The development of a novel Orthodontic Alignment Index and its use to evaluate the effect of residual overjet on the stability of the alignment of the maxillary anterior dentition

BACKGROUND: It is thought that achieving a normal overjet may help to stabilise the alignment of the maxillary anterior dentition. Little’s Irregularity Index is limited in assessing discrete post-orthodontic changes, fails to account for reciprocal rotations and is not sensitive to dental changes in three planes. A more holistic tool for the assessment of post-treatment change is therefore required. AIM: To compare the post-treatment stability of maxillary anterior dental alignment in subjects treated either to a Class I incisor relationship or an increased overjet (> 4 mm) following fixed appliance-based orthodontics using a novel measurement tool. MATERIALS AND METHODS: The Orthodontic Alignment Index (OAI) was developed and validated using a panel of 63 raters. The new index accounts for a range of weighted features including contact point displacement, spacing, reciprocal rotations, inclination, angulation and vertical discrepancy. A retrospective cohort study was undertaken at the Institute of Dentistry, Queen Mary University of London. Recruitment took place over a 4-year period. All participants had removable retainers in the maxillary arch only. The stability of maxillary anterior teeth was assessed using Little’s Irregularity Index (LII) and the OAI. Subjects were recruited at least 12 months following completion of dual-arch fixed appliance-based treatment. RESULTS: Eighty-two participants were included with a positive correlation observed between LII and OAI at the 12-month post-treatment review with a 1-mm increase in LII associated with a 2-point increase in the OAI (P < 0.001). Limited relapse was observed in both groups: normal overjet group (OAI = 1.28; LII = 0.52); residual overjet group (OAI = 0.88; LII = 0.47). Median regression analysis failed to identify a significant association between an increased overjet at debond and the alignment of the maxillary anterior segment when assessed with OAI (P = 0.389) and LII (P = 0.577). Furthermore, age, gender, extraction protocols and retention regime were not predictive of post-treatment change. CONCLUSIONS: Using a novel index (OAI) and LII, there was limited post-treatment relapse in alignment of the maxillary anterior dentition over a 12-month period. Based on this retrospective evaluation, achieving a normal overjet at the end of treatment may have little bearing on the post-treatment stability of maxillary anterior alignment at 12 months.