Avinash Kumar, Arifa Khanam and Hajra Ghafoor
What did the authors aim to do in this study? The main purpose of this study was to evaluate the surface changes of stainless steel archwires after 6 weeks of intraoral use and its influence on frictional resistance during sliding mechanics.
How did the authors evaluate? In this clinical ex-vivo study, ten patients undergoing orthodontic treatment were included & after all 4 extraction was carried out, at the end of leveling & aligning stage, all patients were put on 0.019” × 0.025” stainless steel upper and lower archwires. In order to carry out precise measurements, the Castroviejo caliper was used at the extraction site and the length of the archwire between:
(a) Distal aspect of canine bracket to mesial of premolar bracket,
(b) Distal of premolar to mesial of molar tube,
(c) Mesial of molar tube to the terminal end of the archwire, in all the four quadrants. The wire segment between the premolar bracket and molar tube is considered important for our study because this buccal inter-bracket wire segment is more prone to exposure to saliva, food and sliding of the archwire through the molar tube during space closure. These final archwires, 0.019” × 0.025” stainless steel, of the stage I (leveling and alignment phase of treatment) are left passive intra-orally for about 6 weeks for the expression of torque. After the completion of this duration of 6 weeks, these 0.019” × 0.025” stainless steel archwires are retrieved from patient’s mouth. These 20 archwires, the upper and lower archwires for 10 patients, are then segmented into hemi-arch wires at the incisor area and stored.
Fig 1. Precise measurement of buccal inter‐bracket archwire segment for
the study: (a) Distal aspect of canine bracket to mesial of premolar bracket,
(b) distal of premolar to mesial of molar tube and (c) mesial of molar tube
to the terminal end of the archwire.
The hemi-arch wires were stored in four closed containers 10 each, with a layer of modelling wax onto which the wires were set upright. The tips of the wires that maintained contact with the wax were not used in the examinations. All examinations are performed within 48 hrs of removal of the archwires from oral environment. The 40 hemi-arched wires are then divided into 2 test groups – T1 and T2: T1: Two hemi-arch test wire segments (upper and lower) for the right side of each patient, T2: Two hemi-arch test wire segments (upper and lower) for the left side of each patient i.e., 20 hemi-arch wire segments in each group. C: The 20 as-received stainless steel archwires that serve as control are grouped as: C1 and C2 with 10 control archwires in each group. T1 test archwires, specifically the buccal segments of the archwire between 2nd premolar bracket and molar tube are evaluated for the frictional resistance with the help of Instron Universal Testing Machine (Instron UTM) in comparison with the C1 control archwires and, the same part of the segment of archwire is examined for the surface debris using Scanning Electron Microscopy (SEM) and compared with the surface characteristics of control archwires. T2 test and C2 control archwires are used to evaluate the three dimensional surface characteristics and to quantify the surface roughness of each archwire with the help of Atomic Force Microscope [AFM].
Fig 2. Test setup with acrylic plate model in Instron Universal Testing
Machine for analysis of frictional resistance.
What did the authors find?
The examination of surface characteristics of control stainless steel arch-wires under SEM revealed smooth and regular surfaces. Analysis of debris on these wires showed a complete absence of debris (zero score) for all wire segments at ×200 magnification. However, the SEM micrographs of retrieved test arch-wires showed a marked increase in the degree of debris with an average score of 2.15 (moderate debris) at × 200 magnification. There was also a significant increase in the frictional forces of the test arch-wires when compared with the control arch-wires, corresponding to a 39.7% increase in the friction level.
Spearman correlation analysis showed a highly significant association between kinetic friction and the degree of debris measured at × 200 magnification. Also, there was a significant correlation between surface roughness and kinetic friction of the test arch-wires indicating that as the amount of debris and roughness on the arch-wires increases during their intraoral use, greater becomes the amount of frictional resistance during orthodontic tooth movement. A positive correlation between surface debris and roughness was also determined, to ascertain the surface changes on the test archwires after 6 weeks which cause a considerable increase in the frictional forces.
What did the authors conclude?
This study concludes that rectangular stainless steel archwires exposed to the intraoral environment for about 6 weeks during the final arch wire sequence of levelling and alignment stage for torque expression shows a significant increase in the amount of debris and surface roughness.
The frictional resistance of the archwires also increased markedly after their intraoral exposure when compared to the as-received archwires.
This intraoral aging of the stainless steel orthodontic archwires contributing to the increased degree of debris and roughness on the archwire surface is correlated to the frictional resistance of the archwires, which implies that there would be an increase in friction between the bracket-archwire interfaces during the space closure stage of sliding mechanics.
What do we think about it?
In contemporary orthodontics, many practitioners use sliding mechanics for both aligning irregular teeth and for closing extraction spaces. Tooth movement associated with sliding mechanics involves a dynamic relationship among the archwires, brackets and ligation type in the oral environment. Archwires act as gears to move teeth with light continuous forces. The wires behave elastically to these forces over a period of weeks to months. However, the intraoral use of orthodontic archwires is liable to surface deposits and thus corrosion. These factors affect the surface topography and mechanical properties of the archwires causing an increase in the friction coefficient. Hence, this study provides an insight into the clinical significance & importance of maintaining the archwires ideally to avoid an increase in friction provided by external factors. Decontamination of the archwires has shown to significantly reduce the surface roughness formed by debris and plaque accumulation, but at the same time increases the wire stiffness. Thus, continuing the stainless steel archwire after thorough cleaning of the archwires at every clinical appointment to remove plaque and debris from the archwire surface is not recommended. Using a newer archwire to prevent an increase in friction during mechanical sliding is thus considered desirable.
Kumar A, Khanam A, Ghafoor H. Effects of intraoral aging of arch-wires on frictional forces: An ex vivo study. J Orthod Sci. 2016 Oct-Dec;5(4):109-116.
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