The 3-dimensional zone of the center of resistance of the mandibular posterior teeth segment.

INTRODUCTION: We sought the 3-dimensional (3D) zone of the center of resistance (ZCR) of mandibular posterior teeth groups (group 1: first molar; group 2: both molars; group 3: both molars and second premolar; group 4: both molars and both premolars) with the use of 3D finite element analysis. METHODS: 3D finite element models comprised the mandibular posterior teeth, periodontal ligament, and alveolar bone. In the symmetric bilateral model, a 100-g midline force was applied on a median sagittal plane at 0.1-mm intervals to determine the anteroposterior and vertical positions of the ZCR (where the applied force induced translation). The most reliable buccolingual position of the ZCR was then determined in the unilateral model. The combination of the anteroposterior, vertical, and buccolingual positions was defined as the ZCR. RESULTS: The ZCRs of groups 1-4 were, respectively, 0.48, 0.46, 0.50, and 0.53 of the mandibular first molar root length from the alveolar crest level and located slightly distobuccally at anteroposterior ratios of 2:3.0, 2:2.3, 2:2.4, and 2:2.5 to each sectional arch length and at buccolingual ratios of 2:1.5, 2:1.1, 2:1.6, and 2:2.4 to the first molar's buccolingual width. CONCLUSIONS: The ZCR can be a useful reference for 3D movement planning of mandibular posterior teeth or segments.

A three-dimensional finite element analysis of molar distalization with a palatal plate, pendulum, and headgear according to molar eruption stage.

OBJECTIVE: This study aimed to (1) evaluate the effects of maxillary second and third molar eruption status on the distalization of first molars with a modified palatal anchorage plate (MPAP), and (2) compare the results to the outcomes of the use of a pendulum and that of a headgear using three-dimensional finite element analysis. METHODS: Three eruption stages were established: an erupting second molar at the cervical one-third of the first molar root (Stage 1), a fully erupted second molar (Stage 2), and an erupting third molar at the cervical one-third of the second molar root (Stage 3). Retraction forces were applied via three anchorage appliance models: an MPAP with bracket and archwire, a bone-anchored pendulum appliance, and cervical-pull headgear. RESULTS: An MPAP showed greater root movement of the first molar than crown movement, and this was more noticeable in Stages 2 and 3. With the other devices, the first molar showed distal tipping. Transversely, the first molar had mesial-out rotation with headgear and mesial-in rotation with the other devices. Vertically, the first molar was intruded with an MPAP, and extruded with the other appliances. CONCLUSIONS: The second molar eruption stage had an effect on molar distalization, but the third molar follicle had no effect. The application of an MPAP may be an effective treatment option for maxillary molar distalization.

Clinical application of a stereolithographic surgical guide for simple positioning of orthodontic mini-implants.

AIM: To describe a clinical application of a new surgical guide system that uses cone-beam computed tomography (CBCT) images, an implant-positioning program (SimPlant), and stereolithography to make a surgical guide for accurate placement of orthodontic mini-implants. METHODS: A patient who was planning to have orthodontic mini-implant treatment on the posterior maxilla was recruited to assess the feasibility of using CBCT images in an implant-positioning guide program (SimPlant 9.02, Materilise, Leuven, Belgium). Acquisition slices for the posterior maxilla were 0.15 mm in panoramic mode of PSR 9000N model (Asahi Roentgen, Kyoto, Japan). The surgical guide for the mini-implant was fabricated from the transported CBCT data. RESULTS: The completed surgical guide was easily placed intra-orally and permitted simple and rapid placement of the mini-implant. The site of the implant placement was accurate, while the vector varied slightly from the planned vector. CONCLUSION: A postplacement CBCT demonstrated accurate placement of the mini-implant on the left and a minor discrepancy between the simulated mini-implant position and clinical position on the right. Improvements in the CBCT resolution and laser-scanning technology are likely to eliminate any discrepancies.