Antimicrobial, Biocompatibility, and Durability Outcomes of Nanoparticle-Coated Orthodontic Materials: A Systematic Review of In Vivo Evidence

Objective: To systematically review available in vivo evidence from animal and human studies evaluating the antimicrobial efficacy, biocompatibility, and durability of metallic nanoparticle-coated orthodontic materials, excluding all in vitro research. Methods: Electronic searches were performed in PubMed/MEDLINE, Scopus, Web of Science, Cochrane Library, Ovid, and ClinicalTrials.gov for studies published from January 2015 to August 2025, supplemented by manual and grey-literature searches. Eligible studies included in vivo animal or human investigations assessing surface coatings of orthodontic components—brackets, archwires, ligatures, elastomeric modules, or aligners—with metallic or metal-oxide nanoparticles (Ag, TiN, TiO 2 , N-TiO 2 , ZnO, Au). Primary outcomes were antimicrobial efficacy (CFU, qPCR, or biofilm indices); secondary outcomes included biocompatibility (ion release, cytotoxicity, mucosal response) and coating durability. Quality appraisal used SYRCLE, RoB 2, and ROBINS-I tools; evidence certainty was graded using GRADE. Registration: OSF ( https://doi.org/10.17605/OSF.IO/5CZSG ). Results: Of 1,499 records, eight in vivo studies met the inclusion criteria—six human and two animal. Nanoparticles evaluated were Ag, TiN, TiO 2 , N-TiO 2 , ZnO, and Au. All demonstrated reduced Streptococcus mutans or related bacterial loads compared with uncoated controls. Silver coatings produced early antibacterial effects but released trace Ag + (0.16–0.35 ppb). TiO 2 and TiN coatings reduced bacterial adhesion up to four weeks but showed partial delamination (~60%). N-doped TiO 2 extended efficacy to 60 days, while ZnO coatings-maintained suppression and enamel protection for 12 months despite early surface degradation. Gold nanoclusters exhibited short-term antibacterial and excellent biocompatibility in animal models. Biocompatibility was acceptable across studies, but durability and long-term stability were inconsistently assessed. Meta-analysis was precluded by heterogeneity of outcomes and timepoints. Overall risk of bias ranged from low to serious; certainty of evidence was low to moderate. Conclusions: In vivo data indicate short-term antibacterial and biocompatible benefits of metallic nanoparticle coatings on orthodontic materials; however, evidence for coating durability and long-term safety is limited. Well-designed randomized clinical trials with standardized microbial, biocompatibility, and durability outcomes are essential for clinical translation.