Microstructural, Mechanical, and Adhesive Characterization of Nanofluorapatite-Modified Orthodontic Resin-Modified Glass Ionomer Cement: A Preliminary In Vitro Study

Nanofluorapatite (nFAp) was incorporated into GC Fuji Ortho LC resin-modified glass ionomer cement at 2 wt.% and 5 wt.% to assess its influence on microstructure, local mechanical properties, and orthodontic bond strength. The unmodified and nFAp-modified formulations were characterized by micro-computed tomography, backscattered-electron scanning electron microscopy with energy-dispersive X-ray spectroscopy (BSE-SEM/EDS), instrumented Vickers indentation, shear bond strength (SBS) testing, Weibull analysis, and Adhesive Remnant Index (ARI) scoring after debonding of orthodontic molar tubes bonded to human molars. Micro-CT showed higher porosity and resolved pore number density after nFAp incorporation, while pore dimensions changed only modestly. BSE-SEM/EDS showed microscale compositional heterogeneity and a markedly higher Ca signal in the modified materials. Indentation hardness and modulus did not differ significantly among groups. Mean SBS increased from 12.47 ± 2.41 MPa in the reference group to 17.31 ± 2.34 MPa and 16.66 ± 2.60 MPa for 2 wt.% and 5 wt.% nFAp, respectively; both modified groups differed significantly from the reference group but not from each other. ARI distributions were similar among groups. Overall, nFAp incorporation improved resistance to debonding without a corresponding change in local indentation properties, with 2 wt.% nFAp showing the most favorable descriptive overall mechanical profile among the tested formulations.