Comparison of the shear bond strength of a light-cured glass ionomer and a chemically cured glass ionomer for use as an orthodontic bonding agent Anne M. Compton, DDS, ° Charles E. Meyers, Jr., DDS, b Steven O. Hondrum, DDS, MS, c and Lewis Lorton, DDS, MS ~ Ft. Meade and RocL'ville, Md.

Light-cured glass ionomers with an initial set of 20 seconds may produce higher initial bond strengths, as well as decreased sensitivity to moisture contamination and desiccation, than chemically cured glass ionomers making them attractive for use as orthodontic bonding agents. The purpose of this study was to determine and compare the shear bond strength of stainless steel orthodontic attachments to enamel with a light-cured glass ionomer (Zionomer) tested at 60 minutes and 24 hours, and a rapidly setting chemically cured glass ionomer (Ketac-Bond) tested at 60 minutes and 24 hours. Fifty-two recently extracted human premolars were random!y divided into four groups--l-hour and 24-hour light-cured glass ionomer groups and 1-hour and 24-hour chemically cured glass ionomer groups. Stainless steel lingual buttons were bonded to prepared enamel surfaces, and the samples were placed in a water bath at 37 ° C until ready for testing. The shear bond strength of each sample was determined with a universal testing instrument. Results from the study conclude: (1) The mean shear bond strength of the light-cured glass ionomer is greater than that of the chemically cured glass ionomer at 1 and 24 hours. (2) The mean shear bond strength of both glass ionomers increases from 1 to 24 hours. (3) The mean shear bond strength of the lightcured glass ionomer is not significantly different from 1 to 24 hours, but the shear bond strength of the chemically cured glass ionomer cement is different. The faster setting reaction of the light-cured glass ionomer and its higher initial and sustained bond strength make it more attractive for use as an orthodontic bonding agent. (AMJ ORTHOD DENTOFACORTHOP 1992;101:138-44.)

G l a s s ionomer cements were first introduced in 1972 by Wilson and Kent ~ and have been available for clinical use in the United States since 1977. They have the favorable properties of the silicate cements (insolubility, hardness, the ability to release fluoride) Paper submitted by the first author to the Orthodontic Department, U.S. Army Dental Activity, Ft. Meade, hid. as partial fulfillment of the requirements for a certificate in orthodontics. The views of the author(s) do not support or reflect the views of the Department of the Army or the Depamnent of Defense. 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 necesssarily the best available for the purpose. *Major, U.S, Army Dental Corps; Resident, U.S. Army Orthodontic Residency Program, Ft. George G. Meade, Md. bColonel, U.S. Army Dental Corps; Director, U.S. Army Orthodontie Residency Program, Ft. George G. Meade. Md. 'Colonel, U.S, Army Dental Corps; Chief. Dental Materials, U.S. Army Institute of Dental Research, Ft. George G. Meade, Md. dColonel, U.S. Army Dental Corps (Re/.); Senior Clinical Research Associate, Henry M. Jackson Foundation, Rockville, Md. 811126494

138

and the adhesive properties of polycarboxylate cements.t3 Glass ionomer cements have also been shown to adhere to base metal alloys and to plastics, t'4-6 The properties of glass ionomer cements make them attractive for conventional banding and directly bonded brackets? '57 Unfortunately, glass ionomer cements exhibit a prolonged setting reaction and a late gain of strength, and they are sensitive initially to moisture contamination and later to dehydration, sl° The recent development of light-cured glass ionomer systems may hastefi the setting reaction. This would increase the initial strength and hardness of the cement and decrease its sensitivity to moisture contamination and dehydration, ensuring optimal properties for use as an orthodontic bonding agent.~t Glass ionomer cements consist of two components--a calcium-aluminum fluorosilicate glass powder and a carboxylic acid copolymer, such as polyacrylic acid. ~za3 Itaconic acid copotymers have been used to increase the reactivity of the polyacrylic acid

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Comparison of shear bond strength of glass ionomer cements

to the glass, and small quantities of tartaric acid have been added to improve the rate o f hardening) 4 Modifications to the polyacid, as well as the addition of a small amount of resin, such as hydroxydimethacrylate and BIS-GMA, in combination with a camphorquinone photoinitiator, produce the photosensitive properties and the faster initial set of the light-cured glass ionomers) 5.16 The size of the glass particles may also be altered for the intended use of the material. Large particle sizes (50 i.tm) are used for the restorative cements, and smaller particle sizes (20 gm) are used for the luting cements) °'~4 The chemistry of the mixing reaction is described in the literature. ,7.19 The dispensing, mixing, and placement of glass ionomer cements are technique-sensitive, which may be considered a disadvantage to some clinicians. The powder-liquid ratio is crucial in maximizing the physical properties and the proper setting of the cement. The most common cause of failure o f the glass ionomer cements is incorrect proportioning and mixing) 9 A study by Wong and Bryant :° concluded that the manual dispensing systems provided by the manufacturers were only an approximate guide to the quantities o f powder and liquid required and recommended the use of an encapsulated system to achieve optimum results. Glass ionomer cements should not be contaminated by moisture for 10 to 60 minutes after mixing and should be protected from dehydration for at least 24 hours. During the initial setting stage, moisture contamination will cause the matrix to become chalky and porous and will result in a loss of surface hardness. 7:°:9 During the second phase, the matrix is susceptible to desiccation. '4"21 The fast initial set of the light-cured glass ionomers enables them to be less susceptible to dehydration. 16 Enamel loss occurs during acid etching in the orthodontic bonding process. Localized enamel fractures are created during debonding procedures because of the micromechanical retention produced during the acidetch treatment. 22"23Debonding brackets and cleanup of composite resin residue cause scratches and facets in the enamel that promote plaque and stain formation. 24 Unlike the resin-filling materials, glass ionomer cements can adhere to unetched enamel by physicochemical means, therefore reducing the need for mechanical retention and facilitating d e b o n d i n g : "25 Studies evaluating bond strengths of glass ionomer cement to enamel have concluded that the adhesive bond of the material is stronger than the cohesive strength of the cement, z6"27 The bond strength of glass ionomers to enamel may be enhanced by "conditioning" the tooth surfaces with a weak acid, such as 40% to 10% polyacrylic acid to remove contaminants and debris, z Other effective con-

139

ditioning solutions include tannic acid, dodicin, and surface-active microbial solutions. All have functional groups capable of hydrogen bonding to tooth material, which promote effective cleaning and wetting of the substrate surface. 9 It has been reported that rapidly setting chemically cured glass ionomer cements reach 80% of their 24hour bond strength in 15 minutes. 2s Light-cured glass ionomers with an initial set of 20 seconds may produce higher initial shear bond strengths, as well as decreased sensitivity to moisture contamination and desiccation, making them attractive for use as orthodontic bonding agents. The purpose of this investigation was to determine the shear bond strength of stainless steel orthodontic brackets to enamel with (1) light-cured glass ionomer cement tested at 60 minutes, (2) light-cured glass ionomer cement tested at 24 hours, (3) a rapidly setting chemically cured glass ionomer cement tested at 60 minutes, and (4) a rapidly setting chemically cured glass ionomer cement tested at 24 hours. MATERIALS AND METHODS

Fifty-two recently extracted human premolars were cleaned by means of a rubber cup with flour of pumice and stored in distilled water for a period of less than 6 weeks. Enamel surfaces, approximately 4 × 4 ram, were prepared by successive wet grinding to a 600 grit with silicon carbide paper on a polishing machine (Buehler, Lake Bluff, I11.). The fiat enamel surfaces were then covered with nonabsorbent plastic tape, with a hole 4 mm in diameter to ensure an equal amount of exposed enamel on each tooth. 2~ The teeth were randomly divided into four groups--l-hour and 24-hour light-cured glass ionomer groups and l-hour and 24-hour chemically cured glass ionomer groups. The two types of glass ionomer cement chosen for testing were (1) Zionomer, a two-paste light-cured glass ionomer, (Den Mat Corp., Santa Monica, Calif.) and (2) Ketac-Bond, a rapidly setting chemically cured cement, (Premier Dental Products, Norristown, Pa.). Stainless steel lingual buttons (UNITEK Corp., Monrovia, Calif.) with 3 × 3 mm welded mesh pads, were bonded to each tooth. In the bonding procedure, enamel surfaces for the lightcured glass ionomer samples were conditioned, according to the manufacturer's instructions (with a weak nitric acid solution for 30 seconds), rinsed thoroughly for 30 seconds with an air/water spray, and gently dried with an oil-free air syringe. The light-cured cement was dispensed and mixed, according to the manufacturer's instructions, and applied in a thin layer on the mesh pad of the lingual button with a plastic instrument. The lingual button was positioned and pressed on the enamel surface with cotton forceps until fully seated. Excess cement around the lingual button was removed with an explorer. The cement was light cured (Ortholux, UNITEK Corp., Monrovia, CaliL) for 20 seconds on each of the

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Am. J. Orthod. Dentofac. Orlhop. February 1992 25

Shearing Blade

Glass [onomer

0--0

Light, Cured

• --v

Chemica/ly" Cured

20

~"

~5

Button ~o

,,,J J.

!

Acrylic

Base

IH~R

!

24H~RS

Fig. 1. Sample mounted with lingual button perpendicular to shearing blade of testing instrument.

Fig. 2. Shear bond strength of glass ionomers, light-cured versus chemically cured.

Table I. Mean shear force (MPa) to fracture glass ionomer bond

acrylic (Buehler, Lake Bluff, I11.)enclosed by a 1-inch breakapart rubber cup. A wire guide and surveyor were used to ensure a perpendicular mounting of each sample in the resin base. The shear bond strength of each sample was determined with a universal testing instrument (lnstron Model 1011, Instron Corp., Canton, Mass.) (Fig. 1). The samples were stabilized with a vise. The shearing force was applied at the ligature groove of the lingual button. Crosshead speed was I mm/min. The ultimate shear strength was recorded in kilograms and converted to megapascals. The fracture sites were visually inspected with a stereoscopic light microscope to determine the location of the fracture. The fracture sites of random samples were observed under a scanning electron microscope (SEM), and photographs were made. The data were subjected to one- and two-way analyses of variance (ANOVA) and the post hoc Student-Newman-Keuls multiple comparison test at the 95% level of confidence.

ro,,p CC! CC24 LCl LC24

i

co.,. 13 13 13 13

f

i 8.8 11.8 16.7 17.2

s,, 1.8 2.5 3.0 6.2

CCI = Chemically cured tested at 1 hour; CC24 = chemically cured tested at 24 hours; LCI = light-cured tested at I hour; and LC24 = light-curedtested at 24 hours.

four sides of the button. A new mix was prepared for each sample. In preparation for bonding the chemically cured glass ionomer samples, enamel surfaces were also conditioned according to the manufacturer's instructions (with a 25% polyacrylic acid solution for 10 seconds), rinsed thoroughly for 30 seconds with an air/water spray, and gently dried with an oil-free air syringe. The encapsulated cement was triturated (Caulk/Dentsply, Milford, Del.) for 10 seconds according to the manufacturer's instructions and then applied in a thin layer on the mesh pad of the lingual button with a plastic instrument. The lingual button was placed on the enamel surface as previously described, and the excess material was removed with an explorer. A new mix was prepared for each application. Fifteen minutes after bonding, the samples were placed in a distilled water bath at 37° C until ready for testing. The shear bond strength of the 24-hour groups were tested 24 hours after bonding, and the l-hour groups were tested an hour after bonding. In preparation for testing, the teeth were removed from the water bath and their roots were embedded in a fast-set

RESULTS Results are shown in Table I and Fig. 2. The mean shear bond strength of both the light-cured and the chemically cured glass ionomers increased from 1 to 24 hours, although the mean shear bond strength of the light-cured glass ionomer increased only slightly. Two-way ANOVA shows no significant interaction between material and time (probability o f p = 0.2338, Table II). There was a significant difference between the materials (p = 0.0001). The effect for time was not statistically significant (p = 0.0994), but Fig. 2 illustrates that the time variable was important in the chemically cured material. To illustrate this, the numbers were recast in a one-way ANOVA. One-way ANOVA of the shear bond strengths of the four groups

Comparison of shear bond strength of glass iononter cements

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Table II. Two-way ANOVA

141

Table IIIB. Student-Newman-Keuls

post hoc test at p = 0.05 Time Type Txt Error

37.241 519.647 19.181 633.222

1 1 1 48

37.241 519.647 19.181 13.192

2.823 39.391 1.454

0.0994 0.0001 0.2338

T.rt, Treatment; SS, sum of squares; DF, degrees of freedom; MS, mean square; F, F ratio; p = probability.

[ F

io 1

Between Within

576.069 633.216

3 48

TOTAL

1209.285

51

192.023 13.192

I

Mean

CCI CC24 LCI LC24

8.8 I 1.8 16.7 17.2

Table IV. Frequency of failure site*

Table IliA. One-way ANOVA

I s,

Group

14.556

[ p 0.0001

shows a statistically significant difference (p < 0.0001) among the four groups (Table IliA). The post hoc Student-Newman-Keuls multiple comparison test shows no significant difference between bond strengths of the light-cured glass ionomers tested at 1 and 24 hours at the 95% level of confidence. Significant differences in bond strength do exist between the chemically cured glass ionomer tested at 1 hour, chemically cured glass ionomer tested at 24 hours, and the light-cured glass ionomer pair (Table IIIB). Of the bond fractures, 77% were cohesive (within the cement) in the chemically cured glass ionomer groups, and 50% were cohesive in the light-cured glass ionomer groups (Table IV). Adhesive bond failures at the cement-bracket interface were found only in the light-cured glass ionomer groups, and adhesive bond failures at the cement-enamel interface were found only in the chemically cured glass ionomer samples. Fracture sites of randomly selected teeth were observed under the SEM and photographed (Figs. 3 and 4). A marked physical difference in the two materials was noticed. Information concerning the nature of this difference could not be obtained from the manufacturer. DISCUSSION

The shear bond strengths of glass ionomers have been previously reported to be 35% to 39% of the shear bond strength of composite r e s i n s . :6'3°'31 The maximum tensile bond strength recommended for successful clinical bonding is estimated to be 7 MPa. 3-" According to the results of this study, chemically cured Ketac-Bond and light-cured Zionomer, have sufficient mean shear bond strengths to enable them to be successfully used as orthodontic bonding agents.

Material

Adhesive enamel

Cohesive

Adhesive bracket

Chemically cured Light-cured

6 (23%) 0 (0%)

20 (77%) 13 (50%)

0 (0%) 13 (50%)

*Percentage by material in parentheses.

The chemically cured glass ionomer reached 74% of its 24-hour mean bond strength in the first hour, whereas the light-cured glass ionomer reached 97% of its 24-hour mean bond strength in the first hour. This illustrates that the light-curing properties of the lightcured glass ionomer hasten the setting reaction of the material, enabling it to attain a higher initial bond strength, in comparison with the chemically cured glass ionomer, which has a more prolonged setting reaction and later gain of strength over 24 hours. Although both materials have mean shear bond strengths suitable for use as orthodontic bonding agents, the light-cured glass ionomer would be preferred because of its faster setting reaction and its higher mean bond strength at 1 and 24 hours. Although the mean shear bond strength of the lightcured glass ionomer tested at 24 hours was reported to be slightly higher than that obtained at 1 hour, initial testing of the material at 24 hours resulted in mean shear bond strengths lower than those obtained at 1 hour. Separation of the components of paste A and paste B of the light-cured glass ionomer was noted when they were initially dispensed from each tube. Although the material became more homogeneous as more samples were dispensed, the initial heterogeneity of the material resulted in a lower mean shear bond strength for the light-cured glass ionomer tested at 24 hours than for the light-cured group tested at I hour after bonding. One month later, when a second group of 13 samples was prepared, bonded with the light-cured glass ionomer, and tested 24 hours after bonding, similar observations and results, as in the first 24-hour test group, were obtained. Several months laier, a third group of 13 samples was prepared for bonding and

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Am. J. Orthod. Dentofac. Orthop. February 1992

Fig. 3. Chemically cured glass ionomer observed under SEM.

Fig. 4. Light-cured glass ionomer observed under SEM.

testing at 24 hours. The light-cured glass ionomer pastes, A and B, were first emptied from the manufacturer's packaged tubes into separate containers and labeled. Each paste was then mixed to a homogeneous consistency in its new container, before dispensing and mixing of each sample. The third group was then bonded in the same manner as the first and second

groups and tested 24 hours after bonding. The reported mean shear strength of this light-cured glass ionomer group was greater than other light-cured glass ionomer groups tested in this study. The bond failures of the glass ionomers were primarily cohesive, which supports findings in previous studies that the cohesive strength of the material is

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Comparison of shear bond strength of glass ionomer cements 143

w e a k e r than its adhesive bond. 26-2s'3~T h e a d h e s i v e bond strength o f the light-cured glass i o n o m e r to e n a m e l was greater than that o f the c h e m i c a l l y cured glass i o n o m e r to enamel. T h e small a m o u n t o f resin present in the light-cured glass i o n o m e r material m a y e n h a n c e the b o n d i n g properties o f the light-cured glass i o n o m e r to

enamel.33 T h e faster setting reaction o f the light-cured glass i o n o m e r and its h i g h e r initial and sustained shear bond strength m a k e it m o r e attractive for use as an orthodontic bonding agent than the c h e m i c a l l y cured glass ionomer. I m p r o v e m e n t in the h o m o g e n e i t y o f the lightcured glass i o n o m e r tested in this study is necessary to ensure more optimal properties o f the material, such as shear bond strength. R e s e a r c h is continuing to c o m p a r e the shear bond strength o f different glass i o n o m e r products at different t i m e intervals for use as orthodontic b o n d i n g agents.

CONCLUSIONS O n the basis o f the results o f this study, the following conclusions appear valid: 1. The m e a n shear bond strength o f the light-cured glass i o n o m e r is h i g h e r than the m e a n shear bond strength o f the c h e m i c a l l y cured glass i o n o m e r at 1 and 24 hours. 2. The m e a n shear bond strength o f the c h e m i c a l l y cured glass i o n o m e r and the light-cured glass i o n o m e r increases f r o m I to 24 hours. 3. The m e a n shear bond strength o f the light-cured glass i o n o m e r is not significantly different from 1 to 24 hours, but the shear bond strength o f the c h e m i c a l l y cured glass i o n o m e r is different. We express our gratitude for the research assistance provided by the U.S. Army Institute of Dental Research. Specifically, we thank Dr. Paul R. Cuenin and his assistant, Sergeant Emeterio L. Cerbas, for their assistance with the scanning electron microscope. We also thank Specialist John J. Holoyohoy for his assistance with the testing devices and Mr. Jay L. Mansfield and Sergeant Dan P. Raymundo for their assistance with the graphs and illustrations in the article.

REFERENCES 1. Wilson AD, Kent BE. A new translucent cement for dentistry. Br Dent J 1972;132:133-5. 2. Swift EJ. Glass ionomers: a review for the clinical dentist. Gen Dent 1986, Nov/Dec:468-71. 3. Mizrahi E. Glass ionomer cements in orthodontics: an update. A~.t J ORTttODDENTOFACORTHOP 1988;93:505-7.

4. Hotz P, McLean JW, Seed I, Wilson AD. The bonding of glass ionomer cements to metal and tooth substrates. Br Dent J 1977;142:41-7. 5. Maijer R, Smith DC. A comparison between zinc phosphate and glass ionomer cement in orthdontics. AM J ORTttOD DENTOFAC OR'ntOP 1988;93:273-9.

6. White LW. Glass ionomer cement. J Clin Orthod 1986;20:38791. 7. Norris DS, Mclnnes-Ledoux P, Schwaninger B, Weinberg R. Retention of orthodontic bands with new fluoride-releasing cements. AM J OR'n~Ot) 1986;89:206-11. 8. Knibbs PJ. Glass ionomer: 10 years of clinical use. J Oral Rehabil 1988;15:103-15. 9. Powis DR, Folleras T, Merson SA, Wilson AD. Improved adhesion of a glass ionomer cement to dentin and enamel. J Dent Res 1982;61:1416-22. 10. Swift EJ. An update on glass ionomer cements. Quintessence Int 1988;19:125-30. 1I. Baharav H, Abraham D, Cardash HS, Helft M. Effect of exposure time on the depth of polymerization of a visible lightcured composite resin. J Oral Rehabil 1988;15:167-72. 12. McLean JW, Wilson AD, Prosser HJ. Development and use of water-hardening glass-ionomcr luting cements. J Prosthet Dent 1984;52:175-81. 13. Seeholzer HW, Dasch W. Banding with glass ionomer cement. J Clin Orthod 1988;22:165-9. 14. Mount GJ. Glass ionomer cements: clinical considerations. In: Clinical Dentistry. Philadelphia: Harper & Row, 1984; vol 4, chap 20A: 1-22. 15. Wilson AD. Developments in glass ionomer cements, lnt J Prosthod 1989;2:438-46. 16. Jordon RE, Suzuki M, MacClean D. Light cured glass ionomers. J Esthetic Dent 1989;1:59-61. 17. Barry TI, Clinton DJ, Wilson AD. The structure of a glassionomer cement and its relationship to the setting process. J Dent Res 1979;58:1072-9. 18. Crisp S, Wilson AD. Reactions in glass ionomer cements. I!!. The precipitation reaction. J Dent Res 1974;53:1420-4. 19. McLean JW. Status report on the glass ionomer cements. J Am Dent Assoc 1979;99:221-6. 20. Wong TCC, Bryant RW. Glass ionomer cements: dispensing and strength. Aust Dent J 1985;30:336-40. 2 h Phillips S, Bishop BM. An in vitro study of the effect of moisture on glass-ionomer cement. Quintessence Int 1985;16:175-8. 22. Diedrich P. Enamel alterations from bracket bonding and debonding: a study with the scanning electron microscope. AM J OR~rOO 1981;79:500-22. 23. Pus MD, Way DC. Enamel loss due to orthodontic bonding with filled and unfilled resins using various clean-up techniques. AM J ORTttOD 1980;77:269-83. 24. Gwinnett AJ, Gorelick L. Microscopic evaluation of enamel after debonding: clinical application. AM J ORmoo 1977;71:651-65. 25. Mount GJ. Restoration with glass ionomer cement: requirements for clinical success. Oper Dent 1981;6:59-65. 26. Coury TL, Miranda FJ, Wilier RD. Probst RT. Adhesiveness of glass-ionomer cement to enamel and dentin: a laboratory study. Oper Dent 1981;7:2-6. 27. Eakle WS. Increasing the resistance of teeth to fracture: bonded composite resin versus glass ionomer cement. Dent Mater 1985; 1:228-30. 28. Aboush YEY, Jenkins CBG. An evaluaton of the bonding of glass-ionomer restoratives to dentin and enamel. Br Dent J 1986;161:179-84. 29. Negro MM, Beech DR, Grant AA. An evaluation of mechanical and adhesive properties of polycarboxylate and glass ionomer cements. J Oral Rehabil 1982;9:161-7. 30. Murray GA, Yates JL. A comparison of the bond strengths of composite resins and glass ionomer cements. J Pedod 1984; 8:172-7.

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31. Klockowski R, Davis E, Joynt R, Wieczkowski G, McDonald A. Bond Strength and durability of glass ionomer cements used as bonding agents in the placement of orthodontic brackets. AM J ORTHOD DENTOFAC ORTIIOP 1989;96:60-4. 32. Lopez JI. Retentive shear bond strengths of various bonding attachment bases. AM J ORTIIOD1980;77:669-78. 33. Cook PA, Youngson CC. An in vitro study of the bond strength of a glass ionomer cement in the direct bonding of orthodontic brackets. Br J Orthod 1988;15:247-53.

Am. J. Orthod. Dentofac. Orthop. February 1992

Reprint requests to: Major Anne M. Compton USA DENTAC Dental Clinic No 5 ATTN: Orth. Ft. Riley, KS 66442

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Comparison of the shear bond strength of a light-cured glass ionomer and a chemically cured glass ionomer for use as an orthodontic bonding agent.

Light-cured glass ionomers with an initial set of 20 seconds may produce higher initial bond strengths, as well as decreased sensitivity to moisture c...
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