Comparative evaluation of fracture resistance of endodontically treated maxillary premolars reinforced by customized glass fiber post in two different ways: An in vitro study

Abstract

Context:

Endodontically treated premolars are currently restored with direct bonded techniques in conservative manner enabling them to bear functional stresses homogeneously.

Aim:

The study aimed to evaluate the effect of placement of compactable glass fibers in reinforcing the endodontically treated teeth in a novel conservative manner.

Settings and Design:

Research laboratory, in vitro study.

Materials and Methods:

Seventy-five extracted maxillary premolars were procured. Fifteen teeth were left untreated (Group A) and the remaining teeth were endodontically treated followed by standardized mesio-occluso-distal preparation and randomly assigned to experimental groups (n = 15) as follows: (B) no restoration, (C) restoration with composite, (D) EverStick^® POST followed by composite, and (E) vertical glass fibers within 3 mm of the coronal root canal space and buccopalatal flaring of the coronal fibers followed by composite. After conditioning and thermocycling, specimens were loaded under a universal testing machine to evaluate fracture resistance and fracture pattern of specimens.

Statistical Analysis Used:

Obtained scores were statistically analyzed using one-way analysis of variance test for stress analysis, post hoc Tukey's test for intergroup comparison, and Chi-square test for analysis of favorable and unfavorable fracture.

Results:

The fracture resistance was highest to lowest as follows: Group A > E > C > D > B (P < 0.001).

Conclusion:

EverStick^®POST used in conservative manner improved fracture strength of teeth significantly.

INTRODUCTION

Access preparation during endodontic management quite often results in extensive coronal damage.[1] Extensive dentin destruction in the form of compromised marginal ridges and elimination of pulp chamber roof do not let the endodontically treated teeth defend against the cuspal deflection arising due to masticatory loadings.[2]

A greater occurrence of fracture in endodontically treated maxillary premolars is attributed to their anatomic form, unfavorable crown/root proportion, and their concomitant dealing with shear and compressive forces.[3] Therefore, the final tooth restoration complex must be capable of withstanding cyclic masticatory forces.[4] Hence, it is suggested that complete cuspal coverage is essential to restore endodontically treated posterior teeth.[5]

The substantial remaining dentin loss in endodontically treated teeth during tooth preparation might worsen the situation specially when associated with complex cavities. In such an event, the reinforcing adhesive restorations[6] are advantageous over complete coverage crowns.[7] Restoring endodontically treated tooth with direct resin-bonded composite eliminates the need to sacrifice the precious residual tooth structure that transmits and distributes functional stresses uniformly.[8]

In teeth with extensive coronal destruction, an intraradicular post is essential for coronal retention of the restoration. Very often, these teeth are restored with a custom-built metallic post followed by a cemented crown. However, it holds multiple shortcomings such as possible root fractures, compromised esthetics, difficult retrieval during retreatment, and the risk of corrosion or allergic reactions.[9]

The elastic modulus of intact dentin and enamel is approximately 15 GPa and 80 GPa, respectively.[10] The metallic posts have much higher modulus of elasticity as compared to dentin, which may result in catastrophic unrestorable fracture.[11] Recent research advocates that the rigidity of the post should equate or approximate radicular dentin that assists in the uniform dissemination of masticatory stresses.

The minimal preparation needed for fiber post allows preservation of remaining dentin.[12] Fibers aligned in the longitudinal direction of the posts result in less stress transmission to the resin matrix. The matrix is, therefore, prone to stresses when forces are applied obliquely to the fibers. This is attributed to the stiffness of the fibers in contrast to the surrounding resinous matrix.

A unique post design, EverStick^®POST, comprises adhesive fiber bundles, which can be compacted and adapted directly into the prepared post space to attain customized shape, which is then adhesively luted. This post utilizes a glass fiber network embedded in an interpenetrating polymer network matrix of polymethyl methacrylate. Due to the adaptive characteristic of EverStick^®POST, a relatively uniform, void free, and thin cement layer between the post and the dentin is created that potentially allows even distribution of occlusal forces.[13]

Earlier documented literature suggests the reinforcement of endodontically treated maxillary premolars in an innovative way using either a segment of prefabricated fiber post or occlusal placement of polyethylene or glass fibers in a conservative manner.[1,14] The excellent flexibility and maneuverability of EverStick^®POST appears to be beneficial in obtaining the three-dimensional force distribution of occlusal stresses in a more conservative manner. Therefore, this study was designed to assess the fracture resistance of the endodontically treated maxillary premolars by incorporating anatomic EverStick^®POST with buccopalatal flaring of the coronal segment when preserving radicular dentin and to comparatively evaluate it with conventional postplacement technique utilizing EverStick^®POST. The null hypothesis stated that there will be no statistically significant difference in the fracture resistance of endodontically treated single-rooted maxillary premolars among the groups being tested.

MATERIALS AND METHODS

After obtaining the institutional ethical approval (SVIEC/ON/Dent/BNPG15/D16016), seventy-five intact human single-rooted, straight, single-canal maxillary premolars with similar dimensions extracted for orthodontic/periodontal reasons were selected and disinfected by immersing the specimens in 0.1% thymol solution for 1 week followed by cleaning with ultrasonic scaler. Fifteen teeth were randomly allocated to group A (positive control) and were left untreated. For the remaining sixty teeth, conservative endodontic access was prepared and working length was set at 0.5 mm short of the apical foramen. The biomechanical preparation was done using nickel titanium rotary files up to apical size 40 (0.06 taper) (K3 XF^TM, Sybron Endo, USA). 17% ethylenediaminetetraacetic acid solution (Dentwash, Prime Dental Products, India) was used during the biomechanical preparation. Thereafter, all the canals were finally rinsed with distilled water followed by drying with paper points. Prepared canals were obturated using gutta percha (Dentsply India Pvt Ltd, New Delhi, India) and resin-based sealer (AH Plus^TM, Dentsply India Pvt Ltd, New Delhi, India) by lateral condensation technique. Subsequently, the access cavities were restored with temporary filling material (Samfil G Dentokem, India). The prepared teeth were stored in distilled water for 72 h. Standardized mesio-occluso-distal (MOD) cavities were prepared in all the treated teeth with cylindrical diamond bur under copious air water cooling at high speed by a single operator. For standardization purpose, the preparations were calibrated using digital Vernier caliper. Width of the MOD cavities was kept one-third of intercuspal distance at isthmus and the width of proximal boxes was determined by one-third of total buccopalatal dimension. The depth of cavity preparation was kept 1 mm above CEJ. Bur was replaced after every 6 preparations to ensure high cutting efficacy.

Subsequently, all the 75 teeth were embedded in autopolymerizing resin up to 2 mm apical to cementoenamel junction. Notches were prepared in roots to prevent dislodgement from the embedding material. Periodontal ligament was simulated by immersing the roots into melted wax to a depth of 2 mm below the CEJ and later was embedded in acrylic resin blocks. When primary signs of polymerization were noticed, each tooth was removed from the resin block. A silicon-based impression material (elite P and P, Zhermack, Italy) was used to replace the wax spacer. Thereafter, tooth was repositioned into the resin block and excess impression material was removed with surgical blade. All the prepared samples were randomly assigned to various groups of 15 samples each according to the group distribution protocol as follows [Figure 1a-e]:

Figure 1.

(a-e) Representative specimens of respective corresponding groups

A-Untreated teeth (positive control)

B-Endodontic treatment of teeth followed by MOD preparation without restoration (negative control)

C-Endodontic treatment of teeth followed by MOD preparation and composite restoration

D-Endodontic treatment of teeth followed by MOD preparation, placement of vertical compactable glass fiber post (EverStick^®POST, GC, India) keeping the 5 mm of gutta percha apically followed by composite restoration

E-Endodontic treatment of teeth followed by MOD preparation, placement of vertical glass fiber post (EverStick^®POST, GC, India) within 3 mm of the coronal root canal space and buccopalatal flaring of the remaining coronal fibers followed by composite restoration.

In the teeth assigned to group D, post space was prepared with size 3 peeso reamer keeping 5 mm of gutta percha apically. 0.9 mm size EverStick^®POST was compacted in the canal and luted with self-adhesive dual cure resin cement (G-CEM LINKAGE, GC, India) as per the manufacturer's instructions keeping it 1 mm short of the occlusal cavosurface margin. For the group E, the coronal 3 mm of the gutta percha was removed using heated plugger and flattened. Thereafter, EverStick^®POST was placed and compacted to obtain the shape of the prepared canal space. This was followed by buccopalatal flaring of coronal fibers keeping the fibers 1 mm short of the occlusal cavosurface margin and luted with self-adhesive dual cure resin cement (G-CEM LINKACE, GC, India) as per the manufacturer's instructions. Universal matrix band and retainer assembly was applied to all the prepared samples and the MOD cavities were bonded using self-etch dentin adhesive (G Bond, GC, India) and light cured for 10 seconds, followed by restoration with light-cured composite resin (Solare X, GC, India) and light curing for 30 s. All the restored teeth were finished and polished using flame shaped fine diamond bur (Shofu Inc, Japan). All the specimens were thermocycled for 500 cycles between 5°C and 55°C, dwell time 15 s followed by immersion in physiological saline, and incubation at 37°C for 15 days. All the specimens were quasi-statically loaded with a cross head speed of 1 mm/min in a universal testing machine. The bar was placed parallel to the long axis of the teeth and centered until the bar contacted the occlusal surface [Figure 2a and Figure 2b]. The failure load was determined for the entire specimen, when the force versus time graph showed any abrupt change in load, indicating a sudden decrease in the specimen's resistance to compressive loading. Specimens were visually examined for the type and location of failure, as well as the direction of failure. Fracture of tooth was classified as favorable or unfavorable depending on the fracture line. Fractures above or at the simulated bone level were defined as favorable fracture, whereas fractures below the simulated bone level were designated as unfavorable fracture [Figure 2c and Figure 2d]. Loading of the specimen was carried out by a skilled operator and evaluation of the fractured specimen for type of fracture was done by an experienced dental clinician. Both were blinded regarding the specimen preparation. The collected data were subjected to one-way analysis of variance (ANOVA) test and Tukey's post hoc test.

Figure 2.

(a) Specimen loaded in a universal testing machine; (b) close-up views of the loaded specimen; (c) representative specimen depicting favorable fracture pattern; (d) representative specimen depicting unfavorable fracture pattern

RESULTS

The mean values of the fracture resistance and standard deviations ranged from 287.8 N (+86.38) to 645.1 N (+97.68). The fracture resistance was recorded to be highest in Group A, whereas the lowest fracture resistance was recorded in Group B. Significant difference among the groups (P < 0.001) was discovered by one-way ANOVA test [Table 1]. Intergroup comparison was assessed by Tukey's HSD test, which presented significantly higher fracture resistance values in Group E as compared to other test groups, except Group A (control) [Table 2]. There was no statistically significant difference among the values of Group B, C, and D, respectively. The mode of failure was assessed by visual inspection of all the tested specimens and classified as favorable or unfavorable. All groups (except Group A) demonstrated an unfavorable fracture mode [Table 3]. The 60% specimen in Group E presented unfavorable manner of fracture, whereas the majority of specimens from Group A depicted favorable fracture mode (Fisher's exact test value of 24.469, P < 0.001). All the teeth in Group B, C, and D, respectively, showed an oblique or vertical root fracture extending till the middle or apical third of the root.

Table 1.

One-way ANOVA test for the stress levels

	Number of values	Minimum	25% percentile	Median	75% percentile	Maximum	Mean±SD
Group A	15	500.8	580.3	630.3	700.3	845.7	645.1±97.68
Group B	15	135.2	241.2	300.3	360.3	445.9	287.8±86.38
Group C	15	218.6	279.3	331.2	473.3	603.7	373.2±115.3
Group D	15	196.1	250.6	317	350.3	708.1	321±122.4
Group E	15	390	474.3	552.2	665.4	1078	581.6±168.1
F				25.29
P				<0.01

SD: Standard deviation

Table 2.

Tukey's post hoc test representing inter group comparison

Tukey's multiple comparison test	Mean difference	Q	Significant P<0.05	95% CI of difference
Group A versus Group B	357.4	11.59	Yes	229.8-484.9
Group A versus Group C	271.9	8.820	Yes	144.4-399.5
Group A versus Group D	324.1	10.51	Yes	196.6-451.6
Group A versus Group E	63.49	2.059	No	−64.04-191.0
Group B versus Group C	−85.42	2.770	No	−212.9-42.11
Group B versus Group D	−33.26	1.079	No	−160.8-94.27
Group B versus Group E	−293.9	9.531	Yes	−421.4-−166.3
Group C versus Group D	52.16	1.692	No	−75.37-179.7
Group C versus Group E	−208.5	6.761	Yes	−336.0-−80.93
Group D versus Group E	−260.6	8.453	Yes	−388.1-−133.1

CI: Confidence interval

Table 3.

Chi-square test for the favourable and unfavourable analysis

Group×status cross tabulation
Group	Status		Total
	Favourable fracturre	Unfavourable fracture
Group A
Count	13	2	15
Percentage within group	86.7	13.3	100.0
Percentage within status	40.6	3.4	16.7
Group B
Count	2	13	15
Percentage within group	13.3	86.7	100.0
Percentage within status	6.2	22.4	16.7
Group C
Count	2	13	15
Percentage within group	13.3	86.7	100.0
Percentage within status	6.2	22.4	16.7
Group D
Count	3	12	15
Percentage within group	20.0	80.0	100.0
Percentage within status	9.4	20.7	16.7
Group E
Count	6	9	15
Percentage within group	40.0	60.0	100.0
Percentage within status	18.8	15.5	16.7
Total
Count	26	49	75
Percentage within groups	34.7%	65.3	100.0
Percentage within status	100.0%	100.0	100.0

Fisher's exact test value of 24.469, P<0.001

DISCUSSION

The study results clearly presented higher fracture strength of teeth, in which adhesively luted fiber posts were used in a novel manner. The buccopalatal flaring of EverStick^®POST and minimal radicular extension showed the highest fracture resistance of the tooth among the tested groups. Our results agree with the earlier studies that also demonstrated improved outcome in specimens with occlusally placed fibers.[14]

The dentin-like behavior of the EverStick^®POST enables better stress distribution resulting in high fracture strength values.[15] The high molecular weight polymethyl methacrylate chains in the EverStick^®POST function as stress breaker as it plasticizes the stiffness of highly cross-linked bisphenol A-glycidyl methacrylate (Bis-GMA) matrix, eventually reducing the stress concentration at the interface of fiber-matrix during deflection.[16] Furthermore, during post fabrication, rehabilitating effect of unidirectional impregnated fibers can be produced. The impregnated fibers are soaked in resin matrix in a prestressed tension that is released after curing, which leads to fiber's compression that may absorb the tensile stresses under flexion.[17]

Wiskott et al.[18] in their study concluded that individually formed posts perform better against rotational forces and dislodgement in contrast to standard glass fiber or metal post. In addition, these fibers dissipate the stress and check the crack development.[19]

The results of our study are in accordance with previous research where an insignificant difference was noted where largely compromised teeth were restored using adhesive restorative design either with or without using posts.[20]

In our study, one of the tested groups did not involve any post space preparation but minimal EverStick^®POST extension into the root canal space. This eventually allowed better preservation of clinically crucial residual pericervical dentin. The researchers in the previous studies have also suggested the modification in preparation designs and successfully utilized various forms of reinforced fibers/fiber reinforced composites/fiber posts.[1,14,21,22,23]

Considering the immense improvement in adhesive restorative materials and techniques, reinforced direct composite restorations appear to be a sensible substitute as definite restorative treatment modality in endodontically treated teeth over full-coverage crowns as it sacrifices ample amount of sound dentin. This statement is validated by a previous research that showed no benefit of ceramometal crown coverage on the clinical performance of endodontically treated teeth when compared with placement of a direct composite restoration over a 3-year time span.[7] However, on the contrary, another study identified that endodontically treated teeth without crowns were lost at a 6.0 times greater rate than teeth with crowns; however, it did not specify the type of restorative material used as well as the type of teeth.[5]

As previously suggested by Karzoun et al.,[1] this research also reported favorable fracture pattern in the specimens from intact teeth group, while specimens from remaining test groups exhibited unfavorable fracture. This fracture pattern is possibly attributed to the MOD design that permitted limited volume of residual cervical tooth structure. In addition, the similar elastic modulus of glass fiber post, dentin, and resin composite[24] aided in the redistribution of compressive load. Therefore, the stress principally concentrated in the middle and apical thirds of root, resulting in catastrophic fracture. In addition, the specimens from all the tested groups except control group showed cervical root fracture which propagated toward the post and transferred to the root, thereby suggesting good bonding between the post and the root probably due to the larger bonded surface in the root as compared to crown. This is in accordance with the study by Garoushi et al., where the fractures originated from regions with excessive stress concentrations and propagated toward the mechanically weak areas in the restoration.[19]

The study was carried out on morphologically similar single-rooted maxillary premolars to standardize the treatment procedure[25] as inclusion of multirooted teeth could make the standardization difficult due to the presence of curvatures, anatomic complexities, and possibility of increased frequency of procedural errors. Maxillary premolars were preferred as they are frequently exposed to lateral forces of detrimental nature as compared to molars.[26] In addition, the remaining walls of the tooth influence cusp fracture; hence, complex MOD preparations are highly prone to fracture.

In the dynamic thermal oral environment, marginal leakage at the tooth–restoration interface is common, which is associated with the differing thermal properties of the restorative material and the tooth structure. Hence, to simulate the intraoral conditions, it is important to expose the specimens to thermocyclic changes as followed in our experiment.

The study utilized 0.1% thymol for disinfection of teeth as it provides adequate disinfection without altering the properties of dental tissues. Although the clinical studies portray the realistic outcome, in vitro assessment can overcome certain limitations associated with individual variations. By measuring the fracture resistance of reinforced teeth, we may have an idea about the load-bearing capacity of the strengthening system that provides a foundation for further in vivo studies.

In our study, the acrylic resin was chosen to embed the prepared teeth as its modulus of elasticity is approximately like that of human bone. Furthermore, coating the external radicular surface with light-body silicone simulated the periodontal ligament and the 2 mm of coronal root was left exposed to mimic the level of healthy alveolar bone.

The use of dual cured resin cement during postcementation increases the fracture resistance of reinforced teeth. It also enhances the postadhesion potential resulting in greater toughness, longevity, low solubility, and minimal microleakage as compared to conventional cement.[27]

The functional occlusal forces in the clinical conditions vary from those generated by a universal testing machine. Therefore, the results obtained after the static loading used in the present study may not correspond to the clinical situation. Hence, the outcome of the present laboratory study necessitates the additional clinical trials to validate the results.

CONCLUSION

Within the limitation of this in vitro research, it can be concluded that the use of EverStick^®POST in a conservative manner is beneficial to acquire adequate fracture strength, thus augmenting the clinical longevity of the endodontically treated teeth. However, long-term clinical studies with stringent protocols are essential to validate the above-mentioned results.

Financial support and sponsorship

Self-funded.

Conflicts of interest

There are no conflicts of interest.