## 1 Introduction  
The use of temporary skeletal anchorage devices (TADs) has simplified and enhanced the efficacy of many orthodontic treatments, reducing the risk of undesired tooth movements [1]. Furthermore, they were met with a greater acceptance rate due to the reduced need for patient's compliance and their superior aesthetic appearance [2; 3]. For these reasons, mini-implants are currently applied in a wide range of orthodontic treatments, such as upper molar distalization, mesialization or intrusion, rapid maxillary expansion, and extrusion of impacted teeth [4; 5; 6; 7]. To achieve successful retention, mini-implants should be placed in the optimal insertion site, avoiding contact with dental roots and interference with the intended path of tooth movement. Moreover, they should have a diameter and length adequate to the applied forces [8]. Considering these principles, the palate seems to be the most predictable region for obtaining a successful mini-implant anchorage, with a survival rate ranging between 91.5% and 97.9% for palatal mini-implants, compared to 67.5% for the buccal ones [9; 10]. In the palate, the paramedian region immediately posterior to the palatal rugae and the medial area corresponding to the palatal suture, described as the \"T-Zone\", constitute the most suitable regions for the insertion of palatal mini-implants [8]. This area is generally characterized by the presence of an adequate bone volume, a reduced soft tissue thickness, and the absence of anatomical structures which could be damaged, allowing, in most of cases, a predictable mini-implant insertion without performing cone beam computed tomography (CBCT) [11; 12; 13]. Another parameter strictly related to the stability of mini-implants is the insertion torque, which depends on the bone quality, on the design and size of the mini-implants, and on the predrilling procedure [14; 15]. To reduce biological and mechanical complications, the ideal insertion torque should range between 5 and 10 Ncm for mini-implants with a diameter of 1.6 mm [15; 16]. Although self-drilling mini-implants are associated with higher insertion torque, which provides a higher primary stability compared to the self-cutting ones [17], a tendency to fracture has been reported during the insertion of self-drilling mini-implants in thick or dense cortical bone when excessive insertion torques were applied [18]. Moreover, excessive torque in dense cortical bone may induce heat damage or vessel compression, which might lead to bone resorption and lack of secondary stability [19; 20]. For these reasons, the use of pilot holes with a smaller diameter has been successfully proposed to obtain a good mini-implant primary stability by decreasing insertion torque (at least in adults) [16; 21]. In dental implant surgery, the use of computer-aided design (CAD) milled or three-dimensional (3D) printed surgical insertion guides has been widely implemented in the past years. Indeed, guided implant surgery is currently a reliable and useful method to prevent anatomical structure damage, reduce the surgical trauma, and place the dental implant in the best position for the subsequent prosthetic restoration [22; 23]. More recently, the use of insertion guides has been proposed and are becoming increasingly popular for the insertion of orthodontic mini-implants. Although the \"T-zone\" is generally considered a safe zone for mini-implants placement, in certain conditions, such as the presence of palatally displaced teeth, cleft palate, or minor palatal bone support, the use of insertion guides can be extremely useful. Moreover, the digitally planned mini-implant position allows for the production of the orthodontic supraconstruction together with the guide, enabling its placement in the same appointment [24; 25]. Different designs and techniques have been proposed for the realization of orthodontic implant insertion guides.