Relationship between Shear Bond Strength and Fluoride Content of Fluoridated Teeth as Stated by Scanning Electron Microscopy and X-Ray Fluorescence Analysis

Objectives: To study the effect of fluoride treatment on shear bond strength (SBS) of orthodontic brackets and to investigate the relationship between SBS and fluoride content of fluoridated teeth by means of scanning electron microscopy (SEM) and X-ray fluorescence (XRF) analysis.
Materials and methods: A total of one hundred sound maxillary premolars were thoroughly cleaned and randomly assigned to 5 groups (n=20): control group without fluoride treatment and four groups fluoridated with Profluorid® Varnish 5% NaF (VOCO GmbH. Cuxhaven, Germany) and stored in artificial saliva for varied periods: 1 day, 3 days, 5 days and 7 days. After completing the predetermined storage time, specimens of each group were subdivided into two subgroups (n=10); ten specimens were used for SBS testing and the other ten specimens were submitted to XRF analysis to study their fluoride content. An etchant (phosphoric acid 37%) and a resin adhesive system Transbond XT (TXT) (3M ESPE, St. Paul, Minn) were used to bond the ceramic brackets (Perfect Clear, Orthodontic Sapphire Ceramic Brackets, HUBIT Co., Ltd. South Korea) to the teeth surfaces. Debonding was carried out in a universal testing machine (Lloyd, Type 500, Lloyd Instrument, England) at a cross-head speed of 1 mm/min. After debonding, a stereomicroscope (Olympus SZ 60, Japan) was used to determine the type of fracture of each specimen and scores of adhesive remnant index (ARI) were given. SEM investigation was conducted on representative specimens of debonded tooth surfaces of all groups in a scanning electron microscope (Jeol, JSM-5200LV scanning microscope, Japan). XRF analysis was carried out using AXIOS, WD-XRF Sequential Spectrometer (Malvern Panalytical, 2005, United Kingdom). Crowns of specimens of each subgroup were cut, ground and pressed by the aid of binding wax to form disc-shaped specimens (35 mm in diameter × 3 mm in thickness) that were submitted to the XRF analysis. One-way ANOVA, Tukey test and chi-square test were used to analyze the data. Significance level was established at (p ≤ 0.05).
Results: statistically significant differences were found between studied groups in both SBS values and ARI scores. SEM examination showed that fluoride layer acted as a barrier hindering proper etching of tooth surface that resulted in poorer bonding compared to control group. Longer storage, however, caused fluoride layer to be washed away giving better bonding. XRF analysis revealed greater fluoride content of groups with shorter storage time compared to those with longer storage. Linear regression analysis revealed a strong positive correlation (R2 = 0.936) between SBS and storage time and a strong negative correlation (R2 = 0.959) between SBS and fluoride content of fluoridated groups.
Conclusions: As confirmed by SEM and XRF analysis, fluoride reduced SBS of orthodontic brackets in the first few days of fluoride treatment. As fluoride dissolved in artificial saliva after a period of time, SBS recovered to normal values.
Clinical significance: For patients who are in need to orthodontic treatment, if fluoride treatment is mandatory, bonding of orthodontic brackets should be postponed seven days after treatment to get adequate bonding to the tooth structure.