During which stage does maximum expansion occur in the passive self-ligating system?

To evaluate the stage-specific contribution of each archwire to transverse arch development in a passive self-ligating (PSL) system using multilandmark digital measurements.

Twenty-one adults with mild-to-moderate crowding were treated with a PSL system using a standardized archwire sequence: 0.013-inch CuNiTi (S1), 0.014 × 0.025-inch CuNiTi (S2), 0.018 × 0.025-inch CuNiTi (S3), and 0.019 × 0.025-inch stainless steel (S4). Transverse measurements were made at cuspal, centroid, and palatal/lingual landmarks on digital models at baseline (S0) and after each archwire stage. Pain was evaluated using a visual analogue scale (VAS).

0.014 × 0.025-inch CuNiTi (copper-nickel-titanium) archwire (ΔS2-S1) produced the greatest single-stage expansion in both arches. In the maxillary arch, the largest increment was at the intersecond premolar cuspal width (1.81 ± 0.87 mm; P < .001), followed by interfirst premolar (1.65 ± 0.91 mm; P < .001) and intercanine (1.33 ± 0.73 mm; P < .001). In the mandibular arch, the greatest expansion was at the intercanine cuspal width (1.10 ± 0.68 mm; P < .001) and intersecond premolar (1.03 ± 0.69 mm; P = .012). Intercuspal expansion consistently exceeded centroid and palatal/lingual expansion at canine and premolar widths. 0.013-inch CuNiTi archwire elicited the highest VAS score (6.14 ± 1.62) and stainless steel the lowest (1.95 ± 1.69).

The 0.014 × 0.025-inch CuNiTi archwire was the primary contributor to transverse arch development in the PSL system, producing expansion predominantly through buccal crown tipping. Stainless steel archwires contributed the least expansion and pain, while the initial archwires caused the greatest pain.