Introduction
Our objective was to investigate the effect of archwire cross-section increases on the levels of force applied to teeth during complex malalignment correction with various archwire-bracket combinations using an experimental biomechanical setup.
Methods
The study comprised 3 types of orthodontic brackets: (1) conventional ligating brackets (Victory Series [3M Unitek, Monrovia, Calif] and Mini-Taurus [Rocky Mountain Orthodontics, Denver, Colo]), (2) self-ligating brackets (SmartClip, a passive self-ligating bracket [3M Unitek]; and Time3 [Rocky Mountain Orthodontics, Denver, Colo] and SPEED [Strite Industries, Cambridge, Ontario, Canada], both active self-ligating brackets), and (3) a conventional low-friction bracket (Synergy [Rocky Mountain Orthodontics]). All brackets had a nominal 0.022-in slot size. The brackets were combined with 0.014-in and 0.016-in titanium memory wires, Therma-Ti archwires (American Orthodontics, Sheboygan, Wis). The archwires were tied to the conventional brackets with both stainless steel ligatures of size 0.010-in and elastomeric rings. A malocclusion of the maxillary central incisor displaced 2 mm gingivally (x-axis) and 2 mm labially (z-axis) was simulated.
Results
The forces recorded when using the 0.014-in archwires ranged from 1.7 ± 0.1 to 5.0 ± 0.3 N in the x-axis direction, and from 1.2 ± 0.1 to 5.5 ± 0.3 N in the z-axis direction. When we used the 0.016-in archwires, the forces ranged from 2.6 ± 0.1 to 6.0 ± 0.3 N in the x-axis direction, and from 2.0 ± 0.2 to 6.0 ± 0.4 N in the z-axis direction. Overall, the increases ranged from 16.0% to 120.0% in the x-axis and from 10.4% to 130.0% in the z-axis directions.
Conclusions
Increasing the cross section of the wire increased the force level invariably with all brackets. Wires of size 0.014 in produced relatively high force levels, and the force level increased with 0.016-in wires.
Highlights
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We studied the effect of wire cross sections on the levels of force applied to teeth.
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Displacements of the maxillary central incisors 2 mm gingivally and 2 mm labially were simulated.
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The 0.014-in and 0.016-in titanium memory wires were combined with 6 types of brackets.
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The 0.014-in wires produced relatively high force levels that increased with the 0.016-in wire.
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Small wires are recommended in leveling and alignment during orthodontic treatment.
The first phase of fixed appliance orthodontic treatment is concerned with tooth alignment and leveling; although the biologic factors that affect the effectiveness of tooth movement in this stage are largely outside the control of the orthodontist, more direct influence can be achieved with the choice of the bracket-archwire combination. This stage therefore requires a system that will produce an optimum orthodontic force. Optimum orthodontic forces will produce maximum tooth movement and maximum biologic response, and do not compromise patient comfort. Excessive forces have been associated with undesirable reactions and side effects, including bone hyalinization, root resorption, pain and patient discomfort, and anchorage loss. Calculating the forces applied by a continuous archwire proved that forces are applied by the archwire on all the teeth engaged and ligated to the brackets even if only 1 tooth is malaligned.
With the rapid change in the materials used in orthodontics in recent years, there has been a change from the conventional variable cross-section orthodontics to variable-modulus orthodontics; different alloys have different degrees of stiffness for the same shape and size of the archwire, so the stiffness can be reduced without reducing the cross-sectional dimensions. However, since the introduction of nickel-titanium (NiTi) alloys into orthodontics, there has been an increased tendency to use larger continuous archwires early in treatment. The in-vitro deactivation forces at 2 mm of the 0.016-in superelastic NiTi wire measured 2.55 ± 0.03 N, and the forces of the 0.016-in heat-activated NiTi wire measured 2.47 ± 0.03 N. Also, 2-mm deactivations of stainless steel coaxial or thermalloy archwires generated forces that ranged from 3.4 ± 0.2 to 0.7 ± 0.1 N in the incisogingival direction, and from 4.5 ± 0.3 to 0.5 ± 0.1 N in the labiolingual direction.
Generally, increasing the diameter or the cross section of a wire increases its strength and decreases its springiness. The principle with any supported beam, as is the case for a segment of archwire between 2 teeth, is that as the wire size increases, strength increases as a cubic function, whereas springiness decreases as a fourth power function, and range decreases proportionally. However, studies have found that with NiTi alloys, the flexural rigidity is not strongly linked with the cross-sectional size when there is superelastic behavior during unloading; therefore, it was deemed possible to sometimes start treatment with full-size rectangular archwires and still expect force levels acceptable for both tooth movement and patient comfort.
The objective of this study was to investigate the effect of a cross-section increase of Ni-Ti thermally active archwires on the level of force applied to teeth during complex malalignment correction with various archwire-bracket combinations with the orthodontic measurement and simulation system (OMSS).
Material and methods
The study comprised 3 types of orthodontic brackets (1) conventional ligating brackets (Victory Series [3M Unitek, Monrovia, Calif] and Mini-Taurus [Rocky Mountain Orthodontics, Denver, Colo]), (2) self-ligating brackets (SmartClip, a passive self-ligating bracket [3M Unitek]; Time3, an active self-ligating bracket [American Orthodontics, Sheboygan, Wis]; and SPEED, an active self-ligating bracket [Strite Industries, Cambridge, Ontario, Canada]), and (3) a specially designed conventional low-friction bracket (Synergy [Rocky Mountain Orthodontics]). All brackets had a nominal 0.022-in slot size. The brackets were combined with 2 archwires: 0.014-in and 0.016-in titanium memory wire, Therma-Ti (American Orthodontics). Therma-Ti archwires are Ni-Ti alloys; the chemical composition of the alloy in weight percentage was determined to be 54.4% nickel and 45.6% titanium. The archwires were tied to the conventional brackets with both stainless steel ligatures of size 0.010 in (Advanced Orthodontics, Näpflein, Düsseldorf, Germany) and elastomeric rings (3M Unitek). The study materials and design are presented in Table I .
| Bracket type | Bracket width (central incisor) | Bracket width (lateral incisor) | Type of ligation | Type of wire | Group observations (n) | Total observations (n) |
|---|---|---|---|---|---|---|
| Mini-Taurus | 3.6 mm | 3.1 mm | Stainless steel ligation Elastic ligation |
0.014-in titanium memory wire and 0.016-in titanium memory wire | 20 | 360 |
| Victory series | 3.6 mm | 2.9 mm | Stainless steel ligation Elastic ligation |
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| Synergy | 3.4 mm | 3.4 mm | Stainless steel ligation Elastic ligation |
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| Smart-Clip | 3.8 mm | 3.3 mm | Passive self-ligation | |||
| Time3 | 2.5 mm | 2.4 mm | Active self-ligation | |||
| SPEED | 2.5 mm | 2.3 mm | Active self-ligation |
Resin replicas (Palavit G 4004; Heraeus Kulzer, Hanau, Germany) were constructed from a model (frasaco, Tettnang, Germany) of a normally aligned maxillary arch. The right central incisor was removed from the resin model to allow for placement of the sensor of the testing OMSS ( Fig 1 ).
Brackets were bonded from second premolar to second premolar on the resin models with a cyanoacrylate adhesive. A jig was used to standardize the bonding process of the right central incisor bracket to a bracket holder that would be attached to the first sensor of the OMSS.
The self-ligating brackets in this study were used in the closed position. For the conventional brackets, the stainless steel ligatures were tied using a needle holder; each ligature was first fully tightened around the bracket wings and then loosened by 1 turn to allow free movement of the archwire. The elastomeric ligatures were tied to the bracket wings with a needle holder. A 3-minute waiting period was allocated to allow a reproducible amount of stress relaxation to occur before starting to measure the force level as recommended by Henao and Kusy. New elastomeric rings were used for each test.
The OMSS comprises 2 force-moment sensors capable of registering forces and moments in the 3 planes of space simultaneously. The 2 sensors are mounted on motor-driven positioning tables that can move freely in 3 planes of space. Commands regarding the conditions of the experiment are given to the OMSS through a personal computer. Two microcomputer-based sensor electronics deliver the digital output of the force moment vectors to a personal computer where the resultant force-deflection curves are recorded, thus facilitating a means to study the loads from simulated orthodontic tooth movement. The whole mechanical assembly of the OMSS was built in a temperature-controlled chamber; this is especially important when testing temperature-dependent alloys.
To prepare the setup for measurements, the resin model was mounted on the OMSS table, and the bracket holder with the bracket of the right central incisor bonded to it was fixed to the first sensor. The sensor was then adjusted so the bracket was in the right position in the prepared space in the resin model. The whole assembly now simulated the original aligned arch. The malocclusion simulated in this study comprised intrusion-extrusion movements of 2 mm in 0.01-mm increments and a labiolingual movement of 2 mm in 0.01-mm increments. The intrusion-extrusion movement was represented on the x-axis, and the labiolingual movement was represented on the z-axis on the computer program. Thus, the OMSS was set to move the maxillary right central incisor from the initial position 2 mm gingivally and 2 mm labially and then to move it back to the initial position in 0.01-mm increments. Although the 2 force-moment sensors of the OMSS register forces and moments in the 3 planes of space simultaneously and record them directly with the system software, only the maximum absolute values of the forces generated by bracket displacement in the x-axis and z-axis were included in this study. Before starting the measurements, the initial forces and moments exerted on the bracket and recorded by the sensors were adjusted as close to zero as possible.
Each bracket-archwire combination was tested 20 times, for a total of 360 tests. During testing, the temperature was kept at 37°C (±1°C).
Statistical analysis
Descriptive statistics including means, standard deviations, and maximum and minimum values were calculated for each archwire-bracket combination and then statistically analyzed with 2-way analysis of variance (ANOVA) followed by the least significant test and the Student t test. The percentage change in the force with the increase in cross section of the archwires from 0.014 in to 0.016 in was calculated for each bracket.
Results
Descriptive statistics of the maximum forces exerted in the x-axis, representing the intrusion-extrusion movement, and in the z-axis, representing the labiolingual movement, are presented in Tables II and III and Figure 2 .
| Wire type | n | Mini-Taurus stainless steel ligation | Mini-Taurus elastic ligation | Victory series stainless steel ligation | Victory series elastic ligation | Synergy stainless steel ligation | Synergy elastic ligation | Smart-clip | Time3 | SPEED |
|---|---|---|---|---|---|---|---|---|---|---|
| Force (N), x-axis | ||||||||||
| 0.014 in | 20 | 2.2 ± 0.2 a | 4.7 ± 0.3 b | 2.1 ± 0.2 ac | 5.0 ± 0.3 b | 1.8 ± 0.1 cde | 1.8 ± 0.2 de | 2.0 ± 0.1 acd | 1.7 ± 0.1 e | 2.0 ± 0.2 ac |
| 0.016 in | 20 | 3.5 ± 0.3 a | 6.0 ± 0.3 f | 3.0 ± 0.2 bd | 5.8 ± 0.3 f | 3.0 ± 0.2 b | 2.7 ± 0.1 cd | 3.3 ± 0.2 a | 2.6 ± 0.1 c | 4.4 ± 0.2 e |
| Force (N), z-axis | ||||||||||
| 0.014 in | 20 | 1.9 ± 0.2 a | 4.8 ± 0.3 b | 1.8 ± 0.2 a | 5.5 ± 0.3 b | 1.4 ± 0.1 c | 1.3 ± 0.1 c | 2.0 ± 0.2 a | 1.2 ± 0.1 c | 2.0 ± 0.2 a |
| 0.016 in | 20 | 2.9 ± 0.3 a | 5.3 ± 0.3 f | 2.6 ± 0.2 c | 6.0 ± 0.4 f | 2.2 ± 0.2 bc | 2.0 ± 0.2 b | 3.6 ± 0.3 d | 2.0 ± 0.2 b | 4.6 ± 0.2 e |
| Wire type | n | Mini-Taurus stainless steel ligation | Mini-Taurus elastic ligation | Victory series stainless steel ligation | Victory series elastic ligation | Synergy stainless steel ligation | Synergy elastic ligation | Smart-clip | Time3 | SPEED |
|---|---|---|---|---|---|---|---|---|---|---|
| Force (N), x-axis | ||||||||||
| 0.014 in | 20 | 2.2 ± 0.2 a | 4.7 ± 0.3 a | 2.1 ± 0.2 a | 5.0 ± 0.3 a | 1.8 ± 0.1 a | 1.8 ± 0.2 a | 2.0 ± 0.1 a | 1.7 ± 0.1 a | 2.0 ± 0.2 a |
| 0.016 in | 20 | 3.5 ± 0.3 b | 6.0 ± 0.3 b | 3.0 ± 0.2 b | 5.8 ± 0.3 b | 3.0 ± 0.2 b | 2.7 ± 0.1 b | 3.3 ± 0.2 b | 2.6 ± 0.1 b | 4.4 ± 0.2 b |
| Force (N), z-axis | ||||||||||
| 0.014 in | 20 | 1.9 ± 0.2 a | 4.8 ± 0.3 a | 1.8 ± 0.2 a | 5.5 ± 0.3 a | 1.4 ± 0.1 a | 1.3 ± 0.1 a | 2.0 ± 0.2 a | 1.2 ± 0.1 a | 2.0 ± 0.2 a |
| 0.016 in | 20 | 2.9 ± 0.3 b | 5.3 ± 0.3 b | 2.6 ± 0.2 b | 6.0 ± 0.4 b | 2.2 ± 0.2 b | 2.0 ± 0.2 b | 3.6 ± 0.3 b | 2.0 ± 0.2 b | 4.6 ± 0.2 b |
