Effect of Different Concentrations of Nano Hydroxyapatite on the Microshear Bond Strength of Zirconia to Resin Cement.

Statement of problem: The impact of varying thicknesses and densities of nano hydroxyapatite (n-HA) coating on the microshear bond strength (μSBS) of zirconia ceramics to resin cements is not well investigated.

Purpose: This study aimed to assess the impact of nano hydroxyapatite coating of different concentrations on the microshear bond strength of zirconia ceramics to resin cements.

Materials and method: 48 Cercon Y-TZP blocks (4×4×4 mm) were randomly divided into 4 groups (n=12); 1) Control group, 2) 5gr n-HA coating, 3) 10gr n-HA coating, and 4) 15gr n-HA coating. The specimens were sintered at 1250°C. The thickness of the coatings was characterized using scanning electron microscopy (SEM). Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analyses were carried out to characterize the surface. All specimens were then bonded to Panavia F2.0 cement. Microshear bond testing was carried out using a universal testing machine. Stereomicroscope imaging was used to evaluate the zirconia surface failure mode. The data were analyzed using Kruskall-wallis and Mann-whitney test and P<0.05 was considered statistically significant.

Results: The mean μSBS value of the 5gr n-HA coated group with a thickness of 7.71±2.51 micron was 28.30±3.02 Mpa, which was significantly higher than the control group with the mean value of 19.94±1.86 Mpa (P<0.05). The mean μSBS value of the 10gr and 15gr n-HA with thicker coatings were 21.66±3.63 Mpa and 15.23±3.34 Mpa, respectively. Additionally, the main mode of failure for the 5gr n-HA group was a mixed failue. In contrast, the adhesive mode of failure was more dominant in the control group. EDS and XRD analysis revealed the formation of crystalline HA on zirconia blocks, highlighting the predominant zirconia peak at 32°. The SEM analysis of the coated specimens displayed a uniform layer of n-HA coating with evident cracks which were filled with the resin cement. These cracks themselves can help increase the bond strength between ceramic and resin cement.

Conclusion: A thin coating of nanocrystalline hydroxyapatite on zirconia surface can be effective in enhancing the micro shear bond strength of these ceramics to resin cements. The microshear bond strength of nHA-coated zirconia significantly improved compared to the untreated specimens.