The possibility of “total arch distalization’ or en-masse distalization of the molars for the correction of class II malocclusions remains a challenge in orthodontics. The main advantage of this approach is the possibility of reducing the overall treatment duration while simplifying the distalization steps [8]. However, the distalization of several teeth requires an increase in the force magnitude that might cause undesirable biomechanical effects. Therefore, the purpose of the present study was to compare the biomechanical effects of one versus en-masse two molars distalization using the iPanda.
Stress distribution pattern
For the one and two molars molar distalization models, the stress was concentrated at the furcation area and along the distal surface of all roots. This stress distribution pattern showed that the distalizing force provided by the iPanda can be advantageous for a controlled movement of the molars. This result is in agreement with a previous clinical study that showed the bodily distalization of a single molar with the iPanda [7, 13]. Moreover, since the center of resistance of the molar is located within the furcation area, the desirable resultant distalization vectors should pass to the level of the molar’s center of resistance to minimize the distal tipping of the molars [22]. The stress distribution at the furcation area and along the distal surface of the roots suggests the controlled distal movement of the molar.
In the present study, the use of 50 g force to distalize the single molar was shown to be beneficial to promote homogeneous stress distribution along with the distal surface of the molar roots during distalization with minimal side effects. Moreover, a single tooth movement generates a more simplified force system with reduced undesirable effects in adjacent teeth. The result of this study is in agreement with Kinzinger et al. [6] who investigated the effect of tooth eruption in a composed system during distalization. In their study, it was concluded that the stage of development of the third molar influenced the pattern of tooth movements, such as tipping and extrusion.
For en-masse two molars distalization, the increase in the load at the first molar was not sufficient to generate an increase in the stress along with the distal surface of roots of the second molar that would promote their distalization. FEA showed that the stress in the first molar was approximately 10 times higher than in the second molar, thus indicating that the force delivered to the second molar would not be sufficient to cause the distalization of the second molar. The difference in stress distribution between the first and second molars indicated that the force transfer from the first to the second molar is far beyond ideal; therefore, the force system might be adjusted to allow optimized distalization. The results are in agreement with the previous article. The study Ammoury et al. [23] compared the effect of direct versus indirect anchorage for en masse distalization. They concluded that 150 g is not enough for efficient distalization, since it resulted in low stresses and displacements at the molars. Therefore, the need for a heavier force for the initial molar movement is necessary.
Based on the results of the present study, the suggested approach for the distalization of two molars would be the simultaneous distalization of the first molar with the iPanda (100 g) combined with the distalization of the second molar aided by an open coil spring (50 g) applied with the orthodontic appliance. With this design, both molars are simultaneously distalized with reduced force levels. The distalization force applied to the second molar provided by the open coil spring (50 g) generates undesirable mesialization forces to the first molar. However, this mesialization force is neutralized with the iPanda providing direct distalization forces (100 g) to the first molar, thus resulting in the distalization of both first and second molars. This suggested design might improve the results in terms of biomechanical performance. However, further studies need to be performed to evaluate its effectiveness.
In the present study, a characteristic stress distribution pattern between the mesial and distal miniscrew was observed. A higher-stress concentration was found in the distal miniscrew compared to the mesial miniscrew. The results are in agreement with the previous iPanda FEA study [24]. Moreover, although the stress distribution pattern of the miniscrews in both models was relatively similar, the amount of stress in the one molar was lower than the two molar distalization.
The stress distribution pattern showed the role of each miniscrews that resisted the distalization force in three dimensions. The stress in the mesial and distal surface of the threaded part of the distal miniscrew showed the resistance of the miniscrew to the distalization force in the sagittal direction during iPanda activation. While the stress in the mesial miniscrew which shows at the lateral side of the miniscrew platform showed resistance to the vertical intrusive force. The stress in the distal miniscrew was approximately six times higher than in the mesial miniscrew, thus indicating that the distal miniscrew is more prone to higher stress and more prone to failure.
Although the stress distribution in the miniscrews in both models followed the same pattern, the relationship of the position of the mesial miniscrew and the moving tooth might have effects on the stress in the tooth.
Displacement pattern
Regarding the displacement pattern for one molar distalization, the initial displacement analysis demonstrated a large amount of molar distalization with a small amount of undesirable distal and buccal crown tipping. This result is in agreement with previous clinical studies of molar distalization [13]. The displacement pattern exhibited by one molar distalization under 50 g load represents the desired tooth movement during the distalization of a molar with reduced side effects. Moreover, the results also support the efficiency of the iPanda for the three-dimensional control of the tooth movement during distalization.
In contrast, for the en-masse two molars distalization, a minimum amount of distalization combined with a high tendency towards distobuccal rotation, buccal tipping, and outward tilting of the first molar was observed. Although the iPanda is effective for a single molar distalization, the presence of a second molar generates undesirable resistance to the movement of the first molar which resulted in undesirable moments. Moreover, only a minimal amount of distal movement of the second molar was observed despite a load increase. The main explanation is that the load up to 200 g applied to the first molar was not enough to be transferred to the second molar. According to the results, the load applied to the first molar should higher than the load used in the present study to achieve a sufficient amount of distalization of the second molar. Consequently, the high levels of force ultimately generate various undesirable biomechanical effects, such as uncontrolled tipping and extrusions [25].
In the present study, both one and two molars distalization models exhibited consistent buccal movement of the molars. The first explanation for this buccal movement is the design of iPanda’s power arms that are straight and do not conform with the curved ovoid shape of the skeletal base. Adjustments in the direction of the power arms to conform to the dental arch form are important to minimize this problem. The second explanation is the height of force application, since the distalizing force is applied coronally to the center of resistance of the molars, therefore facilitating the buccal crown tipping of the molars. This results are in agreement with Yu et al. who described the displacement pattern of distalization first and the second molar simultaneously by a palatal plate in FEA, which shows that the first molar moved more widen laterally than distalized only first molar. A more heavy archwire would reduce distortions and minimize these effects [26].
In the present study, for both one and two molars distalization models, a minimal amount of extrusion of the molars was observed. This also can be explained by the height of force application coronally to the center of resistance of the molars and to the vector of force application parallel to the occlusal plane. Therefore, resulting in extrusion displacement. To avoid this effect, adjustments in both height and vector of force application should be performed. Our findings are in accordance to study of Yu et al. which use a palatal plate with various heights of the distalization vectors [26]. The authors further reported that the distalization force close to midpalatal suture, the effect of extrusion was replaced with intrusion and bodily movement might be achieved [26].
The en-masse distalization of first and second maxillary molars has been attempted by several authors with contrasting results [27, 28]. It has been reported that the presence of the second molars increases the duration of the distalization and produces more tipping of the second molars with anchorage loss [28]. In contrast, Bussik and Mcnamara reported that the presence and position of the second maxillary molar did not influence the amount and the type of the maxillary first molar distalization [29].
Moreover, the patient’s age and the stage of the second molar development play an important role in the amount and pattern of molar distalization [6]. Most of the studies that were performed in young patients, when the second molars were not completely erupted, the en-masse distalization of the first and second molars had been successfully performed [4, 30]. Comparing distalization effects according to molar eruption stage had observed that the distalization of the first molar with an unerupted second molar was 20% greater than that when the second molar was erupted [30]. However, it is also suggested that the most efficient timing when using an MPAP is after the full eruption of the second molar, since it decrease the mesial-in rotation of the molars. Therefore, confirming the strong influence of the presence of an erupted tooth on the path of distalization. A different approach for en-masse distalization using a sequential distalization pattern with an individual distalization of the second molar should be employed to obtain the maximum benefits of the en-masse distalization.
In the present study, a particular pattern of displacement between the mesial and distal miniscrews was observed. A couple of force was created with the pair of miniscrews, with the mesial miniscrew receiving an intrusion force and the distal miniscrew receiving an opposing extrusion force [31]. This system allows for the creation of a stable skeletal anchorage to withstand the high magnitude force that is required to perform en-masse distalization forces. Moreover, the pair of miniscrew implants of the iPanda is often placed in the midpalatal suture area, which is often regarded as the preferred site for miniscrew placement in the maxilla for its keratinized soft tissue and sufficient cortical bone [32]. Consequently, high stability values are often obtained with miniscrews placed in the midpalatal suture [33].
Our results suggest that the distalization of one molar is the most efficient treatment approach to obtain a controlled distalization of a molar, with the advantage of applying relatively low forces with reduced dental undesirable effects. On the other hand, the en-masse distalization of two molars requires high distalization force values that produce increased distal tipping, buccal tipping, and extrusion of the first molar. Additionally, the en-masse distalization produces minimal effects for the second molar.
However, the extension of the interpretation for a clinical situation should be considered with limitations. Since the results of FEM just explained the initial effect of stresses and tooth displacements within the PDL space before bone remodeling in one condition, the FEM presents intrinsic limitations. Therefore, evaluation of biomechanical effects in the case of anatomical variation, such as different bone stiffness or bone thickness, might result in different outcomes [23]. Moreover, improvement of the distalization force system should be performed to allow adequate force distribution during the en-masse distalization of the molars. Therefore, further studies are necessary to elucidate the optimum force system to perform the en-masse molar distalization.