Shear bond strength of orthodontic brackets to aged resin-matrix ceramics with different polymer composition and evaluation of surface restorability after debonding and polishing

This study investigated the shear bond strength (SBS) of orthodontic brackets to aged resin-matrix with different polymer compositions and a lithium disilicate-reinforced glass-ceramic and also assessed the surface restorability of the debonded surface to the pre-bond state. Twenty specimens (n = 20) were prepared from Crystal Ultra (CU), Vita Enamic (VE), Cerasmart (CS), Lava Ultimate, and IPS e.max CAD (EM, control) and subjected to thermocycling. The bonding surface of the aged materials was conditioned via tribochemical silica coating, followed by bonding of orthodontic metal brackets. Ten specimens from each material were randomly allocated to baseline (BL) or thermocycled (TC), which underwent additional thermocycling. The shear bond strength of orthodontic brackets was evaluated using a universal testing machine. The debonded specimens were finished and polished to compare the surface restorability to the pre-bond state. Surface roughness was assessed using a profilometer at different intervals. Two-way ANOVA followed by post-hoc Tukey’s HSD test was used to analyze the SBS data. A paired t-test was used to analyze the difference in surface roughness at various intervals. The significance level was set at p < 0.05. Two-way ANOVA revealed significant main effects for material type (F = 28.32, p < 0.001) and thermocycling (F = 224.76, p < 0.001). The interaction effect was not significant (F = 1.98, p = 0.104). VE showed the highest SBS (BL-27.36 ± 2.73 MPa; TC-18.52 ± 4.13), while CS had the lowest (BL-17.54 ± 3.38 MPa; TC-10.97 ± 2.51 MPa). Thermocycling negatively impacted SBS for all the tested materials. Post-debonding restorability varied between materials, with CU and EM demonstrating the highest and lowest restorability (81.7% vs 36.6%), respectively, to their pre-bond state. The varying performance of different resin-matrix ceramics in terms of bond strength and surface restorability has important clinical implications. Resin-matrix ceramics exhibit varying SBS compared to lithium disilicate ceramics, even with similar surface conditioning and bonding protocols. Resin-matrix ceramics can be restored after bracket debonding and finishing and polishing, but the degree of restoration varies among materials depending on the ceramic and polymer content.