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Effects of a newly designed orthodontic miniplate platform for elevating the miniplate over the gingiva: A 3-dimensional finite element analysis
ORIGINAL ARTICLE
Effects of a newly designed orthodontic miniplate platform for elevating the miniplate over the gingiva: A 3-dimensional finite element analysis rı Ulusoy,b and C rı Tu € zb € rko Volkan Aykac¸,a C ¸ ag ¸ ag Ankara, Turkey
Introduction: Miniplates are the treatment of choice for complex orthodontic and orthopedic problems. However, they require surgical placement and removal, and complications such as infection and mobility can occur. The aim of this finite element analysis was to investigate the effects of a newly designed miniplate platform to elevate the miniplate above the gingiva. Methods: A bone block was modeled in 3 dimensions, and 2 N of force was applied on miniplates in 2 scenarios. In scenario 1, the miniplate was fixed with 2 miniscrews on both ends; in scenario 2, miniplate platforms were first seated on the cortical bone surface with their spikes fully penetrating, and then the miniplate was fixed on top with 2 miniscrews. Results: The highest von Mises stress on the cortical bone decreased from 0.5 to 0.3 MPa when miniplate platforms were used. In scenario 2, the principal maximum stresses on the cortical bone around the miniscrews decreased from 0.42 and 0.48 MPa to 0.20 and 0.22 MPa, and the principal minimum stresses decreased from 0.45 and 0.48 MPa to 0.01 MPa. Conclusions: Miniplate platforms used to elevate the miniplate lowered the stresses generated on cortical bone around the miniscrews by distributing the stresses on the cortical bone surface. Patients can clean the miniplate more readily because it is elevated above the soft tissues. Placing the miniplate platforms requires only removing the gingiva with a punch, and their removal does not require flap surgery. (Am J Orthod Dentofacial Orthop 2015;148:110-22)
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temporary anchorage device is temporarily fixed to bone to enhance orthodontic anchorage either by supporting the teeth of the reactive unit or by obviating the need for the reactive unit altogether, and it is subsequently removed after use.1 Kanomi2 used miniscrews for mandibular incisor intrusion and inspired many researchers to give more attention to skeletal anchorage in orthodontics. Although some studies suggested that miniscrews have predictable and stable results,3,4 some others found complications such as pain,5 irritation,6 soft tissue inflammation,7 peri-implantitis, and loosening of screws.8,9 Miyawaki et al5 stated that miniscrews had a success rate of 83.9%, and the authors of most studies found success rates over 80%.5,10 If a miniscrew a
Private practice, Ankara, Turkey. Associate professor, Department of Orthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Address correspondence to: C¸a grı Ulusoy, Department of Orthodontics, Faculty of Dentistry, Gazi University, Emek St, Ankara, Turkey; e-mail, culusoy77@ yahoo.com. Submitted, July 2014; revised and accepted, February 2015. 0889-5406/$36.00 Copyright Ó 2015 by the American Association of Orthodontists. http://dx.doi.org/10.1016/j.ajodo.2015.02.024 b
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loosens, it will not regain stability and will probably need to be removed and replaced.11 Miniplates have been used for treatment of fractures; after their clinical success in orthodontic practice, they have fulfilled the need for a more stable temporary anchorage device for complex orthodontic treatments.12-15 Miniplates may provide more secure anchorage when higher forces are needed.16 Cornelis et al15 made a survey to evaluate patients' and orthodontists' perceptions of miniplates during orthodontic treatment and reported good acceptance from both groups. Miniplate failure rates have been reported to be over 7% in previous studies.5,17,18 Inflammation of the tissues around the miniplates can require miniplate removal. Choi et al17 stated that miniplates might be more reliable tools for orthodontic anchorage if the rate of complications was reduced, possibly by changing the size of the appliance. Oral hygiene is another important factor for success because prevention of inflammation of the peri-implant tissues is a critical factor for miniplate success.5,15 Different designs of platforms and spikes were introduced to increase the stability of miniscrews and miniplates, such as the washer,19 the mini-implant ring,20 and the spiky miniplate.21
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The aim of this finite element analysis study was to investigate the effects of a newly designed miniplate platform (MPP) used to elevate the miniplate above the gingiva. MATERIAL AND METHODS
Finite element analysis was used to evaluate the stress distributions in bone, miniscrews, miniplates, and MPPs. Three-dimensional (3D) models of all instruments, tested in this study, were scanned with a 3D scanner (SCAN ST; Steinbichler, Plymouth, Mich). Marc software (version 2005; MSC Software, Newport Beach, Calif) was used to construct the 3D finite element models for preprocessing and modeling. All final solid meshes were constituted by tetrahedral elements with 4 nodes using the Marc software. Finite element analysis was also performed with this software. In scenario 1, 90801 nodes and 466144 elements were used; in scenario 2, 141442 nodes and 717998 elements were used. All bone, miniscrew, miniplate, and MPP elements were assumed to be isotropic, homogeneous, and linearly elastic. The elastic properties of the materials used in this study are shown in Table I.22-24 Bone models were fixed for displacement and rotation in the x-, y-, and z-axes on 5 surfaces, except for the surface of bone with the appliance. All appliances used in this study were made of grade 5 titanium (Ti-6Al-4V). Three-dimensional finite element models were prepared, and finite element analyses were processed with an Intel Pentium D Windows 7 Pro computer (CPU 2.60 GHz, 32 GB RAM) (Intel, Santa Clara, Calif). Supporting bone models were modeled consisting of cortical and trabecular bone. The modeled block of bone was a rectangular prism 48 mm wide, 30 mm deep, and 13.5 mm high. The upper 1.5-mm slice of the block was modeled as cortical bone, and the remaining 12 mm was modeled as trabecular bone (Fig 1).21,25,26 Cylindrical miniscrews, 1.7 mm in diameter and 8 mm in length (Ortho Easy; Forestadent, Pforzheim, Germany), were used (Fig 2, A). In this study, we simulated osseointegrated miniscrews.21 For simulating such a condition, the miniscrews were fully bonded to the bone along their entire interface. Other than this, all interfaces, including bone-miniplate, miniscrewminiplate, bone-MPP, MPP-miniplate, and MPPminiscrew, were defined as face-to-face contacts. The surgical miniplate (Rita Leibinger, Neuhausen ob Eck, Germany) had a length of 21.6 mm and a height of 0.9 mm, and had 4 holes separated by 6 mm for miniscrew orientation (Fig 2, B). The neck of the
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Table I. Properties of the materials used in the study Cortical bone Trabecular bone Miniscrew MPP Miniplate
Young's modulus (MPa) 13.7 3 103 7.9 3 103 105 3 103 105 3 103 105 3 103
Poisson's ratio 0.3 0.3 0.33 0.33 0.33
miniscrew and the holes of the miniplate were modeled with a perfect adaptation. The MPP, which had 0.7-mm minispikes at the side that faced the cortical bone, was designed to elevate the miniplate above the gingiva. MPPs also increased the contact surface of the appliance with cortical bone, and the spikes of the MPPs fully penetrated into the cortical bone. This round appliance had a height of 2 mm, a diameter of 6 mm, and a hole in the middle that fit perfectly to the neck part of the miniscrew (Fig 2, C). Two scenarios were formed in our study. In scenario 1, miniplates were placed on the cortical bone and fixed with 2 miniscrews on both ends (Fig 1, A). In scenario 2, miniplates were placed over the MPP (Fig 1, B). Miniplates were therefore placed above the gingiva, whereas the lower surfaces of the MPPs were directly in contact with the cortical bone, with their spikes fully penetrating the bone, as if a gingival punch was used to remove the soft tissues before placement of the MPPs. The appliance was fixed with 2 miniscrews on both ends. The MPPs were designed to be 2 mm thick to elevate the miniplate over the gingiva. The tip of the white arrow in Figure 3, A, shows the point of force application of 2 N applied to the midpoint of the upper surface of the miniplate in a sagittal direction from left to right, parallel to the axis between the miniscrew middle points in both scenarios. This experimental setup was designed for easier evaluation and understanding of the stresses generated. Von Mises stresses were measured and evaluated for titanium appliance elements; von Mises, principal maximum (compression), and principal minimum (tension) stresses were measured for cortical and trabecular bone. The miniscrew at the reverse direction of force application was called miniscrew 1, and the other miniscrew at the direction of force application was called miniscrew 2 (Fig 1). Stresses on cortical bone around the neck of miniscrew were measured in the force application direction (1) and in the reverse direction of force application ( ) (Fig 3, C). For example, principal minimum stresses at the cortical bone around miniscrew 1 ( ) signify the principal minimum stresses generated
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Fig 1. A, Orientation and cross-section of the miniplate model with 2 fixation miniscrews on both ends (scenario 1); B, orientation and cross-section of the elevated miniplate model on 2 MPPs with 2 fixation miniscrews on both ends.
at the cortical bone around the miniscrew, which is at the reverse direction of force application. Stresses were measured from several regions. The highest value measured shows the overall highest stress on the material, and the location of this value is presented in the results for each material. Four values around the miniscrews were measured: stresses around or at the necks of miniscrews 1 and 2 at the direction of force application (1) and the reverse direction of force application ( ). Stresses generated at the outer edge and the cortical bone surface contacts were named the edge of the MPP, and stresses generated at the spike and cortical bone contacts were named spikes of the MPP. When measuring the stresses generated around the necks of the miniscrews, the mean of the 4 nodes in contact with the miniscrews at the investigated regions was evaluated, since there was not necessarily a node intersecting the axis of force application direction. Figure 3, C, shows 4 nodes selected for both the 1 and directions. This measurement style was chosen to differentiate the highest value measured overall and the highest stresses generated at comparable regions.
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Overall highest stresses can be close to the direction of force application, but they may not be right on the axis; this makes it hard to evaluate and compare.27 Figure 3, B, demonstrates how the highest stress on cortical bone around the miniscrew can be at a different location; therefore, the mean of the 4 neighboring nodes was selected to make the data comparable (Fig 3, C). The white arrow in Figure 3, D, shows the direction of force application on the close-up images. The white arrows in Figure 3, A and D, represent 0 , meaning that the closeup images were rotated by 90 in a counterclockwise direction to see the 3D effect of the stresses around the miniscrews; the setup simplified the orientation. RESULTS
In scenario 1, the highest von Mises stress was measured as 0.5 MPa at the cortical bone surrounding the neck of miniscrew 2 (Figs 4, A, and 5, A and B). The highest principal maximum value on the cortical bone was 0.6 MPa located at the connection of miniscrew 2 and the cortical bone surface at the reverse
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Fig 2. Dimensions of: A, the miniscrew; B, the miniplate; C, the MPP.
direction of force application. The principal maximum stresses on the cortical bone around the necks of miniscrews 1 and 2 were measured as 0.42 and 0.48 MPa, respectively, at the reverse direction of force application (Fig 6, A and B). The highest principal minimum value on the cortical bone was 0.6 MPa located at the connection of miniscrew 1 and the cortical bone surface at the direction of force application. The principal minimum stresses on the cortical bone around the necks of miniscrews 1 and 2 were measured as 0.48 and 0.45 MPa, respectively, at the direction of force application (Fig 7, A and B) (Table II). In scenario 2, the highest von Mises stress was measured as 0.3 MPa at the surface of the cortical bone, in contact with the outer edge of the MPP, which is located at the direction of force and fixed with miniscrew 2 (Figs 4, B, and 5, C and D). The highest principal maximum value on the cortical bone was 0.31 MPa located at the connection of miniscrew 1 and the cortical bone surface at the reverse direction of force application. The principal maximum stresses on the cortical bone around the necks of miniscrews 1 and 2 were measured as 0.22 and 0.20 MPa, respectively, at the reverse direction of force application (Fig 6, C and D).
The highest principal minimum value on the cortical bone was 0.34 MPa at the surface of the cortical bone, in contact with the outer edge of the MPP, which is located at the direction of force and fixed with miniscrew 2. The principal minimum stresses on the cortical bone around the necks of miniscrews 1 and 2 were measured as almost 0 MPa in both regions at the direction of force application (Fig 7, C and D). The principal minimum stresses were located at the cortical bone surface touching the outer edge of the MPP, and the spikes of the MPP measured 0.3 and 0.1 MPa, respectively (Table II). In scenario 1, the highest von Mises stress was measured as 0.06 MPa at the trabecular bone, where it was in contact with the cortical bone around miniscrew 2 (Fig 8, A). The highest principal maximum value on the trabecular bone was 0.06 MPa located at the connection of miniscrew 2's thread and trabecular bone at the reverse direction of force application. The principal maximum stresses on the cortical bone around miniscrews 1 and 2 were measured as 0.02 MPa in both regions at the reverse direction of force application. The highest principal minimum value on the trabecular bone was 0.04 MPa located at the connection with
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Fig 3. A, Direction of force application (tip of white arrow represents the point of force application); B, highest stress formation deviated from the axis of force application (black arrow represents the direction of force application); C, 4 neighboring nodes used for regional stress measurements; D, 90 counterclockwise rotated close-up image.
the cortical bone around miniscrew 2 at the direction of force application. The principal minimum stresses on trabecular bone around miniscrews 1 and 2 were measured as almost 0 MPa in both regions at the direction of force application (Table III). In scenario 2, the highest von Mises stress was measured as 0.03 MPa at the trabecular bone, where it was in contact with the cortical bone around miniscrew 2 (Fig 8, B). The highest principal maximum value on the trabecular bone was 0.03 MPa located at the trabecular bone, where it was in contact with the cortical bone at the reverse direction of force application under the edge of the MPP. The principal maximum stresses on the cortical bone around miniscrews 1 and 2 were measured as 0.02 MPa in both regions at the reverse direction of force application. The highest principal minimum value on the trabecular bone was 0.04 MPa located at the connection with the cortical bone around miniscrew 2 at the direction of force application. The principal minimum stresses on the trabecular bone around miniscrews 1 and 2 were measured as almost
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0 MPa in both regions at the direction of force application (Table III). In scenario 1, the highest von Mises stress was measured as 1.3 MPa on the miniscrews, located at the neck of miniscrew 2 at the reverse direction of force application. The von Mises stresses on the necks of miniscrews 1 and 2 were measured as 1.12 and 0.42 MPa, respectively, at the direction of force application. The von Mises stresses on the necks of miniscrews 1 and 2 were measured as 0.45 and 1.25 MPa, respectively, at the reverse direction of force application. The highest von Mises stress was measured as 3.3 MPa on the miniplate, located at the middle of the miniplate, where the force was applied on the appliance. The highest von Mises stresses on the miniplate around the necks of both miniscrews 1 and 2 were measured as 1.10 MPa (Table IV). In scenario 2, the highest von Mises stress was measured as 9.6 MPa on the miniscrews, located at the contact between the thread of the miniscrew and the MPP. The von Mises stresses on the necks of miniscrews
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Fig 4. Von Mises stresses on the cortical bone: A, in scenario 1; B, in scenario 2.
1 and 2 were measured as 1.30 and 0.38 MPa, respectively, at the direction of force application. The von Mises stresses on the necks of miniscrews 1 and 2 were measured as 0.30 and 1.28 MPa, respectively, at the reverse direction of force application. The highest von Mises stress was measured as 3.3 MPa on the miniplate, located at the middle of the miniplate, where the force was applied on the appliance. The highest von Mises stresses on the miniplate around the necks of miniscrews 1 and 2 were measured as 1.40 and 1.60 MPa, respectively. The highest von Mises stress was measured as 4.3 MPa on the MPPs, located at the contact between the thread of the miniscrew and the MPP. The von Mises stresses around the necks of miniscrews 1 and 2 were measured as 2.52 and 3.08 MPa, respectively, at the direction of force application on the MPP. The von Mises stresses around the necks of both miniscrews 1 and 2 were measured as 0.1 MPa at the reverse direction of force application on the MPP (Table IV). DISCUSSION
Pilot studies have shown that using a rectangular prism bone block is comparable with the bone modeled
from cone-beam computed tomography images of a real patient. The main problem observed when using real 3D data of a patient was the irregular surface of the cortical bone. Studies have shown the effects of the insertion angle of miniscrews; for this study, the insertion angles needed to be the same and perpendicular to the bone.28,29 The models were linear in this study, meaning that if 5 N of force was applied in the same study instead of 2 N, all data of the stresses would be multiplied by 2.5. This was proved in a previous pilot study and in the study of Huang et al.30 Therefore, the table can be adjusted to any force value application for comparison of results for another study. Using this linear relationship, it can be stated that heavier extraoral forces would also result in lower stresses than the critical threshold values for both models. The highest stress measured was 0.6 MPa for 2 N in this study, and it would be 1.5 MPa if a force of 5 N was applied from the same point to the finite element model. Different levels of osseointegration of miniscrews change the distribution of stresses.31 Further study is needed to investigate the effects of the MPP in different levels of osseointegration conditions.
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Fig 5. Von Mises stresses on the cortical bone in scenario 1: A, around miniscrew 1; B, around miniscrew 2. Von Mises stresses on the cortical bone in scenario 2: C, around miniscrew 1; D, around miniscrew 2.
The rates of stationary anchorage failure of miniscrews under orthodontic loading vary between 11% and 30%.32 If a miniscrew loosens, it will not regain stability and will probably need to be removed and replaced.11 Lee et al33 mentioned a backup plan for replacing failed miniscrews; instead of waiting for the bone to heal, a miniplate was placed through the mucosal hole left after the miniscrew was removed. Only a small horizontal incision at the sulcus was made to adapt and fix the miniplate by miniscrews, leading to a less invasive miniplate placement. Miniplate placement is more complex because of the 2 surgical procedures to the region for placement and removal.18 The surgical interventions for both devices are generally well accepted by patients and providers, but there are some side effects or problems during treatment.15,34 Failure rates of miniplates have been reported between 7% and 11%.15 Choi et al17 stated that the failures might have resulted from soft tissue infections. They also recommended a new design of the transmucosal part of the appliance. This brings the question: Is there a way to lower the risk of infection with a new
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miniplate design? Many miniplates have been designed and evaluated both by finite element analysis for mechanical understanding and in clinical studies for success rates and treatment alternatives.30,35 The main idea behind the MPP was to elevate the miniplate over the soft tissue gingiva for easier placement and a more hygienic appliance that has similar mechanical advantages as a regular miniplate. Three appliances—washer,19 mini-implant ring,20 and spiky miniplate21—were found to be noteworthy in the reviewed literature. The washer had varying long spikes penetrating the gingiva, with the base in contact with the soft tissues, which may not be stable enough or can cause inflammation. The mini-implant ring showed promising results for increasing the stability. A finite element study showed that a spiky miniplate causes lower stresses on the surrounding cortical bone. Two studies investigated the buccal gingival tissue thickness, and both reported values below 2 mm, with the highest mean value at 1.84 mm.36,37 The MPP modeled in our study had a platform height of 2 mm, which was enough to elevate the miniplate from the gingival
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Fig 6. Principal maximum stresses on the cortical bone in scenario 1: A, around miniscrew 1; B, around miniscrew 2. Principal maximum stresses on the cortical bone in scenario 2: C, around miniscrew 1; D, around miniscrew 2.
tissues. Therefore, placement and removal surgeries could be omitted. Only a gingival punch would be needed to prepare the region for miniplate placement with the MPP. Miyawaki et al5 reported that prevention of inflammation of the peri-implant tissue is a critical factor for the success rates of temporary anchorage devices. Cornelis et al15 stated that careful brushing of miniplates and the surrounding mucosa with a soft toothbrush is important to prevent infection and thus to prevent miniplate failure. Therefore, elevating the miniplate over the gingival tissue facilitates cleaning the miniplate; this can reduce the risk of infections. MPP placement seems to offer some advantages; placement and removal surgeries are not necessary, the miniplate can be placed above the gingival tissues, and it lowers the risk of complications. The questions still needing answers were whether this design could provide enough stability and whether there would be any negative effects of generating new moments, caused by the 2-mm elevation of the MPP. It was not necessarily expected to improve the mechanical stability. This new
design was expected to have stability similar to a regular miniplate, which is successful in clinical situations.15 Von Mises stress values were reported for the cortical bone layers to provide an indication of the average stress levels and to compare both scenarios. Von Mises values yield a criterion that applies best to ductile materials such as metals; however, for brittle materials such as bone, they are not favorable.38 Principal maximum and principal minimum values were reported to determine the local risk indicator or physiologic bone failure, and to compare stresses in detail in the different regions of interest. Principal maximum stresses had positive values, indicating tension, and principal minimum stresses had negative values, indicating compression. The results of this study are promising. The highest von Mises stresses on the cortical bone decreased from 0.5 to 0.3 MPa when MPPs were used. Stresses generated on the cortical bone are mainly focused around the necks of the miniscrews.30 Duaibis et al38 stated that the majority of stress in bone is transferred to the cortical layer on the side of compression; because
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Fig 7. Principal minimum stresses on the cortical bone in scenario 1: A, around miniscrew 1; B, around miniscrew 2. Principal minimum stresses on the cortical bone in scenario 2: C, around miniscrew 1; D, around miniscrew 2. Table II. Stress values in cortical bone Cortical bone Scenario 1 (miniplate 1 2 miniscrews) Highest value measured Neck of miniscrew 1 (1) Neck of miniscrew 1 ( ) Neck of miniscrew 2 (1) Neck of miniscrew 2 ( ) Scenario 2 (miniplate 1 2 MPPs 1 2 miniscrews) Highest value measured Neck of miniscrew 1 (1) Neck of miniscrew 1 ( ) Neck of miniscrew 2 (1) Neck of miniscrew 2 ( ) Edge of MPP Spikes of MPP
Von Mises (MPa)
Principal maximum (MPa)
0.50 0.42 0.35 0.38 0.48
0.60 0.00 0.42 0.00 0.48
0.60 0.48 0.00 0.45 0.00
0.30 0.08 0.22 0.08 0.20 0.20 0.10
0.31 0.07 0.22 0.07 0.20 0.00 0.90
0.34 0.00 0.01 0.00 0.01 0.30 0.10
cortical bone absorbs most of the stress, it may be the determining factor in miniscrew stability. Principal maximum stresses on the cortical bone around the miniscrews decreased from 0.42 to 0.48 MPa to 0.20 to 0.22 MPa. This may be caused by the stability provided by the MPP to prevent movement of the
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Principal minimum (MPa)
miniscrew. Principal minimum stresses on the cortical bone around the miniscrews were the focus of this study. These stresses of tension decreased from 0.45 to 0.48 MPa to 0.01 MPa for both sides. This was further investigated by measuring the stresses generated on the cortical bone around the MPPs. The highest
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Fig 8. Von Mises stresses on the trabecular bone: A, in scenario 1; B, in scenario 2. Table III. Stress values in trabecular bone Trabecular bone Scenario 1 (miniplate 1 2 miniscrews) Highest value measured Around miniscrew 1 (1) Around miniscrew 1 ( ) Around miniscrew 2 (1) Around miniscrew 2 ( ) Scenario 2 (miniplate 1 2 MPPs 1 2 miniscrews) Highest value measured Around miniscrew 1 (1) Around miniscrew 1 ( ) Around miniscrew 2 (1) Around miniscrew 2 ( )
Von Mises (MPa)
Principal maximum (MPa)
0.06 0.04 0.03 0.04 0.02
0.06 0.00 0.02 0.00 0.02
0.06 0.04 0.00 0.05 0.00
0.03 0.01 0.02 0.01 0.02
0.03 0.00 0.02 0.00 0.02
0.04 0.00 0.00 0.00 0.00
stresses generated on cortical bone were 0.3 MPa at the edge of the MPP and 0.1 MPa at the spikes. Therefore, it was concluded that when the force was applied on the miniplate, the MPPs transfer the load to the surface of the cortical bone; this allows much less stress to be formed around the miniscrews. Spikes also increased the bone surface and stabilized the system, as shown by Nalbantgil et al.21 The highest von Mises stresses in the trabecular bone decreased from 0.06 to 0.03 MPa. The
Principal minimum (MPa)
principal maximum stresses generated in the trabecular bone around the miniscrew stayed the same, whereas the principal minimum stresses decreased from 0.04 to 0.05 MPa to undetectable levels below 0.01 MPa. Overloading regions were detected in the von Mises stresses generated on the titanium elements of the 2 models. The highest von Mises stresses in the miniscrews increased from 1.3 to 9.6 MPa. This was further investigated, and the reason was the contact between the first
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Table IV. Stress values on miniscrews, MPPs, and miniplates Miniscrew Miniscrew/MPP/miniplate Von Mises (MPa) Scenario 1 (miniplate 1 2 miniscrews) Highest value measured 1.30 Neck of miniscrew 1 (1) 1.12 Neck of miniscrew 1 ( ) 0.45 Neck of miniscrew 2 (1) 0.42 Neck of miniscrew 2 ( ) 1.25 Scenario 2 (miniplate 1 2 MPPs 1 2 miniscrews) Highest value measured 9.60 Neck of miniscrew 1 (1) 1.30 Neck of miniscrew 1 ( ) 0.30 Neck of miniscrew 2 (1) 0.38 Neck of miniscrew 2 ( ) 1.28
MPP Von Mises (MPa)
Miniplate Von Mises (MPa) 3.30 1.10 1.10
4.30 2.52 0.10 3.08 0.10
3.30 1.40 1.60
Fig 9. A, Distance of the closest point of the first thread to the miniscrew head; B, distance of the farthest point of the first thread to the miniscrew head.
thread of the miniscrew and the MPP. MPPs with 2 mm of height elevated the miniscrews as well, causing the threads to be in contact with their inner surfaces. This can be corrected using a miniscrew without threads in the neck region or by extending the neck of the screw, but that was not necessary. Elevation of the miniscrew caused 2 more possible problems. The first was the shortening of the infrabony length, but it was reported that the infrabony length change did not affect the stresses in the cortical bone around the miniscrew.30,38 Second, because of the elevation, the increase of the miniscrew head length could cause an increase in the stresses generated on the cortical bone around the miniscrews, as stated previously in the literature.38,39 Instead of an increase, however, the newly designed system showed significant decreases in the stresses; this shows the mechanical stability provided by MPPs. Another overloading was detected on the miniplates; the von Mises stresses for both miniplates were high because the force application points were localized on them. Stresses on the miniplates around the
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miniscrews were investigated separately to isolate this overloading and to obtain comparable data. The first thread of the miniscrew started at 0.7 mm from the neck of the miniscrew head, and the farthest threadless point was 1.4 mm away (Fig 9). Therefore, the first thread pitch was 0.7 mm, whereas the continuing thread pitches were 0.9 mm (Fig 2). In scenario 1, the cervical threadless part was touching the corner of the cortical bone surface at 0.7 mm from the superior margin; for scenario 2, the threads were in contact with the cortical bone from the beginning of cortical bone penetration (Fig 1). Motoyoshi et al27 investigated this in detail; cervical threadless miniscrews decreased the stresses generated on the cortical bone around the miniscrew neck. They also reported that the first thread relationship in different loading directions causes mechanical anisotropy, which resulted in higher stress formations. In scenario 2, the tip of the thread ridge was located close to the superior margin of the cortical bone (Fig 1, B); this causes higher stresses to be formed when compared with the cervical threadless miniscrew
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relationship.27 Even though there was close thread contact in scenario 2, the MPP's effect in the decrease of stress was much more effective. The middle point of the miniplates was chosen to interpret data more easily and to make the analysis of the results more understandable. The models were designed to be comparable, standardized, easy to understand, and unaffected by any unwanted variable. This system offers a new approach to miniplates; new auxiliaries and designs can be created for future investigations and clinical use for treatments. Critical stress values for cortical bone were reported in several studies. The maximum principal stress theory states that local overloading in the cortical bone was assumed to occur when the peak principal maximum stress reached the ultimate tensile strength of 100 to 130 MPa, or when the peak principal minimum stress reached the ultimate compressive strength of 170 to 190 MPa.38,40,41 The critical threshold for cortical bone resorption was stated as 50 MPa in several studies.27,42,43 Li et al44 created a new mathematic model to simulate overload bone resorption and prepared a graph for underload and overload values for different bone densities that might cause bone resorption around implants. At regions with high bone densities, they reported that stresses over 25 MPa would cause overload bone resorption. Stresses generated in this study in cortical bone were measured below all the critical threshold values stated in the literature. The results of this study showed that the MPP system may provide the desired stability in the clinic as well as many other advantages; however, more clinical studies are needed to further evaluate the system. CONCLUSIONS
This new anchorage device, which elevates the miniplate over the gingiva, can be placed by removing the gingiva with a punch and placing the MPPs before miniplate placement, without the need for flap surgery; MPPs can also be removed without surgery. Within the limitations of this finite element analysis study, we concluded the following. 1. 2.
3.
4.
The newly designed MPP can be a new approach for miniplate placement. When a miniplate was placed over the MPP, the highest von Mises principal maximum and principal minimum stresses decreased in both cortical and trabecular bone. MPPs increased the bone contact area and distributed the stresses on the cortical bone surface, resulting in decreased stresses around the fixation miniscrews. Clinical studies are necessary to confirm these results.
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ACKNOWLEDGMENT
We thank Ahmet Kurt from ASELSAN, Ankara, Turkey, for his help with the finite element analysis. REFERENCES 1. Singh K, Kumar D, Jaiswal RK, Bansal A. Temporary anchorage devices—mini-implants. Natl J Maxillofac Surg 2010;1:30-4. 2. Kanomi R. Mini-implant for orthodontic anchorage. J Clin Orthod 1997;31:763-7. 3. Hong RK, Heo JM, Ha YK. Lever-arm and mini-implant system for anterior torque control during retraction in lingual orthodontic treatment. Angle Orthod 2005;75:129-41. 4. Bae SM, Park HS, Kyung HM, Kwon OW, Sung JH. Clinical application of micro-implant anchorage. J Clin Orthod 2002;36: 298-302. 5. Miyawaki S, Koyama I, Inoue M, Mishima K, Sugahara T, TakanoYamamoto T. Factors associated with the stability of titanium screws placed in the posterior region for orthodontic anchorage. Am J Orthod Dentofacial Orthop 2003;124:373-8. 6. Erverdi N, Keles A, Nanda R. The use of skeletal anchorage in open bite treatment: a cephalometric evaluation. Angle Orthod 2004; 74:381-90. 7. Park HS, Lee SK, Kwon OW. Group distal movement of teeth using microscrew implant anchorage. Angle Orthod 2005;75:602-9. 8. Cheng SJ, Tseng IY, Lee JJ, Kok SH. A prospective study of the risk factors associated with failure of mini-implants used for orthodontic anchorage. Int J Oral Maxillofac Implants 2004;19:100-6. 9. Liou EJ, Pai BC, Lin JC. Do miniscrews remain stationary under orthodontic forces? Am J Orthod Dentofacial Orthop 2004;126: 42-7. 10. Reynders R, Ronchi L, Bipat S. Mini-implants in orthodontics: a systematic review of the literature. Am J Orthod Dentofacial Orthop 2009;135:564.e1-19:discussion, 564-5. 11. Melsen B, Verna C. Miniscrew implants: the Aarhus anchorage system. Semin Orthod 2005;11:24-31. 12. Umemori M, Sugawara J, Mitani H, Nagasaka H, Kawamura H. Skeletal anchorage system for open-bite correction. Am J Orthod Dentofacial Orthop 1999;115:166-74. 13. De Clerck H, Geerinckx V, Siciliano S. The zygoma anchorage system. J Clin Orthod 2002;36:455-9. 14. Erverdi N, Tosun T, Keles A. A new anchorage site for the treatment of anterior open bite: zygomatic anchorage. Case report. World J Orthod 2002;3:147-53. 15. Cornelis MA, Scheffler NR, Nyssen-Behets C, De Clerck HJ, Tulloch JF. Patients' and orthodontists' perceptions of miniplates used for temporary skeletal anchorage: a prospective study. Am J Orthod Dentofacial Orthop 2008;133:18-24. 16. Kircelli BH, Pektas ZO, Uckan S. Orthopedic protraction with skeletal anchorage in a patient with maxillary hypoplasia and hypodontia. Angle Orthod 2006;76:156-63. 17. Choi BH, Zhu SJ, Kim YH. A clinical evaluation of titanium miniplates as anchors for orthodontic treatment. Am J Orthod Dentofacial Orthop 2005;128:382-4. 18. Schatzle M, Mannchen R, Zwahlen M, Lang NP. Survival and failure rates of orthodontic temporary anchorage devices: a systematic review. Clin Oral Implants Res 2009;20:1351-9. 19. Jang HJ, Kwon SY, Kim SH, Park YG, Kim SJ. Effects of washer on the stress distribution of mini-implant. Angle Orthod 2012;82:137-44. 20. Tozlu M, Nalbantgil D, Ozdemir F. Effects of a newly designed apparatus on orthodontic skeletal anchorage. Eur J Dent 2013; 7(Suppl 1):S83-8.
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21. Nalbantgil D, Tozlu M, Ozdemir F, Oztoprak MO, Arun T. FEM analysis of a new miniplate: stress distribution on the plate, screws and the bone. Eur J Dent 2012;6:9-15. 22. Lee NK, Baek SH. Stress and displacement between maxillary protraction with miniplates placed at the infrazygomatic crest and the lateral nasal wall: a 3-dimensional finite element analysis. Am J Orthod Dentofacial Orthop 2012;141:345-51. 23. Chen J, Akyuz U, Xu L, Pidaparti RM. Stress analysis of the human temporomandibular joint. Med Eng Phys 1998;20:565-72. 24. Ludwig B, Baumgaertel S, Zorkun B, Bonitz L, Glasl B, Wilmes B, et al. Application of a new viscoelastic finite element method model and analysis of miniscrew-supported hybrid hyrax treatment. Am J Orthod Dentofacial Orthop 2013;143:426-35. 25. Park J, Cho HJ. Three-dimensional evaluation of interradicular spaces and cortical bone thickness for the placement and initial stability of microimplants in adults. Am J Orthod Dentofacial Orthop 2009;136:314.e1-12:discussion, 314-5. 26. Baumgaertel S, Hans MG. Buccal cortical bone thickness for miniimplant placement. Am J Orthod Dentofacial Orthop 2009;136: 230-5. 27. Motoyoshi M, Inaba M, Ueno S, Shimizu N. Mechanical anisotropy of orthodontic mini-implants. Int J Oral Maxillofac Surg 2009;38: 972-7. 28. Woodall N, Tadepalli SC, Qian F, Grosland NM, Marshall SD, Southard TE. Effect of miniscrew angulation on anchorage resistance. Am J Orthod Dentofacial Orthop 2011;139:e147-52. 29. Suzuki A, Masuda T, Takahashi I, Deguchi T, Suzuki O, TakanoYamamoto T. Changes in stress distribution of orthodontic miniscrews and surrounding bone evaluated by 3-dimensional finite element analysis. Am J Orthod Dentofacial Orthop 2011;140: e273-80. 30. Huang YW, Chang CH, Wong TY, Liu JK. Bone stress when miniplates are used for orthodontic anchorage: finite element analysis. Am J Orthod Dentofacial Orthop 2012;142:466-72. 31. Gracco A, Cirignaco A, Cozzani M, Boccaccio A, Pappalettere C, Vitale G. Numerical/experimental analysis of the stress field around miniscrews for orthodontic anchorage. Eur J Orthod 2009;31:12-20. 32. Kravitz ND, Kusnoto B. Risks and complications of orthodontic miniscrews. Am J Orthod Dentofacial Orthop 2007;131(4 Suppl): S43-51.
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33. Lee JH, Choo H, Kim SH, Chung KR, Giannuzzi LA, Ngan P. Replacing a failed mini-implant with a miniplate to prevent interruption during orthodontic treatment. Am J Orthod Dentofacial Orthop 2011;139:849-57. 34. Kuroda S, Sugawara Y, Deguchi T, Kyung HM, TakanoYamamoto T. Clinical use of miniscrew implants as orthodontic anchorage: success rates and postoperative discomfort. Am J Orthod Dentofacial Orthop 2007;131:9-15. 35. Leung MT, Lee TC, Rabie AB, Wong RW. Use of miniscrews and miniplates in orthodontics. J Oral Maxillofac Surg 2008;66: 1461-6. 36. Cha BK, Lee YH, Lee NK, Choi DS, Baek SH. Soft tissue thickness for placement of an orthodontic miniscrew using an ultrasonic device. Angle Orthod 2008;78:403-8. 37. Cao J, Hu WJ, Zhang H, Liu DG, Le D. Method and its application of gingival thickness measurement based on cone-beam computed tomography. Beijing Da Xue Xue Bao 2013;45:135-9. 38. Duaibis R, Kusnoto B, Natarajan R, Zhao L, Evans C. Factors affecting stresses in cortical bone around miniscrew implants: a three-dimensional finite element study. Angle Orthod 2012;82: 875-80. 39. Liu TC, Chang CH, Wong TY, Liu JK. Finite element analysis of miniscrew implants used for orthodontic anchorage. Am J Orthod Dentofacial Orthop 2012;141:468-76. 40. Natali AN, Pavan PG, Ruggero AL. Evaluation of stress induced in peri-implant bone tissue by misfit in multi-implant prosthesis. Dent Mater 2006;22:388-95. 41. Martin RB, Burr DB, Sharkey NA. Skeletal tissue mechanics. New York: Springer; 1998. 42. Sugiura T, Horiuchi K, Sugimura M, Tsutsumi S. Evaluation of threshold stress for bone resorption around screws based on in vivo strain measurement of miniplate. J Musculoskelet Neuronal Interact 2000;1:165-70. 43. Jung D, Tsutsumi S, Nakai R, Ikeuchi K, Sekel R. Numerical estimation of periprosthesis resorptive bone remodeling caused by high compressive stress in relation to bony ingrowth conditions. Japanese J Clin Biomech 2005;26:247-55. 44. Li J, Li H, Shi L, Fok AS, Ucer C, Devlin H, et al. A mathematical model for simulating the bone remodeling process under mechanical stimulus. Dent Mater 2007;23:1073-8.
American Journal of Orthodontics and Dentofacial Orthopedics
Effects of a newly designed orthodontic miniplate platform for elevating the miniplate over the gingiva: A 3-dimensional finite element analysis rı Ulusoy,b and C rı Tu € zb € rko Volkan Aykac¸,a C ¸ ag ¸ ag Ankara, Turkey
Introduction: Miniplates are the treatment of choice for complex orthodontic and orthopedic problems. However, they require surgical placement and removal, and complications such as infection and mobility can occur. The aim of this finite element analysis was to investigate the effects of a newly designed miniplate platform to elevate the miniplate above the gingiva. Methods: A bone block was modeled in 3 dimensions, and 2 N of force was applied on miniplates in 2 scenarios. In scenario 1, the miniplate was fixed with 2 miniscrews on both ends; in scenario 2, miniplate platforms were first seated on the cortical bone surface with their spikes fully penetrating, and then the miniplate was fixed on top with 2 miniscrews. Results: The highest von Mises stress on the cortical bone decreased from 0.5 to 0.3 MPa when miniplate platforms were used. In scenario 2, the principal maximum stresses on the cortical bone around the miniscrews decreased from 0.42 and 0.48 MPa to 0.20 and 0.22 MPa, and the principal minimum stresses decreased from 0.45 and 0.48 MPa to 0.01 MPa. Conclusions: Miniplate platforms used to elevate the miniplate lowered the stresses generated on cortical bone around the miniscrews by distributing the stresses on the cortical bone surface. Patients can clean the miniplate more readily because it is elevated above the soft tissues. Placing the miniplate platforms requires only removing the gingiva with a punch, and their removal does not require flap surgery. (Am J Orthod Dentofacial Orthop 2015;148:110-22)
A
temporary anchorage device is temporarily fixed to bone to enhance orthodontic anchorage either by supporting the teeth of the reactive unit or by obviating the need for the reactive unit altogether, and it is subsequently removed after use.1 Kanomi2 used miniscrews for mandibular incisor intrusion and inspired many researchers to give more attention to skeletal anchorage in orthodontics. Although some studies suggested that miniscrews have predictable and stable results,3,4 some others found complications such as pain,5 irritation,6 soft tissue inflammation,7 peri-implantitis, and loosening of screws.8,9 Miyawaki et al5 stated that miniscrews had a success rate of 83.9%, and the authors of most studies found success rates over 80%.5,10 If a miniscrew a
Private practice, Ankara, Turkey. Associate professor, Department of Orthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Address correspondence to: C¸a grı Ulusoy, Department of Orthodontics, Faculty of Dentistry, Gazi University, Emek St, Ankara, Turkey; e-mail, culusoy77@ yahoo.com. Submitted, July 2014; revised and accepted, February 2015. 0889-5406/$36.00 Copyright Ó 2015 by the American Association of Orthodontists. http://dx.doi.org/10.1016/j.ajodo.2015.02.024 b
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loosens, it will not regain stability and will probably need to be removed and replaced.11 Miniplates have been used for treatment of fractures; after their clinical success in orthodontic practice, they have fulfilled the need for a more stable temporary anchorage device for complex orthodontic treatments.12-15 Miniplates may provide more secure anchorage when higher forces are needed.16 Cornelis et al15 made a survey to evaluate patients' and orthodontists' perceptions of miniplates during orthodontic treatment and reported good acceptance from both groups. Miniplate failure rates have been reported to be over 7% in previous studies.5,17,18 Inflammation of the tissues around the miniplates can require miniplate removal. Choi et al17 stated that miniplates might be more reliable tools for orthodontic anchorage if the rate of complications was reduced, possibly by changing the size of the appliance. Oral hygiene is another important factor for success because prevention of inflammation of the peri-implant tissues is a critical factor for miniplate success.5,15 Different designs of platforms and spikes were introduced to increase the stability of miniscrews and miniplates, such as the washer,19 the mini-implant ring,20 and the spiky miniplate.21
Aykac¸ , Ulusoy, and T€ urk€oz
The aim of this finite element analysis study was to investigate the effects of a newly designed miniplate platform (MPP) used to elevate the miniplate above the gingiva. MATERIAL AND METHODS
Finite element analysis was used to evaluate the stress distributions in bone, miniscrews, miniplates, and MPPs. Three-dimensional (3D) models of all instruments, tested in this study, were scanned with a 3D scanner (SCAN ST; Steinbichler, Plymouth, Mich). Marc software (version 2005; MSC Software, Newport Beach, Calif) was used to construct the 3D finite element models for preprocessing and modeling. All final solid meshes were constituted by tetrahedral elements with 4 nodes using the Marc software. Finite element analysis was also performed with this software. In scenario 1, 90801 nodes and 466144 elements were used; in scenario 2, 141442 nodes and 717998 elements were used. All bone, miniscrew, miniplate, and MPP elements were assumed to be isotropic, homogeneous, and linearly elastic. The elastic properties of the materials used in this study are shown in Table I.22-24 Bone models were fixed for displacement and rotation in the x-, y-, and z-axes on 5 surfaces, except for the surface of bone with the appliance. All appliances used in this study were made of grade 5 titanium (Ti-6Al-4V). Three-dimensional finite element models were prepared, and finite element analyses were processed with an Intel Pentium D Windows 7 Pro computer (CPU 2.60 GHz, 32 GB RAM) (Intel, Santa Clara, Calif). Supporting bone models were modeled consisting of cortical and trabecular bone. The modeled block of bone was a rectangular prism 48 mm wide, 30 mm deep, and 13.5 mm high. The upper 1.5-mm slice of the block was modeled as cortical bone, and the remaining 12 mm was modeled as trabecular bone (Fig 1).21,25,26 Cylindrical miniscrews, 1.7 mm in diameter and 8 mm in length (Ortho Easy; Forestadent, Pforzheim, Germany), were used (Fig 2, A). In this study, we simulated osseointegrated miniscrews.21 For simulating such a condition, the miniscrews were fully bonded to the bone along their entire interface. Other than this, all interfaces, including bone-miniplate, miniscrewminiplate, bone-MPP, MPP-miniplate, and MPPminiscrew, were defined as face-to-face contacts. The surgical miniplate (Rita Leibinger, Neuhausen ob Eck, Germany) had a length of 21.6 mm and a height of 0.9 mm, and had 4 holes separated by 6 mm for miniscrew orientation (Fig 2, B). The neck of the
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Table I. Properties of the materials used in the study Cortical bone Trabecular bone Miniscrew MPP Miniplate
Young's modulus (MPa) 13.7 3 103 7.9 3 103 105 3 103 105 3 103 105 3 103
Poisson's ratio 0.3 0.3 0.33 0.33 0.33
miniscrew and the holes of the miniplate were modeled with a perfect adaptation. The MPP, which had 0.7-mm minispikes at the side that faced the cortical bone, was designed to elevate the miniplate above the gingiva. MPPs also increased the contact surface of the appliance with cortical bone, and the spikes of the MPPs fully penetrated into the cortical bone. This round appliance had a height of 2 mm, a diameter of 6 mm, and a hole in the middle that fit perfectly to the neck part of the miniscrew (Fig 2, C). Two scenarios were formed in our study. In scenario 1, miniplates were placed on the cortical bone and fixed with 2 miniscrews on both ends (Fig 1, A). In scenario 2, miniplates were placed over the MPP (Fig 1, B). Miniplates were therefore placed above the gingiva, whereas the lower surfaces of the MPPs were directly in contact with the cortical bone, with their spikes fully penetrating the bone, as if a gingival punch was used to remove the soft tissues before placement of the MPPs. The appliance was fixed with 2 miniscrews on both ends. The MPPs were designed to be 2 mm thick to elevate the miniplate over the gingiva. The tip of the white arrow in Figure 3, A, shows the point of force application of 2 N applied to the midpoint of the upper surface of the miniplate in a sagittal direction from left to right, parallel to the axis between the miniscrew middle points in both scenarios. This experimental setup was designed for easier evaluation and understanding of the stresses generated. Von Mises stresses were measured and evaluated for titanium appliance elements; von Mises, principal maximum (compression), and principal minimum (tension) stresses were measured for cortical and trabecular bone. The miniscrew at the reverse direction of force application was called miniscrew 1, and the other miniscrew at the direction of force application was called miniscrew 2 (Fig 1). Stresses on cortical bone around the neck of miniscrew were measured in the force application direction (1) and in the reverse direction of force application ( ) (Fig 3, C). For example, principal minimum stresses at the cortical bone around miniscrew 1 ( ) signify the principal minimum stresses generated
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Fig 1. A, Orientation and cross-section of the miniplate model with 2 fixation miniscrews on both ends (scenario 1); B, orientation and cross-section of the elevated miniplate model on 2 MPPs with 2 fixation miniscrews on both ends.
at the cortical bone around the miniscrew, which is at the reverse direction of force application. Stresses were measured from several regions. The highest value measured shows the overall highest stress on the material, and the location of this value is presented in the results for each material. Four values around the miniscrews were measured: stresses around or at the necks of miniscrews 1 and 2 at the direction of force application (1) and the reverse direction of force application ( ). Stresses generated at the outer edge and the cortical bone surface contacts were named the edge of the MPP, and stresses generated at the spike and cortical bone contacts were named spikes of the MPP. When measuring the stresses generated around the necks of the miniscrews, the mean of the 4 nodes in contact with the miniscrews at the investigated regions was evaluated, since there was not necessarily a node intersecting the axis of force application direction. Figure 3, C, shows 4 nodes selected for both the 1 and directions. This measurement style was chosen to differentiate the highest value measured overall and the highest stresses generated at comparable regions.
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Overall highest stresses can be close to the direction of force application, but they may not be right on the axis; this makes it hard to evaluate and compare.27 Figure 3, B, demonstrates how the highest stress on cortical bone around the miniscrew can be at a different location; therefore, the mean of the 4 neighboring nodes was selected to make the data comparable (Fig 3, C). The white arrow in Figure 3, D, shows the direction of force application on the close-up images. The white arrows in Figure 3, A and D, represent 0 , meaning that the closeup images were rotated by 90 in a counterclockwise direction to see the 3D effect of the stresses around the miniscrews; the setup simplified the orientation. RESULTS
In scenario 1, the highest von Mises stress was measured as 0.5 MPa at the cortical bone surrounding the neck of miniscrew 2 (Figs 4, A, and 5, A and B). The highest principal maximum value on the cortical bone was 0.6 MPa located at the connection of miniscrew 2 and the cortical bone surface at the reverse
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Fig 2. Dimensions of: A, the miniscrew; B, the miniplate; C, the MPP.
direction of force application. The principal maximum stresses on the cortical bone around the necks of miniscrews 1 and 2 were measured as 0.42 and 0.48 MPa, respectively, at the reverse direction of force application (Fig 6, A and B). The highest principal minimum value on the cortical bone was 0.6 MPa located at the connection of miniscrew 1 and the cortical bone surface at the direction of force application. The principal minimum stresses on the cortical bone around the necks of miniscrews 1 and 2 were measured as 0.48 and 0.45 MPa, respectively, at the direction of force application (Fig 7, A and B) (Table II). In scenario 2, the highest von Mises stress was measured as 0.3 MPa at the surface of the cortical bone, in contact with the outer edge of the MPP, which is located at the direction of force and fixed with miniscrew 2 (Figs 4, B, and 5, C and D). The highest principal maximum value on the cortical bone was 0.31 MPa located at the connection of miniscrew 1 and the cortical bone surface at the reverse direction of force application. The principal maximum stresses on the cortical bone around the necks of miniscrews 1 and 2 were measured as 0.22 and 0.20 MPa, respectively, at the reverse direction of force application (Fig 6, C and D).
The highest principal minimum value on the cortical bone was 0.34 MPa at the surface of the cortical bone, in contact with the outer edge of the MPP, which is located at the direction of force and fixed with miniscrew 2. The principal minimum stresses on the cortical bone around the necks of miniscrews 1 and 2 were measured as almost 0 MPa in both regions at the direction of force application (Fig 7, C and D). The principal minimum stresses were located at the cortical bone surface touching the outer edge of the MPP, and the spikes of the MPP measured 0.3 and 0.1 MPa, respectively (Table II). In scenario 1, the highest von Mises stress was measured as 0.06 MPa at the trabecular bone, where it was in contact with the cortical bone around miniscrew 2 (Fig 8, A). The highest principal maximum value on the trabecular bone was 0.06 MPa located at the connection of miniscrew 2's thread and trabecular bone at the reverse direction of force application. The principal maximum stresses on the cortical bone around miniscrews 1 and 2 were measured as 0.02 MPa in both regions at the reverse direction of force application. The highest principal minimum value on the trabecular bone was 0.04 MPa located at the connection with
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Fig 3. A, Direction of force application (tip of white arrow represents the point of force application); B, highest stress formation deviated from the axis of force application (black arrow represents the direction of force application); C, 4 neighboring nodes used for regional stress measurements; D, 90 counterclockwise rotated close-up image.
the cortical bone around miniscrew 2 at the direction of force application. The principal minimum stresses on trabecular bone around miniscrews 1 and 2 were measured as almost 0 MPa in both regions at the direction of force application (Table III). In scenario 2, the highest von Mises stress was measured as 0.03 MPa at the trabecular bone, where it was in contact with the cortical bone around miniscrew 2 (Fig 8, B). The highest principal maximum value on the trabecular bone was 0.03 MPa located at the trabecular bone, where it was in contact with the cortical bone at the reverse direction of force application under the edge of the MPP. The principal maximum stresses on the cortical bone around miniscrews 1 and 2 were measured as 0.02 MPa in both regions at the reverse direction of force application. The highest principal minimum value on the trabecular bone was 0.04 MPa located at the connection with the cortical bone around miniscrew 2 at the direction of force application. The principal minimum stresses on the trabecular bone around miniscrews 1 and 2 were measured as almost
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0 MPa in both regions at the direction of force application (Table III). In scenario 1, the highest von Mises stress was measured as 1.3 MPa on the miniscrews, located at the neck of miniscrew 2 at the reverse direction of force application. The von Mises stresses on the necks of miniscrews 1 and 2 were measured as 1.12 and 0.42 MPa, respectively, at the direction of force application. The von Mises stresses on the necks of miniscrews 1 and 2 were measured as 0.45 and 1.25 MPa, respectively, at the reverse direction of force application. The highest von Mises stress was measured as 3.3 MPa on the miniplate, located at the middle of the miniplate, where the force was applied on the appliance. The highest von Mises stresses on the miniplate around the necks of both miniscrews 1 and 2 were measured as 1.10 MPa (Table IV). In scenario 2, the highest von Mises stress was measured as 9.6 MPa on the miniscrews, located at the contact between the thread of the miniscrew and the MPP. The von Mises stresses on the necks of miniscrews
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Fig 4. Von Mises stresses on the cortical bone: A, in scenario 1; B, in scenario 2.
1 and 2 were measured as 1.30 and 0.38 MPa, respectively, at the direction of force application. The von Mises stresses on the necks of miniscrews 1 and 2 were measured as 0.30 and 1.28 MPa, respectively, at the reverse direction of force application. The highest von Mises stress was measured as 3.3 MPa on the miniplate, located at the middle of the miniplate, where the force was applied on the appliance. The highest von Mises stresses on the miniplate around the necks of miniscrews 1 and 2 were measured as 1.40 and 1.60 MPa, respectively. The highest von Mises stress was measured as 4.3 MPa on the MPPs, located at the contact between the thread of the miniscrew and the MPP. The von Mises stresses around the necks of miniscrews 1 and 2 were measured as 2.52 and 3.08 MPa, respectively, at the direction of force application on the MPP. The von Mises stresses around the necks of both miniscrews 1 and 2 were measured as 0.1 MPa at the reverse direction of force application on the MPP (Table IV). DISCUSSION
Pilot studies have shown that using a rectangular prism bone block is comparable with the bone modeled
from cone-beam computed tomography images of a real patient. The main problem observed when using real 3D data of a patient was the irregular surface of the cortical bone. Studies have shown the effects of the insertion angle of miniscrews; for this study, the insertion angles needed to be the same and perpendicular to the bone.28,29 The models were linear in this study, meaning that if 5 N of force was applied in the same study instead of 2 N, all data of the stresses would be multiplied by 2.5. This was proved in a previous pilot study and in the study of Huang et al.30 Therefore, the table can be adjusted to any force value application for comparison of results for another study. Using this linear relationship, it can be stated that heavier extraoral forces would also result in lower stresses than the critical threshold values for both models. The highest stress measured was 0.6 MPa for 2 N in this study, and it would be 1.5 MPa if a force of 5 N was applied from the same point to the finite element model. Different levels of osseointegration of miniscrews change the distribution of stresses.31 Further study is needed to investigate the effects of the MPP in different levels of osseointegration conditions.
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Fig 5. Von Mises stresses on the cortical bone in scenario 1: A, around miniscrew 1; B, around miniscrew 2. Von Mises stresses on the cortical bone in scenario 2: C, around miniscrew 1; D, around miniscrew 2.
The rates of stationary anchorage failure of miniscrews under orthodontic loading vary between 11% and 30%.32 If a miniscrew loosens, it will not regain stability and will probably need to be removed and replaced.11 Lee et al33 mentioned a backup plan for replacing failed miniscrews; instead of waiting for the bone to heal, a miniplate was placed through the mucosal hole left after the miniscrew was removed. Only a small horizontal incision at the sulcus was made to adapt and fix the miniplate by miniscrews, leading to a less invasive miniplate placement. Miniplate placement is more complex because of the 2 surgical procedures to the region for placement and removal.18 The surgical interventions for both devices are generally well accepted by patients and providers, but there are some side effects or problems during treatment.15,34 Failure rates of miniplates have been reported between 7% and 11%.15 Choi et al17 stated that the failures might have resulted from soft tissue infections. They also recommended a new design of the transmucosal part of the appliance. This brings the question: Is there a way to lower the risk of infection with a new
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miniplate design? Many miniplates have been designed and evaluated both by finite element analysis for mechanical understanding and in clinical studies for success rates and treatment alternatives.30,35 The main idea behind the MPP was to elevate the miniplate over the soft tissue gingiva for easier placement and a more hygienic appliance that has similar mechanical advantages as a regular miniplate. Three appliances—washer,19 mini-implant ring,20 and spiky miniplate21—were found to be noteworthy in the reviewed literature. The washer had varying long spikes penetrating the gingiva, with the base in contact with the soft tissues, which may not be stable enough or can cause inflammation. The mini-implant ring showed promising results for increasing the stability. A finite element study showed that a spiky miniplate causes lower stresses on the surrounding cortical bone. Two studies investigated the buccal gingival tissue thickness, and both reported values below 2 mm, with the highest mean value at 1.84 mm.36,37 The MPP modeled in our study had a platform height of 2 mm, which was enough to elevate the miniplate from the gingival
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Fig 6. Principal maximum stresses on the cortical bone in scenario 1: A, around miniscrew 1; B, around miniscrew 2. Principal maximum stresses on the cortical bone in scenario 2: C, around miniscrew 1; D, around miniscrew 2.
tissues. Therefore, placement and removal surgeries could be omitted. Only a gingival punch would be needed to prepare the region for miniplate placement with the MPP. Miyawaki et al5 reported that prevention of inflammation of the peri-implant tissue is a critical factor for the success rates of temporary anchorage devices. Cornelis et al15 stated that careful brushing of miniplates and the surrounding mucosa with a soft toothbrush is important to prevent infection and thus to prevent miniplate failure. Therefore, elevating the miniplate over the gingival tissue facilitates cleaning the miniplate; this can reduce the risk of infections. MPP placement seems to offer some advantages; placement and removal surgeries are not necessary, the miniplate can be placed above the gingival tissues, and it lowers the risk of complications. The questions still needing answers were whether this design could provide enough stability and whether there would be any negative effects of generating new moments, caused by the 2-mm elevation of the MPP. It was not necessarily expected to improve the mechanical stability. This new
design was expected to have stability similar to a regular miniplate, which is successful in clinical situations.15 Von Mises stress values were reported for the cortical bone layers to provide an indication of the average stress levels and to compare both scenarios. Von Mises values yield a criterion that applies best to ductile materials such as metals; however, for brittle materials such as bone, they are not favorable.38 Principal maximum and principal minimum values were reported to determine the local risk indicator or physiologic bone failure, and to compare stresses in detail in the different regions of interest. Principal maximum stresses had positive values, indicating tension, and principal minimum stresses had negative values, indicating compression. The results of this study are promising. The highest von Mises stresses on the cortical bone decreased from 0.5 to 0.3 MPa when MPPs were used. Stresses generated on the cortical bone are mainly focused around the necks of the miniscrews.30 Duaibis et al38 stated that the majority of stress in bone is transferred to the cortical layer on the side of compression; because
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Fig 7. Principal minimum stresses on the cortical bone in scenario 1: A, around miniscrew 1; B, around miniscrew 2. Principal minimum stresses on the cortical bone in scenario 2: C, around miniscrew 1; D, around miniscrew 2. Table II. Stress values in cortical bone Cortical bone Scenario 1 (miniplate 1 2 miniscrews) Highest value measured Neck of miniscrew 1 (1) Neck of miniscrew 1 ( ) Neck of miniscrew 2 (1) Neck of miniscrew 2 ( ) Scenario 2 (miniplate 1 2 MPPs 1 2 miniscrews) Highest value measured Neck of miniscrew 1 (1) Neck of miniscrew 1 ( ) Neck of miniscrew 2 (1) Neck of miniscrew 2 ( ) Edge of MPP Spikes of MPP
Von Mises (MPa)
Principal maximum (MPa)
0.50 0.42 0.35 0.38 0.48
0.60 0.00 0.42 0.00 0.48
0.60 0.48 0.00 0.45 0.00
0.30 0.08 0.22 0.08 0.20 0.20 0.10
0.31 0.07 0.22 0.07 0.20 0.00 0.90
0.34 0.00 0.01 0.00 0.01 0.30 0.10
cortical bone absorbs most of the stress, it may be the determining factor in miniscrew stability. Principal maximum stresses on the cortical bone around the miniscrews decreased from 0.42 to 0.48 MPa to 0.20 to 0.22 MPa. This may be caused by the stability provided by the MPP to prevent movement of the
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Principal minimum (MPa)
miniscrew. Principal minimum stresses on the cortical bone around the miniscrews were the focus of this study. These stresses of tension decreased from 0.45 to 0.48 MPa to 0.01 MPa for both sides. This was further investigated by measuring the stresses generated on the cortical bone around the MPPs. The highest
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Fig 8. Von Mises stresses on the trabecular bone: A, in scenario 1; B, in scenario 2. Table III. Stress values in trabecular bone Trabecular bone Scenario 1 (miniplate 1 2 miniscrews) Highest value measured Around miniscrew 1 (1) Around miniscrew 1 ( ) Around miniscrew 2 (1) Around miniscrew 2 ( ) Scenario 2 (miniplate 1 2 MPPs 1 2 miniscrews) Highest value measured Around miniscrew 1 (1) Around miniscrew 1 ( ) Around miniscrew 2 (1) Around miniscrew 2 ( )
Von Mises (MPa)
Principal maximum (MPa)
0.06 0.04 0.03 0.04 0.02
0.06 0.00 0.02 0.00 0.02
0.06 0.04 0.00 0.05 0.00
0.03 0.01 0.02 0.01 0.02
0.03 0.00 0.02 0.00 0.02
0.04 0.00 0.00 0.00 0.00
stresses generated on cortical bone were 0.3 MPa at the edge of the MPP and 0.1 MPa at the spikes. Therefore, it was concluded that when the force was applied on the miniplate, the MPPs transfer the load to the surface of the cortical bone; this allows much less stress to be formed around the miniscrews. Spikes also increased the bone surface and stabilized the system, as shown by Nalbantgil et al.21 The highest von Mises stresses in the trabecular bone decreased from 0.06 to 0.03 MPa. The
Principal minimum (MPa)
principal maximum stresses generated in the trabecular bone around the miniscrew stayed the same, whereas the principal minimum stresses decreased from 0.04 to 0.05 MPa to undetectable levels below 0.01 MPa. Overloading regions were detected in the von Mises stresses generated on the titanium elements of the 2 models. The highest von Mises stresses in the miniscrews increased from 1.3 to 9.6 MPa. This was further investigated, and the reason was the contact between the first
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Table IV. Stress values on miniscrews, MPPs, and miniplates Miniscrew Miniscrew/MPP/miniplate Von Mises (MPa) Scenario 1 (miniplate 1 2 miniscrews) Highest value measured 1.30 Neck of miniscrew 1 (1) 1.12 Neck of miniscrew 1 ( ) 0.45 Neck of miniscrew 2 (1) 0.42 Neck of miniscrew 2 ( ) 1.25 Scenario 2 (miniplate 1 2 MPPs 1 2 miniscrews) Highest value measured 9.60 Neck of miniscrew 1 (1) 1.30 Neck of miniscrew 1 ( ) 0.30 Neck of miniscrew 2 (1) 0.38 Neck of miniscrew 2 ( ) 1.28
MPP Von Mises (MPa)
Miniplate Von Mises (MPa) 3.30 1.10 1.10
4.30 2.52 0.10 3.08 0.10
3.30 1.40 1.60
Fig 9. A, Distance of the closest point of the first thread to the miniscrew head; B, distance of the farthest point of the first thread to the miniscrew head.
thread of the miniscrew and the MPP. MPPs with 2 mm of height elevated the miniscrews as well, causing the threads to be in contact with their inner surfaces. This can be corrected using a miniscrew without threads in the neck region or by extending the neck of the screw, but that was not necessary. Elevation of the miniscrew caused 2 more possible problems. The first was the shortening of the infrabony length, but it was reported that the infrabony length change did not affect the stresses in the cortical bone around the miniscrew.30,38 Second, because of the elevation, the increase of the miniscrew head length could cause an increase in the stresses generated on the cortical bone around the miniscrews, as stated previously in the literature.38,39 Instead of an increase, however, the newly designed system showed significant decreases in the stresses; this shows the mechanical stability provided by MPPs. Another overloading was detected on the miniplates; the von Mises stresses for both miniplates were high because the force application points were localized on them. Stresses on the miniplates around the
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miniscrews were investigated separately to isolate this overloading and to obtain comparable data. The first thread of the miniscrew started at 0.7 mm from the neck of the miniscrew head, and the farthest threadless point was 1.4 mm away (Fig 9). Therefore, the first thread pitch was 0.7 mm, whereas the continuing thread pitches were 0.9 mm (Fig 2). In scenario 1, the cervical threadless part was touching the corner of the cortical bone surface at 0.7 mm from the superior margin; for scenario 2, the threads were in contact with the cortical bone from the beginning of cortical bone penetration (Fig 1). Motoyoshi et al27 investigated this in detail; cervical threadless miniscrews decreased the stresses generated on the cortical bone around the miniscrew neck. They also reported that the first thread relationship in different loading directions causes mechanical anisotropy, which resulted in higher stress formations. In scenario 2, the tip of the thread ridge was located close to the superior margin of the cortical bone (Fig 1, B); this causes higher stresses to be formed when compared with the cervical threadless miniscrew
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relationship.27 Even though there was close thread contact in scenario 2, the MPP's effect in the decrease of stress was much more effective. The middle point of the miniplates was chosen to interpret data more easily and to make the analysis of the results more understandable. The models were designed to be comparable, standardized, easy to understand, and unaffected by any unwanted variable. This system offers a new approach to miniplates; new auxiliaries and designs can be created for future investigations and clinical use for treatments. Critical stress values for cortical bone were reported in several studies. The maximum principal stress theory states that local overloading in the cortical bone was assumed to occur when the peak principal maximum stress reached the ultimate tensile strength of 100 to 130 MPa, or when the peak principal minimum stress reached the ultimate compressive strength of 170 to 190 MPa.38,40,41 The critical threshold for cortical bone resorption was stated as 50 MPa in several studies.27,42,43 Li et al44 created a new mathematic model to simulate overload bone resorption and prepared a graph for underload and overload values for different bone densities that might cause bone resorption around implants. At regions with high bone densities, they reported that stresses over 25 MPa would cause overload bone resorption. Stresses generated in this study in cortical bone were measured below all the critical threshold values stated in the literature. The results of this study showed that the MPP system may provide the desired stability in the clinic as well as many other advantages; however, more clinical studies are needed to further evaluate the system. CONCLUSIONS
This new anchorage device, which elevates the miniplate over the gingiva, can be placed by removing the gingiva with a punch and placing the MPPs before miniplate placement, without the need for flap surgery; MPPs can also be removed without surgery. Within the limitations of this finite element analysis study, we concluded the following. 1. 2.
3.
4.
The newly designed MPP can be a new approach for miniplate placement. When a miniplate was placed over the MPP, the highest von Mises principal maximum and principal minimum stresses decreased in both cortical and trabecular bone. MPPs increased the bone contact area and distributed the stresses on the cortical bone surface, resulting in decreased stresses around the fixation miniscrews. Clinical studies are necessary to confirm these results.
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ACKNOWLEDGMENT
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