## 2 Description of the Cases  
Two patients were referred for the extraction of clinically asymptomatic impacted left mandibular third molars. Informed consent was obtained and the two cases belonging to the case collection were approved by the Padova Hospital Ethical Review Board (Prot. 3719/AO/2016). The first patient, a 47-year-old healthy male, was referred for the suspicion of an odontogenic cyst associated with a horizontally oriented impacted tooth 38 incidentally discovered on a panoramic radiograph (case 1) (Figure 1). Case 2 consisted of a horizontally oriented deeply impacted tooth 38 in a 31-year-old healthy male patient, responsible for an increased probing depth distal to the adjacent second molar. In both cases, the unreported teeth were close to the mandibular canal (Figure 2). Preoperative cone-beam computed tomography (CBCT) was performed in both cases, and the main inclusion criteria for the adoption of the technique were evaluated, namely, the considerable thickness of the buccal bone cortex at multiple points, i.e., >2 mm, and the need to enhance the control of the operatory field in the presence of anatomical limits (inferior alveolar nerve). CBCT allowed the design and production of a 3D printed surgical guide following the SafeCut(r) workflow (SafeCut(r), GuidedSurgery2.0 s.r.l, Padova, Italy). The Digital Imaging and Communication in Medicine (DICOM) datasets were processed with dedicated diagnostic and analytical software (3 Diagnosys(r) 4.0, 3 DIEMME(r), Como, Italy). Structures such as the third molar, the pericoronal radiolucat area, the mandibular canal, and the roots of adjacent teeth were identified. Ideal bone-cutting planes were defined (angle and depth) in order to perform the osteotomies as close as possible to the roots of the adjacent teeth while avoiding any contact with them. The projections of the planned cutting planes allowed long-bevel osteotomies and the fashioning of a bone lid of adequate thickness and extent. The final guide design, including the anchorage to the adjacent teeth and the position of the stabilisation screw, was shaped using computer-aided design (CAD) software (PlastyCAD, 3 DIEMME(r), Como, Italy). The surgical guides were made of medical polyamide by means of a computer-aided manufacturing (CAM) process (3 Dfast s.r.l., Padova, Italy) and were designed to have both dental and bone stabilisation. In case 1, the template had an internal cutting profile and an anterior fixation screw (Figure 1), whereas in case 2, the template appeared as a solid with an external guide profile. Furthermore, the fixation screw was envisaged in the centre of the solid, with the expectation of using the same bone hole to stabilise the bone lid at the end of the operation using a longer screw anchored to the lingual cortical bone. The surgical procedures were performed under local anaesthesia and conscious sedation. A full-thickness flap of sufficient width was raised to easily place and secure the guide with a 1.3 mm fixation screw (Synthes GmbH, Oberdorf, Switzerland). Subsequently,osteotomies were performed following the internal (case 1) or external (case 2) contour of the template using a piezoelectric surgical device (Piezosurgery(r) Flex, Mectron S.p.A., Carasco, Italy) with a thin 0.25 mm insert (Mectron, insert OT12S). In case 2, a second insert was used to partially cut the cortical bone apically (Mectron, insert OT8L). The bone lid was detached and immediately placed in sterile saline in case 1, while in case 2, it remained partly anchored to the bone apically. In case 1 (Figure 3), a minimal pericoronal osteotomy was performed at tooth 38 with the piezoelectric insert OT12S, and then it was extracted using elevators without odontomy. Then, the pericoronal lesion was excised and placed in 10% buffered formalin for histological examination. The residual cavity was inspected and washed, and the lid repositioned and fixed back in place with a linear mini-plate stabilised with four fixation screws (Synthes GmbH).