EVALUATION OF STRESS AND DEFORMATION ON PROSTHETIC IMPLANTS DURING MINI- PLATE ASSISTED CANINE AND EN-MASSE RETRACTION IN LINGUAL ORTHODONTICS: A FINITE ELEMENT in silico STUDY

Dr. Shamu Rapheal 1 , Dr. Rohan Mascarenhas 2 , Dr. Satish Shenoy 3 and Dr. Akhter Husain 4 . 1. Post graduate, department of orthodontics and dentofacial orthopedics, Yenepoya Dental College, Mangalore, Karnataka. 2. Professor, department of orthodontics and dentofacial orthopedics, Yenepoya Dental College, Mangalore, Karnataka. 3. Professor and head, department of aeronautical engineering, Manipal Institute of Technology, Manipal, Karnataka. 4. Professor and head, department of orthodontics and dentofacial orthopedics, Yenepoya Dental College, Mangalore, Karnataka. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History


Methodology:-
This study is conducted to determine the stress and deformational changes on implants during mini-plate assisted canine and en masse retraction in lingual orthodontics. The materials required for the study included; bit computer with windows 7, CT scan of upper anterior dentition with its dentoalveolar complex and various softwares (CHART I) Computerized Tomography scan of upper anterior dentition with dentoalveolar complex of patient were selected for the study after obtaining Institutional ethical clearance. The DICOM (Digital Imaging Communications in Medicine) Images obtained after scanning were converted to a finite element model using CATIA, MIMICS & ANSYS software. Pre Processing, where CT Scan of the patient is converted into digital images with the help of MIMICS 14.0 software. Dimensions of implant, mini-plate and minis-crews were obtained using Scanned Electron Microscopy (Fig.1). Implants, mini-plates, mini-screws and arch wires are constructed using CATIA V5 R19 software. Blue light scanner is used for scanning brackets. Upper anterior dentition including, PDL, and surrounding Bone are modeled using MIMICS software's. Brackets, implants, mini-plates and arch wires are modeled using CATIA software. Meshing of all the models is carried out using ANSYS software. Finite element method basically can be divided into 7 steps, which include; Construction of the geometric model, Conversion of geometric model to a Finite Element model, Material property data representation, Defining the boundary condition, Loading Configuration, Solving the system of linear algebraic equation and Interpretation of the results In this study, previously created model of upper anterior dentition with its dentoalveolar complex of an adult male after obtaining the ethical clearance from Yenepoya university ethical committee (YUEC 315/01/2016) CT images for the upper anterior dentition were obtained in DICOM (Digital Imaging Communications in Medicine) format with Philips CT scanner in the regime of 140KV, 800mA, 512 by 512 matrix, field of view (FOV) 14cm by 14cm and slice thickness of 0.7mm. CT images are only in a series of two dimensional images, ant it cannot be directly used for finite element modeling. The images thus obtained were imported in MIMICS software for processing. From the CT image the outlines and borders of teeth and bone required for constructing the Finite Element model was obtained using the MIMICS software and a R19 for construction of surface and solid features of 220 the geometric model. The images thus obtained from MIMICS were in stereo lithographic format (STL format). The surface models of the teeth, periodontal ligament, cortical and cancellous bone were generated individually. Brackets, Mini-plate, Implants, Mini-screws and arch wire were modeled using CATIA software. All these images were imported into CATIA V5 R19 software. These images were then refined and smoothened to produce accurate contours. The discontinuous surface was healed and joined together to form a continuous surface model. Then the surface model was converted into a solid model by volume meshing.  (TABLE I, II, III & IV). The models were meshed with higher order tetrahedral elements of element size 0.5mm. This measurement of mesh was chosen after a trial of multiple mesh sizes since it produced the best and most accurate patterns. The continuous nature of the meshes was checked by manual inspection.      The area of concern is prosthetic implant. This was modeled with good quality high number of elements to capture the result gradients accurately. The number of elements decides the quality of the results and the system used for solving. It is very difficult to have ideal shaped elements embedded in the complex non-specific geometry. Hence the entire Finite Element mesh has to meet a set of quality criteria which decides the nature of the outcome. The quality criteria included was:

Chart I:-Softwares
Element twisting:-Shape collapse:-The result of the Finite Element Analysis is directly proportional to the mesh quality. If the mesh quality is good, the convergence problems are minimal. The number of elements is directly proportional to computational time. Hence careful judgments were made while meshing the significant parts.
Material properties were assigned for different structures such as bone, teeth and periodontal ligament (TABLE  VII). The material properties used in this study was taken from experimental data from previous studies.  V) 880 0.31-0.37 In this study all the tissues are assumed to be isotropic, i.e., the properties of the material are same in all directions.
The model was restrained at the superior border of the maxilla in order to avoid any motion against the loads imposed on the dentoalveolar structures. The final model was confirmatory from an engineering point of view for this study. It is very important to keep in mind that the finite element model will give the results based upon the nature of modeling system and for that reason, the procedure for modeling is most important.
Using MIMICS software maxillary anterior dentition was modeled. Using CATIA V5 R19 software archwires, CAD/CAM bracket systems, mini-plates and implants are modeled. Then all the geometric models were imported to ANSYS software for analysis. In this study, the aim was to determine the stress related changes generated on prosthetic implants during mini-plate assisted canine and en-masse retraction in lingual orthodontics.
The sequential application of the above steps leads to a system of simultaneous algebraic equations where the nodal displacements are the unknowns.
As a last step the results obtained after processing was displayed and graphical output were made. This is a pictorial representation of the results, which gets displayed in a color-coded manner. The maximum and minimum stress and deformation in the implants were recorded. Interpretations of the various analysis were carried out at this stage. The colors represent the magnitude of stress/deformation at that area. The palate of colors along with values is depicted on the side of the images. This enables a quick interpretation of the results. The colours vary from red to blue. Positive values indicate tensile stress and negative values indicates compressive stress. Unit of Stress is MPa. (One MPa is equal to one N/mm2).

Results:-
Using CATIA V5 R19 software implants, mini-plates with two mini-screws, 7th Generation Lingual Bracket System (Anterior teeth) and Maxillary archwire were modeled. Maxillary arch with anterior dentition along with PDL were modelled using MIMICS software. The models were meshed using ANSYS software. Later all the modes were imported to ANSYS software for analysis. Loads and material properties are applied after Finite Element Analysis. It is followed by step solving, where we solve for stress and deformation.
Stress and deformation produced in implants are recorded during mini-plate assisted canine and en-masse retraction 223 at various predetermined force levels (Fig. 5-17). During mini-plate assisted canine retraction and en-masse retraction, point of force application was from hook of the mini-plate to hook of canine bracket and on the arch wire between canine and lateral incisor respectively (Fig. 4). Stress and deformation values generated for 1N, 2N and 3N forces were recorded (TABLE VII & VIII). Graphs were plotted for Force Vs Stress and Force Vs Deformation for comparing canine retraction and en-masse retraction (Fig 17 and Fig 18).

Fig. 4:-Image showing line of force during Canine retraction (Red) and en-masse retraction (Yellow)
The graphical comparisons conveyed that for every increase in the force value there is a linear progression of stress and deformation during canine and en-masse retraction. The observations made from the graphical representations showed stress on implant is higher during canine retraction than en-masse retraction. Deformation of implant is higher during mini-plate assisted canine retraction than mini-plate assisted en-masse retraction. The stress and deformation pattern generated were represented by different colours from blue to red which are expressed MPa (N/mm2) and mm respectively.

Discussion:-
One of the most important determinants in successful orthodontic treatment is anchorage control. Orthodontic anchorage is categorized into maximum, moderate and minimum anchorage, depending on the amount of movement of the anchoring unit during force application [2]. Of the three types of anchorage, maximum anchorage is usually most desirable and, at the same time, the most difficult to achieve. Absolute anchorage is the term used to describe the anchoring unit that remains unmoved under orthodontic forces. With the advent of mini-plate and other temporary anchorage devices the possibility of absolute anchorage is realized [3].
Orthodontic treatment requires optimum force to bring about maximum tooth movement with minimal damage to the root, periodontal ligament, and alveolar bone [4]. This will also bring changes in the adjacent prosthesis if present. These changes are generally given less importance as the stand away from the mainstream of treatment. Hence, the quantification and direction of force is very important during orthodontic treatment. Therefore, the stressed state of the implant needs to be studied in order to understand the effect of different forces.
In this study, stress and deformations of implants during mini-plate assisted canine and en-masse retractions were evaluated. For every increase in the force value, a linear increase in the stress and deformation values were observed. Observations of stress and deformation changes at various sites were tabulated ( Deformation of Implant during Mini-plate assisted retraction