## 1 Introduction
[5] proposed another method, in which they adapted light-curing splint resin over the occlusal and insertion area of the cast, then a hole for a guide sleeve was made. Both articles reported that higher accuracy could be achieved with this method than by using freehand insertion. However, in these methods, clinicians needed to take X-rays before and after positioning the surgical guide to ensure the sleeve was accurately positioned. That was because neither the intra-oral scanner nor the desktop model scanner were common in orthodontic practice at that time, and the superimposition of CBCT and dental geometry was not performed in the planning stage. Since intra-oral scanners have become more common in orthodontic practice in the past decade, and 3D printers are also increasingly common, methods using these devices have been proposed recently [8; 9; 10]. The overall designs of these methods are similar to those of a previous study that used light-curing splint resin, but it was designed using CAD software based on the CBCT and 3D models of the patient's dentition and printed with stereolithography (SLA) or digital light processing (DLP) 3D printers. As the superimposition of the CBCT and the 3D dental model was completed before designing the surgical guide, these methods did not require X-ray validation after making the surgical sprint. The surgical guides for miniscrews are designed either for the self-drilling or the self-tapping methods. Self-drilling is a method in which the miniscrew makes the hole and thread by itself, and the self-tapping method is a method in which a drill bit is used to make the hole, and a miniscrew is used to make the thread in the hole. Although the self-tapping method requires more steps than the self-drilling method, it has some advantages over the self-drilling method. Previous research has suggested that the self-drilling method tends to cause more bone damage around the hole, and that more bone cracks were observed than when using the self-tapping method [11]. Although a systematic review concluded that their success rate was not significantly different, it was also reported that the self-drilling method shows a higher failure rate when root proximity occurs [12]. Another advantage of the self-tapping method is that it can reduce insertion torque. Previous research has suggested that the insertion torque may exceed a yield torque of miniscrews during the placement with a driver without a torque limiter [13]. Pre-drilling before insertion can reduce the insertion torque and reduces the risk of a fracture. However, to our knowledge, no research has been performed for determining the best parameters of surgical guides for the self-tapping method. As of now, most research relating to surgical guides has been about the accuracy of surgical guides for the self-drilling method, and only a few studies have been conducted about the self-tapping method. The objective of this study was to assess the effect of the length of surgical guides on the accuracy of drilling using the self-tapping method. A longer surgical guide should restrict the direction of the drill more tightly, but a longer guide is less easy to handle. In this study, we assessed the accuracy of the hole position and the direction using a model experiment to find the minimal lengths that can achieve acceptable accuracy.