Frictional
Resistance of Ceramic Brackets
When
Subjected to Variable Tipping Moments
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.3-5 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.8-10 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 Omana12 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 Mah13.
 |
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.
 |
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.
References
1.
Rossouw PE, ed. Friction in Orthodontics. Seminars in Orthodontics
2003, 9.
2. Tidy
DC. Frictional forces in fixed appliances. Am J Orthod Dentofacial Orthop
1989; 96:249-54.
3.
Andreasen GF, Quevedo FR. Evaluation of friction forces in the 0.022
x 0.028 edgewise bracket in vitro. J Biomech 1970; 3:151-60.
4.
Frank CA, Nilolai RJ. A comparative study of frictional resistances
between orthodontic brackets and arch wires. Am J Orthod 1980; 78:593-609.
5.
Peterson L, Spencer R, Andreasen GF. A comparison of frictional resistance
for Nitinol and stainless steel wire in edgewise brackets. Quintess Int 1982;
5:563-65.
6.
Drescher D, Bourauel C, Schumacher HA. Frictional forces between brackets
and arch wires. Am J Orthod Dentofacial Orthop 1989; 96:397-404.
7.
Garner JL, Allai WW, Moore BK. A comparison of frictional forces during
simulated canine retraction of a continuous edgewise wire. Am J Orthod Dentofacial
Orthop 1986; 90:199-203.
8.
Kusy RP, Whitley JQ. Coefficients of friction for arch wires in stainless
steel and polycrystalline alumina brackets slots. Am J Orthod Dentofacial
Orthop 1990; 98:300-12.
9.
Pratten D, Popli K, Germane N, Gunsolley J. Frictional resistance of
ceramic and stainless steel orthodontic brackets. Am J Orthod Dentofacial
Orthop 1990; 98:398-403.
10.
Angolkar PJ, Kapile S, Duncanson MG, Nanda RS. Evalusiton of friction
between ceramic brackers and orthodontic wires of four alloys. Am J Orthod
Dentofacial Orthop 1990; 98:499-506.
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.
13.
Acknowledgements
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.
|
Michael Bagby,
D.D.S., Ph.D. |
Peter Ngan,
D.M.D.,Cert Orth, D. Orth. |
Todd Bovenizier,
D.D.S. |
|
Please send
all correspondence to:
|
E-mail:
mbagby@hsc.wvu.edu |