Browsing by Subject "Facial Asymmetry"

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    A Study of Craniofacial Asymmetry in Monozygotic and Dizygotic Twins Using Posteroanterior Radiographs
    (2002) Russell, Jennifer A.; Hartsfield, James K., Jr.; Everett, Eric T.; Roberts, W. Eugene; Shanks, James C.; Ward, Richard E.
    This project investigated craniofacial asymmetry in a prospective study among monozygotic (identical) versus dizygotic (fraternal) twins and sibling pairs. Posteroanterior radiographs that had previously been collected were used in this study and the zygosity of the twins had previously been determined. We hypothesized that in this study measuring craniofacial asymmetry there would be a greater concordance between monozygotic twins than between dizygotic twins for each of the variables measured. If there were some genetic component to asymmetry, we would expect to find a greater correlation between monozygotic versus dizygotic twins and sibling pairs since monozygotic twins are genetically identical and the dizygotic twins and siblings share on average fifty percent of their genes. If there are no statistically significant differences between the monozygotic and dizygotic twin correlations as shown from the heritability value, one could conclude that genetic factors are not important in the development of craniofacial asymmetry. This information is clinically significant. If we find that environment plays the predominant role in the development of asymmetry, one would conclude that clinicians should be vigilant about employing a treatment plan that will modify the environment while an asymmetry is developing. Since the zygomatico-frontal sutures and crista galli were found to be relatively symmetrical landmarks in a previous investigation by Harvold, a horizontal axis was constructed through the zygomatico-frontal sutures and a perpendicular vertical axis was constructed through crista galli. Perpendicular distances of ten bilateral structures were measured from the vertical reference line. Two methods of evaluating subjects' head positioning were used and compared. Any subject that was found to have excessive rotation of their head was excluded from the sample. After a preliminary analysis it was found that the horizontal distances were strongly correlated to patient positioning even after subjects determined to have excessive rotation of the head were eliminated from the sample. For this reason, vertical measurements of eight bilateral landmarks were made perpendicular to the reference line constructed through the zygomatico-frontal sutures. Using Statistical Package for the Social Sciences, the magnitude of craniofacial asymmetry was evaluated by calculating the absolute (non-signed) asymmetry values (IL-RI) for each of the ten horizontal and eight vertical variables. The horizontal and vertical variables were evaluated for any directional asymmetry to determine if the right or left side is consistently larger by calculating signed asymmetry values (L-R) for each of the ten horizontal and eight vertical variables. Since the subjects vary in size, relative differences were subsequently calculated. The relative difference was calculated by dividing the right-left absolute difference by half the sum of the right and left measurements [IL-Rlf.5(L+R)]. The relative difference of each of the ten horizontal and eight vertical variables was calculated for each individual. Using SAS software correlation coefficients were calculated for the relative asymmetry values of the monozygotic and dizygotic twin groups for each of the ten horizontal and eight vertical variables. Subsequently heritability values of craniofacial asymmetry were calculated for each of the eighteen variables using the standard formula h2= 2(rMz-rDz). The data revealed that head positioning had significantly affected our horizontal asymmetry values. We found a significant directionality in the horizontal and vertical dimension with the left side being larger horizontally and the right side being larger vertically. However the horizontal asymmetry values must be interpreted with caution as they are significantly affected by the rotation. After analyzing the data collected, the hypothesis was rejected. We were unable to demonstrate a significant difference in correlation values between monozygotic and dizygotic twins/sibling pairs for most craniofacial asymmetry values. The few variables that did show high heritability were in the horizontal plane. The results of the horizontal data must be interpreted with caution due to the significant effect of head rotation. In contrast it was found that vertical asymmetry values were not significantly correlated to rotation and none of these had notable estimates of heritability. These findings suggest that craniofacial asymmetry is more strongly influenced by environmental factors than genetics. This finding supports treatment by clinicians to improve or prevent developing asymmetries. While genetically controlled malformations can often be improved with treatment, environmentally induced malformations may respond well to treatment that changes the faulty developmental environment. Still it will be the capability of the patient to respond to the change in environment (treatment) that will determine the efficacy of the treatment. The interaction of genetic factors with this change in environment at a particular time as represented by the treatment may have a greater influence on the outcome of the treatment than how the problem was produced in the first place. Future studies analyzing asymmetry need to take extreme care in patient positioning and calibration of the x-ray machine, for they may lead to errors which overpower the data already subject to the error of a two-dimensional representation of a three dimensional object. Keeping these technical caveats in mind, future studies should focus on whether there are genetic factors that influence a patient's response (or the difference in response among patients) to the treatment of asymmetry.
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    Assessment of Dental Asymmetry Utilizing Orthodontic Study Casts Comparing Tooth Positions to the Midpalatal Raphe and Tooth Size Among Antimeres
    (2002) Sprowls, Matthew W.; Hartsfield, James K., Jr.; Baldwin, James; Everett, Eric T.; Hathaway, Ronald; Shanks, James C.; Ward, Richard E.
    Several studies have been conducted in relation to the treatment and etiologies of craniofacial and dental asymmetries in the orthodontic scientific literature. However, limited research has attempted to correlate asymmetry in dental traits to each other. The purpose of this research study was to develop a better understanding of the correlations between dental arch asymmetry and dental tooth size asymmetry and their underlying causes. Understanding asymmetries is essential in orthodontic diagnosis and treatment planning. An orthodontist cannot obtain an optimal occlusal and functional result without properly balancing skeletal and dental relationships. Pretreatment dental study casts of 86 sequential patients from the postgraduate orthodontic clinic at the Indiana University School of Dentistry were used for the analysis. Each patient was analyzed in a single blind fashion to minimize bias. The Adobe Photoshop® ruler and Chicago® brand electronic digital calipers accurate to the nearest tenth of a millimeter were used for digital measurements. Measurements were made to determine the amount and direction of asymmetry along the dental midlines relative to the median palatine raphe. Transverse and sagittal measurements were made to record asymmetries in canine and molar positioning. Furthermore, three sets of antimeric maxillary teeth were measured for length and width. These included the central incisors, the canines, and the first molars. Fluctuating asymmetry was present in all measurements, except molar distance to median palatal raphe. This was found to be directional to the left in the entire sample group. Statistical analysis was subsequently performed and the results showed significant correlations in the individuals in the subsample dentition group. Subjects belonging to this group with a measurable degree of fluctuating asymmetry had a greater tendency toward transverse maxillary discrepancies as measured from the canines and first maxillary molars relative to the median palatal raphe. Moreover, individuals in the subsample group with above average total weighted fluctuating asymmetry values according to the parameters analyzed in this study appeared more likely to have decreased developmental stability leading to the clinical expression of more severe malocclusions.