Abstract Design Systematic review and meta-analysis of clinical trials, and prospective or retrospective cohort. The protocol of the study was registered in advance on PROSPERO.Data sources An electronic search in MEDLINE (PubMed), Web of Science, Scopus, and The Cochrane Library was conducted by two independent authors up to September 2022. Additionally, OpenGrey and www.greylit.org were searched for gray literature, whereas ClinicalTrials.gov was searched for detecting any relevant unpublished data.Study selection The review question was defined in PICOS format as follows: population (P), patients undergoing orthodontic therapy; intervention (I), orthodontic therapy with clear aligner (CA); comparison (C), orthodontic therapy with fixed appliances (FA); outcome (O), periodontal health status and development of gingival recession; studies (S), randomized clinical trials (RCTs), controlled clinical trials, and retrospective or prospective cohort studies. Cross-sectional studies, case series, case reports, studies without a control group, and studies with less than 2 months follow-up were excluded.Data analysis Periodontal health status was assessed as a primary outcome and it was measured in terms of pocket probing depth (PPD), gingival index (GI), plaque index (PI) and bleeding on probing (BoP). Gingival recession (GR) was assessed as secondary outcome, and was measured as the development or progression of GR, shown by the apical migration of the gingival margin occurring between pre- and post-orthodontic treatment. Each periodontal index was assessed in three-time points; short-term (2–3 months from baseline), mid-term (6–9 months from baseline), and long-term (12 months or more from baseline). A descriptive analysis of included articles was performed. Pairwise meta-analyses were conducted to compare outcomes assessed in FA and CA groups and were only performed when studies reported similar periodontal indices at similar follow-ups.Results 12 studies (3 RCTs, 8 prospective cohort studies, 1 retrospective cohort study) were included in the qualitative synthesis, of which, 8 studies were included in the quantitative synthesis (meta-analysis). A total of 612 patients (321 treated with buccal FA and 291 with CA) were assessed. Results from meta-analyses favored CA in regards to PI, demonstrating a significant difference in the mid-term follow-up (number of studies?=?4, standardized mean difference SMD?=??0.99, 95 confidence interval CI?=??1.94 to ?0.03, I2?=?99, P?=?0.04). There was a tendency to report better GI values with CA, specifically in long-term (number of studies?=?2, SMD?=??0.46 95 CI, ?1.03 to 0.11, I2?=?96, P?=?0.11). However, no statistical significance between the two treatment modalities was shown for any follow-up intervals (P?>?0.05). As for PPD, the long-term follow-up showed statistical significance favoring CA (SMD?=??0.93 95 CI, ?1.06 to 0.7, P?
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