# An Efficient Distalization Technique Using Coil Springs and Mini Screws--A Finite Element Analysis
## 4 Discussion
This compressive stress indicates that the distalization force was transmitted by open-coil springs in the Sp model, while it was transmitted by the contact between the teeth and the friction in the NoSp model, and more moment was generated, which caused distal rotation. This rotational force in the Sp model might be helpful for the treatment of class II cases because molars in such cases usually show mesial rotation [18] and have to be rotated distally to achieve a class I molar relationship. The proposed technique has great advantages when one or more teeth do not respond to the treatment. Since the results of the present study suggested that a tooth would not be distalized until its mesial teeth were distalized, if the mesial tooth does not move for some reason--such as ankylosis--the tooth movement sequence will stop at the tooth. This makes it difficult to find which tooth is causing the problem, because all that a doctor can observe is the fact that the grouped distalization failed. On the other hand, it is easy to find which tooth causes the problem with the proposed method, because even if a tooth in the arch absorbs all force transmitted from its mesial teeth, the distal coil spring still appliesdistalization force to its distal tooth, resulting in a gap between the problematic tooth and the distal tooth. To apply this technique in real clinical practice, it must be considered that the actual spring rate is not always at the same level. In the present study, we simulated the force generated by springs and elastics using external forces, so the force level was always the same regardless of the distance between brackets. However, the force generated by a spring will differs based on the amount of activation in the real world. A previous study showed that even if a NiTi coil spring was advertised that could generate continuous force of a certain level, its force increased significantly as the amount of activation increased [19]. Thus, the force-level springs should be measured accurately outside of the mouth before the application of the present mechanics. The simulation of a long-term tooth movement was necessary to clarify the force system of the proposed method. Since the results suggested that the distalization force was not transmitted to the second molar until bone remodeling of the mesial teeth occurred, and that the stress distribution dramatically changed throughout the analysis, we would have overestimated the efficiency of the proposed method if we had evaluated the initial displacement only. The simulation of a long-term tooth movement made it possible to compare the force systems of the conventional grouped distalization method and our proposed method more impartially and accurately. A limitation of the present study was that it did not take bone density into account. It takes more time to resorb alveolar bone of high density. For example, there is a wall of cortical bone in the distal region of the second molar, meaning that more force is required to move the second molar distally or completely block the movement of the second molar. However, the bone remodeling algorithm that we employed in the present study assumed that bone density was uniform everywhere. Thus, the molars could be distalized infinitely for as long as the distalization force was applied and the number of bone remodeling operations was iterated. This behavior was obviously different from that in the real alveolar bone. This is was why we finished the bone remodeling operation after 30 iterations and avoided excessive distalization, which would probably be seen as unrealistic anyway. However, even with this limitation and the use of the simplest way of simulating bone remodeling, we believe that simulated bone remodeling provided more information than an analysis of the initial displacement in the PDL, which most of the other FEM studies employed.