Experimental Investigation of the Biomechanical Properties of a Newly Introduced Self-ligating Bracket

Abstract

Orthodontic tooth movement is the result of a combination of biological and biomechanical phenomena, which occur under the application of specific forces by means of orthodontic appliances. Both the behavior of elastic materials and the mechanical factors, that control and influence the effectiveness of the applied forces, must be thoroughly considered in the design of an optimal orthodontic system. In addition, self-ligating brackets have gained lately a broad acceptation in the scientific society and have been extensively investigated. Nevertheless, in these days of multifaceted versatile brackets, the available evidence concerning the potential forces applied to the teeth by the various combinations of self-ligating mechanisms and archwires is limited. Under the perspective of the biomechanical principles applied to the orthodontic mechanotheraphy, an in-vitro study was designed and carried out, presenting mainly the following objectives: (a) comparative investigation of the forces generated during the initial stages of complex orthodontic tooth malalignment correction with various bracket-archwire combinations, by means of an experimental biomechanical set-up, (b) assessment of torque effectiveness in the sagittal plane during later stages of a simulated orthodontic tooth movement by utilizing diverse bracket designs combined with a variety of rectangular archwires.<br /> In terms of this experimental investigation have been used a newly introduced 0.016 inch twin slot bracket (Swiss Nonligating Bracket/ SNB), as well as 3 different kinds of 0.018 inch slot brackets (Speed, Mini mono, Brilliant). A variety of archwire combinations has been used in order to evaluate the amount of the correction of a complex malalignment and the torque efficiency of the above mentioned bracket systems. These archwires are differentiated basically according to both their material and cross-section dimensions. The Orthodontic Measurement and Simulation System (OMSS) allowed us in each measurement cycle simultaneously a three-dimensional registration of the force and moment systems, which affected the left mandibular incisor during the performance of a mock orthodontic tooth movement.<br /> Analyzing the obtained results on the whole, it is obvious that the various ligation types applied in each bracket system exert a significant influence on the degree of the malalignment’s correction. Specifically, the degree of correction concerning the self-ligating brackets ranges from 72 % - 98 %, respectively, whereas the same measurements for the conventional brackets demonstrate values of 51.7 % - 77.39 %. The constricting alignment capabilities by the conventional brackets can be attributed to the frictional forces generated between bracket and archwire.<br /> In the current study the values referring to the mean maximal forces in the inciso-gingival axis by using the 0.007" NiTi and the 0.009" NiTi combined with all bracket designs did not overcome 0.88 N. However, the relevant values by utilizing the 0.0135" NiTi archwires, as well as those with wider cross-sections, were significantly higher and reached magnitudes up to 6.55 N.<br /> The moments generated in this in vitro investigation approach principally the limits set from the previous investigations, including though some restrictions. In particular, all the evaluated moments exert greater magnitude than 10 N with an exception of the SNB combined with 0.016 x 0.016" BioTorque archwire, where the values are only 4.9 Nmm and cannot display sufficient clinical efficiency. On the contrary, SPEED brackets demonstrate enhanced torque capabilities by expressing torsional movement of 17 Nmm by application of a 0.016" x 0,016" stainless steel archwire and have reached the value of 35.2 Nmm by ligation of a 0.016" x 0.022" stainless steel archwire.<br /> The superiority of the self-ligating brackets against the conventional ones is suggested through this study, as far as the exertion of light and constant forces is concerned. Moreover, the active self-ligating brackets have demonstrated slightly better results as the passive ones. The qualities of the NiTi spring play a prominent role in the demonstration of these advantageous biomechanical properties. Additionally, it should be underlined that the greater impact factor on the expression of better torque capabilities is imposed by the archwire dimension, rather than its material properties.