Change in crown inclination accompanying initial tooth alignment with round archwires

ABSTRACT Objective: To evaluate, in-vitro, the change in crown inclination that occurs during orthodontic leveling and alignment using different archwire-bracket-ligation combinations. Materials and Methods: Four archwire types were tested: (1) 0.012-in stainless steel and (2) 0.0155-in stainless steel multi-stranded, (3) 0.012-in nitinol Orthonol® and (4) 0.012-in nitinol Thermalloy®. Combinations with five types of 0.022-in slot orthodontic brackets were tested: SmartClipTM and Time3® self-ligating brackets, Mini-Taurus® and Victory SeriesTM conventional brackets, and Synergy® conventional-low friction bracket. Conventional brackets were ligated with both stainless steel and elastomeric ligatures. The simulated malocclusion comprised 2.0mm gingival and 2.0mm labial displacements of a maxillary right central incisor. Rotation around the Y-axis (representing labio-palatal inclination) was measured for the different archwire-bracket-ligation combinations. Results: The largest rotation was measured whith Orthonol® and Thermalloy® wires when combined with SmartClipTM brackets (8.07±0.24º and 8.06±0.26º, respectively) and with Synergy® brackets ligated with stainless steel ligatures (8.03±0.49º and 8.0±0.37º, respectively). The lower rotation was recorded when Thermalloy®, multi-stranded, and Orthonol® wires were ligated with elastomeric rings to Mini-Taurus® brackets (1.53±0.18º, 1.65± 0.23º and 1.70±0.28º, respectively) and to Victory SeriesTM brackets (1.68± 0.78º, 2.92± 1.40º and 1.74±0.46º, respectively). Conclusions: All archwire-bracket-ligation combinations produced lingual crown inclination; however, lower changes were observed when the conventional brackets were ligated with elastomeric rings. The multi-stranded archwire produced less rotation with nearly every bracket-ligation combination, compared to the other archwires. The effect of the archwire-bracket-ligation combination on tooth inclination during leveling and alignment should be considered during planning treatment mechanics.


INTRODUCTION
Andrews, 1 in his prominent article introducing the six keys to normal occlusion, described the importance of adjusting the maxillary incisors' labio-palatal inclination to achieve normal overbite and proper posterior occlusion. Since then, controlling maxillary incisors' inclination for the achievement of pleasing esthetics and convenient occlusion has been well established in Orthodontics. 2 In-vitro and in-vivo studies have investigated the relationship between tooth inclination and arch perimeter 3 , smile and facial features, 4,5 forces and moments generated, 6,7 , and perceived tooth color. 8 Emphasis has shifted in Orthodontics from molars to incisors; at first to the mandibular incisors and then to the maxillary incisors.
This shift happened with advances in orthodontic and orthognathic techniques that made the position of the maxillary incisors the start point in certain treatment plans. 9 The crown inclination is a feature that could be manipulated with rectangular archwires at the finishing stage of orthodontic treatment by creating a moment generated by the torsion of a rectangular archwire in the bracket slot. 10 This action requires full-size archwires to avoid play between the archwire and the slot walls. 11,12 However, archwire of a round cross-section is the standard choice for initial leveling and alignment. 13 Round archwires produce tipping movement, thus changing crown inclination, which may be or may not be desired. Archwire material Montasser MA, Keilig L, Bourauel C -Change in crown inclination accompanying initial tooth alignment with round archwires 5 choices for leveling and alignment include, but are not limited to, stainless steel and nitinol. 14 Thus, the objective of this study was to evaluate, in-vitro, the change in crown inclination that occurs during the orthodontic leveling and alignment of a displaced central incisor using different archwire-bracket-ligation combinations. Simulation System (OMSS) is based on two sensors capable of 3D-recording of forces and moments. The OMSS is built in a temperature-controlled compartment. This is especially important for heat-managed alloys; therefore, during testing the nitinol archwires the temperature was kept at 37±1°C.

Material
Further details about the OMSS and its applications in orthodontics can be found in articles devoted to that purpose. 15,16 Preparing the measurement setup: Brackets were bonded with a cyanoacrylate adhesive from the right second premolar to the left second premolar of fabricated maxillary arch resin models. These resin models had the right central incisor

RESULTS
The results of two-way ANOVA (Table 1)   Montasser MA, Keilig L, Bourauel C -Change in crown inclination accompanying initial tooth alignment with round archwires 10  The boxplots compare changes in crown inclination produced by self-ligating brackets to conventional brackets ligated with stainless steel ligatures (Fig 2) and elastomeric rings (Fig 3).
The figures show that, contrary to the two other conventional brackets, the low-friction Synergy ® bracket ligated with elastomeric rings produced rotation comparable to that produced by self-ligating brackets and conventional brackets ligated with stainless steel ligatures.  Evaluating changes in crown inclination produced by each bracket when combined with the different archwires, as seen in Table 2, less rotation was produced by conventional brackets ligated with elastomeric rings. Compared to the two other conventional brackets, Synergy ® brackets ligated with elastomeric rings produced comparable or more rotation. In the current study, the elastomeric ring was tied around the two central wings of the Synergy ® bracket, which are raised, preventing the contact between the archwire and the ligature and thus ensuring free play between the archwire and the bracket slot walls. Meanwhile, steel ligature was tied slightly loose because this is the preferred method during leveling and alignment to allow free sliding and efficient tooth movement with less force exerted on the tooth. The play between the wire and the bracket slot is probably the most important factor influencing the labio-palatal inclination of teeth. 18 Multi-stranded archwires produced less rotation with every bracket-ligation combination. The rotation was significantly higher when Mini-Taurus ® and Victory Series TM brackets were ligated with stainless steel ligatures than elastomeric rings, but Synergy ® brackets showed insignificant difference and significantly higher rotation with multi-stranded archwire However, elastomeric rings deteriorate quickly in the oral cavity to the extent that they have been found to lose 40% of their force in the first 24 hours. 19 Frequent change of the elastomeric rings might be needed to overcome material deterioration and keep the control on rotation. Tight steel ligature would be required.
There was no statistical difference, as shown in Table 3 The rotation produced by the two types of steel archwires was higher with Time3 ® brackets than SmartClip TM brackets, but differences were not significant. The rotation that was produced by the two types of nitinol archwires was higher with SmartClip TM brackets than Time3 ® brackets, and the difference was significant in case of Orthonol ® archwire. It is noted that the active self-ligating Time3 ® brackets showed a wide range of rotation values with multi-stranded and Thermalloy ® archwires, this evidence could be attributed to the interaction of the clip with these archwire materials while exerting a seating force on the archwire. The change in crown inclination in the present study is the result of a rotational moment dependent on the force applied on the tooth and the distance from that point of force application to the designated center of resistance of the simulated misaligned central incisor. In this study design, the distance was standardized for all tests, therefore the changing factor is the force applied to the tooth from each archwire-bracket-ligation combination. These archwire-bracket-ligation combinations showed a significant interaction between the archwire and the bracket type, and a consistently higher force applied on the displaced central incisor when elastomeric rings were used, which consequently affected the correction of the malocclusion. 20,21 Orthodontic tooth movement is the result of a combination of many mechanical factors related to the bracket-archwire-ligation combination, including: bracket prescription, bracket slot dimension, bracket material, archwire dimension, archwire material, inter-bracket distance, and method of ligation (stainless steel wire, elastomeric rings or self-ligation). [22][23][24][25][26] The oral environment, however, adds additional elements to these factors. Saliva, thermocycling, and forces of occlusion lead to aging and deterioration of the orthodontic materials.
Anisotropic periodontal ligament may also complicate the prediction of tooth movement. 27 Even with the application of the same force system, different responses are expected depending on the surrounding environment and involved biological factors that make tooth movement occurs in a certain plane more easily than it occurs in the other planes. 28 Since mechanics is a major factor that affects force/moment systems from the start of treatment, controlling tooth inclination should be planned from the leveling and alignment because of the different requirements among the types of malocclusion and the individual cases in each malocclusion type. Manipulating tooth inclination from the start would help clinicians to limit the need for bends and other auxiliaries in later stages.
Clinically, multiple teeth or arch segments involved in the malocclusion will lead to complex force systems that sometimes might be hard to predict. Realizing the true complexity of the force systems that could result from the used biomechanics in the oral cavity would help clinicians to use biomechanics more efficiently and simply. 28 Clinical relevance: Results could be useful as a guide when comparing orthodontic materials and planning biomechanics.
Results contribute primarily to the understanding of the performance of an orthodontic appliance and how the outcomes are affected by each of the appliance components, as well as