Three-dimensional evaluation of interradicular spaces and cortical bone thickness for the placement and initial stability of microimplants in adults

Introduction: The objective of this retrospective study was to measure interradicular space, thickness of cortical bone, and alveolar process width at prospective microimplant placement sites in order to understand both safety and stability aspects of microimplant placement by using cone-beam 3-dimensional volumetric images. Methods: Initial 3-dimensional images of 60 adult patients (30 men, 30 women; mean age, 27.1 years) were reoriented by using a standardized protocol. Inter-radicular space, cortical bone thickness, and alveolar process width were obtained at the alveolar processes from canine to second molar at 3 different vertical levels from the cement enamel junction (CEJ). Palatal bone thickness was measured along the midpalate at 5 different distances from the distal edge of the incisal foramen. Peripheral palatal bone thickness was also measured at analogous locations, and cortical bone thickness of the retromolar pad was measured. Results: Maxillary interradicular distances ranged from 1.6 to 3.46 mm and tended to increase from the CEJ to the apex. They were the greatest between the second premolar and the first molar. Mandibular interradicular distances ranged from 1.99 to 4.25 mm and tended to be greater than maxillary interradicular spaces. Maxillary and mandibular buccal cortical bone thicknesses were 1.12 to 1.33 mm and 1.25 to 2.98 mm, respectively. In both jaws, buccal cortical bone thickness tended to increase from the CEJ to the apex. Alveolar process widths were 3.74 to 5.78 mm in the maxilla and 3.11 to 7.84 mm in the mandible. Along the midpalate, palatal bone 20 to 25 mm from the incisive foramen had 7.04 mm and 6.99 mm thickness, respectively. The retromolar pad areas showed 1.96 to 2.06 mm thicknesses of cortical bone. Conclusions: In the alveoloar process, 1 mm or more cortical bone thickness can be expected in the posterior dentition area. Safe locations for microimplant placements with adequate interradicular space are between the second premolar and the first molar in the maxillary buccal alveolar bone, between the molars in the maxillary palatal alveolar bone, and interradicular spaces from the first premolar to the second molar in the mandibular buccal alveolar bone. The midpalatal area and the retromolar pad area are also excellent locations for microimplant placement. Because of limited interradicular spaces, the recommended diameter of a microimplant is 1.2 to 1.6 mm for placement in the alveolar bone, and the recommended length is 6-7 mm.