Comparison Between Two Clear Aligner Activation Increments and Conventional Orthodontic Fixed Appliance in Treatment of Lower Incisors Crowding: A Randomized Clinical Trial

This study was a three-arm, parallel-group, randomized clinical trial conducted at the outpatient clinic of the Orthodontics Department, Faculty of Dentistry, Ain Shams University. Twenty-one participants were enrolled and allocated equally to three groups (n = 7 per group) using simple randomization. An allocation sequence was generated with Microsoft Excel by a colleague not involved in the trial. Each participant received a sequential number corresponding to presentation order, and group assignment was determined by matching this number to the pre-generated sequence.

For the aligner groups, intraoral scan datasets were imported into 3Shape software (3Shape Ortho System, 3Shape A/S, Copenhagen, Denmark). Models were spatially oriented using three reference points: the central fossae of the maxillary right and left first molars and the contact point between the central incisors. Model bases were created 4-5 mm apical to the gingival margins. Individual tooth segmentation was performed by identifying mesial and distal contact points, refining tooth splines, and adjusting axial orientation. The diagnostic setup involved adjusting the occlusal plane to reproduce any clinical canting, selecting and refining the arch form, and performing a three-dimensional tooth setup to achieve ideal alignment per American Board of Orthodontics criteria without overcorrection. Incremental staging steps were generated. Extrusive and intrusive movements were set at 0.15 mm per step; rotational, tipping, and torquing movements were set at 1 degree per step. Planar translation increments were the group-specific variable: 0.25 mm per step for Aligner I and 0.35 mm per step for Aligner II. Each staging step was inspected for interproximal or occlusal interferences exceeding 0.2 mm, which were resolved within the software. Attachments were placed according to biomechanical needs: vertical attachments for tip or rotation ≥5 degrees and horizontal attachments for extrusion ≥0.5 mm. Attachments were positioned centrally and as incisally or occlusally as possible without interfering with occlusion. Standard dimensions were 3.5 × 1.5 × 0.8 mm, with adjustments based on tooth morphology or occlusal interferences. Finalized digital models were labeled with the patient's name and exported as STL files.

For aligner fabrication, STL files were sent to a digital lab. Models were printed using Formlabs Grey resin (Formlabs, Somerville, MA, USA) on a Form 2 3D printer (Formlabs Inc., Somerville, MA, USA). Printed samples were washed in a Form Wash tank (Formlabs) for 20 minutes and post-cured in a Form Cure tank (Formlabs). Aligners were thermoformed from 0.8 mm Erkodur-AL material (Erkodent Erich Kopp GmbH, Germany) using a MiniSTAR S machine (Scheu-Dental GmbH, Iserlohn, Germany). Aligners were trimmed in a straight line terminating 2 mm apical to the gingival margins.

Clinically, one week before bonding, patients in the aligner groups received dental prophylaxis. After isolation with cheek retractors, cotton rolls, and suction, enamel surfaces were etched with 37% phosphoric acid for 15 seconds, rinsed, and dried to a frosty appearance. A thin layer of Transbond XT primer (3M Unitek, Monrovia, CA) was applied and light-cured. The attachment template was seated. Flowable composite (Nexcomp Flow, Meta Blomea, Korea) was injected into attachment recesses and light-cured through the template. The template was removed, and excess composite was finished with a carbide bur. Interproximal reduction was performed using metallic abrasive strips as prescribed. Gained space was measured using IPR gauges. Treated surfaces were polished. The first aligner was seated using chewies.

For the control group, conventional fixed appliance therapy was initiated. One week after polishing, teeth were isolated and etched with 37% phosphoric acid for 30 seconds. After rinsing and drying, Transbond XT primer was applied. Stainless steel brackets with a Roth prescription and 0.018-inch slot size, along with bondable molar tubes, were bonded using Transbond XT composite. Each bracket was firmly seated, excess adhesive was removed, and adhesive was light-cured for 20 seconds per tooth. Leveling and aligning were performed using a standardized, progressive sequence of nickel-titanium archwires, advancing to rectangular stainless-steel wires.