Screening the optimal parameters for Er: YAG laser debonding of lithium disilicate veneers: an in vitro study

Although the efficacy of Er: YAG lasers for debonding all-ceramic restorations has been validated, further research is required to optimize the operational parameters for Er: YAG laser debonding of lithium disilicate veneers and to determine whether laser debonding alters the translucency, hardness, and rebonding strength of lithium disilicate veneers.

This study aimed to evaluate the comprehensive debonding performance of an Er: YAG laser operating at a clinically prevalent power of 4.5 W with three different energy and frequency parameter combinations. The evaluation was performed on IPS e.max CAD lithium disilicate ceramic specimens with two thicknesses (0.5 mm and 1.0 mm), focusing on debonding time and intrapulpal temperature variation. Furthermore, the changes in ceramic rebonding strength, surface morphology, translucency, and Vickers hardness following laser debonding were analyzed, so as to identify the optimal parameters for clinical laser debonding of ceramic veneers.

Forty freshly extracted maxillary first premolars were prepared to achieve standardized uniform enamel surfaces. CAD/CAM ceramic blocks were fabricated into two sizes of square specimens: 3 × 3 mm2 square specimens for the assessment of debonding efficiency and rebonding strength, and 10 × 10 mm2 square specimens for translucency and Vickers hardness tests. All specimens were categorized into two groups based on ceramic thickness (0.5 and 1.0 mm). Each thickness group was further subdivided into four subgroups with distinct laser parameters: a non-irradiated control group (E0), 300 mJ/15 Hz (E1), 225 mJ/20 Hz (E2), and 150 mJ/30 Hz (E3). The 3 × 3 mm2 square specimens were bonded to prepared tooth surfaces using Variolink N resin cement and subsequently debonded with an Er: YAG laser according to the corresponding subgroup parameters. After debonding, the specimens were rebonded to the tooth surfaces using the same resin cement. Debonding time and intrapulpal temperature change (∆T) were recorded throughout the procedure. The fracture modes of the veneer bonding interfaces were observed under a stereomicroscope, and shear rebonding strength was measured via a universal testing machine. The 10 × 10 mm2 ceramic specimens were irradiated with the corresponding Er: YAG laser parameters for 30 s. Post-treatment evaluations included observations of surface morphology and quantitative measurements of translucency and Vickers hardness. A two-way analysis of variance was adopted for statistical analysis. The Pearson correlation coefficient was calculated to analyze the correlation between debonding time and intrapulpal temperature variation.

For specimens with the same thickness, the E3 group exhibited significantly longer debonding time and greater intrapulpal temperature elevation compared with the E1 and E2 groups, whereas no significant differences were detected between the E1 and E2 groups. For the same laser parameters, 1.0 mm-thick veneers showed significantly longer debonding time and higher temperature variation than 0.5 mm-thick veneers. A strong positive correlation was identified between debonding time and ∆T (R² = 0.739). The main failure mode of the adhesive surface was classified as type 1. No significant differences in rebonding strength were observed after debonding. In addition, laser irradiation exerted no remarkable influence on the surface morphology, translucency, or Vickers hardness of lithium disilicate ceramic specimens.

At a constant laser power of 4.5 W, both E1 and E2 parameter settings are applicable for the debonding of 1.0 mm-thick lithium disilicate veneers, while the E2 setting is optimal for 0.5 mm-thick veneers. Er: YAG laser debonding treatment has no significant adverse effect on the rebonding strength, surface morphology, translucency, or hardness of lithium disilicate ceramics.

The outcomes of Er: YAG laser debonding for all-ceramic veneers are affected by laser energy, frequency, and veneer thickness. Optimized Er: YAG laser parameters should be selected according to the thickness of all-ceramic veneers. Given the preservation of mechanical strength and translucency after laser debonding, the treated all-ceramic veneers are reusable for clinical application.