Evaluation of Fracture Resistance of Root Canal Treated Teeth Restored with Different Types of Post-Endodontic Restorations: An In Vitro Study

Introduction

Loss of tooth structure due to dental caries, restorations, improper endodontic access preparation, and endodontic instrumentation can lead to fracture of the root canal (RC) treated tooth.^1,2 For this reason, Endodontic treated teeth (ETT) have a greater risk of fracture compared to vital teeth.³ For the restoration of ETT, complete knowledge regarding the tooth anatomy, occlusion, endodontic procedure, and periodontal concerns is required. After RC treatment, reconstruction of the lost tooth structure becomes an essential part of dental restorative treatment.

Endodontic procedures reduce the strength of premolar teeth by 5%. The mean fracture strength for unrestored teeth with Mesio-Occluso-Distal (MOD) preparation was 50% less than that of unaltered premolar teeth. Hence, intracoronal support of the teeth is important to protect against the fracture.^4,5  

Various modalities to restore the ETT are direct resin composites, amalgam fillings, post and core, crowns, etc.^6,7,8 According to Soares PV et al.,1 occlusal forces can cause different types of fractures due to their wedging action between the cusps on the non-restored teeth. Hence to support the remaining tooth structure, strengthening of the cavity with various restorative materials is required.

Recent studies have shown that fiberglass materials can be used to reinforce the restoration.^9,10 However, knowledge regarding various materials in current literature is conflicting and needs to be evaluated; in particular, if polyethylene fiber and glass fiber posts can be useful.

Therefore, this study was conducted to evaluate the fracture resistance of root canal-treated teeth restored with different types of post-endodontic restorations which would reinforce the endodontically weakened teeth.

Materials and Methods

A total of 60 upper premolar teeth were used in this study. The case records of patients were studied and the details were noted in the proforma to exclude pathologies such as attrition and root caries. All the samples were stored in 10% formalin in individual boxes. The study protocol was approved by the institutional ethical committee.

Samples were equally divided into six groups (n=10). Access openings were prepared using high-speed air turbine water-cooled handpiece (NSK, Japan) and Endo access burs (Dentsply, USA) in all the specimens except in group 1 (control group, untreated). Working length was set at 0.5-1 mm short of the apical foramen. Biomechanical preparation was completed using a rotary system (Hyflex CM, Coltene/Whaledent Private Ltd, USA). Sodium hypochlorite solution (3%), and saline were used for irrigation within the canals throughout the instrumentation procedure.

Paper points (Spident; Meta Biomed, Republic of Korea) were used to remove moisture from the canals. Canals were obturated with gutta-percha cones and non-eugenol resin sealer (AH Plus Sealer; Dentsply, USA) by lateral condensation and were temporarily restored (Coltosol F, USA). After root canal treatment, all the samples were stored in distilled water for 72 hours.

To simulate periodontal ligament (PDL) and supporting structures, the roots were dipped in melted wax upto the cement-enamel junction (CEJ) and then rooted into auto-polymerizing resin. To prevent displacement, small depressions were made on the roots. Once the acrylic blocks were set, the wax coating on the samples was substituted with silicon-based impression material (AFFINIS- Coltene/ Whaledent Private Ltd, USA).

Mesio-Occluso-Distal (MOD) cavities were prepared using straight fissure burs (Mani SF-41, Japan) at high speed with air-water spray (depth upto the level of orifice, buccolingual width of 4±0.5 mm, mesiodistal width of 4±0.5 mm at cervical region of proximal box as measured using Digital Caliper) on all the samples, except for Group 1.

Group 1 (G1): (n= 10) Untreated or sound tooth. (Figure 1)

Group 2 (G2): (n= 10) RC treated samples with MOD cavity prepared and not restored. (Figure 1) 

Group 3 (G3): Similar preparation as in group 2. A dentin bonding agent (ONE COAT 7.0, Coltene/ Whaledent Private Ltd, USA) was applied over MOD cavities, cured, and restored with resin composite (BrilliantTMNG,coltene, Brazil) using incremental  technique and cured. A composite finishing kit was used for finishing and polishing the restoration. (Figure 1)

Group 4 (G4): Cavities were restored similar to group 3. This was followed by a tooth reduction of 2 mm and 1.5 mm on the functional cusp and non-functional cusp, respectively. A shoulder finish line of 1 mm was given. After the tooth preparation, a putty impression was recorded followed by a light body to replicate the tooth surface. The impressions were sent to the dental laboratory for the fabrication of porcelain fused metal crown. Once the porcelain fused metal crown was ready, it was cemented to the prepared specimen using self-adhesive cement. (Figure 1)

Group 5 (G5): Teeth received preparations as in group 2. Then on the highest contour of both the buccal and palatal surfaces of the teeth, perforations were made. Round burs were used to prepare the holes (size close to that of the glass fiber post). The glass fiber posts (Hi-rem post, Over-fiber, Brazil) were brushed, cleaned with alcohol, and cemented in the hole using self-adhesive resin cement according to the manufacturer’s instructions. Margins of the post were restricted within the tooth. A dentin bonding agent (ONE COAT 7.0, Coltene/ Whaledent Private Ltd, USA) was applied over the cavities, cured, and a flowable composite (FeltekTM Z350XT,3M ESPE, USA) was used to fill the cavities till the level of post. The rest of the preparation was filled using resin composite (Brilliant^TMNG,coltene, Brazil) using the incremental technique. Finishing and polishing of the restoration was done. (Figure 1)

Group 6 (G6): Samples were restored as in group 3. On the occlusal surface, a cavity of 2 mm in width and depth was prepared buccolingually using straight fissure bur to implant polyethylene fiber (Ribbond, USA). After the cavity completion, the fiber was dipped into dentin adhesive and adapted into the prepared space, and excess was cut. A dentin bonding agent (ONE COAT 7.0, Coltene) was applied over and below the mesh and cured. Cavity was finally restored with composite resin (BrilliantTMNG,coltene, Brazil). Restorations were then finished and polished. (Figure 1)

The specimens were stored in the incubator at 100% humidity after being removed from the acrylic blocks at 37°C.

Loading of the Specimens

A universal testing machine (HEICO, INDIA) was used to check the compressive load resistance of the samples. A round stainless steel probe of 5 mm in cross-section was positioned in a line parallel to the long axis of the sample (Figure 2). With the head speed of 0.1 mm/min, the compressive load was applied on the tooth until it fractured and the reading was recorded in Newton (N). Machine software determined the fracture resistance in force-to-displacement graphs by showing a sudden drop in load, indicating a fracture. The type of failure was visually examined.

Statistical Analysis

Data collected during loading was entered in Microsoft Excel and analyzed using IBM SPSS software version 22.0 (Armonk, NY, USA). Fracture resistance between the experimental groups was compared using the Analysis of variance (ANOVA) test and Tukey's Post hoc test. For a mean description of data, mean, standard deviation, and 95% confidence interval were used as seen in Table 1.

Results

According to Table 1 and Graph 1, the specimens in G1 required the maximum load application to fracture (M=1433.30, SD=24.739). Samples in G2 (M=453.20, SD=28.150) had the least fracture resistance when compared to other groups which were statistically significant (p <.0001). The teeth restored with fiberglass horizontal post (G5) (M=1248.60, SD =38.497) had significantly highest fracture resistance followed by G4 (M=1127.00N, SD= 36.935) and G6. Teeth restored with polyethylene fiber mesh (G6, M=963.80, SD=39.009) needed more force or load application to fracture than the teeth restored with composite (G3, M=963.80N, SD=39.009). 

Following the visual examination of all the specimens for the mode of failure, unfavorable fractures were observed only in group 2, 3 and 5 (Figure 3 and Figure 4).

The fracture patterns were visually determined for all the teeth. Mainly two typical root fracture modes were seen - fracture above the CEJ (favorable mode) and fracture below the CEJ (catastrophic mode). As illustrated in Figure 4 and Table 2, G1 and G6 showed the favorable fractures to the maximum, and specimens in G2, G3, and G5 presented catastrophic fracture mode. Most of the samples in G4 showed a fracture in the porcelain body only. Figure 4 illustrates the fracture type and the frequency.

Discussion

The primary goal of restoring endodontically treated teeth is to enhance structural strength and prevent complicated fractures. The present study aimed to evaluate the fracture resistance of root canal treated teeth restored with different types of post-endodontic restorations which would reinforce the endodontically weakened teeth. The study was conducted using maxillary premolars because of the comparable fracture potential with that of molar and better accessibility in the interproximal margins for inspection and finishing.¹¹

According to the present study, the highest mean fracture resistance was seen in G1 followed by G5, G4, G6, and G3, While the least fracture resistance was observed in G2. The maximum fracture resistance in G1 was due to the highest rigidity and integrity of the tooth structure. Group 2 showed the least fracture resistance due to the lack of restoration to reinforce the tooth. This is in accordance with the study conducted by Monga et al. which reported that unrestored MOD prepared tooth was 50% less stronger than the sound tooth.⁴ 

The samples with glass fiber horizontal post showed increased fracture resistance amongst the other experimental groups, followed by samples restored with porcelain-fused-to-metal (PFM) crowns. Fiberglass posts were chosen for the post endodontic restorations in G5 because of their even distribution of the force along the tooth due to their elastic modulus close to dentin (elastic modulus - 0.8x106 psi).^12,13 The results of the present study revealed that the glass fiber horizontal post within the composite resin enhanced the fracture resistance of ETT premolars which is similar to the findings reported by Karzoun et al.¹⁴ and Bromberg CR et al. ¹⁵ The glass fiber horizontal post within the composite resin showed the ability to absorb the occlusal loads and thereby can withstand fracture.¹⁶

On comparing G5 and G4, the fracture resistance of group 4 was significantly lower. In G4, after compressive loading, fracture occurred through the porcelain body and not through the teeth. Even though PFM crowns are known for their good color stability and esthetics, the porcelain body in the crown is susceptible to fracture. Fractures in the porcelain body are due to the mechanical stresses on the sub-microscopic defects known as Griffith’s microcracks which cause crack propagation and finally lead to fracture of the crown.^17,18

A Class-I type of fracture (i.e Cracks and porcelain body failures) according to the classification for the fracture mode of the crown proposed by Burke et al., was observed in G4.¹⁹ These results are in accordance to the study conducted by Michalakis KX et al.²⁰ 

The fracture resistance of G6 was significantly higher in comparison with G3 due to the presence of ultra-high elastic modulus of reinforced ribbon or polyethylene fiber mesh (Ribbond). When a crack in the resin encounters a fiber, it is deflected and spreads along with the interface due to its efficient transfer of compressive load, thus preventing the fracture and increasing the strength of the restoration.⁵ This proves that the sole purpose of the restoration is to strengthen the tooth rather than fill it. On the contrary, a study conducted by Luthria et al.,²¹ reported conflicting findings with no difference observed in fracture resistance betweenimpregnated glass fibre and polyethylene fibre.

Even though the fiber post insertion increased the fracture resistance of the ETT, the post did not avoid complicated fractures. The post succeeded in holding the remaining walls of the ETT when compared to other groups, but the excessive load created strain in the cervical area leading to catastrophic fractures. However, the force required to induce such fractures is more than the masticatory load of the premolars. All the groups except G2 and G5 had favorable fracture mode which can be restored clinically.

The present study results showed that the technique used in G5 and G6 reinforced the composite resin filling and thereby enhanced the fracture resistance of ETT premolars. Since the study was conducted in vitro, many clinical trials and follow-ups are needed to appreciate the success of this technique. Also, the comparison between horizontal fiberglass posts and vertical fiber posts has to be further evaluated. Thus future studies are needed to determine better clinical outcome of this technique.

Conclusion

Based on the results, the following conclusions can be drawn:

• Fiberglass horizontal post reinforced with composite can be used as an alternative technique for restoring ETT with MOD cavities.

• This technique can be primarily used in financially distressed situations or when no successive crowns are planned.

Conflict of Intreset

None