# An Efficient Distalization Technique Using Coil Springs and Mini Screws--A Finite Element Analysis
## 4 Discussion  
The Sp model achieved more distalization on the molars and the second premolar, while the NoSp model showed slightly more distalization the on first premolar, and the distalization was achieved more efficiently with the open-coil springs; 1.5 times more distalization was observed in the Sp model than the NoSp model, and the first and second premolar are more distalization than the NoSp model. Figure 8: Distribution of compressive stress in the PDL at the initial and final steps. Higher compressive stress was observed in the initial displacement (left). A large, highly compressed region existed at the first premolar in the NoSp model. molars were more evenly distalized in the Sp model, while the second molar was distalized significantly less than the first molar in the NoSp model. The distal displacements of the molars were smaller than those of the premolars in both the Sp and NoSp models. Since the force levels acting on the molars and premolars were not significantly different in the Sp model, this difference in the amount of displacement was probably caused by the difference in the surface area of the PDL. The PDL area of molars is larger than that of premolars, and moving molars requires more force than moving premolars at the same rate. As a result, the molars acted as a speed controller of the grouped distalization. Most of the distalization force was absorbed by the PDL of the first premolar in the NoSp model. The first premolar received 60% of the distalization force in the NoSp model, and the minimum principal stress in the PDL of the first premolar was also significantly higher in the NoSp model--nearly twice as high as that in the Sp model. This high load on the first premolar may cause hyalnization of the PDL and root resorption [17]. At the same time, less than 5% of the distalization force was transmitted to the second molar. This low load on the second molar may cause no movement of the second molar. Although we constructed the model so that the teeth had tight interproximal contact, the contact surface could not transmit the moment, because it only transmitted pushing force and could not pull the adjacent surface. This was because the premolar was rotated by the retraction force and the posterior teeth did not move as one unit. Furthermore, since the simulation using FEM did not consider the threshold of stress that causes bone remodeling in real PDLs, the simulated bone remodeling would occur even if the force was smaller than the threshold. Therefore, the displacement of the second molar in a real patient might be smaller than that in the simulation. The force distribution in the NoSp model was gradually improved over time; however, it was still worse than that in the Sp model at the end of the simulation. After some steps of bone remodeling in the simulation, the force transmitted to the molars gradually increased due to contact between the teeth and frictional forces between the archive and the brackets. Compared to the NoSp model, the force was more evenly distributed over the premolar and the molars in the Sp model. The force transmitted to each tooth was 1.0 N at the initial displacement, and little frictional force existed, as we had expected. This was probably because that the archive and the brackets were not completely engaged until the bone remodeling operation was executed a certain number of times. The stress was also distributed more evenly than in the NoSp model. The peak stress level at the premolar in the Sp model was significantly lower than that in the NoSp model, while higher stress was observed at the molars, suggesting that the Sp model could transmit forces more efficiently. More compressive stress was observed on the distobuccal side of the PDL in the Sp model during the initial phase of the distalization.