.[8] Fractures in endodontically treated teeth have a reduced long term survival rate due to reduced fracture resistance caused by dentin loss during endodontic procedures.[9] Different methodologies have been tried to reinforce the endodontically treated tooth.

MATERIALS AND METHODS

Extracted permanent mandibular first molars were collected, prepared, analyzed using radiovisuography, and twenty-four teeth having mature, intact root apices were selected. Mesiodistal and buccolingual diameters at the level of cementoenamel junction measured and samples weighed. The samples were evenly distributed across experimental groups based on their weights and homogeneity. They were subjected to a statistical test to assess normality of these continuous variables and stored in normal saline at 4°C in separate glass bottles [Table 1].

Table 1.

Sample groups

Groups (n=8)	Posthesign
I	RC treated - No post
II	Horizontal post - Buccolingual
III	Diagonal horizontal post
IV	Natural tooth

RC: Root canal

The cusp tips were marked, access done after marking the outline, analyzed under magnification and access outline refined and standardized across samples.

Working length determined by passively placing a size 15K file (Dentsply Maillefer, Ballaigues, Switzerland) in the canal until tip visualized and adjusted to the apical foramen for samples of Gps I, II, and III. Working length calculated by subtracting 0.5 mm from recorded value and canal preparation done. The instrumentation was initiated with hand files (Dentsply, Maillefer, Ballaigues, Switzerland) up to size 20 followed by ProTaper gold rotary files from size Sx-F2 (Dentsply Maillefer, Ballaigues, Switzerland) with X-smart plus endomotor (Dentsply Maillefer, Ballaigues, Switzerland) as per the manufacturer instructions. The irrigant delivered using a 30-gauge side vent pro-rinse needle (Dentsply, Tulsa Dental) at working length. 1 ml of the irrigant was used for canal irrigation after using each instrument and before proceeding to the next. A total of 8 ml of the irrigant were used during the process. Subsequent to the canal preparation, the samples were irrigated with a final rinse of 5 ml of 17% EDTA (Desmear). The time of exposure of the irrigant was for duration of 3 min. During the 1^st min delivery of the irrigant, the needle was withdrawn to 5 mm inserted back to working length followed by rotation of the needle by 180° three times alternatively. During the 2^nd min, a F3 size gutta percha cone (Dentsply Maillefer, Ballaigues, Switzerland) was inserted to working length and withdrawn three times (Manual Dynamic Activation). This was done to improve the irrigant delivery and replacement to the apical third of the canal space. After 3 min, a postfinal rinse irrigation of 5 ml of distilled water was done to flush out the remaining final rinse from within the canal. Subsequently, obturation was done using AH plus sealer and gutta percha cones using a cold lateral condensation technique. The samples were stored for 24 h after which the standardization of the coronal portion was done. After completion of root canal obturation, the samples were stored for 24 h. The access then was extended into a mesio-occluso distal preparation with a gingival seat of about 2 mm and the width being one-third the intercuspal distance. A defined axial wall was given with the termination of the gingival seat just short of the cementoenamel junction. Subsequently, the cavity surfaces were primed and core build up was done using a dual cure core buildup material for the non postgroup. For the horizontal postgroups, the cavity was further prepared by creating through and through slots running on both the buccal and lingual walls of the cavity so that the post could be placed in a horizontal alignment either buccolingually or diagonally from the distobuccal to the mesio lingual cusp. The posts were then placed in the slots and bonded in place using a dual cure resin cement. The core build up material was then placed in the preparation in increments and the procedure completed. The samples were then stored.

The samples belonging to each group mounted on silicone formers (1.5” ×1.5” ×1.5”) with the help of an 18 gauge stainless steel wire. The tooth embedded vertically in such a way that the crown of the tooth was fully visible and the occlusal surface parallel to the mounting surface. The autopolymerizing resin was then poured and allowed to set. The sample blocks were then coded and stored.

The samples of each group were tested with a universal testing machine using a custom metal indenter (3 mm). A crosshead speed of 1 mm/min was set and the load applied on the surface of the tooth vertically parallel to the occlusal surface from above. The maximum force at which the tooth fractured was recorded. The fracture strength and type of fractures which were observed were analyzed. The results were assessed by two different operators and the results compared and tabulated.

Samples were kept covered by moist cotton to prevent dehydration till they were ready for fracture testing. The type of fractures was observed, grouped, and recorded. The results were assessed by two different operators, compared, and tabulated. The results of fracture test were statistically analyzed using one-way ANOVA test of variance with Tukey's post hoc test for multiple comparisons.

RESULTS

Buccolingual type of fracture was the most common among experimental groups with a percentage of incidence of 37.45%. The cuspal type of fracture was the next most common with a percentage of incidence of 25.0%. The incidence of mesiolingual, transverse, and comminuted type of fracture was not common and had a percentage of incidence of 9.38%, 9.38%, and 9.38%, respectively [Figure 1]. There was no incidence of transverse fractures. There was statistically significant difference between the experimental groups and control group in load values of root fracture (P < 0.05). Among the groups, Group I mesiodistal type of fractures was common with an incidence of 37.5%. In Group II, Group III, and Group IV, the buccolingual type of fracture was common with an incidence of 62.5%, 37.5%, and 50.0%, respectively. The other type of fractures encountered was comminuted, cuspal, and other proximal fractures.

Figure 1.

Fracture types

GP IV had the highest fracture resistance with a mean value of 1279.82 (N). Groups I, II, and III had mean values of 684.16(N), 756.84(N), and 758.17(N) [Figure 2]. There was a statistically significant difference between experimental groups [I, II] and [II, III] in load values of root fracture (P < 0.05). There was a statistically significant difference between Gp IV and Gps (I-III) in load values of root fracture (P < 0.05) [Figure 3].

Figure 2.

Fracture mean

Figure 3.

Fracture distribution

DISCUSSION

Studies of endodontic outcomes suggest that fracture of the tooth after endodontic therapy may be a greater problem than reinfection. Immediate full coverage with or without a post and core preparation is the best option to avoid fractures. Due cost factor involved, this is often postponed, eventually leading to tooth fracture. Postendodontic restorations should reinforce the residual structure of the tooth in a way that it maintains long-term functionality, leading to successful outcomes of therapy. A considerable amount of radicular dentin is lost intentionally during biomechanical preparation. The amount of remaining dentin in a tooth is directly proportional to fracture resistance. Radicular dentin thickness in a buccolingual direction also decides the resistance to fracture. A directly proportional relationship also exists between root thickness and lateral dislodging forces. Reviews of several in vitro studies suggest that a composite restoration reinforced with glass fibers, particularly with fiberglass posts laidhorizontally in a buccolingual direction, significantlyincreased fracture resistance.[10]

Fractures of endodontically treated teeth can be either transverse or vertical. Microcracks on canal walls become areas of stress concentration and extend over a period of time to the surface eventually causing a vertical root fracture.[10] Cracks usually begin at the apex of root and propagate in a cervical direction. There has been a positive correlation between the presence of microcracks and incidence of root fractures. Root fracture is a gradual propagation of craze lines and microcracks in radicular dentin.[8] Microcracks are formed as due to rotational forces acting on canal walls during biomechanical preparation. Increased incidence that has been reported with rotary instrumentation techniques may be related to the design features of rotary instruments. The hand instruments produce less dentinal defects compared to rotary instruments.[11]

Vertical fractures rarely occur as a result of acute trauma, split roots along their long axes, and have a poor prognosis. A potential correlation exists between the rotary instrument design and incidence of vertical fractures.[12] Biomechanical preparation induces craze lines and cracks on radicular canal walls which become areas of high stress. These microcracks may spread slowly over a period of time to the surface of the root eventually resulting in a vertical root fracture.[13] The cracks usually begin at the apex of the root and propagate cervically. The change in the mechanical properties of dentin with age and loss of hydration after endodontic therapy has also been a reason for the reduced tolerance of teeth to fractures. Horizontal or transverse root fractures are more common. They occur due to frontal impact in the anterior region of the jaw. The fracture usually occurs in the middle third of root and prognosis is usually good. Successful management of a tooth that has sustained a transverse root fracture depends on position of the line of fracture, the mobility of the coronal segment of the tooth, and the status of the pulpal tissues. It is very important to differentiate between the kind of fracture which has happened, either vertical or horizontal fractures. There is a poor prognosis in conditions with vertical fractures which split roots along their long axis.

The process of retreatment removes dentin again from within the canal space rendering these retreated teeth more susceptible to fracture. The rotary files used for removal of obturated material from the canal space during the retreatment remove the surface dentin, as well as further enlargement of the apical third of the root to achieve better cleansing leading to a reduced fracture resistance. A positive correlation exists between the removal of dentin during retreatment and reduction of fracture resistance.[14]

The existence of structural defects and craze lines which extend from the external surface of root in radicular dentin has been found to result in fractures, due to amplification of induced stresses at these microcracks.[14] External craze lines and cracks are often found not to connect with canal lumen. One possible reason is that stress generated during instrumentation of canals is transmitted to the external surface of teeth where it overcomes bonds in dentin to form these craze lines, microcracks eventually leading to fracture of root.[15,16]

The position and relationship of fracture line to gingival crevice are the most important factor affecting the long-term prognosis. Coronal and middle third root fractures have a less favorable prognosis than root fractures of the apical third. When fracture is infra-bony having no communication to gingival sulcus and patient practices meticulous oral hygiene, appropriate treatment options can result in successful outcomes.

Failure rates of endodontically treated teeth become almost double in cases without adequate postendodontic restoration.[18] Provision of intra-orifice barrier is an efficient alternative method to decrease coronal leakage and improve fracture resistance.[19,20]

Irrigation procedure during endodontic treatment procedures leads to alteration of chemical composition of organic and inorganic phases of radicular dentin. This affects microhardness, permeability, and solubility of radicular dentin leading to changes in the ability of teeth to resist fracture under varying loads.

Glass fiber posts with different properties have been formulated and have been used to restore endodontically treated teeth, as they are esthetic, faster to execute, and have an elastic modulus near that of tooth structure (30–50 GPa).[15] In the present study, we have used a zirconia reinforced glass fiber post for the evaluation as a horizontal post placed in two different configurations. A composite restoration reinforced with glass fibers, particularly with fiberglass posts especially placed in a horizontally in a buccolingual direction, significantly increased the fracture resistance. Reinforcement of composite resins core buildups with glass fibers or use of fiber posts improves fracture resistance of tooth structure postendontically. The use of horizontal posts along with composite resins postendodontically to restore the tooth was found to be comparable and had a similar performance level as that of teeth that were restored with onlay type of preparations. The authors also observed that sound teeth had the maximum resistance to fracture.[16] Clinical success rates of endodontically treated premolar teeth restored with fiber posts and direct composite restorations after 3 years of service were similar to a treatment of full coverage with metal ceramic full coverage restorations.[17]

Survival of endodontically treated teeth was found to be directly associated with permanent coronal restorations. Tooth loss after endodontic therapy often occurred for those teeth restored with temporary restorations (34.5%).[18] The type of coronal restorations used had a say on the longevity of the teeth involved.[19]

Ideally placing the postendodontic restoration within a span of 2 weeks was found to have a high rate of success.[20,21] When unrestored, endodontically treated teeth have issues that like coronal leakage, reinfection, and fractures. These teeth are at risk for fracture since they become predisposed to structural compromise when compared to their properly restored counterparts. A timely tooth reconstruction following endodontic therapy goes a long way to ensure a favorable prognosis.[22]

There is insufficient evidence to assess the effects of crowns compared to conventional fillings for the restoration of endodontically treated teeth. Until more evidence is found, clinicians should continue to base decisions about postendodontic restorations on their own clinical experience, taking into consideration circumstances and preferences of their patients.[23] Adhesion of obturation materials to root canal wall increases mechanical interlocking reducing risk of fracture.[21] The selection of obturation materials and techniques employed impact the resistance of root obturation fracture.[22]

In this in vitro study, sufficient protocols were followed for standardization of controllable factors due to the large amount of variations in the volume and weight of roots among samples selected. Normality testing of these variables was assessed statistically, and distributions were found to be normal at 5% level of significance. Gp IV which was natural teeth has the highest fracture resistance with a mean of 1279.82 ± 349.9 and Gp I had the least values for fracture resistance where no posts were used with a mean value of 684.17 ± 234.10. It was extremely interesting to note that the natural teeth – Gp IV presented with the highest values for fracture resistance. On analysis of the fractures samples, those with no placement of horizontal posts showed a tendency to fracture mesiodistally, while those in which the posts were placed showed a buccolingual fracture line and the tooth was held together by the post [Figures 4–7]. The fracture of the tooth happened only in the proximal areas of the mesiodistal preparation. The use of horizontal posts seems to structurally stabilize the tooth and have got a potential to replace the full coverage restoration procedures which involve extensive tooth structure removal.

Figure 4.

Buccolingual fracture

Figure 7.

Comminuted fracture

Figure 5.

Mesiodistal fracture

Figure 6.

Proximal fracture

CONCLUSION

Dentin preservation should be done as much as possible during endodontic therapy. The choice of biomechanical instrumentation, technique of obturation, postendodontic restoration choices including core buildup methodologies has to be customized to individual cases based on preoperative analysis which would lead to better long-term outcomes. The use of diagonal horizontal posts seems promising and has the potential to be a conservative postendodontic management option. Further evaluation of different horizontal postdesigns and assessment in a clinical setting is recommended. These designs have the potential to be a conservative postendodontic management option.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.