Effect of pulpal floor perforation repair on biomechanical response of mandibular molar: A finite element analysis.

Background: Evaluation of the biomechanical response of tooth with perforation repair is important to attain predictable prognosis. It may remain altered even after perforation repair due to the loss of tooth structure. Aim: The aim of this study is to assess and compare the effect of pulpal floor perforation repair of different sites with biodentine, on the biomechanical response of mandibular molar through 3-dimensional (3D) finite element analysis (FEA). Materials and
Methods: Five different 3D models were constructed based on the site of perforation on the pulpal floor using cone-beam computed tomographic images of an extracted mandibular molar. Perforation size was standardized and simulated to be repaired with calcium silicate-based cement. A force of 200 N was applied simulating normal occlusal loads. Static linear FEA was performed using the Ansys FEA software. Tensile stresses were evaluated (Pmax). Statistical Analysis Used: The data were evaluated using the independent t-test (P = 0.05).
Results: All the simulated models with perforation repair exhibited higher stress values than their equivalent sites in the control group. The Pmax values of the repaired models were highest in central furcal perforation, followed by buccal furcal perforation. However, there was no statistically significant difference in the stress accumulation among the different repaired perforation sites.
Conclusion: The site of the pulpal floor perforation affected the stress distribution and accumulation. Central and buccal furcal perforation repairs on the pulpal floor with calcium silicate-based cement in mandibular molar are likely to have an increased risk of fracture. Copyright:

INTRODUCTION

Root canal perforations form communications between the root canal system, the periodontium, and/or the oral cavity. They are created during operative procedures leading to iatrogenic complications or as a consequence of pathologic aberrations. Perforations that are iatrogenic in origin usually transpire due to misaligned instruments used during access cavity preparations, root canal negotiation, cleaning and shaping, or post space preparations. Cervical perforations and those of relatively larger sizes can more easily contaminate the root canal system leading to bacterial colonization. Bone resorption at the perforated site occurs as a sequel of the inflammatory process. This, further, negatively influences the prognosis of the tooth biologically as well as mechanically.[1]

A successful treatment outcome of teeth with repaired perforations relies on numerous factors such as perforation size and location, time of occurrence, duration of exposure to the oral environment, and the repair material used.[234] Biomechanical response of the tooth to functional loads is one other such factor. Due to the structural loss at the site of perforation, the stress distribution within the tooth may differ from an intact tooth even after perforation repair.[5]

In addition, the operator can control the choice of the repair material. The advent of bioactive materials such as mineral trioxide aggregate (MTA) and biodentine has been considered to elevate the success rate in such cases. Biodentine is calcium silicate-based cement. It is indicated to be used as a dentin substitute in both coronal and radicular regions of the tooth. It has a compressive strength of 304 MPa, which is better than that of MTA and is close to that of human dentine.[5]

Predicting the biomechanical response of teeth with perforation repairs at different locations on the pulpal floor is important as it would lead to a predictable prognosis of the tooth. Hence, this study aimed to assess and compare the effect of pulpal floor perforation repair of different sites (central, buccal, lingual sites in the furcal region, and mesial and distal sites) on the biomechanical response of a mandibular molar through 3-dimensional finite element analysis (3D FEA). The null hypothesis of the study was that the biomechanical behavior of such teeth does not change in accordance with the site of perforation repair.

MATERIALS AND METHODS

A conventional access cavity was prepared on an extracted mandibular left molar. The working length was established at 0.5 mm from the anatomic root apex using a #10 K file (Dentsply Maillefer, Ballaigues, Switzerland). Cleaning and shaping of the root canals were done to size 25.,06 taper and size 40.,06 taper for mesial and distal canals, respectively, using Protaper Next rotary files (Dentsply Maillefer, Ballaigues, Switzerland). The tooth was then subjected to a cone-beam computed tomography (CS 9300, Carestream Dental, India) scan. Using these cone-beam computed tomographic (CBCT) images; a 3D geometric model was constructed employing modeling software (Materialise Mimics, Version 10.01, Leuven, Belgium). The total length of the tooth model was 21 mm with crown and root length measuring 10 mm and 11 mm, respectively. The root trunk type was Type-A.[6]

This basic geometric model of the mandibular first molar (with access cavity, root canal preparation, and without perforations) was further modified into five different models based on the site of perforation on the pulpal floor.

Group 1 - Model without perforation (Control)

Group 2 – Central furcal perforation;

Group 3 –Buccal furcal perforation;

Group 4 – Lingual furcal perforation;

Group 5 – Mesial perforation and

Group 6 – Distal perforation

All the perforations were 2 mm in diameter. A root canal filled model (RCF) was constructed using the basic model. The root canals were simulated to be filled with gutta-percha extending from 1 mm short of the cementoenamel junction (CEJ) apically to the predetermined working length. The access cavity was simulated to be restored with composite resin (Prodigy Condensable, Packable hybrid composite resin, Sds Kerr, Orange, CA, USA). Using this RCF model, the five models with perforations were simulated to be repaired with Biodentine™ (Biodentine, Septodont, Saint Maur Des Fossés, France).

The geometric models were meshed and contained tetrahedral quadratic elements. All the simulated tissues and materials were assumed to be homogeneous, isotropic, and linear. The material properties (moduli of elasticity and Poisson's ratio) of the tissues and dental materials being simulated were retrieved from the literature.[1] The normal occlusal loads were simulated by applying a force of 200N. The loads were applied at the occlusal contact points, namely, the central fossa, distal marginal ridge, mesiobuccal, distobuccal, and distal cusp tips of all the experimental models. An oblique force, angled at 45° to the occlusal plane and oriented toward the buccal side, was applied. Using the Ansys FEA software (version 15.0) a static linear FEA was performed. Tensile stresses were evaluated in terms of the maximum principal stress values (Pmax). For better visualization of the stress distribution within the models, the numerical data was converted into color graphics [Figure 1].

Figure 1

Three-dimensional geometric model with perforations; (a) Group 2 – Central Furcal perforation; (b) Group 3 – Buccal furcal perforation; (c) Group 4 – Lingual furcal perforation; (d) Group 5 – Mesial perforation; and (e) Group 6 – Distal perforation

Statistical analysis used

The data were evaluated using the independent t-test to determine P in relation to increase in stress concentration between the control group and the repaired sites. The level of significance was set at P = 0.05. Statistical Package for the Social Sciences software (SPSS), version 20 (SPSS, version 20, IBM, Armonk, New York, USA) was used for the statistical analysis.

RESULTS

All models in the perforated models with repair exhibited higher stress values than their equivalent sites in the control group (Group 1). The mean and standard deviation of the control and repaired groups are depicted in Table 1 (P = 0.040). The Pmax values of the repaired models are depicted in Table 2. Based on these observations, the Pmax values of the repaired models, from high to low, were: Central Furcal perforation > Buccal Furcal perforation > Distal perforation > Mesial perforation > Lingual Furcal perforation. The stress accumulation and distribution within the models among the perforated groups that were repaired with calcium silicate cement were highest in the model with central furcal perforation (Group 2). However, there was no statistically significant difference in the stress accumulation among the different repaired perforation sites [Table 1].

Table 1

Group statistics and Independent samples test results

Group statistics
Group		Mean	n	SD	SEM
Control		0.6660	5	0.97728	0.43705
Repaired perforations		1.448	5	1.00480	0.44936
Independent samples test
	F	Significant	t	Significant (two-tailed)	95% CI of the difference
					Lower	Upper
Equal variances assumed	0.086	0.777	−1.248	0.247	2.22752	0.66352

*Significant at 5% level of significance. Mean and SD of the control group was 0.066±0.977 and that of the repaired perforations was 1.44±1.004 (P=0.040); with an improvement of 1.374±0.027. No statistically significant difference among repaired groups based on the site/location of the perforation repair. SD: Standard deviation, SEM: Standard error mean, CI: Confidence interval

Table 2

The maximum principal stresses stress values of the control and repaired perforation models

Location of perforation	Maximum principal stresses (MPa)
	Control (intact model)	Repaired perforations
Central furcal perforation	0.06	1.97
Buccal furcal perforation	0.50	1.56
Lingual furcal perforation	0.20	0.45
Mesial perforation	0.18	0.47
Distal perforation	2.39	1.31

DISCUSSION

The key factor for the increased brittleness and higher occurrence of fractures in endodontically treated teeth appears to be the compromised structural integrity following the loss of tooth structure.[7] An increased cuspal deflection occurs during function further elevating the likelihood of fracture.[8] Iatrogenically created furcal perforations lead to loss of tooth structure in the region of the pericervical dentin, which is vital for the transmission of functional stresses from the crown to the apex via the CEJ. This may alter stress distributions within such teeth during function.[1]

Inadequate literature on the pattern of stress distribution in mandibular molars with perforation repairs at different sites has urged the undertaking of the present study. This study evaluated the effects of perforation repair at different sites on the pulpal floor on the biomechanical response of mandibular first molar repaired with calcium silicate-based cement.

The extracted permanent mandibular molar was subjected to a CBCT scan after the access cavity preparation, and cleaning and shaping of the root canals were done. The CBCT images thus obtained were used to construct the FEA models in the present study. Thus, anatomically accurate models could be constructed as the accuracy and reliability of the results of FEA studies depend on the creation of precise and detailed FEA models.[1]

Since dentin is more vulnerable to tensile forces, the maximum principal stresses (Pmax) were evaluated.[9] The occlusal contact points on the crown were subjected to a load of 200 N to simulate normal occlusal forces.[10]

According to the results obtained, perforated models with repair exhibited higher stress values than their equivalent sites in the control group [Table 2]. This could be ascribed to the difference in the moduli of elasticity of dentine and biodentine which may have influenced the stress distribution and its concentration at the tooth-restoration interface.[11] In addition, it may get pronounced with an increase in the size of the defect. It has been reported that the amount of the residual dental tissue influences the level of residual stress concentration, possibly explaining the occurrence of high-stress concentration in the repaired perforation sites.[12] Among the sites of perforation repair, the central furcal perforation (Group 2) and the buccal furcal perforation (Group 3) showed higher tensile stress concentration [Table 2]. The inner dentine is more porous, more hydrated, and less mineralized compared to the outer dentine. The micromovement of the free water present in this region facilitates a homogenous lateral strain transfer within the bulk of the dentine. The loss of free water from the inner dentine after endodontic therapy can lead to alterations in the stress distribution. This would be higher in case of a central furcal perforation due to its anatomic location and inherent dentine structure variation.[13] Furthermore, on the application of the obliquely directed occlusal load, the stresses increase in an occluso-apical direction referred to as the moment-effect. Since the central furcal perforation leads to tooth loss at a more apical level compared to the other perforation sites [Figure 2], it presented with high-stress values.[14]

Figure 2

Stress distribution patterns in experimental groups and in the corresponding sites in the control groups: (a and b) Group 1 and 2; (c and d) Group 1 and 3; (e and f) Group 1 and 4; (g and h) Group 1 and 5; (i and j) Group 1 and 6, respectively. (To be printed in color)

An oblique force, angled at 45° to the occlusal plane and oriented toward the buccal side was applied to simulate the occlusal load. The increase in stresses at the buccal furcal perforation site could be due to the direction of the force application on the selected occlusal contact points. Calcium silicate-based cements such as biodentine are the most sought-after perforation repair materials. Yet, biodentine seems to be affected by tensile stresses at certain sites of the tooth. The biomechanical behavior of teeth changed with the site of perforation repair. Hence, the null hypothesis is rejected.

The clinical extrapolation of the present study indicates that the increased stresses at the repaired central and buccal perforation sites on the pulpal floor may be associated with a higher risk of fracture when compared with the other sites in an endodontically treated tooth.

FEA is a numerical technique used for evaluating and analyzing stress distribution patterns.[1516] The samples in laboratory fracture resistance tests are not standardized due to the variation in the anatomy of extracted teeth.[316] Conversely, FEA devises comparable standardized 3D models allowing visual assessment of stress distribution. Therefore, it can serve as an optimal tool to estimate the biomechanics of a tooth under simulated occlusal stresses. However, this study has certain limitations. Since the tooth employed in this study had anatomic morphology as reported in the majority of the population, one can expect alterations in the stress distribution pattern in teeth presenting with atypical anatomy. In addition, dentin is an anisotropic material with varying compositions and properties at different sites of the tooth structure. Hence, considering it to be homogeneous and isotropic in nature is anecdotal.

The findings of this study conferred that the presence of perforation at different sites on the pulpal floor after repair affected the stress accumulation and distribution in mandibular molars. A direct correlation between the FEA results and clinical outcome may be debatable as the treatment outcome is also affected by various other patient-related and operator-related factors. The present results only provide qualitative theoretical data and hence are to be evaluated with caution for clinical decision-making in teeth with perforations and their subsequent repair.

However, an important observation from the present study was the change in the biomechanical behavior and altered tensile stress concentration based on the site of pulpal floor perforation repairs.

CONCLUSION

Acknowledging the limitations of the study, it can be concluded that:

The site of pulp floor perforation affected the stress distribution and accumulation within the models and

Mandibular molar teeth repaired with calcium silicate cements in central and buccal furcal perforation are likely to have an increased risk of fracture compared to other perforation sites on the pulpal floor.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.