In this study, to obtain simpler and standardized data in the experiment, pay attention to the influence of thickness on the laser removal results, and avoid the interference of variables such as abutment size, height, taper, therefore choosing the sheet shape zirconia to instead of the crown shape restoration as the clinical. The follow-up study will focus on the clinical experiment of laser removal of dental crowns. Shear bond strength test results for samples that did not come off after 5 min of laser irradiation. For example, the residual bond strength of 2 mm samples in RXL group and 1 mm samples in CSL group are less than 1 MPa, which can be removed with the aid of clinical tools such as crown remover. However, the residual bond strength of 4 mm samples in RXL group and 3 and 4 mm samples in CSL group are more than 10 MPa, which cannot be removed smoothly. It can be seen that the thickness has a certain impact on the residual bond strength of the samples. Studies have shown  among the permanent cements for restorations above implants, the retention force of composite resin cement is greater than that of resin reinforced glass ionomer cement and zinc phosphate cement, this conclusion is consistent with the experimental results.
In addition, the main working principle of the laser to retrieve the restoration is to activate the water and monomer molecules in the adhesive between the crown and the abutment [6,7,8,9], this is consistent with the results observed under the electron microscope in this experiment: the bonding surface of 1 mm and 2 mm had numerous adhesive fracture points, the fracture point was circular and was connected to the crack. The ratio of water and monomer components in different adhesives may also affect the absorption of laser energy by the adhesive, thereby affecting the overall dismantling results. Compared with composite resin adhesives, resin-reinforced glass ionomer adhesives contain higher concentrations of water and monomer molecules , which can better absorb the energy of laser to destroy the polymeric structure of the adhesive and accelerate the debonding of the restoration.
Previously reported removal of zirconia crowns from natural teeth or implant abutment using an Er:YAG laser was typically 1–2 min (resin-reinforced glass ionomer cement) or about 4–5 min (composite resin cement) [7, 12, 16]. Therefore, in this experiment, 5 min was used as the upper limit time of laser irradiation, and then the shear bond strength test of the adhesive force of the samples that did not fall off was tested to determine whether the laser had an effect. The parameters of the laser in this experiment were set to 300 mJ, 15 Hz, 4.5 W, which is consistent with the previous parameter settings for removing lithium disilicate ceramics from titanium/zirconia abutments and natural teeth [13, 14, 16], and zirconia crowns on the natural teeth were also used this parameter . It can be seen from the existing research reports that this parameter will not cause obvious damage to the tooth and the titanium/zirconia abutment, only the temperature rises, but the temperature change does not damage the surrounding tissue . In this experiment, the scanning electron microscopy observed that minor changes of morphological changes could be seen on the surface of zirconia samples with shorter laser irradiation time, but the appearance of burning pits could be seen on the surface of zirconia samples with longer laser irradiation time. The morphological change may has some impact on the compressive strength of zirconia, but it needs to be further studied in combination with clinical. This experiment is the first time to test whether the Er:YAG laser has a penetrating effect on the 3 mm and 4 mm zirconia sheets, combined with the electron microscope results, it can be seen that after the Er:YAG laser is used to irradiate the 3 mm and 4 mm specimens for 5 min, it seems to have no effect on the adhesive under the specimens.
This in vitro study demonstrated the feasibility of Er:YAG laser to retrieve zirconia crowns from the titanium abutments, however, it still has some limitations. First of all, the zirconia sheets and titanium blocks which were used in this experiment are different from the crowns and abutments used in actual clinical work, and more time may be required to retrieve a crown in the clinical application; secondly, in this vitro study, the laser can better contact the surface of the restoration, avoiding the obstruction of soft and hard tissues (such as adjacent teeth and gingiva), so that the laser can better act on the surface and edge of the restoration; third, in this study, only resin-reinforced glass ionomer adhesives (RXL) and composite resin-based adhesives (CSL) were used, and the proportions of water and monomer molecules in different adhesives are different, the resulting demolition results will also be different; fourth, with the rapid development of zirconia materials, many zirconia materials have appeared clinically, different zirconia materials may have different reactions to laser retrieval due to their different physical properties (such as light transmittance); fifth, the skill of the operator affects the success rate and time required for irradiation. Finally, it is important to note that future clinical in vivo studies with varieties of abutment/crown materials, cements and prosthetic designs, and clinician skills will be needed to further optimize and understand the clinical applications of Er:YAG laser.