• This prospective, longitudinal, within-person (split-mouth) clinical study evaluates the influence of two clear aligner trimming-line designs on marginal soft-tissue biofilm distribution and periodontal tissue response over a six-month period.
• Two trimming-line designs are compared. The supragingival design has a straight, horizontal edge that extends approximately 2 mm onto the marginal gingiva, resting in direct contact with the gingival surface but not entering the gingival sulcus. The juxtagingival design follows the scalloped contour of the gingival margin and terminates at the free gingival margin, without covering the gingiva and without entering the sulcus.
• Each participant receives both designs simultaneously, one per dental arch, and therefore serves as their own control. The assignment of each design to an arch is determined by the treating clinician according to the orthodontic treatment indications and is not randomized.
• The primary outcome is the percentage of sites with bleeding on probing per arch at six months. Secondary outcomes are bleeding on probing at three months, mean probing depth, maximum probing depth, and the Turesky-Quigley-Hein plaque index, with three exploratory photographic soft-tissue variables describing marginal gingival plaque presence, vestibular interdental papilla plaque presence, and the marginal demarcation pattern. Clinical and photographic assessments are performed at baseline (before aligner delivery), at three months, and at six months.
• All aligners are fabricated from a tri-layer clear aligner material (CA Pro, SCHEU-DENTAL GmbH, Iserlohn, Germany) by a single orthodontic laboratory.

• BACKGROUND AND RATIONALE
• Clear aligner therapy is widely used in contemporary orthodontics. The geometry of the aligner trimming line at the gingival margin differs between two commonly used designs: a supragingival design that covers a band of marginal gingiva, and a juxtagingival design that follows the gingival contour. Biomechanical investigations have characterized how trimming-line geometry affects force transmission and retention, and a limited number of clinical reports have examined its periodontal effects. However, direct clinical comparisons of trimming-line geometry on periodontal outcomes remain scarce and have been limited largely to adolescent populations and to conventional periodontal indices; within-person comparisons in adults, and the distribution of biofilm on the marginal soft tissues, remain insufficiently characterized. This study addresses that gap.
• THE TWO TRIMMING-LINE DESIGNS The supragingival design has a straight, horizontal edge that extends approximately 2 mm onto the marginal gingiva, resting in direct contact with the gingival surface but not entering the gingival sulcus. The juxtagingival design follows the scalloped contour of the gingival margin and terminates at the free gingival margin, without covering the gingiva and without entering the sulcus. Both designs are produced from the same CE-marked tri-layer clear aligner material (CA Pro, SCHEU-DENTAL GmbH, Iserlohn, Germany) and are standard variations used in routine orthodontic practice. The intervention under study is the trimming-line design, a manufacturing parameter, not the aligner material or device itself.
• STUDY DESIGN This is a prospective, longitudinal, within-person (split-mouth) interventional study. Each participant receives both trimming-line designs simultaneously, one on the maxillary arch and one on the mandibular arch, and therefore serves as their own control. The unit of comparison is the dental arch within the participant. The split-mouth design controls for inter-individual variation in systemic health, oral hygiene, salivary characteristics, and baseline periodontal status. The within-person (split-mouth) design was chosen to maximize statistical efficiency in an exploratory clinical evaluation, with each participant serving as their own control.
• ALLOCATION RULE (NON-RANDOMIZED) Allocation of each design to an arch is not randomized. It follows a pre-specified rule based on the orthodontic treatment plan. The primary criterion is the magnitude of planned dental intrusion per arch, derived from the digital treatment setup prepared before aligner fabrication: the arch with the greater planned intrusion receives the supragingival design. When planned intrusion is equal on both arches or absent, a pre-specified alternating sequence in enrollment order is applied as a tiebreaker, with the first such participant receiving the supragingival design on the maxillary arch, the next on the mandibular arch, and so on. This produces an approximately balanced arch-level distribution. Planned intrusion magnitude is used only as the basis for this allocation rule; it is not entered as a covariate in the analysis, and the study does not analyze the relationship between intrusion magnitude and hard-tissue outcomes (root or alveolar bone).
• CLINICAL AND PHOTOGRAPHIC ASSESSMENTS Assessments are performed at baseline before aligner delivery (T0), at three months (T1, plus or minus 7 days), and at six months (T2, plus or minus 7 days). At each visit, full-mouth periodontal charting (probing depth and bleeding on probing) is performed first, on undisclosed tissue, by a calibrated examiner who is blinded to the trimming-line allocation; conventional plaque assessment is then performed using a two-tone disclosing agent and the Turesky-Quigley-Hein plaque index; and standardized intraoral photographs are obtained afterwards.
• EXPLORATORY PHOTOGRAPHIC VARIABLES Three photographic soft-tissue variables, namely marginal gingival plaque presence, vestibular interdental papilla plaque presence, and the marginal demarcation pattern, were developed for this study to describe the distribution of biofilm on the marginal gingival soft tissues, a dimension not captured by conventional hard-surface plaque indices. These variables are designated as exploratory. The photographs are de-identified, coded, and presented in randomized order to two independent blinded examiners, and inter-examiner agreement is assessed using the Cohen kappa coefficient.
• ANALYSIS The primary comparison is the within-person, between-arch difference in the percentage of sites with bleeding on probing at six months (T2), with baseline (T0) as the reference and each participant serving as their own control. The three-month (T1) assessment is analyzed as a secondary, intermediate time point describing the trajectory of periodontal response. The secondary validated outcomes, probing depth and the plaque index, are analyzed within the same within-person framework across all time points. The exploratory photographic variables are summarized descriptively, and their effect estimates are reported to inform the design of future confirmatory studies. All tests are two-sided, with the significance level set at 0.05.
• SAFETY Both trimming-line designs are standard variations used in routine clinical orthodontic practice with a CE-marked aligner material, and neither constitutes a novel or experimental device. Untoward periodontal or soft-tissue events occurring during the study are recorded at each scheduled visit and may be reported by participants between visits. Predefined clinical criteria are applied for the withdrawal of a participant when continued participation would no longer be in the participant's clinical interest.
• LIMITATIONS Allocation is not randomized and is tied to the planned intrusion per arch. Because intrusion magnitude is not adjusted for in the analysis, between-arch differences may partly reflect the clinically driven allocation rather than the trimming-line design alone; this is acknowledged in the interpretation of the results. The split-mouth design reduces inter-individual variation but cannot exclude arch-level biomechanical differences other than the trimming-line design.

Inclusion Criteria:

• Permanent dentition
• Adult patients
• Mild to moderate crowding not requiring extractions for orthodontic purposes

Exclusion Criteria:

• Mouth breathing (may influence the status of the marginal periodontium)
• Periodontal involvement (gingivitis or periodontitis)
• Presence of fixed prosthetic restorations with incorrect marginal adaptation
• Deficient oral hygiene

Individual participant data will not be made publicly available, as the small single-site cohort (20 participants) cannot be fully de-identified without risk to participant confidentiality. De-identified data may be made available from the principal investigator on reasonable academic request, subject to an appropriate data-sharing agreement and ethics approval. Aggregate results will be reported in the doctoral thesis and in peer-reviewed publication.

Outcome assessor masking. Clinical periodontal assessments (probing depth and bleeding on probing) are performed by a calibrated examiner blinded to the trimming-line allocation; at each visit the participant removes both aligners before the examiner enters, and the examiner has no access to treatment plans or allocation records. Periodontal charting is performed on undisclosed tissue, before the disclosing agent and photography; the plaque-based marginal demarcation has no clinically visible tissue component and is therefore not perceptible at the time of clinical assessment. Photographic outcomes are scored independently by two examiners blinded to allocation, from de-identified images coded with random identifiers and presented in randomized order; inter-examiner agreement is assessed using the Cohen kappa coefficient. The treating clinician, who assigns the designs, is not an outcome assessor.