Tensile bond force of glass ionomer cements in direct bonding of orthodontic brackets: An in vitro comparative study F. Rezk-Lega and B. Ogaard Oslo, Norway Tensile bond force of three glass ionomers was evaluated in vitro. Ketac-Cem and Aqua-Cem, two conventional cements, and light-cured Vitrabond were used in this study. The results were then compared with the values obtained for a composite resin (Concise) by means of the Mann-Whitney two-sample rank test adjusted for ties. The composite resin had a significantly higher bond force (152.5 N) than any of the other adhesives (5.5 to 27.53 N) used. Tensile bond strength was also calculated and the failure bond site investigated on the enamel surface was evaluated. The composite resin and the two conventional glass ionomers used had high cement percentages (66% tO 62%) adhering to the enamel surface. Cement remaining on enamel was lower (20%) for the light-cured glass ionomer. It was concluded that the in vitro bond force of Vitrabond might be adequate for orthodontic bracket bonding. (AMJ ORTHODDENTOFACORTHOP1991 ;100:357-61.)
T h e direct bracket-bonding technique is a standard procedure in treatment with fixed orthodontic appliances. By acid etching technique, composite resins have been widely used in dentistry. Direct bonding of orthodontic brackets with this technique has been used for the last two decades.~ Because of their high bond strength, composites are able to withstand normal oral forces exerted during mastication and traction forces generated by orthodontic appliances. However, pretreatment of the enamel surface with phosphoric acid is required to achieve a satisfactory bond. Several studies have shown enamel loss by acid etching. 25 Another disadvantage of composite resins involves their removal after debonding. Grinding off the adhesive from the tooth surface may lead to enamel alterations: Furthermore, enamel demineralization around brackets and beneath ill-fitting bands represents a serious clinical problem. T M Visible white spot lesions have been observed within 4 weeks in the absence of any fluoride r e g i m e n : Enamel decalcification is attributed to the extended accumulation and retention of bacterial plaque on the enamel surface adjacent to the bands and brackets, t4 In 1972 glass ionomer cements were introduced as dental cements by Wilson and Kent.~5 Since then glass ionomer cements have received a great deal of attention because of the advantages provided to clinical dentistry.
From the Department of Orthodontics, Dental Faculty, University of Oslo. 811125450
Glass ionomers adhere to enamel and dentin as well as to some metals) 6 They have also shown adequate biocompatibility) 7 More recently, they have been used in orthodontics for cementation of bands and bonding of brackets) m3"24 Acid etching of the enamel surface is unnecessary and mineral loss is practically avoided. Another attractive property of these cements is their long-term fluoride release, which is of interest for fixed orthodontic appliance therapy) 822 Few investigations have been performed to shed light on the bond strength of these cements in direct orthodontic bracket bonding. In 1988 Cook and Youngson 23 compared, in vitro, the shear/peel bond strength of a glass ionomer cement with a composite resin. In 1990 Bowser Fajen et a l : ~ evaluated, in vitro, the tensile bond strength of three glass ionomers. A significantly higher bond strength was found for the composite resin. The aim of this study was to compare bond forces of several glass ionomer cements with a commonly used composite resin by means of a tensile test method applied in vitro. In addition, the bond strength and the bond failure site were evaluated.
MATERIALS AND METHODS Forty sound premolars extracted for orthodontic purposes were used in this investigation. Before storage, the teeth were thoroughly washed in running water and all blood and adherent tissue were removed. Immediately afterward, the teeth were stored in distilled water in a refrigerator at 4° C. The 357
358
Rezk-Lega atzd Ogaard
Am. J. Orthod. Oentofiw. Orthop. October 1991
Fig. 1. Bracket design along with sotdered stainless steel ring.
storage medium was replaced periodically to minimize deterioration. The crowns were separated from the roots with a cooled diamond disk in a machine designed for the cutting of hard tissue. The pulp chamber was then enlarged to secure a retention in the holder. Each crown was lightly polished with pumice in a rubber cup, sprayed with water, and dried with a compressed oil-frce airstream for about 15 seconds. Subsequently, the crowns were mounted in plastic molds filled with Epofix (resin and hardener, 15 ml/2 ml, Struers, Copenhagen, Denmark) up to their facial surface, which was left intact. After fixation in the molds, test specimens were transferred to a chamber at 37 ° C and 50% relative humidity for 24 hours to allow hardening of the resin. The next day the specimens were carefully withdrawn from their molds, surplus material around the edge was removed with a scalpel, and cementation of brackets was performed according to the manufacturer's instructions. The bracket type used in this study was a GAC edgewise wide twin bracket (torque, 0; reference No. K232CS22). The bracket base area was 15.75 mm-'. A stainless steel circular ring was soldered at the bracket slot to provide a grip for the hook and allow better control of the type of stress and its distribution along the bond plane (Fig. 1). The premolars were divided into three groups, each con-
Fig. 2. Mounting system on Instron machine. sisting of ten teeth. The different adhesives used for bracket bonding are presented in Table I. Bonding procedures were performed on each tooth by one of the following three methods:
Group 1. Concise composite resin (No. 1994 A and B). The buccal surface of'each tooth was etched for 60 seconds with a semiliquid gel containing 40% phosphoric acid, rinsed with copious amounts of water, and dried in an oil-free airstream. The 15-drop Concise paste/paste system recommended by/krtun and Zachrisson-'~was used in the study. The orthodontic bracket was then bonded with a thin cement film that respected the tensile test alignment block. Group 2. Ketac-Ccm (K-C)/Aqua-Cem (A-C) glass ionomer cements. Before bonding, premolars in this group (10 for each cement), were treated with ChemFil 1I tooth cleanser (De Trey) for '10 seconds. They were then rinsed and dried in an oil-free airstream. Brackets were bonded with either K-C or A-C in a slightly thicker mix than recommended by the manufacturer (double cream consistency). This made bracket seating easier and allowed quicker setting.
Volume tO0 Number4 Group 3. Vitrabond glass ionomer cement. This is a lightcured glass ionomer. The powder component is primarily a radiopaque, ion-leachable fluoroaluminosilicateglass powder made photosensitive by its unique chemistry. The liquid component consists of a light-curable polymer, water, 2hydroxyethylmethacrylate (HEMAL and a photosensitizer. For this group, no pretreatment other than prophylaxis was carried out. The teeth were rinsed and dried as in the other groups. The brackets were cemented as recommended by the manufacturer, thus requiring light activation for polymerization. Each bracket side was exposed for 30 seconds to light from a Visilux 2 light curing unit. No tooth cleanser was used. Tensile bond force All bonding procedures were performed by the same operator (R. L. F.) according to a standardized technique. Each crown was checked to make sure that all excess adhesive was removed with a probe from around the entire periphery of the bracket so that none ovcrlapped the sides of the latter. After initial setting or curing of the different adhesives, the specimens were transferred again to a water bath at 37° C for further storage (24 hours). Specimens were then removed from the water container. They were securely clamped in their appropriate place on the lnstron materialstesting machine (Model 1121 - Serial No. H1875, Instron Ltd. Coronation Bucks, Great Britain HP12 3SY). A stainless steel hook (0.8 mm thick) was then engaged to the circular ring soldered onto the bracket slot. The other end of the hook was tightly screwed to the Instron machine (Fig. 2). The force (in newtons) required to dislodge the bracket was measured at a crosshead speed of 1 mm/minute. Debonding was performed at 23° ± 2° C and 50% ± 5% relative humidity.
Bond failure site The bond failure site was measured as the percentage of adhesive material adhering onto the tooth surface (tracing). This was made possible by a Wild Photomacroscope M-400 (Wildheerbrugg Ltd., CH 9435 Heerbrugg, Switzerland). The latter instrument was connected to a Wild Photoautomat MPS 45. To create a marked contrast between the cement and the enamel surface, specimens were illuminated from two directions by means of a Volpi-Intralux unit 250 HL; Swiss-made, Volpi AG, CH 8902, Urdorf, Zurich). The measurement area was magnified up to 24 times. The image obtained was marked off in a standard grid fashion. The latter consisted of I00 sections of equal size. Those sections with cement residue were identified and a percentage ratio was determined.
RESULTS Tensile bond force Mean tensile bond values (force and strength), along with the corresponding standard deviations, are shown in Table II. Two brackets bonded with Aqua-Cem and
Tensile bond force of glass ionomer cements 359 T a b l e I. Adhesive materials used, manufacturer,
type, and powder/liquid ratio
Adhesive]Manufacturer I
Type
Powder/liquid ratio
Concise
3M
Composite resin
Vitrabond
3M
Ketac-Cem
ESPE
Aqua-Cem
DeTrey Dentsply
Light-cured glass ionomer Conventional Double cream glass ioconsistency homer Conventional Double cream glass ioconsistency nomer
15-drop paste/paste system* 1.4 wt/1.0 wt
*A variation of working and setting times has been used to obtain optimal consistency.~
Table II. Mean tensile bond scores (force and strength) along with the respective standard deviations
Tensile bondforce (IV) Adhesives
Mean
Concise Vitrabond Ketac-Cem Aqua-Cem
152.5 27.53 10.75 5.5
[
SD 51.99 12.63 13.65 8.25
Tensile bond strength* (Nlmm") Mean ] SD 9.68 1.74
0.68 0.34
3.3 0.8 0.86 0.52
*The bond strength was calculatedby dividing the measured forces by the area of bonding (here the bracket area). Bracket area = 15.75 mmL
one bonded with Ketac-Cem broke off on early debonding. The Mann-Whitney two-sample rank test adjusted for ties was used to compare the different adhesives. The significance probability (p) was calculated for t w o sided tests, and no corrections were made for multiple tests on the same data set. When Vitrabond was compared with Ketac-Cem and Aqua-Cem, light-cured Vitrabond had significantly higher bond values than the two conventional glass ionomers with p = 0.003 for K-C and p = 0.04 for A-C. The results also show a clearly significant difference in the tensile bond recordings between the composite resin and Vitrabond (p = 0.0002) in favor of the resin. No significant differences were found between A-C and K-C (p > 0.05).
360
Rezk-Lega and
Am. J. Orthod. Dentofac. Orthop. October 1991
,,(~"aar~/ ,"6
Table III. Mean percentage of bond-failure sites
along with the respective standard deviations Cement percentage adhering to tooth surface Adhesives
Mean
Concise Vitrabond Ketac-Cem Aqua-Cem
85 20 86 62
I
SD 38.2 34.6 39.4 37.9
The bracket area --- 100%.
Bond-failure site
The bond-failure site was recorded in percent, where the bracket base area delimited on the buccal surface represented 100% (Table III). A high percentage o f Ketac-Cem (86%), Concise (85%), and Aqua-Cem (62%) was found adhering to the tooth surface after debonding. This was not the case with the light-cured glass ionomer (Vitrabond); only 20% residual adhesive was detected after debonding. DISCUSSION
The recorded data obtained from the present in vitro study clearly show a significantly higher tensile bond force of the composite resin when compared to the glass ionomer cements. These findings are in agreement with those of Cook and Youngson 23 and Bowser Fajen et al. 2~ Among the glass ionomers tested, Vitrabond showed a significantly higher bond force. This may be due to the onset of polymerization of the light-cured glass ionomer allowing total penetration of the latter to the retentive areas if present. Another reason could be the presence of less air-inclusion areas and, consequently, fewer areas of stress concentration than with chemically cured materials. 26 In the oral cavity, bonded brackets are subject to shear, tensile, and torsion forces. The difficulty of measuring these forces has made it difficult to quantify them precisely. According to Newmann': and Wheeler and Ackerman, 28 orthodontic forces do not surpass 4.45 Newtons (N) per tooth. In another study conducted by Reynolds, 29 these orthodontic forces are unlikely to exceed 14.7 N. Furthermore, the maximum load per tooth probably occurring in a clinical situation is 17.8 N. 27 On the other hand, several other studies indicate that a maximum bond force of 35.6 N 3° or 97.88 N 3~ is adequate for orthodontic brackets and appliances. This wide range of values is probably the result of the variations in the experimental design. Furthermore, in most
situations, the stress on the bond can be defined as either tensile, shear, torsion, or a combination of all these. There are no specific in vitro or in vivo tests that can be valid for all o f the various clinical applications o f adhesive materials. The mean bond force values obtained from the present study allow us to presume that Vitrabond (27.5 N) may be an adequate adhesive for bonding of orthodontic brackets. In addition, maximum and minimum bond values obtained for each cement are also given to represent the best and worst performances that can be expected under clinical conditions. For Concise (212 103 N), Vitra bond 46.5 - 11 N), Ketac-Cem (38 - 3 N), and finally Aqua-Cem (26 - 5 N), in spite of the very high standard deviation, the significant tests give rise to valid conclusions. On the other hand, nonsignificant test results could be due to a combination of small samples and high variation in measurements. A point that may be clinically relevant is that only 20% of Vitrabond was still adherent to the tooth surface. This is an advantage since minimum effort and. enamel surface alteration are achieved during debonding and polishing procedures. In conclusion, the glass ionomers used in the present study had a significantly lower tensile bond force than the composite resin. On the other hand, whether the strongest glass ionomer (Vitrabond) with an average tensile force of 27 N would be adequate in the mouth is quite debatable. Therefore, further clinical investigations are required. We thank Mrs. Ellen Austrheim and Mr. Ketil Kvam at the Scandinavian Institute of Dental Materials, Oslo, Norway, for their generous assistance and collaboration. We also express our gratitude to Dr. Marek ROsier for the statistical calculations. REFERENCES I. Retief DH, Dreyer CJ. Epoxy resins for bonding orthodontic attachments to teeth. J Dent Assoc S Aft 1967;22:338-46. 2. GwinettAJ, Matsui A. A study of enamel adhesives: the physical relation between enamel and adhesive. Arch Oral Biol 1967; 12:1615-20. 3. Fitzpatrick DA, Way DC. The effects of wear, acid etching, and bond removal on human enamel. AM J ORTHOO1977;72:67181. 4. Brown CRL, Way DC. Enamel loss during orthodontic bonding and subsequent loss during removal of filled and unfilled adhesives. AM J ORTHOD1978;74:663-71. 5. Diedrich P. Enaffael alterations from bracket bonding and debonding: a study with the scanning electron microscope. AM J ORTHOD1981,;79:500-22. 6. ZachrissonBU, Artun J. Enamel surfaceappearanceafter various debonding techniques. AM J ORTtIOD1979;75:121-37. 7. ElgaardB, Rolla G, ArendsJ. Orthodonticappliancesand enamel demineralization. Part 1. Lesion development. AM J ORTIIOD DErrrOFACOR'rHoP1988;94:68-73.
Volume 100 Number 4 8. Norris S, Mclnnes-Ledoux P, Schwaninger B, Weinberg R. Retention of orthodontic bands with new fluoride-releasing cements. AM J OR'moP 1986;89:206-10. 9. Gorelick L, Geiger AM, Gwinnett AJ. Incidence of white spot formation after bonding and banding. AM J ORTItOD 1982; 8 i :323-3 I. 10. Ogaard B. Prevalence of white spot lesions in 19-year-olds: a study on untreated and orthodontically treated persons 5 years after treatment. AM J ORTtlOD DENTOFAC ORTIIOP 1989;96: 423-7. 1I. Kvam E, Broch J, Nissen-Meyer IH. Comparison between a zinc phosphate cement and a glass ionomer cement for cementation of orthodoniic bands. Eur J Orthod 1983;5:307-13. 12. Rezk-Lega F, Ogaard B, Arends J. An in vitro study on the merits of two glass ionomers for the cementation of orthodontic bands. AM J OR'rHOD DENTOFACORTttOP 1990;98:(in press). 13. O'Reilly MM, Featherstone JDB. Demineralization and remineralization around orthodontic appliances: an in vitro study. AM J ORTHOD DENTOFACORTtlOP 1987;92:33-40. 14. Zachrisson BU, Zachrisson S. Caries incidence and oral hygiene during orthodontic treatment. Scand J Dent Res 1971;79:394401. 15. Wilson AD, Kent BE. A new translucent cement for dentistry: the glass ionomer cement. Br Dent J 1972;132:133-5. 16. Hood JA, Childs WA, Evans DF. Bond strength of glass ionomer and polycarboxylate cements to dentine. NZ Dent J 1981;77: 141-4. 17. Smith DC. Dental cements: current status and future prospects. Dent Clin North Am 1983;6:763-92. 18. Cranfield M, Kuhn AT, Winter GP. Factors relating to the rate of fluoride-ion release from glass ionomer cement. J Dent 1982;10:333-41. 19. Forsten L. Fluoride release from glass ionomer cement. Scand J Dent Res 1977;85:503-4. 20. Wilson AD, Groffman DR, Kuhn AT. The release of fluoride and other chemical species from a glass ionomer cement. Biomaterials 1985;6:431-3.
Tensile bond force o f glass ionomer cements
36"1
21. Swartz ML, Phillips RW, Clark HE. Long-term fluoride release from glass ionomer cements. J Dent Res 1984;63:158-60. 22. Rezk-Lega F, Ogaard B, R011a G. Availability of fluoride from glass-ionomer luting cements in human saliva. Scand J Dent Res 1991;99:(in press). 23. Cook PA, Youngson CC. An in vitro study on the bond strength of a glass ionomer cement in the direct bonding of orthodontic brackets. Br J Orthod 1988;15:247-53. 24. Bowser Fajen V, Duncanson MG, Nanda RS, Currier F, Angolkar PV. An in vitro evaluation of bond strength of three glass ionomer cements. AM J OR'nIOD DEr,rrOFAC OR'rHOP 1990; 97:316-22. 25. /~rtun J, Zaehrisson B. Improving the handling properties of a composite resin for direct bonding. AM J OR'fHOD 1982;81:26976. 26. O'Brien KD, Watts DC, Read MJF. Residual debris and bond strength; is there a relationship? AM J ORTrtODDEN'rOFACORTttOP 1988;94:222-30. 27. Newmann GV. Epoxy adhesives for orthodontic attachments: progress report. AM J OR'rttoD 1965;51:901-12. 28. Wheeler J J, Ackerman RJ. Bond strength of thermally recycled metal brackets. AM J OR'rrtoD 1983;88:181-6. 29. ReynoldsIR. A review ofdirect orthodontic bonding. BrJOrthod 1975;2:171-8. 30. Keizer S, ten Cate JM, Arends J. Direct bonding of orthodontic brackets. AM J ORTtIOD 1976;69:318-27. 31. Maijer R, Smith DC. A new surface treatment for bonding. J Biomed Mater Res 1979;13:975-85.
Reprint requests to: Dr. Bj6m Ogaard Department of Orthodontics Faculty of Dentistry University of Oslo PO Box 1109 Blindern 0317 Oslo 3 No~'ay
T h e direct bracket-bonding technique is a standard procedure in treatment with fixed orthodontic appliances. By acid etching technique, composite resins have been widely used in dentistry. Direct bonding of orthodontic brackets with this technique has been used for the last two decades.~ Because of their high bond strength, composites are able to withstand normal oral forces exerted during mastication and traction forces generated by orthodontic appliances. However, pretreatment of the enamel surface with phosphoric acid is required to achieve a satisfactory bond. Several studies have shown enamel loss by acid etching. 25 Another disadvantage of composite resins involves their removal after debonding. Grinding off the adhesive from the tooth surface may lead to enamel alterations: Furthermore, enamel demineralization around brackets and beneath ill-fitting bands represents a serious clinical problem. T M Visible white spot lesions have been observed within 4 weeks in the absence of any fluoride r e g i m e n : Enamel decalcification is attributed to the extended accumulation and retention of bacterial plaque on the enamel surface adjacent to the bands and brackets, t4 In 1972 glass ionomer cements were introduced as dental cements by Wilson and Kent.~5 Since then glass ionomer cements have received a great deal of attention because of the advantages provided to clinical dentistry.
From the Department of Orthodontics, Dental Faculty, University of Oslo. 811125450
Glass ionomers adhere to enamel and dentin as well as to some metals) 6 They have also shown adequate biocompatibility) 7 More recently, they have been used in orthodontics for cementation of bands and bonding of brackets) m3"24 Acid etching of the enamel surface is unnecessary and mineral loss is practically avoided. Another attractive property of these cements is their long-term fluoride release, which is of interest for fixed orthodontic appliance therapy) 822 Few investigations have been performed to shed light on the bond strength of these cements in direct orthodontic bracket bonding. In 1988 Cook and Youngson 23 compared, in vitro, the shear/peel bond strength of a glass ionomer cement with a composite resin. In 1990 Bowser Fajen et a l : ~ evaluated, in vitro, the tensile bond strength of three glass ionomers. A significantly higher bond strength was found for the composite resin. The aim of this study was to compare bond forces of several glass ionomer cements with a commonly used composite resin by means of a tensile test method applied in vitro. In addition, the bond strength and the bond failure site were evaluated.
MATERIALS AND METHODS Forty sound premolars extracted for orthodontic purposes were used in this investigation. Before storage, the teeth were thoroughly washed in running water and all blood and adherent tissue were removed. Immediately afterward, the teeth were stored in distilled water in a refrigerator at 4° C. The 357
358
Rezk-Lega atzd Ogaard
Am. J. Orthod. Oentofiw. Orthop. October 1991
Fig. 1. Bracket design along with sotdered stainless steel ring.
storage medium was replaced periodically to minimize deterioration. The crowns were separated from the roots with a cooled diamond disk in a machine designed for the cutting of hard tissue. The pulp chamber was then enlarged to secure a retention in the holder. Each crown was lightly polished with pumice in a rubber cup, sprayed with water, and dried with a compressed oil-frce airstream for about 15 seconds. Subsequently, the crowns were mounted in plastic molds filled with Epofix (resin and hardener, 15 ml/2 ml, Struers, Copenhagen, Denmark) up to their facial surface, which was left intact. After fixation in the molds, test specimens were transferred to a chamber at 37 ° C and 50% relative humidity for 24 hours to allow hardening of the resin. The next day the specimens were carefully withdrawn from their molds, surplus material around the edge was removed with a scalpel, and cementation of brackets was performed according to the manufacturer's instructions. The bracket type used in this study was a GAC edgewise wide twin bracket (torque, 0; reference No. K232CS22). The bracket base area was 15.75 mm-'. A stainless steel circular ring was soldered at the bracket slot to provide a grip for the hook and allow better control of the type of stress and its distribution along the bond plane (Fig. 1). The premolars were divided into three groups, each con-
Fig. 2. Mounting system on Instron machine. sisting of ten teeth. The different adhesives used for bracket bonding are presented in Table I. Bonding procedures were performed on each tooth by one of the following three methods:
Group 1. Concise composite resin (No. 1994 A and B). The buccal surface of'each tooth was etched for 60 seconds with a semiliquid gel containing 40% phosphoric acid, rinsed with copious amounts of water, and dried in an oil-free airstream. The 15-drop Concise paste/paste system recommended by/krtun and Zachrisson-'~was used in the study. The orthodontic bracket was then bonded with a thin cement film that respected the tensile test alignment block. Group 2. Ketac-Ccm (K-C)/Aqua-Cem (A-C) glass ionomer cements. Before bonding, premolars in this group (10 for each cement), were treated with ChemFil 1I tooth cleanser (De Trey) for '10 seconds. They were then rinsed and dried in an oil-free airstream. Brackets were bonded with either K-C or A-C in a slightly thicker mix than recommended by the manufacturer (double cream consistency). This made bracket seating easier and allowed quicker setting.
Volume tO0 Number4 Group 3. Vitrabond glass ionomer cement. This is a lightcured glass ionomer. The powder component is primarily a radiopaque, ion-leachable fluoroaluminosilicateglass powder made photosensitive by its unique chemistry. The liquid component consists of a light-curable polymer, water, 2hydroxyethylmethacrylate (HEMAL and a photosensitizer. For this group, no pretreatment other than prophylaxis was carried out. The teeth were rinsed and dried as in the other groups. The brackets were cemented as recommended by the manufacturer, thus requiring light activation for polymerization. Each bracket side was exposed for 30 seconds to light from a Visilux 2 light curing unit. No tooth cleanser was used. Tensile bond force All bonding procedures were performed by the same operator (R. L. F.) according to a standardized technique. Each crown was checked to make sure that all excess adhesive was removed with a probe from around the entire periphery of the bracket so that none ovcrlapped the sides of the latter. After initial setting or curing of the different adhesives, the specimens were transferred again to a water bath at 37° C for further storage (24 hours). Specimens were then removed from the water container. They were securely clamped in their appropriate place on the lnstron materialstesting machine (Model 1121 - Serial No. H1875, Instron Ltd. Coronation Bucks, Great Britain HP12 3SY). A stainless steel hook (0.8 mm thick) was then engaged to the circular ring soldered onto the bracket slot. The other end of the hook was tightly screwed to the Instron machine (Fig. 2). The force (in newtons) required to dislodge the bracket was measured at a crosshead speed of 1 mm/minute. Debonding was performed at 23° ± 2° C and 50% ± 5% relative humidity.
Bond failure site The bond failure site was measured as the percentage of adhesive material adhering onto the tooth surface (tracing). This was made possible by a Wild Photomacroscope M-400 (Wildheerbrugg Ltd., CH 9435 Heerbrugg, Switzerland). The latter instrument was connected to a Wild Photoautomat MPS 45. To create a marked contrast between the cement and the enamel surface, specimens were illuminated from two directions by means of a Volpi-Intralux unit 250 HL; Swiss-made, Volpi AG, CH 8902, Urdorf, Zurich). The measurement area was magnified up to 24 times. The image obtained was marked off in a standard grid fashion. The latter consisted of I00 sections of equal size. Those sections with cement residue were identified and a percentage ratio was determined.
RESULTS Tensile bond force Mean tensile bond values (force and strength), along with the corresponding standard deviations, are shown in Table II. Two brackets bonded with Aqua-Cem and
Tensile bond force of glass ionomer cements 359 T a b l e I. Adhesive materials used, manufacturer,
type, and powder/liquid ratio
Adhesive]Manufacturer I
Type
Powder/liquid ratio
Concise
3M
Composite resin
Vitrabond
3M
Ketac-Cem
ESPE
Aqua-Cem
DeTrey Dentsply
Light-cured glass ionomer Conventional Double cream glass ioconsistency homer Conventional Double cream glass ioconsistency nomer
15-drop paste/paste system* 1.4 wt/1.0 wt
*A variation of working and setting times has been used to obtain optimal consistency.~
Table II. Mean tensile bond scores (force and strength) along with the respective standard deviations
Tensile bondforce (IV) Adhesives
Mean
Concise Vitrabond Ketac-Cem Aqua-Cem
152.5 27.53 10.75 5.5
[
SD 51.99 12.63 13.65 8.25
Tensile bond strength* (Nlmm") Mean ] SD 9.68 1.74
0.68 0.34
3.3 0.8 0.86 0.52
*The bond strength was calculatedby dividing the measured forces by the area of bonding (here the bracket area). Bracket area = 15.75 mmL
one bonded with Ketac-Cem broke off on early debonding. The Mann-Whitney two-sample rank test adjusted for ties was used to compare the different adhesives. The significance probability (p) was calculated for t w o sided tests, and no corrections were made for multiple tests on the same data set. When Vitrabond was compared with Ketac-Cem and Aqua-Cem, light-cured Vitrabond had significantly higher bond values than the two conventional glass ionomers with p = 0.003 for K-C and p = 0.04 for A-C. The results also show a clearly significant difference in the tensile bond recordings between the composite resin and Vitrabond (p = 0.0002) in favor of the resin. No significant differences were found between A-C and K-C (p > 0.05).
360
Rezk-Lega and
Am. J. Orthod. Dentofac. Orthop. October 1991
,,(~"aar~/ ,"6
Table III. Mean percentage of bond-failure sites
along with the respective standard deviations Cement percentage adhering to tooth surface Adhesives
Mean
Concise Vitrabond Ketac-Cem Aqua-Cem
85 20 86 62
I
SD 38.2 34.6 39.4 37.9
The bracket area --- 100%.
Bond-failure site
The bond-failure site was recorded in percent, where the bracket base area delimited on the buccal surface represented 100% (Table III). A high percentage o f Ketac-Cem (86%), Concise (85%), and Aqua-Cem (62%) was found adhering to the tooth surface after debonding. This was not the case with the light-cured glass ionomer (Vitrabond); only 20% residual adhesive was detected after debonding. DISCUSSION
The recorded data obtained from the present in vitro study clearly show a significantly higher tensile bond force of the composite resin when compared to the glass ionomer cements. These findings are in agreement with those of Cook and Youngson 23 and Bowser Fajen et al. 2~ Among the glass ionomers tested, Vitrabond showed a significantly higher bond force. This may be due to the onset of polymerization of the light-cured glass ionomer allowing total penetration of the latter to the retentive areas if present. Another reason could be the presence of less air-inclusion areas and, consequently, fewer areas of stress concentration than with chemically cured materials. 26 In the oral cavity, bonded brackets are subject to shear, tensile, and torsion forces. The difficulty of measuring these forces has made it difficult to quantify them precisely. According to Newmann': and Wheeler and Ackerman, 28 orthodontic forces do not surpass 4.45 Newtons (N) per tooth. In another study conducted by Reynolds, 29 these orthodontic forces are unlikely to exceed 14.7 N. Furthermore, the maximum load per tooth probably occurring in a clinical situation is 17.8 N. 27 On the other hand, several other studies indicate that a maximum bond force of 35.6 N 3° or 97.88 N 3~ is adequate for orthodontic brackets and appliances. This wide range of values is probably the result of the variations in the experimental design. Furthermore, in most
situations, the stress on the bond can be defined as either tensile, shear, torsion, or a combination of all these. There are no specific in vitro or in vivo tests that can be valid for all o f the various clinical applications o f adhesive materials. The mean bond force values obtained from the present study allow us to presume that Vitrabond (27.5 N) may be an adequate adhesive for bonding of orthodontic brackets. In addition, maximum and minimum bond values obtained for each cement are also given to represent the best and worst performances that can be expected under clinical conditions. For Concise (212 103 N), Vitra bond 46.5 - 11 N), Ketac-Cem (38 - 3 N), and finally Aqua-Cem (26 - 5 N), in spite of the very high standard deviation, the significant tests give rise to valid conclusions. On the other hand, nonsignificant test results could be due to a combination of small samples and high variation in measurements. A point that may be clinically relevant is that only 20% of Vitrabond was still adherent to the tooth surface. This is an advantage since minimum effort and. enamel surface alteration are achieved during debonding and polishing procedures. In conclusion, the glass ionomers used in the present study had a significantly lower tensile bond force than the composite resin. On the other hand, whether the strongest glass ionomer (Vitrabond) with an average tensile force of 27 N would be adequate in the mouth is quite debatable. Therefore, further clinical investigations are required. We thank Mrs. Ellen Austrheim and Mr. Ketil Kvam at the Scandinavian Institute of Dental Materials, Oslo, Norway, for their generous assistance and collaboration. We also express our gratitude to Dr. Marek ROsier for the statistical calculations. REFERENCES I. Retief DH, Dreyer CJ. Epoxy resins for bonding orthodontic attachments to teeth. J Dent Assoc S Aft 1967;22:338-46. 2. GwinettAJ, Matsui A. A study of enamel adhesives: the physical relation between enamel and adhesive. Arch Oral Biol 1967; 12:1615-20. 3. Fitzpatrick DA, Way DC. The effects of wear, acid etching, and bond removal on human enamel. AM J ORTHOO1977;72:67181. 4. Brown CRL, Way DC. Enamel loss during orthodontic bonding and subsequent loss during removal of filled and unfilled adhesives. AM J ORTHOD1978;74:663-71. 5. Diedrich P. Enaffael alterations from bracket bonding and debonding: a study with the scanning electron microscope. AM J ORTHOD1981,;79:500-22. 6. ZachrissonBU, Artun J. Enamel surfaceappearanceafter various debonding techniques. AM J ORTtIOD1979;75:121-37. 7. ElgaardB, Rolla G, ArendsJ. Orthodonticappliancesand enamel demineralization. Part 1. Lesion development. AM J ORTIIOD DErrrOFACOR'rHoP1988;94:68-73.
Volume 100 Number 4 8. Norris S, Mclnnes-Ledoux P, Schwaninger B, Weinberg R. Retention of orthodontic bands with new fluoride-releasing cements. AM J OR'moP 1986;89:206-10. 9. Gorelick L, Geiger AM, Gwinnett AJ. Incidence of white spot formation after bonding and banding. AM J ORTItOD 1982; 8 i :323-3 I. 10. Ogaard B. Prevalence of white spot lesions in 19-year-olds: a study on untreated and orthodontically treated persons 5 years after treatment. AM J ORTtlOD DENTOFAC ORTIIOP 1989;96: 423-7. 1I. Kvam E, Broch J, Nissen-Meyer IH. Comparison between a zinc phosphate cement and a glass ionomer cement for cementation of orthodoniic bands. Eur J Orthod 1983;5:307-13. 12. Rezk-Lega F, Ogaard B, Arends J. An in vitro study on the merits of two glass ionomers for the cementation of orthodontic bands. AM J OR'rHOD DENTOFACORTttOP 1990;98:(in press). 13. O'Reilly MM, Featherstone JDB. Demineralization and remineralization around orthodontic appliances: an in vitro study. AM J ORTHOD DENTOFACORTtlOP 1987;92:33-40. 14. Zachrisson BU, Zachrisson S. Caries incidence and oral hygiene during orthodontic treatment. Scand J Dent Res 1971;79:394401. 15. Wilson AD, Kent BE. A new translucent cement for dentistry: the glass ionomer cement. Br Dent J 1972;132:133-5. 16. Hood JA, Childs WA, Evans DF. Bond strength of glass ionomer and polycarboxylate cements to dentine. NZ Dent J 1981;77: 141-4. 17. Smith DC. Dental cements: current status and future prospects. Dent Clin North Am 1983;6:763-92. 18. Cranfield M, Kuhn AT, Winter GP. Factors relating to the rate of fluoride-ion release from glass ionomer cement. J Dent 1982;10:333-41. 19. Forsten L. Fluoride release from glass ionomer cement. Scand J Dent Res 1977;85:503-4. 20. Wilson AD, Groffman DR, Kuhn AT. The release of fluoride and other chemical species from a glass ionomer cement. Biomaterials 1985;6:431-3.
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21. Swartz ML, Phillips RW, Clark HE. Long-term fluoride release from glass ionomer cements. J Dent Res 1984;63:158-60. 22. Rezk-Lega F, Ogaard B, R011a G. Availability of fluoride from glass-ionomer luting cements in human saliva. Scand J Dent Res 1991;99:(in press). 23. Cook PA, Youngson CC. An in vitro study on the bond strength of a glass ionomer cement in the direct bonding of orthodontic brackets. Br J Orthod 1988;15:247-53. 24. Bowser Fajen V, Duncanson MG, Nanda RS, Currier F, Angolkar PV. An in vitro evaluation of bond strength of three glass ionomer cements. AM J OR'nIOD DEr,rrOFAC OR'rHOP 1990; 97:316-22. 25. /~rtun J, Zaehrisson B. Improving the handling properties of a composite resin for direct bonding. AM J OR'fHOD 1982;81:26976. 26. O'Brien KD, Watts DC, Read MJF. Residual debris and bond strength; is there a relationship? AM J ORTrtODDEN'rOFACORTttOP 1988;94:222-30. 27. Newmann GV. Epoxy adhesives for orthodontic attachments: progress report. AM J OR'rttoD 1965;51:901-12. 28. Wheeler J J, Ackerman RJ. Bond strength of thermally recycled metal brackets. AM J OR'rrtoD 1983;88:181-6. 29. ReynoldsIR. A review ofdirect orthodontic bonding. BrJOrthod 1975;2:171-8. 30. Keizer S, ten Cate JM, Arends J. Direct bonding of orthodontic brackets. AM J ORTtIOD 1976;69:318-27. 31. Maijer R, Smith DC. A new surface treatment for bonding. J Biomed Mater Res 1979;13:975-85.
Reprint requests to: Dr. Bj6m Ogaard Department of Orthodontics Faculty of Dentistry University of Oslo PO Box 1109 Blindern 0317 Oslo 3 No~'ay
