comparison of different bonding materials for laser debonding

Comparison of different bonding materials for laser debonding

Hiroshi Mimura, DDS, PhD, = Toshio Deguchi, DDS, MSD, PhD, b Akihiko Obata, DDS," Toshio Yamagishi, DDS, ° and Michio Ito, MS, DDSc ~ Nagano, Japan

The laser-aided removal of ceramic brackets from enamel surfaces was compared between two different adhesives. The selected bonding materials were Bis-GMA composite resin and 4-META MMA resin. Debonding forces were measured as the shear bond strength, perpendicular to the brackets. Debonding force, debonding time, total illuminated laser energy, and Modified Adhesive Remnant Index were discussed. Laser illumination was very effective for debonding in both resin groups. Enamel fracture was not observed in either laser illuminated groups, whereas two teeth were cracked in the Bis-GMA control group. For MMA resin, debonding force was sufficiently decreased at 3 watts output, whereas 7 watts output was needed for Bis-GMA resin samples. Total illuminated energies until the removal of the brackets were statistically lower in the MMA groups than in the Bis-GMA groups. Laser-focused adhesives tended to be removed with the brackets in the Bis-GMA groups, whereas they tended to remain on the tooth surface in the MMA groups. We concluded that debonding MMA resin with a laser is safer than debonding Bis-GMA resin with a laser. (AM J ORTHOD DENTOFAC ORTHOP 1995;108:267-73.)

1 4 a o r esthetic reasons, orthodontic treat- ment with translucent ceramic brackets is now popular. However, it is well-known that debonding ceramic brackets too often can cause enamel fracture and bracket failure. 1-3 Manufacturers changed the bonding mechanism between brackets and bonding material from a chemical bond to a mechanical one to decrease the adhesive strength. Nevertheless, some difficulties in the ceramic debonding remained. So, electrothermal debonding was developed. 4"5 Thereafter , Strobl et al. 6 origi- nated laser-aided debonding, a safe easy ceramic debonding method, and discussed the differences between COz and Y A G laser. Tocchio et al. 7 investigated the debonding mechanism of polycrystalline alumina and single crystal alumina brackets, and clarified the differences between these two brackets. However, no report explains the differences in the way laser debonding is affected by adhesives.

This study investigated the differences in the laser-aided debonding mechanism between M M A

From the Matsumoto Dental College. "Assistant Professor, Department of Orthodontics. bprofessor and Chairman, Department of Orthodontics. CAssistant Professor, Department of Biomaterials, Institute of Dental Science. dAssociate Professor, Department of Biomaterials, Institute of Dental Science. Copyright © 1995 by the American Association of Orthodontists. 0889-5406/95/$5.00 + 0 8/1/53962

resin and Bis-GMA resin. Two different bonding materials were selected for this study. Concise (3M, St. Paul, Minn.), the most popular in the U.S.A., is made from Bis-GMA and a quartz filler (77%). Super-bond (Sunmedical, Kyoto, Japan), the most familiar in Japan, contains no filler and is made of pure M M A containing 4-META.

MATERIALS AND METHODS Laser. The Laser selected for this study was a CO2

(10.6 i~m) Laser (LX-20, Luxar, Bothell, Wash.). The illuminating output was controlled at 3 watts and 7 watts.

Brackets. Throughout this study, we used the most popular laser-aided debonding resistant ceramic bracket, 7 polycrystalline alumina brackets (Transcend series 6000, Unitek/3M, Monrovia, Calif.). Maxillary premolar brackets were used.

Bonding materials. Two different bonding materials were selected for this study. One was a Bis-GMA composite resin, Concise (3M, St. Paul, Minn.), which has a quartz filler (77%). The other bonding material was 4META-MMA resin, Super-Bond (Sunmedical, Kyoto, Japan).

Preparation and bonding of the teeth. One hundred twenty-three maxillary premolars were stored in 70% ethanol. All teeth were implanted in resin blocks to easily measure the bond strength. Before bonding, the teeth were washed, scaled off calculus and soft tissue, polished, rinsed, and dried. According to each manufacturers instructions, the 63 teeth for Concise were etched with a 37% orthophosphoric acid solution for 1 minute, 8 and the other 60 teeth for Super-Bond were etched with

267

268 Mimura et al. American Journal of Orthodontics and Dentofacial Orthopedics September 1995

Fig. 1. Experimental set-up for measuring shear bond strength. Laser tip was placed just apart from bracket.

65% orthophosphoric acid for 45 seconds. 9 All teeth were rinsed with water for 20 seconds, and completely dried with an air-water syringe.

Concise is composed of a sealant resin and two pastes, and both sealant resin and pastes are catalyzed by equivalence mixing. Super-Bond is composed of polymer (PMMA), monomer (4-META + MMA), and catalyst (TBB), and instructions indicate one drop of catalyst is to be mixed with four drops of monomer, and then the brush-on technique is used. However, for accurate measurement, 0.07 gm of powder was mixed in the dispensing dish with the mixture of monomer and catalyst. ~

Then all bonded teeth were preserved in 37 ° C water for 24 hours. All these procedures were performed at a constant temperature and humidity (23 ° C, 50%).

Shear bond strength measurement. The teeth implanted resin blocks were secured on an Autograph (AG-5000D, Shimadzu, Kyoto, Japan) (Fig. 1). A ther- mally insulated blade (Teflon tape wrapped around the blades) touched the bracket, and an initial shear force (3 kgf) was applied to the brackets with this blade. The application of shearing force perpendicular to the bracket-enamel interface was the standard debonding technique used throughout this study. The laser was set to the desired intensity just apart from the bracket. The blade fell at a speed of 1 ram/rain, while the laser was simultaneously applied. When the force transmitted from the blade immediately decreased, the peak force was determined as the shear bond strength. Shear bond strength and debonding times were recorded, and the products of laser beam output (watts) and debonding times (seconds) were calculated to determine the level of laser energy applied (joules).

Modified Adhesive Remnant Index (MAR1) and damage observation. Bracket failure was defined as any break-

age of the bracket, partial or complete, in either the bracket base or the tie wings. The amount of residual adhesive was classified in compliance with a Modified Adhesive Remnant Index (MARl), which uses a scale of l to 4 to rate the amount of resin adhering to the enamel surface.

When a bond failure was detected at the enamel- adhesive interface, a stereoscope (X17.5) (SZH-111, Olympus, Tokyo, Japan) was used to assess the damage to the enamel surface.

Measurement of thermal expansion. To determine the debonding mechanism in laser debonding, thermal expansion properties of the brackets and adhesives were examined with a thermomechanical analyzer (TMA-50, Shimadzu, Kyoto, Japan) with a range from 50 ° C to 120 ° C. Thermal expansion of the bonding materials was measured in cylindrical blocks (4 mm in diameter, 8 mm in height, each five samples) of the bonding material.

RESULTS Control data (brackets debonded without laser).

F o r t y - t h r e e b o n d e d tee th were p r e p a r e d to conf i rm the efficiency of laser debonding . Twen ty - th ree t ee th were p r e p a r e d with Concise and the o the r 20 with Super -Bond .

E n a m e l f rac ture occu r red in two t ee th in the Concise group. C o m p l e t e b r acke t fa i lure at the slot was obse rved in a too th b o n d e d by Concise. The re - fore these t h ree t ee th were omi t t ed f rom the data . T h e da ta o b t a i n e d a re s u m m a r i z e d in Tab les I and II. The shea r b o n d s t rength of the two cont ro l samples was not significantly d i f ferent (Tab le II1). The M A R I scores a re ind ica t ed in Tab le IV. The two bond ing ma te r i a l s had s imilar adhesive rein-

American Journal of Orthodontics and Dentofacial Orthopedics M i m u r a et al. 269 Volume 108, No. 3

Table I. Shear bond strength (Kgf) needed for the bracket removal

n Mean SD n Mean SD

Super-Bond Conc~e

Laser output (watts)

0 (control) 20 12.49 3.97 20 14.81 3.91 3 20 3.63 0.41 20 5.85 1.41 7 20 3.41 0.15 20 4.01 0.38

SD = Standard deviation,

Table II. Measurement of debonding time (seconds)


Super-Bond

n Mean SD

Conc&e

n Mean SD

0 (control) 20 21.05 9.90 20 23.16 10.99 3 20 2.33 0.69 20 6.62 4.18 7 20 1.22 0.31 20 3.35 0.69

SD = Standard deviation.

Table I I I . Comparison of bond strength values between the different products and laser illumination by t test (level of significance p < 0.05)

Super-Bond Concise

Control 3 watts 7 watts Control 3 watts 7 watts

Super-Bond Control 3 watts p < 0.01 - 7 watts p < 0.01 p < 0.05

Concise Control ns p < 0.01 p < 0.01 - 3 watts p < 0.01 p < 0.01 p < 0.01 p < 0.01 - 7 watts p < 0.01 p < 0.01 p < 0.01 p < 0.01 p < 0.01

Table IV. Frequency distribution of various Modified Adhesive Remnant Index (MARI) scores

MAR1 scores 1 2 3 4 Total

Super-Bond Control 0/20 2/20 6/20 12/20 20 3 watts 0/20 1/20 6/20 13/20 20 7 watts 0/20 1/20 3/20 16/20 20

Concise Control 0/20 3/20 5/20 12/20 23* 3 watts 2/20 6/20 7/20 5/20 20 7 watts 0/20 6/20 9/20 5/20 20

*Two teeth were omitted because of the enamel fracture, and one tooth because of complete bracket failure. 1: All of the adhesive removed from enamel surfaces. 2: Less than half of adhesive remained. 3: More than half of adhesive remained. 4: All of the adhesive remained.


Table V. Comparison of debonding time between the various combinations of adhesives and laser setting by t test (level of significance p < 0.05).

Super-Bond

Control 3 watts

Super-Bond C o n t r o l

3 w a t t s p < 0.01 -

7 w a t t s p < 0.01 p < 0.01

Concise

7 watts Control I 3 watts 7 watts I

Concise C o n t r o l ns p < 0.01 p < 0.01 -

3 w a t t s p < 0.01 p < 0.01 p < 0.01 p < 0.01 -

7 w a t t s p < 0.01 p < 0.01 p < 0.01 p < 0.01 p < 0.01

Table VI. Calculated total illuminated energy (joules) needed for bracket removal


Super-Bond

n [ Mean SD

Concise

n Mean SD

3 20 6.98 2.08 20 19.85 12.55

7 20 8.54 2.16 20 23.49 4.83

S D = S t a n d a r d dev ia t ion .

Table VII. Comparison of total illuminated energy values between the various combinations of adhesives and laser settings by t test (level of significance p < 0.05)

Super-Bond Concise

3 watts 3 watts I 7 watts l

Super-Bond 3 w a t t s

7 w a t t s p < 0.05

7 watts

Concise 3 w a t t s p < 0.01 p < 0.01 -

7 w a t t s p < 0.01 p < 0.01 ns

nant properties, almost all resin remained on the tooth surface.

Experimental data (bracket debonded with the aid of a CO 2 laser). The illuminated laser energy was absorbed and converted to heat, and softened bonding material.

The debonding forces were significantly reduced in both adhesives illuminated by the CO 2 laser. Both illumination intensities (3 watts and 7 watts) were efficient, and in the Concise groups, the application of 7 watts made debonding easier than by using only 3 watts (p < 0.01) (Table III). In the Super-Bond group, there were significant differences (p < 0.05) between 3 watts and 7 watts. At 7 watts output, the Super-Bond group was much

easier to debond than the Concise group, and decreasing the output to 3 watts, increased this difference.

Debonding time data showed almost the same results in shearing forces. There was no significant difference in control groups, and debonding times were significantly reduced in both adhesives illuminated by the CO z laser (Tables II and V).

The applied laser energies were then compared. There was a marked difference between Concise and the Super-Bond groups. Among the four combinations of laser output and adhesives, 3 watts applied to the Super-Bond group allowed debonding with the minimum energy (Tables VI and VII).

There was no enamel fracture in either laser

American Journal of Orthodontics and Dentofacial Orthopedics Mimura et aL 271 Volume 108, No. 3

0.10

0 . 0 8

.~ 0 . 0 6 W C

X 0

0.04

E

I- 0 .02

. . . . . . . Transcend series 6000

.... ..... 4 4

t . j 1 I I I f I T I

50 60 70 80 90 100 110 120

Temperature °C

Fig. 2. Thermal expansion properties of ceramic bracket.

illuminated group, and a slight bracket failure at the tie wing was observed in only one tooth in the Concise-3 watts group. However, bond strength data were obtained from this sample.

Damage observations. During laser-aided debonding, there was no damage to tooth enamel. Laser illuminated adhesives showed a difference in remnant tendencies between Bis-GMA and MMA resin (Table IV). The laser-focused MMA samples tended to remain on the teeth surface, whereas the Bis-GMA samples tended to be partially removed with the brackets.

In the Super-Bond samples, much more adhesive remained in the high energy group. However, the laser-focused Concise group showed a similar pattern.

Thermal-expansion properties. Polycrystalline ceramic brackets (Transcend) showed almost linear expansion as the temperature rising (Fig. 2). The thermal expansion properties of the two adhesives were quite different. Concise samples also expanded almost linearly to the temperature increase, but the expansion was fourfold that of Transcend, whereas Super-Bond started to contract at higher temperatures (Fig. 3). Super-Bond expanded until 80 ° C, having peaked at 60 ° C. When the temperature increased above 80 ° C, Super-Bond began to contract.

DISCUSSION

The effects of laser-aided ceramic brackets debonding were investigated with the use of a CO 2 laser.

Enamel fracture and bracket failures. To ease the debonding of ceramic brackets, the bonding mechanism between the bracket and the bonding material

was changed from chemical bonding to mechanical interlocking. However, two control teeth in the Concise group had enamel fracture, whereas in the Super-Bond control, there was no enamel damage. In the Concise control group and the 3 watts laser illuminated Concise group, there were a complete bracket failure and a partial bracket failure, respec- tively. This indicated that laser output remains insufficient for the laser-aided debonding of the ceramic bracket when bonded by Concise. Concise includes more filler and is harder than Super-Bond. With Concise, instructions advise using 37% phos- phoric acid, which provides stronger etching. 1°'11 So, these differences may have affected the prob- ability of the enamel fracture.

Debonding force. Shear bond strength clearly decreased during the laser-aided debonding procedures. Among the various combinations, the 7 watts output applied to the Super-Bond group showed the lowest force level. In the Concise samples, the force reduction between the 7 watts group and the 3 watts group was obvious, whereas 3 watts illuminating the Super-Bond group was sufficient to weaken the bond strength. These differences are thought to be due to the compositions of the two materials. Concise includes considerable filler (77%), which affects its expansion properties. Su- per-Bond begins to contract above 80 ° C differing from the expansion profile of the polycrystalline alumina brackets.

Just after the laser debonding, illumination of the laser beam created hot brackets in every combination. Tocchio et al. 7 concluded the debonding mechanism of the polycrystalline alumina brackets was based on thermal softening of the resin adhesive, whereas that of monocrystalline brackets was


0.4

0.2

0 o ¢0

~. -0.2

0=

E -o.4

I-,

-0.1

-0. I

60 70

Temperature °C

. . . . . . . Concise

Super-Bond

. . . . - - ~ - ~ . . . .

o

910 1~o 1~o 1~o

Fig. 3. Thermal expansion properties of two different adhesives.

based on either thermal ablation or photoablation resulting from direct interaction of the light beam with resin. From our results of MARI, laser-illuminated Super-Bond tended to be apart from brackets and to remain on the teeth surface, and Concise tended to remove with brackets. These results indicates that the differences of the thermal expansion between MMA resin and ceramic brackets facilitate the debonding. Therefore, we conjecture that the MMA contraction from the brackets com- bined with the thermal softening of the resin formed the basis of the laser-aided debonding mechanism in Super-Bond.

Supplied laser energy. There was considerable difference between the Concise group and the Super-Bond group. The Concise group required much stronger laser energy than the' Super-Bond group, because of its resistance to the shearing force.

These differences also indicate that the most efficient laser output was 7 watts for Concise, and 3 watts for Super-Bond. Strobl et al. 6 reported that Transcend brackets illuminated by a CO2 laser beam at 14 watts for 2 seconds were easy to debond and calculated that the laser energy supplied (28 joules) was lower than the total energy of the electrothermal device (30 joules). However, our results indicate the combination of Transcend and Super-Bond can reduce illuminating laser power during debracketing. So this combination is safer

for laser-aided debonding than that of Transcend and Concise.

Thermal-expansion properties. After laser illumination, temperatures of the adhesive and brackets were raised and thermal softening of the adhesive is thought to be the main mechanism easing the debonding difficulties. At the higher temperature, Super-Bond began to contract, whereas Concise and Transcend were still expanding. However, under the critical temperature (80 ° C), the difference in the thermal expansion between Transcend and Super-Bond is quite large. This difference should affect the shear bond strength and the illuminated energy of both adhesives.

Although of the enamel surface might be ex- posed to a higher temperature, the pulp is pro- tected by enamel and dentin, and blood circulation is cooling in the pulp. They do have some heat resistance capacities. Temperature rise on the enamel surface and in the pulpal cavity after the laser irradiation for debonding is a project of further research.

C O N C L U S I O N

1. Laser-aided debonding was very efficient for debonding ceramic brackets.

2. No enamel fracture was observed in the laser-debonding groups, whereas two teeth of the Bis-GMA (Concise) control group were fractured at debonding.

American Journal of Orthodontics and Dentofacial Orthopedics Mimura et aL 273 Volume 108, No. 3

3. MMA (Super-Bond) resin was easier to debond with the laser beam than Bis-GMA (Concise). The applied energy was significantly lower in MMA (Super-Bond) than Bis-GMA (Concise).

4. Laser-focused adhesives tended to be removed with the bracket in the Bis-GMA (Concise) groups, whereas they tended to remain on the tooth surface in the MMA (Super-Bond) groups.

5. Ceramic brackets bonded with MMA (Su- per-Bond) resin can be removed with lower laser energy and lighter shearing force. We conclude that Super-Bond (4-META MMA) can be debonded more safely than Concise (Bis-GMA).

6. Further research is necessary to determine the effects of heat generated on the pulp.

REFERENCES

1. Joseph VP, Rossouw E. The shear bond strength of stainless steel and ceramic brackets used with chemically and light- activated composite resins. AM J ORTHOD DENTOFAC ORTHOP 1990;97:121-9.

2. Viazis AD, DeLong R, Bevis RR, Rudney JD, Pintado MR. Enamel abrasion from ceramic orthodontic brackets under an artificial oral environment. AM J ORTHOD DENTOFAC ORTHOP 1990;98:103-9.

3. Viazis AD, Cavanaugh G, Bevis RR. Bond strength of ceramic brackets under shear stress: an in vitro report. AM J ORTHOD DENTOFAC ORTHOP 1990;98:214-21.

4. Bishara SE, Trulove TS. Comparisons of different bonding techniques of ceramic brackets: an in vitro study. Part I. Background and methods. AM J ORTHOD DENTOFAC ORTHOP 1990;98:145-53.

5. Bishara SE, Trulove TS. Comparisons of different bonding techniques of ceramic brackets: an in vitro study. Part II. Findings and clinical implications. AM J ORTHOD DENTO- FAC ORTHOP 1990;98:263-73.

6. Strobl K, Bahns TL, Williams L, Bishara SE, Stwalley WC. Laser-aided debonding of orthodontic ceramic brackets. AM J ORTHOD DENTOFAC ORTHOP 1992;101:152-8.

7. Tocchio RM, Williams PT, Mayer FJ, Standing KG. Laser debonding of ceramic orthodontic brackets. AM J ORTHOD DENTOFAC ORTHOP 1993;102:155-62.

8. Concise Orthodontic Bonding System, Dental Products/3M. St. Paul, Minn: 3M Co.

9. Superbond C&B, Superbond D-Liner, their basis and clinical applications, Sunmedical. Kyoto, Japan: Sunmedical Co. LTD.

10. Nakagawa K. Studies on the direct bonding of the orthodontic resin bracket to the tooth enamel: part 2 the effect of pretreatment on the enamel surface. J Jpn Orthod Soc 1969;28:278-85.

11. Silverstone LM. Acid etch technique: the acid etch technique; in vitro studies with special reference to the enamel surface and the enamel resin interface, St Paul: North Central Publishing, 1975:13-39.

Reprint requests to: Dr. Hiroshi Mimura Department of Orthodontics Matsumoto Dental College 1780 Gohbara Hirooka Shiojiri Nagano, 39%07, Japan

comparison of different bonding materials for laser debonding

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