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Mechanical and bond strength properties of light-cured and chemically cured glass ionomer cements
ORIGINAL ARTICLES
Mechanical and bond strength properties of light-cured and chemically cured glass ionomer cements Michael F. McCarthy, DMD," and Steven O. Hondrum, DDS, MS b Fort George G. Meade, Md.
The purpose of this study was to evaluate the mechanical and bond strength properties of a commercially available light-cured glass ionomer cement and of a chemically cured glass ionomer cement. Sixty recently extracted human molars were randomly divided into six equal groups, and the bond strengths of the two cement types were evaluated at 1 hour, 24 hours, and 7 days. Stainless steel lingual buttons were bonded to prepared enamel surfaces, and the samples were placed in a water bath at 37 ° C until testing. The shear bond strength of each sample was determined with a universal testing instrument. The mechanical strength properties of the two cements were then evaluated. The transverse flexural strength, compressive strength, rigidity, and diametral tensile strength were tested for each cement at 1 hour, 24 hours, and 7 days. The results of the mechanical property strength tests were then compared with the results of the bond strength tests. The results from the study conclude: (1) The light-cured cement achieves maximum tensile strength faster than the chemicalIy cured material; (2) the bond strengths of both chemically cured and light-cured glass ionomer cements increase with time; (3) the bond strength of light-cured cement was greater than the proposed minimum level for clinical success at all test times whereas the bond strength of the chemically cured material did not reach minimum levels until after 24 hours; (4) the bond failures of the glass ionomer cements are primarily cohesive; and, (5) there is a high correlation between the diametral tensile strength and the bond strength for glass ionomer cements at 1 hour and 24 hours (r = 0.98). (AMJ ORTHOD DENTOFACORTHOP 1994;105:135-41.)
S i n c e the introduction of acid etching of enamel by Buonocore' in 1955, the use of composite resins has become widely accepted as a medium for cementing orthodontic brackets to a tooth. 2 Bonded orthodontic brackets are more advantageous than bands in that they have no interproximal contact; they are both easier to place and to remove; they are more esthetic and hygienic; and they are less irritating to the gingiva. 3 However, the use of composite resins as the bonding medium in orthodontics also has its disadvantages. During the composite resin bonding procedure, surface enamel may be lost through preetching pumicing, as well as through dissolution and demineralization of the superficial enamel layer during the acid etch. Enamel can also be lost during the cleanup of residual resin after the bonded bracket has been removed from the tooth, as well as during rebonding procedures. This is
The views and opinions of the authors expressed herein do not necessarily state or reflect those of the United States government or the Department of Defense. 'Major, US Army Dental Corps, U.S. Army Orthodontic Residency Program. ~'Colonel, US Army Dental Corps; Chief, Dental Materials, U.S. Army Institute of Dental Research. 811142490
a concern since the concentration of fluoride is greatest at the surface of the enamel: Another risk associated with the use of orthodontic bands and composite resin bonded brackets is the decalcification of adjacent tooth structure. This decalcification, which may occur within 4 weeks of fitting the appliance, has been attributed to the prolonged accumulation and retention of bacterial plaque next to the orthodontic bands and brackets: A bonding material that could make the tooth structure more resistant to caries, yet retain the bonding strengths and properties of composite resins without the loss of enamel, may prove to be beneficial. One potential dental adhesive is the glass ionomer cement. Glass ionomer cements are a relatively new class of dental materials which possess a unique combination of properties. They adhere to the enamel surface without the need of acid etching or enamel surface conditioning. They serve as a reservoir of fluoride ions that protect against decalcification of the surrounding tooth structure. 68 The glass ionomer cements are also easier to remove from enamel than composite resins. Since the adhesion of the cement does not involve the acid etching of enamel from the tooth surface, the cement can be dessicated with a stream of air from an air syringe 135
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McCarthy and Hondrttm
and easily removed. 9 These characteristics o f the glass ionomer cement may reduce the amount o f enamel surface loss often associated with bonding procedures that use composite resins. The glass ionomer cements have been shown to have significantly higher tensile and compressive strengths than zinc phosphate cements as a cementing medium for orthodontic bands, t° T h e y have also been shown to have retentive strengths similar to the zinc phosphate and polycarboxylate cements. In studying the site o f cement failure, it has been shown that the glass ionomer failed more often at the cement-band interface. Failure at this location is considered to be most favorable, as the fluoride-releasing cement may offer protection against decalcification under loose bands. ".12 These studies have been supported by clinical findings which show that orthodontic bands cemented with the glass ionomer cements have a significantly lower failure rate than those cemented with zinc phosphate or polycarboxylate cements. 9a3"~4 In vitro bond strength studies ~5,16 have shown that the bracket bond strength o f composite resins are significantly higher than those o f the glass ionomers. However, White 9 has estimated that there were no more bracket failures with glass ionomers than with composite resins. If adherence to the enamel alone is the main requisite in orthodontic bonding, then composite resins alone should be used for bonding. However, glass ionomer cements possess a number o f advantageous properties over composite resins that may outweigh its relatively inferior bond strength. The glass ionomer cements have been investigated for use as a cementing medium for orthodontic bands and bonded brackets. 9.12"17They are presently marketed as both chemically cured and light-cured systems. The purpose o f this study is to evaluate the mechanical and bond strength properties o f a commercially available light-cured glass ionomer cement and o f a chemically cured glass ionomer cement. It will be further determined whether mechanical properties can be correlated with bond strength.
METHODS AND MATERIALS Two cements were evaluated (Table I). A universal testing machine (Model 1011, Instron Corp., Canton, Mass.) was used in determination of the mechanical and bond strength properties.
Transverse flexural strength Each cement was mixed or cured according to the manufacturer's instructions and placed into a rectangular Teflon mold measuring 2 × 2 × 10 mm. The Teflon mold was placed on a celluloid-lined, flat glass plate. The mix was condensed, and the mold slightly overfilled within 90 seconds after the start of the mix. A second celluloid-lined, fiat glass plate was then placed on top of the mold and the plates tightly
American Journal of Orthodontics and Dentofacial Orthopedics February 1994
clamped together. The light-cure samples were cured with a visible curing light (Prismetics, Caulk, Milford, Del.) placed against the upper glass plate in a series of three overlapping steps for 20 seconds each (total 60 seconds). Two minutes after mixing/curing, the samples were placed in an oven at 95% --- 5% relative humidity and 37 ° --- 1° C. Fifteen minutes after mixing/curing, the test samples were removed from the oven, and the excess flash removed. The depth and width of each sample were measured to within 0.01 mm, and the samples were stored in distilled water at 37" + 1° C until testing. Ten specimens from each cement were tested at 1 hour, 24 hours, and 7 days after mixing. Each sample was placed between the bearers of a three-point beam apparatus and continuously loaded to failure at a crosshead speed of 10 mm/min. Flexural strength, in megapascals (MPa), was calculated as TFS = 3Fll2bd 2, where F is the failure load in newtons, 1 is the test span (8 mm), b is the specimen width, and d is the specimen thickness.
Ultimate compressive strength Each cement was mixed/cured according to the manufacturer's instructions and placed into a cylindrical Teflon mold measuring 6 mm high and 6 mm in diameter. The Teflon mold was placed on a celluloid-lined, flat glass plate. The mix was condensed and the mold slightly overfllled within 90 seconds after the start of the mix. A second celluloidlined, fiat glass plate was then placed on top of the mold and the plates tightly clamped together. The light-cure samples were cured for 60 seconds with a visible curing light placed against the fiat plates at each end of the mold. Two minutes after mixing/curing, the samples were placed in an oven at 95% - 5% relative humidity and 37 ° ± 1° C. One hour after mixing/curing, the samples were removed from the oven, and the cylinder was ground flat with wet 240 grit silicon carbide paper. The specimens were then removed from the molds, and the diameter was recorded within 0.01 mm. The samples were then stored in distilled water at 37 ° 4- I ° C until testing. Ten specimens from each cement were evaluated at 1 hour, 24 hours, and 7 days after mixing. Each specimen was placed on its end between parallel platens of the testing machine, and a thin piece of moist blotting paper was inserted between the ends of the specimen and the platens. The test samples were then loaded to the breaking point at a crosshead speed of I mm/min. Ultimate compressive strength, in megapascals, was calculated as UCS = 4F/-/rd2, where F is the failure load in newtons, and d is the specimen diameter.
Rigidity Rigidity was calculated as the slope of the straight line (elastic) portion of the compressive strength stress-strain curve for each sample.
Diametral tensile strength Each cement was mixed/cured according to the manufacturer's instructions and placed into a cylindrical Teflon mold measuring 3 mm high and 6 mm in diameter. The Teflon mold was placed on a celluloid-lined, fiat glass plate. The mix was condensed, and the mold slightly overfilled within
McCarthy and Hondrum
American Journal of Orthodontics and Dentofacial Orthopedics Volume 105, No. 2
Table I. Materials --
[
Description
Manufacturer
Ketae-Bond
Encapsulated, chemically cured glass ionomer cement
Zionomer
Two-paste, light-cured glass ionomer cement
Premier Dental Products Norristown, Pa. Den Mat Corporation Santa Monica, Calif.
Material
90 seconds of mixing. A second celluloid-lined, flat glass plate was then placed on top of the mold and the plates tightly clamped together. The light-cured samples were cured for 60 seconds with a visible curing light placed against the flat plates at each end of the mold. Two minutes after mixing/curing, the samples were placed in an oven at 95% --- 5% relative humidity and 37 ° __+ 1° C. One hour after mixing/curing, the samples were removed from the oven, removed from the molds, and the length and diameter of the specimens recorded to within 0.01 mm. The samples were then stored in distilled water at 37 ° + 1° C until testing. Ten specimens from each cement were evaluted at 1 hour, 24 hours, and 7 days after mixing. Each specimen was placed on its side between the parallel platens of the testing machine, and a thin piece of moist blotting paper was inserted between the ends of the specimen and the platens. The specimen was then loaded to failure at a crosshead speed of 1 mm/min. Diametral tensile strength, in megapascals, was calculated as DTS = 2F/rrdl, where F is the failure load in newtons, d is the specimen diameter, and 1 is the specimen length.
Bond strength Sixty recently extracted human molars were cleaned, by using a robber cup with flour of pumice, and stored in distilled water. Enamel surfaces, approximately 4 × 4 mm, were prepared by successive wet grinding to a 600 grit with silicon carbide paper on a polishing machine (Buehler, Lake Bluff, II1.). The teeth were randomly divided into equal groups. Stainless steel lingual buttons (Unitek Corp., Monrovia, I11.), with 3 × 3 nun welded mesh pads, were bonded to each tooth. In the bonding procedure for the light-cured cements, the enamel surfaces were conditioned for 30 seconds with a weak nitric acid, aluminum oxalate, polyacrylic acid solution (DenMat Corp., Santa Monica, Calif.), rinsed thoroughly for 30 seconds with an air/water spray, and gently dried with an oil-free air syringe, according to the manufacturer's instructions. The cement was then mixed and applied in a thin layer on the mesh pad of the lingual pad with a plastic instrument. The lingual button was positioned and pressed onto the enamel surface with cotton forceps until fully seated. The cement was then light-cured for 20 seconds on each of the four sides of the pad. A new mix was prepared for each sample. The chemically cured samples were similarly prepared. The enamel surfaces were conditioned for 10 seconds with 25% polyacrylic acid solution (Premier Dental, Norristown, Pa.), rinsed thoroughly for 30 seconds with an air/water spray, and gently dried with an oil-free air syringe, according to the manufacturer's instructions. The encapsulated cement
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was triturated for 10 seconds and 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. A new mix was prepared for each application. Immediately after bonding, the samples were placed for 15 minutes in an oven at 37 ° - 1° C and 95% - 5% relative humidity. After 15 minutes, the samples were placed in a distilled water bath at 37 ° -- 1° C until ready for testing at 1 hour, 24 hours, and 7 days. In preparation for testing, the teeth were removed from the water bath, and the roots embedded in a fast-set acrylic resin (Fastray, Bosworth Co., Skokie, I11.). A surveyor was used to ensure perpendicular mounting of each sample in the resin base. The shear bond strength of each sample was determined with the universal testing instrument. The shearing force was applied at the ligature groove of the lingual button at a crosshead speed of 1 mm/min. The ultimate shear strength was recorded (MPa), and the fracture sites were visually inspected with a stereoscopic light (Teknique Dentalscope, Dated, Inc., Clearwater, Fla.) to determine the location of the fracture. The arithmetic means of the I hour bond strength tests were compared with the minimum bond strength (7 MPa) recommended for successful clinical bonding, t' For the cement having a mean 1 hour bond strength greater than 7 MPa, the bond strength test was repeated at I5 minutes after curing to determine whether the cement had sufficient bond strength to be used clinically at that time.
Data analysis Data from each test were subjected to two-way analysis of variance (ANOVA) at the 95% level of confidence. Data from ihe mechanical property and bond strength tests were compared to determine association between mechanical properties and bond strength for glass ionomer cements. The ratio of changes in mechanical strength properties were compared with the changes in bond strength from 1 to 24 hours. The correlation coefficient, by using the difference between the means, the average of the means, and the standard deviations, was calculated.
RESULTS Table II is a legend for abbreviations used in the tables.
Transverse flexural strength (Table III) A strength o f 0 M P a was given to chemically cured glass i o n o m e r c e m e n t at 1 hour because samples could
138 McCarthy and Hondrum
Table II. Legend CCI = Chemically cured glass ionomer cement tested at I hour. CC24 = Chemically cured glass ionomer cement tested at 24 hours. CC7D = Chemically cured glass ionomer cement tested at 7 days. LC15m = Light-cured glass ionomer cement tested at 15 minutes. LCI = Light-cured glass ionomer cement tested at 1 hour. LC24 = Light-cured glass ionomer cement tested at 24 hours. LC7D = Light-cured glass ionomer cement tested at 7 days. DTS = Diametral tensile strength. TFS = Transverse flexural strength. UCS = Compressive strength.
not be removed from the molds without fracture; a total of 30 samples were made for evaluation at 1 hour, all of which fractured. Two-way ANOVA (Table IV) reveals a significant difference between the materials; the effect of time was not significant.
Compressive strength (Table V) Two-way ANOVA (Table VI) indicates that there is no significant difference between the materials; the effect of time was significant.
Rigidity (Table VII) Two-way ANOVA (Table VIII) reveals a significant difference between the groups with respect to time and materials.
Diametral tensile strength (Table IX) Two-way ANOVA (Table X) reveals that there is a significant difference between the groups with respect to time and materials.
Bond strength (Table XI) Two-way ANOVA (Table XII) reveals a significant difference between the groups with respect to time and materials. The 1-hour mean bond strength of the light-cured glass ionomer cement (but not the chemically cured) was greater than the minimum bond strength (7 MPa) recommended for successful clinical bonding. Therefore the bond strength of the light-cured Cement was evaluated at 15 minutes.
Comparison of mechanical and bond strength tests The ratios of the mechanical strength tests to the bond strength tests were evaluated at 1 and 24 hours.
American Journal of Orthodonticsand DentofacialOrthopedics February 1994
The results indicate a close association between the diametral tensile and bond strength tests; the diametral tensile strength was found to be approximately twice the value of the bond strength for the two cements at both 1 hour and 24 hours. The correlation coefficient, by using the difference between the means, the average of the means, and the standard deviations, was calculated. There was strong correlation between the diametral tensile strength and the bond strength for the values measured at 1 and 24 hours (r = 0.98).
Location of bond failure With the chemically cured cement, 73% of the bond fractures were cohesive (within the cement), and the remaining 27% were adhesive bond failures at the cement-enamel interface. All of the light-cured cement failures were cohesive.
DISCUSSION The mechanical strength properties (with the exception of rigidity) of the chemically cured cement increase with time. This is in accordance with previous work by Wilson and McLean ts that suggests a continuous increase in strength over time. Conversely, the mechanical strength properties of the light-cured cement decrease slightly over time. This suggests that although maximum strength is achieved early, the lightcured cement may be susceptible to water degradation after its initial set. The mean bond strengths of both the light-cured and chemically cured cements increased with time, although the light-cured material is substantially stronger at all time periods. This is in agreement with the results of the chemically cured mechanical strength tests, but not those of the light-cured cement. A probable explanation for this is the amount of cement surface area that is exposed to water in the bath. Compared with the mechanical strength tests, a relatively small amount of cement surface area was exposed to water under the brackets. This would minimize the adverse effect that water has on strength. Transverse flexural strength data mirror that of the diametral tensile strength test. This might be anticipated as both tests are designed to determine tensile strength of brittle materials. The transverse flexural results are more dramatic than the diametral; this probably relates to sample geometry and fragility. Compressive strengths are approximately the same between materials and over time, and there appeared to be little association between compressive strength and bond strength. This might be expected as brittle materials traditionally fracture tension rather than compression. Rigidity appears to be inversely proportional to ten-
McCarthy and Hondrum 139
American Journal of Orthodontics and Oentofaciat Orthopedics Volume 105, No. 2
T a b l e III. D e s c r i p t i v e s t a t i s t i c s m t r a n s v e r s e flexural strength test
Group
Mean (MPa)
CCI CC24 CC7D LCI LC24 LC7D
0 11.22 12.70 60.61 45.46 43.18
I I
Standarddeviation (MPa)
Minimum (MPa)
Maximum (Mea)
-10.22 8.82 5.37 6.08 12.97
-0 0 53.50 38.89 21.69
28.14 28.08 70.36 55.98 59.5
I I
Coefficientvariant (%) 91.5 69.4 8.9 13.4 30.1
T a b l e IV. T w o - w a y A N O V A m t r a n s v e r s e flexural strength
--Source
I
DF
Mat(A) Time (B) AxB Error
I
Sum of squares
1 2 2 54
26181.019 64.275 2695.778 3754.882
I
Mean square
I
F-test
26181.019 32.137 1347.889 69.535
I
376.517 0.462 19.384
P value 0.0001 0.6324 0.0001
T a b l e 7. D e s c r i p t i v e s t a t i s t i c s - - c o m p r e s s i v e strength |
Group
Mean (MPa)
[ [
CCI CC24 CC7D LCI LC24 LC7D
100.47 132.673 133.916 126.104 130.953 112.42
Standarddeviation (MPa)
Minimum (MPa)
11.889 15.355 33.477 10.104 12.126 10.392
83.16 99.39 68.18 109.94 113.96 93.36
Maximum (MPa)
Coefficientvariant (%)
120.92 150.18 170.65 138.57 148.67 133.28
11.833 11.573 24.999 8.013 9.259 9.244
T a b l e Vl. T w o - w a y A N O V A - - c o m p r e s s i v e strength
Source
I
Mat (A) Time (B) AxB Error
[
DF
Sum of squares
Mean square
9.567 3341.153 5435.832 16553.29
9.567 1670.576 2717.916 312.326
1 2 2 53
I
I
F test 0.031 5.349 8.702
I
I
P value 0.8617 0.0077 0.0005
T a b l e VII. D e s c r i p t i v e s t a t i s t i c s - - r i g i d i t y
Group CCI CC24 CC7D LCI LC24 LC7D
sile strength the inherent cally cured whereas the
Mean (MPa) 1227 1553.5 1206.2 648.1 643.8 449.9
[ I
Standarddeviation (MPa)
Minimum (MPa)
Maximum (MPa)
216.28 193.977 122.738 131.644 62.057 28.215
1016 1228 916 500 541 450
1512 1811 1341 866 770 541
and to b o n d strength. T h i s m a y relate to nature o f the materials tested; the c h e m i material b e h a v i n g as a brittle c e m e n t , light-cured material b e h a v e s m o r e as a
I I
Coefficientvariant (%) 17.627 12.486 10.176 20.312 9.639 5.644
resin, i . e . , u n t i m a t e l y stronger yet less rigid. In addition, rigidity decreases after 24 hours for both c e m e n t s . This m a y again be related to w a t e r degradation. A strong association was found to exist b e t w e e n the
140
McCarthy and Hondrum
AmericanJournalof Orthodontics and Dentofacial Orthopedics February1994
Table VIII. Two-way ANOVA--rigidity
I
Source
DF
Mat (A) Time (B) AxB Error
]
1 2 2 52
Sum of squares
Mean square
7708137.616 621247.631 264926.777 999459.5
7708137.616 310623.815 132463.388 19220.375
I
F test
J
401.04 16.161 6.892
P value 0.0001 0.0001 0.0022
Table IX. Descriptive statistics--diametral tensile strength Group
I
Mean (MPa)
CC 1 CC24 CC7D LCI LC24 LC7D
9.741 14.493 15.87 22.246 21.509 19.371
I
Standarddeviation (MPa)
Minimum (MPa)
Maximum (MPa)
Coefficientvariant (%)
2.303 .965 3.356 2.387 2. i 19 3.096
5.47 13.11 i 1.24 17.39 18.29 13.78
12.75 15.84 21.84 25.0 25.19 23.58
23.638 6.659 21.147 10.73 9.85 15.985
Table X. Two-way ANOVA--diametral tensile strength Source Mat (A) Time (B) AxB Error
I
DF 1 2 2 56
I
|
Sum of squares
Mean square
908.606 46.601 206. l 17 344.407
908.606 23.3 103.059 6.15
]
F test 147.738 3.789 16.757
!
]
P value
0.000l 0.0286 0.0001
Table Xi. Descriptive statistics--bond strength
Group
Mean (MPa)
Standard deviation (MPa)
CCI CC24 CC7D LCI5m LCI LC24 LC7D
4.33 6.28 8.71 I 1.23 12.17 12.58 12.53
1.32 1.47 2.21 1.81 2.69 1.46 3.27
diametral tensile strength and bond strength tests. This finding may allow prediction of bond strength on the basis of mechanical properties. The results of the bond strength tests indicate that the light-cured cement possesses sufficient strength at 15 minutes to be used clinically to bond orthodontic brackets. The chemically cured cement did not achieve a mean of 7 MPa for more than 24 hours; this has clinical implications for the orthodontist. The dramatic difference in tensile and bond strength between light-cured and chemically cured materials in this study may relate not only to curing method, but
Minimum(MPa)
[
2.40 3.63 4.58 8.60 7.87 9.02 5.26
Maximum(MPa) [ 7.30 8.49 11.18 14.45 ! 6.45 14.36 16.18
Coefficient (%)variant 30.4 23.4 25.3 16.1 22.1 11,6 26.1
the chemical nature of the primary materials themselves. If the light-cured material, despite its superior mechanical and early bonding strength, has properties more characteristic of composite resins than of glass ionomer cements, traditional advantages of glass ionomers, such as fluoride release and ease of removal, may be compromised. Further study is needed to resolve these questions. CONCLUSIONS
The light-cured cement achieves maximum tensile strength faster than the chemically cured material.
American Journalof Orthodonticsand DentofacialOrthopedics Volume 105, No. 2
McCarthy and Hondrum
141
Table XII. Two-way ANOVA--bond strength Source Mat(A) Time (B) AxB Error
I
,ean
DF
Sum of squares
squares
1 2 2 54
537.902 56.241 41.027 259.543
537.902 28.12 20.513 4.806
The bond strengths of both chemically cured and light-cured glass ionomer cements increase with time. The bond strength of light-cured cement was greater than the proposed minimum level for clinical success at all test times; the bond strength of chemically cured materials did not reach minimum levels until after 24 hours. The bond failures of the glass ionomer cements are primarily cohesive, light-cured more than chemically gured. There is a high correlation between the diametral tensile strength and the bond strength for glass ionomer cements at 1 hour and 24 hours (r = 0.98). REFERENCES 1. Buonocore MG. A simple method of increasing the adhesion of acrylic filling material to enamel surfaces. L Dent Res 1955;34:849-53. 2. Retief DH, Sadowski PL. Clinical experience with the acid-etch technique in orthodontics. AM J OR'moP 1975;79:645-65. 3. Proffit W. Contemporary orthodontics. St. Louis: CV Mosby, 1986:287. 4. Thompson RE, Way DC. Enamel loss due to prophylaxis and multiple bonding/rebonding of orthodontic attachments. AM J OR'roOD 1981;79:282-95. 5. Zachrisson BU, Zachrisson S. Caries incidence and oral hygiene during orthodontic treatment. Scand J Dent Res 1971;79:394401. 6. Forsten L. Fluoride release from a glass ionomer cement. Scand J Dent Res 1977;85:503-4. 7. Kidd EAM. Cavity sealing ability of composite glass ionomer cement restorations. Br Dent J 1978;144:139-42. 8. Maldonato A, Swartz ML, Phillips RW. An in vitro study of certain properties of a glass ionomer cement. J Am Dent Assoc 1978;145:67-71.
I
F-test 111.915 5.851 4.268
I
P value 0.0001 0.005 0.019
9. White LW. Glass ionomer cement. J Clin Orthod 1986;20:38791. 10. McComb D, Sirisko R, Brown J. Scientific comparison of commercial glass ionomer cements. J Can Dent Assoc 1984;9:69970. 11. Norris DS, Mclnnes-Ledoux P, Schwaninger B, Weinberg R. Retention of orthodontic bands with new fluoride-releasing cements. AM J OR~IOD 1986;89:206-11. 12. Copenhavcr DJ. In vitro comparison of glass ionomer cements ability to inhibit decalcification under orthodontic bands. A~.I J OR'rHOD 1986;89:528. 13. Maijer R, Smith DC. A comparison between zinc phosphate and glass ionomer cement in orthodontics. AM J ORTHODDENTOFAC ORTtIOP 1988;93(4):273-9. 14. Seeholzer HW, Dasch W. Banding with a glass ionomer cement. J Clin Orthod 1988;22:165-9. 15. Murray GA, Yates JL. A comparison of the bond strengths of composite resins and glass ionomer cements. J Pedodont 1984;8:172-7. 16. Klockowski R, Davis EL, Joynt RB, Wieczkowski G, MacDonald A. Bond strength and durability of glass ionomer cements used as bonding agents in the placement of orthodontic brackets. AM J ORTHOD DENTOFACORTHOP 1989;96:60-4. 17. Ferguson JW, Read MJF, Watts DC. Bond strengths of an integral bracket-base combination: an in vitro study. Eur J Orthod 1984;6:267-76. 18. McLean JW, Wilson AD. The clinical development of the glass ionomer cement. Formulations and properties. Aust Dent J 1977;22(1):31-6. 19. Lopez Jl. Retentive shear bond strength of various bonding attachment bases. AM J OR'I'HOD 1980;77:669-78.
Reprint requests to: Maj. Michael F. McCarthy 665th MED Co (DS) (Prov) Unit #15659 APO AP 96218
Mechanical and bond strength properties of light-cured and chemically cured glass ionomer cements Michael F. McCarthy, DMD," and Steven O. Hondrum, DDS, MS b Fort George G. Meade, Md.
The purpose of this study was to evaluate the mechanical and bond strength properties of a commercially available light-cured glass ionomer cement and of a chemically cured glass ionomer cement. Sixty recently extracted human molars were randomly divided into six equal groups, and the bond strengths of the two cement types were evaluated at 1 hour, 24 hours, and 7 days. Stainless steel lingual buttons were bonded to prepared enamel surfaces, and the samples were placed in a water bath at 37 ° C until testing. The shear bond strength of each sample was determined with a universal testing instrument. The mechanical strength properties of the two cements were then evaluated. The transverse flexural strength, compressive strength, rigidity, and diametral tensile strength were tested for each cement at 1 hour, 24 hours, and 7 days. The results of the mechanical property strength tests were then compared with the results of the bond strength tests. The results from the study conclude: (1) The light-cured cement achieves maximum tensile strength faster than the chemicalIy cured material; (2) the bond strengths of both chemically cured and light-cured glass ionomer cements increase with time; (3) the bond strength of light-cured cement was greater than the proposed minimum level for clinical success at all test times whereas the bond strength of the chemically cured material did not reach minimum levels until after 24 hours; (4) the bond failures of the glass ionomer cements are primarily cohesive; and, (5) there is a high correlation between the diametral tensile strength and the bond strength for glass ionomer cements at 1 hour and 24 hours (r = 0.98). (AMJ ORTHOD DENTOFACORTHOP 1994;105:135-41.)
S i n c e the introduction of acid etching of enamel by Buonocore' in 1955, the use of composite resins has become widely accepted as a medium for cementing orthodontic brackets to a tooth. 2 Bonded orthodontic brackets are more advantageous than bands in that they have no interproximal contact; they are both easier to place and to remove; they are more esthetic and hygienic; and they are less irritating to the gingiva. 3 However, the use of composite resins as the bonding medium in orthodontics also has its disadvantages. During the composite resin bonding procedure, surface enamel may be lost through preetching pumicing, as well as through dissolution and demineralization of the superficial enamel layer during the acid etch. Enamel can also be lost during the cleanup of residual resin after the bonded bracket has been removed from the tooth, as well as during rebonding procedures. This is
The views and opinions of the authors expressed herein do not necessarily state or reflect those of the United States government or the Department of Defense. 'Major, US Army Dental Corps, U.S. Army Orthodontic Residency Program. ~'Colonel, US Army Dental Corps; Chief, Dental Materials, U.S. Army Institute of Dental Research. 811142490
a concern since the concentration of fluoride is greatest at the surface of the enamel: Another risk associated with the use of orthodontic bands and composite resin bonded brackets is the decalcification of adjacent tooth structure. This decalcification, which may occur within 4 weeks of fitting the appliance, has been attributed to the prolonged accumulation and retention of bacterial plaque next to the orthodontic bands and brackets: A bonding material that could make the tooth structure more resistant to caries, yet retain the bonding strengths and properties of composite resins without the loss of enamel, may prove to be beneficial. One potential dental adhesive is the glass ionomer cement. Glass ionomer cements are a relatively new class of dental materials which possess a unique combination of properties. They adhere to the enamel surface without the need of acid etching or enamel surface conditioning. They serve as a reservoir of fluoride ions that protect against decalcification of the surrounding tooth structure. 68 The glass ionomer cements are also easier to remove from enamel than composite resins. Since the adhesion of the cement does not involve the acid etching of enamel from the tooth surface, the cement can be dessicated with a stream of air from an air syringe 135
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McCarthy and Hondrttm
and easily removed. 9 These characteristics o f the glass ionomer cement may reduce the amount o f enamel surface loss often associated with bonding procedures that use composite resins. The glass ionomer cements have been shown to have significantly higher tensile and compressive strengths than zinc phosphate cements as a cementing medium for orthodontic bands, t° T h e y have also been shown to have retentive strengths similar to the zinc phosphate and polycarboxylate cements. In studying the site o f cement failure, it has been shown that the glass ionomer failed more often at the cement-band interface. Failure at this location is considered to be most favorable, as the fluoride-releasing cement may offer protection against decalcification under loose bands. ".12 These studies have been supported by clinical findings which show that orthodontic bands cemented with the glass ionomer cements have a significantly lower failure rate than those cemented with zinc phosphate or polycarboxylate cements. 9a3"~4 In vitro bond strength studies ~5,16 have shown that the bracket bond strength o f composite resins are significantly higher than those o f the glass ionomers. However, White 9 has estimated that there were no more bracket failures with glass ionomers than with composite resins. If adherence to the enamel alone is the main requisite in orthodontic bonding, then composite resins alone should be used for bonding. However, glass ionomer cements possess a number o f advantageous properties over composite resins that may outweigh its relatively inferior bond strength. The glass ionomer cements have been investigated for use as a cementing medium for orthodontic bands and bonded brackets. 9.12"17They are presently marketed as both chemically cured and light-cured systems. The purpose o f this study is to evaluate the mechanical and bond strength properties o f a commercially available light-cured glass ionomer cement and o f a chemically cured glass ionomer cement. It will be further determined whether mechanical properties can be correlated with bond strength.
METHODS AND MATERIALS Two cements were evaluated (Table I). A universal testing machine (Model 1011, Instron Corp., Canton, Mass.) was used in determination of the mechanical and bond strength properties.
Transverse flexural strength Each cement was mixed or cured according to the manufacturer's instructions and placed into a rectangular Teflon mold measuring 2 × 2 × 10 mm. The Teflon mold was placed on a celluloid-lined, flat glass plate. The mix was condensed, and the mold slightly overfilled within 90 seconds after the start of the mix. A second celluloid-lined, fiat glass plate was then placed on top of the mold and the plates tightly
American Journal of Orthodontics and Dentofacial Orthopedics February 1994
clamped together. The light-cure samples were cured with a visible curing light (Prismetics, Caulk, Milford, Del.) placed against the upper glass plate in a series of three overlapping steps for 20 seconds each (total 60 seconds). Two minutes after mixing/curing, the samples were placed in an oven at 95% --- 5% relative humidity and 37 ° --- 1° C. Fifteen minutes after mixing/curing, the test samples were removed from the oven, and the excess flash removed. The depth and width of each sample were measured to within 0.01 mm, and the samples were stored in distilled water at 37" + 1° C until testing. Ten specimens from each cement were tested at 1 hour, 24 hours, and 7 days after mixing. Each sample was placed between the bearers of a three-point beam apparatus and continuously loaded to failure at a crosshead speed of 10 mm/min. Flexural strength, in megapascals (MPa), was calculated as TFS = 3Fll2bd 2, where F is the failure load in newtons, 1 is the test span (8 mm), b is the specimen width, and d is the specimen thickness.
Ultimate compressive strength Each cement was mixed/cured according to the manufacturer's instructions and placed into a cylindrical Teflon mold measuring 6 mm high and 6 mm in diameter. The Teflon mold was placed on a celluloid-lined, flat glass plate. The mix was condensed and the mold slightly overfllled within 90 seconds after the start of the mix. A second celluloidlined, fiat glass plate was then placed on top of the mold and the plates tightly clamped together. The light-cure samples were cured for 60 seconds with a visible curing light placed against the fiat plates at each end of the mold. Two minutes after mixing/curing, the samples were placed in an oven at 95% - 5% relative humidity and 37 ° ± 1° C. One hour after mixing/curing, the samples were removed from the oven, and the cylinder was ground flat with wet 240 grit silicon carbide paper. The specimens were then removed from the molds, and the diameter was recorded within 0.01 mm. The samples were then stored in distilled water at 37 ° 4- I ° C until testing. Ten specimens from each cement were evaluated at 1 hour, 24 hours, and 7 days after mixing. Each specimen was placed on its end between parallel platens of the testing machine, and a thin piece of moist blotting paper was inserted between the ends of the specimen and the platens. The test samples were then loaded to the breaking point at a crosshead speed of I mm/min. Ultimate compressive strength, in megapascals, was calculated as UCS = 4F/-/rd2, where F is the failure load in newtons, and d is the specimen diameter.
Rigidity Rigidity was calculated as the slope of the straight line (elastic) portion of the compressive strength stress-strain curve for each sample.
Diametral tensile strength Each cement was mixed/cured according to the manufacturer's instructions and placed into a cylindrical Teflon mold measuring 3 mm high and 6 mm in diameter. The Teflon mold was placed on a celluloid-lined, fiat glass plate. The mix was condensed, and the mold slightly overfilled within
McCarthy and Hondrum
American Journal of Orthodontics and Dentofacial Orthopedics Volume 105, No. 2
Table I. Materials --
[
Description
Manufacturer
Ketae-Bond
Encapsulated, chemically cured glass ionomer cement
Zionomer
Two-paste, light-cured glass ionomer cement
Premier Dental Products Norristown, Pa. Den Mat Corporation Santa Monica, Calif.
Material
90 seconds of mixing. A second celluloid-lined, flat glass plate was then placed on top of the mold and the plates tightly clamped together. The light-cured samples were cured for 60 seconds with a visible curing light placed against the flat plates at each end of the mold. Two minutes after mixing/curing, the samples were placed in an oven at 95% --- 5% relative humidity and 37 ° __+ 1° C. One hour after mixing/curing, the samples were removed from the oven, removed from the molds, and the length and diameter of the specimens recorded to within 0.01 mm. The samples were then stored in distilled water at 37 ° + 1° C until testing. Ten specimens from each cement were evaluted at 1 hour, 24 hours, and 7 days after mixing. Each specimen was placed on its side between the parallel platens of the testing machine, and a thin piece of moist blotting paper was inserted between the ends of the specimen and the platens. The specimen was then loaded to failure at a crosshead speed of 1 mm/min. Diametral tensile strength, in megapascals, was calculated as DTS = 2F/rrdl, where F is the failure load in newtons, d is the specimen diameter, and 1 is the specimen length.
Bond strength Sixty recently extracted human molars were cleaned, by using a robber cup with flour of pumice, and stored in distilled water. Enamel surfaces, approximately 4 × 4 mm, were prepared by successive wet grinding to a 600 grit with silicon carbide paper on a polishing machine (Buehler, Lake Bluff, II1.). The teeth were randomly divided into equal groups. Stainless steel lingual buttons (Unitek Corp., Monrovia, I11.), with 3 × 3 nun welded mesh pads, were bonded to each tooth. In the bonding procedure for the light-cured cements, the enamel surfaces were conditioned for 30 seconds with a weak nitric acid, aluminum oxalate, polyacrylic acid solution (DenMat Corp., Santa Monica, Calif.), rinsed thoroughly for 30 seconds with an air/water spray, and gently dried with an oil-free air syringe, according to the manufacturer's instructions. The cement was then mixed and applied in a thin layer on the mesh pad of the lingual pad with a plastic instrument. The lingual button was positioned and pressed onto the enamel surface with cotton forceps until fully seated. The cement was then light-cured for 20 seconds on each of the four sides of the pad. A new mix was prepared for each sample. The chemically cured samples were similarly prepared. The enamel surfaces were conditioned for 10 seconds with 25% polyacrylic acid solution (Premier Dental, Norristown, Pa.), rinsed thoroughly for 30 seconds with an air/water spray, and gently dried with an oil-free air syringe, according to the manufacturer's instructions. The encapsulated cement
137
was triturated for 10 seconds and 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. A new mix was prepared for each application. Immediately after bonding, the samples were placed for 15 minutes in an oven at 37 ° - 1° C and 95% - 5% relative humidity. After 15 minutes, the samples were placed in a distilled water bath at 37 ° -- 1° C until ready for testing at 1 hour, 24 hours, and 7 days. In preparation for testing, the teeth were removed from the water bath, and the roots embedded in a fast-set acrylic resin (Fastray, Bosworth Co., Skokie, I11.). A surveyor was used to ensure perpendicular mounting of each sample in the resin base. The shear bond strength of each sample was determined with the universal testing instrument. The shearing force was applied at the ligature groove of the lingual button at a crosshead speed of 1 mm/min. The ultimate shear strength was recorded (MPa), and the fracture sites were visually inspected with a stereoscopic light (Teknique Dentalscope, Dated, Inc., Clearwater, Fla.) to determine the location of the fracture. The arithmetic means of the I hour bond strength tests were compared with the minimum bond strength (7 MPa) recommended for successful clinical bonding, t' For the cement having a mean 1 hour bond strength greater than 7 MPa, the bond strength test was repeated at I5 minutes after curing to determine whether the cement had sufficient bond strength to be used clinically at that time.
Data analysis Data from each test were subjected to two-way analysis of variance (ANOVA) at the 95% level of confidence. Data from ihe mechanical property and bond strength tests were compared to determine association between mechanical properties and bond strength for glass ionomer cements. The ratio of changes in mechanical strength properties were compared with the changes in bond strength from 1 to 24 hours. The correlation coefficient, by using the difference between the means, the average of the means, and the standard deviations, was calculated.
RESULTS Table II is a legend for abbreviations used in the tables.
Transverse flexural strength (Table III) A strength o f 0 M P a was given to chemically cured glass i o n o m e r c e m e n t at 1 hour because samples could
138 McCarthy and Hondrum
Table II. Legend CCI = Chemically cured glass ionomer cement tested at I hour. CC24 = Chemically cured glass ionomer cement tested at 24 hours. CC7D = Chemically cured glass ionomer cement tested at 7 days. LC15m = Light-cured glass ionomer cement tested at 15 minutes. LCI = Light-cured glass ionomer cement tested at 1 hour. LC24 = Light-cured glass ionomer cement tested at 24 hours. LC7D = Light-cured glass ionomer cement tested at 7 days. DTS = Diametral tensile strength. TFS = Transverse flexural strength. UCS = Compressive strength.
not be removed from the molds without fracture; a total of 30 samples were made for evaluation at 1 hour, all of which fractured. Two-way ANOVA (Table IV) reveals a significant difference between the materials; the effect of time was not significant.
Compressive strength (Table V) Two-way ANOVA (Table VI) indicates that there is no significant difference between the materials; the effect of time was significant.
Rigidity (Table VII) Two-way ANOVA (Table VIII) reveals a significant difference between the groups with respect to time and materials.
Diametral tensile strength (Table IX) Two-way ANOVA (Table X) reveals that there is a significant difference between the groups with respect to time and materials.
Bond strength (Table XI) Two-way ANOVA (Table XII) reveals a significant difference between the groups with respect to time and materials. The 1-hour mean bond strength of the light-cured glass ionomer cement (but not the chemically cured) was greater than the minimum bond strength (7 MPa) recommended for successful clinical bonding. Therefore the bond strength of the light-cured Cement was evaluated at 15 minutes.
Comparison of mechanical and bond strength tests The ratios of the mechanical strength tests to the bond strength tests were evaluated at 1 and 24 hours.
American Journal of Orthodonticsand DentofacialOrthopedics February 1994
The results indicate a close association between the diametral tensile and bond strength tests; the diametral tensile strength was found to be approximately twice the value of the bond strength for the two cements at both 1 hour and 24 hours. The correlation coefficient, by using the difference between the means, the average of the means, and the standard deviations, was calculated. There was strong correlation between the diametral tensile strength and the bond strength for the values measured at 1 and 24 hours (r = 0.98).
Location of bond failure With the chemically cured cement, 73% of the bond fractures were cohesive (within the cement), and the remaining 27% were adhesive bond failures at the cement-enamel interface. All of the light-cured cement failures were cohesive.
DISCUSSION The mechanical strength properties (with the exception of rigidity) of the chemically cured cement increase with time. This is in accordance with previous work by Wilson and McLean ts that suggests a continuous increase in strength over time. Conversely, the mechanical strength properties of the light-cured cement decrease slightly over time. This suggests that although maximum strength is achieved early, the lightcured cement may be susceptible to water degradation after its initial set. The mean bond strengths of both the light-cured and chemically cured cements increased with time, although the light-cured material is substantially stronger at all time periods. This is in agreement with the results of the chemically cured mechanical strength tests, but not those of the light-cured cement. A probable explanation for this is the amount of cement surface area that is exposed to water in the bath. Compared with the mechanical strength tests, a relatively small amount of cement surface area was exposed to water under the brackets. This would minimize the adverse effect that water has on strength. Transverse flexural strength data mirror that of the diametral tensile strength test. This might be anticipated as both tests are designed to determine tensile strength of brittle materials. The transverse flexural results are more dramatic than the diametral; this probably relates to sample geometry and fragility. Compressive strengths are approximately the same between materials and over time, and there appeared to be little association between compressive strength and bond strength. This might be expected as brittle materials traditionally fracture tension rather than compression. Rigidity appears to be inversely proportional to ten-
McCarthy and Hondrum 139
American Journal of Orthodontics and Oentofaciat Orthopedics Volume 105, No. 2
T a b l e III. D e s c r i p t i v e s t a t i s t i c s m t r a n s v e r s e flexural strength test
Group
Mean (MPa)
CCI CC24 CC7D LCI LC24 LC7D
0 11.22 12.70 60.61 45.46 43.18
I I
Standarddeviation (MPa)
Minimum (MPa)
Maximum (Mea)
-10.22 8.82 5.37 6.08 12.97
-0 0 53.50 38.89 21.69
28.14 28.08 70.36 55.98 59.5
I I
Coefficientvariant (%) 91.5 69.4 8.9 13.4 30.1
T a b l e IV. T w o - w a y A N O V A m t r a n s v e r s e flexural strength
--Source
I
DF
Mat(A) Time (B) AxB Error
I
Sum of squares
1 2 2 54
26181.019 64.275 2695.778 3754.882
I
Mean square
I
F-test
26181.019 32.137 1347.889 69.535
I
376.517 0.462 19.384
P value 0.0001 0.6324 0.0001
T a b l e 7. D e s c r i p t i v e s t a t i s t i c s - - c o m p r e s s i v e strength |
Group
Mean (MPa)
[ [
CCI CC24 CC7D LCI LC24 LC7D
100.47 132.673 133.916 126.104 130.953 112.42
Standarddeviation (MPa)
Minimum (MPa)
11.889 15.355 33.477 10.104 12.126 10.392
83.16 99.39 68.18 109.94 113.96 93.36
Maximum (MPa)
Coefficientvariant (%)
120.92 150.18 170.65 138.57 148.67 133.28
11.833 11.573 24.999 8.013 9.259 9.244
T a b l e Vl. T w o - w a y A N O V A - - c o m p r e s s i v e strength
Source
I
Mat (A) Time (B) AxB Error
[
DF
Sum of squares
Mean square
9.567 3341.153 5435.832 16553.29
9.567 1670.576 2717.916 312.326
1 2 2 53
I
I
F test 0.031 5.349 8.702
I
I
P value 0.8617 0.0077 0.0005
T a b l e VII. D e s c r i p t i v e s t a t i s t i c s - - r i g i d i t y
Group CCI CC24 CC7D LCI LC24 LC7D
sile strength the inherent cally cured whereas the
Mean (MPa) 1227 1553.5 1206.2 648.1 643.8 449.9
[ I
Standarddeviation (MPa)
Minimum (MPa)
Maximum (MPa)
216.28 193.977 122.738 131.644 62.057 28.215
1016 1228 916 500 541 450
1512 1811 1341 866 770 541
and to b o n d strength. T h i s m a y relate to nature o f the materials tested; the c h e m i material b e h a v i n g as a brittle c e m e n t , light-cured material b e h a v e s m o r e as a
I I
Coefficientvariant (%) 17.627 12.486 10.176 20.312 9.639 5.644
resin, i . e . , u n t i m a t e l y stronger yet less rigid. In addition, rigidity decreases after 24 hours for both c e m e n t s . This m a y again be related to w a t e r degradation. A strong association was found to exist b e t w e e n the
140
McCarthy and Hondrum
AmericanJournalof Orthodontics and Dentofacial Orthopedics February1994
Table VIII. Two-way ANOVA--rigidity
I
Source
DF
Mat (A) Time (B) AxB Error
]
1 2 2 52
Sum of squares
Mean square
7708137.616 621247.631 264926.777 999459.5
7708137.616 310623.815 132463.388 19220.375
I
F test
J
401.04 16.161 6.892
P value 0.0001 0.0001 0.0022
Table IX. Descriptive statistics--diametral tensile strength Group
I
Mean (MPa)
CC 1 CC24 CC7D LCI LC24 LC7D
9.741 14.493 15.87 22.246 21.509 19.371
I
Standarddeviation (MPa)
Minimum (MPa)
Maximum (MPa)
Coefficientvariant (%)
2.303 .965 3.356 2.387 2. i 19 3.096
5.47 13.11 i 1.24 17.39 18.29 13.78
12.75 15.84 21.84 25.0 25.19 23.58
23.638 6.659 21.147 10.73 9.85 15.985
Table X. Two-way ANOVA--diametral tensile strength Source Mat (A) Time (B) AxB Error
I
DF 1 2 2 56
I
|
Sum of squares
Mean square
908.606 46.601 206. l 17 344.407
908.606 23.3 103.059 6.15
]
F test 147.738 3.789 16.757
!
]
P value
0.000l 0.0286 0.0001
Table Xi. Descriptive statistics--bond strength
Group
Mean (MPa)
Standard deviation (MPa)
CCI CC24 CC7D LCI5m LCI LC24 LC7D
4.33 6.28 8.71 I 1.23 12.17 12.58 12.53
1.32 1.47 2.21 1.81 2.69 1.46 3.27
diametral tensile strength and bond strength tests. This finding may allow prediction of bond strength on the basis of mechanical properties. The results of the bond strength tests indicate that the light-cured cement possesses sufficient strength at 15 minutes to be used clinically to bond orthodontic brackets. The chemically cured cement did not achieve a mean of 7 MPa for more than 24 hours; this has clinical implications for the orthodontist. The dramatic difference in tensile and bond strength between light-cured and chemically cured materials in this study may relate not only to curing method, but
Minimum(MPa)
[
2.40 3.63 4.58 8.60 7.87 9.02 5.26
Maximum(MPa) [ 7.30 8.49 11.18 14.45 ! 6.45 14.36 16.18
Coefficient (%)variant 30.4 23.4 25.3 16.1 22.1 11,6 26.1
the chemical nature of the primary materials themselves. If the light-cured material, despite its superior mechanical and early bonding strength, has properties more characteristic of composite resins than of glass ionomer cements, traditional advantages of glass ionomers, such as fluoride release and ease of removal, may be compromised. Further study is needed to resolve these questions. CONCLUSIONS
The light-cured cement achieves maximum tensile strength faster than the chemically cured material.
American Journalof Orthodonticsand DentofacialOrthopedics Volume 105, No. 2
McCarthy and Hondrum
141
Table XII. Two-way ANOVA--bond strength Source Mat(A) Time (B) AxB Error
I
,ean
DF
Sum of squares
squares
1 2 2 54
537.902 56.241 41.027 259.543
537.902 28.12 20.513 4.806
The bond strengths of both chemically cured and light-cured glass ionomer cements increase with time. The bond strength of light-cured cement was greater than the proposed minimum level for clinical success at all test times; the bond strength of chemically cured materials did not reach minimum levels until after 24 hours. The bond failures of the glass ionomer cements are primarily cohesive, light-cured more than chemically gured. There is a high correlation between the diametral tensile strength and the bond strength for glass ionomer cements at 1 hour and 24 hours (r = 0.98). REFERENCES 1. Buonocore MG. A simple method of increasing the adhesion of acrylic filling material to enamel surfaces. L Dent Res 1955;34:849-53. 2. Retief DH, Sadowski PL. Clinical experience with the acid-etch technique in orthodontics. AM J OR'moP 1975;79:645-65. 3. Proffit W. Contemporary orthodontics. St. Louis: CV Mosby, 1986:287. 4. Thompson RE, Way DC. Enamel loss due to prophylaxis and multiple bonding/rebonding of orthodontic attachments. AM J OR'roOD 1981;79:282-95. 5. Zachrisson BU, Zachrisson S. Caries incidence and oral hygiene during orthodontic treatment. Scand J Dent Res 1971;79:394401. 6. Forsten L. Fluoride release from a glass ionomer cement. Scand J Dent Res 1977;85:503-4. 7. Kidd EAM. Cavity sealing ability of composite glass ionomer cement restorations. Br Dent J 1978;144:139-42. 8. Maldonato A, Swartz ML, Phillips RW. An in vitro study of certain properties of a glass ionomer cement. J Am Dent Assoc 1978;145:67-71.
I
F-test 111.915 5.851 4.268
I
P value 0.0001 0.005 0.019
9. White LW. Glass ionomer cement. J Clin Orthod 1986;20:38791. 10. McComb D, Sirisko R, Brown J. Scientific comparison of commercial glass ionomer cements. J Can Dent Assoc 1984;9:69970. 11. Norris DS, Mclnnes-Ledoux P, Schwaninger B, Weinberg R. Retention of orthodontic bands with new fluoride-releasing cements. AM J OR~IOD 1986;89:206-11. 12. Copenhavcr DJ. In vitro comparison of glass ionomer cements ability to inhibit decalcification under orthodontic bands. A~.I J OR'rHOD 1986;89:528. 13. Maijer R, Smith DC. A comparison between zinc phosphate and glass ionomer cement in orthodontics. AM J ORTHODDENTOFAC ORTtIOP 1988;93(4):273-9. 14. Seeholzer HW, Dasch W. Banding with a glass ionomer cement. J Clin Orthod 1988;22:165-9. 15. Murray GA, Yates JL. A comparison of the bond strengths of composite resins and glass ionomer cements. J Pedodont 1984;8:172-7. 16. Klockowski R, Davis EL, Joynt RB, Wieczkowski G, MacDonald A. Bond strength and durability of glass ionomer cements used as bonding agents in the placement of orthodontic brackets. AM J ORTHOD DENTOFACORTHOP 1989;96:60-4. 17. Ferguson JW, Read MJF, Watts DC. Bond strengths of an integral bracket-base combination: an in vitro study. Eur J Orthod 1984;6:267-76. 18. McLean JW, Wilson AD. The clinical development of the glass ionomer cement. Formulations and properties. Aust Dent J 1977;22(1):31-6. 19. Lopez Jl. Retentive shear bond strength of various bonding attachment bases. AM J OR'I'HOD 1980;77:669-78.
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