|Year : 2023 | Volume
| Issue : 1 | Page : 18
Factorial analysis of variables affecting bone stress adjacent to mini-implants used for molar distalization by direct anchorage—A finite element study
Veena Kovuru1, Kaladhar R Aileni1, Jaya P Mallepally1, K Siva Kumar1, Swathi Sursala2, Vattipalli Pramod1
1 Department of Orthodontics and Dentofacial Orthopedics, SVS Institute of Dental Sciences, Appanapally, Yenugonda, Mahabubnagar, Telangana, India
2 Private Practitioner, Hyderabad, Telangana, India
|Date of Submission||25-Aug-2022|
|Date of Decision||25-Oct-2022|
|Date of Acceptance||10-Nov-2022|
|Date of Web Publication||18-Mar-2023|
SVS Institute of Dental Sciences, Appanapally, Yenugonda, Mahabubnagar, Telangana
Source of Support: None, Conflict of Interest: None
OBJECTIVE: The aim of this study was to investigate the stresses on mini-implant, cortical bone, and cancellous bone for maxillary molar distalization using an orthodontic implant in a finite element model for different angulations and depths of insertion.
METHODS: A three-dimensional finite element method was used to simulate overall orthodontic tooth movements by using ANSYS software. The maxillary bone and the molars were reproduced using CT scan images and conversion of the same into STL file was done. Finite element model was generated and the effect of forces was studied on the model for different depths and angulations of mini-implant insertions. The distalization force was exerted by an open-coil spring and the direct skeletal anchorage was provided by a mini-implant. Mini-implants were placed in depths of 5 mm, 7 mm, and 9 mm inside the bone and insertion angles of 30°, 60°, and 90°. Stresses on mini-implant and extent of stress on the surrounding bone were assessed by the software.
RESULTS: 1. Least stress was found when the mini-implant was inserted at an angle of 30°, as it is nearer to the stronger cortical bone. 2. As the length of the mini-implant increases, accompanied by the increase in the depth of insertion, a decrease in stress in the mini-implant, cortical bone, and cancellous bone was noticed.
CONCLUSION: An increase in the insertion angle from 30° to 90° increases the stresses on both the implant and the cortical bone. A higher depth of thread in the bone helps in reducing the stress on the implant, cortical bone, and cancellous bone. This helps in improving the primary stability of the mini-implant and its life.
Keywords: Angulations of the mini-implant, depth of min implant, FEM, orthodontic mini-implant
|How to cite this article:|
Kovuru V, Aileni KR, Mallepally JP, Kumar K S, Sursala S, Pramod V. Factorial analysis of variables affecting bone stress adjacent to mini-implants used for molar distalization by direct anchorage—A finite element study. J Orthodont Sci 2023;12:18
|How to cite this URL:|
Kovuru V, Aileni KR, Mallepally JP, Kumar K S, Sursala S, Pramod V. Factorial analysis of variables affecting bone stress adjacent to mini-implants used for molar distalization by direct anchorage—A finite element study. J Orthodont Sci [serial online] 2023 [cited 2023 Mar 25];12:18. Available from: https://www.jorthodsci.org/text.asp?2023/12/1/18/371974
| Introduction|| |
Anchorage during orthodontic treatment is a major challenge for an orthodontist. Traditionally, the principles of mechanics in orthodontic treatment are based on the anchorage provided by the surrounding dental units from which reactive forces may result in anchorage loss.
Orthodontic implants have become broadly accepted as alternatives to extraoral devices in patients who either have insufficient dental support suitable for orthodontic anchorage or are not compliant with wearing extraoral devices. Compared with traditional anchorage reinforcements such as transpalatal arches and extraoral appliances, mini-implants are advantageous because of their smaller size, convenient insertion and removal procedures, relatively low cost, and the fact that immediate orthodontic loading is possible.
A practical issue with the mini-implants would be its loosening, which can compromise the success of the procedure. The failure rate of mini-implant for orthodontic anchorage is reportedly up to 30%. Research studies have investigated biological and mechanical factors, other than infection, that might be related to the failure of mini-implants. These factors include orthodontic force level, site of implantation, cortical bone thickness, and patient-dependent oral conditions. Although a mini-implant is used as a temporary anchorage device, its primary stability is crucial to the long-term success of the associated treatment. Because of limited interdental space, the mini-implant can be close to the root, and orthodontic forces can be transmitted directly to the alveolar bone via the mini-implant. Moreover, the development of a stress field incorporating the alveolar bone around the mini-implant is reportedly correlated with mini-implant failure.
Some relevant factors that may affect the primary stability of orthodontic mini-implants (OMIs) are bone quality, implant design, insertion method, cortical bone thickness, and insertion angle. Studies have reported that mini-implants that are inclined in the bone surface provide greater contact with the cortical bone, resulting in increased mechanical retention and stability of the implant.,
The finite element method is a computational system for continuum mechanics that estimates the deformation (fully detailed changes of the position of all component particles) that are expected to result from a specified pattern of stresses (forces) upon a mechanical system.
The aim of this study was therefore to investigate the stresses on mini-implant, cortical bone, and cancellous bone for maxillary molar distalization using an orthodontic implant in a finite element model for different angulations and depths of insertion.
Model design and forces
The FEM model was designed according to the study done by Lu et al. The distalization force was exerted by open-coil spring and the direct skeletal anchorage was provided by a mini-implant. Brackets were placed on all teeth except the second premolar. A 0.019 × 0.025-inch SS arch wire was engaged. To distalize the first molar, 150 g of force was applied by open-coil spring between the first premolar and the first molar. To prevent labial movement of anterior teeth, a force of 150 g was applied between a virtual position of a mini-implant (8 mm apical to arch wire and 2 mm lateral to the alveolar bone surface) and a 0.8-mm SS retraction hook attached to the arch wire between the lateral incisor and canine [Figure 1] and [Figure 2]. The implants were placed in the interdental space between the second premolar and the first molar in the maxilla as suggested by Chen et al. and Ishii et al.
| Materials and Methods|| |
In this study, the CT scan (DICOM format data) of the maxilla and the maxillary dentition in the axial plane was taken. The cut sections were taken starting from 1 mm from the apex and moving incisally at a thickness of 1 mm. The processing was done using MIMICS (Materialize's Interactive Medical Image Control System) software and then maxilla was exported to STL (Stereolithography) format. This STL format is imported into rapid form software to create the surface data. Surface data are converted to IGES (Initial Graphic Exchange Specification) and is exported to HYPER MESH. Approved by ethical committe & form uploaded Date of approval 05-12-2017.
Steps involved in the finite element model
- Construction of the geometric model
- Conversion of the geometric model to a finite element model
- Material property of the data representation
- Defining the boundary condition
- Loading configuration
- Interpretation of results.
Utilizing a mock approach, the mini-implant was placed between the root of the second premolar and first molar teeth into the bone model. The suitable site of arch wire placement was assumed in measuring the position of the implant. Three different angles of implant insertions were considered: 30°, 60°, and 90°. Three different depths of insertion 5 mm, 7 mm, and 9 mm were considered. The area of the simulation was the interdental region between the maxillary second premolar and first permanent molar. The placement of the mini-screw was 5 mm gingival from the intercrestal bone level between the two teeth [Figure 3].
| Results|| |
The analysis was done to check the Von Mises stresses on the implant, cortical bone, and cancellous bone while performing direct distalization of the maxillary first molar with a force of 150 gm/side. The Von Mises stresses on the implant, cortical bone, and cancellous bone for different angulations and depths were calculated and the results were tabulated.
Stress values for different angulations of insertion of mini-implant
The different inclinations of implant insertions give a significant influence on the stress distribution and its magnitude within bones and implant. Within all implant inclinations, the pattern of stress seemed to be approximately similar to each other [Figure 4], [Figure 5], [Figure 6], [Figure 10].
|Figure 10: Graphical representation showing the effect of inclination on the stress generation|
Click here to view
The stress was more concentrated at the neck and first thread of the implant in the mesial direction [Table 1]. The results obtained were; for a depth of 5 mm insertion the stress on the implant was found to be 1.02294 MPa, for a depth of 7 mm insertion the stress on the implant was found to be 0.67637 MPa, and for a depth of 9 mm insertion the stress on the implant was least at 0.512657 MPa. With the increase in the length of the mini-implant and the subsequent increase in depth of insertion, the Von Mises stress values reduced gradually [Table 2]. For a depth of 5 mm insertion the stress on the cortical bone is 0.101577 MPa, for a depth of 7 mm the stress was 0.071402 MPa, and for a depth of 9 mm insertion the stress on the implant was found to be 0.054146 MPa. The values of Von Mises stress in the cortical bone also decreased gradually as the depth of insertion of the mini-implant increased. For a depth of 5 mm insertion the stress on the cancellous bone was found to be 0.002199 MPa, for a depth of 7 mm insertion the stress was found to be 0.001572 MPa, and for a depth of 9 mm insertion the stress was found to be 0.001276 MPa. The values of Von Mises stress in the cancellous bone also decreased gradually as the depth of the mini-implant increased [Figure 7], [Figure 8], [Figure 9],[Figure 11].
|Figure 11: Graphical representation showing the effect of depth of implant on the bone|
Click here to view
This clearly shows that with the increase in length of the mini-implant inside the bone, the stability of the implant increases with reduced stress.
| Discussion|| |
Mini-implant failure and loosening rates for orthodontic tooth movement range from 6.9 to 28.0%, and their success depends on several factors including the magnitude and direction of the applied force; operator experience; insertion site; quality of cortical bone; surface contact area in cortical bone; length, depth, diameter, thread configuration, the shape of the mini-implant; and patient's age.
The study conducted shows that the stress is concentrated at the neck of the mini-implant with Maximum stress of 0.000702 MPa observed at the implant cancellous interface. The maximum stress is localized due to implant and cancellous geometrical interface due to stress concentration. The least stress in the mini-implant is noted for 30° inclination as it is nearer to the stronger cortical bone, ensuring maximum support available for load transfer. Laursen et al. in their study found a similar finding that changing the insertion angle from 90° to 45° increased the cortical bone-to-implant contact by an average of 47% and ensured increased primary stability of the mini-implant. Kravitz et al. said that the major problem associated with angulated insertion is that it does not allow complete insertion of the OMI threads into the bone, leading to a larger lever arm formed off-the-bone, negatively contributing to primary and secondary stability, as well as increasing the possibility of forming niches where food may accumulate. For all the different depths of insertion, there was greater cortical bone deformation when compared to the cancellous bone. But the greatest strain was located on the mini-implant. Tension on the mini-implant was located in its cervical region. It was noticed that as the depth of the implant increased, the Von Mises stress values decreased on all the parameters assessed. This indicated that the greater the implant length and the accompanied depth, the greater is its primary stability. Chiatzigianni et al. found a similar result in their study and concluded that both increased diameter and length of the orthodontic mini-implant resulted in decreased mini-screw mobility. Lin et al. compared exposure length, insertion angle, and direction of the force on cortical bone stresses. The results showed that exposure length has the highest influence (82.38%) followed by insertion angle (6.03%). It was found that the orthodontic stress was mainly borne by the cortical bone.
They also found that the primary stability of mini-implants is positively correlated with the quality and thickness of the cortical bone at the insertion site. The study parameters assessed were positively correlated with the factors which increased the contact area with the cortical and increased depth of OMI in the cancellous bone. The closer the screw head is to the attached mucosa, the less destructive will be the applied load.,
| Conclusions|| |
- As the angle of insertion increases from 30° to 90°, the stresses on both the implant and the cortical bone increased.
- Depth of insertion has a significant effect on the stresses generated.
- As the length of the mini-implant increases accompanied by the increase in the depth of insertion, the stresses in the mini-implant, cortical bone, and cancellous bone decrease.
- A higher depth of thread in the bone helps in reducing implant stress, cortical stress, and cancellous stress. Finally, this helps in improving the life and stability of the implant.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11]
[Table 1], [Table 2]