To determine the effect of material fatigue of aligners in oral environment and to describe and quantify the type and amount of Orthodontic tooth movement produced by clear aligners

Introduction: The following observations regarding tooth movement with Invisalign aligners resulted from a randomized and controlled clinical trial1. In a two-week prescription cycle of wearing an aligner, more orthodontic tooth movement (OTM) was seen in the first week as compared to the second week.1. In a two-week prescription cycle of wearing an aligner, more orthodontic tooth movement (OTM) was seen in the first week as compared to the second week.2. The exact expected result from the comparison was not obtained.3. Different individuals depicted different OTM.The results in this study have certain clinical implications which require further research and a more thorough study.Aim: The primary aim of this study was to determine whether these observations were due to material fatigue of the aligners in the oral environment. The secondary aim of this study was to describe and quantify the type and amount of OTM produced by clear aligners.Material and Method: A single centre prospective clinical trial was completed, comparing 15 subjects treated over a course of eight weeks with a control group of 37 subjects study with similar population demographics and nearly identical study design. An upper central incisor was programmed to move two mm over eight weeks, or 0.5 mm every two weeks, using Invisalign® aligners. The treatment subjects changed to a fresh aligner with the same prescription after one week, and the control subjects wore each aligner for the prescribed two weeks. Every week impressions were taken with polyvinyl siloxane (PVS) and models were fabricated digitally to measure OTM. In addition, initial and final cone beam computed tomography (CBCT) images were obtained from the treatment subjects only, to describe and measure OTM.Result: No significant difference was seen found in OTM in individuals who were wearing the initial aligner for a period of two weeks vs. those individuals who switched to a fresh aligner after a period of one week. The OTM variability in the second week was seen due to the material wear and fatigue present. There was, however, a significant difference in OTM during the first week vs. the second week of any given two-week cycle, for both groups. When OTM was measured from the centre of the clinical crown, the two mm prescription of the aligners was not fully expressed in any of the 15 treated subjects.Conclusion: High variability was observed. In addition, CBCT data indicated that: 1) the target teeth experienced tipping that was uncontrolled in nature, with the median rotation center at 41% of the length of the root apical to the alveolar crest, 2) although only one central incisor was planned to move, the contralateral central incisor felt a reactive force and got rotated in the direction opposite that of the target tooth, 3) seven of the 15 subjects experienced a net anteroposterior (A-P) change of less than 1.9 mm between the midpoint of the incisal edge of the target tooth and the contralateral central incisor, and 4) the results of an exploratory data analysis attempting to correlate biologic variables with OTM revealed some trends, but additional research is required before making conclusions.