The Influence of Finishing Clear Aligner Marginal Termination Design on Subgingival Periodontal Microbiota Composition During Orthodontic Treatment: A Prospective Longitudinal Split-Mouth Study.

The goal of this clinical trial is to learn whether the edge design of a finishing orthodontic clear aligner at the gumline affects the bacteria living in the groove between the tooth and the gum (the gingival sulcus) in patients undergoing the finishing phase of orthodontic treatment.

The main question it aims to answer is:

- Does a supragingival aligner edge design - which covers approximately 2 mm of gum tissue - lead to higher levels of disease-associated bacteria in the gingival sulcus compared to a juxtagingival edge design that follows the gumline exactly, after 4 and 8 weeks of aligner wear?

Researchers will compare the supragingival trimming-line design to the juxtagingival festooned design to see if covering gum tissue with the aligner edge creates conditions that favor the growth of anaerobic bacteria associated with gum disease.

Participants will:

• Wear finishing clear aligners with both designs simultaneously - one design on the upper jaw and one on the lower jaw - for 8 weeks
• Provide fluid samples from the gum groove at 3 visits: at the start of treatment (baseline), at 4 weeks, and at 8 weeks.

Two aligner edge designs are compared: a supragingival design, which has a straight horizontal edge positioned approximately 2 mm over the gumline, covering approximately 2 mm of gum tissue with direct contact but without entering the gum groove itself, and a juxtagingival design, which follows the natural scalloped shape of the gumline exactly, terminating at the free gingival margin without covering gum tissue and without entering the groove.

The biological rationale for comparing these two designs is the following: the supragingival design, by covering 2 mm of gum tissue, creates a partially enclosed space at the entrance of the gum groove, potentially reducing the clearance of saliva and limiting oxygen access to that area. These conditions may favor the growth of anaerobic bacteria associated with gum disease. The juxtagingival design, by following the gum contour exactly at its edge, leaves the gum groove entrance more accessible to saliva and oxygen, potentially maintaining a less favorable environment for those bacteria.

Each participant receives both designs at the same time - one on the upper jaw and one on the lower jaw - and serves as their own comparison group. This within-person (split-mouth) approach eliminates differences between individuals in general health, oral hygiene habits, and saliva composition, making the comparison between the two designs more precise.

Samples are analyzed using the PeriodontScreen Real-TM real-time PCR kit (Sacace Biotechnologies Srl, Como, Italy; CE-marked in vitro diagnostic device), which detects and quantifies seven bacteria known to cause gum disease: Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis, Tannerella forsythia, Treponema denticola, Porphyromonas endodontalis, Fusobacterium nucleatum, and Prevotella intermedia.

The main measurement is the number of these seven bacteria that exceed the clinically significant concentration threshold defined by the kit at each jaw arch and time point. A score of 0 means none of the seven bacteria exceed the threshold; a score of 7 means all seven do.

The study hypothesis is that the supragingival trimming-line design - by covering gingival tissue and partially enclosing the sulcus entrance - creates microenvironmental conditions that favor anaerobic periodontal pathogen colonization, resulting in higher pathogen burden scores compared to the juxtagingival design after 4 and 8 weeks of finishing aligner wear.

The study is conducted during the finishing phase of orthodontic treatment, when planned tooth movements are minimal (no more than 0.5 mm per tooth), so that any differences in bacterial levels can be attributed to aligner edge design rather than to tooth movement forces.