Torque expression capacity of 0.018 and 0.022 bracket slots by changing archwire material and cross section
Progress in Orthodontics
Torque expression capacity of 0.018 and 0.022 bracket slots by changing archwire material and cross section
Angela Arreghini 0
Luca Lombardo 2
Francesco Mollica 1
Giuseppe Siciliani 2
0 Postgraduate School of Orthodontics, University of Ferrara , Via Montebello 31, 44100 Ferrara , Italy
1 Department of Engineering, University of Ferrara , Via Montebello 31, 44100 Ferrara , Italy
2 Postgraduate School of Orthodontics, University of Ferrara , Ferrara , Italy
Background: The aim of this study is to calculate and compare the play and torque expression of 0.018 and 0.022 bracket slots when engaged with archwires of different size, cross section and material. Methods: Eight orthodontic brackets, two of slot height 0.018 and six of slot height 0.022, from different manufacturers, were measured and fixed to a vertical support. Twenty-four archwires of differing size, cross section and material were selected, measured and tested in each bracket of compatible slot width. Compression testing by Instron dynamometer and geometric calculations enabled us to determine the play angle of each bracket/archwire combination, and the angle at which a clinically efficacious force couple, sufficient for dental movement, is exerted. Results: All bracket/archwire combinations considered were found to have play angles far above the ideal. This is ascribable to the slots being oversized with respect to the manufacturers' claims. Likewise, some archwires were found to be oversized, while others undersized. When the same archwire was tested with brackets from different manufacturers, the play and torque expression differed, despite the same nominal dimensions of the slots. When the same bracket was tested with the same size archwires, their construction material was found to influence the torque expression, due to the difference in elastic modulus, but not the wire/slot play. Conclusions: The dimensional precision of orthodontic brackets and archwires and the rigidity of the latter have a profound influence on the torque expression of pre-angled appliances.
Torque expression; Slot precision; Play
Background
In the straight-wire technique, brackets are pre-programmed
with first-, second- and third-order information, which is
expressed thanks to the interplay between the archwire
and slot, a function of their respective geometries and
sizes. When an undersized archwire is inserted into a
bracket slot, the wire can rotate clockwise or
anticlockwise. The angle of freedom of the wire within the bracket
slot is known as ‘play’, and this increases as the difference
in size between the archwire and the slot [
1
].
Within this range of rotation, no dental movement
occurs, so to transmit third-order information to the
tooth, the archwire must come into contact with the
walls of the slot and then undergo further torsion,
generating a force couple through which a moment, or torque,
is expressed. In 1982, Burstone stated that a clinically
efficacious moment is between 5 and 20 Nmm [
2
], i.e.
no tooth movement occurs under 5 Nmm, and torque
exceeding 20 Nmm is likely to damage the periodontal
tissues.
Hence, the effective size of the slot is of fundamental
importance in orthodontic biomechanics. The earliest
edgewise appliances designed by Angle in 1925 [
3
]
featured brackets with slots of height 0.022 inch. In 1930,
however, with the introduction of more rigid steel alloys,
archwire diameters began to get smaller. This led Steiner,
in 1952, to design a bracket with slot 0.018, which were
threaded with working archwires of cross section .017 × .025
Torque
−1°
0°
0°
0°
0°
0°
0°
0°
and full-thickness archwires of .018 × .025 [
4
]. From the
1970s onwards, first, Andrews then Roth introduced and
perfected the straight-wire technique, using working wires
of dimensions .019 × .025 and greater thickness wires of
.021 × .025 in slot size 0.022 [
5
]. This marked the start of a
divergence between two of the most widespread
orthodontic systems: those involving 0.022 inch slots and those
relying on 0.018 inch slots.
Over the same period, the archwires also began to
evolve. In the 1930s, the first chromium/nickel/steel alloy
wires were introduced [
6,7
], and in the 1950s, the Elgin
Watch Company developed chromium/cobalt archwires,
whose rigidity increases when heat-treated [8]. In the
1960s, the US Navy created a revolutionary
‘shapememory’ alloy, Nitinol, which is 20% more elastic than
conventional steel and has a much broader field of
action. This was followed, in 1980, by the Ormco
Corporations launch of beta-titanium (TMA) archwires, made
of a formable alloy of elasticity between steel and nickel
titanium (NiTi) [
9
].
The .022 inch system has mechanical advantages in
some clinical situations, such as during sliding
mechanics when a .019 × .025″ SS archwire is used, nevertheless,
.018 inch system seems to be superior in the amount of
the couple it is able to express, when a .017 × .025″ SS
archwire is e (...truncated)