Comparative analysis of the biomechanical behavior of the maxillary central incisors restored with glass fiber post and cast metal post and core submitted to orthodontic forces: A study with finite elements

Introduction

Different types of intraradicular restorations and their insertion have an impact on teeth biomechanics. This study aimed to analyze the biomechanical behavior of maxillary central incisors restored with glass fiber post (GFP) and cast metal post and core (CMP) subjected to buccolingual and mesiodistal orthodontic forces using the finite element method.

Methods

Two models of the maxillary central incisor with periodontal ligament, cortical bone, and trabecular bone were made. One of the models included intraradicular restoration with GFP, whereas, in the other, the incisor was restored with CMP. After creating the tridimensional mesh of finite elements, applying 2 orthodontic forces were simulated: 65 g of buccolingual force and 70 g of mesiodistal force. The forces were applied parallel to the palatal plane in the region of the bracket slot, located 4 mm to the incisal edge.

Results

The maximum stresses generated in the GFP-restored root were 3.642 × 10 ⁻¹ MPa and 4.755 × 10 ⁻¹ MPa from the buccolingual and mesiodistal forces, respectively. Likewise, the stresses in the CMP restored root were 2.777 × 10 ⁻¹ MPa and 3.826 × 10 ⁻¹ MPa. The radicular area with higher stress on both models was located in the cervical third: on the buccal surface when the buccolingual force was applied and on the mesial surface when the mesiodistal force was applied. The highest stress levels were found on the CMP structure.

Conclusions

The incisor restored with cast metal post revealed lower stress values transferred to the root than the one restored with GFP. The stresses on the structure of the GFP were lower and more homogeneous than the ones found on the cast metal post. The difference among the stress values in the materials is within a safe margin for using both materials in relation to orthodontic forces.

Highlights

•

The post material interfered with the stress level transmitted to the root.
•

The root restored with cast metal post and core suffered less stress.
•

The glass fiber post absorbed less stress than the cast metal post and core.
•

The cervical third was the root area with higher stress, regardless of the material.

Orthodontic movement of teeth submitted to endodontic treatment has become more usual because the search for orthodontic treatment by elderly patients, who present a high level of endodontically treated teeth, has increased. Depending on the coronal destruction level, an intraradicular post to support the restoration might be necessary. The intraradicular restorations mostly used for that purpose are the cast metal post and core (CMP) and the glass fiber post (GFP).

For many years, the CMP was the first option because of its high modulus of elasticity and high resistance to fracture. The GFP, which is more flexible, has been gaining use with the increase of prosthetic crowns usage, which is free of metal, and because it is more practical, as it is prefabricated. ^,

Teeth biomechanics changes according to the intraradicular post insertion, altering the stress distribution, such as tensile stress and compression, resulting from masticatory forces. In the presence of these forces, the incisors present a higher risk of failures in intraradicular restorations when compared with posterior teeth.

Orthodontic forces also produce stress in different places in the inner portion of the root and along the periodontal ligament. ^, Researchers that compared stress distribution among different types of intraradicular posts used loads simulating the application of masticatory forces, which presented distinct magnitude and direction of the orthodontic forces. ^, ^, Although some studies pointed out that the use of materials with high levels of elasticity reduces the radicular stress, ^, others showed that a lower level of elasticity, close to the value found in the dentin, is better. ^,

The finite element method (FEM) is a digital tool that defines the stresses and deformations suffered by structures when submitted to simulated forces. Experimental methods using a physical sample instead of a computational model are ideal, as they represent the problem physiologically. However, direct experimental approaches cannot reveal the stress on the tooth root or the interface between post and dentin. That is why it is necessary to use models developed from FEM. ^,

This study analyzed, by using FEM, the biomechanical behavior of the maxillary central incisors restored with GFP and CMP when they are submitted to buccolingual and mesiodistal orthodontic forces, comparing the levels of stress and distribution mode along the root and the intraradicular posts.

Material and methods

The development of the model employed in the application of FEM was done at Renato Archer Information Technology Center. The following components were obtained from a maxillary model previously developed through BioCAD techniques ( Fig 1 ): cortical bone, trabecular bone, periodontal ligament, root, and crown of the maxillary central incisor. The long axis of the incisor was positioned at a 90° angle in relation to the palatine plane from the coronal view and 110° from the sagittal view. These components underwent a surface refinement through Rhinoceros 3D software (version 7; McNeel North America, Seattle, Wash) to correct any inaccuracies.

A cavity was opened in the maxillary central incisor to represent the root canal, and the region was prepared to receive the post. Considering the dimensions of an ideal intraradicular restoration, it was decided that a post with two-thirds of the radicular length and one-third of the root diameter would be used. ^, Zirconia was the material placed in the incisor crown, representing a prosthetic crown ( Fig 2 ). Using the model described as the basis, 2 models were created: the first restored with GFP and composite resin core, and the second restored with CMP.

A simplified bracket model was created with stainless steel mechanical properties, and the slot was positioned 4 mm from the incisal edge to finish the modeling process. This simplification was possible because the bracket is not a region of interest in the analysis. The finished models were exported to HyperMesh software to create a 3-dimensional tetrahedral finite element mesh ( Fig 3 ). From creating the mesh, a model of a restored incisor with GFP was obtained containing 340,352 elements and 509,099 nodes and a model of a restored incisor with CMP containing 351,454 elements and 523,731 nodes.

The properties of the materials used were attributed to each component. Young’s modulus value and Poisson coefficient were determined according to Table I . The materials were considered elastic, linear, and isotropic, except for the GFP, which has an orthotropic characteristic.

Table I

Mechanical properties attributed to the structures of the model

Material	Young’s modulus (GPa)	Poisson coefficient
Dentine	18.60	0.31
Zirconia	205.00	0.30
Periodontal ligament ^,	6.89 × 10 ⁻⁵	0.45
Cortical bone	13.70	0.30
Trabecular bone	1.37	0.30
Stainless steel	210.00	0.30
Composite resin	12.00	0.33
CMP (NiCr)	200.00	0.33
GFP	Ex = 37.00	Vxz = 0.34
	Ey = 9.50	Vxy = 0.27
	Ez = 9.50	Vyz = 0.27

The settings of the contacts among the components were defined, and the following surfaces were considered perfectly attached: bracket with the buccal surface of the incisor crown, incisor crown with the composite resin core, composite resin core with the GFP coronal portion, GFP with the wall of the root canal, incisor crown with CMP and CMP with the wall of the root canal.

Finally, a simulation of the application of orthodontic forces was performed with the OptiStruct software. Two forces, parallel to the palatal plane, were applied separately on fixed points in the bracket slot region ( Fig 4 ): a 65 g of buccolingual force in the lingual direction and a 70 g of mesiodistal force in the distal direction.