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Interfacial bond strengths between layers of visible light-activated composites
Interfacial bond strengths between layers of visible light-activated composites Anthony Loma Linda
H. L. Tjan, Dr. Dent., D.D.S.,* and James F. Glancy, B.A.** University,
School of Dentistry,
Loma Linda,
Calif
lhe primary limitation of visible light-activated composites is the light beam penetration and the consequent depth of cure. The degree of polymerization of any resin system is crucial for the long-term performance of composite restorations.’ Incomplete polymerization may lead to premature degradation of the composite, retention failure, and to possible adverse pulp reactions. The in vitro depth of cure for visible light-activated composites has been reported in the range of 2 to 8 mm.2 In large restorations or deep cavities where the curing depth of the material is less than the thickness of the restoration, incremental placement of the restorative material is recommended for building up the restoration. A chemical bonding between increments is ensured by the use of the air-inhibited surface layer.3 The incremental or layering technique minimizes the effect of the ;polymerization shrinkage (setting contraction), which may create considerable tensile stresses at the interface between resin and tooth. These stresses severely strain the bond between the low-viscosity resin bonding agent and the acid-etched enamel at the cavity margins, which may lead to early marginal failure and possible enamel fractures along the bonded interface.3-6It has been suggested that incremental application of light-activated composites aids in maintaining adaptation to the cavity wall.’ Obviously the clinical applications of the layering technique are also numerous. The technique is commonly used for complex restorative procedures, such as closing diastemas and veneering, which require extensive freehand contouring of the composite. The technique is also applied in Class IV restorations to attain optimal strength and esthetics by using a conventional composite as the substructure and adding an overlay of a microfilled resin to achieve maximal translucency, Another related procedure is the addition of a new layer of composite to repair fracture or discoloration of older restorations. The purpose of this investigation was to evaluate the interfacial bond strengths between increments of (1) posterior composites, (2) a combination of traditional or
*Professor and Director of Biomaterials Research, Department of Restorative Dentistry. **Dental Student, Work-study Dental Research Program. THE JOURNAL
OF PROSTHETIC
DENTISTRY
Table I. Visible light-activated used in study Composite
Code Shade
Batch No.
composites
Manufacturer
Heliosit
HL
30
120384
Vivadent (U.S.A.), Inc. Tonawanda, N.Y.
Visio-Dispers
VD
L
0017
ESPE-Premier Corp., Norristown, Pa.
Micro-Fine
MF
B52
010985
The L. D. Caulk Co., Milford, Del.
&ray-Match
CM
95
DD-453
Sci-Pharm, Inc., Duarte, Calif.
Visio-Fil
VF
STD
0057
ESPE-Premier
Prisma-Fil
PF
B59
1127842 The L. D. Caulk Co.,
Gray-Fil
CF
65
99-420
Sci-Pharm,
P30
P
XL
5A5P
3M Co., St. Paul, Minn.
Herculite
HR
L
5-1834
Kerr/Sybron, Romulus, Mich.
Curay II
CP
55
BB389
Sci-Pharm,
Corp.,
Inc.
Inc.
hybrid composites with microfilled composites of the same brands, and (3) a combination of composites of different brands.
MATERIAL
AND METHODS
The visible light-activated composites used in this study were three brands of posterior and seven brands of anterior composites that included three microfilled composites. The product names, batch numbers, and manufacturers are shown in Table I. The interfacial bond strengths of the 10 paired composite systems were evaluated by using a direct tensile test. They consisted of three posterior components bonded to themselves; three macrofiiled (or hybrid) composites bonded to microfilled composites of the same brands; and four macrofilled (or hybrid) composites bonded to microfilled composites of different brands, respectively referred to by the following codes: 1. HR-HR (Herculite to Herculite) 25
TJAN
AND
GLANCY
LIGHT
LIGHT 9
Fig. 1. Schematic diagram for specimen preparation. A, Composite is inserted in lower section of metal mold, condensed, and irradiated to form first half of specimen or adherend: I, metal mold for first layer; 2, first layer or adherend; and 3, jig. B, Upper section of mold is placed on top of lower section and second half of specimen or adhesive layer polymerized: 4, second layer or adhesive portion; 2, first layer in A now forms substrate or adherend in B; and 5, upper section of mold.
2. P-P (P30 to P30) 3. CP-CP (Curay II to Curay II)
4. VF-VD (Visio-Fil to Visio-Dispers) 5. PF-MF (Prisma-Fil to Micro-Fine) 6. CF-CM
(Curay-Fil
to Curay-Match)
7. CF-MF (Curay-Fil to Micro-Fine) 8. PF-CM (Prisma-Fil to Curay-Match) 9. CF-HL (Curay-Fil to Heliosit) 10. PF-HL (Prisma-Fil to Heliosit) Heliosit (HL) is a urethane dimethacrylate-based microfilled composite.* Curay-Match is a low-viscosity hybrid composite. To simulate actual clinical conditions of curing composite in deep cavity preparations in which the light can be applied only from one direction, a two-level mold, consisting of a lower and upper part, was designed and used to prepare the specimens. The composite was inserted in the lower section and condensed to form the first half of the specimen (substrate or adherend) which was exposed to a visible light (Elipar Visio, ESPEPremier Sales Corp., Norristown, Pa.) for 40 seconds (Fig. 1, A). The upper part of the mold, which was placed on top of the lower part, was filled immediately with either the same composite or with another composite, condensed, and then irradiated for an additional 40 seconds to form the adhesive layer (Fig. 1, B). The material was premeasured to produce a consistent thickness (2.5 mm) of the composite. The diameter of the mold at the interface was approximately 7 mm. While curing, the top of the activating light was held 1 mm from the specimen surface. All bonded specimens were 26
then stored at 37” C at 100% relative humidity for 24 hours before testing. All specimens remained in the molds.
Tensile testing The specimen was held in a specially designed device that permitted only pure axial loading and was mounted to the Riehle Universal testing machine (American Machine and Metal Inc., East Moline, Ill.) with a crosshead speed of 1 mm/min (Fig. 2). Peak force values at fracture were recorded. For the diametral tensile strengths of the composites, specimen preparation and testing were done in accordana with ADA Specification No. 27. One-way analysis of variance (ANOVA) was used to analyze the data and multiple comparisons were performed by using Duncan’s multiple range tests. RESULTS Table II presents the means and standard deviations of the load required to induce a fracture to the specimens by a direct tensile testing method. The diametral tensile strength values of the 10 light-activated composites used in the study were also determined for the sake of comparison and are shown in Table III. With the exception of the two groups that combined BIS-GMA and urethane dimethacrylate-based composite (PF-HL and CF-HL), all the specimens broke at sites other than the interfaces (cohesive failure). With the combination of PF-HL and CF-HL, however, the fractures took place either at the interface (complete or JANUARY
1988
VOLUME
59
NUMBER
1
INTERFACIAL
ROND
STRENGTHS
‘2
‘--1
-2 -3 t----4
Fig. 2. A, Device used to induce tensile loading. B, Schematic diagram of device: 1, specimen; 2, two-level metal mold; 3, threaded rod; and 4, metal cylinder sleeve.
partial adhesive failure) or at sites in close proximity to the junctions but in the microfilled region and with significantly lower fracture strengths at p < .Ol compared with the other groups. In the groups where hybrid composites were bonded to microfilled composites (PF-MF and VF-VD), the cohesive break with a direct tensile testing occurred consistently within the microfilled portion at approximately 30% of their respective diametral tensile strengths. This was consistent with the commonly lower diametral strengths of the microfilled composites compared with those of the hybrid composites. In the combination of a high and low viscosity hybrid-type composite (CF-CM), the cohesive failure occurred at the high viscosity resin (CF), which also had relatively lower diametral strength. Of the three posterior composites tested, P (P30) yielded the highest cohesive strength at p < .Ol , whereas CP and HR demonstrated comparable tensile strengths THE JOURNAL
OF PROSTHETIC
DENTISTRY
with this true tension test that were approximately 25% of their respective diametral tensile strengths. The diametral tensile strength of P was also considerably higher than that of CP or HR.
DISCUSSION As mentioned, the clinical indications for using the incremental or layering technique are numerous, but the primary advantage is in overcoming the problem of limiting curing depth. If the curing depth of the composite is less than the thickness of the restoration and cannot be compensated for by irradiation from different directions, the restoration requires incremental insertion. The successof such a method is dependent on the ability of the added material to adhere to the already polymerized composite. The interfacial bonding between layers of chemically cured composites has been reported in some studies to be almost as strong as the material itself.’ Other studies, 27
TJAN
Table II. Tensile load required
AND
GLANCY
to induce fracture Load at fracture
Layer*
Fracture sitet Bottom
Top Visio-Dispers Micro-Fine Curay-Match P30 Herculite Curay II Micro-Fine Curay-Match Heliosit Heliosit
Top
Bottom
5 5 0 0 2 1 3 4 5 5
0 0 5 5 3 4 2 1 0 0
Visio-Fil Prisma-Fil Curay-Fil P30 Herculite Curay II Curay-Fil Prisma-Fil Curay-Fil Prisma-Fil
MN/m’ (SD) 10.8 13.1 13.4 17.2 12.6 12.6 8.9 11.4 2.1 4.7
Lbfin’ (SD)
(1.3) (0.7) (1.8) (3.0) (0.9) (0.8) (1.1) (3.0) (1.2) (2.5)
1562 1894 1938 2499 1824 1824 1288 1647 300 680
(192) (100) (264) (435) (136) (114) (164) (433) (175) (361)
MN = meganewtons. *Bottom layer forms the substrate or adherend with the top layer as the adhesive. *Frequency of fracture at indicated site.
Table III. Diametral
tensile strength
of
composites studied Load at fracture Material Visio-Dispers Visio-Fil Heliosit Prisma-Fil Micro-Fine Curay-Match Curay-Fil Curay II Herculite P-30
Batch No.
MN/m2
Lb/in2
0017 0057 120384 1127842 01985 DD-453 GG-420 BB 389 5-1834 5A5P
40 48 38 62 42.5 47 42 56.4 55.4 72
5829 6960 5510 8990 6162 6815 6138 8178 8033 10440
however, have indicated a reduced bond strength unless a bonding agent is used.‘O-” There have been several in vitro studies of the bond strength of composite repairs.“-” Either a direct tensile test using a dumbbellshaped specimen or the transverse strength test was usually used to evaluate the bond strength. The interfacial bonding to aged composites has generally been reported as significantly lower than the cohesive strength of the materials.“~‘3-15 The results of this study agreed with the findings of other investigators that interfacial bond strength to freshly polymerized composite did not differ from the cohesive strength.11p’7*‘8 The data indicated that the interfacial bond strength was greater, or at least equal to the cohesive strength of the material when a composite was cured to itself. The data also indicated that when two types of composites were bonded together, the bond strength was greater than the cohesive strength of the weaker material. Some correlation was observed between the values of the cohesive strength measured with true tension and the diametral tensile strength of the material. The results 28
also indicated that a urethane dimethacrylate-microfilled composite bonded weakly to BIS-GMA composite and failures of an adhesive nature were generally observed. These findings were consistent with the findings of Chan and Boyer.13 The cohesive strengths of the composites measured with direct tension were significantly lower than their diametral tensile strengths, which varied from 22 to 32% of the diametral tensile strength depending upon the brands of the composites. Composites are brittle and therefore much weaker in tension or transverse loading than in compression. In addition, a direct tensile strength is adversely influenced by the presence of void spaces or cracks. Brittle materials fail by the propagation of external or internal cracks because stress concentrations remain high around such flaws. Tensile or bending stresses tend to extend cracks, whereas compressive stresses do not. Obviously the presence of voids or flaws is one of the greatest potential problems inherent to composite restorations because the material is relatively viscous, does not flow readily, and therefore tends to trap air. The commonly used dumbbell-shaped specimens for tensile testing with true tension are considered appropriate for self-cured composites. However, for light-activated composites, this may not accurately reflect the actual conditions of curing a deep restoration in the mouth where the material can only be irradiated from one direction. In this study some modification was made to the traditional dumbbell method to simulate the curing of composite in an actual cavity preparation that allows the light beam to penetrate only from the upper surface of the specimen.
SUMMARY AND CONCLUSIONS The bond strengths between composite layers either cured to themselves or to other types (classes) of composJANUARY
1988
VOLUME
59
NUMBER
1
INTERFACIAL
BOND STRENGTHS
ites of similar or different brands were measured by using a direct tensile test (true tension). The diametral compression test for tension for each material tested was also conducted according to ADA specification No. 27. The values were used for comparison. 1. The interfacial bond strengths were generally found to be higher than the cohesive strengths of the weaker materials when cured to different types of composites or of the weak region in the specimen when composites were cured to themselves. 2. The cohesive tensile failure of the materials occurred at much lower stress levels than their corresponding diametral tensile strength (ranging from ‘/ to % of the diametral strength). 3. When two types of composites were bonded together the cohesive failure occurred consistently within the materials with lower diametral strength. Correlation was observed between the values of cohesive strength of material measured with true tension and the diametral test. 4. A urethane dimethacrylate microfilled composite bonded weakly to BIS-GMA composite, therefore, their combined use should be avoided. 5. Incremental placement produced a clinically acceptable bond strength because it exceeded or was at least comparable to the cohesive strength of the material.
5.
6.
7.
8.
9. 10.
11. 12.
13. 14. 15. 16. 17.
REFERENCES 1.
Lutz F, Phillips RW. A classification and evaluation of composite resin systems. J PROSTHET DENT 1983;50:480. 2. Kilian RJ, Mullen TJ. Light-cured composites: dependence of test results on test parameters [Abstract]. J Dent Res i980;59:318. 3. Craig RG. Restorative dental materials. 7th ed. St Louis: The CV Mosby Co, 1985: 231-241. 4. Bowen RL, Nemoto K, Rapson JE. Adhesive bonding of various
Effect of provisional provisional resins
18.
materials to hard tooth tissues: forces developing in composite materials during hardening. J Am Dent Assoc 1983;106:475. Jorgenson KD, Asmussen E, Shimokobe H. Enamel damages caused by contracting restorative resins. Stand J Dent Res 1975;83:120. Davidson CL, DeGee AJ. Relaxation of polymerization contraction stresses by flow in dental composites. J Dent Res 1984;163:146. Davidson CL, DeGee AJ, Feilzer A. The competition between the composite-dentin bond strength and the polymerization contraction stress. J Dent Res 1984;63:1396. DeLange K, Davidson CL. Current status of posterior composite resins in Europe. In Vanherle G, Smith DC editors. Posterior composite resin dental restorative materials. Netherland: Peter Szulc Publishing Co, 1985: 181. Chiba K. Adhesion of the subsequently added composite resin [Abstract]. J Dent Res 1983;62:471. Miranda FJ, Duncanson Jr MG, Dilts WE. Interfacial bonding strengths of paired composite systems [Abstract]. J Dent Res 1982;61:215. Boyer DB, Chan KC, Torney DL. The strength of multilayer and repaired composite resin. J PROSTHET DENT 1978;39:63. Boyer DB, Chan KC, Reinhardt JW. Build-up and repair of light-cured composites: bond strength. J Dent Res 1984; 63:1241. Chan KC, Boyer DB. Repair of conventional and microfilled composite resins. J PROSTHET DENT 1983;50:345. Reisbick MH, Brodsky JF. Strength parameters of composite resins. J PROSTHET DENT 1971;26:178. Forsten L, Valiaho ML. Transverse and bond strength of restorative resins. Acta Odontol Stand 1971;29:527. Causton BE. Repair of abraded composite fillings. Br Dent J 1975;139:286. Lloyd CH, Baigrie DA, Jeffrey IW. The tensile strength of composite repairs. J Dent 1980;8:171. Marshall TD, Murrey AJ, and Norling BK. Shear bond strength of additions to composite resin [Abstract]. J Dent Res 1982;61:302.
Reprint requeststo: DR. ANTHONY H. L. TJAN LOMA LINDA UNIVERSITY SCH~L OF DENTISTRY LOMA LINDA, CA 92350
cementing agents on
Stephen F. Rosenstiel, B.D.S., M.S.D.,* and A. G. Gegauff, D.M.D.* Ohio State University,
College of Dentistry,
Columbus, Ohio
A
utopolymerizing resins are commonly used for making provisional restorations in fixed prosthodontic treatment. They are fixed to the abutment teeth with a *Assistant Professor, Department of Restorative and Prosthetic Dentistry. THE JOURNAL
OF PROSTHETIC
DENTISTRY
cementing agent that should have low strength and an obtundent effect on freshly prepared dentin. Zinc oxide-eugenol (ZOE) cement is popular because it has low strength and is an obtundent. However, eugenol softens different types of resins.‘e3 Eugenol-free cementing agents are commercially 29
H. L. Tjan, Dr. Dent., D.D.S.,* and James F. Glancy, B.A.** University,
School of Dentistry,
Loma Linda,
Calif
lhe primary limitation of visible light-activated composites is the light beam penetration and the consequent depth of cure. The degree of polymerization of any resin system is crucial for the long-term performance of composite restorations.’ Incomplete polymerization may lead to premature degradation of the composite, retention failure, and to possible adverse pulp reactions. The in vitro depth of cure for visible light-activated composites has been reported in the range of 2 to 8 mm.2 In large restorations or deep cavities where the curing depth of the material is less than the thickness of the restoration, incremental placement of the restorative material is recommended for building up the restoration. A chemical bonding between increments is ensured by the use of the air-inhibited surface layer.3 The incremental or layering technique minimizes the effect of the ;polymerization shrinkage (setting contraction), which may create considerable tensile stresses at the interface between resin and tooth. These stresses severely strain the bond between the low-viscosity resin bonding agent and the acid-etched enamel at the cavity margins, which may lead to early marginal failure and possible enamel fractures along the bonded interface.3-6It has been suggested that incremental application of light-activated composites aids in maintaining adaptation to the cavity wall.’ Obviously the clinical applications of the layering technique are also numerous. The technique is commonly used for complex restorative procedures, such as closing diastemas and veneering, which require extensive freehand contouring of the composite. The technique is also applied in Class IV restorations to attain optimal strength and esthetics by using a conventional composite as the substructure and adding an overlay of a microfilled resin to achieve maximal translucency, Another related procedure is the addition of a new layer of composite to repair fracture or discoloration of older restorations. The purpose of this investigation was to evaluate the interfacial bond strengths between increments of (1) posterior composites, (2) a combination of traditional or
*Professor and Director of Biomaterials Research, Department of Restorative Dentistry. **Dental Student, Work-study Dental Research Program. THE JOURNAL
OF PROSTHETIC
DENTISTRY
Table I. Visible light-activated used in study Composite
Code Shade
Batch No.
composites
Manufacturer
Heliosit
HL
30
120384
Vivadent (U.S.A.), Inc. Tonawanda, N.Y.
Visio-Dispers
VD
L
0017
ESPE-Premier Corp., Norristown, Pa.
Micro-Fine
MF
B52
010985
The L. D. Caulk Co., Milford, Del.
&ray-Match
CM
95
DD-453
Sci-Pharm, Inc., Duarte, Calif.
Visio-Fil
VF
STD
0057
ESPE-Premier
Prisma-Fil
PF
B59
1127842 The L. D. Caulk Co.,
Gray-Fil
CF
65
99-420
Sci-Pharm,
P30
P
XL
5A5P
3M Co., St. Paul, Minn.
Herculite
HR
L
5-1834
Kerr/Sybron, Romulus, Mich.
Curay II
CP
55
BB389
Sci-Pharm,
Corp.,
Inc.
Inc.
hybrid composites with microfilled composites of the same brands, and (3) a combination of composites of different brands.
MATERIAL
AND METHODS
The visible light-activated composites used in this study were three brands of posterior and seven brands of anterior composites that included three microfilled composites. The product names, batch numbers, and manufacturers are shown in Table I. The interfacial bond strengths of the 10 paired composite systems were evaluated by using a direct tensile test. They consisted of three posterior components bonded to themselves; three macrofiiled (or hybrid) composites bonded to microfilled composites of the same brands; and four macrofilled (or hybrid) composites bonded to microfilled composites of different brands, respectively referred to by the following codes: 1. HR-HR (Herculite to Herculite) 25
TJAN
AND
GLANCY
LIGHT
LIGHT 9
Fig. 1. Schematic diagram for specimen preparation. A, Composite is inserted in lower section of metal mold, condensed, and irradiated to form first half of specimen or adherend: I, metal mold for first layer; 2, first layer or adherend; and 3, jig. B, Upper section of mold is placed on top of lower section and second half of specimen or adhesive layer polymerized: 4, second layer or adhesive portion; 2, first layer in A now forms substrate or adherend in B; and 5, upper section of mold.
2. P-P (P30 to P30) 3. CP-CP (Curay II to Curay II)
4. VF-VD (Visio-Fil to Visio-Dispers) 5. PF-MF (Prisma-Fil to Micro-Fine) 6. CF-CM
(Curay-Fil
to Curay-Match)
7. CF-MF (Curay-Fil to Micro-Fine) 8. PF-CM (Prisma-Fil to Curay-Match) 9. CF-HL (Curay-Fil to Heliosit) 10. PF-HL (Prisma-Fil to Heliosit) Heliosit (HL) is a urethane dimethacrylate-based microfilled composite.* Curay-Match is a low-viscosity hybrid composite. To simulate actual clinical conditions of curing composite in deep cavity preparations in which the light can be applied only from one direction, a two-level mold, consisting of a lower and upper part, was designed and used to prepare the specimens. The composite was inserted in the lower section and condensed to form the first half of the specimen (substrate or adherend) which was exposed to a visible light (Elipar Visio, ESPEPremier Sales Corp., Norristown, Pa.) for 40 seconds (Fig. 1, A). The upper part of the mold, which was placed on top of the lower part, was filled immediately with either the same composite or with another composite, condensed, and then irradiated for an additional 40 seconds to form the adhesive layer (Fig. 1, B). The material was premeasured to produce a consistent thickness (2.5 mm) of the composite. The diameter of the mold at the interface was approximately 7 mm. While curing, the top of the activating light was held 1 mm from the specimen surface. All bonded specimens were 26
then stored at 37” C at 100% relative humidity for 24 hours before testing. All specimens remained in the molds.
Tensile testing The specimen was held in a specially designed device that permitted only pure axial loading and was mounted to the Riehle Universal testing machine (American Machine and Metal Inc., East Moline, Ill.) with a crosshead speed of 1 mm/min (Fig. 2). Peak force values at fracture were recorded. For the diametral tensile strengths of the composites, specimen preparation and testing were done in accordana with ADA Specification No. 27. One-way analysis of variance (ANOVA) was used to analyze the data and multiple comparisons were performed by using Duncan’s multiple range tests. RESULTS Table II presents the means and standard deviations of the load required to induce a fracture to the specimens by a direct tensile testing method. The diametral tensile strength values of the 10 light-activated composites used in the study were also determined for the sake of comparison and are shown in Table III. With the exception of the two groups that combined BIS-GMA and urethane dimethacrylate-based composite (PF-HL and CF-HL), all the specimens broke at sites other than the interfaces (cohesive failure). With the combination of PF-HL and CF-HL, however, the fractures took place either at the interface (complete or JANUARY
1988
VOLUME
59
NUMBER
1
INTERFACIAL
ROND
STRENGTHS
‘2
‘--1
-2 -3 t----4
Fig. 2. A, Device used to induce tensile loading. B, Schematic diagram of device: 1, specimen; 2, two-level metal mold; 3, threaded rod; and 4, metal cylinder sleeve.
partial adhesive failure) or at sites in close proximity to the junctions but in the microfilled region and with significantly lower fracture strengths at p < .Ol compared with the other groups. In the groups where hybrid composites were bonded to microfilled composites (PF-MF and VF-VD), the cohesive break with a direct tensile testing occurred consistently within the microfilled portion at approximately 30% of their respective diametral tensile strengths. This was consistent with the commonly lower diametral strengths of the microfilled composites compared with those of the hybrid composites. In the combination of a high and low viscosity hybrid-type composite (CF-CM), the cohesive failure occurred at the high viscosity resin (CF), which also had relatively lower diametral strength. Of the three posterior composites tested, P (P30) yielded the highest cohesive strength at p < .Ol , whereas CP and HR demonstrated comparable tensile strengths THE JOURNAL
OF PROSTHETIC
DENTISTRY
with this true tension test that were approximately 25% of their respective diametral tensile strengths. The diametral tensile strength of P was also considerably higher than that of CP or HR.
DISCUSSION As mentioned, the clinical indications for using the incremental or layering technique are numerous, but the primary advantage is in overcoming the problem of limiting curing depth. If the curing depth of the composite is less than the thickness of the restoration and cannot be compensated for by irradiation from different directions, the restoration requires incremental insertion. The successof such a method is dependent on the ability of the added material to adhere to the already polymerized composite. The interfacial bonding between layers of chemically cured composites has been reported in some studies to be almost as strong as the material itself.’ Other studies, 27
TJAN
Table II. Tensile load required
AND
GLANCY
to induce fracture Load at fracture
Layer*
Fracture sitet Bottom
Top Visio-Dispers Micro-Fine Curay-Match P30 Herculite Curay II Micro-Fine Curay-Match Heliosit Heliosit
Top
Bottom
5 5 0 0 2 1 3 4 5 5
0 0 5 5 3 4 2 1 0 0
Visio-Fil Prisma-Fil Curay-Fil P30 Herculite Curay II Curay-Fil Prisma-Fil Curay-Fil Prisma-Fil
MN/m’ (SD) 10.8 13.1 13.4 17.2 12.6 12.6 8.9 11.4 2.1 4.7
Lbfin’ (SD)
(1.3) (0.7) (1.8) (3.0) (0.9) (0.8) (1.1) (3.0) (1.2) (2.5)
1562 1894 1938 2499 1824 1824 1288 1647 300 680
(192) (100) (264) (435) (136) (114) (164) (433) (175) (361)
MN = meganewtons. *Bottom layer forms the substrate or adherend with the top layer as the adhesive. *Frequency of fracture at indicated site.
Table III. Diametral
tensile strength
of
composites studied Load at fracture Material Visio-Dispers Visio-Fil Heliosit Prisma-Fil Micro-Fine Curay-Match Curay-Fil Curay II Herculite P-30
Batch No.
MN/m2
Lb/in2
0017 0057 120384 1127842 01985 DD-453 GG-420 BB 389 5-1834 5A5P
40 48 38 62 42.5 47 42 56.4 55.4 72
5829 6960 5510 8990 6162 6815 6138 8178 8033 10440
however, have indicated a reduced bond strength unless a bonding agent is used.‘O-” There have been several in vitro studies of the bond strength of composite repairs.“-” Either a direct tensile test using a dumbbellshaped specimen or the transverse strength test was usually used to evaluate the bond strength. The interfacial bonding to aged composites has generally been reported as significantly lower than the cohesive strength of the materials.“~‘3-15 The results of this study agreed with the findings of other investigators that interfacial bond strength to freshly polymerized composite did not differ from the cohesive strength.11p’7*‘8 The data indicated that the interfacial bond strength was greater, or at least equal to the cohesive strength of the material when a composite was cured to itself. The data also indicated that when two types of composites were bonded together, the bond strength was greater than the cohesive strength of the weaker material. Some correlation was observed between the values of the cohesive strength measured with true tension and the diametral tensile strength of the material. The results 28
also indicated that a urethane dimethacrylate-microfilled composite bonded weakly to BIS-GMA composite and failures of an adhesive nature were generally observed. These findings were consistent with the findings of Chan and Boyer.13 The cohesive strengths of the composites measured with direct tension were significantly lower than their diametral tensile strengths, which varied from 22 to 32% of the diametral tensile strength depending upon the brands of the composites. Composites are brittle and therefore much weaker in tension or transverse loading than in compression. In addition, a direct tensile strength is adversely influenced by the presence of void spaces or cracks. Brittle materials fail by the propagation of external or internal cracks because stress concentrations remain high around such flaws. Tensile or bending stresses tend to extend cracks, whereas compressive stresses do not. Obviously the presence of voids or flaws is one of the greatest potential problems inherent to composite restorations because the material is relatively viscous, does not flow readily, and therefore tends to trap air. The commonly used dumbbell-shaped specimens for tensile testing with true tension are considered appropriate for self-cured composites. However, for light-activated composites, this may not accurately reflect the actual conditions of curing a deep restoration in the mouth where the material can only be irradiated from one direction. In this study some modification was made to the traditional dumbbell method to simulate the curing of composite in an actual cavity preparation that allows the light beam to penetrate only from the upper surface of the specimen.
SUMMARY AND CONCLUSIONS The bond strengths between composite layers either cured to themselves or to other types (classes) of composJANUARY
1988
VOLUME
59
NUMBER
1
INTERFACIAL
BOND STRENGTHS
ites of similar or different brands were measured by using a direct tensile test (true tension). The diametral compression test for tension for each material tested was also conducted according to ADA specification No. 27. The values were used for comparison. 1. The interfacial bond strengths were generally found to be higher than the cohesive strengths of the weaker materials when cured to different types of composites or of the weak region in the specimen when composites were cured to themselves. 2. The cohesive tensile failure of the materials occurred at much lower stress levels than their corresponding diametral tensile strength (ranging from ‘/ to % of the diametral strength). 3. When two types of composites were bonded together the cohesive failure occurred consistently within the materials with lower diametral strength. Correlation was observed between the values of cohesive strength of material measured with true tension and the diametral test. 4. A urethane dimethacrylate microfilled composite bonded weakly to BIS-GMA composite, therefore, their combined use should be avoided. 5. Incremental placement produced a clinically acceptable bond strength because it exceeded or was at least comparable to the cohesive strength of the material.
5.
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8.
9. 10.
11. 12.
13. 14. 15. 16. 17.
REFERENCES 1.
Lutz F, Phillips RW. A classification and evaluation of composite resin systems. J PROSTHET DENT 1983;50:480. 2. Kilian RJ, Mullen TJ. Light-cured composites: dependence of test results on test parameters [Abstract]. J Dent Res i980;59:318. 3. Craig RG. Restorative dental materials. 7th ed. St Louis: The CV Mosby Co, 1985: 231-241. 4. Bowen RL, Nemoto K, Rapson JE. Adhesive bonding of various
Effect of provisional provisional resins
18.
materials to hard tooth tissues: forces developing in composite materials during hardening. J Am Dent Assoc 1983;106:475. Jorgenson KD, Asmussen E, Shimokobe H. Enamel damages caused by contracting restorative resins. Stand J Dent Res 1975;83:120. Davidson CL, DeGee AJ. Relaxation of polymerization contraction stresses by flow in dental composites. J Dent Res 1984;163:146. Davidson CL, DeGee AJ, Feilzer A. The competition between the composite-dentin bond strength and the polymerization contraction stress. J Dent Res 1984;63:1396. DeLange K, Davidson CL. Current status of posterior composite resins in Europe. In Vanherle G, Smith DC editors. Posterior composite resin dental restorative materials. Netherland: Peter Szulc Publishing Co, 1985: 181. Chiba K. Adhesion of the subsequently added composite resin [Abstract]. J Dent Res 1983;62:471. Miranda FJ, Duncanson Jr MG, Dilts WE. Interfacial bonding strengths of paired composite systems [Abstract]. J Dent Res 1982;61:215. Boyer DB, Chan KC, Torney DL. The strength of multilayer and repaired composite resin. J PROSTHET DENT 1978;39:63. Boyer DB, Chan KC, Reinhardt JW. Build-up and repair of light-cured composites: bond strength. J Dent Res 1984; 63:1241. Chan KC, Boyer DB. Repair of conventional and microfilled composite resins. J PROSTHET DENT 1983;50:345. Reisbick MH, Brodsky JF. Strength parameters of composite resins. J PROSTHET DENT 1971;26:178. Forsten L, Valiaho ML. Transverse and bond strength of restorative resins. Acta Odontol Stand 1971;29:527. Causton BE. Repair of abraded composite fillings. Br Dent J 1975;139:286. Lloyd CH, Baigrie DA, Jeffrey IW. The tensile strength of composite repairs. J Dent 1980;8:171. Marshall TD, Murrey AJ, and Norling BK. Shear bond strength of additions to composite resin [Abstract]. J Dent Res 1982;61:302.
Reprint requeststo: DR. ANTHONY H. L. TJAN LOMA LINDA UNIVERSITY SCH~L OF DENTISTRY LOMA LINDA, CA 92350
cementing agents on
Stephen F. Rosenstiel, B.D.S., M.S.D.,* and A. G. Gegauff, D.M.D.* Ohio State University,
College of Dentistry,
Columbus, Ohio
A
utopolymerizing resins are commonly used for making provisional restorations in fixed prosthodontic treatment. They are fixed to the abutment teeth with a *Assistant Professor, Department of Restorative and Prosthetic Dentistry. THE JOURNAL
OF PROSTHETIC
DENTISTRY
cementing agent that should have low strength and an obtundent effect on freshly prepared dentin. Zinc oxide-eugenol (ZOE) cement is popular because it has low strength and is an obtundent. However, eugenol softens different types of resins.‘e3 Eugenol-free cementing agents are commercially 29