Micrographic study and torsional strength of grade 23 titanium mini-implants recycled for orthodontic purposes

Introduction: The aim of this study was to identify the fracture resistance of grade 23 Titanium mini-implants recycled for orthodontic purposes and having stayed in the mouth during different periods of time. Materials and methods: A total of 52 hybrid-designed titanium mini-implants, threaded length L = 8 mm, self-drilling and self-taping were used in 32 patients presenting orthodontic anomalies which required the use of mini-implants. Two types of tests were performed: polarizing optical imagery to assess the surface characteristics of the mini-implants (polarizing optical microscopy) and (mechanical) torsional strength tests, more quantitative, in order to establish the breaking point of the mini-implants during their re-use. Results: For the mini-implants which had been in the mouth for a short period of time, (immediate removal or after two months of use), it must be noted that their surface characteristics revealed no defect at micron scale, the drilling head did not show any alteration, with a breaking point record of about 53 Newton/cm(2) (N/cm(2)). In contrast, the mini-implants, which had stayed in the mouth for 12 and 14 months, showed surface alterations especially at the "screw-gingiva" interface and the rupture stress ranges from 42 to 39 N/cm(2), respectively. Discussion: Our results show that the resistance to fracture of the re-used mini-implants is inversely proportional to the duration of stay in the mouth. (C) 2018 CEO. Published by Elsevier Masson SAS. All rights reserved