ORIGINAL ARTICLE

Diode laser debonding of ceramic brackets

Paul J. Feldon,a Peter E. Murray,b James G. Burch,c Malcolm Meister,c and Matthew A. Freedmand

Boca Raton and Fort Lauderdale, Fla

Introduction: Our objective was to investigate the effectiveness of debonding ceramic brackets with a diodelaser. Methods: Two types of ceramic brackets (monocrystalline and polycrystalline) were bonded to bovinemaxillary central incisors. The diode laser was applied to brackets in the experimental groups for 3 seconds.Shear bond strength and thermal effects on the pulp chamber were assessed at 2 laser energy levels: 2 and 5W per square centimeter. Analysis of variance (ANOVA) was used to determine significant differences in shearbond strength values. Results: The diode laser was ineffective with polycrystalline brackets and effective withmonocrystalline brackets in significantly (P\0.05) lowering the shear bond strength. There were no significantadhesive remnant index score differences between any groups tested. Conclusions: Diode laser use signif-icantly decreased the debonding force required for monocrystalline brackets without increasing the pulpchamber temperature significantly. Diode lasers did not significantly decrease the debonding force requiredfor polycrystalline brackets. (Am J Orthod Dentofacial Orthop 2010;138:458-62)

Ceramic brackets have been the preferred choicefor esthetically conscious clinicians and pa-tients since their introduction in the 1980s.

Their popularity continues, as evidenced by the recentwave of ceramic self-ligating bracket systems intro-duced to the specialty. Discussion of the clinical chal-lenges of ceramic bracket use remains an active topicof debate.1-20 A major concern is bracket debonding.The increased bond strength in attachment of ceramicbrackets to enamel might increase the potential forenamel damage or bracket fractures at debonding.11,21-31

Various methods have been developed to aid in de-bonding ceramic brackets. These methods use specialpliers32 for mechanical debonding, degrading the bond-ing resin with electro-thermal debonding devices,33-35

and lasers.36-41 There are 4 major types of lasers.They are classified mostly by their lasing mediums,defined by their state: gas, liquid, solid, andsemiconductor (or laser diode).

A laser diode, also known as an injection laser ora diode laser, is a semiconductor device that producescoherent radiation (in which the waves are all at the

aPrivate practice, Boca Raton, Fla.bDirector of biological research, College of Dental Medicine, Nova Southeastern

University, Fort Lauderdale, Fla.cProfessor, Department of Orthodontics, Nova Southeastern University,

Fort Lauderdale, Fla.dPrivate Practice, Naples, Fla.

The authors report no commercial, proprietary, or financial interest in the

products or companies described in this article.

Reprint requests to: Paul J. Feldon, 595 S. Federal Hwy. Suite 130, Boca Raton,

FL 33432; e-mail, paul@feldonortho.com.

Submitted, May 2008; revised and accepted, November 2008.

0889-5406/$36.00

Copyright � 2010 by the American Association of Orthodontists.

doi:10.1016/j.ajodo.2008.11.028

458

same frequency and phase) in the visible or infraredspectrum when current passes through it. Laser diodesare used in optical fiber systems, compact disk players,laser printers, and remote-control devices. Laser diodesdiffer from other laser types in several important ways:small size and low weight, current, voltage, intensity,and power requirements.

Previous studies have shown that use of carbon di-oxide,37-40 neodymium: yttrium-aluminum garnet,40,41

or xenon chlorine excimer36 lasers can be effective insignificantly lowering the shear bond strength (SBS)of ceramic brackets. The purpose of this study was toinvestigate the effect of diode laser use in debonding2 types of ceramic brackets.

MATERIAL AND METHODS

Two maxillary left central incisor ceramic bracketswere used in this study: Inspire ICE (Ormco, Orange,Calif), a monocrystalline bracket; and Clarity (3MUnitek, Monrovia, Calif), a polycrystalline bracket. Asingle-paste visible light-cured orthodontic adhesivesystem, Transbond XT (3M Unitek), was used.

Sixty bovine incisors were obtained from a slaugh-terhouse. The teeth were stored in distilled water ina freezer at –20�C until used. When needed, the teethwere thawed in warm water and embedded in auto-polymerizing polymethyl methacrylate in molds(25 3 25 3 25 mm) so that the crown was exposed.After polymerization of the polymethyl methacrylate,the teeth were stored for 24 hours in distilled water towhich crystals of thymol were added to inhibit bacterialgrowth. Before bonding, access cavities, 2 mm indiameter, were drilled into the pulp chamber by using

Table I. Experimental groups

Group Bracket type Debonding method

1 Clarity Force only

2 Inspire ICE Force only

3 Clarity Diode laser 2 W/cm2/3sec and force

4 Inspire ICE Diode laser 2 W/cm2/3 sec and force

5 Clarity Diode laser 5 W/cm2/3 sec and force

6 Inspire ICE Diode laser 5 W/cm2/3 sec and force

American Journal of Orthodontics and Dentofacial Orthopedics Feldon et al 459Volume 138, Number 4

a high-speed hand piece into the center of the cingulumon the lingual surfaces.

The middle third of the labial surface was flattenedwith sandpaper. The flattened enamel surface was driedwith hot air, and Transbond Plus Self Etching Primer(3M Unitek) was prepared for application to the toothsurface according to the manufacturer’s instructions.A saturated applicator tip was rubbed on the toothsurface for 4 seconds followed by a gentle air burst.Transbond XT was then placed on the bracket base,and the bracket was placed on the prepared enamelsurface so that the slot was parallel to the incisal edgeof the incisor.

The guide pin of a nonadjustable Hanau articulator(Teledyne Water Pik, Fort Collins, Colo), with a stonecylinder of 600 g attached to its upper member, waspositioned to engage the bracket slot. This ensured thatthe brackets were seated under constant pressure andallowed the investigator to remove, with a sharp dentalexplorer, any surplus cement extruded from the peripheryof the base without compromising the onset of the poly-merization process. The cement was then cured witha 75-W halogen curing light (3M ESPE, St Paul, Minn)for 40 seconds (10 seconds on each side of the bracket).The teeth were then stored for 24 hours in distilled waterwith thymol in an incubator at 37�C.

The embedded teeth and ceramic bracket were posi-tioned in a universal testing machine (Instron, Canton,Mass) so that the bracket slot was parallel to the hori-zontal. The brackets were lased at this time accordingto the protocol of the group in which they were catego-rized (Table I). The laser was applied directly to thecenter of the Inspire ICE brackets and just superior tothe stainless steel slot of the Clarity brackets. A typeK thermocouple (Extech Instruments, Waltham, Mass)was placed in the pulp chamber, and temperatureswere recorded before and after lasing.

Immediately after lasing, a sharp chisel blade wasplaced at the bracket base-enamel interface, and, witha load cell of 1 kN and a crosshead speed of 0.5 mmper minute, the bracket was shear tested to failure.The force producing failure was recorded in newtonsand converted to force per unit area (megapascals) bydividing the measured force values by the mean surfacearea of the brackets. Immediately after debonding, thebracket and tooth were examined with a light micro-scope under 10 times magnification to determine theadhesive remnant index (ARI) score42: 0, all adhesivewas removed with the bracket; 1, adhesive remnantscovered less than 50% of the former bracket site; 2,adhesive remnants covered more than 50% of formerbracket site; and 3, adhesive remained on tooth surfacewith a clear imprint of the bracket base.

Statisical analysis

Statistical analysis of the data was then carried out,including calculating the mean SBS values, ARI scores,and standard deviations for each group. A 1-way analy-sis of variance (ANOVA) established whether therewere any differences in the mean SBS values for eachbracket type. If there were differences, the Tukey hon-estly significant difference test was done to establishwhich groups differed significantly. A chi-square testwas calculated to analyze the mean ARI score foreach bracket group. Signed rank tests were used to com-pare each bracket group’s temperature change againststandard increases of 1.8�C and 5.5�C.

RESULTS

The mean SBS values and standard deviations aregiven in Table II. The data were then used to comparethe means of the groups for each bracket type.

No statistically significant differences (P \0.05)were found when comparing the mean SBS values ofthe 3 groups of Clarity brackets. There was a statisticallysignificant difference in the mean SBS values of the 3groups of Inspire ICE brackets. The means of the 2 lasedgroups were statistically significantly lower (P \0.05)than those of the nonlased group. However, the differ-ence in the means of the lased groups was not statisti-cally significant (P \0.05; Fig 1).

There were no significant differences in the ARIscores of 3 groups when either the Clarity (chi-square,P 5 0.37) or the Inspire ICE (chi-square, P 5 0.79)brackets were tested. All groups in the study hadmean ARI scores of 3; this indicates that almost alladhesive was left behind on the tooth surfaces witha clear imprint of bracket base.

The mean increases in pulp chamber temperaturefor groups 3, 4, and 5 were statistically significantlyless (P \0.01) than the 5.5�C increase threshold andnot significantly different (P \0.01) from the 1.8�Cstandard. Group 6 had a mean pulp chamber increasesignificantly greater than the 1.8�C standard and notsignificantly different (P \0.01) from the 5.5�Cstandard (Fig 2).

Table II. Mean SBS values (MPa)

Clarity Inspire Ice

Group 1 Group 3 Group 5 Group 2 Group 4 Group 6

n 10 10 10 10 10 10

Mean 9.79 9.68 7.24 15.99 9.27 8.88

SD 3.20 2.63 3.60 4.91 3.67 3.04

Fig 1. Mean shear bond strengths of Clarity and Inspire ICE brackets.

460 Feldon et al American Journal of Orthodontics and Dentofacial Orthopedics

October 2010

DISCUSSION

Previous studies have shown that lasers can signifi-cantly reduce ceramic bracket debonding force. In thisstudy, we investigated the effects of using a diode laserfor debonding. The relatively small size, weight, powerrequirements, and lower cost make this laser a morepractical addition to an orthodontic practice.

The use of the diode laser was ineffective in signif-icantly lowering the required debonding force for thepolycrystalline brackets (Clarity). Two levels of lasingwere tested, and neither resulted in a significantly differ-ent debonding force when compared with the nonlasedcontrol.

The use of the diode laser was effective in signifi-cantly lowering the required debonding force whenmonocrystalline brackets (Inspire ICE) were tested.Both the 3-W and the 5-W per square centimeter laserprotocols yielded significantly lower debonding forcethan the nonlased control group. There was, however,no significant difference in debond force when the2 laser power levels were compared.

The significant reduction in the debonding forceof the monocrystalline brackets after lasing, but notof polycrystalline brackets, might be explained bytheir uniform crystal structure that enables high

transmissibility, thereby limiting energy loss. Further-more, the polycrystalline brackets used in this study(Clarity) have a stainless steel slot. This most likelyshielded the bracket base and the adhesive layer fromsome laser energy; it might have absorbed or reflectedthis energy away.

Tocchio et al36 described 3 methods of debonding:thermal softening, thermal ablation, and photo ablation.The diode laser debonding protocol we used did notproduce any of the explosive ‘‘blow offs,’’ noticeablecarbonization-like changes to the remnant resin, ordecomposition of the bracket base as reported byHayakawa41 when using an Nd: YAG laser. It appearsthat the effect of the diode laser was to provide thermalsoftening of the adhesive.

There were no significant ARI score differences be-tween any of the groups tested. Uniformly, they all hada mean ARI score of or close to 3. In almost all samples,all or most of the adhesive remained on the tooth, withan imprint of the bracket pad after debonding.

The mean temperature changes of the pulp chamberwalls of the laser groups were compared with the exper-imental results reported by Zach and Cohen.43 Theyfound no pulp damage with an intrapulpal temperatureincrease of 1.8�C when external heat was applied to

Fig 2. Mean temperature increases in groups 3 through 6.

American Journal of Orthodontics and Dentofacial Orthopedics Feldon et al 461Volume 138, Number 4

teeth. With an increase in pulpal temperature of 5.5�C,they found pulpal necrosis in 15% of teeth. Only group6 had a mean pulp chamber wall temperature increasethat was not significantly below 5.5�C (P \0.01). Theother laser groups had mean pulp chamber wall temper-ature increases that were significantly below thisthreshold (P \0.01).

We used a bovine tooth model. It is possible thata human tooth model would have yielded differentresults. In today’s world of conservative dental practice,obtaining large quantities of extracted human teeth ina suitable condition for bonding studies is difficult.Using different bonding agents than we used in thisstudy, Nakamichi et al44 found that the adhesion toenamel and the superficial layer of dentin showed nostatistically significant differences between human andbovine teeth.

Many different ceramic brackets are available today.We tested only 1 polycrystalline bracket and 1 mono-crystalline bracket. Further testing should be done todetermine whether there are differences among otherbrackets in these 2 groups.

Other variables that could be studied are the thick-ness of the bracket base and the thickness of the bracketitself. Brackets with larger profiles would be expected totransmit less laser energy to the bracket base and theunderlying layer of adhesive.

We used 2 laser outputs: 3 and 5 W per square cen-timeter for 3 seconds. Alternatively, a future study couldinvestigate the effect of varying the lasering time onSBS and keeping the output power constant. However,it can be expected that the heat generated in the pulpchamber would increase with the time of exposure tothe laser output. Another potential study would be toincrease the laser output power from those tested anddecrease the exposure time. In theory, the SBS valueswould be further reduced without an adverse effect onpulp chamber temperature.

CONCLUSIONS

1. Diode laser use significantly decreased the debond-ing force required for monocrystalline bracketswithout increasing the internal pulp chamber walltemperature significantly.

2. Diode laser use did not significantly decrease thedebonding force required for polycrystallinebrackets with stainless steel slots.

3. The use of the diode laser did not alter the amountof adhesive remaining on the tooth surface afterdebonding.

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