Mechanical restorative D. A. covey,
properties materials DDS,
MSa
of heat-treated
S. R. Tahaney,
composite
BS,b and J. M. Davenport,
resin
PhD”
Virginia CommonwealthUniversity, Medical Collegeof Virginia, School of Dentistry, Richmond, Va. Clinical methods for heat treating composite resin restorations have been developed. In this investigation, the effect of heat treatments on the diametral tensile strength of composite resin was determined. The composite resin restorative materials were selected according to the manufacturers’ suggested use for anterior or posterior teeth, filler pa.rticle composition, and light-cured or chemical polymerization. Samples were prepared according to American Dental Association specification No. 27, and heat treatments were accomplished with a Coltene DI 500 oven for curing at approximately 120° C for 7 minutes. Heat treatment substantially increased the diametral tensile strength tested, with the exception of the anterior hybrid particle (p < 0.05). Composite resins with fine-particle inorganic fillers were significantly stronger than hybrid and microfilled composite resins. (J PROSTHET DENT 1992;68:458-61.)
T
he physical properties of composite resin are greatly affected by the monomer to polymer conversion during the curing process.l-s Visible-light irradiation of composite resin restorative materials at intraoral temperatures is commonly insufficient for maximal polymerization.4: 5 Exposure to elevated temperatures after light irradiation improves the conversion and physical properties of composite resins.6-g Techniques for laboratory and chairside fabrication of heat-treated composite resin restorations have been introduced.10-12 The number of composite resins specifically formulated to incorporate the postcured methods is presently limited. This study evaluated the diametral tensile strength (DTS) of composite resin restorative materials exposed to postirradiation heat tempering. MATERIAL
AND
METHODS
Six light-cured and one chemically cured composite resin test groups were evaluated. Representative composite resins were selected on the basis of type, such as anterior or posterior composites and filler composition of microfilled, hybrid, or fine particle. Group A consisted of an anterior microfilled composite resin, Silux Plus (3M Co., St. Paul, Minn.); group B, an anterior hybrid particle, Multifil VS (Kulzer & Co., GmbH, Bad Homburg, West Germany); group C, an anterior fine particle, PrismaFil (L. D. Caulk Co., Milford, Del.); group D, a posterior microfilled composite resin, Heliomolar (ViPresentedat the American Associationof Dental Researchmeeting, San Francisco,Calif. “Assistant Professor,Department of Restorative Dentistry. bSeniorDental Student. CAssociateProfessor,Department of Mathematical Sciences. 10/1/37746 458
Table I. Diametral tensile strength test results Light-cured
only
Heat treated
Composite group
A
B C
D E F G
MPa
SD
MPa
SD
40.4 42.5 58.4 41.3 45.1
1.6 3.9 4.7 3.2 4.9 4.3 2.8
48.9 45.9 65.8 52.4 54.2 69.2 71.2
3.9 2.2 7.1 2.7 5.6 4.1 6.6
59.8 63.8
A, Anterior microfilled; B, anterior hybrid particle; C, anterior fine particle; D, posterior microfilled; E, posterior hybrid; F, posterior fine particle; G, chemically cured posterior fine particle.
vadent, Schaan, Lichtenstein); group E, a posterior hybrid particle, Coltene DI (Coltene AG, Alstatten, Switzerland); group F, a posterior fine particle, Ful-Fil (L. D. Caulk Co., Milford, Del.); and group G, a chemically cured posterior fine particle, P-10 (3M Co., St. Paul, Minn.). Ten cylindrical samples of each composite resin group were made with a stainless steel split mold, according to American Dental Association (ADA) specification No. 27. The mold reproduced samples 6 mm in diameter and 3.3 mm in height. The mold was secured on a glass slab with a plastic strip between the mold and the glass. The mold was half filled with composite resin and cured for 40 seconds with an Optilux 50 light-curing unit (Dematron Research Co., Danbury, Conn.). A second increment of composite resin was placed to fill the mold. The final increment was covered with a plastic strip, and a glass microscope slide was positioned to create a flat, even surface. The microscope slide was removed and the composite resin cured for an additional 40 seconds. The chemically cured composite resin was prepared according to the manufacturer SEPTEMBER
1992
VOLUME
68
NUMBER
3
NEAT-TREATED
COMPOSITE
RESINS
80
MPa 1
a
LIGHT ONLY LIGHT/HEAT
60
Composite Fig.
Table
II.
Three-way
1. DTS of composite resin restorative
analysis of variance for light-cured
Source
DF
Model
48 59
856.713 5994.033
1 1
size
total
Sum of squares
2 2 2 2
1
materials.
composite resin restorative
5137.319 168.029 994.323 43.334 3856.427 33.661 31.372 10.172
11
Type Treatment Type*treatment Particle size Type*particle size Treatment*particle size Type*treatment*particle Error Corrected
Resin Test Groups
Mean
square
467.029
materials F value
26.17 9.41 55.71 2.43 108.03 0.94 0.88 0.28
PR>F
0.0001 0.0035 0.0001 0.1258 0.0001 0.3966 0.4218 0.7533
17.848
*Type, Recommended use (anterizr, posterior); treatment, polymerization method (light, light and heat).
and inserted in the mold as one increment and allowed to polymerize for 10 minutes. After removal from the mold, the ends of the cylindrical samples were polished with 240-grit wet silicon carbide paper on an automated polisher (Minimet Buehler, Lake BIuff, Ill.), producing a sample 3 mm in height and 6 mm in diameter. Five samples from each composite resin group received an additional postirradiation, heat-tempering treatment. Within 10 minutes of fabrication, the samples were placed in a Coltene DI 500 curing oven (Coltene AG). This curing oven with a high-intensity light bulb produced a maximal temperature of 120” C during a ‘7-minute cycle. The composite resins were stored in 37O C distilled water for 24 hours before testing. DTS tests were performed with an Instron Universal testing machin.e (Instron Corp., Canton, Mass.). A double layer of 0.003-inch thick lead foil was positioned between the samples and each platten of the testing machine. The samples were loaded continuously in THE
JOURNAL
OF PROSTHETIC
DENTISTRY
compression at a cross-head speed of 0.02 inch/min until fracture. The effects of experimental factors such as particle size and polymerization treatment on DTS were evaluated by means of a three-way analysis of variance (ANOVA). The effect of heat treatment on DTS was determined with the F statistic for contrasting of group means with General Linear Models (SAS Institute, Cary, N.C.). The StudentNewman-Keuls test was used to compute in multiple the comparisons of experimental factors.
RESULTS DTS in the light-cured and chemically cured composite resin test groups was increased by means of postirradiation heat treatment (Table I, Fig. 1). The average increase in DTS of the seven groups tested was 16.6%. A three-way ANOVA excluding the chemically cured groups revealed no interaction between experimental factors (Table II). However, the main effects, composite resin 459
COVEY,TAHANEY,ANDDAVENPORT
Table
III.
F statistic
comparison
of conventional
SOW%?
DF
Model Contrast Particle size-linear Particle size--quad Particle size-lin + quad
and heat-treated sum
of squares
13
7343.888
1 1
3082.012
Type
2 1
3856.427 168.029
Treatment AM light vs light/heat AH light vs light/heat AF light vs light/heat PM light vs light/heat PH light vs light/heat PF light vs light/heat PFC vs PFC/heat PF light vs PFC PF light/heat vs PFC PF light/heat vs PFC/heat
1 1 1 1 1 1 1 1 1 1 1
Error Corrected total
group means Mean
square
F value
564.914
29.77
994.323 179.225 28.890 139.120
0.0001
0.0001 0.0001 0.0043
52.41 9.45
0.0001 0.0033 0.2224 0.0090 0.0002 0.0012 0.0017
1.52 7.33 16.08
305.147
219.952
11.59
206.867
10.90
135.621
7.15
0.0098
45.653 71.580
2.41 3.77 0.53
0.1265 0.0571 0.4671
10.145
56
0.0001
162.44 40.82 101.63 8.86
774.414
1062.488 8406.377
69
PR>F
18.973
AM, Anterior microfilled; AH, anterior hybrid; AF, anterior fme particle; PM, posterior microfilled; PH, posterior hybrid particle; PF, posterior fine particle; PRY’, posterior fine particle chemically cured.
PV. Student-Newman-Keuls for composite resin type
multiple
Table
Grouping
A
B
Mean
53.7 50.3
comparison
Table
V. Student-Newman-Keuls
for particle N
30 30
Type
Posterior Anterior
Number of meant 2 Critical range 2.193 (alpha = 0.05, DF = 48, MSE = 17.848)
Grouping
A B B B
Mean
A positive correlation has been demonstrated between the degree of monomer to polymer conversion (DC) and the mechanical properties of composite resins. Asmussen,4
460
comparison
N
Particle
type
63.3
20
46.9
20
Fine Hybrid
45.8
20
Microfilled
Number of means Critical range (alpha = 0.05, DF = 48, MSE = 17.848
type, particle size, and polymerization were statistically significant. The contrasts of the F statistics of the light-only and the light and heat-treated group means demonstrated, with the exception of group B, a statistically significant increase in the DTS (Table III). The contrasts between light-cured and chemically cured fine-particle composite resins were not significantly different. The multiple comparison tests identified significant mean differences between anterior and posterior composite resins (Table IV). The DTS of fine-particle composites, was significantly greater than that of the hybrid or microfilled composites (Table V). The differences in DTS between hybrid and microfilled composite resins were not significant.
multiple
size
2 2.686
3 3.231
with the use of infrared spectroscopy, reported a DC of 57% to 77% for chemically cured anterior composite resins. Chung and Greener5 reported a similar 43 % to 64 % for light-cured posterior composites. Ferracane and Greener3 found that composite resin formulations that increase the DC result in a higher DTS. Bausch et al.” increased the DTS of chemically cured composites by treating the composites with infrared heat. The application of heat increased the mobility of both polymer segments and reactive free radicals formed during polymerization This allowed a greater degree of conversion and increased cross-linking of polymer units. The DTS of light-cured composite resin was also improved by heat treatment.2 Boyer et a1.13reported that the mechanical properties, including DTS of composite resin, depended greatly on the concentration and particle size of the filler. In this study, the fine-particle composite resins with a high volumetric filler fraction recorded the greatest DTS.
SEPTEMBER
1992
VOLUME
68
NUMBER
3
HEAT-TREATED
COMPOSITE
RESINS
The specialized inlay composite resin (Brilliant DI) did not exhibit significantly greater tensile strength than other posterior composite resins with or without heat tempering. Watts,r4 by means of dynamic mechanical thermal analysis, discovered that specialized and conventional visible light-cured composite resins were similar when exposed to elevated temperatures. Wendt15 investigated the effects of heat-treatment duration and temperature on the DTS of composite resins and concluded that a temperature of 125“ C for 7.5 minutes was optimal for the composites tested. Similar conditions were produced by the oven in this study.16 The Coltene oven generated heat with a xenon light bulb by exposing the composite resin to additional light irradiation. This additional light is not necessary for increased polymerization because use of dry-heat ovens resulted in similar elevations in DTS.7, I5 Leung et alI7 believed that the timing of the testing was critical when determining the the properties of composite polymerization. Results of postirradiation studies showed that the polymerization process continued after the initial gelation of the composite. Studies of surface hardness of composite resins indicated that polymerization continued for a week or more before a termination point was reached.r7-lg Exposure to elevated temperatures appeared to accelerate this process, but whether the final polymerization was improved remains unknown. KancazO reported that after 1 week, the surface hardness of light-cured composite resin had reached that of heat-treated composite resin. The effect of prolonged postirradiation polymerization on the DTS of composite resins has not been determined. Results of this study demonstrated that postcuring methods improved the mechanical properties of composite resins. However, the clinical significance of heat treating composite resin restorative materials during the fabrication of restorations has not been established. Nevertheless, in vitro7, 21,22and in vivo23 research has demonstrated that heat-treated restorations exhibit less microleakage than conventional direct restorations. It is postulated that extraoral completion of the curing process minimizes tooth restoration interface stresses and marginal gap formation caused by polymerization shrinkage. In vitro abrasion studies have also indicated that heat-treated composite restorations should increase the wear resistance.8 However, results of in vivo surface wear studies have not confirmed a substantial improvement.lg, 23
1. Heat treatments improved the DTS of light- and chemically cured composite resins. 2. Specialized composite resin.s were not superior to conventional light-cured composite resin when heat treated.
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
3. Composite resins with a high inorganic filler content recorded a significantly greater tensile strength than hybrid or microfilled resins. REFERENCES 1. Asmussen E. Restorative resins: hardness and strength vs quantity of remaining double bonds. Stand J Dent Res 1982;90:484-9. 2. Cook WD, Johannson M. The influence of postcuring on the fracture properties of photo-cured dimethacrylate based dental composite resin. J Biomed Mater Res 1987;21:979-89. 3. Ferracane JL, Greener EH. The effect of resin formulation on the degree of conversion and mechanical properties of dental restorative resins. J Biomed Mater Res 1986;20:121-31. 4. Asmussen E. Factors affecting the quantity of remaining double bonds in restorative resin polymers. Stand J Dent Res 1982;90:490-6. 5. Chung K, Greener EH. Degree of conversion of seven visible light-cured posterior composites. J Oral Rehabii 1988;15:555-60. G. Bausch JR, de Lange C, Davidson CL. The influence of temperature on some physical properties of dental composites. J Oral Rehabil 1981; 8:309-17. 7. Wendt SL. The effect of heat used as a secondary cure upon the physical properties of three composite resins. I. Diametral tensile strength, compressive strength, and marginal dimensional stability. Quintessence Int 1987;18:265-71. 8. Wendt SL. The effect of heat used as secondary cure upon the physical properties of three composite resins. II. Wear, hardness, and color stability. Quintessence Int 1987;18:351-6. 9. Wu W. Factors controlling the degree of polymerization in dental resins [Abstract]. J Dent Res 1983;62:285. 10. James DF, Yarovesky U. An esthetic inlay technique for posterior teeth. Quintessence Int 1983;7:725-31. 11. Blankenau RJ, Kelsey WP, Gavel WT. A direct posterior restorative resin inlay technique. Quintessence Int 1984;5:515-6. 12. Jackson RD, Ferguson RW. An esthetic, bonded inlay/onlay technique for posterior teeth. Quintessence Int 1990;21:7-12. 13. Boyer DB, Chalkley Y, Chan KC. Correlation between strength of bonding to enamel and mechanical properties of dental composites. J Biomed Mater Res 1982;16:775-83. 14. Watts DC. Dynamic mechanical behavior of post-cured composite resins [Abstract]. J Dent Res 1989;68:574. 15. Wendt SL. Time as a factor in the heat curing of composite resins. Quintessence Int 1989;20:259-63. 16. Dionysopoulos P, Watts DC. Dynamic mechanical properties of an inlay composite. J Dent 1989;17:140-4. 17. Leung RL, Fan PL, Johnston WM. Post-irradiation polymerization of visible light-activated composite resin. J Dent Res 1983;62:363-5. 18. Watts DC, Amer OM, Combe EC. Surface hardness development in light-cured composites. Dent Mater 1987;3:265-9. 19. Watts DC, McNaughton V, Grant AA. The development of surface hardness in visible light-cured posterior composites. J Dent 1986;14:16974. 20. Karma J. The effect of heat on the surface hardness of light-activated composite resins. Quintessence Int 1989;20:899-901. 21. Sheth PJ, Jensen ME, Sheth JJ. Comparative evaluation of three inlay techniques: microleakage studies. Quintessence Int 1989;20:831-6. 22. Douglas WH, Fields RP, Fundingsland J. A comparison between the microleakage of direct and indirect composite restorative systems. J Dent 1989;17:184-8. 23. Wendt SL, Leinfelder FK. The clinical evaluation of heat-treated composite resin inlays. J Am Dent Assoc 1990;120:177-81. Reprint requests to: DR. DAVID A. COVEY MEDICAL COLLEGE OF VIRGINIA-VIRGINIA Box 566, MCV STATION RICHMOND, VA 23298-0566
COMMONWEALTH UNIVERSITY
461
properties materials DDS,
MSa
of heat-treated
S. R. Tahaney,
composite
BS,b and J. M. Davenport,
resin
PhD”
Virginia CommonwealthUniversity, Medical Collegeof Virginia, School of Dentistry, Richmond, Va. Clinical methods for heat treating composite resin restorations have been developed. In this investigation, the effect of heat treatments on the diametral tensile strength of composite resin was determined. The composite resin restorative materials were selected according to the manufacturers’ suggested use for anterior or posterior teeth, filler pa.rticle composition, and light-cured or chemical polymerization. Samples were prepared according to American Dental Association specification No. 27, and heat treatments were accomplished with a Coltene DI 500 oven for curing at approximately 120° C for 7 minutes. Heat treatment substantially increased the diametral tensile strength tested, with the exception of the anterior hybrid particle (p < 0.05). Composite resins with fine-particle inorganic fillers were significantly stronger than hybrid and microfilled composite resins. (J PROSTHET DENT 1992;68:458-61.)
T
he physical properties of composite resin are greatly affected by the monomer to polymer conversion during the curing process.l-s Visible-light irradiation of composite resin restorative materials at intraoral temperatures is commonly insufficient for maximal polymerization.4: 5 Exposure to elevated temperatures after light irradiation improves the conversion and physical properties of composite resins.6-g Techniques for laboratory and chairside fabrication of heat-treated composite resin restorations have been introduced.10-12 The number of composite resins specifically formulated to incorporate the postcured methods is presently limited. This study evaluated the diametral tensile strength (DTS) of composite resin restorative materials exposed to postirradiation heat tempering. MATERIAL
AND
METHODS
Six light-cured and one chemically cured composite resin test groups were evaluated. Representative composite resins were selected on the basis of type, such as anterior or posterior composites and filler composition of microfilled, hybrid, or fine particle. Group A consisted of an anterior microfilled composite resin, Silux Plus (3M Co., St. Paul, Minn.); group B, an anterior hybrid particle, Multifil VS (Kulzer & Co., GmbH, Bad Homburg, West Germany); group C, an anterior fine particle, PrismaFil (L. D. Caulk Co., Milford, Del.); group D, a posterior microfilled composite resin, Heliomolar (ViPresentedat the American Associationof Dental Researchmeeting, San Francisco,Calif. “Assistant Professor,Department of Restorative Dentistry. bSeniorDental Student. CAssociateProfessor,Department of Mathematical Sciences. 10/1/37746 458
Table I. Diametral tensile strength test results Light-cured
only
Heat treated
Composite group
A
B C
D E F G
MPa
SD
MPa
SD
40.4 42.5 58.4 41.3 45.1
1.6 3.9 4.7 3.2 4.9 4.3 2.8
48.9 45.9 65.8 52.4 54.2 69.2 71.2
3.9 2.2 7.1 2.7 5.6 4.1 6.6
59.8 63.8
A, Anterior microfilled; B, anterior hybrid particle; C, anterior fine particle; D, posterior microfilled; E, posterior hybrid; F, posterior fine particle; G, chemically cured posterior fine particle.
vadent, Schaan, Lichtenstein); group E, a posterior hybrid particle, Coltene DI (Coltene AG, Alstatten, Switzerland); group F, a posterior fine particle, Ful-Fil (L. D. Caulk Co., Milford, Del.); and group G, a chemically cured posterior fine particle, P-10 (3M Co., St. Paul, Minn.). Ten cylindrical samples of each composite resin group were made with a stainless steel split mold, according to American Dental Association (ADA) specification No. 27. The mold reproduced samples 6 mm in diameter and 3.3 mm in height. The mold was secured on a glass slab with a plastic strip between the mold and the glass. The mold was half filled with composite resin and cured for 40 seconds with an Optilux 50 light-curing unit (Dematron Research Co., Danbury, Conn.). A second increment of composite resin was placed to fill the mold. The final increment was covered with a plastic strip, and a glass microscope slide was positioned to create a flat, even surface. The microscope slide was removed and the composite resin cured for an additional 40 seconds. The chemically cured composite resin was prepared according to the manufacturer SEPTEMBER
1992
VOLUME
68
NUMBER
3
NEAT-TREATED
COMPOSITE
RESINS
80
MPa 1
a
LIGHT ONLY LIGHT/HEAT
60
Composite Fig.
Table
II.
Three-way
1. DTS of composite resin restorative
analysis of variance for light-cured
Source
DF
Model
48 59
856.713 5994.033
1 1
size
total
Sum of squares
2 2 2 2
1
materials.
composite resin restorative
5137.319 168.029 994.323 43.334 3856.427 33.661 31.372 10.172
11
Type Treatment Type*treatment Particle size Type*particle size Treatment*particle size Type*treatment*particle Error Corrected
Resin Test Groups
Mean
square
467.029
materials F value
26.17 9.41 55.71 2.43 108.03 0.94 0.88 0.28
PR>F
0.0001 0.0035 0.0001 0.1258 0.0001 0.3966 0.4218 0.7533
17.848
*Type, Recommended use (anterizr, posterior); treatment, polymerization method (light, light and heat).
and inserted in the mold as one increment and allowed to polymerize for 10 minutes. After removal from the mold, the ends of the cylindrical samples were polished with 240-grit wet silicon carbide paper on an automated polisher (Minimet Buehler, Lake BIuff, Ill.), producing a sample 3 mm in height and 6 mm in diameter. Five samples from each composite resin group received an additional postirradiation, heat-tempering treatment. Within 10 minutes of fabrication, the samples were placed in a Coltene DI 500 curing oven (Coltene AG). This curing oven with a high-intensity light bulb produced a maximal temperature of 120” C during a ‘7-minute cycle. The composite resins were stored in 37O C distilled water for 24 hours before testing. DTS tests were performed with an Instron Universal testing machin.e (Instron Corp., Canton, Mass.). A double layer of 0.003-inch thick lead foil was positioned between the samples and each platten of the testing machine. The samples were loaded continuously in THE
JOURNAL
OF PROSTHETIC
DENTISTRY
compression at a cross-head speed of 0.02 inch/min until fracture. The effects of experimental factors such as particle size and polymerization treatment on DTS were evaluated by means of a three-way analysis of variance (ANOVA). The effect of heat treatment on DTS was determined with the F statistic for contrasting of group means with General Linear Models (SAS Institute, Cary, N.C.). The StudentNewman-Keuls test was used to compute in multiple the comparisons of experimental factors.
RESULTS DTS in the light-cured and chemically cured composite resin test groups was increased by means of postirradiation heat treatment (Table I, Fig. 1). The average increase in DTS of the seven groups tested was 16.6%. A three-way ANOVA excluding the chemically cured groups revealed no interaction between experimental factors (Table II). However, the main effects, composite resin 459
COVEY,TAHANEY,ANDDAVENPORT
Table
III.
F statistic
comparison
of conventional
SOW%?
DF
Model Contrast Particle size-linear Particle size--quad Particle size-lin + quad
and heat-treated sum
of squares
13
7343.888
1 1
3082.012
Type
2 1
3856.427 168.029
Treatment AM light vs light/heat AH light vs light/heat AF light vs light/heat PM light vs light/heat PH light vs light/heat PF light vs light/heat PFC vs PFC/heat PF light vs PFC PF light/heat vs PFC PF light/heat vs PFC/heat
1 1 1 1 1 1 1 1 1 1 1
Error Corrected total
group means Mean
square
F value
564.914
29.77
994.323 179.225 28.890 139.120
0.0001
0.0001 0.0001 0.0043
52.41 9.45
0.0001 0.0033 0.2224 0.0090 0.0002 0.0012 0.0017
1.52 7.33 16.08
305.147
219.952
11.59
206.867
10.90
135.621
7.15
0.0098
45.653 71.580
2.41 3.77 0.53
0.1265 0.0571 0.4671
10.145
56
0.0001
162.44 40.82 101.63 8.86
774.414
1062.488 8406.377
69
PR>F
18.973
AM, Anterior microfilled; AH, anterior hybrid; AF, anterior fme particle; PM, posterior microfilled; PH, posterior hybrid particle; PF, posterior fine particle; PRY’, posterior fine particle chemically cured.
PV. Student-Newman-Keuls for composite resin type
multiple
Table
Grouping
A
B
Mean
53.7 50.3
comparison
Table
V. Student-Newman-Keuls
for particle N
30 30
Type
Posterior Anterior
Number of meant 2 Critical range 2.193 (alpha = 0.05, DF = 48, MSE = 17.848)
Grouping
A B B B
Mean
A positive correlation has been demonstrated between the degree of monomer to polymer conversion (DC) and the mechanical properties of composite resins. Asmussen,4
460
comparison
N
Particle
type
63.3
20
46.9
20
Fine Hybrid
45.8
20
Microfilled
Number of means Critical range (alpha = 0.05, DF = 48, MSE = 17.848
type, particle size, and polymerization were statistically significant. The contrasts of the F statistics of the light-only and the light and heat-treated group means demonstrated, with the exception of group B, a statistically significant increase in the DTS (Table III). The contrasts between light-cured and chemically cured fine-particle composite resins were not significantly different. The multiple comparison tests identified significant mean differences between anterior and posterior composite resins (Table IV). The DTS of fine-particle composites, was significantly greater than that of the hybrid or microfilled composites (Table V). The differences in DTS between hybrid and microfilled composite resins were not significant.
multiple
size
2 2.686
3 3.231
with the use of infrared spectroscopy, reported a DC of 57% to 77% for chemically cured anterior composite resins. Chung and Greener5 reported a similar 43 % to 64 % for light-cured posterior composites. Ferracane and Greener3 found that composite resin formulations that increase the DC result in a higher DTS. Bausch et al.” increased the DTS of chemically cured composites by treating the composites with infrared heat. The application of heat increased the mobility of both polymer segments and reactive free radicals formed during polymerization This allowed a greater degree of conversion and increased cross-linking of polymer units. The DTS of light-cured composite resin was also improved by heat treatment.2 Boyer et a1.13reported that the mechanical properties, including DTS of composite resin, depended greatly on the concentration and particle size of the filler. In this study, the fine-particle composite resins with a high volumetric filler fraction recorded the greatest DTS.
SEPTEMBER
1992
VOLUME
68
NUMBER
3
HEAT-TREATED
COMPOSITE
RESINS
The specialized inlay composite resin (Brilliant DI) did not exhibit significantly greater tensile strength than other posterior composite resins with or without heat tempering. Watts,r4 by means of dynamic mechanical thermal analysis, discovered that specialized and conventional visible light-cured composite resins were similar when exposed to elevated temperatures. Wendt15 investigated the effects of heat-treatment duration and temperature on the DTS of composite resins and concluded that a temperature of 125“ C for 7.5 minutes was optimal for the composites tested. Similar conditions were produced by the oven in this study.16 The Coltene oven generated heat with a xenon light bulb by exposing the composite resin to additional light irradiation. This additional light is not necessary for increased polymerization because use of dry-heat ovens resulted in similar elevations in DTS.7, I5 Leung et alI7 believed that the timing of the testing was critical when determining the the properties of composite polymerization. Results of postirradiation studies showed that the polymerization process continued after the initial gelation of the composite. Studies of surface hardness of composite resins indicated that polymerization continued for a week or more before a termination point was reached.r7-lg Exposure to elevated temperatures appeared to accelerate this process, but whether the final polymerization was improved remains unknown. KancazO reported that after 1 week, the surface hardness of light-cured composite resin had reached that of heat-treated composite resin. The effect of prolonged postirradiation polymerization on the DTS of composite resins has not been determined. Results of this study demonstrated that postcuring methods improved the mechanical properties of composite resins. However, the clinical significance of heat treating composite resin restorative materials during the fabrication of restorations has not been established. Nevertheless, in vitro7, 21,22and in vivo23 research has demonstrated that heat-treated restorations exhibit less microleakage than conventional direct restorations. It is postulated that extraoral completion of the curing process minimizes tooth restoration interface stresses and marginal gap formation caused by polymerization shrinkage. In vitro abrasion studies have also indicated that heat-treated composite restorations should increase the wear resistance.8 However, results of in vivo surface wear studies have not confirmed a substantial improvement.lg, 23
1. Heat treatments improved the DTS of light- and chemically cured composite resins. 2. Specialized composite resin.s were not superior to conventional light-cured composite resin when heat treated.
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
3. Composite resins with a high inorganic filler content recorded a significantly greater tensile strength than hybrid or microfilled resins. REFERENCES 1. Asmussen E. Restorative resins: hardness and strength vs quantity of remaining double bonds. Stand J Dent Res 1982;90:484-9. 2. Cook WD, Johannson M. The influence of postcuring on the fracture properties of photo-cured dimethacrylate based dental composite resin. J Biomed Mater Res 1987;21:979-89. 3. Ferracane JL, Greener EH. The effect of resin formulation on the degree of conversion and mechanical properties of dental restorative resins. J Biomed Mater Res 1986;20:121-31. 4. Asmussen E. Factors affecting the quantity of remaining double bonds in restorative resin polymers. Stand J Dent Res 1982;90:490-6. 5. Chung K, Greener EH. Degree of conversion of seven visible light-cured posterior composites. J Oral Rehabii 1988;15:555-60. G. Bausch JR, de Lange C, Davidson CL. The influence of temperature on some physical properties of dental composites. J Oral Rehabil 1981; 8:309-17. 7. Wendt SL. The effect of heat used as a secondary cure upon the physical properties of three composite resins. I. Diametral tensile strength, compressive strength, and marginal dimensional stability. Quintessence Int 1987;18:265-71. 8. Wendt SL. The effect of heat used as secondary cure upon the physical properties of three composite resins. II. Wear, hardness, and color stability. Quintessence Int 1987;18:351-6. 9. Wu W. Factors controlling the degree of polymerization in dental resins [Abstract]. J Dent Res 1983;62:285. 10. James DF, Yarovesky U. An esthetic inlay technique for posterior teeth. Quintessence Int 1983;7:725-31. 11. Blankenau RJ, Kelsey WP, Gavel WT. A direct posterior restorative resin inlay technique. Quintessence Int 1984;5:515-6. 12. Jackson RD, Ferguson RW. An esthetic, bonded inlay/onlay technique for posterior teeth. Quintessence Int 1990;21:7-12. 13. Boyer DB, Chalkley Y, Chan KC. Correlation between strength of bonding to enamel and mechanical properties of dental composites. J Biomed Mater Res 1982;16:775-83. 14. Watts DC. Dynamic mechanical behavior of post-cured composite resins [Abstract]. J Dent Res 1989;68:574. 15. Wendt SL. Time as a factor in the heat curing of composite resins. Quintessence Int 1989;20:259-63. 16. Dionysopoulos P, Watts DC. Dynamic mechanical properties of an inlay composite. J Dent 1989;17:140-4. 17. Leung RL, Fan PL, Johnston WM. Post-irradiation polymerization of visible light-activated composite resin. J Dent Res 1983;62:363-5. 18. Watts DC, Amer OM, Combe EC. Surface hardness development in light-cured composites. Dent Mater 1987;3:265-9. 19. Watts DC, McNaughton V, Grant AA. The development of surface hardness in visible light-cured posterior composites. J Dent 1986;14:16974. 20. Karma J. The effect of heat on the surface hardness of light-activated composite resins. Quintessence Int 1989;20:899-901. 21. Sheth PJ, Jensen ME, Sheth JJ. Comparative evaluation of three inlay techniques: microleakage studies. Quintessence Int 1989;20:831-6. 22. Douglas WH, Fields RP, Fundingsland J. A comparison between the microleakage of direct and indirect composite restorative systems. J Dent 1989;17:184-8. 23. Wendt SL, Leinfelder FK. The clinical evaluation of heat-treated composite resin inlays. J Am Dent Assoc 1990;120:177-81. Reprint requests to: DR. DAVID A. COVEY MEDICAL COLLEGE OF VIRGINIA-VIRGINIA Box 566, MCV STATION RICHMOND, VA 23298-0566
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