# An Efficient Distalization Technique Using Coil Springs and Mini Screws--A Finite Element Analysis
## 1 Introduction  
Molar distalization is a commonly used procedure to obtain aligning space in orthodontic treatment, where doctors utilize the obtained space to correct crowding, Angle's class II molar relationship, or incisor proclination. Various types of appliances have been proposed to distalize molars efficiently, such as the headgear, the pendulum appliance, the Greenfield molar distalizer, the Carriere distalizer, etc. Today, distalization using orthodontic anchor screws is becoming more common because it is less dependent on patients' adherence, more predictable than the conventional appliances [1] by providing solid skeletal anchorage, and its force system can be modified easily by changing the position of the anchor screws. For efficient distalization, it is preferable to apply well-distributed distalization force over premolars and molars, and to ensure that stress concentration does not occur at a specific tooth. If such stress concentration occurs on a premolar, the molars cannot receive sufficient distalization force, while if it occurs on a molar, the premolars will be left behind. Moreover, high compressive stress caused by the stress concentration in the periodontal ligament (PDL) slows down the tooth movement due to hyalinization of the PDL. To avoid such a phenomenon, some techniques employ two-step distalization [2; 3; 4; 5], where they first distalize the molars using the screw anchorage directly or indirectly, and then distalize the rest of teeth. This method can apply force to molars more securely, but the two-step procedure will increase the total time required for whole-arch distalization, since thepremolars are suspended while the molars are being distalized. On the other hand, some previous studies have reported that it is possible to distalize the whole arch as one group by using anchor screws [6; 7; 8]. This method is called grouped distalization and can shorten the required treatment period if the force distribution is appropriate, i.e., all premolars and molars receive the ideal level of distalization force, and no stress concentration occurs on a specific tooth. However, [PERSON] et al. [9] reported that higher stress was observed in the periodontal ligaments of canines and premolars while lower stress was observed in molars when the grouped distalization method was employed, suggesting that grouped distalization does not generate optimal force distribution. To improve the force distribution over posterior teeth and achieve efficient distalization, we propose a novel technique for grouped distalization. In the conventional technique for grouped distalization, distalization force is applied to a single tooth, which is typically a canine or a premolar, and the rest of teeth receive force through tooth-tooth contact and friction between the bracket and archive. However, the contact force between teeth gradually decreases as the force is transmitted to the distal tooth because part of the contact force is supported by the PDL. This is why canines and premolars receive more force and molars receive less force in the grouped distalization method, and the molars will not move until the premolars are distalized. To overcome this issue, we propose a novel method where we insert open-coil springs with different spring rates between the posterior teeth. A retraction force of 4 N is applied to the first premolar, an open-coil spring of 3 N is inserted between the premolars, another spring of 2 N is inserted between the second premolar and the first molar, and a 1 N spring is inserted between the molars (Figure 1). Since adjacent springs cancel one another's forces, each of the premolars and molars should receive 1 N, meaning that the distalization force will be applied more directly and securely to the molars. Although the proposed technique generates better force distribution theoretically, its force system might not be as ideal as expected. Since the brackets of the posterior teeth are not in a straight line, the actual force distribution might be different from what is expected.