| Objective:The treatment purposes of patients with maxillary protrusion is achieving intrusion and retraction of the maxillary incisor, which needs strong anchorage of posterior area. Implanting mini-implants into the bone is an effective means to provide absolute orthodontic anchorage. The most common implant sites appear to be the palate, the palatal aspect of the maxillary alveolar process and the buccal cortical plate of the posterior arch. For a clinical evaluation of the data, it is important to combine the interradicular distance measurements and palatal bone depth with the mini-implants’ diameters or length. On the other hand, the thickness of cortical bone is an important factor in mini-implant stability.The purpose of this study were to investigate the cortical bone thickness and mesiodistal distances between the dental roots, and both bone depth and cortical bone thickness in the palate quantitatively to aid clinicians in planning successful mini-implant placements in females with maxillary protrusion. Methods:Volumetric tomographic images of31patients with maxillary protrusion(range16-36years, mean21.81±5.25years) taken with the NewTom3G were examined.The patients were selected according to the following inclusion criteria:1. Female, native place was Guangdong province2. Age≧16years3. Associated with protrusive maxillary dentition in Angle Class II Division1malocclusions and Class I malocclusions4. Without severe crowding and without missing teeth (excluding third molars) in posterior area of maxillary arch5. Without mental restoration in mouth6. Without periodontal disease(determined from radiographic signs of alveolar bone resorption) and systemic disease related to bone metabolism.7. Without congenital dental-maxillo-craniofacial deformity8. Having not yet started orthodontic treatment or once did it9. Excluding cyst or tumor in maxillaThe cone-beam computed tomography data were imported into3-dimensional software (Simplant Pro13.0), the following measurements of maxilla were performed (1) cortical bone thickness both buccally and palatally of every posterior interdental area at2,4,6and8mm from the alveolar crest;(2) mesiodistal spaces both on buccal side and palatal side of every posterior interroots at2,4,6and8mm from the alveolar crest;(3) overall bone depth and cortical bone thickness at4levels and38palatal placement sites.On6randomly selected patients, all measurements were made twice to assess intra-rater reliability,4weeks apart. The paired Student t test was used to test for differences between measurements on the left and right sides. If there were no statistically significant differences were found, for all future analyses, data from the left and right sides were pooled. Repeated measurement analysis of variance was used for data analysis, and the paired Student t test was used to test for differences between measurements on buccal and palatal cortical bone thickness and mesiodistal spaces. The level of significance was set at P<0.05.Results:(1) The paired Student t test (P>0.05) suggested high reliability for all the measurements and no statistically significant differences between left and right sides. For all future analyses, data from the left and right sides were pooled.(2) The buccal and palatal measurements around the first molar at8mm depth were missing range22.6%to51.6%, while there were only6groups measurements at2mm distal from middle sagittal suture of ML1in palate.(3) Obtain the interradicular distribution value for cortical bone thickness and root proximity of31person, overall bone depth and cortical bone thickness on palate of29person, who are maxillary protrusive from Guangdong province.(4) Cortical bone thickness at maxillary posterior interradicular sites:The highest buccal cortical bone thickness was found between the1st/2st molars at8mm depth (1.59±0.40mm), while the lowest was at2mm depth (1.09±0.25mm).The highest palatal cortical bone thickness was found between the1st/2st premolars at6mm depth (1.90±0.28mm), and the lowest was the1st/2st molars at2mm depth (1.13±0.26mm).The cortical bone thickness both buccally and palatally increased with increasing distance from the alveolar crest(P<0.001), and there were no statistically significant differences of buccal cortical bone thickness between different roots(P>0.05), except the canine/1st premolar and the1st/2st molars at2mm cut (F=3.172, P=0.028).On the palatal side, the cortical bone thickness decreased from canine to mesial of the2st molar (P<0.001). The paired Student t test showed differences (P<0.001) between buccal and palatal cortical bone thickness at the canine/1st premolar and the1st/2st premolars sites, suggesting the palatal measurements were greater.(5) Mesiodistal distance at maxillary posterior interradicular sites:On the buccal side, the greatest amount of mesiodistal bone was between the1st/2st at8mm depth (3.64±0.97mm),while the least amount of bone was between the1st/2st at4mm depth (2.30±0.74mm). The mesiodistal spaces between the2st premolar and1st molar at all cuts were sufficient(3.02-3.43mm) and larger than the other interroots below4mm cut. On the palatal side, the greatest amount of mesiodistal bone was between the2st premolar/1st molar at8mm depth (5.81±1.27mm),while the least amount of bone was between the1st/2st molars at2mm depth (2.54±0.61mm). The results of this study showed a consistent increase in the mesiodistal distances both buccally and palatally in most of the studied sites when moving apically. One exception was the mesiodistal buccal distance between the maxillary1st/2st molars. The palatal mesiodistal distances at the2st/lst premolar (3.80～5.81mm)and1st/2st molars (3.46～4.12mm) were distinctly larger than the anteriorer area.The paired Student t test (P<0.001) showed differences between buccal and palatal mesiodistal bone amount,suggesting the palatal measurements were greater around molars area.(6) On the palate:As far as the distance of2mm from median-sagittal is concerned, there was no significance found between MLs for bone depth. Further than2mm distant from the median-line, the overall bone depth was greatest at ML2(7.65±8.54mm), followed closely by ML1,ML3,and ML4. The amount of total BD to anchor an orthodontic mini-implant when placed perpendicular to the bone surface generally decreased with increasing distance from the midsagittal plane except for ML1. These findings were statistically significant only at ML4. However, the most peripheral measurements at MLs3, and4showed marginal increases.Palatal cortical bone thickness decreased from the midsagittal plane to the margin of alveolar bone. The mean value at a distance of2mm from the median line were distinctly larger than other. Within the distance of6mm from the median line, the posterior MLs (ML3and ML4) showed greater mean value than the anterior MLs (ML1and ML2), while there were not statistically significant at far away6mm region from the median line.Conclusions:1. Interdental spaces and cortical bone thicknes of maxillary posterior area varies appears to be a distinct pattern:①Aroud the first molar more than6mm above the alveolar crest, the frequent presence of the maxillary sinus was observed, showing that areas are off limits for implantation.②For all mearements, the results of this study showed a consistent increase in most of the studied sites when moving apically, which means the more apical the site, the safer the placement.③Because of insufficient bone at the canine/1st premolar and the1st/2st premolars region, the sites below4mm cuts from the alveolar are not a choice for iplantation. The area between the2st premolar and the1st molar both buccally and palatally and the palatal side of the1st/2st molars are optimal sites for implantation as they have the advantage of the greatest amount of mesiodistal bone(more than3.0mm).2. Because of the incisive foramen,the2mm site away from the middle line at ML1shoud be avoiding implant. The2mm near the midsagittal plane region extend to the first molar level can be a valid placement site since it has sufficient bone and bicortical bone. Bone depth and cortical bone thickness of the palate are most favorable for mimi-implants placement at the level of the first and second premolars.This information may aid clinicians in choosing suitable insertion sites for orthodontic mini-implants in maxilla of females with maxillary protrusion. |
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