Effect of yttrium/lanthanum-doped ultrasonically assisted nano-hydroxyapatite on remineralization and bracket bond strength in artificial enamel lesions

This in vitro study aimed to evaluate the remineralization efficacy of ultrasonically assisted yttrium fluoride-doped (Ult-YF₃-nHAP) and lanthanum fluoride-doped (Ult-LaF₃-nHAP) nano-hydroxyapatite (nHAP) on artificially induced enamel lesions (aWSLs), and to compare their performance with acidulated phosphate fluoride (APF) gel, fluoride varnish, casein phosphopeptide-amorphous calcium phosphate (CPP-ACP), and resin infiltrant (ICON). This in vitro study followed a four-phase design: enamel lesion creation, application of remineralization agents, a 14-day treatment protocol, and post-treatment analyses using QLF, Micro-CT, SEM-EDX, and SBS testing. This study included 168 extracted human premolars, divided into eight experimental groups (n = 21 per group): (1) Demineralized control (no remineralization treatment), (2) Acidulated phosphate fluoride (APF) gel, (3) Fluoride varnish, (4) Casein phosphopeptide-amorphous calcium phosphate (CPP-ACP), (5) Ultrasonically assisted nHAP (Control nHAP), (6) Ult-YF₃-nHAP, (7) Ult-LaF₃-nHAP, and (8) Resin infiltrant (ICON). The aWSLs were created under laboratory conditions. Brackets were bonded to the teeth with composite material, and aWSLs were created under laboratory conditions. After lesion formation and at the end of the experimental process, micro-computed tomography (Micro-CT) and laser-assisted quantitative light fluorescence (QLF) analysis were performed to assess lesion progression and remineralization. Additionally, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and shear bond strength (SBS) tests were conducted at the end of the study. Statistical analysis was performed using one-way ANOVA, Kruskal-Wallis, and Mann–Whitney U tests, with a significance level of p < 0.05. The bracket bond strength test data showed no significant differences between the groups (p = 0.156). Significant differences were found among groups for QLF fluorescence recovery (ΔF, p < 0.001), with the Ult-YF₃-nHAP group showing the greatest increase (median: +0.5, IQR: -1.4 to + 0.7), while the control group showed the greatest decrease (median: -12.1, IQR: -12.4 to -10.2). Micro-CT analysis also revealed significant differences between groups (p = 0.008). The APF Gel group showed values comparable to those of all other experimental groups. The highest remineralization values were recorded in the Ult-YF₃-nHAP group (6.87 ± 3.03 mm³), whereas the lowest values were found in the Varnish group. The demineralized control group had significantly higher values than the Varnish group, but lower than the Ult-LaF₃-nHAP group. SEM-EDX analysis revealed that fluoride weight was significantly lower in the Tooth Mousse and Varnish groups compared to the other experimental groups (p < 0.001). Ca/P ratio was significantly lower in the demineralized control, Varnish, and Ult-YF₃-nHAP groups than in other experimental groups (p = 0.002). Ult-YF₃-nHAP showed higher efficacy in remineralization of aWSLs compared to fluoride-based treatments, CPP-ACP, and resin infiltrant. The highest remineralization was detected in the Ult-YF₃-nHAP group by micro-CT and QLF analysis, while fluoride varnish gave the lowest result.