CATEGORY AUSTRALASIAN DENTIST 91 CLINICAL Digital planning All patients eligible for rehabilitation with subperiosteal implants underwent cone beam computed tomography (CBCT) of the maxilla and mandible (slice thickness 0.1– 0.3 mm) with a sufficiently large field of view to include the entire mandible, including the mandibular angles. The case study was completed with the acquisition of digital scans of both dental arches and with the diagnostic wax-up of the prosthetic project necessary to establish the orientation and length of the implant abutments. The prosthetic plan was prepared and executed by the same experienced prosthodontist (A.B.). The DICOM and STL files were then sent to the company responsible for manufacturing the implant (B&B Dental, San Pietro in Casale, Italy). The DICOM files obtained from the CBCT scan were extracted and imported into B&B Dental GS software (B&B Dental) which allows the bone segments to be reconstructed in three dimensions (3D). The 3D images were cleaned of scattering and other inaccuracies, and the course of the IAN was identified and marked. The STL files of the dental arches and of the diagnostic wax-up were then merged with the 3D model of the jaws. The 3D files were imported into Meshmixer software (Autodesk, San Rafael, CA, USA) and the implant was designed in accordance with the surgeon’s indications. The positions of the screw holes were determined taking into account the position of the IAN in order to avoid neurological damage during fixation. On the basis of the prosthodontist’s preferences, cementable or multiunit abutments integrated into the structure of the implant were designed and manufactured. The abutments were always housed in slots created in the alveolar crest so that they could rest on the basal bone, which is less prone to further resorption over time. The length and orientation of the abutments were established in accordance with the diagnostic wax-up and the thickness of the gingiva, as determined by the scan of the arches. The implant was always designed with the goal of keeping the majority of the plate and the fixation screws as far away from the surgical wound as possible. For this reason, if all of the abutments were behind the emergence of the mental nerve, the implant was made to run below the nerve (Fig. 1). On the lingual side, a connection between the abutments was provided in all cases in which the mylohyoid crest, a limit that cannot be crossed by the implant, was not too superficial. The 3D models of the bones, gums, prostheses, and implants were then re-imported into B&B Dental GS software and submitted for the surgeon’s approval. Implant manufacturing After approval of the project by the surgeon, the implant was made in grade V titanium by double laser melting technology (MYSINT100; Sisma, Piovene Rocchette, Italy). The implant then underwent a passage in a sintering oven (Nabertherm GmbH, Lilienthal, Germany) at 840 °C for 4 h and then at 500 °C for a further 2 h to make the metal stable and free of porosity. During this process, the heat distributed over the implant does not induce any dimensional changes. The abutments were then finished with the aid of a 5-axis milling machine (Datron D5; Datron Dynamics Inc., Milford, NH, USA), and the internal threads of the multiunit abutments, if any, were finally washed in the organic acid Dowclene 1601 (Dow Chemical Company, Midland, MI, USA) to eliminate all possible impurities and finally sterilised. The templates for preparing the slots for the housing of the abutments in the alveolar crest were milled in cobalt chrome (Datron D5) in order to be resistant and at the same time thin and easy to position. Finally, a stereolithography model of the jaw was 3D-printed in resin (Stratasys Objet30; Stratasys, Eden Prairie, MN, USA) and provided to the surgeon. Surgery The surgeries were performed by the same surgeon (L.A.V.), under local anaesthesia, without the aid of intravenous sedation. Local anaesthesia was performed with articaine with 1:100,000 epinephrine (Supplementary Material Video S1). An IAN block was not routinely performed as in any posterior jaw implant surgery. The vestibular fornix was infiltrated from the trigone to the incisor region both superficially and deeply, up to the inferior border of the mandible. Local anaesthesia was then completed with infiltration of the mandibular lingual side to cover the entire surgical site. A full- thickness mucosal incision was made following the course of the edentulous crest, extending mesially, intrasulcularly, or paramarginally on the residual teeth. At the level of the edentulous crest, it is essential that the incision splits the residual keratinized mucosa, often reduced to a strip of a few millimetres, ensuring that at least 2 mm is left on the buccal side so as to provide an adequate lining to the abutments. Two vestibular releases were then performed. The posterior release was performed at least 5 mm from the distal abutment in order to favour the healing of the mucosa around the latter. The position of the anterior release was instead established based on the length of the anterior arm of the implant; this should be more mesial than the nearest screw hole, to reduce the risk of exposure during healing. After the mucosal incision, a full thickness buccal flap was elevated, and the mental nerve was identified and preserved. The buccal side of the mandible was extensively skeletonised from the trigone region to the parasymphyseal area, extending inferiorly below the external oblique line towards the lower margin of the mandible. If the course of the implant required it, the mental nerve was isolated at 360° with the aid of a curved dissector. Finally, the periosteum on the lingual side of the mandible was elevated up to the mylohyoid crest, which represents the limit of the dissection. The cutting guide Fig. 1.(A) Digital planning and (B) intraoperative view of a subperiosteal implant that runs below the mental nerve; case 14 (female, age 49 years). (C) Digital planning and (D) intraoperative view of a subperiosteal implant that runs over the mental nerve; case 10 (female, age 65 years). A C B D
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