En masse retraction after extraction of the first premolar can be conducted using continuous or segmented mechanics. Conventional methods for anterior en masse retraction in sliding mechanics produce extrusion of the upper incisors and clockwise rotation of the occlusal plane, thus causing problems in applying to patients with vertical dento-alveolar excess or gummy smile [27]. Extrusion of molars is not suitable for hyperdivergence patients. Thus, the employment of an appropriate mechanic that controls the extrusion of molars is essential especially in vertical grower patients. However, using the mini screw for anterior segment retraction has minimum (or no) effects on posterior teeth, reducing the adverse side effects of treatment. Upadhyay et al. [21] reported significant improvement in bi-alveolar protrusion patients who were treated with mini screws. Significant reduction in the vertical dimension by intrusion in the maxillary incisors and molars was also obtained.
The center of resistance (CR) for anterior teeth could not be clearly defined because it would change with tooth movement. Melsen et al. [27] indicated the CR of anterior teeth was located 13.5 mm posteriorly and 9 mm superiorly from the center of the arch wire. True translation will occur if the force passes through the CR whereas if the force vector passes below the center of resistance of anterior dentition, uncontrolled tipping of all anterior teeth would be inevitable. Some other investigators [24, 27, 28] estimated the center of resistance of six maxillary anterior teeth to be 13.5 mm apical and 14 mm posterior to the incisal edge of central incisors.
In order to achieve the desired type of tooth movement, altering the height of the anterior retraction hook can make the force application close to the CR. Moreover, different heights of mini implants quantify the torque control from different levels of force vectors [18].
Numerous positioning of mini screws have been experimented. Lim [29] stated that in order to improve the vertical force vector, the mini implant should be inserted between the first and second premolars. Lee et al. [30] also reported that greater intrusion of all of the incisor tips and root apexes resulted following insertion of the mini implant into the mesial second premolar area. We employed two appropriate positions for the mini screw in the mesial and distal of the second premolar, 6 mm above the arch wire. Forces were applied from the mini screw to four different levels of anterior hook height: 0, 3, 6, and 9 mm. Force direction from the mini screw to the anterior power arms in this study has been demonstrated in schematic Figs. 3 and 4. In order to conduct en masse retraction of anterior teeth, a force of 150 gm per side was applied and shown to be in physiologic limits for anterior teeth retraction [18, 24, 25].
Schematic force diagram and ?
1 angle in the mesial placement of the mini screw
Schematic force diagram and ?
2 angle in the distal placement of the mini screw (?
1???
2)
The angle formed between the force direction of the mini screw and the horizontal component is called the ? angle. Changing the ? angle will alter the paradigm of biomechanics. Increasing the height of the anterior power arm or the distal placement of the mini screw would cause a decrease in the ? angle but would increase the horizontal force (horizontal force?=?force?×?cos?). The amount of vertical force is also dependent on the ? angle, i.e., decreasing the ? angle can reduce the vertical force (vertical force?=?force?×?sin?) [16].
In our study, maximum intrusion in both positions of the screw occurred with 0 mm of the power arm (the largest ? angle) whereas with 3 mm of the power arm, the intrusion decreased (following decrease in the ? angle), and with 9 mm of the power arm, the entire anterior dentition was slightly extruded. Theoretically, according to the ? angle, with 6 mm of the power arm, anterior dentition must be neither intruded nor extruded because the position of the mini screw and the edge of the power arm are at the same vertical level (??=?0). But in our finite element analysis, insignificant intrusion was observed which can be interpreted due to some distance from the CR. In addition, when sliding mechanics are employed, the effect of arch wire deflection acting on a tooth can play a role and should be taken into consideration [15].
Mesial displacement (larger ? angle) of the mini screw caused greater moment than distal. At a power arm height of 6 mm in combination with distal positioning of the mini screw, minimum effects on the vertical plane resulted. This was consistent with the reports of Lim [29] and Lee et al. [30] which emphasized that insertion of the mini screw between the first and second premolars increases the vertical force vector.
The evaluation of initial tooth movement in the sagittal plane showed that uncontrolled tipping with 0, 3, and 6 mm of the power arm occurred in both positions of the mini screw. Line of action in all these cases passed below the estimated center of resistance of the anterior teeth segment. Obviously, the clockwise moment on the anterior dentition decreases with an increase of the length of the power arm (less distance between the point of action and the center of resistance). These findings are in line with those of the finite element study done by Kojima et al. [23]. They observed that increasing the height of the power arm reduced the clockwise moment of the anterior teeth segment during retraction.
During en masse retraction in the case of the 9-mm power arm, bodily movement (unequal crown and root tipping in the same direction) occurred as the total force passes close to the estimated center of resistance of the anterior teeth [30].
Slight extrusion happened when applying force to the 9-mm power arm. It can be assumed that bodily movement in the anterior dentition occurred, but because of the difference in the vertical level of the mini screw and the power arm, some extrusion was observed. In a FEM study conducted by Tominaga et al. [15] at a level of 5.5 mm of the power arm, no rotation was produced and bodily movement of the anterior segment occurred. Lingual root tipping was observed when the retraction arm was above 5.5 mm.
This side effect in the vertical plane is not suitable in patients with deep overbite or gummy smile. However, the long anterior power arm is uncomfortable and requires good patient cooperation.
Our results indicated that rotation and bodily movements of the anterior dentition were more obvious in the distal placement of the mini screw as compared to the mesial placement.
As a result, with patients who need extraction of the first premolar with different discrepancies in the sagittal and vertical planes, a precision treatment plan with fewer adverse side effects should be chosen with respect to the existing malocclusion.
It is, in fact, the patient’s requirements, such as esthetic, occlusion, function, intensity of discrepancy, and comfort, that guide us to choose the best position of the mini screw and anterior power arm height for having a more satisfactory treatment outcome.
Finite element analysis calculated the initial tooth movement by using an accurate method [31]. These useful information increase our knowledge with regard to the response of tooth displacement to a specific force direction, but this might not be enough for predicting orthodontic tooth movement in clinical practice. Finite element is based on mechanical law [32] without considering the oral cavity condition such as saliva, chewing force, and habit.
Geometric modeling of the bone and PDL is a limitation of finite element study. In this study, the bone was modeled as a solid body and the difference between cancellous and cortical bone was not defined. Also, PDL was modeled as a uniform layer with the same thickness, but even through the root, it is not monotonous.