The wettability, microhardness and bonding performance of newly developed orthodontic primers under different ageing models

Objectives: To investigate the properties of the newly developed graphene oxide/hydroxyapatite (GOHA) and graphite fluoride/bioactive glass (GFBAG) primers and compare them with Transbond XT orthodontic primer, which served as the control primer. Design: In vitro study. Setting: Department of Orthodontics, College of Dentistry, University of Baghdad. Methods: After primer preparation, primer wettability was investigated by measuring the static contact angle (n = 8). The hardness was evaluated by the Vickers hardness of the primer disks (n = 6). The shear bond strength (SBS), adhesive remnant index (ARI) and enamel damage index (EDI) were investigated using 240 premolars that were randomly divided into three groups (control, GOHA, GFBAG) and bonded in a standardised method. Each group was further subdivided (n = 20) for evaluation using different ageing models (initial bonding, water storage, acid challenge and thermocycling). Results: The experimental primers exhibited higher contact angles (25.59° and 27.83° for GOHA and GFBAG, respectively) and lower wettability than the control primer (20.48°), with a significant reduction in wettability for the GFBAG primer ( P = 0.008). In addition, a non-significant increase in microhardness was observed in the experimental primers. The values of SBS, ARI and EDI were comparable to the control primer ( P > 0.05), except for the acid challenge group, which showed a significant reduction in the SBS compared to the control primer ( P = 0.046 and P = 0.014 for GOHA and GFBAG, respectively). Conclusions: The wettability and hardness of the newly developed GOHA and GFBAG primers were comparable to those of the control primer, while the bonding strength was within clinically acceptable levels, with comparable ARI and EDI to the control primer. Since in vitro studies cannot provide an exact simulation of the complex oral environment, the new primers could be promising candidates for further in-vitro and clinical evaluation during orthodontic treatment, as patients will be at high risk of developing white spot lesions.