## 4 Discussion
However, this part is the farthest from the gingival surface, which indicates that the guide hole must be long, and therefore bulky. Such a bulky guide might affect the accuracy of the insertion, due to pressure from the surrounding soft tissue during insertion. On the other hand, in the self-tapping system, since the surgical guide will be removed after drilling, the hole can be the same diameter as the drill bit. Thus, the surgical guide for the self-tapping method can be much thinner, and still achieve the same accuracy. However, in some situations, the self-drilling method has advantages over the self-tapping method. For example, self-drilling is probably preferable for an insertion site with low bone density to obtain enough insertion torque and mechanical retention of the screw. In such cases, surgical guides for the self-drilling method will be needed, and it is our intention to investigate this by comparing the two types of surgical guides in future. A surgical guide can be more generic when it is designed for self-tapping than for self-drilling. There are numerous miniscrew products in the market, and they all have different diameters, taper angles, head shapes, and drivers for insertion. Thus, surgical guides must be optimized for each product to achieve better accuracy. This can be difficult because manufacturers do not always provide their product's measurements in order to avoid reverse engineering. On the other hand, the parameter of a drill bit is its diameter only. Only the cutter part of a drill bit goes through the surgical guide, and the cutter part does not have a taper. This means that, as long as the hole diameter is configuredcorrectly, a surgical guide will fit any miniscrew system. This is of great advantage for dental laboratories, which often receive various requests from different doctors. In a preliminary experiment, we tested different diameters of holes for the surgical guides and decided to make the holes slightly smaller than the diameter of the drill. As a 3D printer is not 100% accurate, the hole diameter on the printed surgical guide was smaller than the diameter of its original data. Therefore, initially we printed the guide with a hole of 1.1 and 1.2 mm to allow the drill bit of 1.0 mm to pass through the hole. However, it increased the gap between the guide and the drill bit, and the accuracy seemed to decrease. Thus, we set the hole diameter at 1.0 mm in the original data, which was printed slightly smaller than 1.0 mm, then we enlarged the hole diameter using another drill bit of 1.0 mm before the experiment. It should be noted that the freehand drilling might be less accurate in a real-world patient than in the results of this study. We employed micro-motor and checked the insertion angle from various angles before drilling, which was an easier setting to work in, compared with an intra-oral environment. In a real-life setting, the viewpoints are limited, and the driver is pushed by buccal mucosa. Thus, the accuracy will probably decrease. As the objective of this study was to assess the differences created by the guide lengths, we controlled these environmental factors. However, using phantoms and having the drilling performed by a less experienced doctor should be better for evaluating the effect of a surgical guide. Drilling using a surgical guide is less sensitive to the skills of a doctor and can reproduce the same results; however, the fit of surgical guide should be inspected carefully before the drilling. If the guide does not fit the tooth surface perfectly, the error will be greater and might be worse than using freehand insertion. To avoid this issue, we designed the guide to expose a part of the crown, so that the gap between the tooth and the guide can be observed. As the surgical guide is removed in the self-tapping method when a miniscrew is inserted, it is easier for doctors to observe the distance between the neck of a miniscrew and the gingival tissue.