Background: Clear aligner (CA) therapy has been increasingly adopted as an alternative to fixed orthodontic appliances. However, evidence regarding its periodontal effects, including gingival inflammation, biofilm composition, and the potential role of aligner marginal edge design, remains insufficiently mapped. The objective of this scoping review was to map and synthesize available clinical and laboratory evidence on periodontal and biological outcomes associated with CA therapy, with particular attention to the potential influence of aligner marginal edge design. Methods: This scoping review followed PRISMA-ScR and the Population–Concept–Context (PCC) framework. PubMed, Scopus, and Web of Science were searched from 1 January 2015 to 24 October 2025. A total of 1587 records were identified; after deduplication, 770 unique records were screened by title and abstract, followed by full-text assessment of potentially eligible articles. Twenty-five primary studies published between 2020 and 2025 met eligibility criteria and were included in the review. Results: The final synthesis comprised clinical investigations, laboratory studies, and case reports. Clinical periodontal indices and inflammatory biomarkers were assessed using heterogeneous protocols and timepoints. Only three studies specifically addressed aligner marginal edge design; one directly compared margin configurations, reporting differences in local gingival response, with substantial diversity in study design precluding quantitative synthesis. Conclusions: Available evidence on periodontal outcomes during CA therapy is methodologically heterogeneous. Investigations of aligner marginal edge design remain scarce. Standardized assessment protocols and targeted clinical studies are needed to establish the periodontal relevance of trimline configurations.

Most meta-analyses report less external apical root resorption (EARR) with clear aligners (CAs) than fixed appliances (FAs), but pooled differences are sub-millimetric and rest on few recurring studies. This scoping review (PRISMA-ScR) mapped how 2020–2026 evidence on CA–FA comparisons, treatment-related factors and imaging modalities was generated, and checked data consistency, without pooling. Its unregistered protocol (version 1.0) was reconstructed during manuscript preparation. PubMed, Scopus and Web of Science were searched; paired reviewers screened 1688 records and 263 full texts and included 176 reports (174 studies). Fifteen reports, including eight of 32 direct-comparison reports, were excluded because outcome data were not attributable to a distinct sample or not identifiable; in one, pooled by six syntheses, aligner values equal another study’s fixed-appliance values. Of 21 included comparisons, 17 had serious or high risk of bias, one accounted for clustering and three reported measurement error in millimetres; differences were 0.01–1.12 mm except in one. Fifteen of 17 studies not described as randomised reported less EARR with CAs; the only concealed-allocation trial of appliances alone found no difference. Two imaging comparisons disagreed on whether panoramic radiography under- or overestimates EARR. Differences should be judged against measurement error and design, and data consistency should be checked before pooling.