Some biomechanical considerations in treatment with the lingual technique

Dietmar Segner 

This paper was presented at the 2nd International Conference of the JLOA in Tokyo
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www.lingualnews.com   Vol 5 No 1 May 2007   

Abstract
The mechanical properties of the bracket-wire-interface influences the efficiency of any fixed appliance, especially the lingual appliance. Up to now few studies have looked into mechanical properties of the bracket-archwire-interface. Laboratory tests were devised to check on the following aspects of the brackets and ligation methods: friction, rotational control, and force development. A number of measurements were carried out to get more information on what to expect in the treatment with the lingual appliance.

Part 1 - Friction
Friction is created between the archwire and the bracket for two main reasons (Segner and  1995). The first is the possible pull of the ligature. It depends on the type of ligation method and can be influenced by the choice of it. The second is more complicated to understand but inevitable. It is induced by the task of the bracket to prevent undesired tipping and rotation or -depending on the task- the intentional application of a derotation moment. To create these moments, the wire has to press on the entrance of the bracket slot (Fig. 1). This kind of friction is proportional to the moment that is created by the archwire-bracket combination and inversely proportional to the width of the bracket. Since lingual brackets are narrower, they display relatively short slots and thus create more friction than labial brackets.

Fig. 1: Creation of an uprightening moment involves the archwire pressing against the bracket at the edges of the bracket slot at its end. This in turn inevitably induces friction when the tooth and the archwire are moved relative to each other during sliding mechanics.

Fig. 2:Test bracket mounted on the axis of the friction test apparatus. Test wire is pulled through the bracket-ligature combination while at the same time defined tipping moments can be applied by weights on the back side of the axis (see inset).

For the present study the influence of the ligation was to be investigated. To do this a test wire (16x22 stainless steel) was pulled through the test bracket in a specially designed test apparatus (Fig. 2). The position of the wire was controlled by ball-bearings. To simulate the uprighting moments, the bracket was mounted passively using light cured composite on an axis that can turn in more ball bearings. On the other side of the axis moments simulating a tipping action can be created by hanging weights onto a pulley. For the current investigation friction was tested with no moment and with a moment of 1000 cNmm.

The results are shown in curves that describe the friction force while the wire was pulled through the bracket-ligature combination (Figures 3 and 4). Fig. 3 shows the situation of a bicuspid bracket (Ormco 7th generation) with different ligation methods. It shows clearly that the only ligation method that creates little friction is the steel ligature. The friction values are almost as low as if there would be no ligature. A simply tied element of an elastomeric chain (3M-Unitek) and the ligation with an elastomeric ligature (O-ring, Ormco, size 120) showed similar friction values of about 85 cN. If a double-over-tie ligature from elastomeric chain (Alastik grey, 3M-Unitek) was used, the friction increased more than two-fold to 290 cN. If a tipping moment of 1000 cNmm was simulated the friction values increased by between 60 and 95 cN for all ligation methods.

Fig. 3:Friction curves for different ligation methods with an Ormco 7th generation bicuspid bracket. 

Fig. 4: Friction curves for different ligation methods with an Ormco 7th generation molar twin bracket.

A similar situation exists for the molar brackets (Fig. 4). If no ligature was used, if a steel ligature was used or if a hinge-cap bracket was used there was almost no friction. With elastic ligatures friction increased to between 80 and 150 cN. The highest value was created when 2 elastomeric rings were used, one on the mesial two wings and one on the distal ones. Applying the tipping moment increased friction measurements by about 70 cN.

Looking at these results the following conclusions may be drawn:

•       as in labial treatment technique elastomeric ligation induces severe friction

•       specific to the lingual technique any use of the double-over-tie ligature (DOT) will involve much more friction than simple elastic ligation

•       if sliding mechanics is used, DOT should not be used in the lateral segments

•       the use of steel ligatures or hinge-cap brackets is highly recommendable

Part 2 - Rotational control
A paper by Bednar and Gruendemann (1993) showed that significant differences exist in the ability of certain ligation methods to control rotation or actively effect derotation. In their study they showed that in labial brackets elastic ligation proved much less efficient in rotational control than steel ligature ligation. To test how commonly used ligation methods in lingual technique perform a test apparatus was devised the can measure how much rotational moment the archwire-bracket-ligature combination can create for certain rotation angles (Fig. 5 a) and b)). The crown of a Frasaco canine tooth was mounted on a steel rod that could be connected to a ball bearing. A lingual bracket (Ormco 7th generation, lower left canine) was bonded to the plastic tooth in a way that the rotation axis of the steel rod was passing right through the bracket slot. The whole assembly could be rotated by applying weights to a pulley on the other end of the axis. A test wire was fixed to the left and right of the test bracket using two labial brackets and ligatures. The test wires were ligated to the test bracket using different ligation methods.

Fig. 5: a) Ormco 7th generation test bracket with ligature mounted on a frasaco tooth connected to a steel rod. The long axis of the assembly and its rotation axis passes through the slot of the bracket. b) Defined moments are applied to the steel rod. They rotate the tooth-bracket combination until the ligature creates enough resistance to withstand further rotation. The buccal twin brackets are used to hold the test wire in place.

The results are summarized in Fig. 6. The double-over-tie ligature (from grey Alastik chain short; 3M/Unitek) had very little rotational control. If the tooth was rotated 10°, which is clinically a severe rotation, the ligature generated a mere 100 CNmm of rotational moment. Even at 25° the moment was still below 500 cNmm. For efficient rotation of teeth a moment of at least 500 cNmm and for many teeth rather 1000 to 1500 cNmm seem desirable.

Fig. 6: Diagram showing how much rotational moment the different ligature systems can withstand/develop at certain rotational angles.

Fig. 7: Double-over-tie steel ligature shows a significant amount of play. With very careful ligating and the use of thin (0.2 mm) ligature wire the amount of play can be significantly reduced.

The so called lasso-ligature that passes from the bracket labially around the tooth and back to the lingual archwire was the only ligation or auxillary that could produce meaningful moments. At 10° rotation it developed 800 cNmm and even when the tooth was not rotated any more it still showed 380 cNmm. However, it was important, that the lasso-ligature was rather tight (4 short chain elements) as the results for a longer (6 short elements) and therefore less tight lasso-ligature were much less favorable.

One of the aims of the investigation was to find out whether modern highly elastic leveling wires in combination with steel ligatures could render the lasso redundant. This would be highly appreciated as the lasso tends to discolor and the part on the labial side of the tooth is visible. The test of a NEO-Sentalloy F80 wire ligated with a double-over steel ligature (ligature wire 0.25 mm; Dentaurum) were not very promising (see red broken ling in Fig. 6). The curve runs on a rather low level up to around 20°, when it becomes much steeper. The curve indicates that there is slack between the archwire and the ligature. Closer visual inspection could verify this (Fig. 7). The ligature was then re-made and it was tried to make it as tight as possible. However, the stiffness of the ligature wire makes it very difficult to pull the wire material close around the bracket. The continuous line in Fig. 7 shows that some slack could be eliminated, but a play of still more than 10° before meaningful rotational moments were created, are clinically far from optimal.

With a round 16 Nitinol Classic archwire the results were better but the resulting moments were still below 500 CNmm for rotations of up to 13°. The following conclusions could be drawn from the results:

•         Derotation of teeth is a problem in lingual orthodontics

•         Elastomeric ligation has very little effect, even the DOT ligature

•         If using steel ligatures, care must be taken to eliminate all slack (which is difficult with today’s ligature wires)

•         Lasso elastics or elastics from hook to hook are the most effective derotation methods with the Ormco 7th Generation brackets

•         Vertical slot brackets have a clear advantage regarding de-rotations (see Fig. 8 as an example)

Fig. 8: Efficient rotational control using occlusal slot brackets (Conceal 2, Creekmore Enterprises, Houston). There are 4 months between the two pictures. No the almost perfect de-rotation of the right central incisor with only one archwire (Sentalloy light 0.014; GAC) and no auxiliaries or rotational elastics.

Part 3 - Control of forces

Superelastic wire materials have been integrated into lingual orthodontics widely in the desire for less permanent deformation of the archwires, more constant forces, and shorter treatment times. The phase transition of the nickel-titanium wire material during the application of stress promises these properties of modern wire materials (Segner and Ibe, 2000). However, other studies have reported that many wires either do not show good superelastic properties or that they develop forces that are too high (Segner and Ibe, 1995). NiTi wires of identical diameter (0.016 round) showed force between 100 and 400 g. Wire materials of rectangular cross-section often exerted forces of more than 1000 g. As it is the responsibility of the orthodontist to assure that the forces created by his appliance do not exceed a safe limit, it is of interest how the commonly used superelastic wires in lingual technique perform.

Orthodontic wires are tested in bending in a test described by a norm of the American Dental Association (Specification 32) and by the ISO norm 15841. The center plunger moves down and thereby elastically bends the test wire. The test machine registers the displacement of the center plunger and the force with which the wire resists to the bending. The result are force deflection curves as in figures 9 and 10 to 12. Figure 9 shows the relationship between the curvature of the wire and the measured curves. The tighter the curvature and the smaler its curve radius, the more bending strain is in the wire and the further to the right on the curve is the operating point. In the clinical situation depicted with “C1” in figure 9 the curvature is wide and such a large curve radius would be found on the force-deflection diagram in the left part. Especially in lingual treatments the leveling archwire must often be formed into rather tight curvatures (“C3” in figure 9). Such a small curve radius would be found far to the right in the force-deflection diagram. In Fig. 9 it becomes apparent, why a superelastic wire with relatively constant forces  over a large range of deflections (= bending radii) can be advantageous in lingual orthodontics. With a conventional wire material  (stainless steel, TMA, non-superelastic nickel-titanium) the forces would increase with tighter curvatures of the wire, while in superelastic nickel-titanium wires the force increases only a little.

Fig. 9: Relationship between curvature of the elastically bent archwire and the force-deflection-diagram of the superelastic wire material. See text for detailed explanation.

Fig. 10: Force-deflection-diagrams for superelastic leveling wires of the dimension 16 round.

Fig. 10 shows the results for a number of leveling wires of the dimension 16 round. The lowest force is exerted by the Sentalloy light material. This wire only creates forces of 40 to 50 grams.  Nitinol Heat Activated, Tensic, Sentalloy medium , and Copper-NiTi 35° have all forces of between 80 and 130 gram. Rematitan light and Nitinol superelastic have plateaus at around 175 gram while Nitinol Classic does not show a superelastic curve. This wire is made of nickel-titanium but as it will not perform a phase transition between the austenitic and martensitic phases it is not superelastic and reacts more like a “classic” linear wire. In the initial stages of the treatment forces above 80 gram might already be undesirably large, especially for the lower anteriors.

In Fig. 11 the difference between really superelastic wire materials (here: Sentalloy light) and non-superelastic materials (here: Nitinol Classic; both in 16 round) become apparent. In this diagram the measured results for different spans between the outer supports (10 mm vs. 12 mm) are shown. The smaller span leads to larger deformations and tighter curvatures with the same deflection. It thereby resembles the smaller inter-bracket spans in lingual technique in relation to the larger spans in the labial technique.

Fig. 11: Influence of wire span on the force development of superelastic and non-superelastic nickel-titanium wires. While the force increases significantly with the smaller span in conventional wires, the span almost doesn’t affect the force in superelastic wire materials.

As can be seen clearly the non-superelastic wires have a significantly higher force. According to the laws of physics (i.e. Hooke’s law) the shorter span should create forces that are higher by the factor of 1/((10/12)^3) or 73% more. The measured curves only show between 30% and 50% more, which still is significantly more. With the lingual technique the span differences and hence the force differences are expected to be even larger. With the superelastic wire materials there is not only a much lower force, but it is also almost identical for the two spans. So the shorter spans have almost no negative effect. This truly is a significant advantage of superelastic archwires used in the leveling of lingual cases.

The last figure (Fig. 12) shows the measured curves for 4 different superelastic nickel-titanium wires with a rectangular cross-section (i.e. 16x22). Today such wires are often used as initial leveling wires both in the labial and the lingual technique.  

Fig. 12: Force-deflection-diagrams for superelastic wires with rectangular cross-section of the dimension 16x22. Note the large hysteresis and the fact that on the deactivation curves it takes a deactivation of around 1 mm from the maximum activation until the force level reaches the plateau. Before that the forces are significantly higher.

Again large differences show. While the NEO-Sentalloy 80 and the Copper-NiTi 40° show forces of around 50 gram on the plateau, the other 2 wires (Copper-NiTi 35° and Nitinol Heat Activated) exert forces of between 150 and 300 gram which seems to be too high for almost all orthodontic leveling applications.

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Adult and Lingual Orthodontics
EDITORS:
Dr. Silvia Geron D.M.D., M.Sc
Dr. Rafi Romano D.M.D., M.Sc
Dr. Pablo Echarri D.M.D., M.Sc