September, 2004
Frictional Resistance of Ceramic Brackets When Subjected to Variable Tipping Moments
Michael Bagby, DDS, PhD. and Peter Ngan, DMD and Todd Bovenzier, DDS.
Introduction
As a bracket and tooth move along an archwire, friction opposes such movement. Friction can be a major factor determining the efficiency of an orthodontic appliance. Friction increases the force necessary to move teeth, slows tooth movement and contributes to the loss of anchorage. A number of factors, both physical and biological, effect friction in orthodontics: bracket properties (material, manufacturing process, design), archwire properties (material and cross section), ligation method, patient factors (bracket-archwire angulation, dynamic forces of the mouth) and biological films. [1,2]
Studies have shown that greater tipping angulation between the archwire and bracket yielded greater friction. [3,4,5] The effect of angulation on friction is more pronounced with stainless steel archwires than nickel titanium archwires. This can be explained by the lower stiffness of Ni-Ti wires. Increased friction with increased bracket angulation is often attributed to binding rather than true friction. [6,7]
There is continued interest in improving the esthetic orthodontic appliance such as the ceramic bracket. The reduction of friction is one of the goals for the new generation of ceramic brackets. Improvements to decrease roughness include coating the bracket slot with silica (GAC’s Mystique) or placing a metal insert into the slot (Unitek’s Clarity). Also, the bracket slot edge has been rounded to reduce binding and friction.
In most studies, stainless steel brackets produced the least amount of friction when compared to traditional ceramic brackets. [8,9,10] Rose and Zernik [11] demonstrated that rounding the corners of ceramic brackets would significantly lower the frictional resistance. There are few studies that compare the frictional resistance of the new generation of ceramic brackets with stainless steel brackets.
In addition, most friction testing models pulled a straight wire through the bracket slot. This does not simulate the clinical situation in which there is tipping and binding of the archwire with the bracket slot. At 0° of tip, there can be little or no contact between brackets and archwires depending on the ligation utilized. With increased tipping angles, which ensured bracket and archwire contact, friction can be significantly higher especially with the ceramic brackets.
Materials and Methods
Two archwires .019 x .025 Ni-Ti and stainless steel (GAC), were tested using upper bicuspid brackets with a .022 slot. Four types of brackets were tested: GAC’s Mystique ceramic bracket and MicroArch stainless steel bracket along with 3M/Unitek’s Clarity and Transbond ceramic brackets (Figure 1). The Transcend bracket was added as a positive control, a bracket known to have high frictional resistance. The MicroArch bracket served as a negative control as stainless steel brackets are known to have low frictional resistance. [12]
Figure 1. Brackets tested.
Brackets were bonded to one end of quarter inch diameter acrylic rods with cyanoacrylate cement. A dental surveyor and modified pin were utilized to position the bracket slot in the middle of the acrylic rod. In addition, the surveyor pin positioned each bracket’s slot perpendicular to the long axis of the acrylic rod negating the effects of bracket prescription (Figure 2).
Figure 2. Bracket being positioned with a machined surveyor pin.
The testing apparatus designed by Omana [12] was modified to enable the application of a variable tipping torque to be applied to the bracket while recording the friction, tipping torque and angle similar to the method of Mah. [13]
Figure 3. Mechanical testing machine and accessory equipment.
The bracket-acrylic rod assembly was mounted in the test fixture. The archwire was inserted into the bracket slot and attached to the crosshead of the testing machine. The wire was guided by the bearings of the test fixture (Figure 4). No ligatures were used since ligatures were found to be a confounding variable. Without ligatures, an extraneous variable was eliminated. The measurements focused on the friction of the bracket/wire interface. In the test fixture, the acrylic rod was connected to the lever arm of the offset cam. The rotating cam moved the lever up and down on one end, rotating the acrylic rod and the bracket on the other end (Figure 5).
Figure 4. Archwire in bracket slot and guide bearings.
The bracket was tipped to four fixed angles: 0, 2.7, 4.9 and 5.9 degrees by hand and friction measured by pulling the wire through the bracket. After measuring friction at each fixed angle, variable cyclical tipping angle and torque from 0 to 5.9 degrees was applied via the rotating cam and level arm, and dynamic friction measured. Each combination of bracket and archwire was tested three (Ni-Ti) or five (SS) times. Each test run utilized a new bracket and wire segment.
Figure 5. Rotating cam lifted the lever arm resulting in bracket rotation.
Multiple data channels were recorded via an analog to digital conversion board in a PC. Data was analyzed with MS Excel and JUMP statistical software. Typical results are shown in Figure 6.
Figure 6. Graph of typical data showing points averaged.
Results and Discussion
Friction and torque increased with increased tipping angle (Figure 6). As the cam turned, the lever moved up and down, and the acrylic rod/bracket rotated. As the bracket rotated, the bracket and archwire were forced into greater contact. This increased contact increased friction as expected from classical theories of friction.
Friction with Ni-Ti wire was low (less than 20 gm) for all brackets and judged not to be clinically significant (Figure 7).
Figure 7. Average fiction for the Ni-Ti wire.
When the stainless steel wire was tested, friction increased with increasing angle for all brackets (Figure 8). ANOVA analysis of the stainless steel wire data showed a significant interaction of bracket and stage factors, p=0.006. Therefore, friction for the brackets was statistically analyzed at each angle.
Figure 8. Average fiction for the stainless steel wire.
No significant differences were found between any brackets at 0 and 2.7 degrees (Figure 9). No differences between brackets were found for dynamic friction. At 4.9 and 5.9 degrees, friction of the Transcend was significantly greater than that of the MicroArch. There was no difference between the Clarity and Mystique brackets at any angle. The rounded corners of these brackets’ edges reduced binding with the archwire.
Figure 9. Graph showing the increase in friction with increased tipping angle.
The results show that the methods utilized were able to distinguish between the positive and negative controls. The data indicated that the Transcend bracket has the highest friction and the stainless steel bracket (GAC MircoArch) has the lowest, as expected. The Mystique and Clarity brackets fall somewhere between the high friction and the low friction brackets. Differences between the Mystique and Clarity brackets were small and are not likely to be clinically relevant. Most differences in friction between brackets were not statistically significant. This is probably due to the high variability of the data with the different tipping angles as shown in the graph.
Conclusions
The Mystique bracket with the silica treatment of the slots has similar friction to the Unitek Clarity bracket but less friction than the “first generation” ceramic bracket (Unitek Transcend). The metal MicroArch bracket had the lowest friction measured of the four brackets studied.
Acknowledgements
GAC International Inc, Bohemia NY for donation of brackets and wires, and sponsoring the research project.
Vince Kish, WVU, School of Medicine, Department of Orthopedics for help constructing test equipment.
Dr. Jerry Hobbs, WVU, School of Medicine, Department of Community Medicine for assistance with the statistical analyses.
References
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11. Rose CM, Zernik JH. Reduced resistance to sliding in ceramic brackets. J Clin Ortho 1996, 30:78-84.
12. Omana HM, Moore RN, Bagby MD. Frictional properties of metal and ceramic brackets during simulated cuspid retraction. Journal of Clinical Orthodontics 1992, 36:425-32.
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Contributed by:
Michael Bagby, D.D.S., Ph.D.
Professor, West Virginia University School of Dentistry, Division of Pediatric Dentistry, Department of Orthodontics
Peter Ngan, D.M.D.,Cert Orth, D. Orth.
Professor and Chair, West Virginia University School of Dentistry, Department of Orthodontics
Todd Bovenizier, D.D.S.
Orthodontic Resident, West Virginia University School of Dentistry, Department of Orthodontics
