Supervisors info:
Γεώργιος Ηλιάδης, Καθηγητής, Τμήμα Οδοντιατρικής, Σχολή Επιστημών Υγείας, ΕΚΠΑ
Τριαντάφυλλος Παπαδόπουλος, Καθηγητής,Τμήμα Οδοντιατρικής, Σχολή Επιστημών Υγείας, ΕΚΠΑ
Σπυρίδων Ζηνέλης, Αναπληρωτής Καθηγητής, Τμήμα Οδοντιατρικής, Σχολή Επιστημών Υγείας, ΕΚΠΑ
Summary:
Introduction: Orthodontic lingual brackets were introduced in clinical practice in order to satisfy the demand for invisible treatment. Τhe Au alloy fabricated customized Incognito lingual brackets (3M Unitek) show superior tooth adaptation and biocompatibility in comparison to the stainless steel conventional ones. But, does this superiority apply for their galvanic behavior when coupled to different alloys as well? In the relevant literature there are a lot of studies regarding stainless steel fixed appliances but this is not the case for lingual brackets and in particular made of Au alloy where the research data are scarce.
Aim: The aim of this study was to measure the potential difference of coupled lingual orthodontic brackets and arch wires in different electrolyte media containing chlorides and fluorides.
Materials and Methods: The study included three types of orthodontic lingual brackets of 0.022” (0.5588 mm) slot size corresponding to central incisors. These were IncognitoTM (3M Unitek, USA), In-Ovation L (DENTSPLY GAC, USA) and STbTM-L (Light Lingual System, ORMCO, USA). Furthermore, they were used rectangular stainless steel orthodontic archwires 0.017 x 0.025” (0.4318 x 0.635 mm) (Steel Arch Wires, FORESTADENT, GERMANY) as well as NiTi of similar size (Neo Sentalloy, DENTSPLY GAC, USA).
The surface of all brackets and wires tested were initially imaged by SEM (SEM, and then all materials were embedded in epoxy resin molds with their longitudinal axis oriented vertically and parallel to the horizontal plane, respectively. Subsequently, the fixed appliances were ground and polished in a grinding/polishing machine. The surface characterization was accomplished by acquisition of secondary (SEI) and compositional backscattered (BEI) electron images under high vacuum conditions. X-ray energy dispersive spectroscopy (EDS) was used for the elemental analysis of orthodontic wires and brackets, whereas X-ray diffraction analysis (XRD) was employed for the analysis of their crystallographic structure.
Proper length stainless steel metallic rods were laser welded to the bases of lingual orthodontic brackets. The orthodontic archwires, the bases of the brackets and a portion of the welded rods were insulated with non-conductive epoxy resin and elastic rubber tube on top. The galvanic cell apparatus consisted of a glass container for the electrolyte, a plexiglass top permitting the entrance of a coupled bracket-orthodontic archwire, a voltometer and a personal computer. The construction of electrochemical cell and the measurement procedure of galvanic potential difference complied with the ASTM G71-81 (reapproved 2009) standard. The measurement of galvanic potential difference was performed for 48 h until a plateau was reached. Two water electrolyte solutions were used. The first one was an acidic (pH=2.3) solution of 0.1M lactic acid and 0.1M NaCl according to ISO 10271/2011. The second one was a neutral (pH=6.5) water based solution of 0.3% NaF corresponding to 1394 ppm F⁻. In total six combinations of orthodontic wires and brackets were used for each electrolyte.
Results: Incognito were three-phases, single piece, Au alloy brackets. The heavy phase was mainly characterized by Ir. STb and In-Ovation brackets were two piece appliances made of steel of different consistency for the base and wings. The two bracket pieces were joined together with Au alloy brazing (STb) or welded (In-Ovation L). In-Ovation L base was composed of a three-phases alloy, while the wing and both parts of the STb were single-phase compounds. The elemental analysis showed that the bases of both brackets match the composition of martensitic steel AISI 316 (Fe, Cr: 16-18%, Ni: 10-14%, Mo: 2-3% P,S, C-ASTM A240) and the wings austenitic steel PH 17-4 (Fe, Cr: 15-17,5%, Ni: 3-5%, Cu: 3-5%, C, P, S, Si, Mn, Nb-ASTM Α693). Furthermore, the stainless steel Forestadent orthodontic archwires did not show mean atomic number difference areas and were close to austenitic steel composition AISI 304 (Fe, Cr: 18-20%, Ni: 8-10,5%, P, S, Si, C). Similarly, the NeoSentalloy archwires were single-phase compounds made of 52,5% Ni and 47,5% Ti. The crystallographic analysis showed the presence of martensitic and austenitic phases for the stainless steel and the predominance of austenitic phase for the NiTi archwires.
Electrochemically, Incognito brackets showed the lowest potential difference in NaCl for both archwire combinations (SS=-19 mV, NiTi= +3.5 mV) while STb the highest (SS=-58 mV, NiTi=-94.5 mV). In between were placed In-Ovation L brackets (-23.5 mV, -8.5 mV). On the other hand, the NaF electrolyte had minimal effects on SS-bracket couples but this was not the case for NiTi. In particular, Incognito brackets showed the highest potential difference (+319 mV) followed by STb (+202.5 mV) and In-Ovation L (+55 mV).
Conclusions: Under the limitations of this experimental research the following conclusions could be extracted:
• In contrast to Cl⁻ ions, F⁻ ones influenced negatively all the NiTi lingual bracket couples galvanic potential.
• All bracket couples with SS orthodontic archwire under research showed high resistance to galvanic phenomena irrespective to the electrolyte used.
• The combination of Au alloy (either as a bracket or as a brazing) with NiTi orthodontic archwire, in the presence of F⁻ ions, is vulnerable to galvanic corrosion.
