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Fracture resistance of ceramic brackets during arch wire torsion
American Journal of ORTHODONTICS and DENTOFACIAL ORTHOPEDICS Volume 99 Number 4
Founded in 1915
April 1991
Copyright © 1991 by Mosby-Year Book, Inc.
ORIGINAL ARTICLES
Fracture resistance of ceramic brackets during arch wire torsion Mark H. Holt, DDS, MS,* Ram S. Nanda, DDS, MS, PhD,** and Manville G. DuncansOn, Jr., DDS, PhD*** Oklahoma Ci~', Okla. The purpose of this study was to determine the fracture resistance of commemially available ceramic brackets during arch wire torsion. Lingual root torque was applied at the distal side of upper central incisor brackets with 0.022-inch slots by means of a 0.0215 × 0.028oinch arch wire. A specially designed apparatus was used to test six types of ceramic bracket in sample groups of 30. The amount of torque and degrees of torsional rotation at fracture were measured. The ceramic brackets could be separated into three statistically different groups with mean torques at fracture ranging from 3,706 to 6,177 gm-mm. The mean torsional rotation at fracture ranged from 9.5 ° to 17.8 °. The single-crystal alumina bracket had the most intragroup variation. Eight to ten degrees of torsional rotation of the arch wire produced sufficient orthodontic force to achieve the torque. The fracture resistance of the ceramic brackets appears to be adequate for clinical use. The Starfire, Allure III, and Transcend brackets had the highest fracture resistance values. (AMJ ORTHOD DENTOFAC ORTHOP 1991 ;99:287-93.)
T h e introduction of the ceramic bracket was a much-heralded development in the treatment of adult patients. The ceramic brackets offer improved esthetics and are well suited to the oral environment. Their acceptance by patients has been unprecedented in the practice of orthodontics. However, orthodontists have noticed problems with the resistance to fracture of the ceramic brackets. To address these professional concerns, the American Association of Orthodontists conducted a survey t among the orthodontists on their experiences with the ceramic brackets and discovered that From the University of Oklahoma Health Sciences Center. Supported by grants from the Foundation for Orthodontic Research, Pacific Palisades, Calif., and the Presbyterian Health Foundation, Oklahoma City, Okla. *Former Graduate Resident, Department of Orthodontics, College of Dentistry; in private practice, Sacramento, California. **Professor and Chairman, Department of Orthodontics, College of Dentistry. ***Professor and Chairman, Department of Dental Materials, College of Dentistry. 811116887
significant problems existed with respect to fracturing of these brackets. The association has recommended that a disclosure to the patients regarding the difficulties experienced with the ceramic brackets is appropriate. Ceramic brackets are made from A120~, which is referred to as alumina or aluminum oxide. There are two types of ceramic bracket on the market: (1) Polycrystalline alumina brackets, the most common type available, are translucent and may match tooth color. (2) Single-crystal alumina or sapphire brackets are clear and manufactured from single-crystal man-made c~-alumina. 2 Ceramics show very little elastic or plastic deformation, and the notch sensitivity is high. 3 Ideal singlecrystal alumina has higher tensile strength than polycrystalline alumina4; however, the latter has a higher fracture toughness, s'6 Bulk tensile strength o f ceramics is dependent on the surface condition and hence should not be compared to that of metals. Fracture toughness, the ability of a material to resist fracture along a crack
287
288
Holt, Nanda, and Dtmcanson
Am. J. Orthod. Dentrfiw. Orthop. April 1991
Fig. 1. Facial and distal views of six types of ceramic brackets. The brackets in the top row have bases that extend under the tie wings. They are (left to right).: Starfire (single-crystal), Transcend, (polycrystalline), and Quasar (polycrystalline). The brackets in the bottom row are all polycrystalline, and the bases do not extend under the lie wings. They are (left to right): 20/20, Fascination, and Allure IlL
or groove, is an important property of ceramics, much as yield strength is among metals. 7 The finishing techniques can cause microcracks, which can make the brackets more susceptible to fracture. It is, therefore, necessary to compare actual brackets, rather than bracket materials. Orthodontic brackets are subjected to various types of force during treatment. During torquing and tipping, the arch wire pushes the occlusal and gingival wings in opposite directions, and this pressure may lead to bracket fracture, which increases treatment time and may be harmful to patients. 2 In orthodontics, torque refers to the labiolingual inclination of the root or crown. Torque is commonly applied as torsional rotation to maxillary anterior teeth during and after their retraction. It is a moment resulting from a force acting at a distance from an axis of rotation within a body and is equal to the force times the perpendicular distance from the axis of rotation. ~ Although the fracture of ceramic brackets has been indicated to be a major clinical problem, the literature lacks scientific data on the fracture resistance of these
brackets because of the various forms of orttlodontic force that may be applied to them. This study was designed to test the various ceramic brackets available on the market for their resistance to fracture during arch wire torsion. MATERIALS AND METHODS
We tested ceramic brackets for maxillary right central incisors, with 0.022-inch slots, by applying lingual root torque on the distal side with an 0.0215 × 0.028inch arch wire. Six types of ceramic bracket were tested in sample sizes of 30 each: Single-crystal alumina Starfire ("A"-Company, San Diego, Calif.) Polycrystalline alumina Allure III (GAC International, Central Islip. N.Y.) Transcend (Unitek, Glendora, Calif.) Quasar (Rocky Mountain Orthodontics. Denver, Col. This is the same bracket as Intrigue--Lancer Orthodontics and Illusion--Ortho Organizers) Fascination (Dentaurum. Newton, Pa.) 20/20 (American Orthodontics. Sheboygan. Wis.)
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Fracture resistance of ceramic brackets during arch wire torsion
289
_
Crossbar
SupportPosts --.-
"Holder
1__
I
Base Drawn to Scale
Fig. 2. Schematic diagram of torsional test apparatus used to apply torque through rotation of crossbar attached to sprocket. The pin is used to measure torsional rotation.
Each bracket had + 5 ° of angulation and + 12° of torque, except for the Fascination bracket, which was a standard edgewise type (0 ° angulation, 0 ° torque). Fig. 1 shows the facial and distal views of these brackets and demonstrates that the brackets are of variable size and shape.
Testing apparatus A testing apparatus was designed to maintain the orientation of the bracket to the arch wire in all three planes of space (Figs. 2 and 3). A base with support posts was used to mount a crossbar that could hold and twist an arch wire without moving in any other dimension. A split-die specimen holder was also attached to the base. A chain-and-sprocket apparatus was attached to one crossbar for twisting the wire. The other crossbar held the other end of the wire in place and could rotate freely. The end of the chain was attached to the load cell at the top of the Instron Universal testing machine (Instron, Canton, Mass.) during testing. The crossbar was hemisectioned and notched on the end to permit the arch wire to fit in only one way and to be centered each time. The wire was placed with the
widest part held in a vertical position during preparation of the specimen. A pin (Fig. 2) was located on the rotating crossbar to determine how far the crossbar had rotated.
Specimen preparation Porcelain denture teeth were used to hold the brackets during the test. The facial surface of the denture tooth was roughened with a diamond bur on a highspeed handpiece to secure bonding. Ceramic Primer, marketed by the 3M Company (St. Paul, Minn.), was applied to each porcelain tooth according to the manufacturer's directions. Concise bonding material (3M Company) was then used to bond the brackets to the denture teeth. The brackets were allowed to sit a sufficiently long time for the adhesive to set (10 minutes) before the wires were placed. A 33.5 mm length of 0.0215 × 0.028-inch resilient stainless steel wire (Unitek, Monrovia, Calif.) was ligated in each bracket with an elastic ligature (Ormco, Glendora, Calif.). Both the wire and the elastic ligature remained in place throughout the preparation and testing procedure.
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Am. J. Orthod. Dentofac. Onhop.
Holt, Nanda, attd Duncanson
April 1991
Fig. 3. A specimen in the test apparatus,
To achieve a repeatable fixed orientation of the bracke(to the test apparatus, the arch wire was held in place in the apparatus while the split-die specimen holder was filled with die-stone (Super-Die, Whip Mix, Louisville, Ky.). Since the bracket-tooth mounting was done with the arch wire held with its widest side vertical, all of the bracket slots were in the same orientation to the arch wire, i.e., 0 ° angulation and 0 ° torque. The existing torque or angulation of the bracket had no effect on the experiment. The bracket and tooth were placed with 6 mm of space from the end of the crossbar wire holder to the middle of the bracket. This distance was chosen because it was considered to be an average interbracket distance for a clinical situation.
from the X-Y Recorder was multiplied by the radius of the sprocket (485 mm) to obtain the torque in grammillimeters. At the time of fracture, the Instron machine was stopped, and the number of degrees of rotation was measured with a protractor that was aligned with the support post and a pointer attached to the rotating crossbar. Also, the location of fracture was recorded. The means and standard deviations were calculated for the torque and number of degrees of rotation at failure. An analysis of variance was used to determine whether there were significant differences between the groups. Duncan's multiple-range test was used to distinguish which groups were different at the p < 0.05 level.
Testing procedure
RESULTS
At the time of testing, the apparatus base was clamped to the Instron crosshead while the end of the chain was fastened to the load cell above. The lnstron crosshead was moved down at a rate of 1 in per minute for each sample until the bracket fractured. The force on the chain was recorded graphically on an X-Y Recorder (Hewlett-Packard, Palo Alto, Calif.). The highest point on the graph was taken as the point of failure of the ceramic brackets. The force obtained
Table l shows the mean torque required to fracture the ceramic brackets. On the basis of their resistance to fracture, the brackets could be divided into three groups. The differences between the groups were significant at p < 0.05. The first group included the Starfire, Allure III, and Transcend brackets, which fractured at mean torques of 6177 gm-mm, 6042 gm-mm, and 5771 gm-mm, respectively. The second group consisted of the Quasar and Fascination brackets, which failed at
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Fracture resistance of ceramic brackets during arch ,wire torsion 291
_~G
Table I. Means, standard deviations, and minimum and maximum torque values at failure for all brackets tested (gm-mm)
Bracket Starfire Allure III Transcend Quasar Fascination 20120
[ Mean ± SO 6177 6042 5771 4748 4545 3706
± 1317 ± 809 ± 475 ± 741 ± 691 ± 767
Ma.rimum
I Minimum
9682 7482 6866 6447 5875 4268
3301 4049 4753 3147 3191 1980
Starfire
Allure !II
Transcend
Quasar
D
Table II. Means, standard deviations, and minimum and maximum rotation at failure for all brackets tested (degrees)
Bracket Starfire Transcend Allure III Fascination Quasar 20/20
Degrees (mean ~ SD) 17.8 16.6 16.1 15.2 14.5 9.5
± ± ± ± ±
4.4 2.7 2.0 3.0 3.0 2.6
Mininzum I1 13 il 10 6 5
I
Mctrinnun 28 25 21 25 20 19
4748 gm-mm and 4545 gm-mm, respectively; these brackets were significantly weaker than the first group. The third group, the 20/20 bracket, failed at a mean torque of 3706 gm-mm, which was significantly weaker than any of the other brackets. The Transcend bracket had the smallest standard deviation (475 gm-mm), while the single-crystal bracket, Starfire, showed the most variability, with a standard deviation of 1317 gm-mm. The minimum torquing force at failure ranged from 1980 gm-mm for 20/20 to 4753 gm-mm for Transcend. The mean rotation at failure ranged from 17.8 ° for Starfire to 9.5 ° for 20/20 brackets (Table II). The 20/20 bracket had the lowest mean number of degrees of rotation at failure. The differences between the 20/20 bracket and all other brackets were found to be statistically significant. The differences in the number of degrees of rotation among the other brackets were small and statistically insignificant. The Starfire showed the most variation, with a standard deviation of 4.4 °. The minimum torsional rotation at which a 20/20 bracket fractured was 5 °, in comparison with a Transcend bracket, which fractured at a low of 13°. These were the minimum values of the range in a sample of 30. Fig. 4 shows the location of fracture for all ceramic brackets. Starfire was the only true twin bracket. The
2••M Fascination
D
M
20/20
Fig. 4. Outlines of facial-surface bracket shape are used to depict fracture-location frequency for each ceramic bracket type. The number on the bracket indicates the number of brackets that broke at that location. A number centered mesiodistally means that the whole incisal or gingival half fractured. The numbers on the Starfire bracket add up to more than 30 because this bracket often broke in more than one place.
two wings of this twin bracket had varying susceptibility to fracture. Among these brackets, ten fractured at both gingival wings, while nine fractured at the distoincisal wing, and three fractured at both incisal wings. All other brackets were not true twins because the mesial and distal wings were connected. These brackets usually fractured o f f a whole incisal or gingival section. Incisal fracture predominated in these brackets, except for the Quasar bracket, which broke predominantly at the gingival side. DISCUSSION
This experiment was designed to study the fracture resistance of various commercially available ceramic brackets during orthodontic torquing. The findings indicated that ceramic brackets fractured at mean torques that ranged from 3706 to 6177 gm-mm. This range for the six different brands appears to be above the amount of force recommended for effectively torquing maxillary incisors. Reitan 9 suggested that a force of 130 gm should be used at the root apex during torquing movements. The
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Am. J. Orthod. Dentofac. Orthop. April 1991
Holt, Nanda, and Duncanson
average distance from the bracket to the root apex on an upper central incisor is 18.25 mm ~°, so the applied torque would be 2373 gm-mm. Wainwright ~ studied physiologic tooth movement in Macaca speciosa monkeys and concluded that 2000 gm-mm was a physiologic force for torquing human central incisors. Nikolai ~-"suggested that, for an average-size maxillary incisor segment, the total torque requirement is 3000 to 3500 gm-mm. It can be concluded that the range of force required for successful torquing movement varies from about 2000 to 3500 gm-mm. In this regard, the maximum force recommended is still less than the lowest mean torque (3706 gm-mm) that is required for fracturing 2 0 / 2 0 ceramic brackets. Even though the mean figures are well within the levels of force for physiologic torsional movement, clinicians may be using heavier forces in their quest for better and faster results. In addition, the findings of this study suggest that the amount of torque at which a ceramic bracket of the same type will fracture is rather v a r i a b l e - - e . g . , the 20/20 bracket had the range o f 1980 to 4268 gm-mm. Therefore, in the selection of a suitable ceramic bracket one must consider the variability and minimum value at which a bracket will fail so as to minimize the chances of an occasional bracket failure caused by a weak bracket. This study shows that, barring the 20/20 bracket, all of the other types did not have a single failure below 3000 gm-mm o f torque. Transcend and Allure III brackets sid not have a single bracket failure below 4000 gm-mm. Flores ~3 reported the results of a study on torque applied to brackets with 0.018-inch slots. The mean torque at bracket failure (in gram-millimeters for each sample o f ten brackets) was Transcend 6,083 ± 553; Allure III 5,253 ± 829; Starfire 9,228 ± 5,426. The comparable torque (in gram-millimeters) in our study was similar for Transcend and Allure III b r a c k e t s - i.e., 5771 ± 475 and 6042 __. 809, respectively. However it was different for Starfire; in our study it was 6177 ± 1317 gm-mm. The differences in the results with respect to the Starfire bracket may be caused by the manner of application of the torsional force. The true twin design of the Starfire bracket makes the mesial and distal wings independent. Flores j3 applied the torque to both sides simultaneously. The torque application from just one side in the present study was believed to simulate the clinical situation more accurately. During the treatment procedure, the torque is usually applied to all four incisors simultaneously. Placing the wire in the brackets of all the incisors diminishes the impact of the torque
toward the mesial aspect, since the resistance to the torque is distributed over several brackets. ~4 Evaluation of the torsional rotation o f the wire at bracket failure indicated that the mean rotation at which the various ceramic brackets failed varied from 9.5 ° for 2 0 / 2 0 to 17.8 ° for Starfire. The physiologic force required for effecting torque can be attained at just 8 ° to 10° of torsional rotation with most o f the ceramic brackets. To avoid failure of the ceramic brackets during torquing, one has to exercise caution and avoid excessive torsional rotation of the wire. It may be necessary to apply torque in increments no larger than 10°. Thus, the orthodontist may be required to make more frequent adjustments. Manufacturing processes, structural designs, and materials may explain the differences among the different brands. The resistance to fracture of ceramic brackets in relation to torsional force seems to be adequate for clinical use. However, some brackets that showed a relatively lower resistance to fracture could be improved by better production or quality control. CONCLUSIONS
1. During lingual-root torque of central incisor brackets, the ceramic bracket's could be separated into three groups based on the mean torque at fracture. The first group contained the Starfire, Allure III, and Transcend brackets. The second group included the Quasar and Fascination brackets. The 2 0 / 2 0 bracket showed the least resistance to fracture. 2. The single-crystal bracket, Starfire, showed more intragroup variation than the polycrystalline bracket types. 3. The fracture resistance of the ceramic brackets during arch wire torsion appears to be adequate for clinical use. However, to prevent ceramic bracket fracture, the clinician must avoid applying excessive force on an arch wire. We acknowledge the fine work of Mr. John Welch in constructing the experimental apparatus. We also appreciate the donations of the ceramic brackets from the manufacturers. REFERENCES
I. Lindquist JT. Letter to members gives results of AAO survey on ceramic brackets. AAO Bulletin 1989;7:3. 2. Swartz ML. Ceramic Brackets. J Clin Orthod 1988;22:82-8. 3. Van Vlack LH. Elements of materials science and engineering, 3rd ed. Menlo Park, Calif., Addison-WesleyCo., 1975:259-91. 4. Metals and Ceramics InformationCenter. Engineering property data on selected ceramics. Vol. III. Single oxides. Defense Information Analysis Center. Columbus, Ohio: Battelle Memorial Institute, July 1981. 5. Hertzberg RW. Deformation and fracture mechanics of engi-
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Fracture resistance o f ceramic brackets during arch wire torsion
neering materials, 2nd ed. New York: John Wiley & Sons, 1983:353-422. 6. Iwasa M, Bradt RC. Fracture toughness of single-crystalalumina, In: WD Kingery,ed. Structureand properties of MgO and AI:O3. Advances in ceramics, vol. 10. Columbus: American Ceramic Society, 1986:767-78. 7. Scott GE. Fracture toughness and surface cracks--the key to understandingceramic brackets. Angle Orthod 1988;58:5-8. 8. Spiegel MR. Vector analysis and an introductionto tensor analysis. New York: Schaum Publishing, 1959:16-26. 9. Reitan K. Some factors determiningthe evaluationof forces in orthodontics. AM J ORTttOD1957;43:32-45. I0. Wheeler RC. A textbook of dental anatomyand physiol~y, 4th ed. Philadelphia:WB Saunders, 1965:136. 11. WainwrightWM. Faciolingualtooth movement:its influenceon the root and cortical plate. AM J OR'tHUD1973;64:278-302.
293
12. Nikolai RJ. Bioengineeringanalysis of orthodontic mechanics. Philadelphia: Lea & Febiger, 1985:299-305. 13. Flores DA. The fracture strength of ceramic brackets: a comparative study [Master's Thesis]. Loma Linda, California: Loma Linda University, 1988. 14. SteynCL. Measurementof edgewise torque force in vitro. AM J ORTHOD1977;71:565-73. Reprint requests to:
Dr. Ram S. Nanda Department of Orthodontics Universityof Oklahoma College of Dentistry P.O. Box 26901 I001 Stanton L. Young Blvd. Oklahoma City, OK 73190
BOUND VOLUMES AVAILABLE TO SUBSCRIBERS Bound volumes of the AMERICAN JOURNAL OF ORTHODONTICS AND DENTOFACIAL ORTHOPEDICS are available to subscribers (only) for the 1991 issues from the Publisher, at a cost of $48.00 ($59.00 international) for Vol. 99 (January-June) and Vol. 100 (July-December). Shipping charges are included. Each botind volume contains a subject and author index and all advertising is removed. Copies are shipped within 60 days after publication of the last issue in the volume. The binding is durable buckram with the journal name, volume number, and year stamped in gold on the spine. Payn~ent m u s t a c c o m p a n y all orders. Contact M o s b y - Y e a r Book, Inc., Circulation Department, 11830 Westline Industrial Drive, St. Louis, MO 63146-3318, USA; telephone (800)325-4177, ext. 4351. S u b s c r i p t i o n s m u s t be in force to qualify. B o u n d volumes are n o t available in place of a r e g u l a r J o u r n a l s u b s c r i p t i o n .
Founded in 1915
April 1991
Copyright © 1991 by Mosby-Year Book, Inc.
ORIGINAL ARTICLES
Fracture resistance of ceramic brackets during arch wire torsion Mark H. Holt, DDS, MS,* Ram S. Nanda, DDS, MS, PhD,** and Manville G. DuncansOn, Jr., DDS, PhD*** Oklahoma Ci~', Okla. The purpose of this study was to determine the fracture resistance of commemially available ceramic brackets during arch wire torsion. Lingual root torque was applied at the distal side of upper central incisor brackets with 0.022-inch slots by means of a 0.0215 × 0.028oinch arch wire. A specially designed apparatus was used to test six types of ceramic bracket in sample groups of 30. The amount of torque and degrees of torsional rotation at fracture were measured. The ceramic brackets could be separated into three statistically different groups with mean torques at fracture ranging from 3,706 to 6,177 gm-mm. The mean torsional rotation at fracture ranged from 9.5 ° to 17.8 °. The single-crystal alumina bracket had the most intragroup variation. Eight to ten degrees of torsional rotation of the arch wire produced sufficient orthodontic force to achieve the torque. The fracture resistance of the ceramic brackets appears to be adequate for clinical use. The Starfire, Allure III, and Transcend brackets had the highest fracture resistance values. (AMJ ORTHOD DENTOFAC ORTHOP 1991 ;99:287-93.)
T h e introduction of the ceramic bracket was a much-heralded development in the treatment of adult patients. The ceramic brackets offer improved esthetics and are well suited to the oral environment. Their acceptance by patients has been unprecedented in the practice of orthodontics. However, orthodontists have noticed problems with the resistance to fracture of the ceramic brackets. To address these professional concerns, the American Association of Orthodontists conducted a survey t among the orthodontists on their experiences with the ceramic brackets and discovered that From the University of Oklahoma Health Sciences Center. Supported by grants from the Foundation for Orthodontic Research, Pacific Palisades, Calif., and the Presbyterian Health Foundation, Oklahoma City, Okla. *Former Graduate Resident, Department of Orthodontics, College of Dentistry; in private practice, Sacramento, California. **Professor and Chairman, Department of Orthodontics, College of Dentistry. ***Professor and Chairman, Department of Dental Materials, College of Dentistry. 811116887
significant problems existed with respect to fracturing of these brackets. The association has recommended that a disclosure to the patients regarding the difficulties experienced with the ceramic brackets is appropriate. Ceramic brackets are made from A120~, which is referred to as alumina or aluminum oxide. There are two types of ceramic bracket on the market: (1) Polycrystalline alumina brackets, the most common type available, are translucent and may match tooth color. (2) Single-crystal alumina or sapphire brackets are clear and manufactured from single-crystal man-made c~-alumina. 2 Ceramics show very little elastic or plastic deformation, and the notch sensitivity is high. 3 Ideal singlecrystal alumina has higher tensile strength than polycrystalline alumina4; however, the latter has a higher fracture toughness, s'6 Bulk tensile strength o f ceramics is dependent on the surface condition and hence should not be compared to that of metals. Fracture toughness, the ability of a material to resist fracture along a crack
287
288
Holt, Nanda, and Dtmcanson
Am. J. Orthod. Dentrfiw. Orthop. April 1991
Fig. 1. Facial and distal views of six types of ceramic brackets. The brackets in the top row have bases that extend under the tie wings. They are (left to right).: Starfire (single-crystal), Transcend, (polycrystalline), and Quasar (polycrystalline). The brackets in the bottom row are all polycrystalline, and the bases do not extend under the lie wings. They are (left to right): 20/20, Fascination, and Allure IlL
or groove, is an important property of ceramics, much as yield strength is among metals. 7 The finishing techniques can cause microcracks, which can make the brackets more susceptible to fracture. It is, therefore, necessary to compare actual brackets, rather than bracket materials. Orthodontic brackets are subjected to various types of force during treatment. During torquing and tipping, the arch wire pushes the occlusal and gingival wings in opposite directions, and this pressure may lead to bracket fracture, which increases treatment time and may be harmful to patients. 2 In orthodontics, torque refers to the labiolingual inclination of the root or crown. Torque is commonly applied as torsional rotation to maxillary anterior teeth during and after their retraction. It is a moment resulting from a force acting at a distance from an axis of rotation within a body and is equal to the force times the perpendicular distance from the axis of rotation. ~ Although the fracture of ceramic brackets has been indicated to be a major clinical problem, the literature lacks scientific data on the fracture resistance of these
brackets because of the various forms of orttlodontic force that may be applied to them. This study was designed to test the various ceramic brackets available on the market for their resistance to fracture during arch wire torsion. MATERIALS AND METHODS
We tested ceramic brackets for maxillary right central incisors, with 0.022-inch slots, by applying lingual root torque on the distal side with an 0.0215 × 0.028inch arch wire. Six types of ceramic bracket were tested in sample sizes of 30 each: Single-crystal alumina Starfire ("A"-Company, San Diego, Calif.) Polycrystalline alumina Allure III (GAC International, Central Islip. N.Y.) Transcend (Unitek, Glendora, Calif.) Quasar (Rocky Mountain Orthodontics. Denver, Col. This is the same bracket as Intrigue--Lancer Orthodontics and Illusion--Ortho Organizers) Fascination (Dentaurum. Newton, Pa.) 20/20 (American Orthodontics. Sheboygan. Wis.)
Voh~me 99 Number 4
Fracture resistance of ceramic brackets during arch wire torsion
289
_
Crossbar
SupportPosts --.-
"Holder
1__
I
Base Drawn to Scale
Fig. 2. Schematic diagram of torsional test apparatus used to apply torque through rotation of crossbar attached to sprocket. The pin is used to measure torsional rotation.
Each bracket had + 5 ° of angulation and + 12° of torque, except for the Fascination bracket, which was a standard edgewise type (0 ° angulation, 0 ° torque). Fig. 1 shows the facial and distal views of these brackets and demonstrates that the brackets are of variable size and shape.
Testing apparatus A testing apparatus was designed to maintain the orientation of the bracket to the arch wire in all three planes of space (Figs. 2 and 3). A base with support posts was used to mount a crossbar that could hold and twist an arch wire without moving in any other dimension. A split-die specimen holder was also attached to the base. A chain-and-sprocket apparatus was attached to one crossbar for twisting the wire. The other crossbar held the other end of the wire in place and could rotate freely. The end of the chain was attached to the load cell at the top of the Instron Universal testing machine (Instron, Canton, Mass.) during testing. The crossbar was hemisectioned and notched on the end to permit the arch wire to fit in only one way and to be centered each time. The wire was placed with the
widest part held in a vertical position during preparation of the specimen. A pin (Fig. 2) was located on the rotating crossbar to determine how far the crossbar had rotated.
Specimen preparation Porcelain denture teeth were used to hold the brackets during the test. The facial surface of the denture tooth was roughened with a diamond bur on a highspeed handpiece to secure bonding. Ceramic Primer, marketed by the 3M Company (St. Paul, Minn.), was applied to each porcelain tooth according to the manufacturer's directions. Concise bonding material (3M Company) was then used to bond the brackets to the denture teeth. The brackets were allowed to sit a sufficiently long time for the adhesive to set (10 minutes) before the wires were placed. A 33.5 mm length of 0.0215 × 0.028-inch resilient stainless steel wire (Unitek, Monrovia, Calif.) was ligated in each bracket with an elastic ligature (Ormco, Glendora, Calif.). Both the wire and the elastic ligature remained in place throughout the preparation and testing procedure.
290
Am. J. Orthod. Dentofac. Onhop.
Holt, Nanda, attd Duncanson
April 1991
Fig. 3. A specimen in the test apparatus,
To achieve a repeatable fixed orientation of the bracke(to the test apparatus, the arch wire was held in place in the apparatus while the split-die specimen holder was filled with die-stone (Super-Die, Whip Mix, Louisville, Ky.). Since the bracket-tooth mounting was done with the arch wire held with its widest side vertical, all of the bracket slots were in the same orientation to the arch wire, i.e., 0 ° angulation and 0 ° torque. The existing torque or angulation of the bracket had no effect on the experiment. The bracket and tooth were placed with 6 mm of space from the end of the crossbar wire holder to the middle of the bracket. This distance was chosen because it was considered to be an average interbracket distance for a clinical situation.
from the X-Y Recorder was multiplied by the radius of the sprocket (485 mm) to obtain the torque in grammillimeters. At the time of fracture, the Instron machine was stopped, and the number of degrees of rotation was measured with a protractor that was aligned with the support post and a pointer attached to the rotating crossbar. Also, the location of fracture was recorded. The means and standard deviations were calculated for the torque and number of degrees of rotation at failure. An analysis of variance was used to determine whether there were significant differences between the groups. Duncan's multiple-range test was used to distinguish which groups were different at the p < 0.05 level.
Testing procedure
RESULTS
At the time of testing, the apparatus base was clamped to the Instron crosshead while the end of the chain was fastened to the load cell above. The lnstron crosshead was moved down at a rate of 1 in per minute for each sample until the bracket fractured. The force on the chain was recorded graphically on an X-Y Recorder (Hewlett-Packard, Palo Alto, Calif.). The highest point on the graph was taken as the point of failure of the ceramic brackets. The force obtained
Table l shows the mean torque required to fracture the ceramic brackets. On the basis of their resistance to fracture, the brackets could be divided into three groups. The differences between the groups were significant at p < 0.05. The first group included the Starfire, Allure III, and Transcend brackets, which fractured at mean torques of 6177 gm-mm, 6042 gm-mm, and 5771 gm-mm, respectively. The second group consisted of the Quasar and Fascination brackets, which failed at
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Fracture resistance of ceramic brackets during arch ,wire torsion 291
_~G
Table I. Means, standard deviations, and minimum and maximum torque values at failure for all brackets tested (gm-mm)
Bracket Starfire Allure III Transcend Quasar Fascination 20120
[ Mean ± SO 6177 6042 5771 4748 4545 3706
± 1317 ± 809 ± 475 ± 741 ± 691 ± 767
Ma.rimum
I Minimum
9682 7482 6866 6447 5875 4268
3301 4049 4753 3147 3191 1980
Starfire
Allure !II
Transcend
Quasar
D
Table II. Means, standard deviations, and minimum and maximum rotation at failure for all brackets tested (degrees)
Bracket Starfire Transcend Allure III Fascination Quasar 20/20
Degrees (mean ~ SD) 17.8 16.6 16.1 15.2 14.5 9.5
± ± ± ± ±
4.4 2.7 2.0 3.0 3.0 2.6
Mininzum I1 13 il 10 6 5
I
Mctrinnun 28 25 21 25 20 19
4748 gm-mm and 4545 gm-mm, respectively; these brackets were significantly weaker than the first group. The third group, the 20/20 bracket, failed at a mean torque of 3706 gm-mm, which was significantly weaker than any of the other brackets. The Transcend bracket had the smallest standard deviation (475 gm-mm), while the single-crystal bracket, Starfire, showed the most variability, with a standard deviation of 1317 gm-mm. The minimum torquing force at failure ranged from 1980 gm-mm for 20/20 to 4753 gm-mm for Transcend. The mean rotation at failure ranged from 17.8 ° for Starfire to 9.5 ° for 20/20 brackets (Table II). The 20/20 bracket had the lowest mean number of degrees of rotation at failure. The differences between the 20/20 bracket and all other brackets were found to be statistically significant. The differences in the number of degrees of rotation among the other brackets were small and statistically insignificant. The Starfire showed the most variation, with a standard deviation of 4.4 °. The minimum torsional rotation at which a 20/20 bracket fractured was 5 °, in comparison with a Transcend bracket, which fractured at a low of 13°. These were the minimum values of the range in a sample of 30. Fig. 4 shows the location of fracture for all ceramic brackets. Starfire was the only true twin bracket. The
2••M Fascination
D
M
20/20
Fig. 4. Outlines of facial-surface bracket shape are used to depict fracture-location frequency for each ceramic bracket type. The number on the bracket indicates the number of brackets that broke at that location. A number centered mesiodistally means that the whole incisal or gingival half fractured. The numbers on the Starfire bracket add up to more than 30 because this bracket often broke in more than one place.
two wings of this twin bracket had varying susceptibility to fracture. Among these brackets, ten fractured at both gingival wings, while nine fractured at the distoincisal wing, and three fractured at both incisal wings. All other brackets were not true twins because the mesial and distal wings were connected. These brackets usually fractured o f f a whole incisal or gingival section. Incisal fracture predominated in these brackets, except for the Quasar bracket, which broke predominantly at the gingival side. DISCUSSION
This experiment was designed to study the fracture resistance of various commercially available ceramic brackets during orthodontic torquing. The findings indicated that ceramic brackets fractured at mean torques that ranged from 3706 to 6177 gm-mm. This range for the six different brands appears to be above the amount of force recommended for effectively torquing maxillary incisors. Reitan 9 suggested that a force of 130 gm should be used at the root apex during torquing movements. The
292
Am. J. Orthod. Dentofac. Orthop. April 1991
Holt, Nanda, and Duncanson
average distance from the bracket to the root apex on an upper central incisor is 18.25 mm ~°, so the applied torque would be 2373 gm-mm. Wainwright ~ studied physiologic tooth movement in Macaca speciosa monkeys and concluded that 2000 gm-mm was a physiologic force for torquing human central incisors. Nikolai ~-"suggested that, for an average-size maxillary incisor segment, the total torque requirement is 3000 to 3500 gm-mm. It can be concluded that the range of force required for successful torquing movement varies from about 2000 to 3500 gm-mm. In this regard, the maximum force recommended is still less than the lowest mean torque (3706 gm-mm) that is required for fracturing 2 0 / 2 0 ceramic brackets. Even though the mean figures are well within the levels of force for physiologic torsional movement, clinicians may be using heavier forces in their quest for better and faster results. In addition, the findings of this study suggest that the amount of torque at which a ceramic bracket of the same type will fracture is rather v a r i a b l e - - e . g . , the 20/20 bracket had the range o f 1980 to 4268 gm-mm. Therefore, in the selection of a suitable ceramic bracket one must consider the variability and minimum value at which a bracket will fail so as to minimize the chances of an occasional bracket failure caused by a weak bracket. This study shows that, barring the 20/20 bracket, all of the other types did not have a single failure below 3000 gm-mm o f torque. Transcend and Allure III brackets sid not have a single bracket failure below 4000 gm-mm. Flores ~3 reported the results of a study on torque applied to brackets with 0.018-inch slots. The mean torque at bracket failure (in gram-millimeters for each sample o f ten brackets) was Transcend 6,083 ± 553; Allure III 5,253 ± 829; Starfire 9,228 ± 5,426. The comparable torque (in gram-millimeters) in our study was similar for Transcend and Allure III b r a c k e t s - i.e., 5771 ± 475 and 6042 __. 809, respectively. However it was different for Starfire; in our study it was 6177 ± 1317 gm-mm. The differences in the results with respect to the Starfire bracket may be caused by the manner of application of the torsional force. The true twin design of the Starfire bracket makes the mesial and distal wings independent. Flores j3 applied the torque to both sides simultaneously. The torque application from just one side in the present study was believed to simulate the clinical situation more accurately. During the treatment procedure, the torque is usually applied to all four incisors simultaneously. Placing the wire in the brackets of all the incisors diminishes the impact of the torque
toward the mesial aspect, since the resistance to the torque is distributed over several brackets. ~4 Evaluation of the torsional rotation o f the wire at bracket failure indicated that the mean rotation at which the various ceramic brackets failed varied from 9.5 ° for 2 0 / 2 0 to 17.8 ° for Starfire. The physiologic force required for effecting torque can be attained at just 8 ° to 10° of torsional rotation with most o f the ceramic brackets. To avoid failure of the ceramic brackets during torquing, one has to exercise caution and avoid excessive torsional rotation of the wire. It may be necessary to apply torque in increments no larger than 10°. Thus, the orthodontist may be required to make more frequent adjustments. Manufacturing processes, structural designs, and materials may explain the differences among the different brands. The resistance to fracture of ceramic brackets in relation to torsional force seems to be adequate for clinical use. However, some brackets that showed a relatively lower resistance to fracture could be improved by better production or quality control. CONCLUSIONS
1. During lingual-root torque of central incisor brackets, the ceramic bracket's could be separated into three groups based on the mean torque at fracture. The first group contained the Starfire, Allure III, and Transcend brackets. The second group included the Quasar and Fascination brackets. The 2 0 / 2 0 bracket showed the least resistance to fracture. 2. The single-crystal bracket, Starfire, showed more intragroup variation than the polycrystalline bracket types. 3. The fracture resistance of the ceramic brackets during arch wire torsion appears to be adequate for clinical use. However, to prevent ceramic bracket fracture, the clinician must avoid applying excessive force on an arch wire. We acknowledge the fine work of Mr. John Welch in constructing the experimental apparatus. We also appreciate the donations of the ceramic brackets from the manufacturers. REFERENCES
I. Lindquist JT. Letter to members gives results of AAO survey on ceramic brackets. AAO Bulletin 1989;7:3. 2. Swartz ML. Ceramic Brackets. J Clin Orthod 1988;22:82-8. 3. Van Vlack LH. Elements of materials science and engineering, 3rd ed. Menlo Park, Calif., Addison-WesleyCo., 1975:259-91. 4. Metals and Ceramics InformationCenter. Engineering property data on selected ceramics. Vol. III. Single oxides. Defense Information Analysis Center. Columbus, Ohio: Battelle Memorial Institute, July 1981. 5. Hertzberg RW. Deformation and fracture mechanics of engi-
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Fracture resistance o f ceramic brackets during arch wire torsion
neering materials, 2nd ed. New York: John Wiley & Sons, 1983:353-422. 6. Iwasa M, Bradt RC. Fracture toughness of single-crystalalumina, In: WD Kingery,ed. Structureand properties of MgO and AI:O3. Advances in ceramics, vol. 10. Columbus: American Ceramic Society, 1986:767-78. 7. Scott GE. Fracture toughness and surface cracks--the key to understandingceramic brackets. Angle Orthod 1988;58:5-8. 8. Spiegel MR. Vector analysis and an introductionto tensor analysis. New York: Schaum Publishing, 1959:16-26. 9. Reitan K. Some factors determiningthe evaluationof forces in orthodontics. AM J ORTttOD1957;43:32-45. I0. Wheeler RC. A textbook of dental anatomyand physiol~y, 4th ed. Philadelphia:WB Saunders, 1965:136. 11. WainwrightWM. Faciolingualtooth movement:its influenceon the root and cortical plate. AM J OR'tHUD1973;64:278-302.
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12. Nikolai RJ. Bioengineeringanalysis of orthodontic mechanics. Philadelphia: Lea & Febiger, 1985:299-305. 13. Flores DA. The fracture strength of ceramic brackets: a comparative study [Master's Thesis]. Loma Linda, California: Loma Linda University, 1988. 14. SteynCL. Measurementof edgewise torque force in vitro. AM J ORTHOD1977;71:565-73. Reprint requests to:
Dr. Ram S. Nanda Department of Orthodontics Universityof Oklahoma College of Dentistry P.O. Box 26901 I001 Stanton L. Young Blvd. Oklahoma City, OK 73190
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