• The goal of this clinical study is to learn whether the gumline edge design of a finishing orthodontic clear aligner affects how much the aligner's color changes after 14 days of wear.
• The main question is: does a supragingival edge design - a straight edge covering about 2 mm of gum tissue - lead to a different amount of color change in the aligner compared with a juxtagingival edge design that follows the gumline exactly, after 14 days of wear?
• Each participant wears finishing clear aligners with both edge designs at the same time - one design on the upper jaw and the other on the lower jaw - and serves as their own comparison (split-mouth). This within-person approach removes differences between individuals in oral hygiene, diet, and saliva.
• The aligner's color over six selected teeth (three upper, three lower) is measured with a dental spectrophotometer (VITA Easyshade V) at placement (day 0) and after 14 days. Each measurement is repeated three times under standardized optical conditions. The amount of color change is expressed as a color-difference value (ΔE).
• The study hypothesis is that marginal termination design (supragingival versus juxtagingival) is associated with a difference in the 14-day colorimetric change (ΔE00) of the finishing aligner.
• The study is carried out during the finishing (refinement) stage of orthodontic treatment, when only small residual tooth movements remain, so that color changes can be attributed to wear and edge design rather than to tooth movement.

• BACKGROUND AND RATIONALE
• Two trimming-line (edge) designs are commonly used during the finishing (refinement) stage of clear aligner therapy. The supragingival design uses a straight edge positioned approximately 2 mm apical to the free gingival margin, covering approximately 2 mm of gingival tissue. The juxtagingival festooned design follows the natural scalloped contour of the free gingival margin, without covering gingival tissue. Color stability of the aligner during wear is relevant to esthetics and patient acceptance. Whether marginal termination design influences the colorimetric stability of the aligner during wear has not been investigated.
• STUDY DESIGN
• This is a prospective, within-person (split-mouth) interventional study. Each participant receives both designs simultaneously: the supragingival design on one dental arch and the juxtagingival festooned design on the contralateral arch. The split-mouth design controls for inter-individual variation in oral hygiene, diet, and salivary composition. The aligner material (CA Pro tri-layer, SCHEU-DENTAL GmbH, Iserlohn, Germany; CE-marked thermoplastic) is identical for both designs; only the marginal termination geometry differs.
• ALLOCATION RULE (NON-RANDOMIZED)
• Allocation follows a pre-specified hierarchical rule documented prior to enrollment. Primary criterion: the dental arch with the greater magnitude of planned dental intrusion (mm, from the digital treatment setup) receives the supragingival design. Tiebreaker (equal or zero planned intrusion on both arches): a pre-specified systematic alternating sequence in enrollment order. Planned intrusion magnitude per arch is recorded as a covariate.
• TARGET TEETH AND MEASUREMENT
• Colorimetric measurements of the aligner are performed extraorally at six target-tooth positions per participant: three maxillary (1.4, 1.1, 2.3) and three mandibular (4.3, 3.1, 3.4). Measurements are obtained with a dental spectrophotometer (VITA Easyshade V) operated in Custom Color mode, which simultaneously displays the CIE L, a, and b color coordinates. To ensure reproducibility, the aligner is measured under standardized optical conditions using a standardized optical substrate and a fixed positioning reference (methodological details reported separately). Each target position is measured in triplicate at each visit.
• VISITS
• Measurements are obtained at T0 (day 0, at placement of the finishing aligner) and T1 (day 14 ± 2 of wear), yielding 36 L, a, b measurements per participant (6 positions × 3 repetitions × 2 visits).
• OUTCOME
• The primary outcome is the colorimetric change (ΔE00, CIEDE2000) of the aligner between T0 and T1, computed per dental arch from the measured L, a, b coordinates, compared between the supragingival-design arch and the juxtagingival-design arch. The CIELAB ΔEab value is reported descriptively for comparability with prior literature.
• STUDY CONTEXT
• The study is conducted during the finishing (refinement) stage, when only small residual tooth movements remain, so that observed color changes can be attributed to wear and marginal termination design rather than to substantial tooth movement.
• RELATIONSHIP TO COMPANION STUDY
• A subset of participants may be concurrently enrolled in the companion study PATHO-ALIGN (ClinicalTrials.gov NCT07533058), a split-mouth study of finishing-aligner marginal termination design and subgingival periodontal microbiota conducted by the same investigator at the same sites. Co-enrolled participants wear the same finishing clear aligners with the same arch-level marginal-design allocation in both studies; the present study adds only a non-invasive, extraoral colorimetric assessment of the aligners and does not interfere with the companion study. The two studies have distinct outcomes.
• STATISTICAL ANALYSIS
• Primary confirmatory analysis: The confirmatory endpoint is the arch-level aligner color change (ΔE00) between T0 and T1. The supragingival-design arch and the juxtagingival-design arch are compared within participants using a paired test (two-sided, alpha = 0.05). Normality of the paired differences is assessed (Shapiro-Wilk); a paired t-test is used if the assumption holds, otherwise the Wilcoxon signed-rank test. A single confirmatory comparison is pre-specified, controlling the type I error rate without multiplicity correction.
• Adjusted analysis: A linear mixed-effects model is fitted to arch-level ΔE00, with marginal termination design (supragingival vs juxtagingival), dental arch (maxillary vs mandibular), planned intrusion magnitude per arch, and self-reported daily wear time as fixed effects, and a participant-level random intercept to account for the paired (split-mouth) structure. Including dental arch as a covariate separates a potential arch-level effect from the design effect.
• Secondary analyses (descriptive): Component color changes (ΔL, Δa, Δb) per arch between T0 and T1; overall aligner color change across all target positions (ΔE00 and ΔEab); and the proportion of arch-level changes exceeding the Paravina (2015) perceptibility (ΔE00 = 0.8) and acceptability (ΔE00 = 1.8) thresholds.
• Measurement reliability: Intra-position reproducibility across the three repeated measurements is quantified using the intraclass correlation coefficient (ICC).
• Missing data and quality control: Repeated measurements failing pre-specified inter-repetition agreement criteria are excluded, and the mean of the remaining repetitions is used; analyses use available per-protocol data.
• SAMPLE SIZE JUSTIFICATION
• Sample size was determined for a paired within-subject (split-mouth) comparison of arch-level aligner color change (ΔE00). In the absence of prior data specific to marginal termination design effects on aligner color, parameters were selected conservatively: a minimum meaningful between-arch difference of ΔE00 = 0.8 (50:50% perceptibility threshold; Paravina 2015), an assumed standard deviation of paired differences of approximately 0.9 (derived from an assumed arch-level standard deviation with an intra-subject correlation rho = 0.4), and a two-sided alpha = 0.05 (paired test). Under these assumptions, 50 participants provide power exceeding 90% to detect the specified difference and remain adequately powered (approximately 85%) for smaller between-arch differences (ΔE00 ≈ 0.4). The enrolled sample (50 participants; 100 aligners, two per participant) also serves to estimate within-subject colorimetric variability to refine power assumptions for subsequent confirmatory studies.

Inclusion Criteria:

• Adolescent and adult patients (age 15 years or older at enrollment)
• Undergoing a finishing (refinement) stage of clear aligner therapy, with only small residual tooth movements remaining, including both patients completing active orthodontic treatment and patients with prior orthodontic treatment who require additional refinement (e.g., minor relapse or loss of retention)
• Adherence to the prescribed oral hygiene instructions
• Written informed consent; for participants aged 15-17 years, written consent from a parent or legal guardian

Exclusion Criteria:

• Deficient oral hygiene
• Excessive consumption of intensely staining beverages (coffee, tea, red wine, colored carbonated drinks)
• Systemic conditions or medications that may affect salivary secretion or tooth color
• Non-compliance with the aligner / orthodontic appliance wear protocol

De-identified individual participant data (participant codes, arch-level color change values ΔE00 and ΔE*ab, component changes ΔL*/Δa*/Δb* and baseline covariates) will be made available as a CSV or Excel file upon reasonable request to the corresponding author following publication of study results.

Beginning 6 months after publication of primary study results.

Upon reasonable request to the corresponding author (PI); requests reviewed within 30 days.

The outcomes assessor is masked. (1) Treating clinician (orthodontist, PI): assigns the marginal termination designs to the arches at baseline, holds the participant code and allocation key, and performs the extraoral colorimetric measurements of the aligners under standardized optical conditions; not masked, as allocation is inherent to treatment planning. (2) Data manager / outcomes assessor: receives the recorded L*, a*, b* values labeled only by participant code and arch (e.g., "SPC001 maxillary", "SPC001 mandibular"), with no reference to which design was assigned to which arch, and performs all data centralization and the between-arm comparison; completely blinded to allocation. The arch-to-design key is held by the treating clinician and released only after the colorimetric dataset is locked. Probe positioning is standardized by protocol (fixed measurement height and fixed positioning references) and each target position is measured in triplicate, limiting operator influence on