Enamel abrasion from ceramic orthodontic brackets under an artificial oral environment Anthony D. Viazis, DDS, MS," Ralph DeLong, DDS, MS, PhD, b Richard R. Bevis, DDS, PhD, c Joel D. Rudney, PhD, d and Maria R. Pintado, MPH °

Minneapolis, Minn. The purpose of this investigation was to examine the potential enamel abrasion on contact with stainless steel and various ceramic orthodontic brackets under a simulated oral environment. Three groups of eight lower premolar ceramic brackets and one group of eight stainless steel brackets were used from four different manufacturers. An upper premolar was brought in contact with the bracket bonded to a lower premolar tooth and subjected to a lateral excursion type of movement by the artificial oral environment. A constant load of approximately 2 Ib was used for the masticatory force. The rate of chewing was 1 cycle/sec. The teeth were subjected to 15, 60, and 100 masticatory cycles. The before-and-after occlusal surfaces of the upper premolars were compared by means of a computerized profiling system and the enamel volume loss was calculated. Qualitative changes, such as rate of enamel wear, were examined visually by means of computer graphics and the scanning electron microscope. Abrasion scores (mean __ SD) in mm 3 were 0.015 -,- 0.01 from the metal brackets and 0.135 _+ 0.103, 0.255 -4- 0.242, and 0.581 -4- 0.524 from the three ceramic bracket groups. The abrasion scores were significantly different at p < 0.05. Ceramic brackets caused significantly greater enamel abrasion than stainless steel brackets. Artificial mouth in vitro testing gave a good indication of clinical performance of orthodontic brackets. (AM J ORTHOD DENTOFACORTHOP 1990;98:103-9.)

B e c a u s e of the very recent introduction of ceramic brackets, little information is available in the orthodontic literature regarding these appliances.]7 The amount of published data unfortunately is too sparse to make the clinician feel comfortable with these brackets. There are two types of ceramic brackets-polycrystalline and single-crystal alumina, both composed of aluminum oxide (99.9% to 99.5% AI2 O3). ]'3'4 In his recent article, 3 Swartz describes the basic differences between these two types. A very important physical property of ceramic brackets is the extreme high hardness values of aluminum oxide. It is generally thought that the harder a material is, the more it will wear an opposing material softer than itself. 8 The Knoop hardness number (KHN) for ceramic brackets is in the range of 2400 to 2450, almost nine times as hard as stainless steel brackets (KHN approximately 280) or enamel (KHN 343). 3

In partial fulfillment of the requirements for the degree of master of science From the School of Dentistry, University of Minnesota. 'Assistant Professor, Department of Orthodontics. bAssistant Professor, Biomaterial Program-Department of Fixed Prosthodontics. CProfessor, Department of Orthodontics dAssistant Professor, Department of Oral Biology. eAssistant Professor, Biomaterials Program. 811113150

Table I. Commercially available orthodontic

brackets used in study

Bracket A B C D

I Commercial I name Company Transcend Allure Gem

Unitek/3M GAC Ormeo American

Type Polycrystalline alumina Polycrystalline alumina Single crystal alumina Stainless steel

Monasky and Taylor8 investigated the wear of porcelain against enamel and found that the highest rates of enamel wear occurred with unglazed porcelain and that the wear rates of opposing teeth decreased with time as a result of a functional polishing of the porcelain. Serious consideration should be given to the possibility of enamel contact with an opposing ceramic bracket and the detrimental effects it may have on the integrity of the enamel. Accordingly, the purpose of this investigation is to examine the potential enamel damage on contact with stainless steel and various ceramic appliances in a simulated in vitro oral environment. METHODS AND MATERIALS

Sixty-four human premolar teeth were obtained from the Deparment of Oral Surgery, School of Den103


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Am. J. Orthod. Dentofac. Orthop. August 1990

Fig. 1. Three-dimensional profile of the occlusal surface of an upper premolar. Fig. 2. The teeth in the artificial chamber. Note the 1 mm of interference caused by the ceramic bracket. Fig. 3. Computer graphics of the path of the lateral excursion movement. I, Initial; ,4, after bracket placement; B, after 15 cycles; C, after 60 cycles; D, after 100 cycles. Fig. 4. No detection of enamel abrasion from stainless steel brackets by the computer system used. Fig. 5. Abrasion of stainless steel bracket from opposing tooth enamel.

tistry, University of Minnesota. Immediately after extraction, the teeth were stored in plastic bottles containing a solution of 0.9% sodium chloride (normal saline), which was changed periodically to prevent bacterial growth. Each tooth was examined for surface irregularities under a Zeiss 40-power stereomicroscope.

Any tooth with damaged enamel or restoration was excluded from the study. The 32 lower right second premolar teeth were divided into four groups, each consisting of eight teeth. Three groups of eight lower right second premolar ceramie brackets were used from three different manu-

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Table II. Enamel volume loss due to abrasion from opposing bracket


A (Transcend)


B (Allure)

C (Gem)


D (American)

Volume loss in rran~ after 100 cycles

0.2335 0.0226 0.0269 0.2870 0.2306 0.1348 0.0637 0.0864

0.0869 0.0300 0.1029 0.7441 0.4251 0.3262 0.2600 0.0674

0.0116 0.0506 1.3181 0.2359 1.2985 0.8670 0.3442 0.5235

0.00 0.0196 0.0228 0.00 0.0601 0.00 0.0146 0.00

Mean SD Min. Max.

0.135 0.103 0.0226 0.2870

0.255 0.242 0.030 0.7441

0.581 0.524 0.0116 1.3181

0.015 0.021 0.130 0.0228

facturers. The fourth group consisted of eight lower right second premolar stainless steel brackets (Table I). The square area of the base of the brackets was not a factor in this study. The ceramic brackets were used as provided by the manufacturers, whereas the metal brackets were cleansed in carbon tetrachloride to remove any contaminating oils from the mesh surface area. The "artificial mouth" developed at the University of Minnesota Dental School in conjunction with MTS Systems Corporation, Minneapolis, is a system that enables investigators to evaluate dental materials by synthesizing mastication under simulated conditions of an oral environment at a statistically significant level. 9-~2 Being a closed-loop system, it has the capacity to dynamically make force level adjustments while a test is in progress. 9t2 In the design of this project, the groups of two human fight premolar teeth, one lower second premolar opposed by one upper second premolar, were embedded in orthodontic acrylic resin and mounted in maximum intercuspation (centric occlusion) with the use of a modified Hanau articulator. Positioning pins secured the installation of the mounted setup without loss of alignment in the environmental chamber. In evaluating enamel wear characteristics, it is necessary to have a method to determine the volume of removed material. This is accomplished by profiling the tooth surface before and after testing with the use of a computerized controlled stylus with an accuracy of -4-5 ~tm, previously described in the literature. 912 The profiles are then overlapped and the volume loss calculated. Before the maxillary tooth was placed in the oral chamber, it was subjected to a threedimensional profiling of its occlusal surface (Fig. 1). An impression of the occlusal surface of the maxillary tooth was also taken with an additional silicone

material. Acrylic resin was poured into the impression and a replica of the tooth surface was obtained to be examined subsequently with the scanning electron microsocpe (SEM). Before the simulated masticatory test, the artificial mouth was configured to record the occlusal intercuspation of both the mandibular and maxillary elements, also previously described in the literature. 9-12 The occlusal surfaces were mapped with a three-dimensional system. This enabled visual observation of the wear by means of computer graphics. The masticatory cycle simulated by the artificial mouth was a lateral excursion type of movement in which the palatally inclined plane of the buccal cusp of the upper premolar would slide along the bucally inclined plane of the buccal cusp of the lower premolar until the tip of the latter cusp obtained maximum intercuspation, simulating centric occlusion. This type of movement is most likely to occur when an interference is present. Deionized water at 37 ° C was sprayed on the teeth at all times. After the path of mastication was established, a ceramic bracket was bonded to the lower canine with the acid-etch technique using the Transbond (Unitek/3M Company, Monrovia, Calif.) light-cured orthodontic adhesive. The bracket was positioned on the buccal surface to produce a slight occlusal interference of approximately 1 mm when the teeth came in contact (Fig. 2). Apart from the Unitek group, all the other brackets were supported with a rectangular stainless steel wire through their slots, covered by resin to avoid debonding during testing. The teeth were brought in contact again and the path of mastication was redrawn to verify that the buccal cusp was in contact with the ceramic bracket (Fig. 3). The difference between curves I and A of Fig. 3 is caused by the presence of the bracket that makes the opposing cusp "ride" over it during the sliding masticatory movement. Since


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Fig. 6. A, Dramatic enamel abrasion from single-crystal ceramic bracket. B, In vitro contact of tooth enamel with opposing single-crystal ceramic bracket. C, Severe enamel damage after 100 cycles. D, SEM photograph of the same area before testing. L~, SEM photograph of the same area after testing.

average masticatory forces range between 2 and 40 lb,n13 a constant load of approximately 2 lb was applied on the premolars by the computer throughout the experiment. The rate of chewing used was I cycle/sec.

The teeth were subjected to 15, 60, and 100 masticatory cycles. The occlusal surface was profiled and the volume of enamel loss determined with the aid of a computer program. The same surfaces were also examined

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Fig. 7. Enamel abrasion from polycrystalline ceramic bracket after 100 cycles.

with the SEM. Statistical analysis of enamel loss data was carried out with the BMDP package of statistical programs. =4

RESULTS The results from total enamel volume loss from each

maxillary sample tested were obtained by comparing the "before" and "after" profiles of the teeth using a computer that isolated the wear area. The mean and standard deviation of the total enamel loss of the maxillary teeth after contact with the various groups of brackets are shown in Table II. One-way analysis of variance (ANOVA) was used to evaluate differences among means of all the various bracket groups. ,5 Group variances were substantially unequal, which is a violation of the assumptions of ANOVA. This was corrected by reexpressing volume loss scores as logarithms. =5 Since some teeth showed zero scores, a constant of 1/6 was added to all scores before logs were taken.16 The overall test for differences among means was significant at p < 0.05. The enamel volume loss from the metal brackets was significantly less (p < 0.05) than that caused by the ceramic brackets when pairs of groups were compared by the Bonferroni t test. This also was confirmed by using the KruskalWallis nonparametric ANOVA test together with a pairwise nonparametric procedure (ceramic brackets would not be distinguished by these pair-wise tests). =4'=5 Qualitative changes such as rate of enamel wear from the various groups of brackets were examined visually with computer graphics. Apart from the stainless steel brackets, almost all ceramic brackets, irrespective of their groups, abraded the opposing enamel very rapidly within the first 15 cycles of the experiment (Fig. 3).

DISCUSSION The artificial mouth demonstrates clinical wear on a significant level as described extensively in the literature. 9=3 It is not an exaggeration to correlate the type of abrasion demonstrated in this investigation to a saw

Fig. 8. Aggressive enamel abrasion from ceramic brackets (grinding marks and delamination of half of the abraded area).

blade against a hard surface area. The low occlusal force of approximately 2 lb and the minimal number of masticatory cycles (100) that were used in the laboratory study imply that clinically visible enamel abrasion from ceramic appliances might occur during a single meal. The rate of enamel abrasion must have been very high during the initial cycles since it was very distinctly documented from the computer graphics after 15 cycles as compared with 60 to 100 cycles (Fig. 3). This was not anticipated in the start of the study. Another experiment that will measure the abrasion after 2, 5, and 10 cycles would be most useful. It is apparent from the results of this investigation that the metal brackets induce the least amount of enamel abrasion not clinically visible. In fact, half of the samples demonstrated no damage since it was not even detectable from the compuer system used (Fig. 4). In one sample the metal bracket was worn down by the enamel of the opposing tooth (Fig. 5). On the other hand, all ceramic bracket groups caused enamel abrasion on a significant level (p < 0.05), with enamel volume loss as high as 1.3 mm 3 (Table II). The ceramic appliances made of single-crystal alumina Gem (Ormco Corp., Glendora, Calif.) induced the highest abrasion scores when compared with the rest of the groups, indicating that these appliances are


Am. J. Orthod. Dentofac. Orthop. August 1990

Viazis et al.

In a previous article, ~s we presented a clinical case based on our initial findings and we discussed the potential implications of instant enamel abrasion from ceramic brackets in the practice of clinical orthodontics. The results of this investigation showed a marked similarity to clinical reality.

CONCLUSIONS The following conclusions can be stated from the results of this investigation: 1. Stainless steel brackets induce the least amount of enamel abrasion visible. In fact, wear of the metal appliance from the opposing enamel is equally probable. 2. Ceramic brackets cause significantly greater (p < 0.05) enamel abrasion than stainless steel brackets. 3. Although not statistically significant, there is a trend for single-crystal ceramic brackets to cause more extensive enamel abrasion than polycrystalline ones.


Fig. 9. Mild (absence of grinding marks) enamel abrasion from metal brackets.

the hardest of the available appliances tested (Fig. 6, A through E). There also seems to be a correlation between the design of the ceramic brackets and the damag e of the opposing enamel. Although made of the same material (polycrystalline alumina), the Transcend (Unitek/3M Company, Monrovia, Calif.) appliances caused less abrasion than the Allure (GAC International, Central Islip, N.Y.) brackets (Table II). Enamel variability, anatomy of the various teeth, individual differences of the brackets within the groups, and possible experimental errors could have contributed to these results. This investigation demonstrated clinically visible enamel abrasion from all ceramic appliances, which suggests that all bracket wing designs have a detrimental effect on tooth structure because of the inherent hardness of these materials (Fig. 7). Comparing the enamel abrasion caused by the metal brackets, the tooth that was worn down most extensively by a metal bracket appears to have been "softer" than normaP 7 and the abrasion seems to have been gradual and not immediate (absence of grinding marks) (Figs. 8 and 9).

1. Scott GE. Ceramic brackets. J Clin Orthod 1987;21:872. 2. Scott GE. Fracture toughness and surface cracks--the key to understandingceramic brackets. Angle Onhod 1988;1:3-8. 3. Swartz ML. Ceramic brackets. J Clin Orthod 1988;22:82-8. 4. PhillipsHW. The advent of ceramics: the editor's comer. J Clin Orthod 1988;22:69-70. 5. GwinnetAJ. A comparisonof shear bond strengths of metal and ceramic brackets. Ar,f J ORTHODDENTOFACORTHOP1988;93: 346-8. 6. Odegaard J, Segner D. Shear bond strength of metal brackets compared with a new ceramic bracket. A.,,tJ ORI"HODDENTOFAC ORTHOP1988;94:201-6. 7. Kusy RP. Morphology of polycrystallinealumina brackets and its relationshipto fracture toughnessand strength. Angle Onhod 1988;58:197-203. 8. Monasky GE,Taylor DF. Studies on the wear of porcelain, enamel and gold. J Prosthet Dent 1971;25:299-306. 9. DeLongR, Douglas WH. Developmentof an artificialenvironment for the testing of dental restoratives: bio-axial force and movement control. J Dent Res 1983;62:32-6. 10. DeLong R, Pintado M, Douglas WH. Measurementof change in surface contourby computergraphics. Dent Mater 1985;1:2730. I 1. DeLongR, Sakaguchi RL, DouglasWH, PintadoMR. The wear of dental amalgam in an artificialmouth: a clinical correlation. Dent Mater 1985;1:238-42. 12. SakaguchiRL, DouglasWH, DeLongR, PintadoMR. The wear of a posterior composite in an artificial mouth: a clinical correlation. Dent Mater 1986;2:234-40. 13. DeLongR, DouglasWH, SakaguchiRL, PintadoMR. The wear of dental porcelain in an artificial mouth. Dent Mater 1986;2: 214-9. 14. Dixon WJ. BMDP statisticalsoftware 1981. Berkeley, California: Universityof CaliforniaPress, 1981. 15. Sokal RR, Rohlf FJ. Biometry. 2nd ed. San Francisco: WH Freeman, 1981.

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16. Mosteller FW,TukeyJW. Data analysis and regression addition. Reading, Massachusetts: Wesley, 1977. 17. l_zmbrechtsP, Braem M, Vanherle G. Quantitive in vivo wear of human enamel as acceptance standard for posterior composites. J Dent Res 1987:IADRAbstract 605. 18. Viazis AD, DeLong R, Bevis RR, Douglas WH, Speidel TM. Enamel surface abrasion from ceramic orthodontic brackets: a special case report. A~,i J ORTHOD DENTOFAC ORTHOP 1989;96:514-8.

Reprint requests to

Dr. Anthony Viazis Department of Orthodontics School of Dentistry University of Minnesota Moos Tower 6th Floor 515 Delaware St. S.E. Minneapolis, MN 55455

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Enamel abrasion from ceramic orthodontic brackets under an artificial oral environment.

The purpose of this investigation was to examine the potential enamel abrasion on contact with stainless steel and various ceramic orthodontic bracket...
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