Biomechanical Properties of Maxillary Second Molar with Different Endodontic Cavity Designs - A Finite Element Analysis

ABSTRACT

Aim:

To evaluate the biomechanical properties of maxillary second molars with three different cavity designs – Traditional, Conservative, and Extended – endodontic cavities using the finite element analysis method.

Materials and Methods:

Three finite element models of a maxillary second molar with three different types of endodontic cavities were designed and restored. Each model was subjected to three different force loads directed at the occlusal surface. The stress distribution patterns and the maximum von Mises (VM) stresses were calculated and compared.

Results:

Vertical force of multipoint load on the occlusal surface and lateral forces to the palatal cusp showed the maximum stress values in the extensive cavity design, followed by the traditional cavity design and then the conservative cavity design.

Conclusion:

The VM stress distribution in the conservative endodontic cavity was minimal when compared to other access cavity designs.

INTRODUCTION

Endodontics has undergone tremendous advancements over the past few decades with the aid of newly emerging techniques, materials, and instruments. Endodontic therapy comprises access cavity preparation, cleaning, shaping, debridement, disinfection, and obturation of the root canal system.[1] Access cavity preparation is the most important and challenging step during the endodontic treatment procedure, with a prime objective of better visualization, debridement, and instrumentation, though the treatment outcome is to obtain a perfect seal coronally and apically.[2] Endodontically treated teeth have a compromised fracture resistance due to the removal of a large amount of tooth structure, rendering the teeth more susceptible to fracture.[3]

Structural integrity is an important factor that determines the biomechanical properties of an endodontically treated tooth. Preservation of the maximum tooth structure is essential to optimize its biomechanical behavior.[4] Earlier studies emphasized the importance of preserving peri-cervical dentin and insisted that loss of the marginal ridge or dental cusp mainly affected the biomechanics of the endodontically treated teeth.

In an attempt to understand the biomechanics of endodontically treated teeth regarding the endodontic cavity design, knowledge and understanding of the effects of different endodontic cavities designs are important. Three-dimensional (3D) finite element analysis (FEA) is a promising method to investigate the dental biomechanical process. It can be used to determine the distribution of stress when the tooth structure is subjected to force using computer-generated models acquired from cone-beam computed tomography (CBCT). This study evaluates and compares the biomechanical properties of maxillary second molars with different endodontic cavities like Conservative, Traditional, and Extensive endodontic cavity designs using the FEA method.

MATERIALS AND METHODS

An intact, non-carious, mature second maxillary molar was scanned using the Orthophos SL 3D high-resolution CBCT (Dentsply Sirona, Sirona Dental Systems GmbH, Bensheim, Germany). The scanning parameters were 60–90 k and 3–16 mA. An interactive medical image control system, ezdicom, was used to view the images obtained from CBCT and to identify the different hard tissues visibly. These files were then refined with reverse engineering software using Free CAD 0.19. The Free CAD 0.19, CalculiX 2.19, and LISA 8 software were used to combine the tooth structures like enamel and dentin. CalculiX and Lisa 8 were used to build the finite element models for post-processing, and a 3D model was obtained. Consistent with previous studies, the teeth and materials were considered homogeneous, linear, elastic, and isotropic.[5] The endodontic access cavities were then designed on the 3D model with the same software.

Cavity design

Totally three 3D models were obtained, and one model was allotted for each endodontic cavity design: the Traditional, the Conservative, and the Extended endodontic cavities. In the Traditional Endodontic cavity (TEC) model, the entire roof of the pulp chamber was removed with a part of the cervical dentin to provide straight-line access to the middle third of the root canals. In the Conservative Endodontic Cavity (CEC) model, minimal tooth structure was removed to provide a path for the endodontic instrument to enter into each root canal orifice, thereby preserving the tooth structure, the oblique ridge, and the pulp chamber roof to a great extent. The Extended Endodontic Cavity (ECE) was designed based on the traditional cavity, in which a remaining dentin thickness of 2.0 mm was left intact. [Figure 1a-c] All the 3D models’ mesh was incorporated, and then the root canals were filled with gutta-percha up to 2 mm below the canal orifice. The canal entrance was filled with flowable composite resin, and the cavity was restored with composite resin. Properties (elastic modulus and Poisson ratio) were assigned for each corresponding material and tooth structure [Table 1].

Figure 1.

(a) Conservative Endodontic Cavity; (b) Traditional Endodontic; (c) Extensive Endodontic Cavity; (d) Vertical force of 250 N was applied to the central groove area of the model; (e) The force of 800 N was applied to five points; (f) A force of 225 N was applied to the palatal plane of the palatal cusp at 45⁰ to the longitudinal axis of the model

Table 1.

Properties (elastic modulus and Poisson ratio) assigned for each material and tooth structure

	Elastic modulus (E; GPa)	Poisson ratio (m)
Enamel	84.1	0.002
Dentin	18.6	0.31
Gutta-percha	0.00069	0.45
Composite resin	12	0.3
Flowable composite resin	5.1	0.27

The Loading Process

1. A vertical occlusal force at a constant intensity of 250 N was applied to the central groove area of the model to simulate a normal vertical mastication load. [Figure 1d]

2. A total force of 800 N was applied to five different points on the occlusal surface of the model to simulate the maximum mastication force. [Figure 1e]

3. A total force of 225 N was applied to simulate the lateral mastication load. The force was applied on the palatal plane of the palatal cusp at 45 degrees to the longitudinal axis of the tooth. [Figure 1f].

RESULTS

On application of the vertical force of load (250 N) at one point on the occlusal surface, the peak von Mises (VM) stress occurred at the central groove in all models and was closer to the cavity margins [Figure 2A1-C1]. Under the vertical force of 800 N at the multipoint on the occlusal surface, the peak VM stress in all the models occurred at the mesial marginal ridge. [Figure 2A2-C2] and the palatal cusp [Figure 2A3-C3]. The force of 225 N was applied on the palatal cusp at 45 degrees to the longitudinal axis of the tooth. The stress on the peri-cervical dentin in models A, B, and C shows an increase in stress with the gradual enlargement of the access cavities [Figure 3a-c] [Table 2].

Figure 2.

(A1-C1) The distribution of VM stress on the occlusal surfaces of the A, B, C models, respectively, under a one-point vertical force of 250 N. (A2-C2) The peak VM stress on the mesial marginal ridge of the A, B, and C models, respectively, under a total multipoint vertical force of 800 N. C: (A3- C3) The peak VM stress on the palatal cusp of the A, B, and C models, respectively, under a total multipoint vertical force of 800 N

Figure 3.

The distribution of VM stress on the peri-cervical dentin of the (a-c) models when a force of 225 N was applied on the palatal cusp at 45⁰ to the longitudinal axis of the tooth

Table 2.

The maximum von Mises (VM) stresses calculated and compared for different cavity designs

Cavity design	259 N Vertical force at the central groove	800 N Vertical force at five points on the occlusal surface	225 N at 450 on the palatal cusp
Conservative endodontic cavity model (A)	205.10 MPa	320.63 MPa	92.86 MPa
Traditional endodontic cavity model (B)	204.89 MPa	426.77MPa	96.09 MPa
The Extended endodontic cavity model (C)	153.58 MPa	585.27 MPa	101.95 MPa

DISCUSSION

The longevity of the endodontically treated tooth may be compromised by lowered fracture resistance due to extensive loss of hard tissue, which could weaken the rigidity of the tooth.[3] The study evaluates and compares the biomechanical properties and stress distribution of maxillary second molars with different endodontic cavities using the FEA method.

The TEC design for each tooth has remained unchanged for decades, with very minimal modifications done to date. In TEC preparation, no compromise is made to create straight-line access to the apex, which prevents endodontic iatrogenic procedural errors like incomplete cleaning and shaping, ledge formation, and instrument separation.[6] It involves the removal of the entire roof of the pulp chamber and a part of the cervical dentin to provide straight-line access to the middle third of root canals, which leads to the destruction of healthy tooth structure and peri-cervical dentin.

Studies have suggested that preservation of a higher volume of coronal tooth structure and preservation of peri-cervical dentine are essential to improving the fracture resistance of endodontically treated teeth.[4] Clark and Khademi modified the endodontic access cavity design to minimize the tooth structure removal, and it was known as the Conservative Endodontic access cavity. CEC focuses on minimizing tooth structure removal by providing a path for the endodontic instrument to enter into each root canal orifice, thereby preserving the tooth structure, the oblique ridge, the pulp chamber roof, and peri-cervical dentine to a great extent. The peri-cervical dentin, which is a prime factor in the distribution of functional stresses on teeth, is located 4 mm above and 4 mm below the crestal bone.[7] The peri-cervical region is a zone critical for transferring load from the occlusal table to the root, which signifies that preserving pre-cervical dentin may increase the tooth’s resistance to coronal fracture.[4]

The objectives of access cavity preparation are controversial. Many factors determine the access cavity design, such as the outline form, the convenience form, the removal of the carious dentin, the toilet of the cavity, and the preservation of the coronal tooth structure.[4] The conservative access may increase the risks of inefficient canal instrumentation and the occurrence of procedural errors, whereas the traditional and extensive cavity always provides a convenient form for a better approach to the canal.[8] The extensive endodontic cavity can be justified by Black’s concept of ‘extension for prevention, which promotes the sacrifice of additional tooth structure to prevent iatrogenic complications and to best achieve the ultimate goals.[2]

3D FEA is used to determine the distribution of stress when a structure is subjected to force using computer-generated models utilizing data from CBCT. It is based on the principle of dividing a structure into a finite number of small elements. It is useful for specifying the mechanical aspects of biomaterials and human tissues that cannot be measured in vivo. It has various advantages, such as it can be compared with studies on real models, and the tests are repeatable, with accuracy, and without ethical concerns.[9]. With this technology, previous studies have investigated the stress distributions in the canals of endodontically treated teeth but have yet to consider the impact of such stresses based on the size of the access cavity in the maxillary second molar. This study evaluates and compares the biomechanical properties and stress distribution of maxillary second molars with different endodontic cavities using the FEA method with the application of three different types of loading processes.

The force of 250 N loaded on the central groove elicited the relationship between the stress distribution and the position of the force load. Cavity margins closer to the force load experienced the greatest stress. The CEC model had its cavity margin very close to the load point, and the peak stress observed was 205.10 MPa; this value was higher than the stress recorded in the TEC value of 205.10 MPa and the EEC model stress value of 153.58 MPa. This result was similar to the study by Jiang et al., where different cavity designs on the maxillary first molar were evaluated.[5] However, the study result is not in accordance with the study by Moore et al. and Yuan et al., who concluded that conservative cavity design had no advantage over traditional cavity design as it did not improve fracture resistance.[10,11]

To simulate the maximum masticatory force, such as chewing action, a force of 800 N at 5 different sites on the models was applied. The peak VM stress in all three models occurred at the mesial marginal ridge and the palatal cusp. The peak VM stress was significantly highest in the EEC model at 585.27 MPa, followed by the TEC model at 426.77 MPa, and the CEC model at 320.60 MPa. The study result was in accordance with the study of Plotino et al., where the fracture strength of endodontically treated teeth with various access cavity designs was concluded that TEC designs showed lower fracture strength than the CEC designs.[12]

The force of 225 N was applied on the palatal plane of the palatal cusp at 45 degrees to the longitudinal axis of the tooth to simulate lateral masticatory forces. The peak VM stress in the palatal cusp region was significantly higher in the EEC model 102.95 MPa than in the TEC model 96.09 MPa and the CEC model 92.86 MPa. From the observed data, it was evident that protecting the mesial marginal ridge, the oblique ridge, and the conservation of enamel are very important in increasing fracture strength after endodontic therapy. This may be attributable to the fact that enamel with a higher elastic modulus can resist elastic deformation and accommodate greater stress internally.[5] With the larger access cavity design, the composite resin material used for the restoration is more. The Composite resin has an Elastic modulus lesser than the dentine, so with the distortion of the resin, the stress on the underlying dentine would have increased.[13]

The stress on the peri-cervical dentin showed an increasing trend with the gradual enlargement of the access cavity size [Figure 3]. Gaining access to the root canal for convenience and better instrumentation, cleaning, and shaping is one of the prime requirements of an access cavity. In conservative preparation, there may be a compromise in the instrumentation efficacy, which plays an important factor in a successful endodontic therapy.

CONCLUSION

Within the limitations of this study, there was not much variation in the VM stress value on the occlusal surface among the three endodontic cavity designs. However, the conservative cavity model, which preserved more amount of tooth structure, demonstrated better fracture resistance.

As the cavity dimension increased, the peri-cervical region demonstrated more stress value. Therefore, it can be concluded that the conservative endodontic cavity design could reduce stress distribution, especially in the cervical area.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.