The effects of an Anterior Bolton Discrepancy on the long term stability of lower incisors
Burlingame, Michelle L.
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Relapse in the form of lower incisor crowding is a significant concern to the practicing orthodontist as it tends to be a frequent complication following treatment. Many orthodontists now routinely recommend permanent retention to their patients due to the fact that lower incisor crowding is prominent and unpredictable. It has yet to be evaluated if an Anterior Bolton Discrepancy has an affect on lower incisor relapse. The present study will evaluate if there is a correlation between Anterior Bolton Ratio and long term lower incisor stability. Subjects included 30 cases treated by Dr. R.G. “Wick” Alexander with records available on average 11 years post retention. In the first measurement protocol the mesiodistal widths of all anterior teeth on maxillary models were recorded. Measurements were then recorded for mandibular anterior teeth before and after interproximal reduction. Actual tooth width reduction was measured in millimeters. Anterior Bolton Ratios were then calculated for each case before and immediately following interproximal reduction. In the second measurement protocol, Little’s Irregularity Index was calculated at Initial Records, Final Records, and Post Retention Records. The difference between the values at Final Records and Post Retention Records were used as a measure of relapse in the form of mandibular anterior crowding. The correlation between Anterior Bolton Discrepancy and lower incisor relapse was analyzed by the Pearson correlation. The group was then divided into 2 groups based on having above or below normal Anterior Bolton Ratios. The intergroup differences in lower incisor relapse in the form of crowding were then analyzed using the Independent t test. It was found that Dr. Alexander routinely reduces tooth structure between the mandibular canines by an average of 1.05mm and, in this sample, ranged from 0.23mm to 2.82mm. An average of 1.47mm (SD = 1.60) of relapse in the form of crowding was found to have occurred within the post retention time. The values ranged from -1.21mm (improvement) during the time to 5.91mm (relapse). The value of a patient’s Anterior Bolton Ratio was found to have a strong positive correlation with the amount of lower incisor relapse ( r = 0.5). In addition the group with below average Anterior Bolton Ratios was found to have significantly less lower incisor relapse than the group with above average ratios ( p = 0.001).