Fracture strength of ceramic brackets during arch wire torsion

Philippe C. Aknin, DDS, MS, = Ram S. Nanda, DDS, MS, PhD, b Manville G. Duncanson, Jr., DDS, PhD, c G. Fr~ins Currier, DDS, MSD, MEd, d and Pramod K. Sinha, BDS, MS = Lyon France, and Oklahoma Ci~ Okla.

This study evaluated the fracture strengths of eight new vintage ceramic brackets with application of torsional forces. Palatal root torque was applied at the distal side of right maxillary central incisor brackets with 0.022-inch slots by means of a 0.0215 × 0.027-inch rounded edge stainless steel arch wire. A specially designed apparatus that attached to an Instron machine was used to test the ceramic brackets. The amount of torque, degress of torsion at failure, and fracture locations were measured. The monocrystalline bracket did not break when the torquing test was applied; the portion of the wire outside the slot of the bracket twisted on itself. The mean torquing forces at failure ranged from 5755.2 gm-mm to 9316.5 gm-mm and could be separated into three statistically different groups. The mean torsional rotation at fracture ranged from 32.7 ° to 68.1 ° for the polycrystalline brackets. The results suggested that all the brackets studied were sufficiently strong to withstand the commonly accepted magnitudes of arch wire torquing forces. The present investigation showed higher angulation values for all the brackets than those reported by Holff who used the same apparatus with older style brackets. (AM J ORTHOD DENTOFAC ORTHOP 1996;109:22-7.)

M o n o c r y s t a l l i n e and polycrys ta l l ine ce- ramic b racke t s p rov ide good to excel len t co lor fideli ty and ma tch ing wi th na tu ra l too th color. However , ce ramic b racke t s d e m o n s t r a t e severa l de- f iciencies tha t res t r ic t the i r cl inical use, mos t no ta - bly the i r low f rac tu re s t rength. 1 Because of its br i t t leness , ce ramic b racke t s a re p r o n e to f rac tu re dur ing to rs iona l and t ipp ing movements . 2'3 W h e n ce ramic b racke t s b r e a k dur ing o r thodon t i c t r ea t - ment , the pa t i en t is sub jec ted to inc reased discom- for t and chai r t ime. T h e r e is also a po t en t i a l r isk of swallowing or asp i ra t ing r ad io lucen t b r acke t frag- ments . To ove rcome these deficiencies, manufac - tu re rs have i n t roduced new p roduc t s wi th c la ims of improved b r acke t des ign and manufac tu r ing pro- cess. This s tudy eva lua t ed the f rac tu re s t rengths of the new vin tage ce ramic b racke t s wi th the appl ica- t ion of to r s iona l forces.

Based on a thesis completed in partial fulfillment for the degree of Master of Science, Department of Orthodontics, College of Dentistry, University of Oklahoma. aFormer graduate resident, Department of Orthodontics; in private practice in Lyon, France. bProfessor and Chair, Department of Orthodontics, University of Ok- lahoma. CProfessor and Chair, Department of Dental Materials, University of Oklahoma. dProfessor, Department of Orthodontics, University of Oklahoma. cClinical Assistant Professor, Department of Orthodontics, University of Oklahoma. Copyright © 1996 by the American Association of Orthodontists. 0889-5406/96/$5.00 + 0 8/1/55660

MATERIALS AND METHODS Brackets tested

The 0.022-inch slot brackets for right maxillary per- manent central incisor were tested with a torsional force applied by 0.0215 × 0.027-inch rounded edge stainless steel arch wire. All brackets had a positive 12 ° palatal root torque and a positive 5 ° distal root angulation.

Eight different ceramic brackets were tested. A pilot study of three brackets for each type was used initially for calibration. Then 27 brackets were studied for each of the different ceramic bracket types for a total of 216 specimens. One of them was a single crystalline alumina bracket, whereas the other seven were made from poly- crystalline alumina. Of the seven polycrystalline brackets, two types were characterized by a flexible plastic bonding base. See Table I and Fig. 1. The testing apparatus and specimen preparation were identical to those used in the study conducted by Holt et al. 2

Testing procedure

At the time of testing, the apparatus base was clamped to the Instron crosshead, while the end of the chain was fastened to a tension load cell above. The Instron crosshead moved down at a rate of 2.54 cm/min (1.0 inch/minute). The radius of the chainsprocket on which the chain was pulled was 4.85 cm (1.91 in). The specimens were placed in the split-die specimen one at a time while lingual root torque was applied to the distal of the bracket. A 4.54 km(10 lbs) tension load cell was used on the Instron machine. The data were recorded on the x-y recorder with force recorded on the y-axis and time on the x-axis. The highest point on the graph was recorded in grams as the point of failure of the ceramic

2 2

American Journal of Orthodont&~ and Dentofacial Orthopedics A k n i n et aL 23 Volume 109, No. 1

Fig. 1. Facial and distal views of eight types of ceramic brackets. Brackets in top row are all polycrys- talline, and are, left to right, Allure IV, Contour, Ceramflex Advant-Edge (with polycarbonate base). Brackets in middle row are, left to right, Ceramflex Straight-Edge (polycrystalline with polycarbonate bonding base), Starfire TMB (monocrystalline), and Lumina (polycrystalline). Both brackets in bottom row are polycrystalline and are, left to right, Signature and Transcend series 2000.

bracket. The Instron machine was stopped at the time of fracture, and a reading of the number of degrees of rotation was recorded on the protractor aligned with the support post and a pointer attached to the ro- tating crossbar. The location of fracture was also re- corded.

The mean, standard deviation, minimum and maximum values of torquing force, and degrees of angu- lation at failure were obtained for each of the bracket types. A one-way analysis of variance was used to exam- ine the possible differences among the means for the eight brands. A Student-Newman-Keuls test was con- ducted to group and rank order the data at p < 0.05.

RESULTS Torque at failure

The monocrystalline Starfire TMB ceramic bracket (A Company, San Diego, Calif.) did not fracture on application of torquing force. The por- tion of the wire outside the slot of the bracket twisted on itself. The values for force and an- gulation during the torquing procedure for the bracket therefore yielded no definitive data. This bracket was found to be the strongest relative to torquing forces among all the brackets considered for testing in this experiment. The remaining dis- cussion will be related to the other seven types of brackets.

Table II shows the mean data for the amount of torsional force necessary to fracture the brackets in

Table I. The eight brackets used in this study. They are identified by the commercial name, manufacturer, and their abbreviation used in this study. The Starfire bracket was the only monocrystalline bracket used, whel~eas the other seven were polycrystalline

Name of bracket Manufacturer Abbreviation

Allure IV GAC, Central Islip, N.Y. P-A Contour Class I Orthodontics, P-C

Lubbock, Texas Ceramflex TP laboratories, P-Cx-A

Advant-Edge La Porte, Ind. Ceramflex TP laboratories, P-Cx-S

Straight-Edge LaPorte, Ind. Lumina Ormco, Glendora, Calif. P-L Signature RMO, Denver, Colo P-S Transcend series Unitek/3M, Monrovia, P-T

2000 Calif. Starfire TMB A-Company, Johnson S-S

and Johnson, Inc., San Diego, Calif.

gram-millimeters. The mean torsional force at fail- ure for the brackets ranged from 5755.2 gm-mm for Transcend 2000 to 9316.5 gm-mm for Signature. The minimum torsional force recorded at failure during the testing procedures was 3,881 gm-mm for the Transcend 2000 and the maximum torquing force recorded was 12,614 gm-mm for Signature.

24 Aknin et al. American Journal of Orthodontics and Dentofacial Orthopedics January 1996

Table I I . Mean torque values at fracture in gram-millimeters with standard deviations, minimums, and maximums. The seven polycrystalline ceramic brackets could be divided into three statistically significant groups (p < 0.05) on the basis of their fracture strength. No values for the monocrystalline S-S brackets were recorded as they did not fracture

Group Bracket Mean S.D. Minimum Maximum

1 P-S 9,316.5 1,416.6 6 ,792 12,614

P - C X - S 8,709.2 997.4 6 ,986 10,576

P - C 8,510.0 1,645.0 5 ,142 11,352

P - A 8,382.1 1,388.6 6 ,210 11,255

2 P - C X - A 6,840.5 900.7 5 ,240 8,441

P - L 6,677.0 1,179.7 4 ,366 9,945

3 P - T 5 ,755.2 1,495.8 3,881 9,024

Table I I I , Mean angulations in degrees at fracture on application of torsional forces with standard deviations, minimums, and maximums. Seven polycrystalline ceramic brackets could be divided into five statistically significant groups (p < 0.05) on the basis of the angulation at bracket failure. No values for the monocrystalline S-S brackets were recorded as they did not fracture

Group I Bracket Mean S.D.

1 P - C X - S 68.1 ° 12.3 °

2 P - C X - A 62.4 ° 7.7 °

3 P - C 55.0 ° 11.8 °

P-S 51.6 ° 8.7 °

4 P - L 43.7 ° 13.3 °

5 P - A 35.7 ° 8.7 °

P - T 32.7 ° 5.9 °

Minimum Maximum

45.0 ° 90.0 °

49.0 ° 75.0 °

29.0 ° 83.0 °

34.0 ° 66.0 °

31.0 ° 90.0 °

24.0 ° 62.0 °

18.0 ° 47.0 °

On the basis of the Student-Newman-Keuls Test, three groups could be identified for significant differences in the torque force at failure. The first group, the strongest, included four different brack- ets with a descending order of strength from Sig- nature, Ceramflex Straight-Edge, Contour, and Al- lure IV. The second group included Cermaflex Advant-Edge and Lumina. The third group consti- tuted Transcend 2000.

Angulation at failure

The mean degrees of rotation at which the seven types of brackets fractured varied from 32.7 ° for Transcend 2000 to 68.1 ° for Ceramflex Straight- Edge. See Table III. The minimum angulation at failure was 18 ° for Transcend 2000 and the maxi- mum 90 ° for Lumina and Ceramflex Straight-Edge.

An analysis of variance showed significant dif- ferences among the means of the groups at p < 0.05. On the basis of the Student-Newman- Keuls test, four groups could be identified for significant differences in the angulation at failure of the brackets. The strongest bracket in group one was Ceramflex Straight-Edge. Ceramflex Advant-

Edge constituted the second group, whereas Con- tour and Signature brackets were in the third group, Lumina was the fourth group, and the last included Allure IV and Transcend 2000.

Location of bracket fracture with torque

Fig. 2 shows the locations of fractures for dif- ferent types of brackets on application of torsional force. Incisal fracture predominated in the poly- crystalline brackets with lingual root torque. The bracket that showed less variation in fracture loca- tion was the Ceramflex Straight-Edge, whereas the Ceramflex Advant-Edge bracket showed the most variation in fracture.

DISCUSSION

All manufacturers have agreed that every de- sign and type of ceramic bracket can fracture? To improve the quality of brackets, most manufactur- ers have introduced modifications in their design and manufacture. The ceramic brackets presently available to the clinician are claimed to possess improved qualities and higher resistance to frac- ture. This study was designed to test the new

American Journal of Orthodontics and Dentofacial Orthopedics Aknin et al. 25 Volume 109, No. 1

P-CX-A

= 27 = 27 = 27

P-CX-S S-S P-L

B S = 27 = 27

= 27 CERVICAL

= 27 INCISAL

MESIAL

P-T

= 2 7

Fig. 2. Facial view of brackets with frequency of fracture locations. Numbers on wings of brackets indicate that fractures at that location. Number centered ~nesiodistal means that whole incisal or cervical half fractured. Monocrystalline S-S bracket had no fractures during test.

vintage of ceramic brackets during torque applica- tion and to evaluate the improvements claimed by the manufacturers.

The orthodontic force values needed to torque maxillary central incisors with an edgewise appli- ance have been estimated by various authors. Their findings 5-12 are summarized in Table IV. The range of torquing force is large. There is a minimum of 941 gm-mm reported by Steyn 1~ to a maximum of 3500 gm-mm reported by Nikolai? 2

All of the brackets tested in this study had a mean fracture force far above the maximum level suggested in the literature and therefore should be able to withstand the optimal level of torquing force.

Flores et alJ 3 studied bracket failure by ligating a rectangular arch wire into the 0.018-inch slot bracket. These older styles and types of brackets included two monocrystalline brackets, Gem and Starfire, and also two polycrystalline brackets, Al- lure III and Transcend. No comparisons were pos- sible for the Gem bracket which is no longer available on the market and Starfire whose newer style (Starfire TMB) did not break in the present

Table IV. Recommended maxillary incisor torquing values

Torque (gm-mm) Teeth involved References

2373 Central incisor Rei tan 5

2008 Central incisor Newman 6

1035 to 1665 Central incisor Neuger 7

2000 Central incisor Wainwright s

1624 to 2190 Central incisor Hammond 9

2824 Incisor segment Schrody 1° 941 to 2113 Incisor segment Steyn 11

3000 to 3500 Incisor segment Nikolai 12

Table V. A comparison of mean torquing values at failure in gram-millimeter for the brackets studied here and those reported by Flores 13 a n d G u n n 14

Present study (0.022 inch)

Flores et al. (0.018 inch) Bracket type

P-A 8,382 5,233 NA

P-T 5,755 6,082 6,400

Gunn and Powers (0.022

inch)

26 Aknin et al. American Journal of Orthodontics and Dentofacial Orthopedics January 1996

Table VI. A comparison of mean, minimum, and maximum torque values in gram-millimeter at failure and angulation in degrees for the brackets studied here and those reported by Holt 2. The current modified version is compared with the older style

Mean

Older bracket type Quasar Transcend Allure III

Newer bracket type present study Signature Transcend 2000 Allure IV

4,748 g m - m m a t 14.5 °

5,771 g in - r am a t 16.6 °

6,042 g m - m m a t 16.1 °

9,316 g m - m m at 51.5 °

5,755 g m - m m at 32.7 °

8 ,382 g m - m m at 35.7 °

Minimum Maximum

3,147 g m - m m at 6 °

4,753 g m - m m at 13 °

4 ,049 g m - m m at 11 °

6,792 gm-mm at 34 ° 3,881 gm-mm at 18 ° 6,016 gm-mm at 24 °

6,147 gm-mm at 20 ° 6,866 gm-mm at 25 ° 7,482 gm-mm at 21 °

12,614 gm-mm at 66 ° 9,024 gm-mm at 47 °

11,255 gm-mm at 62 °

study. The findings for Transcend that had a mean torquing force of 6082 grn-mm at failure were similar to the results with Transcend 2000 that had a mean at 5755 gm-mm as shown in Table V. The Allure bracket showed a mean increase in the bracket fracture torque value by 3000 gm-mm from 5233 gm-mm for the older style to 8382 gm-mm for the new style. These differences in the results may be due to multiple factors. The most important one is that Flores' study was reported in 1989 with the older style brackets. Since then, the brackets have been modified by the manufacturers. Another im- portant factor is that the slot size used by Flores was 0.018-inch slot as compared with the 0.022-inch slot in this study. The result may also have been affected by the manner of application of torsional forces and the design of the experiment.

Gunn and Powers 14 studied the load at bracket failure with the application of torsional forces on

f o u r types of 0.022-inch slot ceramic brackets for the maxillary central incisor, i.e., Gem, Quasar, Starfire, and Transcend. Gem and Starfire, for the same reasons as in Flores' study, have been ex- cluded from this discussion. The mean torquing values at failure for Transcend decreased from 6400 gm-mm (Transcend) to 5755 (Transcend 2000) but increased with the RMO products from 4800 gm-mm (Quasar) to 9316 gm-mm (Signature) as shown in Table V.

This study used the same apparatus and similar methods to the one used by Holt 2 to evaluate the new vintage brackets. The only change in the method was the use of a 0.0215 x 0.027-inch stain- less steel rounded edgewise wire. A comparison of results from the present study are given in Table VI. No appreciable differences in fracture strength were evident between Transcend (1989) and Transcend 2000 (1993). However, Allure IV increased in its

resistance to fracture over Allure III as did Signa- ture over Quasar. Starfire TMB did not break in this study as compared with Starfire (1989) that did. The present investigation also.showed higher angulation values for the number of degrees of rotation at fail- ure of the bracket as compared with Holt 's study. 2 This may be attributed to the differences in struc- tural design or the manufacturing process. However, there may also be a difference because of the wire used in this study at 0.0215 x 0.0217 inches with rounded edges as compared with the 0.0215 x 0.028-inch standard rectangular used in Holt's study.

The Starfire TMB, which was the only monocrystalline bracket studied, did not break during arch wire torsion.

2. All the polycrystalline brackets studied were sufficiently strong to withstand the com- monly accepted magnitudes of arch wire torquing forces, ranging from 5755.2 gm-mm up to 9316.5 gm-mm. An increase in fracture strengths was evident with Allure IV and Signature brackets when compared with their earlier counterparts products.

3. All seven brackets tested showed that the angulations for the torquing wire ranged from 32 ° to 68 ° and were larger than previous reported studies.

REFERENCES

1. Scott GE. Fracture toughness and surface cracks the key to unders tand ceramic brackets. Angle Orthod 1988;55:5-8.

2. Holt HM, Duncanson MG, Nanda RS. Fracture strength of ceramic brackets during archwire torsion. AM J ORTHOD DENTOFAC ORTHOP 1991;99:287-93.

3. Rhodes RK, Duncanson MG, Nanda RS, Currier GF. Fracture strengths of ceramic brackets subjected to mesial-

CONCLUSIONS

1.

American Journal of Orthodontics and Dentofacial Orthopedics A k n i n et al. 27 Volume 109, No. 1

distal archwire tipping forces. Angle Orthod 1992;62:67-75. 4. Bramble LM. A paradigm of the market place. AM J

ORTHOD DENTOFAC ORTHOP 1988;94:354-5. 5. Reitan K. Some factors determining the evaluation of forces

in orthodontics. AM J ORTHOD 1957;43:32-45. 6. Newman GV. A biomechanical analysis of the Begg light

arch wire technique. AM J ORTHOD 1963;49:721-40. 7. Neuger RL. The measurement and analysis of moments

applied by a light wire torquing auxiliary and how these moments change magnitude with respect to various changes in configuration and application. AM J ORTHOD 1967;53: 492-543.

8. Wainwright WM. Faciolingual tooth movement: its influ- ence on the root and cortical plate. AM J ORTHOD 1973; 64:278-302.

9. Hammond M, Rock W. Forces produced by auxilliary torqu- ing springs in the Begg technique. Br Dent J 1991;18:219-23.

10. Schrody DW. A mechanical evaluation of buccal segment reaction to edgewise torque. Angle Orthod 1974;44:120-6.

11. Steyn CL. Measurement of edgewise torque force in vitro. AM J ORTHOD 1977;71:565-73.

12. Nikolai RJ. Bioengineering analysis of orthodontic mechan- ics. Philadelphia: Lea & Febiger, 1985:299-305.

13. Flores DA, Caruso JM, Scott GE, Jeiroudi M. The fracture strength of ceramic brackets: a comparative study. Angle Orthod 1990;60:269-76.

14. Gunn S, Power JM. Strength of ceramic brackets in shear and torsion test. J Clin Orthod 1991;25:355-8.

Reprint requests to: Dr. Ram S. Nanda Professor and Chair Department of Orthodontics College of Dentistry University of Oklahoma 1001 S. L. Young Blvd. Oklahoma City, OK 73190

AAO MEETING CALENDAR

1996 - Denver, Colo., May 11 to 15, Colorado Convention Center 1997 - Philadelphia, Pa., May 3 to 7, Philadelphia Convention Center 1998 - Dallas, Texas, May 16 to 20, Dallas Convention Center 1999 - San Diego, Calif., May 15 to 19, San Diego Convention Center 2000 - Chicago, II1., April 29 to May 3, McCormick Place Convention Center 2001 - Toronto, Ontario, Canada, May 5 to 9, Toronto Convention Center 2002 - Baltimore, Mcl., April 20 to 24, Baltimore Convention Center