Investigation On Laser Cleaning Of Orthodontic Brackets

This research was commissioned by Beijing Stomatological Hospital and theresults of this research have been proved to be feasible through the test of clinicalapplications.Due to many factors, failure of orthodontic bracket occurs frequently inorthodontic practice. The usual way is to recycle shed orthodontic bracket by usingblasting and other methods in order to reduce the financial burden on patients and awaste of resources. However, there are some drawbacks for conventional methods,such as damage of bracket base, and cannot effectively recycle ceramic orthodonticbrackets. Based on the demands of brackets refurbishment and understanding ofexisting literature on the utility of laser in refurbishing orthodontic brackets, weinitially proposed the method of using KrF excimer laser and picosecond laser torecycle three kinds of orthodontic bracket, including stainless bracket and ceramicbracket. By analyzing the cleaning process and the recycle result, we indicated avariety of laser cleaning mechanism for each method and the series emergence ofphenomenon during the research was also discussed.Three laser cleaning methods were utilized in the article:(1) KrF excimer laserirradiation;(2) picosecond laser irradiation;(3) picosecond laser scanning.3Mstainless bracket, Mini Sprint stainless bracket and3M ceramic bracket wereexperimented respectively. It was found that the excimer laser irradiation methodwould be a proper way to achieve the recycling of3M and Mini Sprint stainlessbracket. By the comparison of bond strength with new stainless bracket, it wasverified that stainless bracket could be recycled with excimer laser in clinicalapplications. Besides, picosecond laser irradiation and scanning method could be wellperformed in refurbishing3M ceramic bracket, and the appearance remain inaesthetics by scan method, but a black spot could be observed after ps laser irradiation.Compared with new bracket, there is no statistically significant to bond strength forirradiation method and scan method. Thus, the bond strength shares the same level with the new metal bracket, and ceramic bracket recycled by ps laser remains clinicalapplicability.By analyzing the results of experiment along with the record of cleaning process,the cleaning mechanism for each of the three methods was also confirmed anddiscussed. For KrF excimer laser irradiation method, the mechanism was bondbreaking ablation; the elastic waves generated by ultra-short high-frequency pulses isfound to be the main cleaning mechanism for picosecond laser irradiation, andspallation also played a role in it; picosecond laser scanning method is based on thecold ablation to achieve recycling of orthodontic brackets.In addition, some phenomena found in experiments were explained and analyzed.3M ceramic orthodontic brackets changed in appearance after excimer laserirradiation, which it was caused by the concentration change of oxygen vacancy inbracket base. The discoloration can be restored after annealing, but had difficulties inclinical applications for cost of time.By the picosecond laser irradiation at wavelength of532nm, the top of MiniSprint stainless orthodontic bracket retention surface emerged micro-pitmicrostructure. It is suggested that localized surface polaritons (LSPs) induced by theoriginal point defects on bracket base enhance the energy distribution of laser input,which result in the formation of micro-pit structures on the bracket surface. By FDTDelectromagnetic simulations, it was verified the matching of different wavelengthsand different scales of defects, and coincided with the actual situation.The last section of this paper briefly introduced another engineering project, inwhich I participated under the guidance of my tutor.