A retrospective cohort study was carried out in the Orthodontic Department at the Institute of Dentistry, Barts and The London School of Medicine and Dentistry, Queen Mary University of London. Approval was obtained from the Barts NHS Health Trust Clinical Effectiveness Unit (ID 6274).
A convenience sample was recruited at least 12 months following the completion of dual-arch pre-adjusted fixed appliance-based treatment. Participants of all ages, skeletal and dental relationships having received removable retention (vacuum-formed retainers or Hawley retainers) in the maxillary arch were included. Participants were excluded if they had history of functional appliance therapy, cleft lip and/or palate and/or other craniofacial syndromes and fixed retention in the maxillary arch.
Data were extracted from clinical records and reference models obtained pre-treatment (T0), at the end of active treatment (T1) and 12 months post-treatment (T2). All orthodontic study model-based measurements were taken by two investigators using a TESA SHOP-Cal digital callipers (Resolution 0.01 mm).
The stability of the maxillary anterior teeth was assessed using two assessment tools: Little’s Irregularity Index and the Orthodontic Alignment Index (OAI). Little’s Index involves a cumulative score of contact point displacement involving five contact points in the inter-canine region. The OAI was developed and piloted to provide a more detailed assessment of minor orthodontic issues affecting maxillary labial segment alignment (“Appendix 1”).
A comparison of stability was made between subjects with a Class I incisor relationship (2–4 mm) and a residual overjet (> 4 mm) at least 12 months following the completion of fixed appliance-based treatment. Two investigators (DP and CD) were calibrated using both indices. Intra-examiner and inter-examiner reliability was assessed using the intra-class correlation coefficient (ICC).
All data were entered into Microsoft Excel™ for descriptive analysis and later transferred to the Statistical Software Stata 17™ (StataCorp, TX, USA). Descriptive statistics included mean values and standard deviations for continuous data. Participants were categorised dichotomously into normal (2–4 mm) and increased (in excess of 4 mm) overjet groups. Data were normally distributed; therefore, linear regression analysis was used to assess the effect of overjet at debond (T1) on the stability of maxillary anterior alignment 12 months post-treatment (T2). Statistical significance was set at P < 0.05.
Development of the Orthodontic Alignment Index (OAI)
Twenty pre- and post-treatment models were obtained from local archives to evaluate the possible manifestations of instability in the maxillary anterior segment. Based on the previous research , a number of possible features of orthodontic instability including both horizontal and vertical change, reciprocal rotations and tip and torque changes have been shown to evoke negative responses from patients. Each occlusal feature was ranked in order of severity. A supplementary instability feature (spacing) was identified in the upper labial segment. These findings were used to develop a method of grading occlusal discrepancy according to six categories: horizontal discrepancy, spacing, vertical displacement, reciprocal rotations, tip and torque anomalies.
A panel of 10 qualified orthodontic clinicians was randomly selected to assess 10 sets of pre-treatment study models using the newly developed index. Feedback was used to refine the scoring system. The index was further piloted by a panel of 63 orthodontic clinicians recruited at the British Orthodontic Conference, in order to validate the index and determine inter-examiner reliability.
Participants were asked to score three maxillary pre-treatment plaster study models using the new index and to provide feedback on whether the scoring system fairly reflected aesthetic issues relating to the alignment of teeth. Free-text boxes for suggestions on how the system could be improved were also included.
Intra-class correlation coefficient (ICC) estimates and their 95% confidence intervals were calculated using StataCorp 15 TM (LLC Stata Statistical Software, 2017) using the pilot data. The intra-class correlation coefficient (ICC) was 0.72 for the 63 orthodontists. Hence, there was moderate-to-good inter-rater agreement , and further refinement of the index was undertaken.
Feedback from the pilot studies was used to refine the scoring system and assign weighting factors to each category according to severity to finalise the index (“Appendix 1”). The categories were refined as follows:
An objective assessment of the most severe contact point displacement was made with a maximum score of 12 for more than one tooth with both contact points displaced more than 2 mm in the same direction.
An objective assessment of the presence or absence of spacing in the upper labial segment, with a maximum score of four points for spacing present in more than two areas.
Objective assessment of the most severe vertical discrepancy between pairs of incisors, i.e. upper central incisors, upper lateral incisors, and the adjacent lateral and central incisor (more than 1.5 mm in either direction), with a maximum score of eight points.
Obvious rotations without contact point displacement, i.e. adjacent teeth rotated in the same direction without contact point displacement, with the score being given to the distance measured from the maximum point of rotation to the arch form (in mm), with a maximum score of two points.
A subjective assessment of mesio-distal angulation on any tooth in the upper labial segment, with a maximum score of two points given for more than two teeth involved.
Subjective perception of bucco-palatal orientation on any tooth in the upper labial segment, with a maximum score of two points given for more than two teeth involved.