• The trimming-line design of clear aligners (supragingival versus juxtagingival) is a routine clinical decision whose influence on patient-reported comfort has not been established; available evidence on aligner comfort derives almost exclusively from comparisons with fixed appliances.
• This observational, cross-sectional study compares two clear aligner trimming-line designs with respect to patient-reported comfort: a supragingival design, whose edge rests on the marginal gingiva, and a juxtagingival design, whose edge follows the gingival contour without covering it. Because the two designs sit differently against the gingival soft tissue, they may be experienced differently during wear.
• Participants are patients already wearing one trimming-line design on one dental arch and the other on the contralateral arch; each participant thus serves as their own control. After a minimum of four weeks of wear, participants complete a 20-item questionnaire once, rating the frequency of defined oral experiences for the maxillary and mandibular arch separately on a three-point ordinal scale (0 = none, 1 = occasional, 2 = frequent).
• The questionnaire is anonymous; the design worn on each arch is identified from which of two identical forms a participant completes. Responses for the two designs are compared within each participant. The study evaluates whether trimming-line design is associated with differences in patient-reported comfort.

• BACKGROUND AND RATIONALE
• Clear aligner therapy is an established modality in contemporary orthodontics. At the gingival margin, aligners are commonly trimmed according to one of two trimming-line designs, supragingival or juxtagingival. While the biomechanical and periodontal implications of trimming-line geometry have received attention, its influence on patient-reported outcomes remains uncharacterized. Available evidence on the comfort and oral health-related quality of life associated with aligner therapy derives predominantly from comparisons between aligners and fixed appliances, rather than from comparisons among aligner design parameters. The present study was designed to address this gap by evaluating whether trimming-line design is associated with differences in patient-reported comfort.
• THE TWO TRIMMING-LINE DESIGNS
• The supragingival design presents a straight marginal edge extending approximately 2 mm onto the marginal gingiva, in contact with the gingival surface without entering the gingival sulcus. The juxtagingival (festooned) design follows the scalloped contour of the gingival margin and terminates at the free gingival margin without overlying the gingiva. Both designs are fabricated from the same tri-layer thermoplastic aligner material (CA Pro, SCHEU-DENTAL GmbH, Iserlohn, Germany; CE-marked).
• STUDY DESIGN AND POPULATION
• This is an observational, cross-sectional, within-subject (split-mouth) study. Participants are patients in active clear aligner treatment who wear the two trimming-line designs, one per dental arch, prescribed as part of their orthodontic care. Each participant concurrently wears both designs and serves as their own control; the unit of comparison is the dental arch within the participant. Eligibility requires a minimum of four weeks of wear of the two designs. The planned enrollment is 70 participants.
• STUDY INSTRUMENT AND ADMINISTRATION
• The study instrument is a 20-item questionnaire developed for this study. The items address the functional and soft-tissue dimensions of aligner wear, including speech and phonation, mucosal and gingival sensation, retention and insertion, foreign-body sensation, and salivation and taste, corresponding to the oral surfaces in contact with the aligner margin. Each item is rated for frequency of occurrence during wear on a three-point ordinal scale (0 = none, 1 = occasional, 2 = frequent) and is answered separately for the maxillary and mandibular arch. The questionnaire is administered once, online, and is anonymous; no personal or identifying data are collected. Participants are aware that they are wearing two distinct trimming-line designs but are not informed which design corresponds to which arch, in order to limit response (expectation) bias.
• DESIGN ALLOCATION AND ANALYSIS
• The trimming-line design allocated to each arch varies among participants. Two identical questionnaire forms are employed: one is completed by participants in whom the maxillary arch carries the supragingival design (mandibular juxtagingival), and the other by participants with the reciprocal allocation. The form completed thereby encodes the arch-to-design correspondence without recourse to a participant identifier or any identifying data, preserving anonymity. Responses are classified by design accordingly, and supragingival and juxtagingival responses are compared within each participant. The primary analysis is an item-level comparison between designs using a paired non-parametric test (Wilcoxon signed-rank, two-sided, alpha = 0.05), supplemented by descriptive statistics (frequencies, means, standard deviations). A pre-specified secondary analysis compares the per-design total score (range 0 to 40), with assessment of internal consistency (Cronbach's alpha).
• RELATIONSHIP TO THE RESEARCH PROGRAM
• This study is the patient-experience component of a coordinated doctoral research program of four parallel split-mouth investigations of clear aligner trimming-line design (periodontal status, subgingival microbiota, optical stability, and patient-reported comfort); findings are interpreted alongside the companion studies at the group level.

Inclusion Criteria:

• Adolescent or adult patient (aged 15 years or older)
• Active orthodontic treatment with clear aligners, including finishing (refinement) aligners
• Wearing two clear aligners with different trimming-line designs, one per dental arch: a supragingival design on one arch and a juxtagingival design on the contralateral arch
• At least 4 weeks of wear of the two designs at the time of questionnaire completion
• Good treatment compliance
• Willing and able to complete the online questionnaire and provide the required consent

Exclusion Criteria:

• Mixed orthodontic treatment (aligners combined with fixed appliances)
• Pre-existing neurological, cognitive, or speech disorders
• Systemic conditions affecting the oral cavity (e.g., xerostomia, mucosal disorders)

Adolescent and adult patients (aged 15 years or older) in active orthodontic treatment with clear aligners, including finishing (refinement) aligners, in whom a supragingival trimming-line design is worn on one dental arch and a juxtagingival design on the contralateral arch. Eligible participants have worn the two designs for at least four weeks at the time the questionnaire is completed. Recruitment is conducted at a single orthodontic practice.

The study collects anonymous questionnaire responses, with no participant code or identifying data; the trimming-line design per arch is derived from which of two identical forms a participant completed. A minimal de-identified dataset (per-design total discomfort scores) will be made available upon reasonable request to the corresponding author following publication. Companion-study data are not linked.

Beginning 6 months after publication of primary study results.

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