Orthodontic lingual treatment is a feasible way to treat several malocclusion especially in adult patients, who are most likely to seek an aesthetic solution . Lingual straight wire method simplified orthodontic mechanisms and increases the predictability of orthodontic outcomes, and the impact of this revolution on lingual orthodontics was comparable to that brought about by the introduction of Andrew’s labial straight wire technique in 1972 .
Scuzzo and Takemoto realised that if they cut the clinical crowns off a plaster cast, the buccolingual distances at the gingival margin did not vary substantially between canine and first premolar. This led them to conclude that lingual straight wire method was a feasible way if the brackets were placed as close to the gingival margin as possible. Thus, they identified the lingual straight plane (L-S plane), the optimal plane on which to place vertical bracket slots .
This method made possible a frictional space closure with sliding mechanism to correct arch length discrepancy, anteroposterior jaw relationship and to improve the soft-tissue profile [6, 7, 19].
A key factor during this type of treatment, especially during a frictional en masse retraction, is good anchorage control. This will prevent pre-contacts between the maxillary lingual brackets and the mandibular incisors that would otherwise induce posterior dysfunction and inhibit retraction. Moreover, posterior segments tend to tip mesially, leading to lateral open bite at the premolars and loss of lateral function. This phenomenon is known as the vertical bowing effect and can occur when the stiffness of the archwire is overcome by the active forces exerted by the power chains. This can also cause distobuccal molar and mesiobuccal canine rotation and arch expansion at the premolars, causing the so-called transverse bowing effect [6, 7].
Above-mentioned side effects could be counteract modulating elastic chains forces to an optimum level and by using straight lingual archwires with suitable stiffness. We set out the test to prove the hypothesis that lingual straight wire are most suitable than mushroom lingual archwires to take under control these side effects thanks to their major stiffness due to their minor length by a modified three bending test performed through an Instron 4467 dynamometer (Instron, Norwood, Mass).
The recommended lingual archwires for partial canine and en masse retraction of the six anterior teeth are 0.016-in. SS and 0.016 × 0.022-in. SS in the mandibular arch, whereas stiffer wires are recommended for the maxillary arch, specifically 0.017 × 0.025 SS and 0.0175 × 0.0175 TMA, which should enable better control of maxillary incisor torque [19, 20, 21].
In this respect, according to our measurements, all straight wire samples were stiffer than their mushroom counterparts. Although we acknowledge the limitations of conducting an in vitro rather than in vivo study, we also show that the difference in stiffness between the two archwire shapes is particularly relevant in the maxilla, where there is a greater difference in their lengths (about 11.5 %). Indeed, the forces exerted by the UMWs was far lower than that measured for the corresponding USWs, whereas the relative forces exerted by the LMWs and the corresponding LSWs were more similar, and statistically significant differences were not seen in all cases. This could be an advantage in the lower arch with some kinds of archwires, but not all. Indeed, the TMA samples were deflected to a far greater extent than the stainless steel samples, in accordance with the literature, which states that the modulus of elasticity (E) of TMA is about 30 % with respect to that of stainless steel, and TMA wires therefore exert lower forces at the same amount of deflection [12, 13, 22].
The literature also states that the force released by an archwire increases proportionally with its diameter but decreased proportionally with its length. Hence, the stiffness of an archwire depends on both its modulus of elasticity (E), i.e. the alloy, and geometric factors, its second moment of inertia (I). For archwires with a rectangular cross section, I is h3w/12, whereas for those with a round cross section, it is Πr
4/64 [11, 12, 22, 23].
In a maximum anchorage case, when sliding mechanics are used, better anchorage control in the posterior segment can be achieved with a stiffer wire. To this end, it is interesting to note how a straight 0.016 × 0.016-in. SS for the upper arch provides 35 % more stiffness with respect to a mushroom wire. When 0.018 × 0.018-in. SS SWs and UMWs were compared, this difference in stiffness rose to 60 %. As regards the type of alloy, we confirm the findings that TMA wires are not rigid enough to counteract the elastic forces exerted by power chains and should therefore be reserved for non-frictional space closure rather than sliding mechanics. Moreover, microscopic analysis of TMA wires has revealed a very rough surface with the worst coefficient of friction of any of the orthodontic archwires, in which also makes them unsuitable for use in frictional extractive space closure . As a stiff wire provides greater control of the system, minimising vertical and transversal bowing and the other unwanted effects mentioned above [6, 7], it follows that the straight wire would seem to be preferable to the mushroom wire in this respect, particularly in maxillary arch. Incidentally, mushroom wires are also plagued by the difficult management of bending in the non-friction extractive space closure due to the small inter-bracket distances and the greater tendency of adult patients to experience irritation of the soft tissues provoked by loops [24, 25]. Concerning the archwire cross section, it is preferable to choose an archwire with a cross-sectional area enough large to guarantee the stability of the system, but not so large as to increase friction at the wire/bracket interface.