The Effect of Elastic and Metallic Ligation Methods on the Unloading Forces for Three Different Types of Nickel-Titanium Archwires Inserted Into Metallic Brackets: An In-vitro Study

Introduction: One of the unique properties of the nickel-titanium (NiTi) archwires is the release of light continuous forces with a wide range of activation, which is affected by the method of ligating the wire to the bracket. Both elastomeric modules and metallic ligatures are the most common components to ligate the archwire to the bracket. This study aimed to evaluate the effect of elastomeric versus metallic ligatures on the released forces of three types of rectangular 0.016- by 0.022-inch NiTi wires. Materials and Methods: This investigation evaluated three different situations of ligating the NiTi archwires to metallic brackets. The produced forces by these wires were evaluated using a modified three-point bending test (a universal testing machine: Testometric 350M(®), Instron, Lincoln Close, Rochdale, England). A 0.022-inch slot-size premolar bracket was used for this investigation with zero torque and zero angulation (Master Series®, American Orthodontics, Sheboygan, USA). The first situation involved no ligation to the metallic brackets using three types of NiTi wires, whereas the second situation (the elastic ligation) involved the use of colored elastomeric modules with a 0.045-inch inner diameter and a 0.115-inch outer diameter (Color Ligatures(®), American Orthodontics, Sheboygan, USA) to ligate the archwire into the metallic bracket's slot. In the third situation (metallic ligation), stainless steel metallic ligatures with a 0.012-inch (Preformed Ligature Wire(®), American Orthodontics, Sheboygan, USA) were used to perform the ligation. The employed NiTi wires were 0.016 x 0.022-inch in diameter (American Orthodontics(®), Sheboygan, Wisconsin, USA), and they were of three different types: (1) the superplastic wires (NT3-SE(®)), the thermally activated wires at 25°C (Thermal Ti-D(®)), and the thermally activated wires at 35°C (Thermal Ti-Lite(®)). Results: The thermally activated wires at 35°C (Thermal Ti-Lite(®)) recorded the lowest force levels for the three ligation methods, while the highest result appeared with the superelastic type (NT3-SE(®)) and the thermally activated type at 25°C (Thermal Ti-D(®)) which showed a varied force for different ligation methods. Regarding the three different NiTi wires used, the elastic and metallic ligation increased the force levels for the superelastic type (NT3-SE(®)) over non-ligation by 50% and 110%, respectively, whereas the elastic ligation raised only the forces for the thermal types (Thermal Ti-D(®) and Thermal Ti-Lite(®)) by 75% and 22%, respectively. Both thermally activated types (Thermal Ti-D(®) and Thermal Ti-Lite(®)) released forces in the elastic ligation method greater than that of the metallic method by 125% and 20%, respectively. Conclusion: The elastic ligation method raised the unloading forces in comparison with the non-ligation for all tested archwires, whereas the metallic ligation method raised the released forces for the superelastic type (NT3-SE(®)) and decreased those related to the activated types (Thermal Ti-D(®), Thermal Ti-Lite(®)) due to the increased friction and the instability of the crystalline structure. Only the heat-activated type at 35°C (Thermal Ti-Lite(®)) reflected the unique properties of the NiTi wires in producing light continuous forces with a wide range of activations regardless of the type of ligation.