Samir E. Bishara

Impacted maxillary canines: A review

An overview of the incidence and sequelae, as well as the surgical, periodontal, and orthodontic considerations in the management, of impacted canines is presented. The clinician needs to be familiar with the differences in the surgical management of the labially and palatally impacted canines, the best method of attachment to the canine for orthodontic force application, the advantages of one-arch versus two-arch treatment, and the implications of canine extraction. The various factors that influence all these decisions are discussed.

Maxillary expansion: Clinical implications

Clinicians frequently expand the maxilla to correct certain malocclusions. The effects of expansion on facial structures, dentition, and periodontium are reviewed. The implications of these findings for the treatment of patients who need maxillary expansion are discussed.

Dental and facial asymmetries: a review

Asymmetry in the face and dentition is a naturally occurring phenomenon. In most cases facial asymmetry can only be detected by comparing homologous parts of the face. The etiology of asymmetry includes: a) Genetic or congenital malformations e.g. hemifacial microsomia and unilateral clefts of the lip and palate; b) Environmental factors, e.g. habits and trauma; c) Functional deviations, e.g. mandibular shifts as a result of tooth interferences. Dental asymmetries and a variety of functional deviations can be treated orthodontically. On the other hand, significant structural facial asymmetries are not easily amenable to orthodontic treatment. These problems may require orthopedic correction during the growth period and/or surgical management at a later point. Patient complaints and desires need to be addressed since they may vary from unrealistic expectations to a lack of concern even in the presence of large deviations. With mild dental, skeletal and soft tissue deviations the advisability of treatment should be carefully considered.

Arch width changes from 6 weeks to 45 years of age

The purpose of this study was to evaluate on a longitudinal basis, the changes in intercanine and intermolar widths over a 45-year span. The subjects in this study were from two pools of normal persons: (1) 28 male and 33 female infants evaluated longitudinally at approximately 6 weeks, 1 year, and 2 years of age (before the complete eruption of the deciduous dentition); and (2) 15 male and 15 female subjects from the Iowa facial growth study evaluated at ages 3, 5, 8, 13, 26, and 45. Arch width measurements on maxillary and mandibular dental casts were obtained independently by two investigators. Intraexaminer and interexaminer reliability were predetermined at 0.5 mm. From the findings in the current study, the following conclusions can be made: (1) Between 6 weeks and 2 years of age, i.e., before the complete eruption of the deciduous dentition, there were significant increases in the maxillary and mandibular anterior and posterior arch widths in both male and female infants. (2) Intercanine and intermolar widths significantly increased between 3 and 13 years of age in both the maxillary and mandibular arches. After the complete eruption of the permanent dentition, there was a slight decrease in the dental arch widths, more in the intercanine than in the intermolar widths. (3) Mandibular intercanine width, on the average, was established by 8 years of age, i.e., after the eruption of the four incisors. After the eruption of the permanent dentition, the clinician should either expect no changes or a slight decrease in arch widths. In conclusion, although the dental arch widths undergo changes from birth until midadulthood, the magnitude as well as the direction of these changes do not provide a scientific basis for expanding the arches, in the average patient, beyond its established dimensions at the time of the complete eruption of the canines and molars. Both patients and clinicians should be aware of these limitations.

Biodegradation of orthodontic appliances. Part I. Biodegradation of nickel and chromium in vitro

The purpose of this study is to compare in vitro the corrosion rate of a standard orthodontic appliance consisting of bands, brackets and either stainless steel or nickel-titanium arch wires. The corrosion products analyzed were nickel and chromium. Evaluation was conducted with the appliances immersed for 4 weeks in a prepared artificial saliva medium at 37 degrees C. Ten identical sets were used, each simulating a complete orthodontic appliance used on a maxillary arch with a full complement of teeth. Five sets were ligated to stainless steel arch wires, and the other five sets were ligated to nickel-titanium arch wires. Nickel and chromium release was quantified with the use of a flameless atomic absorption spectrophotometry. The analysis of variance was used to determine if differences existed between the nickel and chromium release according to arch wire type, as well as with time (days 1, 7, 14, 21, and 28). The results indicate that (1) orthodontic appliances release measurable amounts of nickel and chromium when placed in an artificial saliva medium. (2) The nickel release reaches a maximum after approximately 1 week, then the rate of release diminishes with time. On the other hand, chromium release increases during the first 2 weeks and levels off during the subsequent 2 weeks. (3) The release rates of nickel or chromium from stainless steel and nickel-titanium arch wires are not significantly different. (4) For both arch wire types, the release for nickel averaged 37 times greater than that for chromium. How much of these corrosive products are actually absorbed by patients still needs to be determined.

Comparisons of different debonding techniques for ceramic brackets: An in vitro study

A series of tests of three different debonding techniques applied to three different types of ceramic brackets revealed the following: (1) The incidence of bracket failure during debonding was significantly greater with conventional debonding recommended by the manufacturer (10-35%), as compared with the incidence associated with either the ultrasonic or the electrothermal methods (0%). (2) Bond failure at the bracket-adhesive interface occurred with significantly greater frequency for the Starfire brackets when debonding was performed with the electrothermal instrument and with significantly less frequency when the debonding pliers were used. Combination bond failures, in which part of the adhesive stayed on the enamel and part stayed on the bracket, occurred with significantly greater frequency when Transcend and Starfire brackets were debonded with debonding wrenches than when other methods were used. Combination-bond failures occurred with significantly less frequency when the brackets were removed with ultrasonic tips or with the electrothermal instrument. (3) The debonding times for the ultrasonic method were significantly greater than the times for either the conventional or the electrothermal methods. There were no significant differences among the debonding times for the three bracket types. There were no significant differences in the debonding times between the electrothermal method (means = 3.0 seconds) and the conventional bracket-removal method (means = 1.0 seconds). (4) Enamel loss as a result of adhesive removal was not significantly different among the three adhesive-removal techniques tested. Post-treatment roughness of the enamel surface was greater for the high-speed adhesive-removal technique than for either the low-speed or ultrasonic adhesive-removal methods.

Longitudinal changes in three normal facial types

The purpose of this study was to describe and compare the dentofacial relationships of three normal facial types (long, average, and short). Comparisons of the absolute and incremental changes between 5 years and 25.5 years of age were made both longitudinally and cross-sectionally. The subjects consisted of 20 males and 15 females for whom complete sets of data were available for the period of this study. All subjects had clinically acceptable occlusion and had not undergone previous orthodontic treatment. Descriptive statistics summarized the changes in 48 parameters, including that of height for males and females at 5, 10, 15, and 25.5 years of age. Longitudinal comparisons of the growth curves evaluated the curve profiles and curve magnitudes for the three facial types for both males and females. The analysis of variance was also used to compare the absolute and incremental changes at ages 5, 10, 15, and 25.5 years. The investigation resulted in the following findings. (1) Most persons (77%) have been categorized as having the same facial type at 5 and at 25.5 years of age. There is a strong tendency to maintain the original facial type with age. (2) Comparisons of the growth curves of the different parameters--with the exception of the incremental curves for MP:SN and Pog:NB in males--consistently demonstrated parallelism of the curves, regardless of the facial type. On the other hand, comparisons of curve magnitude indicated significant differences among the three facial types. (3) The persons within each facial type expressed a relatively large variation in the size and relationships of the various dentofacial structures. (4) Significant differences in the dentofacial parameters were present between males and females with the same facial type. The differences among facial types were not identical in males and females. (5) Longitudinal analysis of the data lends more consistent and, therefore, more meaningful results than cross-sectional comparisons when facial growth trends need to be evaluated. This is because growth changes are often subtle and of magnitudes not readily observed when the data are evaluated cross-sectionally. Standards that are age-, sex- and facial type-specific are presented.

Comparisons of mesiodistal and bnccolingnal crown dimensions of the permanent teeth in three populations from Egypt, Mexico, and the United States

The purpose of this study is to examine the mesiodistal and buccolingual crown dimensions in three populations--57 subjects (35 boys and 22 girls) from Iowa City, Iowa; 54 subjects (30 boys and 24 girls) from Alexandria, Egypt; and 60 subjects (26 boys and 34 girls) from Chihuahua, Mexico. All subjects had normal Class I occlusion, with no history of orthodontic treatment. Comparisons of single teeth as well as sums of groups of teeth were performed between boys and girls within and between the two populations. The analysis of variance general linear models procedure was used for statistical comparisons. The findings from this investigation indicated that (1) differences between antimeres are of small magnitude and of no statistical significance; (2) all populations have significant differences in tooth dimensions between the sexes with boys having larger canines and first molars; (3) there is greater variation in the buccolingual than in the mesiodistal dimensions among the three populations; (4) there is a greater similarity in tooth dimensions among the boys from the three populations than among the girls, but the magnitude of these differences is considered to be of little clinical significance; and (5) standards for the buccolingual diameters were developed for the three populations. As a result, it was concluded that prediction equations used for space analysis in the mixed dentition to determine tooth size-arch length discrepancies in the Iowa population can also be used for persons from Egypt and from the northern part of the Mexican Republic, with some suggested modifications.

Functional appliances: A review

The purpose of this review was to evaluate the scientific studies that describe the effects of functional appliances on the dentofacial structures in the treatment of Class II malocclusions. The review is limited to two appliances: the activator and the functional regulator.

Changes in the maxillary and mandibular tooth size-arch length relationship from early adolescence to early adulthood. A longitudinal study.

The purpose of this study was to determine the association between the changes in maxillary and mandibular tooth size-arch length discrepancies (TSALD) and various dentofacial variables for 18 male and 14 female subjects with normal occlusion. All subjects were participants in the Iowa Longitudinal Growth Study and records were evaluated at two stages of dental development: stage I, when the permanent second molars initially erupted into occlusion (X age = 13.3 years); and stage II, at early adulthood (X age = 26.0 years). The following sets of variables were evaluated: mesiodistal crown diameters of single and groups of permanent teeth, dental arch widths and lengths, curve of Spee, maxillary and mandibular anterior and total crowding or spacing, anterior tooth rotations, and various cephalometric dentofacial parameters. Students t test were used to compare subjects with the most and least changes. Regression analyses also were used to assess the relationships between these parameters and the changes in the maxillary and mandibular tooth size-arch length relationship. The most consistent finding from the t test comparisons is the significantly greater reduction in the available arch length in the group with the most TSALD at early adulthood. No other variables were found to be consistently different in the comparisons between the two groups. The results of the regression analysis indicated that a number of dentofacial variables are associated with the changes in the maxillary and mandibular TSALD--for example, the mesiodistal diameter of different teeth and the changes in anterior and posterior facial heights. The clinical implications of the present findings are discussed.