Frictional Resistance of the Damon SL Bracket
Orthodontic brackets have been modified in several ways to decrease frictional resistance and improve the efficiency of sliding mechanics. These changes initially focused on bracket width,1-5 interbracket distance,5,6 and ligation technique.5,7-10 In recent years, self-ligating brackets have been developed to further minimize frictional forces.11-13
This study was designed to compare the kinetic frictional force of a new self-ligating bracket with that of a conventional twin bracket.
Materials and Methods
Twenty Damon SL self-ligating* brackets and 20 Mini-Twin* brackets were tested. All samples were .0225" X .030" maxillary first premolar brackets with standard Andrews prescriptions. The wires used were 55mm lengths of .018" X .025" nickel titanium and .019" X .025" stainless steel*.
Each bracket was bonded perpendicularly to a cylindrical Plexiglas jig, which was then fixed in a specially designed apparatus (Fig. 1). The apparatus was secured to the base of an Instron Universal Testing Machine**. The wire was attached through a Jacobs chuck to a tension load cell on the crosshead of the testing machine. This allowed it to slide through a single bracket slot without any influence from bracket tip or torque. To maintain uniformity of ligation forces on the Mini-Twin brackets, .110" elastomeric power modules were applied with a Straight Shooter***.
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Each test was carried out for two minutes at a crosshead speed of .02"/minute. Frictional forces were measured and analyzed using the SAS program†. T-tests were conducted to evaluate the significance of differences between mean values.
Results
The Damon SL bracket showed significantly lower kinetic frictional forces (p < .0001) than the Mini-Twin bracket with both wires (Fig. 2, Table 1). With the nickel titanium wires, the Damon SL brackets had a mean friction of 15.0g, compared to 41.2g for the Mini-Twin brackets. With the stainless steel wires, the Damon SL brackets produced a mean friction of only 3.6g, compared to 61.2g for the Mini-Twin brackets.
Fig. 1 A. Testing apparatus. B. Bracket-wire setup in testing apparatus (1 = C-shape rod connected to chuck and load cell; 2 = bracket-wire assembly mounted on plastic pedestal; 3 = stainless steel rod with plastic pedestal attached; 4 = roller bearings for linear and rotational displacements; 5 = wing nuts to hold wire and plastic pedestal).
Fig. 2 Mean kinetic frictional forces (DB = Damon SL bracket; MT = Mini-Twin bracket; SS = stainless steel wire; NiTi = nickel titanium wire).
Discussion
These results corroborate the findings of previous studies of self-ligating brackets.11-13 Several design and manufacturing features may account for the difference in bracket friction. First, the Damon SL bracket has a locking spring-clip slide over the slot that holds the archwire securely in place (Fig. 3).
Fig. 3 Scanning electron microscopic views of Damon SL bracket with closed spring clip (A) and conventional Mini-Twin bracket (B).
Unlike the conventional elastomeric ligature, this slide allows the wire to lie passively in the slot, reducing the normal component of force (Fig. 4).
Fig. 4 Scanning electron microscopic views of .019" x .025" stainless steel archwires held in Damon SL bracket slot by spring clip (A) and in Mini-Twin bracket slot. by elastomeric ligature (B).
Another contributing factor may be the difference in surface conditions within the bracket slots. Under a scanning electron microscope, the Damon SL bracket (Fig. 5) shows smoother surface detail than the Mini-Twin (Fig. 6). Although both brackets are manufactured from 17-4 PH stainless steel, the Damon SL bracket is made by metal injection molding, while the Mini-Twin is investment cast.
Fig. 5. Scanning electron microscopic views of Damon SL bracket slot surface. A. 100x magnification B. 300x magnification.
Fig. 6 Scanning electron microscopic views of Mini-Twin bracket slot surface. A. 100x magnification. B. 300x magnification.
The lower friction measured between the self-ligating bracket and the stainless steel wire, compared to the nickel titanium wire, confirms previous reports.11 The lower frictional resistance of the Mini-Twin bracket with nickel titanium wire is more difficult to explain in light of other studies.5,15 Our results could be explained by the bracket manufacturing process; the presence of microbonds between wire and bracket; the sharper mesial and distal edges of the Mini-Twin bracket slot, causing point contact between wire and bracket and allowing the wire to be held more tightly in the slot by the elastomeric ligature; or the greater cross-section of the stainless steel wire (.019" X .025" vs. .018" X .025").
Conclusion
The results of this study indicate that self-ligating brackets not only make archwire placement more convenient and secure, but also have lower kinetic frictional forces than conventional brackets. These features can be substantial advantages for orthodontists who use sliding mechanics.
FOOTNOTES
- *Ormco/"A" Company Orthodontics, 1717 W. Collins Ave., Orange, CA 92667. Damon SL is a trademark.
- **Model No. 4468, Instron Corporation, Canton, MA.
- ***Trademark of TP Orthodontics, Inc., 100 Center Plaza, LaPorte, IN 46350.
- †SAS Institute, Cary, NC.
REFERENCES
- 1. Andreasen, G.F. and Quevedo, F.R.: Evaluation of friction forces in the 0.022 x 0.028 edgewise bracket in vitro, J. Biomech. 3:151-160, 1970.
- 2. Drescher, D.; Bourauel, C.; and Schumacher, H.A.: Frictional forces between bracket and arch wire, Am. J. Orthod. 96:397-404, 1989.
- 3. Ogata, R.; Nanda, R.S.; Duncanson, M.G.; Sinha, P.K.; and Currier, G.F.: Frictional resistances in stainless steel bracket-wire combinations with effects of vertical deflections, Am. J. Orthod. 109:535-542, 1996.
- 4. Bednar, J.R.; Gruendeman, G.W.; and Sandrik, J.L.: A comparative study of frictional forces between orthodontic brackets and arch wires, Am. J. Orthod. 100:513-522, 1991.
- 5. Frank, C.A. and Nikolai, R.J.: A comparative study of frictional resistances between orthodontic bracket and arch wire, Am. J. Orthod. 78:593-609, 1980.
- 6. Creekmore, T.D.: The importance of interbracket width in orthodontic tooth movement, J. Clin. Orthod. 10:530-534, 1976.
- 7. Echols, P.M.: Elastic ligatures: Binding forces and anchorage taxation (abstr.), Am. J. Orthod. 67:219-220, 1975.
- 8. Ireland, A.J.; Sherriff, M.; and McDonald, F.: Effect of bracket and wire composition on frictional forces, Eur. J. Orthod. 13:322-328, 1991.
- 9. Riley, J.L.: Evaluation of frictional forces with plastic and metal 0.022 x 0.028 edgewise brackets ligated with stainless steel ties and plastic modules, research project, Virginia Commonwealth University, 1977.
- 10. Tselepis, M.; Brockhurst, P.; and West, V.C.: Frictional resistance between brackets and arch wires, Am. J. Orthod. 106:131-138, 1994.
- 11. Berger, J.L.: The influence of the SPEED bracket's self-ligating design on force levels in tooth movement: A comparative in vitro study, Am. J. Orthod. 97:219-228, 1990.
- 12. Kemp, D.W.: A comparative analysis of frictional forces between self-ligating and conventional edgewise orthodontic brackets, master's thesis, University of Toronto, 1992.
- 13. Shivapuja, P.K. and Berger, J.: A comparative study of conventional ligation and self-ligation bracket systems, Am. J. Orthod. 106:472-480, 1994.
- 14. Kusy, R.P. and Whitley, J.Q.: Effect of surface roughness on frictional coefficients of arch wires, J. Dent. Res. 67:205-215, 1988.
- 15. Kusy, R.P. and Whitley, J.Q.: Effects of surface roughness on the coefficients of friction in a model orthodontic system, J. Biomech. 23:913-925, 1990.
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DR. KAPUR -
DR. SINHA -
DR. NANDA
Dr. Kapur is a former graduate student, Dr. Sinha is an Assistant Professor, and Dr. Nanda is Professor and Chairman, Department of Orthodontics, University of Oklahoma, P.O. Box 26901, 1001 S.L. Young Blvd., Oklahoma City, OK 73190.