Effects of Time-Elapsed Bleaching on the Surface and Mechanical Properties of Dentin Substrate Using Hydrogen Peroxide-Free Nanohydroxyapatite Gel.

Abstract

Introduction: There is a critical need to address concerns surrounding the potential impact of bleaching gels specifically on the tooth substrate. Therefore, this laboratory investigation aimed to assess the impact of a hydrogen peroxide (HP)-free bleaching (HiSmile^TM) in comparison to an HP-based bleaching (Opalescence Regular^TM) on the surface and mechanical characteristics of tooth substrate.

Methods: Sixty sound human premolar teeth were sectioned to produce dentin fragments and divided into two primary groups based on the bleaching agent used. Each group was subdivided into three subgroups (n = 10) per distinct bleaching regimens: (T₁) fragments underwent a 7-day immersion in distilled water at 37°C without any bleaching treatment, (T₂) fragments underwent a 7-day immersion in distilled water at 37°C, with the application of bleaching gel occurring on the seventh day for 10 minutes, and (T₃) fragments underwent a bleaching regimen for seven consecutive days, each session lasting for 10 minutes. The initial and final evaluations of surface roughness, nano-hardness, and elastic modulus were performed. Following the bleaching regimens of T₃, a composite stub was fabricated on the dentin fragments for the shear bond strength (SBS) test. Statistical testing was accomplished using the analysis of variance (P < 0.05).

Results: HP-based bleaching gel showed significant differences between measurement intervals in surface roughness, elastic modulus, and SBS parameters (P < 0.05). In contrast, HP-free bleaching gel showed insignificant differences within the group (P > 0.05). The SBS between dentin-composite was significantly affected with the use of HP-based bleaching gel, while HP-free bleaching gel showed insignificant difference between measurement intervals. The qualitative validation of the treatment's impact was further demonstrated using the scanning electron microscopy.

Conclusion: The findings suggest that the bonding stability of composite restorations to dentin may be compromised after bleaching with an HP-based gel, whereas immediate bonding procedures can be safely conducted following the application of an HP-free bleaching gel.