Subject recruitment
The institutional review board at the University of Campania “Luigi Vanvitelli” granted ethical approval for this prospective study (No. 308 dated 20/51/2019).
The sample size was estimated based on preliminary data. A minimum sample of 39 subjects was needed in order to achieve 80% power, with an alpha of 5% to detect a 0.5 mm difference (SD 0.5 mm).
A total of 10 orthodontists were randomly recruited using the Doctor Locator (DL), an application developed on own website by Align Technology (San Jose, California). Inclusion criteria set for the selection were at least 5 years of experience in CAT; execution of IPR with manual strips; at least 20 patients treated with clear aligner last year. Ten Italian ZIP code have been randomly drawn and entered in the DL application. From the list of providers in that area, the first ten doctors that agreed to participate were included. For each provider, the last four consecutive patients started with clear aligner, and manual stripping was selected for a total of 40 subjects and for a potential total of 80 dental arches.
Patients were recruited according to the following inclusion criteria: adult patients with full permanent dentition, non-extraction orthodontic treatment with CAT, use of composite attachments, treatment plan including IPR (between 0.1 mm and 0.5 mm per tooth), and no visible anomaly of enamel.
3D casts and treatment protocol
Before and after stripping, models with silicone impressions were acquired and they were scanned by a 3-dimensional (3D) laser scanner (3Shape, Copenhagen, Denmark). Initial setups were obtained using ClinCheck (Align Technology, San Jose, California), and IPR was planned by each clinician according to the individual patient’s treatment needs. The amount of planned IPR was recorded in an Excel file. Patients were instructed to wear aligners for 22 h per day, except during meals and oral hygiene procedures. Patients were asked to replace aligners on average every 10 days. IPR was performed at the programmed stage according to the virtual treatment staging.
At the scheduled appointment for IPR, separator rings were placed between the teeth for 10 min before the procedure, to make space, to improve visibility and for access to the contact point. Enamel reduction was achieved using single-sided diamond-coated strips (Hopf, Ringleb & Co. GmbH & CIE, Berlin, Germany) and the amount of space obtained was checked with metal gages (Aestetika S.R.L., Terni, TR Italy). Subsequently, polishing strips were used to remove all irregularities. Topical fluoride was also used and left on the reduced teeth for 5 min.
Measurement protocol
STL files (Standard Triangulation Language) of the dental casts at the beginning and at the end of aligners planned were exported from ClinCheck and imported into Ortho Analyzer (3Shape, Copenhagen, Denmark). For each patient, we analyzed upper and lower arches before (T0) and after (T1) treatment with planned CAT and IPR. Measurements were performed by the same operator from the second bicuspid to the second bicuspid of each arch. Mesiodistal tooth dimensions were obtained according to the following procedure: as a first step, an operator defined the tooth long axis with a plane, then the software measured the distance from this plane to the farthest points mesially and distally on the tooth (Figs. 1 and 2). Then for each arch, the mesio-distal teeth dimensions were measured from second to second premolar before and after IPR (Fig. 3). Full arch amount of IPR performed was obtained through the difference between the length mesio-distal tooth diameters before and after treatment. The following formula was used to quantify the accuracy of IPR:
$$ \mathrm{IPR}\ \mathrm{accuracy}:\mathrm{percentage}\ \mathrm{of}\ \mathrm{accuracy}\ 100\%\left[\mathrm{IPR}\ \mathrm{planned}-\mathrm{performed}/\mathrm{planned}\kern0.5em \times \kern0.5em 100\%\right] $$
Thus, an index of the accuracy of each movement was obtained: the closer the value to 1, the more precise the IPR was performed by the operator (100% of the prescription). The mean accuracy index, standard deviation, and mean standard error were calculated, and the Student’s t test for single samples (P < 0.05) was applied in cases in which the accuracy of the IPR was significantly different to 1, i.e., significantly lower than 100% of the prescription.
To evaluate if the amount of anterior crowding could affect the accuracy of IPR, the Little’s irregularity index was calculated for each arch.
Statistical analysis
For continuous variables, means and standard deviations were calculated. For categorical variables, absolute numbers were reported. Categorical variables were compared with χ2.
A one-way analysis of variance was used to determine the influence of gender, amount of initial crowding (Little’s irregularity index), and their interactions.
Comparison of continuous variables between time points was made through unpaired t tests, while average changes within individual cases were tested through paired t tests.
The level of statistical significance was set at P < 0.05 for all statistical tests.
Method of error
A digital caliper (Schieblehre digital 59112; Fino, Bad Bocklet, Germany) was used to verify the accuracy of the virtual measurements comparing them with in vivo mesiodistal premolar widths [8]. Five measurements were performed after 1 month by the same operator to perform error analysis. Measurements were also repeated on eight randomly selected digital models (4 upper and 4 lower arches).
Dahlberg’s D was calculated to quantify the measurement error, and Student’s t test for paired data to identify any systematic error.