Three lingual bracket systems with equivalent slot sizes and orientations were selected. One type of CL-LB (7th Generation; Ormco, Orange, CA, USA), one type of CL-LB with a narrow bracket width (STb; Ormco), and one type of self-ligating lingual bracket (In-Ovation L; GAC, Dentsply Corp., York, PA, USA) were tested under three tooth displacement conditions: no displacement (control); a 2-mm palatal displacement of the maxillary right lateral incisor (MXLI); and a 2-mm gingival displacement of the maxillary right canine (MXC) (Figure 1 ). Therefore, a total of nine groups were created by the combination of these factors (n = 7 per group).
In this study, a stereolithographic typodont system used in previous studies12 (link)17 (link)18 (link) was refabricated. This typodont system had a full maxillary dentition fixed to an archshaped metal frame, which allowed each tooth to move in the occluso-gingival (up and down) and labio-palatal (forward and backward) directions. At the zero position, all teeth were aligned in the ideal position according to an ovoid arch form (OrthoForm III-Ovoid, reference no. 701-723; 3M Unitek, Monrovia, CA, USA). Each tooth had its periodontal ligament space filled with Imprint™ II Garant™ Light Body Vinyl Polysiloxane Impression Material (3M ESPE, Seefeld, Germany), which emulates the mobility of human teeth and absorbs mechanical stress.12 (link)17 (link)18 (link)
The characteristics of the lingual brackets tested in this study are listed inTable 1 . After the 7th Generation, STb, and In-Ovation L brackets were positioned with full-size preformed straight lingual archwire at the center of the lingual surface, customized resin bases for the brackets were fabricated by curing Transbond XT (3M Unitek). Then, the archwire was removed and individual transfer trays were made. To minimize wire-related bias, 0.013-inch copper-nickel-titanium (Cu-NiTi) preformed lingual archwires were used (STb straight wire small, 204-2101; Ormco).
For ligation of the maxillary anterior teeth, a double over-tie of powerchain was used for the 7th Generation group (Clear Generation II Power Chain, 639-0002; Ormco) and a single tie of elastic modules was applied to the STb group (AlastiK Easy-To-Tie Ligature; 3M Unitek) according to the manufacturer's guide.21 (link)22 (link) For ligation of the maxillary posterior teeth, elastic modules (AlastiK Easy-To-Tie Ligature; 3M Unitek) were used in both CL-LB groups (the 7th Generation and STb groups) according to the manufacturer's guide.21 (link)22 (link) After the ligation of all brackets, a 3-minute waiting period was allowed to obtain reproducible amounts of stress relaxation and ligation force.11 (link)12 (link)15 (link)16 (link)18 (link) Since the In-Ovation L bracket was self-ligating, it was closed with an active clip.
The typodont was then attached to a metal plate fixed to a mechanical testing machine (Model 4466; Instron, Canton, MA, USA). After artificial saliva (Taliva®; Hanlim Pharm. Co., Ltd., Seoul, Korea) was sprayed onto the bracket, the end of the archwire extruding from the maxillary right second molar tube was gripped with a custom-designed adaptor. SFF and KFF were measured while drawing the archwire through the brackets at a speed of 0.5 mm/min for 5 minutes. Tests were conducted in a chamber maintained at 36.5 ± 0.3℃ (Figure 2 ).
After each test, the typodont system was immediately washed with distilled water and alcohol to remove the artificial saliva and then dried with an air syringe. Each group was tested seven times, and a new wire was used each time.
The definitions of SFF and KFF are presented inFigure 3 . SFF was measured at the maximal point of the initial rise. KFF was calculated by averaging frictional forces from after the maximal point of the initial rise to the end of the test.12 (link)17 (link)18 (link)
A power analysis was performed to determine the sample size using a sample size determination program (version 2.0.1; Seoul National University Dental Hospital, Registration No. 2007-01-122-004453, Seoul, Korea). The Shapiro-Wilk test was performed to assess the normality of the distributions in the experimental groups. The existence of normal distributions was confirmed in all nine groups. If equal variances were assumed by the Levene's test, a one-way analysis of variance (ANOVA) with Tukey's honest significant difference (HSD) post hoc test was performed for the statistical analysis. When equal variances were not assumed by Levene's test, Welch's variance-weighted ANOVA with Dunnett's T3 post hoc test was used.
In this study, a stereolithographic typodont system used in previous studies12 (link)17 (link)18 (link) was refabricated. This typodont system had a full maxillary dentition fixed to an archshaped metal frame, which allowed each tooth to move in the occluso-gingival (up and down) and labio-palatal (forward and backward) directions. At the zero position, all teeth were aligned in the ideal position according to an ovoid arch form (OrthoForm III-Ovoid, reference no. 701-723; 3M Unitek, Monrovia, CA, USA). Each tooth had its periodontal ligament space filled with Imprint™ II Garant™ Light Body Vinyl Polysiloxane Impression Material (3M ESPE, Seefeld, Germany), which emulates the mobility of human teeth and absorbs mechanical stress.12 (link)17 (link)18 (link)
The characteristics of the lingual brackets tested in this study are listed in
For ligation of the maxillary anterior teeth, a double over-tie of powerchain was used for the 7th Generation group (Clear Generation II Power Chain, 639-0002; Ormco) and a single tie of elastic modules was applied to the STb group (AlastiK Easy-To-Tie Ligature; 3M Unitek) according to the manufacturer's guide.21 (link)22 (link) For ligation of the maxillary posterior teeth, elastic modules (AlastiK Easy-To-Tie Ligature; 3M Unitek) were used in both CL-LB groups (the 7th Generation and STb groups) according to the manufacturer's guide.21 (link)22 (link) After the ligation of all brackets, a 3-minute waiting period was allowed to obtain reproducible amounts of stress relaxation and ligation force.11 (link)12 (link)15 (link)16 (link)18 (link) Since the In-Ovation L bracket was self-ligating, it was closed with an active clip.
The typodont was then attached to a metal plate fixed to a mechanical testing machine (Model 4466; Instron, Canton, MA, USA). After artificial saliva (Taliva®; Hanlim Pharm. Co., Ltd., Seoul, Korea) was sprayed onto the bracket, the end of the archwire extruding from the maxillary right second molar tube was gripped with a custom-designed adaptor. SFF and KFF were measured while drawing the archwire through the brackets at a speed of 0.5 mm/min for 5 minutes. Tests were conducted in a chamber maintained at 36.5 ± 0.3℃ (
After each test, the typodont system was immediately washed with distilled water and alcohol to remove the artificial saliva and then dried with an air syringe. Each group was tested seven times, and a new wire was used each time.
The definitions of SFF and KFF are presented in
A power analysis was performed to determine the sample size using a sample size determination program (version 2.0.1; Seoul National University Dental Hospital, Registration No. 2007-01-122-004453, Seoul, Korea). The Shapiro-Wilk test was performed to assess the normality of the distributions in the experimental groups. The existence of normal distributions was confirmed in all nine groups. If equal variances were assumed by the Levene's test, a one-way analysis of variance (ANOVA) with Tukey's honest significant difference (HSD) post hoc test was performed for the statistical analysis. When equal variances were not assumed by Levene's test, Welch's variance-weighted ANOVA with Dunnett's T3 post hoc test was used.