Introduction

The use of visible light cured composites for direct bonding of orthodontic brackets has increased in popularity since it was first described in 1979 by Tavas and Watts ¹.

The advantages of light cured direct bonding have been highlighted by Read ². Its unlimited working time and command setting advantages offset the conflicting reports of weaker bond strength and higher bond failure ranging from 5-23%.^3,4

Many factors influence the bond strength of directly bonded attachments and are hence implicated in bond failure. These include:

There are many light cured adhesives available on the market, differing in the amount of filler particles and viscosity including dual cured composites and glass ionomer cements. All, however, are subject to bond failure because of bracket base contamination and inconsistent amounts of adhesive applied to the bracket. APC^TM Adhesive Coated Brackets have overcome this problem, providing uniformity in adhesive placement.

Figures 1 and 2.

Prior to the introduction of the APC system, the authors' Department had routinely been using another light-cured composite (Vivadent Heliosit^TM). This is a visible light cured system containing resins with a di-ketone and an amine, which interact with light wavelength between 460-480nm to form free radicals to promote resin setting.

APC adhesive consists of a Bis-GMA resin, glass filler particles, amine and a catalyst (camphorquinone) which reacts to a bluelight wavelength of 450-500nm.

Numerous reports appear in the literature relating bond strength and failure rate of various adhesives/bracket type combinations and methods of bonding. However, the majority are in-vitro studies with relatively few clinical studies, especially prospective ones. Published clinical studies relating to the precoated adhesive system are limited to handling techniques ⁶, and the use of APC brackets in indirect bonding techniques⁷.

The aim of the present study was to compare the bond failure rate of two light cured adhesives, the APC adhesive coated bracket system and Heliosit/non-coated bracket system, used in the Department.

Materials and Method
52 fixed appliance patients, 35 female and 17 male between the ages of 11-30 years were enrolled in the study (mean age 14.3 years, S.D. 3.4 years) as they came off the waiting list. Once consent was obtained, patients were randomly allocated to receive adhesive coated or control brackets. There were 28 patients in the adhesive coated group and 24 in the control group.
Two graduate students with previous, but similar, experience of direct bonding carried out the treatment which compared two different Roth prescription bracket systems and adhesives. 3M Unitek APC Mini Uni-Twin^TM Brackets* were compared with TP Advant-Edge^TM Brackets** bonded with Heliosit***. Both composite resins are cured by light-activation of 440-480 nm wavelength.

* 3M Unitek Corporation, Monrovia, CA

** TP Orthodontics Inc., LaPorte IN

*** Vivadent, Leichtenstein

The direct bonding procedure involved the usual method of pre-etch cleaning of the tooth surface, isolation, etching with 37% phosphoric acid for 30 seconds, washing and drying. For the APC brackets, using the sponge dispenser, primer was applied to the etched tooth surface. Each bracket was removed from its individual blister pack immediately prior to placement, positioned on the tooth surface and pressed firmly in place. Excess adhesive flash was removed prior to light curing. For the Advant-Edge (control) brackets, Heliosit was applied to the bracket base by the nursing assistant and passed to the operator for placement on the tooth and then excess flash removed prior to light activation for 40 seconds for each material.

Routine fixed appliance treatment procedures were carried out as normal. All loose brackets during treatment were recorded as bond failures. Patients were asked to check for loose brackets and report to the Department as soon as possible. The same material was used to rebond brackets as that used for the initial bond up. The period of investigation for individual patients ranged from 12 to 24 months.The total number of brackets bonded was 837, (454 APC brackets and 383 controls).

Results

65.4% (34 patients) had at least one bond failure during the duration of the study, 50% (14 patients) of the APC group had at least one bond failure, while the figure for the control group was 83.3% (20 patients).

The total failure rate of individual bonds for both adhesives combined, was 15.4% (129/837). This was comprised of 48 APC brackets (11 brackets in one patient!) and 81 control brackets failed (15 brackets in one patient!).

This gave a bond failure rate of 10.6% for APC brackets and 21.1% for controls. If the two outliers in each group are excluded, then the adjusted failure rate was 8.4% (37/438) for APC brackets and 18% (66/367) for controls (figure 3).

Figure 3

Discussion

Removal of flash was notably easier with the APC adhesive than with the low viscosity Heliosit, leaving little residual material. A reduction in risk of demineralization may be assumed but was not investigated in this study.

Bracket floatation with the APC brackets was minimal, compared with the control. When bracket floatation occurred, it was usually associated with excess use of primer. At debond, however, most of the residual adhesive remained on the tooth surface, leaving a substantial amount to remove with the debonding bur. The higher filler content in the APC adhesive makes it slightly more time consuming to debond, but better bond strength means less bond failure which offsets the debonding time.

The Heliosit bond failure result for this study (21.1%) is comparable with the prospective study by Lovius et al ⁴ ,which involved operators with minimal experience and found a 23% failure rate. This contrasts with the combined bond failure results of both adhesives for each of two operators in this study, which were 12.8% and 18.4%.

Direct comparison of Heliosit and APC adhesive in an in-vitro study⁸ found that APC adhesive gave a stronger bond; the clinical findings in this study support this assertion.

Conclusions

The bond failure rate of the APC adhesive coated system was less than half that of the Heliosit light cure system (8.4% for APC adhesives and 18% for Heliosit, excluding outliers), when used by operators of comparable experience. This may in part be due to inherent differences between the two adhesives, but absence of contamination of the bracket base may also be important. In addition, consistency in the amount and thickness of adhesive as well as ease of excess material removal, means that the APC adhesive system has indisputable clinical advantages.

Acknowledgement

The study was supported by the 3M Unitek Corporation, Monrovia, CA

 APC and Uni-Twin are trademarks of 3M Unitek Corporation

Advant-Edge is a trademark of TP Orthodontics Inc.

Heliosit is a trademark of Vivadent

References

1. Tavas, M.A. and Watts, D.C.: Bonding of orthodontic brackets by transillumination of a light activated composite: An in-vitro study, Br. J. Orthod. 6:207-208, 1979.

2. Read, M.J.F.: The bonding of orthodontic attachments using a visible light cured adhesive, Br.J.Orthod. 11:16-20, 1984.

3. O'Brien, K.D.; Read, M.J.F.; Sandison, R.J. and Roberts, C.T.: A visible light-activated direct-bonding material: an in vivo comparative study, Am.J.Orthod.Dentofacial Orthop. 95:348-351, 1989.

4. Lovius, B.B.; Pender, N.; Hewage, S.; O'Dowling, I. and Tomkins, A.: A clinical trial of light activated bonding material over an 18 month period, Br.J.Orthod. 14:11-20, 1987.

5.Millett, D.T. and Gordon, P.H.: A 5 year clinical review of bond failure with a no-mix adhesive (Right on), Eur.J.Orthod. 16:203-211, 1994.

6. Cooper, R.B., Goss, M. and Hamula, W.: Direct bonding with light-cured adhesive precoated brackets, J.Clin.Orthod. 26:477-479, 1992.

7. Cooper, R.B. and Sorenson, N.A.: Indirect bonding with adhesive precoated brackets, J.Clin.Orthod. 27:164-167, 1993.

8. Bradburn,G.and Pender,N.: An in vitro study of the bond strength of two light-cured composites used in the direct bonding of orthodontic brackets to molars, Am.J.Orthod.Dentofacial Orthop.102:418-426, 1992.