Bolton discrepancy in an Iranian population and its relation with maxillary lateral incisors' size.

BACKGROUND: Bolton's two main ratios describing the proportional size of upper and lower teeth, could contribute to estimating the excess or deficiency of tooth size necessary to obtain an ideal occlusion. However, the mean Bolton values are not the same among different societies. Determining the prevalence of tooth size deviations from population-specific Bolton indices might help local orthodontists to have a more concise treatment plan. OBJECTIVE: The study aimed to define the prevalence of clinically significant tooth size discrepancies (TSD) in an Iranian population and to evaluate the influence of lateral incisors' size on this discrepancy. METHODS: This cross-sectional study was conducted on study casts of orthodontic patients attending Imam Reza Dental Clinic from September 2008 to December 2016. The sample comprised of 150 randomly selected pre-treatment study casts (64 males and 86 females from 17 to 28). The mesiodistal diameter of all permanent teeth from the first molar on the right to the first molar on the left was measured using 2 similar digital calipers, and Bolton analysis was calculated. Subjective visual estimation of Bolton discrepancy was also performed. SPSS v18.0, Wilcoxon signed ranks test, Pearson correlation and Receiver Operating Characteristic (ROC) curve analysis were used for statistical analysis. A p<0.05 was considered statistically significant. RESULTS: In the sample group, 34.7% had anterior Bolton index (ABI) and 20.7% had total Bolton index (TBI) greater than 2 Standard Deviations (2SDs) of Bolton's means, and about half of them required correction of the ABI considering the actual size of discrepancies (mm). The sensitivity of estimating clinically significant tooth size discrepancy more than 2SDs of Bolton's ABI and the visual judgment was 96.0% and a cut-off point of -0.12mm was obtained. CONCLUSION: Bolton's analysis should be routinely performed in all orthodontic patients, and visual estimation of TSD would be suggested as a screening method in the first visit prior to measurements and set-ups.

The Effects of Different Miniscrew Thread Designs and Force Directions on Stress Distribution by 3-dimensional Finite Element Analysis.

STATEMENT OF THE PROBLEM: The use of miniscrew as an absolute anchorage device in clinical orthodontics is growing increasingly. Many attempts have been made to reduce the size, to improve the design, and to increase the stability of miniscrew. PURPOSE: The purpose of this study was to determine the effects of different thread shapes and force directions of orthodontic miniscrew on stress distribution in the supporting bone structure. MATERIALS AND METHOD: A three-dimensional finite element analysis was used. A 200-cN force in three angles (0°, 45°, and 90°) was applied on the head of the miniscrew. The stress distribution between twelve thread shapes was investigated as categorized in four main groups; buttress, reverse buttress, square, and V-shape. RESULTS: Stress distribution was not significantly different among different thread shapes. The maximum amount of bone stress at force angles 0°, 45°, and 90° were 38.90, 30.57 and 6.62 MPa, respectively. Analyzing the von Mises stress values showed that in all models, the maximum stress was concentrated on the lowest diameter of the shank, especially the part that was in the soft tissue and cervical cortical bone regions. CONCLUSION: There was no relation between thread shapes and von Mises stress distribution in the bone; however, different force angles could affect the von Mises stress in the bone and miniscrew.

Spatial changes of the chin in the vertical and sagittal planes after superior repositioning of the maxilla.

PURPOSE: The mandible autorotates after maxillary superior repositioning. The aim of this study was to address the changes in chin position in the vertical and sagittal planes after maxillary superior repositioning. MATERIALS AND METHODS: This cross-sectional study assessed participants who had class I occlusion with vertical maxillary excess and underwent maxillary superior repositioning. Two lateral cephalograms were taken in central occlusion and natural head position. The amount of maxillary superior repositioning was documented for every participant according to lateral cephalometric indices. The distance between the most prominent point of the chin and a perpendicular line from N to the Frankfort line was used to determine the sagittal changes of the chin before and after surgery. The distance from the N point to Me was used to assess the vertical changes of the chin before and after operation. The Pearson correlation test was used to determine the correlation between the amount of maxillary superior repositioning and the vertical and horizontal changes of the chin. The linear regression model was applied to predict the changes of the chin (dependent factor) according to the vertical change of the maxilla (predictive factor). The occlusal plane angle change, mandibular length, and mandibular plane angle were considered as variable factors. RESULTS: Twenty participants were studied. Analysis of the data demonstrated a significant correlation between the maxillary superior repositioning (predictive factor) and the horizontal and vertical changes of the chin. For every 1  mm of vertical change in the maxilla, the chin could be expected to move 0.21  mm horizontally. For a standard deviation increase of 1 in the maxillary position, the chin advanced by 0.753 of the standard deviation (ß = 0.753). For every 1-mm change of the maxilla vertically, it could be estimated that the chin moved 0.71  mm vertically when the amount of maxillary impaction was 8  mm or less. For an increase in standard deviation of 1 in the position of the maxilla, the chin moved superiorly by 0.711 of a standard deviation (ß = 0.711). In maxillary superior repositioning greater than 8  mm, for every 1  mm of superior repositioning, the chin moved 0.44  mm superiorly. There was a positive correlation between the occlusal plane change, mandibular length, mandibular plane angle as well as the vertical and horizontal changes of the pogonion (P = 0.001). CONCLUSIONS: The chin position after maxillary superior repositioning can be predicted according to the amount of maxillary vertical changes. The vertical change of the chin is more predictable than the horizontal change.

Skeletal and dentoalveolar features in patients with deep overbite malocclusion.

OBJECTIVE: An increased overbite may be due to a skeletal or dental etiology that may influence treatment. The purpose of this study was to evaluate the skeletal and dentoalveolar features in patients with deep bite malocclusion in an Iranian population and to determine the most and least effective and contributory variables causing deep bite. MATERIALS AND METHODS: Lateral cephalograms and study casts of normal (n=85) and deep bite (n=85) subjects were used to evaluate skeletal and dentoalveolar variables. Data were analyzed statistically by independent t-test. The percentages of each variable within normal limits, less and more than one standard deviation were calculated for deep bite subjects. RESULTS: The most significant skeletal contributing factors were gonial and basal angles, as well as the posterior facial height, ramus length, lower anterior facial height and upper anterior facial height. An increased curve of spee and decreased mandibular first molar height were predominant dental variables in the deep bite group. The variables with the greatest variances from the normal limit were the ratio of the lower anterior facial height to the total anterior facial height, the lower anterior facial height to the upper anterior facial height and the ramus length. CONCLUSION: The counterclockwise rotation of the mandible and the increased curve of spee were the dominant feature of deep bite malocclusion.

Association between maxillary canine impaction and arch dimensions.

OBJECTIVE: To compare palatal height index, arch width, and arch length characteristics in Iranian patients presenting with palatal and buccal canine impaction with a matched control group. MATERIALS AND METHODS: The casecontrol study examined 53 patients with canine impaction. The subjects were divided into two groups determined by buccal or palatal impaction which were compared with 53 control subjects presenting without impaction. Subjects in the experimental groups were matched with individuals in the control group according to age, gender, crowding and type of malocclusion. Palatal height and arch length were measured with a Korkhaus three-dimensional divider. Arch width was determined in the anterior and posterior portions of the maxillary arch with a digital caliper. Data were compared with paired t-tests. RESULTS: The buccal canine impaction group exhibited mean differences in arch length between the case and control groups of 0.8 mm (SD 1.63, p = 0.041). The differences between the case and control groups in intermolar width, interpremolar width, intercanine width, palatal depth, and palatal height index were not statistically significant. The palatal impaction group showed no statistically significant differences between the case and control group in any of the dependent variables (p < or = 0.05). In a retest examination of arch dimensions, Bland-Altman plots showed no differences between the first and second measurements. CONCLUSIONS: Arch length in the buccal canine group was the only statistically significant variable. The difference was small and was considered not clinically significant.