Introduction: Titanium is used in dentistry both for crowns, fixed and removable partial dentures, suprastructures and for implants. In such applications, titanium may come in contact with fluorides. Prophylactic agents such as toothpastes and gels have effects on titanium that are dependent on their acidity and fluoride concentration. In order to obtain rapid bone growth and a strong bond between the implant and the bone, fluorides are used to modify the surfaces of titanium implants1. The aim of this investigation was to study the release of ions and the effects of fluorides on surfaces of titanium cp-II in saline solutions of different pF and pH using a) a brush test and b) electrochemical techniques including impedance spectroscopy2.
Materials and Methods: Brush test. An electric toothbrush was used to brush the titanium electrodes for 2 min in aqua solutions of 0.9 % NaCl with 0, 0.3, 0.6 and 1 % NaF. The pH of the solutions was adjusted by adding ultra pure HNO3 and 1 M NaOH. In order to avoid interference with the chemical analyses, the solutions were not buffered. The solutions were divided into two all-embracing groups, pH 4.5 and pH>5.3, with respect to their acidity. An atomic absorption spectrophotometer equipped with a graphite atomizer with Zeeman effect background correction was used to detect titanium in the NaCl solutions.
Electrochemical tests. Planar titanium electrodes with surface area of 0.28 cm2 served as working electrodes and were wet-ground with SiC paper through 1200 grit using standard metallographic procedures prior to exposure to the various electrolyte solutions. The solutions employed were similar to those used in the previous experiments, i.e. phosphate-buffered 0.9 % NaCl solutions with a pH between 4 and 7, containing 0, 0.3, 0.6 and 1 % NaF. An ordinary three-electrode setup was used with Ag/AgCl, sat KCl as a reference electrode and a platinum sheet as a counter electrode. The open circuit potential was recorded for 18–20 hours, and the electrode was then polarized to –0.18 V vs. ref. for five minutes. After this an impedance spectrum was recorded at –0.18 V vs. ref in the frequency range 100 000 Hz to 5 mHz and an amplitude of 10 mV peak to peak. Finally a linear anodic polarization curve was recorded from –0.5 V to 1 V vs ref at a sweep rate of 2 mV/sec.
Results: The result of the brush tests is shown in Table 1. The highest amount of titanium was detected in the NaCl solutions with 0.6 or 1.0 % NaF at pH 4.5.
Titanium remains passive in neutral solutions irrespective of the addition of fluoride and also at pH 4 without addition of fluoride. This is indicated in all three electrochemical experiments: the final electrode potential at open circuit is around -0.1 V vs ref after 18 hours, the impedance spectrum shows an almost pure capacitive behavior and the current at anodic polarization is very small. At low pH with addition of fluoride, titanium becomes active, as evidenced by a negative final electrode potential at open circuit of around -1.0 V vs ref after 18 hours, an impedance spectrum indicating a non-blocking electrode and a high current at anodic polarization. At medium pH and with an addition of fluoride the electrochemical experiments indicate the formation of a porous non-passivating surface film. In the latter case the final open circuit potential is around -0,5 V vs ref, the impedance spectra show a non-blocking electrode with a large charge-storing capacity and the current at anodic polarization is close to the values obtained for the active electrode at pH 4. The results of the electrochemical tests are summarized in Table 2