Evaluation of the efficacy of sliding mechanisms on round versus rectangular archwires in canine retraction after first premolar extraction in class II malocclusion: a randomized clinical trial

OBJECTIVE(S): To compare the efficacy of sliding mechanics on a 0.020-inch round posterior segment incorporated into a bi-dimensional archwire (BA) versus conventional rectangular-section archwire (CONV) for maxillary canine retraction, rotation, angulation, anchorage loss, overbite change, and external apical root resorption (EARR). DESIGN: Randomized, parallel-group, two-arm clinical trial with 1:1 allocation. SETTING: Department of Orthodontics, University of Damascus; recruitment and treatment July-December 2023. PARTICIPANTS AND INTERVENTIONS: Patients aged 18-25 years with mild-moderate skeletal Class II (ANB + 5° to + 7°) and 5-10 mm overjet scheduled for camouflage treatment with extraction of maxillary first premolars were randomized (N = 38) to: (a) BA-bi-dimensional archwire (0.021 × 0.025″ rectangular in the incisor segment; 0.020″ round in canine/premolar/molar slots) or (b) CONV-conventional rectangular-section retraction mechanics. MAIN OUTCOME MEASURE(S): Primary outcome was monthly maxillary canine retraction rate measured on digitally scanned casts (3D analysis). Secondary outcomes included canine rotation rate, canine angulation change (lateral cephalograms), anchorage loss, overbite change, and EARR (panoramic radiographs). SEQUENCE GENERATION: Computer-generated variable block randomization (block sizes 4 and 6) produced by an independent statistician. ALLOCATION CONCEALMENT: Sequentially numbered, opaque, sealed envelopes prepared and stored by the independent statistician; opened after eligibility and baseline assessment. BLINDING: Participants and treating clinicians were unblinded; outcome assessors and the study statistician were blinded. Radiographs and digital models were anonymized and allocation was held independently until primary analyses were locked. RESULTS: Thirty-eight participants (19 per arm) completed the study. Observed median retraction rates (T0→TF; T0 = baseline, TF = end of retraction) were BA 1.17 mm/month (IQR 0.20; bootstrap 95% CI 1.08-1.24) versus CONV 0.82 mm/month (IQR 0.13; bootstrap 95% CI 0.76-0.90). Linear mixed-effects modelling adjusted for baseline overjet showed a significant group effect favoring BA (p < 0.001). BA reduced total canine angulation change by 2.90° (95% CI - 4.97 to - 0.82; p = 0.007). Secondary outcomes (rotation, anchorage loss, overbite, EARR) showed only small differences or non-significant trends after Bonferroni adjustment. CONCLUSIONS: Incorporating a 0.020″ round posterior segment within a bi-dimensional archwire accelerated canine retraction (~ 43% relative increase) and improved mesiodistal angulation control compared with conventional rectangular mechanics, without clear clinically meaningful increases in rotation, anchorage loss, overbite change, or root resorption. Findings warrant confirmation in larger, multicenter trials with extended 3D imaging follow-up. FUNDING: University of Damascus (Reference no.: 501100020595). Trial registration ClinicalTrials.gov NCT07243509 (retrospectively registered 24 November 2025).