) 1991 European Orthodontics Societ;
European Journal of Orthodontics 13 (1991) 187-191
Glass ionomer cements used as bonding materials for metal orthodontic brackets. An in vitro study J. 0. 0en*, N. R. Gjerdet**, and P. J. Wisth* 'Department of Orthodontics and Facial Orthopedics, and "Department of Dental Materials, School of Dentistry, University of Bergen, Bergen, Norway
The forces required to debond orthodontic brackets on human teeth were studied in vitro. The brackets were bonded with different glass ionomer cements (GICs), and a composite resin (Concise®). The shear force required to remove the brackets was recorded at different time intervals after bonding. The bond strength was considerably lower for the GICs compared with brackets bonded with the composite resin, at all time intervals that were studied. Moreover, the bond strength increased more slowly for the GICs compared with the resin. Different GICs displayed a varying setting time. The bonding strength of GICs increased more than 50 per cent between 10 and 20 minutes setting time. SUMMARY
Bonding of orthodontic brackets, using composite resin on acid-etched enamel as introduced by Newman (1965), was a major advance. However, a disadvantage when using composite as bonding material, is the occasional occurrence of decalcification of the enamel, caused by plaque accumulation along the brackets, specially buccogingivally (Gorelick et al., 1982; 0gaard, 1989; Sonis and Snell, 1989). Mechanical damage of the enamel during debonding and removal of resin remnants has also been reported (Farquhar, 1986). Glass ionomer cements (GICs), bond chemically to both enamel and dentine without etching (Hotz et al., 1977; Aboush and Jenkins, 1986; Walls, 1986). Moreover, they release fluorides (Forsten, 1977; Swartz et al., 1984; Valk and Davidson, 1987). GICs have so far been little used for bracket bonding (White, 1986; Valk and Davidson, 1987). Bands cemented with GIC show equal or better overall failure rates than those cemented with polycarboxylate cement (Mizrahi, 1988), or zinc phosphate cement (Fricker and McLachlan, 1987; Maijer and Smith, 1988; Norris et al., 1986; Seeholzer and Dasch, 1988). Recently, some in vitro studies on bracket bonding with GICs have been presented (Cook and Youngson, 1988; Klockowski et al., 1989). Present-day GICs contain tartaric acid to
improve setting properties and, thereby, make the GICs more clinically applicable. The watermixed GICs also have improved handling properties (McLean, 1988). Dual setting GICs with an initial light activation have also been introduced. These improvements have made the products more attractive for use as bonding materials for orthodontic brackets. The aim of this investigation was to measure the bond strength at different time intervals of orthodontic brackets bonded using different GICs and a commonly used composite resin material. Materials and methods Specimens Non-carious premolars extracted for orthodontic reasons and stored in a 2 per cent buffered formaldehyde solution for up to 1 year were immersed in water for at least 1 week before being used in the experiment. Newly extracted teeth were stored in 0.9 per cent NaCl solution, refrigerated, and used for testing of potential influence of storing method. Before bonding, all buccal surfaces of the teeth were polished for 20 seconds with White-Bristle Brushes® soaked in a slurry of pumice and water. The tooth surfaces were rinsed and the teeth were stored in water at 20°C. The roots were prepared for proper embedding in autopolymerizing acrylic resin. The buccal surface of
Downloaded from by guest on June 25, 2015
Introduction
188
J. O. 0EN ET AL.
BRACKET
TOOTH
1 mm
used. Vitrabond® was cured, after mixing, by exposure to light (Visilux 2®, Dental Products/3M). Excess material was removed prior to placing the light source at the occlusal, cervical, distal, and buccal margins of the bracket base for 30 seconds each site. Brackets were attached with the slot 4 mm from the incisal tip of the buccal cusp, and the base perpendicularly to the long axis of the crown. For all teeth bonded with GICs a varnish was applied 5 minutes after bonding of the bracket. The teeth in the 24 hours and 4 months experiments were kept dry for up to 15 minutes, and were then placed in deionized and distilled water and stored at 37°C until the scheduled testing time. Shear strength testing testing was done in a mechanical testing machine (Instron 1193, High Wycombe, UK) with a wire attached to the movable part of the machine. The blocks with teeth were fixed to the base of the tester and a round stainless steel 0.4 mm (0.016 inch) diameter wire was passed through the bracket slot to form a loop. The two ends of the wire were gripped in pneumatic action chucks (Fig. 1). A cross-head speed of 1 mm/minute was used. The shear force was recorded and the load (N) at bond failure was recorded. The site of bond failure was assessed by using the adhesive remnant index (ARI, Artun and Bergland, 1984). The handling of the materials and the mechanical testing took place at 21 ±2°C.
Y/////////////////.The MOVING CROSS-HEAD
Figure 1 Specimen in testing machine
Bonding The composite bonding material tested was Concise®. It is a two-paste heavily filled autopolymerizing composite resin. The Concise® mixture used in the experiment was paste A mixed with 15 drops of Resin A and Paste B mixed with 15 drops of resin B to get a consistency suitable for bonding. Four different GICs were tested, representing Type I (for cementation), Type II (for fillings), and Type III (for lining/basing). One of the Type III GICs was dual-setting using initial light activation (Vitrabond®, Table 1). Pre-angulated stainless steel maxillary premolar brackets (DynaLock®, Unitek/3M) were used according to the Bergen technique (Wisth et at., 1985). The base area was approximately 0.126 cm 2 . The teeth to be bonded with Concise were etched with an etching gel (Scotchbond® phosphoric gel, Dental Products/3 M) according to the manufacturers' instructions, rinsed with water, and dried. Concise was mixed separately for each attachment, using a measuring device (Smile-O-Meter®, Kerr) to ensure a constant mixing ratio. The GIC products were handled according to the manufacturers' instructions. For BaseLine® the medium consistency was
Statistics The results were analysed by the Minitab Statistical Computing System. Mean values and standard deviations for each group were computed. Statistical comparison of the different groups was made by the Mann-Whitney test. Results Shear bond strength, after a setting time of 24 hours, both for brackets bonded with Concise® and AquaCem®, showed no statistical differences (^ = 0.81 and P=1.00) when comparing newly extracted teeth stored in saline and teeth stored in formaldehyde solution. Concise® attained the highest mean shear bond strength at all setting intervals (Fig. 2). After 5 minutes the mean bond strength was
Downloaded from by guest on June 25, 2015
each tooth was placed perpendicularly to the base of the acrylic block (Fig. 1).
189
BONDING STRENGTH OF GLASS IONOMERS
Table 1 Bonding products used in this investigation. Product
Mode of activation
Code
Manufacturer
Batch no.
AquaCem® (Type I)
powder/water
AC
De Trey Dentsply Konstanz, Germany
880944
Baseline® (Type III)
powder/water
BL
De Trey Dentsply Konstanz, Germany
8808104
Ketac-Fil® (Type II)
capsulated
CF
Espe GmbH, Seefeld, Germany
0016
Vitrabond® (Type III) —powder —liquid
light cured
VI
3M Dental Products Division St. Paul, MN, USA 7512 P 7512 L
c
Concise® (composite filling material) —Paste A —Paste B Bond System (unfilled resin) —Resin A —Resin B
7AJ2 7AK1
s=507
100 -
50-
0
J
5min
10min
20min
4 mo
Figure 2 Bonding strength (in Newtons, N) of GICs at different time intervals after bonding
69.9 N and increased to 95.4 N after 10 minutes (Fig. 2). The percentage increase in bond strength was then rather low both up to 24 hours and 4 months after bonding. For the GICs the initial mean shear bond strength at 10 minutes was about one-third of that attained by Concise®. Ketac-Fil® and Vitrabond® attained the highest shear bond strength of 31.4 N and 31.2 N at a setting time of 10 minutes (Fig. 2).
Among the GICs at 20 minutes the mean value was lowest for Vitrabond® (36.9 N) and highest for BaseLine® (49.1 N) (Fig. 2). At a setting time of 24 hours the highest average strength was obtained by Ketac-Fil® (60.4 N). The average shear bond strength for the glassionomer group at 24 hours was 55.4 N. This value is 42 per cent lower compared to the value for Concise® at the same time interval. In the 4-month experiment the shear bond strength values had increased for the GICs. The highest mean value of 63.8 N was found for Ketac-Fil®, about 12 per cent higher than the mean value found for AquaCem®. For Concise the bond failures occurred at the enamel-resin interface at all time intervals. For the GICs all failures occurred cohesively in the cement after 10 minutes. After 20 minutes more failures occurred at the GlC-enamel interface. In the 24-hour and 4-month series separation took place mainly at the GlC-enamel interface.
Discussion The storing method of teeth before bonding did not influence the bond strength. Thus, it is justified to use newly extracted teeth and teeth
Downloaded from by guest on June 25, 2015
P880420 P880504
1S0 -i •Concise HAquaCemHKetacFil •BaselineBVitrabond
3M Dental Products Division St. Paul, MN, USA
190
application at different margins is time consuming. One problem when following the manufacturer's instruction was the initial low viscosity for most of the GICs. The manufacturer of BaseLine® recommends more powder if a thicker consistency is desired. The consistency of the capsulated material (Ketac-Fil®) appears to be clinically acceptable. The strengths of the GICs were always lower than those of the composite resin. However, the scatter in the 4-month results indicates that the resin exhibits greater variations in bond strength than do the GICs, in particular the capsulated product. This may imply that the hydrophilic nature of GICs makes the bond more predictable than that of the resin, even under in vitro conditions. The use of GIC implies new clinical procedures for the orthodontist during the bracket bonding procedure. It is easier and less time consuming to prepare the teeth for bonding, but after the bracket is placed it takes more time to keep the area dry from saliva until the varnish can be applied. The findings also indicate that orthodontic wires should not be placed until 20 minutes after bonding, or preferably even 24 hours after bonding, in order to have a sufficient clinical strength. A favourable characteristic of the GICs is the fluoride release which may prevent white spot formation along the bracket base. This has been reported to be a problem in some patients (Gorelick et al, 1982; 0gaard, 1989). Furthermore, it is easier to clean up the enamel after debonding when GIC is used as a bonding material (Farquhar, 1986; White, 1986), and remnants are easily detected. The findings of this study indicate that a somewhat higher bracket failure rate may be expected during clinical treatment when GICs are used. However, with proper handling of the products and more patience before loading the brackets with wires, the present findings are promising for clinical use of the GICs. Address for correspondence
Professor Per Johan Wisth Department of Orthodontics Orthopedics School of Dentistry Arstadveien 17 N-5009 Bergen Norway
and
Facial
Downloaded from by guest on June 25, 2015
stored in formaldehyde solution for testing of both resin and glass ionomer bonding materials. The loading during testing subjects the bonding material to shear forces along the tooth surface as well as tensile forces at the gingival aspect of the bracket base. It is likely that this loading pattern resembles that of a clinical situation, although the loading rate is different. The results are presented in force units as it has been shown that calculation of apparent stress in bonding strength tests may result in misleading values (van Noort et al., 1989). It is known that the diluted composite resin filling material used here forms strong bonds (Pender et al., 1988; Schulz et al., 1985) and is used extensively in clinical practice (Schulz et al., 1985; Oliver and Howe, 1989). For metal brackets it has been shown that a highly filled bonding diacrylate cement exhibits higher tensile bond strength compared with several other resin systems (Buzzitta et al., 1982; Schulz et al., 1985). In this study the resin exhibited a rapid setting rate, being stronger than any of the GICs at 10 minutes after mixing. Most of the failures occurred in the resin/bracket interface. Under clinical conditions, failures at the resin/enamel interface might be more common due to difficulties related to moisture contamination. The strength developed considerably slower for the GICs compared with that of the resin, making it impossible to measure strength at 5 minutes. After 20 minutes the GIC strength had reached about 80 per cent of the values at 24 hours. The bonding strength measured in the present study was in agreement with an in vitro study on Ketac-Cem® as a bonding material for brackets (Cook and Youngson, 1988). A rapidsetting GIC exhibited a bond strength after 15 minutes of more than 80 per cent of its 24-hour value (Aboush and Jenkins, 1986). At 10 minutes setting time all bond failures occurred cohesively in the GIC materials, indicating that a setting time of at least 20 minutes is needed for GICs. The capsulated glass ionomer cement (Ketac-Fil®) attained higher bonding strength than the other GICs after 10 minutes. One reason for this may be that the rise in temperature noted after the mechanical mixing accelerates the setting reaction. The capsulated product was convenient to handle and to apply to the bracket base. The light activated GIC obtained strength similarly to the capsulated product at 10 minutes. However, the light
J. O. 0EN ET AL.
BONDING STRENGTH OF GLASS IONOMERS
References
Newman G V 1965 Epoxy adhesives for orthodontic attachments: Progress report. American Journal of Orthodontics 51: 901-912 Norris S D, Mclnnes-Ledoux P, Schwaninger B, Weinberg R 1986 Retention of orthodontic bands with new fluoride-releasing cements. American Journal of Orthodontics 89: 206-211 0gaard B 1989 Prevalence of white spot lesions in 19-yearolds: A study on untreated and orthodontically treated persons 5 years after treatment. American Journal of Orthodontics and Dentofacial Orthopedics 96: 423-427 Oliver R G, Howe G S 1989 Scanning electron microscope appearance of the enamel/composite/bracket areas using different methods of surface enamel treatment, composite mix, and bracket loading. British Journal of Orthodontics 16: 39-46 PenderN, Dresner E, Wilson S, Vowles R 1988 Shear strength of orthodontic bonding agents. European Journal of Orthodontics 10: 374-379 Schulz R P, Mayhew R B, Oesterle L J, Pierson W P 1985 Bond strengths of three resin systems used with brackets and embedded wire attachments. American Journal of Orthodontics 87: 75-80 Seeholzer H W, Dasch W 1988 Bonding with a glass ionomer cement. Journal of Clinical Orthodontics 12: 165-169 Sonis A L, Snell W 1989 An evaluation of a fluoridereleasing, visible light-activated bonding system for orthodontic bracket placement. American Journal of Orthodontics and Dentofacial Orthopedics 95: 306-311 Swartz M L, Phillips R W, Clark HE 1984 Long-term F release from glass ionomer cements. Journal of Dental Research 63: 158-160 Valk J W P, Davidson C L 1987 The relevance of controlled fluoride release with bonded orthodontic appliances. Journal of Dentistry 15: 257-260 Van Noort R, Noroozi S, Howard I C, Cardew G 1989 A critique of bond strength measurements. Journal of Dentistry 17: 61-67 Walls AWG 1986 Glass polyalkenoate (glass-ionomer) cements: a review. Journal of Dentistry 14: 231-246 White L W 1986 Glass ionomer cement. Journal of Clinical Orthodontics 20: 387-391 Wisth PJ, Dathkunakorn S, Pattarawadee K 1985 Introduction to the Bergen technique. Department of Orthodontics and Facial Orthopedics, University of Bergen, Norway. ISBN 82-7249-042-0
Downloaded from by guest on June 25, 2015
Aboush Y E Y, Jenkins C B G 1986 An evaluation of the bonding of glass-ionomer restoratives to dentine and enamel. British Dental Journal 161: 179-184 Artun J, Bergland S 1984 Clinical trials with crystal growth conditioning as an alternative to acid-etch enamel pretreatment. American Journal of Orthodontics 85: 333-340 Buzzitta J V A, Hallgren S E, Powers J M 1982 Bond strength of orthodontic direct-bonding cement-bracket system as studied in vitro. American Journal of Orthodontics 81: 87-92 Cook PA, Youngson CC 1988 An in vitro study of the bond strength of a glass ionomer cement in the direct bonding of orthodontic brackets. British Journal of Orthodontics 15: 247-253 Farquhar R B 1986 Direct bonding comparing a polyacrylic acid and a phosphoric acid technique. American Journal of Orthodontics and Dentofacial Orthopedics 90: 187-194 Forsten L 1977 Fluoride release from a glass ionomer cement. Scandinavian Journal of Dental Research 85: 503-504 Fricker J P, McLachlan M D 1987 Clinical studies on glass ionomer cements. Part 2-A two year clinical study comparing glass ionomer cement with zinc phosphate cement. Australian Orthodontic Journal 10: 12-14 Gorelick L, Geiger A M, Gwinnett A J 1982 Incidence of white spot formation after bonding and banding. American Journal of Orthodontics 81: 93-98 Hotz P, McLean J W, Seed I, Wilson A D 1977 The bonding of glass ionomer cements to metal and tooth substance. British Dental Journal 142: 41-47 Klockowski R, Davis E L, Joynt R B, Wieczkowski G, MacDonald A 1989 Bond strength and durability of glass ionomer cements used as bonding agents in the placement of orthodontic brackets. American Journal of Orthodontics and Dentofacial Orthopedics 96: 60-64 Maijer R, Smith DC 1988 A comparison between zinc phosphate and glass ionomer cement in orthodontics. American Journal of Orthodontics and Dentofacial Orthopedics 93: 273-279 McLean J W 1988 Glass-ionomer cements. British Dental Journal 164: 293-300 Mizrahi E 1988 Glass ionomer cements in orthodontics— An update. American Journal of Orthodontics and Dentofacial Orthopedics 93: 505-507
191
European Journal of Orthodontics 13 (1991) 187-191
Glass ionomer cements used as bonding materials for metal orthodontic brackets. An in vitro study J. 0. 0en*, N. R. Gjerdet**, and P. J. Wisth* 'Department of Orthodontics and Facial Orthopedics, and "Department of Dental Materials, School of Dentistry, University of Bergen, Bergen, Norway
The forces required to debond orthodontic brackets on human teeth were studied in vitro. The brackets were bonded with different glass ionomer cements (GICs), and a composite resin (Concise®). The shear force required to remove the brackets was recorded at different time intervals after bonding. The bond strength was considerably lower for the GICs compared with brackets bonded with the composite resin, at all time intervals that were studied. Moreover, the bond strength increased more slowly for the GICs compared with the resin. Different GICs displayed a varying setting time. The bonding strength of GICs increased more than 50 per cent between 10 and 20 minutes setting time. SUMMARY
Bonding of orthodontic brackets, using composite resin on acid-etched enamel as introduced by Newman (1965), was a major advance. However, a disadvantage when using composite as bonding material, is the occasional occurrence of decalcification of the enamel, caused by plaque accumulation along the brackets, specially buccogingivally (Gorelick et al., 1982; 0gaard, 1989; Sonis and Snell, 1989). Mechanical damage of the enamel during debonding and removal of resin remnants has also been reported (Farquhar, 1986). Glass ionomer cements (GICs), bond chemically to both enamel and dentine without etching (Hotz et al., 1977; Aboush and Jenkins, 1986; Walls, 1986). Moreover, they release fluorides (Forsten, 1977; Swartz et al., 1984; Valk and Davidson, 1987). GICs have so far been little used for bracket bonding (White, 1986; Valk and Davidson, 1987). Bands cemented with GIC show equal or better overall failure rates than those cemented with polycarboxylate cement (Mizrahi, 1988), or zinc phosphate cement (Fricker and McLachlan, 1987; Maijer and Smith, 1988; Norris et al., 1986; Seeholzer and Dasch, 1988). Recently, some in vitro studies on bracket bonding with GICs have been presented (Cook and Youngson, 1988; Klockowski et al., 1989). Present-day GICs contain tartaric acid to
improve setting properties and, thereby, make the GICs more clinically applicable. The watermixed GICs also have improved handling properties (McLean, 1988). Dual setting GICs with an initial light activation have also been introduced. These improvements have made the products more attractive for use as bonding materials for orthodontic brackets. The aim of this investigation was to measure the bond strength at different time intervals of orthodontic brackets bonded using different GICs and a commonly used composite resin material. Materials and methods Specimens Non-carious premolars extracted for orthodontic reasons and stored in a 2 per cent buffered formaldehyde solution for up to 1 year were immersed in water for at least 1 week before being used in the experiment. Newly extracted teeth were stored in 0.9 per cent NaCl solution, refrigerated, and used for testing of potential influence of storing method. Before bonding, all buccal surfaces of the teeth were polished for 20 seconds with White-Bristle Brushes® soaked in a slurry of pumice and water. The tooth surfaces were rinsed and the teeth were stored in water at 20°C. The roots were prepared for proper embedding in autopolymerizing acrylic resin. The buccal surface of
Downloaded from by guest on June 25, 2015
Introduction
188
J. O. 0EN ET AL.
BRACKET
TOOTH
1 mm
used. Vitrabond® was cured, after mixing, by exposure to light (Visilux 2®, Dental Products/3M). Excess material was removed prior to placing the light source at the occlusal, cervical, distal, and buccal margins of the bracket base for 30 seconds each site. Brackets were attached with the slot 4 mm from the incisal tip of the buccal cusp, and the base perpendicularly to the long axis of the crown. For all teeth bonded with GICs a varnish was applied 5 minutes after bonding of the bracket. The teeth in the 24 hours and 4 months experiments were kept dry for up to 15 minutes, and were then placed in deionized and distilled water and stored at 37°C until the scheduled testing time. Shear strength testing testing was done in a mechanical testing machine (Instron 1193, High Wycombe, UK) with a wire attached to the movable part of the machine. The blocks with teeth were fixed to the base of the tester and a round stainless steel 0.4 mm (0.016 inch) diameter wire was passed through the bracket slot to form a loop. The two ends of the wire were gripped in pneumatic action chucks (Fig. 1). A cross-head speed of 1 mm/minute was used. The shear force was recorded and the load (N) at bond failure was recorded. The site of bond failure was assessed by using the adhesive remnant index (ARI, Artun and Bergland, 1984). The handling of the materials and the mechanical testing took place at 21 ±2°C.
Y/////////////////.The MOVING CROSS-HEAD
Figure 1 Specimen in testing machine
Bonding The composite bonding material tested was Concise®. It is a two-paste heavily filled autopolymerizing composite resin. The Concise® mixture used in the experiment was paste A mixed with 15 drops of Resin A and Paste B mixed with 15 drops of resin B to get a consistency suitable for bonding. Four different GICs were tested, representing Type I (for cementation), Type II (for fillings), and Type III (for lining/basing). One of the Type III GICs was dual-setting using initial light activation (Vitrabond®, Table 1). Pre-angulated stainless steel maxillary premolar brackets (DynaLock®, Unitek/3M) were used according to the Bergen technique (Wisth et at., 1985). The base area was approximately 0.126 cm 2 . The teeth to be bonded with Concise were etched with an etching gel (Scotchbond® phosphoric gel, Dental Products/3 M) according to the manufacturers' instructions, rinsed with water, and dried. Concise was mixed separately for each attachment, using a measuring device (Smile-O-Meter®, Kerr) to ensure a constant mixing ratio. The GIC products were handled according to the manufacturers' instructions. For BaseLine® the medium consistency was
Statistics The results were analysed by the Minitab Statistical Computing System. Mean values and standard deviations for each group were computed. Statistical comparison of the different groups was made by the Mann-Whitney test. Results Shear bond strength, after a setting time of 24 hours, both for brackets bonded with Concise® and AquaCem®, showed no statistical differences (^ = 0.81 and P=1.00) when comparing newly extracted teeth stored in saline and teeth stored in formaldehyde solution. Concise® attained the highest mean shear bond strength at all setting intervals (Fig. 2). After 5 minutes the mean bond strength was
Downloaded from by guest on June 25, 2015
each tooth was placed perpendicularly to the base of the acrylic block (Fig. 1).
189
BONDING STRENGTH OF GLASS IONOMERS
Table 1 Bonding products used in this investigation. Product
Mode of activation
Code
Manufacturer
Batch no.
AquaCem® (Type I)
powder/water
AC
De Trey Dentsply Konstanz, Germany
880944
Baseline® (Type III)
powder/water
BL
De Trey Dentsply Konstanz, Germany
8808104
Ketac-Fil® (Type II)
capsulated
CF
Espe GmbH, Seefeld, Germany
0016
Vitrabond® (Type III) —powder —liquid
light cured
VI
3M Dental Products Division St. Paul, MN, USA 7512 P 7512 L
c
Concise® (composite filling material) —Paste A —Paste B Bond System (unfilled resin) —Resin A —Resin B
7AJ2 7AK1
s=507
100 -
50-
0
J
5min
10min
20min
4 mo
Figure 2 Bonding strength (in Newtons, N) of GICs at different time intervals after bonding
69.9 N and increased to 95.4 N after 10 minutes (Fig. 2). The percentage increase in bond strength was then rather low both up to 24 hours and 4 months after bonding. For the GICs the initial mean shear bond strength at 10 minutes was about one-third of that attained by Concise®. Ketac-Fil® and Vitrabond® attained the highest shear bond strength of 31.4 N and 31.2 N at a setting time of 10 minutes (Fig. 2).
Among the GICs at 20 minutes the mean value was lowest for Vitrabond® (36.9 N) and highest for BaseLine® (49.1 N) (Fig. 2). At a setting time of 24 hours the highest average strength was obtained by Ketac-Fil® (60.4 N). The average shear bond strength for the glassionomer group at 24 hours was 55.4 N. This value is 42 per cent lower compared to the value for Concise® at the same time interval. In the 4-month experiment the shear bond strength values had increased for the GICs. The highest mean value of 63.8 N was found for Ketac-Fil®, about 12 per cent higher than the mean value found for AquaCem®. For Concise the bond failures occurred at the enamel-resin interface at all time intervals. For the GICs all failures occurred cohesively in the cement after 10 minutes. After 20 minutes more failures occurred at the GlC-enamel interface. In the 24-hour and 4-month series separation took place mainly at the GlC-enamel interface.
Discussion The storing method of teeth before bonding did not influence the bond strength. Thus, it is justified to use newly extracted teeth and teeth
Downloaded from by guest on June 25, 2015
P880420 P880504
1S0 -i •Concise HAquaCemHKetacFil •BaselineBVitrabond
3M Dental Products Division St. Paul, MN, USA
190
application at different margins is time consuming. One problem when following the manufacturer's instruction was the initial low viscosity for most of the GICs. The manufacturer of BaseLine® recommends more powder if a thicker consistency is desired. The consistency of the capsulated material (Ketac-Fil®) appears to be clinically acceptable. The strengths of the GICs were always lower than those of the composite resin. However, the scatter in the 4-month results indicates that the resin exhibits greater variations in bond strength than do the GICs, in particular the capsulated product. This may imply that the hydrophilic nature of GICs makes the bond more predictable than that of the resin, even under in vitro conditions. The use of GIC implies new clinical procedures for the orthodontist during the bracket bonding procedure. It is easier and less time consuming to prepare the teeth for bonding, but after the bracket is placed it takes more time to keep the area dry from saliva until the varnish can be applied. The findings also indicate that orthodontic wires should not be placed until 20 minutes after bonding, or preferably even 24 hours after bonding, in order to have a sufficient clinical strength. A favourable characteristic of the GICs is the fluoride release which may prevent white spot formation along the bracket base. This has been reported to be a problem in some patients (Gorelick et al, 1982; 0gaard, 1989). Furthermore, it is easier to clean up the enamel after debonding when GIC is used as a bonding material (Farquhar, 1986; White, 1986), and remnants are easily detected. The findings of this study indicate that a somewhat higher bracket failure rate may be expected during clinical treatment when GICs are used. However, with proper handling of the products and more patience before loading the brackets with wires, the present findings are promising for clinical use of the GICs. Address for correspondence
Professor Per Johan Wisth Department of Orthodontics Orthopedics School of Dentistry Arstadveien 17 N-5009 Bergen Norway
and
Facial
Downloaded from by guest on June 25, 2015
stored in formaldehyde solution for testing of both resin and glass ionomer bonding materials. The loading during testing subjects the bonding material to shear forces along the tooth surface as well as tensile forces at the gingival aspect of the bracket base. It is likely that this loading pattern resembles that of a clinical situation, although the loading rate is different. The results are presented in force units as it has been shown that calculation of apparent stress in bonding strength tests may result in misleading values (van Noort et al., 1989). It is known that the diluted composite resin filling material used here forms strong bonds (Pender et al., 1988; Schulz et al., 1985) and is used extensively in clinical practice (Schulz et al., 1985; Oliver and Howe, 1989). For metal brackets it has been shown that a highly filled bonding diacrylate cement exhibits higher tensile bond strength compared with several other resin systems (Buzzitta et al., 1982; Schulz et al., 1985). In this study the resin exhibited a rapid setting rate, being stronger than any of the GICs at 10 minutes after mixing. Most of the failures occurred in the resin/bracket interface. Under clinical conditions, failures at the resin/enamel interface might be more common due to difficulties related to moisture contamination. The strength developed considerably slower for the GICs compared with that of the resin, making it impossible to measure strength at 5 minutes. After 20 minutes the GIC strength had reached about 80 per cent of the values at 24 hours. The bonding strength measured in the present study was in agreement with an in vitro study on Ketac-Cem® as a bonding material for brackets (Cook and Youngson, 1988). A rapidsetting GIC exhibited a bond strength after 15 minutes of more than 80 per cent of its 24-hour value (Aboush and Jenkins, 1986). At 10 minutes setting time all bond failures occurred cohesively in the GIC materials, indicating that a setting time of at least 20 minutes is needed for GICs. The capsulated glass ionomer cement (Ketac-Fil®) attained higher bonding strength than the other GICs after 10 minutes. One reason for this may be that the rise in temperature noted after the mechanical mixing accelerates the setting reaction. The capsulated product was convenient to handle and to apply to the bracket base. The light activated GIC obtained strength similarly to the capsulated product at 10 minutes. However, the light
J. O. 0EN ET AL.
BONDING STRENGTH OF GLASS IONOMERS
References
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