Ceramic bracket bonding: A comparison of bond strength with polyacrylic acid and phosphoric acid enamel conditioning A. J. Maskeroni, DDS,* Charles E. Meyers, Jr., DDS.,** and Lewis Lorton, DDS, MSD*** Fort Meade. Md.
The purpose of this study was to compare in vitro shear bond strength with three different enamel surface preparations: (1) 37% phosphoric acid etch, (2) sulfated polyacrylic acid etch with removal of crystals by vigorous rinsing and (3) polyacrylic acid etch with crystal growth. Forty extracted human premolar teeth were divided into four groups of ten. Ceramic brackets were bonded to teeth in each of three groups. The fourth group used was bonded with metal brackets and a phosphoric acid enamel preparation. The same lightly filled resin cement was used for all groups. A shearing force was applied to the teeth. The results demonstrated that the shear force needed to debond with ceramic brackets was 21% greater than the shear force with metal brackets. The polyacrylic acid crystal growth group had shear bond strength values approximately one half as great as the phosphoric acid etch group when ceramic brackets were used. Bond failures in the phosphoric acid etch group were at the bracket/resin interface with the bulk of the resin remaining on the tooth compared with the polyacrylic acid crystal 'growth group in which the bond failure was at the enamel resin interface. Enamel fractures were not found when healthy nonrestored teeth were subjected to the shearing force. In a preliminary test using phosphoric acid etch and teeth with compromised enamel (large restorations involving three or more surfaces), half of the teeth fractured during debonding. The study demonstrated that a polyacrylic acid conditioning of the enamel surface produces different retentive surfaces, depending on the presence or absence of crystal growth. The polyacrylic crystal growth reduced the strength of the bond to enamel by 50% when ceramic brackets were used. (AMJ ORTHOODENTOFACORTHOP1990;97:168-75.)
C r y s t a l bonding, a new alternative to direct bonding of brackets to enamel, has been recently reported.t'2 A sulfated polyacrylic acid solution is applied to the dental enamel, resulting in the growth of crystals from the enamel surface that forms a raised retentive surface to which brackets may be bonded. Smith and Cartz 3 were the first to describe the crystalline deposit formation on enamel when a polyacrylic acid solution With a sulfate ion was applied. The crystals were identified as CaSO4 • 2H20 (gypsum). Chemical analysis of Paper submitted to the Orthodontic Department, USA DENTAC, Fort Meade, Md., in partial fulfillment of the requirement for a certificate in orthodontics. Commercial materials and equipment are identified in this report to specify the investigative procedure. Such identification does not imply recommendation or endorsement or that the materials and equipment are necessarily the best available for the purpose. The views of the author(s) do not support or reflect the views of the Department of the Army/Navy or the Department of Defense. *Lieutenant Commander, Dental Corps., US Navy, US Army Orthodontic Residency Program. **Colonel, US Army Dental Corps; Director, Orthodontic Residency, US Army Orthodontic Residency Program. ***Colonel, US Army Dental Corps; Chief, Bioengineering Branch, US Army Institute of Dental Research. 8/1/i1275
168
the surface of the enamel revealed that calcium ions had been released into the polyacrylic acid and a chemical reaction with the sulfate ions had occurred at the enamel surface. There was a chemical bond between the enamel and the gypsum crystals. The calcium sulfate dihydrate crystals have been reported to be 20 i~m in length and 2 to 5 I.tm in thickness. D.C. Smith, who introduced the polyacrylic acid crystal growth technique, reported several advantages of the polyacrylic acid technique compared to conventional acid etching with phosphoric acid: (1) the enamel surface is not significantly damaged, (2) debonding and cleanup are easier, (3) there is minimal loss of the fluoride-rich outer enamel, and (4) few if any resin tags are left in enamel after debonding.l Maijer and Smith t reported an in vitro study demonstrating that the polyacrylic acid formed crystals that produced bond strength comparable to that of the conventional phosphoric acid etch. The tensile bond strength with polyacrylie acid was reported to be 62 kg/cmL In 1980-81, in abstracts of their ongoing research, Smith and Maijer 5"6 reported crystals with a longer shape when the sulfate ion concentration was
Voh~me 97 Number 2
changed. The longer crystals provided a better retentive surface into which an unfilled resin could flow. They reported tensile bond strength with the new formulation similar to that obtained from conventional phosphoric acid etch. Ceramic brackets are more rigid and do not flex or bend as do the metal brackets during removal. Because of the inability of the ceramic brackets to flex, higher forces may be required to remove the bracket during debonding. In addition, some ceramic brackets have a silane coupling agent on the base of the pad, which is intended to increase the bond strength of the resin to the bracket. Manufacturers have reported ceramic brackets to be stronger than the metal brackets. Scott 4 reported that this is misleading because the ceramic materials, while they are higher in strength, have a low fracture toughness. Lack of reported research on the new ceramic brackets leaves many questions unanswered. Are stronger forces required to remove ceramic brackets? At which interface will bond failure occur? Will the enamel frac=ture, or will tears be found in the enamel when the ~brackets are debonded? In a recent article, Swartz 7 aptly describes the problem: "A rigid, brittle ceramic bracket 15onded to a rigid, brittle enamel has little ability to absorb stresses. If the bracket to adhesive interface is too strong, then failure can only occur within the ceramic, within the adhesive, or within the enamel." In vitro bond studies indicate that the primary site of bond failures with phosphoric acid etch is at the bracket/resin interface, s~4 This is in contrast with the fracture site reported with the polyacrylic acid technique. Farquhar 2 reported the fracture site for the polyacrylic acid in vitro to be at the enamel/resin interface. Little resin or crystals remained on the surface of the tooth; the crystals and resin remained on the bracket. Burkey's thesis 15 reported that the polyacrylic acid crystal growth method resulted in the majority (90%) of the teeth having less than 50% of the resin remaining on the tooth. Because the polyacrylic acid crystal growth technique has been reported to produce slightly lower shear bond strength than conventional acid etching, the polyacrylic acid technique may be of value by reducing the bond strength at the enamel/resin interface when ceramic brackets are used. This may reduce the force required to remove the brackets, reduce the chances of enamel fracture, and still achieve adequate clinical bond strength. The purpose of this study was to evalute two techniques of enamel surface preparation with a polyacrylic acid solution and to compare the shear strength values with those of the conventional acid etch technique with
Polyacrylic acM bonding 169 ceramic brackets. Metal brackets were used for comparison. The enamel surface was evaluated after the ceramic brackets were debonded. MATERIALS AND METHODS
In a preliminary test, ten ceramic brackets were bonded with conventional acid etch to teeth with restorations and to teeth that were extracted for periodontal reasons. Even though these teeth were kept in tap water, 50% of them fractured during debonding. After that finding, only virgin, unrestored teeth were used for this study. Forty extracted human premolar teeth were stored in tap water. The teeth were randomly divided into four groups of 10. All teeth were cleaned with a flour of pumice slurry applied with a rubber polishing cup for I0 to 20 seconds. Ceramic brackets (Transcend Unitek Corp., Monrovia, Calif.) were bonded with three different enamel surface preparations: (I) phosphoric acid etch (Dynabond bonding system, Unitek), (2) polyacrylic acid etch (solution obtained from D.C. Smith, University of Toronto), (3) potyacrylic acid crystal growth (solution obtained from D.C. Smith). The fourth group consisted of metal brackets ("A" Company, San Diego, Calif.) bonded after tooth conditioning with a conventional acid etch with phosphoric acid. All four groups used the same resin cement bonding system (Dynabond). This system included a liquid 37% phosphoric acid, a liquid unfilled resin applied to the tooth, and a microfilled resin paste applied to the bracket. The bracket base area was similar for both the metal ("A" Company) and ceramic brackets (Transcend). After the teeth were bonded, they were mounted in plastic cylinders and placed in tap water at 37" C for 72 hours. Each bracket was subjected to a shearing force with the use of a United testing instrument with a cross head speed of 0.2 in/min. The force was applied in a vertical direction with an upward pull on the bracket after a wire was looped under the gingival wings of the bracket (Fig. 1). Teeth from each group were evaluated with a scanning electron microscope. The surface of the enamel was evaluated after phosphoric acid etching, polyacrylic acid etching, and polyacrylic acid crystal growth. Group 1 consisted of teeth with the conventional acid etch conditioning bonded with a ceramic bracket. A 37% liquid phosphoric acid was applied for 60 seconds with a cotton pellet. Each tooth was rinsed for 15 seconds with an air/water syringe and then dried with air for 10 to 15 seconds until a frosted white surface was obtained. Equal portions of unfilled resin base and catalyst were mixed and applied with a brush to the etched enamel surface. Equal portions of a filled resin
170
Maskeroni, Meyers, and Lorton
Am.J. Orthod.Oentofac.Orthop. FebruaO"1990
Fig. 1. United testing instrument with bonded sample.
Table !. Shear strength
Group 1 2 3 4
Method Acid PAA PAA Acid
etch etch crystals etch
Bracket
I
Ceramic Ceramic Ceramic Metal
MPa Mean(SD) 13.88 13.74 6.71 11.44
(+-- 2.03) (--- 3.53) ( ~ 1.86) (4- 2.27)
t
Fracturi site % Bracketl resin Enamell resin 100 50 23 100
0 50 77 0
I MPa = 145 psi = 1 4 . 3 k g / c m 2.
paste, base, and catalyst were mixed and applied to the ceramic bracket pad, which was then placed on the tooth with cotton forceps. Excess resin around the bracket was carefully removed with an explorer. The 10 brackets were bonded to the teeth within 5 to 10 minutes to simulate a clinical situation. The resin was allowed to set for 5 minutes, and the teeth were placed in tap water at 37 ° C for 72 hours before debonding. Group 2 teeth were etched with the polyacrylic acid. A solution of polyacrylic acid was applied with a brush to the cleaned enamel surface for 30 seconds. The tooth surface was washed with a vigorous air/water spray for 10 seconds in an effort to remove the crystals formed and produce a polyacrylic acid etched surface as described by Burkey. ~s The surface was then dried with air from an air/water syringe for I0 to 15 seconds. The same procedure for application of the resin and ceramic brackets was followed as in Group 1. Group 3 teeth were subjected to the crystal growth method. The teeth were treated exactly as those in group
2, except that the polyacrylic acid solution, after being applied for 30 seconds, was gently washed with water for 15 seconds; no air pressure was used during rinsing of the teeth. In an effort to enhance the growth of the crystals, once the polyacrylic acid solution was applied, no attempt was made to disturb the solution. Group 4 was treated by the same conventional acid etch method as group 1, except that metal brackets were bonded instead of the ceramic ones. RESULTS
The results of the shear bond strength test (Table I and Fig. 2) indicate that enamel preparation with phosphoric acid (group 1) is similar to the bond obtained when the polyacrylic acid solution is washed vigorously (group 2). The crystal growth (group 3) had shear strength values that were half as strong as those of the conventional acid etch with bonded ceramic brackets (group 1). The crystal bond with ceramic brackets (group 3) was 59% as strong as the bond with metal
Pol)'act3'lic acid bonding
Volume 97 Nttmber 2
171
20
15 O
I
I0
I 1
O
0 CRYSTALS CERAMIC
ACID ETCH METAL
post hoe
Bartlett
PAA ETCH CERAMIC Student Newman-Keuls
Test For Homogeneity
Approximate F = 1.418
Analysis Source
ACID ETCH CERAMIC
of Group Variances
DF = 3
Probability
= 4.460
= .236
of Variance
Sum of Squares DF Mean Square
Between Groups
150.196
3
50.065
Within Groups
103.098
f14
3.032
F 16.511
Probability .000
Fig. 2. Comparison of shear bond strength.
brackets (group 4). The ceramic bracket bond was 21% stronger than the metal bracket bond when conventional phosphoric acid was used (groups 1 and 4). The data were analyzed by means of simple oneway analysis of variance and a Student-Newman-Keuls post hoc test. The ceramic brackets used in this study demonstrated low breakage. Only 2 of 30 ceramic brackets fractured within the ceramic during debonding. Representative scanning electron micrographs are shown in Figs. 3, 4, and 5. An enamel sample etched with polyacrylie acid and gently washed (group 3) is shown in Fig. 3. The gypsum crystals shown in Fig. 3 are 10 to 20 I.tm long and 2 to 3 I-tm thick. The crystals are seen projecting in many different directions and are dense, covering the prepared enamel surface. There are areas that show a spherilitic growth, which seems to begin from a common nucleation point. The enamel beneath the crystals appears mildly etched. The enamel after etching and vigorous washing is shown in Fig. 4. The roughened surface of the enamel appears mildly etched. Some areas show linear patterns 10 to 20 Ixm long, as though the crystals were once embedded in the enamel.
Fig. 5, A shows a micr0graph of the enamel surface after the ceramic bracket has been removed from the crystal growth group (group 3). The enamel appears to have short crystals 2 to 3 I.tm in length remaining attached to the surface after debonding; little resin is seen on the surface of the tooth. The micrograph in Fig. 5, B shows the bracket pad after debonding. Resin is interspersed between the crystals. The crystals are approximately I0 ttm long. DISCUSSION
This research evaluated the bonds of ceramic brackets to enamel, hypothesizing that the bond strength would be high, in part because of the rigidity of the ceramic material. The results indicate that the removal of ceramic brackets will require more force when a conventional phosphoric acid etch is used. Removal of the metal brackets is actually less damaging to the enamel because the fracture site is at the bracket/resin interface. A technique that increases the adhesion of bracket to resin and displaces the fracture site to the enamel/resin interface could potentially cause more enamel fracture. The polyacrylic acid crystal growth proved to be a method of reducing the bond strength
172
M a s k e r o n i , M e y e r s , a m l Lorton
Am. J. Ortho
The purpose of this study was to compare in vitro shear bond strength with three different enamel surface preparations: (1) 37% phosphoric acid etch, (2) sulfated polyacrylic acid etch with removal of crystals by vigorous rinsing and (3) polyacrylic acid etch with crystal growth. Forty extracted human premolar teeth were divided into four groups of ten. Ceramic brackets were bonded to teeth in each of three groups. The fourth group used was bonded with metal brackets and a phosphoric acid enamel preparation. The same lightly filled resin cement was used for all groups. A shearing force was applied to the teeth. The results demonstrated that the shear force needed to debond with ceramic brackets was 21% greater than the shear force with metal brackets. The polyacrylic acid crystal growth group had shear bond strength values approximately one half as great as the phosphoric acid etch group when ceramic brackets were used. Bond failures in the phosphoric acid etch group were at the bracket/resin interface with the bulk of the resin remaining on the tooth compared with the polyacrylic acid crystal 'growth group in which the bond failure was at the enamel resin interface. Enamel fractures were not found when healthy nonrestored teeth were subjected to the shearing force. In a preliminary test using phosphoric acid etch and teeth with compromised enamel (large restorations involving three or more surfaces), half of the teeth fractured during debonding. The study demonstrated that a polyacrylic acid conditioning of the enamel surface produces different retentive surfaces, depending on the presence or absence of crystal growth. The polyacrylic crystal growth reduced the strength of the bond to enamel by 50% when ceramic brackets were used. (AMJ ORTHOODENTOFACORTHOP1990;97:168-75.)
C r y s t a l bonding, a new alternative to direct bonding of brackets to enamel, has been recently reported.t'2 A sulfated polyacrylic acid solution is applied to the dental enamel, resulting in the growth of crystals from the enamel surface that forms a raised retentive surface to which brackets may be bonded. Smith and Cartz 3 were the first to describe the crystalline deposit formation on enamel when a polyacrylic acid solution With a sulfate ion was applied. The crystals were identified as CaSO4 • 2H20 (gypsum). Chemical analysis of Paper submitted to the Orthodontic Department, USA DENTAC, Fort Meade, Md., in partial fulfillment of the requirement for a certificate in orthodontics. Commercial materials and equipment are identified in this report to specify the investigative procedure. Such identification does not imply recommendation or endorsement or that the materials and equipment are necessarily the best available for the purpose. The views of the author(s) do not support or reflect the views of the Department of the Army/Navy or the Department of Defense. *Lieutenant Commander, Dental Corps., US Navy, US Army Orthodontic Residency Program. **Colonel, US Army Dental Corps; Director, Orthodontic Residency, US Army Orthodontic Residency Program. ***Colonel, US Army Dental Corps; Chief, Bioengineering Branch, US Army Institute of Dental Research. 8/1/i1275
168
the surface of the enamel revealed that calcium ions had been released into the polyacrylic acid and a chemical reaction with the sulfate ions had occurred at the enamel surface. There was a chemical bond between the enamel and the gypsum crystals. The calcium sulfate dihydrate crystals have been reported to be 20 i~m in length and 2 to 5 I.tm in thickness. D.C. Smith, who introduced the polyacrylic acid crystal growth technique, reported several advantages of the polyacrylic acid technique compared to conventional acid etching with phosphoric acid: (1) the enamel surface is not significantly damaged, (2) debonding and cleanup are easier, (3) there is minimal loss of the fluoride-rich outer enamel, and (4) few if any resin tags are left in enamel after debonding.l Maijer and Smith t reported an in vitro study demonstrating that the polyacrylic acid formed crystals that produced bond strength comparable to that of the conventional phosphoric acid etch. The tensile bond strength with polyacrylie acid was reported to be 62 kg/cmL In 1980-81, in abstracts of their ongoing research, Smith and Maijer 5"6 reported crystals with a longer shape when the sulfate ion concentration was
Voh~me 97 Number 2
changed. The longer crystals provided a better retentive surface into which an unfilled resin could flow. They reported tensile bond strength with the new formulation similar to that obtained from conventional phosphoric acid etch. Ceramic brackets are more rigid and do not flex or bend as do the metal brackets during removal. Because of the inability of the ceramic brackets to flex, higher forces may be required to remove the bracket during debonding. In addition, some ceramic brackets have a silane coupling agent on the base of the pad, which is intended to increase the bond strength of the resin to the bracket. Manufacturers have reported ceramic brackets to be stronger than the metal brackets. Scott 4 reported that this is misleading because the ceramic materials, while they are higher in strength, have a low fracture toughness. Lack of reported research on the new ceramic brackets leaves many questions unanswered. Are stronger forces required to remove ceramic brackets? At which interface will bond failure occur? Will the enamel frac=ture, or will tears be found in the enamel when the ~brackets are debonded? In a recent article, Swartz 7 aptly describes the problem: "A rigid, brittle ceramic bracket 15onded to a rigid, brittle enamel has little ability to absorb stresses. If the bracket to adhesive interface is too strong, then failure can only occur within the ceramic, within the adhesive, or within the enamel." In vitro bond studies indicate that the primary site of bond failures with phosphoric acid etch is at the bracket/resin interface, s~4 This is in contrast with the fracture site reported with the polyacrylic acid technique. Farquhar 2 reported the fracture site for the polyacrylic acid in vitro to be at the enamel/resin interface. Little resin or crystals remained on the surface of the tooth; the crystals and resin remained on the bracket. Burkey's thesis 15 reported that the polyacrylic acid crystal growth method resulted in the majority (90%) of the teeth having less than 50% of the resin remaining on the tooth. Because the polyacrylic acid crystal growth technique has been reported to produce slightly lower shear bond strength than conventional acid etching, the polyacrylic acid technique may be of value by reducing the bond strength at the enamel/resin interface when ceramic brackets are used. This may reduce the force required to remove the brackets, reduce the chances of enamel fracture, and still achieve adequate clinical bond strength. The purpose of this study was to evalute two techniques of enamel surface preparation with a polyacrylic acid solution and to compare the shear strength values with those of the conventional acid etch technique with
Polyacrylic acM bonding 169 ceramic brackets. Metal brackets were used for comparison. The enamel surface was evaluated after the ceramic brackets were debonded. MATERIALS AND METHODS
In a preliminary test, ten ceramic brackets were bonded with conventional acid etch to teeth with restorations and to teeth that were extracted for periodontal reasons. Even though these teeth were kept in tap water, 50% of them fractured during debonding. After that finding, only virgin, unrestored teeth were used for this study. Forty extracted human premolar teeth were stored in tap water. The teeth were randomly divided into four groups of 10. All teeth were cleaned with a flour of pumice slurry applied with a rubber polishing cup for I0 to 20 seconds. Ceramic brackets (Transcend Unitek Corp., Monrovia, Calif.) were bonded with three different enamel surface preparations: (I) phosphoric acid etch (Dynabond bonding system, Unitek), (2) polyacrylic acid etch (solution obtained from D.C. Smith, University of Toronto), (3) potyacrylic acid crystal growth (solution obtained from D.C. Smith). The fourth group consisted of metal brackets ("A" Company, San Diego, Calif.) bonded after tooth conditioning with a conventional acid etch with phosphoric acid. All four groups used the same resin cement bonding system (Dynabond). This system included a liquid 37% phosphoric acid, a liquid unfilled resin applied to the tooth, and a microfilled resin paste applied to the bracket. The bracket base area was similar for both the metal ("A" Company) and ceramic brackets (Transcend). After the teeth were bonded, they were mounted in plastic cylinders and placed in tap water at 37" C for 72 hours. Each bracket was subjected to a shearing force with the use of a United testing instrument with a cross head speed of 0.2 in/min. The force was applied in a vertical direction with an upward pull on the bracket after a wire was looped under the gingival wings of the bracket (Fig. 1). Teeth from each group were evaluated with a scanning electron microscope. The surface of the enamel was evaluated after phosphoric acid etching, polyacrylic acid etching, and polyacrylic acid crystal growth. Group 1 consisted of teeth with the conventional acid etch conditioning bonded with a ceramic bracket. A 37% liquid phosphoric acid was applied for 60 seconds with a cotton pellet. Each tooth was rinsed for 15 seconds with an air/water syringe and then dried with air for 10 to 15 seconds until a frosted white surface was obtained. Equal portions of unfilled resin base and catalyst were mixed and applied with a brush to the etched enamel surface. Equal portions of a filled resin
170
Maskeroni, Meyers, and Lorton
Am.J. Orthod.Oentofac.Orthop. FebruaO"1990
Fig. 1. United testing instrument with bonded sample.
Table !. Shear strength
Group 1 2 3 4
Method Acid PAA PAA Acid
etch etch crystals etch
Bracket
I
Ceramic Ceramic Ceramic Metal
MPa Mean(SD) 13.88 13.74 6.71 11.44
(+-- 2.03) (--- 3.53) ( ~ 1.86) (4- 2.27)
t
Fracturi site % Bracketl resin Enamell resin 100 50 23 100
0 50 77 0
I MPa = 145 psi = 1 4 . 3 k g / c m 2.
paste, base, and catalyst were mixed and applied to the ceramic bracket pad, which was then placed on the tooth with cotton forceps. Excess resin around the bracket was carefully removed with an explorer. The 10 brackets were bonded to the teeth within 5 to 10 minutes to simulate a clinical situation. The resin was allowed to set for 5 minutes, and the teeth were placed in tap water at 37 ° C for 72 hours before debonding. Group 2 teeth were etched with the polyacrylic acid. A solution of polyacrylic acid was applied with a brush to the cleaned enamel surface for 30 seconds. The tooth surface was washed with a vigorous air/water spray for 10 seconds in an effort to remove the crystals formed and produce a polyacrylic acid etched surface as described by Burkey. ~s The surface was then dried with air from an air/water syringe for I0 to 15 seconds. The same procedure for application of the resin and ceramic brackets was followed as in Group 1. Group 3 teeth were subjected to the crystal growth method. The teeth were treated exactly as those in group
2, except that the polyacrylic acid solution, after being applied for 30 seconds, was gently washed with water for 15 seconds; no air pressure was used during rinsing of the teeth. In an effort to enhance the growth of the crystals, once the polyacrylic acid solution was applied, no attempt was made to disturb the solution. Group 4 was treated by the same conventional acid etch method as group 1, except that metal brackets were bonded instead of the ceramic ones. RESULTS
The results of the shear bond strength test (Table I and Fig. 2) indicate that enamel preparation with phosphoric acid (group 1) is similar to the bond obtained when the polyacrylic acid solution is washed vigorously (group 2). The crystal growth (group 3) had shear strength values that were half as strong as those of the conventional acid etch with bonded ceramic brackets (group 1). The crystal bond with ceramic brackets (group 3) was 59% as strong as the bond with metal
Pol)'act3'lic acid bonding
Volume 97 Nttmber 2
171
20
15 O
I
I0
I 1
O
0 CRYSTALS CERAMIC
ACID ETCH METAL
post hoe
Bartlett
PAA ETCH CERAMIC Student Newman-Keuls
Test For Homogeneity
Approximate F = 1.418
Analysis Source
ACID ETCH CERAMIC
of Group Variances
DF = 3
Probability
= 4.460
= .236
of Variance
Sum of Squares DF Mean Square
Between Groups
150.196
3
50.065
Within Groups
103.098
f14
3.032
F 16.511
Probability .000
Fig. 2. Comparison of shear bond strength.
brackets (group 4). The ceramic bracket bond was 21% stronger than the metal bracket bond when conventional phosphoric acid was used (groups 1 and 4). The data were analyzed by means of simple oneway analysis of variance and a Student-Newman-Keuls post hoc test. The ceramic brackets used in this study demonstrated low breakage. Only 2 of 30 ceramic brackets fractured within the ceramic during debonding. Representative scanning electron micrographs are shown in Figs. 3, 4, and 5. An enamel sample etched with polyacrylie acid and gently washed (group 3) is shown in Fig. 3. The gypsum crystals shown in Fig. 3 are 10 to 20 I.tm long and 2 to 3 I-tm thick. The crystals are seen projecting in many different directions and are dense, covering the prepared enamel surface. There are areas that show a spherilitic growth, which seems to begin from a common nucleation point. The enamel beneath the crystals appears mildly etched. The enamel after etching and vigorous washing is shown in Fig. 4. The roughened surface of the enamel appears mildly etched. Some areas show linear patterns 10 to 20 Ixm long, as though the crystals were once embedded in the enamel.
Fig. 5, A shows a micr0graph of the enamel surface after the ceramic bracket has been removed from the crystal growth group (group 3). The enamel appears to have short crystals 2 to 3 I.tm in length remaining attached to the surface after debonding; little resin is seen on the surface of the tooth. The micrograph in Fig. 5, B shows the bracket pad after debonding. Resin is interspersed between the crystals. The crystals are approximately I0 ttm long. DISCUSSION
This research evaluated the bonds of ceramic brackets to enamel, hypothesizing that the bond strength would be high, in part because of the rigidity of the ceramic material. The results indicate that the removal of ceramic brackets will require more force when a conventional phosphoric acid etch is used. Removal of the metal brackets is actually less damaging to the enamel because the fracture site is at the bracket/resin interface. A technique that increases the adhesion of bracket to resin and displaces the fracture site to the enamel/resin interface could potentially cause more enamel fracture. The polyacrylic acid crystal growth proved to be a method of reducing the bond strength
172
M a s k e r o n i , M e y e r s , a m l Lorton
Am. J. Ortho
