The Study of Arch Change with Rocking-chair Archwire

Objectives:"Rocking-chair" archwire which is also called reverse curve of Spee archwire is often used for leveling arch and opening bite in Straight Wire Technique. The purpose of this study was to observe the three-dimensional tendency of arch change when different depth rocking-chair archwire were used. Methods: The Oriental Preadjusted Appliance-KOSAKA (OPA-K) brackets (4.57mm slot, TOMY Company, Japan) and arch form were used in this study. Model teeth (B605, China) whose shape was same as the standard teeth were chosen. Aligned teeth in Typedont according to OPA-K arch form. Ligated 2mm, 4mm, 6mm .018*.025 inch (4.57mm*6.35mm) stainless steel "rocking-chair" archwire. (2mm, 4mm, 6mm were the depth of rocking-chair archwire which means the distance from the lowest point to the line connected the front and back edge of the archwire when observed laterally.) to simulate clinical process. Three-dimensional arch changes before and after ligating different depth rocking-chair archwire were measured by three-dimensional measure instrument. (YM-2115, China). Results: There was no significant change in arch length and width when 2mm rocking-chair archwire was used (P>0.05); The width between molars and arch length increased when the depth of rocking-chair archwire was up to 4mm and 6mm (P<0.05). At the same time, incisors and molars were intruded, premolars were extruded (P<0.05). Conclusion: The result revealed that rocking-chair archwire could level the curve of Spee and have the tendency to change arch length and width at the same time. According to the increase of the "rocking-chair depth", the width between molars and arch length increased.