Comparison of Frictional Forces of Orthodontic Lingual Brackets and Archwires by a Friction Tester

Frictional force tends to rapidly increase as the relative angle between the bracket slot and the archwire increases beyond critical contact angle. Objectives: The purpose of this study was to measure the frictional forces between lingual orthodontic brackets and orthodontic archwires with a friction tester (FPR 2000, Rhesca, Japan). The influence of the presence of artificial saliva was determined additionally. Methods: Two different lingual brackets (FJT: Fujita, Japan; ORM: Ormco, USA) and one labial standard bracket (CTL: TOMY, Japan) as a control were used in this study. Archwires of three different alloys (SS: Stainless steel, Ormco, USA; TM: b–Titanium, Ormco, USA; EL: Cobalt-chrome, RMO, USA) with 0.016 x 0.022 inch and 0.017 x 0.025 inch were used in this study. Each bracket-archwire couple was tested with a friction tester in the dry and artificial saliva environments. Each measurement was conducted with a angular velocity of 0.1 rpm for 90 seconds under the normal force of 100 gf at 25^oC for the dry and 34^oC for the artificial saliva environment. Results: For SS and EL archwires, frictional force was greater with FJT bracket than with ORM bracket (p<0.01). Compared with SS and TM archwires, 0.016 x 0.022 inch EL archwire showed higher frictional forces with two lingual brackets (p<0.01). There was significant difference in frictional forces between the dry and artificial saliva environment (p<0.05); however, the effects of artificial saliva were different depending on the bracket-archwire combinations. ORM bracket showed less frictional forces than FJT bracket in many cases. Experimental critical contact angles were greater than theoretically calculated values. Conclusions: Friction tester can measure the frictional force continuously when the angle between the bracket and arch wire changes.