Bond strength of light-cure fluoride-releasing base-liners as orthodontic bracket adhesives James W. McCourt, DMD,* Robert L. Cooley, DMD, MS,** and Sean Barnwell, BS*** San Antonio, Texas

Fluoride-releasing bracket adhesives are desirable for their ability to minimize the potential for subsurface enamel demineralization adjacent to a bonded bracket. Self-applications with topical fluoride rinses, pastes, and gels have been documented to minimize and eliminate subsurface caries adjacent to bonded brackets. However, the success of these mediums are limited by patient compliance. A urethane with fluoride (TimeLine) and a glass ionomer with methyl methacrylate (Vitrabond), both of which are light cured and exhibit sustained fluoride ion release, were compared with a non-fluoride-releasing light-cured bracket adhesive (Transbond). Premolar brackets with mesh pads (A-Company) were positioned on the buccal surface of the premolarsand placed in a PVC ring with polymethyl methacrylate, Two groups of 10 samples each of the tested material were prepared and immersed in distilled water immediately after in vitro bonding. Samples of each material were evaluated for enamel shear strength (Instron) at 24 hours and at 30 days. Bond strengths to enamel at 24 hours measured in megapascals (MPa) were 5,98 for TimeLine, 11.58 for Vitrabond, and 11.35 for Transbond. Bond strength to enamel at 30 days was found to be significantly less for TimeLine and Vitrabond: 3.05 for TimeLine, 5,39 for Vitrabond, and 10,80 for Transbond. The two fluoride-releasing, light-cured materials tested have low bond strengths after 30 days and are not acceptable as orthodontic bracket bonding agents, However, for patients with high caries risk, these materials may be placed around already bonded brackets, (AMJ ORTHODDENTOFACORTHOP1991; 100:47-52.)

F i x e d orthodontic therapy presents a high and continuous cariogenic challenge to the involved enamel surfaces. 1 It alters plaque content by promoting a facultative bacterial population2 that is an obstacle to remineralization? White spot lesions around bonded brackets are an optical phenomenon that is particularly apparent when the affected enamel surfaces are dried with air. i These lesions are initiated by plaque, which varies in quantity and quality from site to site on the same tooth. 3 This may produce variations seen in the dissolution process of the enamel surface.l Several studies have documented the use of fluoride exchange in retarding demineralization. In 1987 O'Reilly and Featherstone4 showed that microscopic subsurface demineralization occurred adjacent to bonded brackets within 1 month of their placement, even though the patients were motivated to maintain good oral hygiene. Further, they found the use of fluoride toothpaste (1100 ppm fluoride) only did not inhibit From the University of Texas Health Science Center. *Assistant Professor, Department of Pediatric Dentistry. **Associate Professor, Department of General Practice. ***Senior Dental Student, 8/1/19043

these microscopic incipient carious lesions. Prevention occurred only with the concomitant use of a daily fluoride rinse (0.05% sodium fluoride) with fluoride toothpaste (1100 ppm sodium fluoride). Prevention of white spot lesions adjacent to bonded brackets is beneficial to both the patient and the orthodontist? Underwood et al. 6 examined an experimental fluoride-exchanging resin and found a 93% reduction in the first stages of enamel alteration. Enamel infusion by fluoride has also been documented by Tanaka7 from an experimental fluoride-releasing sealant. However, the relevance of demineralized enamel around bracket placement was brought into question by Southard, 8 who speculated that remineralization of white spot lesions occurred spontaneously in orthodontically treated patients. In his study of newly eniisted Naval recruits, less caries was seen in the orthodontically treated patient when compared with the nontreated patient. He attributed this to better oral health in the orthodontically treated patient and a low number of orthodontically treated patients with a high caries rate. The purpose of this study is to evaluate the bond strengths of two fluoride-releasing materials when used as an orthodontic bracket adhesive. Both materials to 47

48 McCourt, Cooley, a n d B a r n w e l l

Am, ,I, Orthod, Derma.fat. Orthop. July 1991

.

Table I. B o n d strength of bracket adhesives Sample no.

Fig, 1. Diagram of Instron impacting bracket with insert demonstrating rounded resin bracket pad to enamel junction.

b e tested, a urethane dimethacrylate with sodium fluoride (TimeLine, Caulk/Dentsply, Milford, Del.) and a glass i o n o m e r - l i n k e d methyl methacrylate (Vitrabond, 3 M Dental Products, St. Paul, M i n n . ) , have docu m e n t e d sustained fluoride release. `) Both of these materials are light cured and have the potential to reduce or eliminate the white spot lesion. However, it is not k n o w n whether these materials will provide sufficient b o n d strength for retention of brackets during treatment.

METHODS AND MATERIALS Two light-cured fluoride-releasing materials--a urethane (TimeLine) and a glass ionomer (Vitrabond)--were evaluated for their ability to bond orthodontic brackets to extracted teeth. These two materials were compared to a light-cured BISGMA orthodontic adhesive (Transbond, Unitek/3M, Monrovia, Calif.). Premolar brackets with mesh backings ("A" Company, San Diego, Calif.) were used for the test brackets. Sixty extracted human maxillary and mandibular premolars were collected and stored in distilled water. The teeth were separated into six random groups of 10 teeth each. Each material was tested on 20 teeth with brackets; one group of 10 was tested at 24 hours and the second group of 10 was tested at 4 weeks. A dental surveyor was used to orient each tooth so that the facial height of contour was in the vertical plane as it was mounted in a PVC ring with polymethyl methacrylate (Coe Labs, lne,, Chicago, Ill.). The facial surface of each tooth was cleaned for 30 seconds with an oil-free fine flour of pumice paste in a prophy cup with a low-speed handpiece. The surfaces were then

Timelhw

Vitrabond

Transbond

1 2 3 4 5 6 7 8 9 10 Mean

24 Hour readings (MPa) 3.84 14.57 6.29 7,95 4.17 13,77 5.70 11.26 5,43 17.42 7,42 11.92 10,46 9.60 5,56 11,72 4,37 8,61 6,56 8,94 5,98 11.58

10.53 15.56 12.98 9.93 7,55 16,56 10.93 I 1.59 7,95 9,93 11.35

1 2 3 4 5 6 7 8 9 10 Mean

4 week readings (MPa/ 1,32 4,04 ,33 4,64 4.11 5,30 1.23 4,14 1.26 9.14 6.59 1,13 6,82 6.82 1.44 8,61 5,09 5.96 2,32 4,11 3,05 5,39

8,08 5.75 9.93 4.50 5.93 12.58 15,23 14,70 8.21 7,02 9.19

rinsed under water and dried with an oil-free air spray for 30 seconds. The facial surface of each tooth was etched with 37% phosphoric acid for 60 seconds, rinsed with an air-water spray for 30 seconds, and air dried. The orthodontic brackets were mounted by one operator. The bracket was placed on the buccal vertical long axis of each tooth, with file bracket pad as a horizontal plane of reference. The glass ionomer (Vitrabond) was mixed according to the manufacturer's recommendations and was applied to two bracket pads with each mix. The glass ionomer was applied to a total of 20 teeth. The urethane (TimeLine) and BIS-GMA (Transbond) required no mixing and, after being placed on the bracket pad, were applied to 20 teeth each. Flash was removed before setting. All materials were light cured for 20 seconds with a new 75watt visible-light curing unit (Ortholux, Unitek/3M, Monrovia, Calif.). The light wand was applied directly againsl the bracket. The specimens were stored in distilled water at 37 ° C until testing. After 24 hours, 10 specimens from each of the product groups were tested on an [nstron universal testing machine (instron Engineering Co., Canton, Mass.), which was calibrated to a full-scale load of 100 newtons. The shearing blade was placed to contact the bracket at its base (Fig. 1) and was placed under continuous loading at a crosshead speed of 0.5 mm/minute until shearing of the bracket from the tooth occurred. The shearing force required was recorded in newtons and converted to megapascals using the surface

Volume 100

Light-cure bracket adhesives

49

Number 1

(MPa) 15-

T

141312-

11 10 9-

E

r'a

TIMELINE

m

24 Hours

VITRABOND

~

TF:IN'JSBOND

4 Weeks

Fig. 2. Mean shear bond strengths at 24 hours and at 4 weeks.

area of the bracket that had been determine d with a measuring microscope. Four Weeks later this procedure was repeated for the remaining product groups, with data recorded in a similar fashion. The data from each test time were subjected to an analysis of variance (ANOVA) and the Scheffe test to determine any statistically significant differences in bond strength at 24 hours and at 4 weeks. A t test (two-tailed) was used to determine any statistical difference between 24-hour bond strengths and 4-week bond strengths for each material. RESULTS

The results of this study are shown in Table I and Fig. 2. Table I illustrates the bond strength of each individual specimen in megapascals (MPa). When the specimens were tested at 24 hours, the urethane (TimeLine) developed the lowest bond strength (5.98 M P a - 1.93), whereas the glass ionomer (Vitrabond) and the BIS-GMA (Transbond) were considerably

stronger and approximately equal (i 1.58 MPa __ 2.99 and 11.35 MPa -- 2.95, respectively). Fig. 2 shows a graphic comparison of the mean bond strengths (MPa) at 24 hours and at 4 weeks. At 4 weeks, the two fluoride-releasing materials lost bond strength. For the urethane (3.05 MPa _+ 2.40) and glass ionomer (5.39 MPa +_ 2.37), the drop was significant, whereas the nonfluoride-releasing B I S - G M A showed little change (9.19 MPa --- 3.80). When all data were subjected to an analysis of variance (ANOVA), there was a significant difference in the bond strength of t h e materials ( p < 0.0I). A Scheffe test of the data showed a statistical difference (p < 0.05) in the bond strength of the materials. At 24 hours the urethane was significantly weaker than the glass ionomer and the BIS-GMA, whereas at 4 weeks the urethane was not significantly different from the glass ionomer but was significantly weaker than the

50

McCourt, Cooley, and Barnwell

BIS-GMA. The glass ionomer was not significantly different from the BIS-GMA at either 24 hours or 4 weeks. When the 24-hour and 4-week data for each material were compared by means of a t test (twotailed), it was evident that both the urethane and the glass ionomer became significantly weaker (p < 0.01). The bond strength for the BIS-GMA was not significantly different at 24 hours and at 4 weeks. All brackets sheared at the bracket-adhesive interface, leaving the material bulk on the tooth. This indicated a stronger bond at the tooth-adhesive interface than at the bracket-adhesive interface. DISCUSSION Both TimeLine and Vitrabond have been marketed as light-cured base/liners that release fluoride. TimeLine has the potential to bond to enamel since it is a urethane dimethaorylate that contains sodium fluoride. Vitrabond has the potential to bond to enamel since it is a glass ionomer with methyl methacrylate. Both materials contain a photosensitizer to give them a lightcure capability. These materials have been shown to release fluoride for 5 months. 9 Initially, these materials exhibited a "burst effect" in which larger amounts of fluoride were released. Fluoride release decreased during the first 5 days and then stabilized, with Vitrabond releasing more fluoride than TimeLine. This fluoride release could prove beneficial in reducing or eliminating the "white spot" lesion that occurs around some orthodontic brackets. As a part of the normal technique, the bracket mesh pad with the bonding material was placed on etched enamel, The bracket adhesive was placed with a uniform thickness greater than 25 p.m on a mesh pad with rounded undercuts. These ideals mentioned by various authors permit adequate adhesion with minimal internal adhesive stress, lo-12 A rounded rather than a flat interface was formed by all tested materials at the mesh pad border and the enamel (Fig. 1). This prevented engagement of the Instron blade at the welded mesh pad-tooth interface and required the Instron blade to be engaged at the bracket-to-pad interface (Fig. 1). The resultant shear force had a slight peel component. This peeling effect may be significant since an unpublished study (Norling RB, personal communication) reports higher shear bond strengths for Transbond with a thicker mesh pad that allowed for direct engagement of the Instron blade on the pad. All the tested materials exhibited failure predominantly at the adhesive-to-mesh pad interface. This was consistent with several previous resin bonding studi e s . m 3 However, it was in contrast to a glass ionomer

Am. J.

Orthod. Dentofac. Orthop. July 1991

bracket bond study that reported failure of the glass ionomer cement within the cement and at its enamel junction. 14 The bond strengths of TimeLine at both the 24hour and 4-week periods were considerably less than 10 MPa. Vitrabond had a 24-hour bond strength that was equivalent to that of Transbond, but at 4 weeks the bond strength decreased below 10 MPa. Transbond maintained approximately a 10 MPa bond strength over the 4-week test period. A bonded orthodontic bracket must resist forces that consistently change during the clinical course of treatment. Proffit j5 states in his book that forces produced during mastication are highly variable, with a range up to 50 kg, and that the controlled force required to move a tooth orthodontically through bone ranges approximately from 15 to 150 gin. The successful bracket adhesive has adequate shear bond strength for its continued attachment during the required clinical period and can be removed easily with no enamel damage. After a 2-year clinical study Muira et a1.16 indicated that the required bracket bond shear strength (resistance to peeling) was 5.1 MPa. Lightcured adhesives produce incomplete bond strengths for the initial loading of brackets. Greenlaw 17reported that at 1 hour a light-cured composite adhesive had less than Va the shear force strength recorded at 30 hours. This problem is overcome by commercial materials that have a bracket shear bond strength higher than 10 MPa.18'19 In the clinical setting, a bracket bond failure rate higher than 5% is unacceptable. In a long-term clinical study Sonis 5 showed bracket bond failures occur 92% of the time within the first 3 weeks of treatment. For him, bracket adhesion failure ranged between 2.43% (FluorEver, a fluoride-releasing composite) and 3.4% (Aurafill, a composite). These are better results than those of other studies that relate bracket adhesion and tooth position in the dental arch to failure rate5 °-zz A urethane resin material, such as TimeLine with its low percentage of filler particles, requires little pressure to seat a bracket but has the disadvantage of bracket "float" before light cure. The absence of large filler particles offers the advantage of easy removal 17 and a smooth surface. The glass ionomer, Vitrabond, also produces bracket "float" before cure. It has the disadvantage of solubility, which may produce its decreasing bond strength. The duration and intensity of light-activated cure is a problem, since light-cured resins under metal bracket pads must receive reflected light. Since the color of translucent materials affects light reflection23 and enamel varies in color, its ability to reflect light varies for each patient. Light cure is unpredictable,

Volume 100

Light-cure bracket adhesives

51

Number 1

since reflected light is weaker than the direct light, and directly dependent on distance from the bracket pad. It is also predictable that the material in the undercut region of the p a d is not cured initially. The resultant weak link for orthodontic bracket adhesives is at the mesh pad interface. This was our finding for all the materials tested in this study. In his transillumination study of tooth tissue, Cheng '-4 showed that surface hardness was increased only slightly when light exposure time was increased from 20 seconds to 60 seconds. For this reason, we used a 20-second light exposure perpendicular to the buccal surface o f the bracket. This is in contrast to studies in which a light wand was placed in three positions (buccal, lingual, and occlusal) with a 60-second total for each bracket.2s'26 Recent literature states the advantage of fluoridereleasing resins in the prevention of demineralization. 6 Fluoride loading in resins requires ion exchange, which affects bond strength. High fluoride ion exchange in a resin weakens the enamel shear bond strength, whereas a low fluoride ion release minimally modifies shear bond strength. This formulation of low and slow fluoride-release capability is exemplified by FluorEver (Macro Chem Corp., Billerica, M a s s . ) 2 It may be the amount o f fluoride ion exchange in our two tested fluoride-releasing light-cured materials, TimeLine and Vitrabond, that produces a low bond strength at the end o f 4 weeks. This makes them unacceptable as bracket bonding agents. However, patients with a high caries index or risk secondary to inadequate oral hygiene may be benefited by a modified bonding method using TimeLine or Vitrabond. This requires application o f an acceptable bracket adhesive followed b y a thin application around the bracket pad periphery of a low-viscosity fluoride-releasing material such as TimeLine or Vitrabond. These materials will form a smooth finish around a composite resin bracket adhesive material, which typically has a rough finish and high bond strength. SUMMARY

Two light-cured, fluoride-releasing materials, TimeLine and Vitrabond, were evaluated for their ability to bond orthodontic brackets to extracted teeth and compared with a light-cured orthodontic adhesive, Transbond. TimeLine developed the lowest mean bond strength at both 24 hours and 4 weeks. Vitrabond developed a bond strength equivalent to that of Transbond at 24 hours, but this decreased significantly at 4 weeks. The non-fluoride-releasing resin, Transbond, did not change bond strength significantly over the 4-week test period.

A l l brackets broke at the adhesive-to-mesh pad interface for the three materials tested. T h e weakness of the mechanical bond at the metal p a d m a y be related to the curing light position and the quantity and intensity of reflected light under the pad. The results of this study indicate that a urethane with fluoride, TimeLine, and a glass ionomer with methyl methaerylate, Vitrabond, are not suitable as orthodontic bracket adhesives on the basis of a clinically suggested minimal shear bond strength of 10 MPa. A modified bracket bond method in which a thin layer o f fluoride-releasing low-viscosity material is placed around the already bonded bracket may meet the treatment needs of the individual patient with a high caries risk or poor oral hygiene habits.

REFERENCES I. ¢lgaard B, Rolla G, ArendsJ. Orthodontic appliances and enamel demineralization, part 1, lesion development..AMJ ORTHODDENTOFACORTHOP1988;94:68-73. 2. Bloom R, Brown L. A study of the effects of orthodontic appliances on the oral microbial flora. Oral Surg Oral Med Oral Pathoi 1964;17:658-67. 3. Creanor SL, Macfarlane TW, Mackenzie D, Weetman DA, Strang R, Stephen KW. Microbiology and acid/anion profiles of enamel surface plaque from an in situ caries appliance. Caries Res 1986;20:392-7. 4. O'Reilly MM, Featherstone JDB. Demineralization and remineralization around orthodontic appliances: an in vivo study. AM J ORTHOD DENTOFAC ORTHOP 1987;92:33-40.

5. Sonis AI. Snell W. An evaluation of a fluoride-releasing, visiblelight-activated bonding system for orthodontic bracket placement. AM J ORTHODDENrOFACORTHOP1989;95:306-11. 6. Underwood M, Rawls HR, Zimmerman B. Clinical evaluation of a fluoride-exchangingresin as an orthodontic adhesive. AM J ORTUOD DENTOFACORTHOP 1989;96:93-9.

7. Tanaka M, One H, Kadoma Y, Imai Y. Incorporation into human enamel of fluoride slowly released from a sealant in vivo. J Dent Res 1987;66:1591-3. 8. Southard TE, Cohen ME, Rails SA, Rouse LA. Effects of fixedappliance orthodontic treatment on DMF indices. AM J ORTHOD DEN'rOFACORTHOP1986;90:122-6. 9. McCourt J, Cooley R, Huddleston A. Fluoride release from fluoride containing liners/bases. Quintessence Int. 1990;21: 41-5. 10. 0degaard J, Segner D. Shear bond strength of metal brackets compared with a new ceramic bracket. AM J ORTHODDENTOFAC ORTHOP 1988;94:201-6. 11. Barnes R, Moon P, Eshleman JR, Button G. Comparison of shear bond strength and film thickness of bonding resins for resin-retained appliances. Gen Dent 1986;34:228-9 May/June. 12. Knoll M, Gwinnett AI, Wolff MS. Shear strength of brackets bonded to anterior and posterior teeth. AM J ORTHODDENTOFAC ORTHOP 1988;89:476-8. 13. Gwinnett AJ. A comparison of shear bond strengths of metal and ceramic brackets. AM I ORTHODDENTOFACORTHOP1988; 93:346-8. 14. Klockowski M, Davis EL, Joynt RB, Wieczkowski G, MacDonald A. Bond strength and durability of glass ionomer

52

15. 16. 17.

18. 19.

20, 21. 22.

McCourt, Cooley, and B a r n w e l l

cements used as bonding agents in the placement of orthodontic brackets. AM J ORTHOD DENTOV^CORTHOP 1989;96:60-9. ProffitWR. Contemporary Orthodontics, First Edition. StLouis: CV Mosby, 1986:229-36. Miura F, Nakagawa K, Masuhara E. A new direct bonding system for plastic brackets. AM J ORTHOD 197 t;59:350-61. Greenlaw R, Way DC, Khadry AG. An in vitro evaluation of a visible light-cured resin as an alternative to conventional resin bonding systems. AM J OIt.TI-IODDENTOFACORTItOP 1989;96:21420. Rux W, Cooley R, Hicks J. Evaluation of Panavia as a bracket adhesive. Submitted to Quintessence [in press]. Dclport A, Grobler S. A laboratory evaluation of the tensile bond strengths of some orthodontic bonding resins to enamel. AM J ORTHOD DENTOFACORTHOP 1988;93:133-7. Geiger L, Gorelick L, Gwinnett AJ. Bond failure rates of facial and lingual attachments. J Clin Orthod 1983;17:165. Mizrahi E. Success and failure of banding and bonding: a clinical study. Angle Orthod 1982;52:113-21. O'Brien KD, Read MJF, Sandison R J, Roberts CT. A visible

Am. J. Orthod. Dentofac. Orthop. July 1991 light-activated direct-bonding material: an in vivo comparative study, AM J ORTHOD DENTOFACORTHOP 1989;95:348-51. 23. ten Bosch JJ, Borsboom PC, van der Burgt T, Kortsmit W. Measurement of reflectivity and color of translucent materials. Soc Photo-Optical Instrum Engineers 1984;494:493-9. 24. Cheng L, Ferguson JW, Jones P, Wilson HJ. 'An investigation of the polymerization of orthodontic adhesives by the transilluminatlon of tooth tissue. Br J Orthod 1989; 16:183-8. 25. King L, Smith RT, Wendt SL, Behrents RG. Bond strengths of lingual orthodontic brackets bonded with light-cured composite resins cured by transillumination. AM J ORTHODDENTOFACORT~-IOP 1987;91:3 12-5. 26. Andreasen G, Chan KC, Fahl JA. Shear strength comparison of autopolymerizing and light cured resins used for orthodontic bonding. Dent Sci Res 1984;10:1081-6. Reprint requests to: Dr. James W. McCourt Texas Health Science Center 7703 Floyd Curl Dr. San Antonio, TX 78284-7888