PATTERSONETAL
3. Klausner LH, Cartwright CB, Charbeneau GT. Polished versus autoglazed porcelain surfaces. J PROSTHET DENT 1982;47:157-62. 4. Smith GA, Wilson NHF. The surface finish of trimmed porcelain. Br Dent J 1981;151:222-4. 5. Sulik WD, Plekavich EJ. Surface finishing of dental porcelain. J PROS-
used. Even a 15 pm grit (yellow band) extra-fine grade FG diamond bur, used with copious water coolant before application of a commercial porselain refinishing kit of the type used in this study, will not permit recreation of a surface of similar smoothness to that of the original glazed surface, nor will it recreate the original intact surface glaze layer. Grades of diamond instruments finer than the existing yellow band types should be investigated if refinishing kits are to be employed.
THET DENT 1981;46:21’7-21.
6. Clyde JS, Gilmour A. Porcelain veneers: a preliminary review. Br Dent J 1988;164:9-14. 7. Patterson CJW, McLundie AC, Stirrups DR, Taylor WC. Refinishing of porcelain by using a refinishing kit. J PROSTHET DENT 1991;65:383-8. 8. Barghi N, King CJ, Draughn RA. A study of porcelain surfaces as utilized in fixed prosthodontics. J PROSTHET DENT 1975:34:314-g. 9. Goldstein RE. Finishing of composites and laminates. Dent Clin North Am 1989;33:305-18. 10. Clayton JA, Green E. Roughness of pontic materials and dental plaque. J PROSTHET DENT 1970;23:407-11. 11. Haywood VB, Heymann HO, Kusy RP, Whitley JQ, Andreaus SB. Polishing porcelain veneers: an SEM and specular reflectance analysis. Dent Mater 1988;4:116-21. 12. Bessing C, Wiktorsson A. Comparison of two different methods of polishing porcelain. Stand J Dent Res 1983;91:482-7.
We express our sincere thanks to Professor R. J. Scothorne and Mr. J. McGadey, Department of Anatomy, for access to the SEM and assistance in the preparation of specimens, and to Mr. J. B. Davies and staff, Department of Dental Illustration, University of Glasgow; to Mr. A. Robertson and staff, Photographic Department, University of Leeds Dental School; also to Mr. A. U. Stuart, Reader, Department of Design, Manufacture, and Engineering Management, and Mr. McMahon, Metrology Laboratory, University of Strathclyde, Glasgow, for the profilometric testing of specimens.
Reprint requests to; DR. C. J. W. PATTERSON DEPARTMENT, OF RESTORATIVE DENTISTRY UNIVERSITY OF LEEDS DENTAL SCHOOL CLARENDON WAY, LEEDS ENGLAND LS2 9LU
REFERENCES 1. Barghi N, Alexander L, Draughn RA. When to glaze-an electron microscope study. J PROSTHET DENT 1976;35:648-53. 2. Monasky GE, Taylor DF. Studies on the wear of porcelain, enamel, and gold. J PROSTHET DENT 1971;25:299-306.
Physical indirect William Joseph
properties composite A. Gregory, B. Dennison,
School of Dentistry,
and repair resins
DDS, MS,a Sue Berry,b DDS, MS”
The University
of Michigan,
bond strength Ettiene
Duke,b
of direct
and
and
Ann Arbor, Mich.
Use of composite resins now includes indirect curing methods. Surface chemistry, repair bond strength, plus location of failure and five physical properties of two direct and three indirect composite resins were determined. There were statistically significant differences in flexural strength of materia.ls and in hardness values. Repair bond strength failures were not significantly different; failures occurred primarily at the interface. Multiple internal reflection spectroscopy confirmed the presence of unpolymerized material after cure in all products. Indirect cure of the direct composite resin increased the degree of cure. (J PROSTHET DENT 1992;68:406-11.)
T he mtroduction :
of reinforced cidyl methacrylate (Bis-GMA) resin restorative material,l coupled with nique,2 greatly improved the esthetics
Bis phenol and glyas a composite resin the acid-etch techand longevity of di-
aLecturer, Department of Cariology and General Dentistry. bFourth year dental student, University of Wales College of Medicine, Heath Park, Cardiff, Wales. cProfessor and Chairman, Department of Cariology and General Dentistry. 10/l/37919
406
rect anterior restorations. These products initially utilized ceramic particles such as quartz, as large as 35 pm, for filler. Subsequent formulations have included various glasses and submicron silica particles, either alone or in a mix, to enhance the physical properties. The application of these composite resins soon expanded beyond the original intent as a replacement for silicate cement in conservative cavity preparations
to a variety
of other
procedures,
including
posterior restorations3 and anterior esthetic overlays or as leakage, veneers. 4 Clinical results were disappointing, wear, microscopic and gross fracture, and color change
SEPTEMBER1992
VOLUME68
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REPAIR BOND STRENGTHS OF COMPOSITE RESINS
Table
I.
Materials used in the study
Product
(symbol)
Herculite XR (HXR) Heliomolar (HM) Concept (C) Visio-Gem (VG) Dentacolor (DC)
Table
II.
1 2 3 4 5
Filler
Type
Hybrid Microfill Microfill Microfill Microfill
weight
Cure method
78% 79% 73% 38% 72%
Tests used in the study
Flexure strength* Radiopacity* Water sorption* Water solubility* Depth of cure*
6 7 8 9
Opacity Surface hardness Repair flexure strength Degree of cure
problems were common.5-8The lack of acceptable physical properties under function has supported the search for a form of composiLe resin that would be suitable for longterm anterior and posterior use. The curing process of composite resins creates large, three-dimensional polymers. The conversion of the available bonds of Bis-GMA restorative resins for direct intraoral use has been shown to be in the range of 52 % to 78% and it increases with heat application.g,10 Improved physical properties, such as surface hardness, were found to correlate directly with the degree of cure of the resin matrix.ll High-intensity light application, vacuum, or heat have been used in the curing of laboratory-processed composite resins.12 Composite resin materials for indirect fabrication of laminates, full crowns, panties, provisional restorations, inlays, onlays, three-quarter crowns facings, custom denture teeth, and miscellaneous repairs to fixed and removable prostheses are now marketed commercialIy.ls This study examined resin conversion and compared seven physical properties and repair bond strength of two direct restorative, one indirect restorative, and two indirect veneer composite resins. AND
METHODS
The materials used in this study are listed in Table I. Polymerization was accomplished according to manufacturers’ instructions. A visible-light irradiation of 20 seconds (Translux, Kulzer Inc., Irvine, Calif.) for each increment application was used for the direct materials. The indirect materials were cured in the appropriate specialized units: Concept (C; Williams Dental, Amherst, N.Y.) utilizing heat and pressure (Concept 250 heat-integrated processor); Visio-Gem (VG; Espe/Premier, Norristown, Pa.) light and vacuum (ESPE Visio Alfa light and Beta unit); Dentacolor (DC; Kulzer, Inc.) light (Dentacolor XS unit). The tests applied to each sample are listed in Table II. Tests for flexural strength, radiopacity, water sorption,
THE
JOURNAL
72105 379401 400287 408022 0005
OF PROSTHETIC
DENTISTRY
Manufacturer
KerrlSybron, Romulus, Mich. Vivadent, Tonawanda, N.Y. Williams Dental, Amherst, N.Y. ESPE/Premier, Norristown, Pa. Kulzer, Irvine, Calif.
Table III. Mean and standard deviation of transverse flexural strength and repair flexural strength Flexural strength ( MN/m2) Product
DC HM
*rso/Dts tests.
MATERIALS
Direct Direct Indirect Indirect Indirect
Batch number
VG HXR C
No.
5 5 5
5 5
Mean
56.94 89.80 90.37 114.97 151.96 I
Repair strength (MN/m2)
Std. dev.
Mean
Std. dev.
4.76 14.23 27.03 14.18 24.03
43.22 36.22 52.26 18.18 9.18
34.70 11.96 22.74 5.13 13.86
Vertical Lines,No significant differencebetweenproducts,p 5 Abbreviations as in body of Table I.
0.05.
water solubility, and depth of cure, which are included in the International Standards Organization document for resin-based filling materials (ISO/DIS 4049, 1988), were conducted according to those protocols. Bond strengths were evaluated for the five materials repaired with Herculite XR (HXR; Sybron/Kerr, Romulus, Mich.). Half-bars of composite resin were prepared in a mold 25 mm (f 0.2) x 2 mm (f 0.1) x 2 mm (+- 0.1) and were poly-
merized. After 24 hours’ storage in distilled water at 37” C (-f lo), one face of each specimen was abraded with a coarse aluminum oxide disk (Soflex, 3M Dental Products, Minneapolis, Minn.), cleaned for 60 seconds with 37% phosphoric acid, rinsed with running water for 30 seconds, dried with compressed air, and replaced in the mold. Herculite XR (HXR) bonding agent (batch No. 601 F, Kerr/ Sybron, Romulus, Mich.) was applied to the abraded surface and air-dispersed. HXR composite resin was packed into the mold to complete the bar and was light-cured. Following 24 hours of storage in water at 37” C, the dimensions of the specimens were recorded to an accuracy of F 0.01 mm. The bars were subjected to three-point bending in a universal testing machine (model TT-BM, Instron Corp, Canton, Mass.) according to the IS0 protocol for flexural
strength.
The fractures
were examined
with 13
power magnification and the fracture location was recorded. Opacity was evaluated using three specimens of each material.
Disks were formed is a mold 15 mm (t
0.1) mm
in diameter by 1 (-t 0.05) mm thick. The molds were slightly overfilled and were pressed together between glass slides placed on either side and were held together using 407
GREGORY ET AI,
Fig. 1. Radiopacity comparisons of 2 mm composite resin disks and aluminum IS0 standard. Superscript a indicates acceptable radiopacity. Disk of resin D was not sufficiently radiopaque to be recorded.
Table IV. Repair bond fracture type Primary Resin Repair Resin HXRiHXR HhUHXR C/HXR VG/HXR DC/HXR
Cohesive 0 0 0 0 0
Adhesive 3 3 5 4 1
Mixed /2* l/,/l* 0 1/* 1/,/4*
Abbreviations as in body of Table I. *Primary resin fracture/repair resin fracture.
spring clips. When necessary, the specimens were ground with 600 grade silicon carbide paper after cure to the required dimensions. The specimens were stored for 24 hours in water at 37OC. Opacity can be represented by a contrast ratio, C0.70, between the daylight apparent reflectance of specimens backed by a black backing and backed by a white backing, with a daylight apparent reflectance of 70% relative to magnesium oxide. A comparison of the opacities of the specimen and two glass standards with Co.70 values of 0.35 and 0.55 respectively, was made by placing the specimen against a variegated black and white background. The specimen, the standards, the space between them, and the black-and-white background were covered by a film of distilled water during observation. Comparisons were made by two evaluators. Opacity of all three specimens of each material between or equal to the opacities of the glass standards was called acceptable. To measure surface hardness, five disks, 15 (F 0.1) mm in diameter by 1 (t 0.65) mm thick, were made of each material. They were stored for 24 hours in water at 37’ C and were then measured for surface hardness (Wilson 408
Tukon Tester, Wilson Instruments, New York, N.Y.). Five measurements were made on each disk with a 300 kg indentor load. The Knoop hardness number {KHN) was calculated as the mean of the means for the five disks of each material. Multiple internal reflection infrared spectroscopy (MIR) was conducted on all five composite resins after polymerization by their prescribed method. A specimen of HXR cured with the Dentacolor XS oven was also examined. The use of the MIR technique for analysis of the surface chemistry of a composite resin has been described in detail elsewhere.lO The selective absorption of radiation in the 5 pm of surface material exhibits absorption bands that may be used to determine the presence of carbon-carbon double bonds, an indicator of unpolymerized resin. Two samples of each composite resin were prepared in a four-sided mold with an internal size of 10 X 40 X 1 mm. Glass slabs and Mylar strips (DuPont Co., Wilmington, Del.) were used for the top and bottom of the mold to produce smooth surfaces and to prevent inhibition of surface polymerization by oxygen during curing. Materials were tested on an SX-60 Fourier transform infrared spectrometer (Nicolet Instrument Corp., Madison, Wise.) in a reflectance mode. Two samples were secured in close contact to each side of a thallium-bromide-iodide crystal approximately the sample size. This combination was analyzed by midrange infrared light (5000 to 400 cm-l). The spectra were analyzed by peaks specific to the chemical groups present on the surface of the samples. Residual carbon-carbon bonds of nonpolymerized resin were identified by a peak at 1638 cm-l. The means and standard deviations of the samples in the tests for flexural strength, repair flexural strength, solubility, sorption, and KHN were calculated. The data were analyzed by analysis of variance (ANOVA) and were sub-
SEPTEMBER
1992
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68
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REPAIR
BOND
STRENGTHS
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RESINS
Table V. Sorption and solubility means and standard deviations of composite resins after T-day immersion (mg/cm’) Product
C
I Ia_I
1
1629
1643
1657
VG HM DC HXR
1671
Solubility
Sorption
0.01 (0.01) 0.02 (0.02)
0.3 (0.01) 0.2 (0.01)
0.05
(0.01)
0.2
0.05 (0.01) 0.08 (0.02)
(0.01)
0.2 (0.02) 0.2 (0.03)
Vertical lines, No significant difference between products, p 5 0.05. Abbreviations as in body of Table I.
VI. Mean Knoop hardness number of test materials (kg/mm2)
Table
Composite
VG
bl
‘4 1629
l&3
l&s7 WAVENUMBERS
lb71
2. Postpolymerization MIR infrared analysis spectra of: a, Visio-Gem (dotted line); Dentacolor (open circles); and Concept (solid line), the usual method of cure. b, Herculite (solid line), usual method of cure; (open circles), strobe light cure; and Heliomolar (dotted line), usual method of cure. Residual C-C bonds identified at 1638 cm-l. Fig.
jetted to the SchefIe contrast.14 Coefficients were calculated for correlations between physical properties. RESULTS The mean transverse flexural strengths and the mean transverse repair flexural strengths of each material are reported in Table III. Statistical analysis of the mean flexural strengths demonstrated significant differences: HXR and C were stronger than Heliomolar (HM; Vivadent, Tonawanda, N.Y.), VG and DC (p i 0.01). Repair of VG and DC showed the largest mean repair values. Three of the five repaired C samples fractured upon removal from the mold, and contributed to the lowest mean of any of the materials. The HXR repair with HXR was less strong than the HM repaired with HXR. There was no statistical difference among any of the means. The type of repair bond failures are listed in Table IV. Possible fractures would be cohesive in the primary resin or in the repair resin, adhesive at the repair interface or mixed, with portions of either of the materials remaining in an otherwise adhesive failure. None of the fractures occurred entirely cohesively. All of the mixed failures involved small fragments of the broken resin; the failure interface was mainly adhesive. All of the C failures were adhesive; the others all had both adhesive and mixed fractures.
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HM DC HXR C
KHN
(S.D.)
14 (1) 30 (10) 31 (3) 48 (5)
52 (1) I
Vertical line, No significant difference, p i 0.05. KHN, Knoop hardness number; other abbreviations as in body of Table I.
Comparison of the radiopacity of the materials is shown in Fig. 1. Radiopacity was found to be acceptable in the 2 mm samples of HXR, HM, and C, as compared with the same thickness of aluminum. VG and DC were less radiopaque than the 2 mm aluminum IS0 standard. The mean solubility and water sorption values are reported in Table V. The solubility of HXR, HM, and DC were not significantly different from one another, but all demonstrated significantly more solubility than C and VG. C and VG were not significantly different in solubility. C demonstrated significantly more sorption than the other materials, which were not different from one another. In the depth of cure test two of the indirect materials, C and VG, were cured the full 6 mm depth of the mold; DC cured to a mean depth of 1.9 mm in the mold. The two direct materials polymerized to a depth slightly more than 4 mm, significantly less than C or VG, but more than twice the IS0 minimum standard of 2 mm. The opacity of HXR, HM, C and VG was judged acceptable with all samples. DC was judged to be too translucent to be acceptable. Table VI reports the mean Knoop hardness for the materials tested. VG was significantly less hard than C, HXR, and DC at the 99 % confidence level, and less hard than HM at the 95 % level. C was statistically harder than all other materials except HXR; HXR was harder than the other three materials (p i 0.05). Analysis of the surfaces of the five composite resins by 409
GREGORY
MIR spectroscopy confirmed the presence of unpolymerized materinl at the surface after cure by the presence of a C-C double bond peak found at 1638 cm-i wave numbers. (Fig. 2). The sample of HXR cured with the Dentacolor strobe light oven demonstrated a lower level of unconverted carbon bonds than the sample cured with the intraoral light. DISCUSSION The materials examined in this study are different in several aspects, which will contribute to differences in physical properties. Of the two direct placement composite resins, HXR is a hybrid, HM, a microfill, with manufacturers’ reported load by weight of 78% and 79%, respectively. The three indirect materials are all microfills. VG’s load is reported as 38 % , while the load of C and DC are reported to be 73 % and 72 % . C is marketed as a posterior composite resin; VG are DC are crown and bridge veneer materials. Methods of cure are different: C is oven polymerized at 250° F and 6 psi for 10 minutes; VG has a IO-second visible light-cure followed by 15 minutes’ light exposure in a chamber under vacuum (5 to 6 mbar); DC is cured under a xenon light for 90 seconds. The mean flexural strengths of the direct materials were 90 MN/m2 (HM) and 115 MN/m2 (HXR), similar to those reported by McLean. I5 The mean flexural strength of C, 152, was significantly higher (p = 0.01) than that of all others except HXR. The heat and pressure cure method for C may be more conversion-effective, improving flexural strength. Flexural strength is an important factor with respect to resistance to restoration deformation or rupture by occlusal loads, as well as maintenance of the marginal seal.16 This property is less important for facing veneers. Lightcured composite resins are brittle in thin sections and require bulk for occlusal stress areas.12Conscientious cavity design will contribute to successful material use, and indirect composite resin restorations should not necessarily be based on preparations designed for metal castings. Studies of composite resin repairs have demonstrated that about 50 % of diametral tensile strengths can be generated without physical retention.17 Repairs rely upon residual unconverted carbon bonds for chemical bonding plus mechanical retention. The 600 grit finish to the primary bar in this study limited the repair bond to chemical bonding. Less conversion of or increases in resin matrix in the primary specimen should provide increased linkages for a chemical bonded repair. Lack of any cohesive failure of repairs indicts the repair interface. C, which did not repair well, had high flexural strength and KHN and a low solubility, all of which suggest a high degree of conversion. A -0.549 correlation coefficient of flexural strength and repair flexural strength, though not significant, reflects the inverse relationship of these two properties. Materials with higher repair flexural strengths (although not significantly different) generally had lower flexural strengths. Of 410
ET AL
the mixed failures, three left portions of the primary resin on the repair bar while seven left repair resin on the primary bar. These results substantially agree with the findings of another investigation of repair failures.‘s Radiopacity of materials is essential to radiographic differentiation of tooth and restoration, particularly in the posterior dentition. The IS0 depth of cure test does not apply well to the indirect materials because of their varied polymerization methods, but its minimum requirement may be used as a standard for comparisons. Statistical significance was demonstrated between the mean solubility values of C and VG as compared with HXR, HM, and DC (p = 0.05). A correlation coefficient of -0.2 between flexural strength and solubility suggests that factors contributing to loss of uncured resin, that is, low degree of cure and high percentage of resin present, contribute to lower flexural strength. The higher mean sorption value of C than of the other four products was significant (p 5 O.Ol), perhaps because of its unique polymerization process. A correlation coefficient of 0.76 between flexural strength and sorption was significant. While one might expect that low sorption, partially dependent upon low adsorption and low solubility, would produce an opposite result, the short time of this test may be inaccurate with respect to their long-term effects. Sorption has been found to be a poor predictor of resin conversion and did not vary with different conversion levels.lg Other studies20,21have demonstrated solubility and sorption levels of microfilled resins higher than those of hybrids and have related them to lower filler content and smaller size particles. Agreater degree of cure of microfilled resins should decrease leaching of matrix components. The value of hardness tests of composite resins is compromised by the biphasic nature of the material. The test stylus may fall on an area not representative of the matrix/ filler proportions. The hardness of the filler itself may adversely affect the results. KHNs of 600 kg/mm2 for quartz filler particles and 400 for barium glass have been reported.22Mean KHN value has been found to be a good test of the degree of polymerization for a specific resin, but not for comparing polymerization of different resins.llaig The higher KHNs of C and HXR may be the result of the type of cure and particle size, respectively. The low KHN of VG may relate to its low filler load (38 % ), exposing more matrix to the testing device. MIR analysis cannot be used to compare the degree of resin conversion of different products because formulations are so diverse, but it can measure the effects of polymerization techniques on individual materials. As indicated by the wave presence at 1638 cm-l in the spectra, none of the techniques eliminated residual unpolymerized resin. The greater degree of cure of HXR using a strobe light cure (a smaller wave peak as measured from the adjacent trough) compared with the intraoral light is consistent with other research on the effect of heat and light on the degree of cure.1° Any technique to improve the cure of indirect or
SEPTEMBER
1992
VOLUME
68
NUMBER
3
REPAIR
BOND
STRENGTHS
direct materials tics.
OF COMPOSITE
RESINS
may improve their functional
characteris-
CQNCLUSIONS 1. The laboratory use of pressure and heat or light and vacuum for indirect polymerization of microfilled composite resin significantly increased flexural strength and KHN, and decreased solubility compared with polymerization by light for intraoral application. 2. The laboratory use of pressure and heat for polymerization of a micro611 composite resin significantly increased flexural strength and KHN, and decreased solubility compared with laboratory polymerization of two microfill resins by strobe light and vacuum or by strobe light alone. 3. Materials demonstrating higher flexural strength of composite resin had lower repair bond strengths. 4. Of the directly cured resins, the hybrid composite resin demonstrated higher flexural strength, KHN, and lower solubility than the microfill. 5. Solubility and sorption did not directly correlate in composite resins for either direct or indirect use. 6. Laboratory methods of resin cure did not eliminate residual unreacted carbon-carbon bonds. Strobe light application to a directly cured hybrid resin achieved a greater degree of carbon bond conversion than cure with the intraoral light. REFERENCES 1. Bowen RL. Dental filling material comprising vinyl silane treated fused silica and a binder consisting of the reaction product of Bis phenol and glycidyl acrylate. 1962 United States Patent No. 3,066,112. 2. Buonocore MG. A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J Dent Res 1955;34:849-53. 3. Phillips RW, Avery DR, Mehra R, Schwartz MI, McCune RS. Observations on a composite resin for class II restorations: two-year report. J PROSTHET DENT 1972;28:164-9. 4. Klaff M, Ward G. Composite technique for restoration of malformed teeth. Dent Surv 1973;49:34-7.
Bound
volumes
available
5. Ameye C, Lambrechts R, Vanherle G. Conventional and microfilled composite resins. Part I. Color stability and marginal adaptation. J PROSTHET DENT 1981;46:623-30. 6. Leinfelder KF. Composite resins in posterior teeth. Dent Clin North Am 1981;25:357-64. 7. Leinfelder KF, Sleder TB, Sockwell CL, Wall JT. Clinical evaluation of composite resins as anterior and posterior restorative materials. J PROSTHET DENT 1975;33:407-16. 8. Loeys K, Lambrechts P, Vanherle G, Davidson CL. Material development and clinical performance of composite resins. J PROSTHET DENT 1982;48:664-72. 9. Craig RG. Restorative dental materials. St. Louis: CV Mosby Co, 1985:137. 10. Van Kerckhoven H, Lambrechts P, Van Beylen M, Davidson CL, Vanherle G. Unreacted methacrylate groups on the surfaces of composite resins. J Dent Res 1982;61:791-5. 11. Ferracane JL. Correlation between hardness and degree of conversion during the setting reaction of unfilled dental restorative resins. Dent Mater 1985;1:11-4. 12. Raffeto RF, Greenberg JR. Practical applications of composite resins in the modern dental laboratory. Trends Techniques 1985;5:15-7. 13. Greenberg JR, Rafetto RF. Laboratory light-cured composite resins: a clinical study. Part I. Compend Contin Educ 1985;6:402-2. 14. Scheffe H. The analysis of variance. New York: John Wiley & Sons, 1959:67. 15. McLean JW. Cermet cements. Am Dent Assoc J 1990;120:43-7. of cavities and 16. Jorgensen KD, Matono R, Shimokobe H. Deformation resin fillings in loaded teeth. Stand J Dent Res 1976;84:46-50. 17. Boyer DB, Chan DC, Torney DL. The strength of multi-layer and repaired composite resin. J PROSTHET DENT 1978;39:63-7. 18. Gregory WA, Pounder B, Bakus E. Bond strengths of chemically dissimilar repaired composite resins. J PROSTHET DENT (In press) 19. Rueggeberg FA, Craig RG. Correlation of parameters used to estimate monomer conversion in a light-cured composite. J Dent Res 1988;67:932-7. 20. van Fraunhofer JA, Hammer DW. Microleakage of composite resin restorations. J PROSTHET DENT 1984;51:209-13. 21. van Fraunhofer JA. The physical and mechanical properties of anterior and posterior composite restorative materials. Dent Mater 1989;5:365-8. 22. Leinfelder KF. Current developments in posterior composite resins. Adv. Dent Res 1988;2:111-21.
Reprint requests to: DR. WILLIAM A. GREGORY SCHOOL OF DENTISTRY THE UNIVERSITY OF MICHIGAN ANN ARBOR, MI 48109-1078
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Bound volumes of THE JOURNALOFPROSTHETICDENTISTRY are available to subscribers (only) for the 1992 issues from the publisher at a cost of $55.00 ($68.00 international) for Vol. 67 (January-June) and Vol. 68 (July-December). Shipping charges are included. Each bound volume contains a subject and author index, and all advertising is removed. Copies are shipped within 30 days after publication of the last issue in the volume. The binding is durable buckram with the journal name, volume number, and year stamped in gold on the spine. Volumes 65 and 66 are also available. Payment must accompany all orders. Contact Mosby-Year Book, Inc., Subscription Services, 11830 Westline Industrial Drive, St. Louis, MO 63146-3318, USA; phone (800) 325-4177, ext. 4351, or (314)453-435X. Subscriptions must be in force to qualify. Bound volumes are not available in place of a rlegular JOURNAL subscription.
THE
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411
3. Klausner LH, Cartwright CB, Charbeneau GT. Polished versus autoglazed porcelain surfaces. J PROSTHET DENT 1982;47:157-62. 4. Smith GA, Wilson NHF. The surface finish of trimmed porcelain. Br Dent J 1981;151:222-4. 5. Sulik WD, Plekavich EJ. Surface finishing of dental porcelain. J PROS-
used. Even a 15 pm grit (yellow band) extra-fine grade FG diamond bur, used with copious water coolant before application of a commercial porselain refinishing kit of the type used in this study, will not permit recreation of a surface of similar smoothness to that of the original glazed surface, nor will it recreate the original intact surface glaze layer. Grades of diamond instruments finer than the existing yellow band types should be investigated if refinishing kits are to be employed.
THET DENT 1981;46:21’7-21.
6. Clyde JS, Gilmour A. Porcelain veneers: a preliminary review. Br Dent J 1988;164:9-14. 7. Patterson CJW, McLundie AC, Stirrups DR, Taylor WC. Refinishing of porcelain by using a refinishing kit. J PROSTHET DENT 1991;65:383-8. 8. Barghi N, King CJ, Draughn RA. A study of porcelain surfaces as utilized in fixed prosthodontics. J PROSTHET DENT 1975:34:314-g. 9. Goldstein RE. Finishing of composites and laminates. Dent Clin North Am 1989;33:305-18. 10. Clayton JA, Green E. Roughness of pontic materials and dental plaque. J PROSTHET DENT 1970;23:407-11. 11. Haywood VB, Heymann HO, Kusy RP, Whitley JQ, Andreaus SB. Polishing porcelain veneers: an SEM and specular reflectance analysis. Dent Mater 1988;4:116-21. 12. Bessing C, Wiktorsson A. Comparison of two different methods of polishing porcelain. Stand J Dent Res 1983;91:482-7.
We express our sincere thanks to Professor R. J. Scothorne and Mr. J. McGadey, Department of Anatomy, for access to the SEM and assistance in the preparation of specimens, and to Mr. J. B. Davies and staff, Department of Dental Illustration, University of Glasgow; to Mr. A. Robertson and staff, Photographic Department, University of Leeds Dental School; also to Mr. A. U. Stuart, Reader, Department of Design, Manufacture, and Engineering Management, and Mr. McMahon, Metrology Laboratory, University of Strathclyde, Glasgow, for the profilometric testing of specimens.
Reprint requests to; DR. C. J. W. PATTERSON DEPARTMENT, OF RESTORATIVE DENTISTRY UNIVERSITY OF LEEDS DENTAL SCHOOL CLARENDON WAY, LEEDS ENGLAND LS2 9LU
REFERENCES 1. Barghi N, Alexander L, Draughn RA. When to glaze-an electron microscope study. J PROSTHET DENT 1976;35:648-53. 2. Monasky GE, Taylor DF. Studies on the wear of porcelain, enamel, and gold. J PROSTHET DENT 1971;25:299-306.
Physical indirect William Joseph
properties composite A. Gregory, B. Dennison,
School of Dentistry,
and repair resins
DDS, MS,a Sue Berry,b DDS, MS”
The University
of Michigan,
bond strength Ettiene
Duke,b
of direct
and
and
Ann Arbor, Mich.
Use of composite resins now includes indirect curing methods. Surface chemistry, repair bond strength, plus location of failure and five physical properties of two direct and three indirect composite resins were determined. There were statistically significant differences in flexural strength of materia.ls and in hardness values. Repair bond strength failures were not significantly different; failures occurred primarily at the interface. Multiple internal reflection spectroscopy confirmed the presence of unpolymerized material after cure in all products. Indirect cure of the direct composite resin increased the degree of cure. (J PROSTHET DENT 1992;68:406-11.)
T he mtroduction :
of reinforced cidyl methacrylate (Bis-GMA) resin restorative material,l coupled with nique,2 greatly improved the esthetics
Bis phenol and glyas a composite resin the acid-etch techand longevity of di-
aLecturer, Department of Cariology and General Dentistry. bFourth year dental student, University of Wales College of Medicine, Heath Park, Cardiff, Wales. cProfessor and Chairman, Department of Cariology and General Dentistry. 10/l/37919
406
rect anterior restorations. These products initially utilized ceramic particles such as quartz, as large as 35 pm, for filler. Subsequent formulations have included various glasses and submicron silica particles, either alone or in a mix, to enhance the physical properties. The application of these composite resins soon expanded beyond the original intent as a replacement for silicate cement in conservative cavity preparations
to a variety
of other
procedures,
including
posterior restorations3 and anterior esthetic overlays or as leakage, veneers. 4 Clinical results were disappointing, wear, microscopic and gross fracture, and color change
SEPTEMBER1992
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REPAIR BOND STRENGTHS OF COMPOSITE RESINS
Table
I.
Materials used in the study
Product
(symbol)
Herculite XR (HXR) Heliomolar (HM) Concept (C) Visio-Gem (VG) Dentacolor (DC)
Table
II.
1 2 3 4 5
Filler
Type
Hybrid Microfill Microfill Microfill Microfill
weight
Cure method
78% 79% 73% 38% 72%
Tests used in the study
Flexure strength* Radiopacity* Water sorption* Water solubility* Depth of cure*
6 7 8 9
Opacity Surface hardness Repair flexure strength Degree of cure
problems were common.5-8The lack of acceptable physical properties under function has supported the search for a form of composiLe resin that would be suitable for longterm anterior and posterior use. The curing process of composite resins creates large, three-dimensional polymers. The conversion of the available bonds of Bis-GMA restorative resins for direct intraoral use has been shown to be in the range of 52 % to 78% and it increases with heat application.g,10 Improved physical properties, such as surface hardness, were found to correlate directly with the degree of cure of the resin matrix.ll High-intensity light application, vacuum, or heat have been used in the curing of laboratory-processed composite resins.12 Composite resin materials for indirect fabrication of laminates, full crowns, panties, provisional restorations, inlays, onlays, three-quarter crowns facings, custom denture teeth, and miscellaneous repairs to fixed and removable prostheses are now marketed commercialIy.ls This study examined resin conversion and compared seven physical properties and repair bond strength of two direct restorative, one indirect restorative, and two indirect veneer composite resins. AND
METHODS
The materials used in this study are listed in Table I. Polymerization was accomplished according to manufacturers’ instructions. A visible-light irradiation of 20 seconds (Translux, Kulzer Inc., Irvine, Calif.) for each increment application was used for the direct materials. The indirect materials were cured in the appropriate specialized units: Concept (C; Williams Dental, Amherst, N.Y.) utilizing heat and pressure (Concept 250 heat-integrated processor); Visio-Gem (VG; Espe/Premier, Norristown, Pa.) light and vacuum (ESPE Visio Alfa light and Beta unit); Dentacolor (DC; Kulzer, Inc.) light (Dentacolor XS unit). The tests applied to each sample are listed in Table II. Tests for flexural strength, radiopacity, water sorption,
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Manufacturer
KerrlSybron, Romulus, Mich. Vivadent, Tonawanda, N.Y. Williams Dental, Amherst, N.Y. ESPE/Premier, Norristown, Pa. Kulzer, Irvine, Calif.
Table III. Mean and standard deviation of transverse flexural strength and repair flexural strength Flexural strength ( MN/m2) Product
DC HM
*rso/Dts tests.
MATERIALS
Direct Direct Indirect Indirect Indirect
Batch number
VG HXR C
No.
5 5 5
5 5
Mean
56.94 89.80 90.37 114.97 151.96 I
Repair strength (MN/m2)
Std. dev.
Mean
Std. dev.
4.76 14.23 27.03 14.18 24.03
43.22 36.22 52.26 18.18 9.18
34.70 11.96 22.74 5.13 13.86
Vertical Lines,No significant differencebetweenproducts,p 5 Abbreviations as in body of Table I.
0.05.
water solubility, and depth of cure, which are included in the International Standards Organization document for resin-based filling materials (ISO/DIS 4049, 1988), were conducted according to those protocols. Bond strengths were evaluated for the five materials repaired with Herculite XR (HXR; Sybron/Kerr, Romulus, Mich.). Half-bars of composite resin were prepared in a mold 25 mm (f 0.2) x 2 mm (f 0.1) x 2 mm (+- 0.1) and were poly-
merized. After 24 hours’ storage in distilled water at 37” C (-f lo), one face of each specimen was abraded with a coarse aluminum oxide disk (Soflex, 3M Dental Products, Minneapolis, Minn.), cleaned for 60 seconds with 37% phosphoric acid, rinsed with running water for 30 seconds, dried with compressed air, and replaced in the mold. Herculite XR (HXR) bonding agent (batch No. 601 F, Kerr/ Sybron, Romulus, Mich.) was applied to the abraded surface and air-dispersed. HXR composite resin was packed into the mold to complete the bar and was light-cured. Following 24 hours of storage in water at 37” C, the dimensions of the specimens were recorded to an accuracy of F 0.01 mm. The bars were subjected to three-point bending in a universal testing machine (model TT-BM, Instron Corp, Canton, Mass.) according to the IS0 protocol for flexural
strength.
The fractures
were examined
with 13
power magnification and the fracture location was recorded. Opacity was evaluated using three specimens of each material.
Disks were formed is a mold 15 mm (t
0.1) mm
in diameter by 1 (-t 0.05) mm thick. The molds were slightly overfilled and were pressed together between glass slides placed on either side and were held together using 407
GREGORY ET AI,
Fig. 1. Radiopacity comparisons of 2 mm composite resin disks and aluminum IS0 standard. Superscript a indicates acceptable radiopacity. Disk of resin D was not sufficiently radiopaque to be recorded.
Table IV. Repair bond fracture type Primary Resin Repair Resin HXRiHXR HhUHXR C/HXR VG/HXR DC/HXR
Cohesive 0 0 0 0 0
Adhesive 3 3 5 4 1
Mixed /2* l/,/l* 0 1/* 1/,/4*
Abbreviations as in body of Table I. *Primary resin fracture/repair resin fracture.
spring clips. When necessary, the specimens were ground with 600 grade silicon carbide paper after cure to the required dimensions. The specimens were stored for 24 hours in water at 37OC. Opacity can be represented by a contrast ratio, C0.70, between the daylight apparent reflectance of specimens backed by a black backing and backed by a white backing, with a daylight apparent reflectance of 70% relative to magnesium oxide. A comparison of the opacities of the specimen and two glass standards with Co.70 values of 0.35 and 0.55 respectively, was made by placing the specimen against a variegated black and white background. The specimen, the standards, the space between them, and the black-and-white background were covered by a film of distilled water during observation. Comparisons were made by two evaluators. Opacity of all three specimens of each material between or equal to the opacities of the glass standards was called acceptable. To measure surface hardness, five disks, 15 (F 0.1) mm in diameter by 1 (t 0.65) mm thick, were made of each material. They were stored for 24 hours in water at 37’ C and were then measured for surface hardness (Wilson 408
Tukon Tester, Wilson Instruments, New York, N.Y.). Five measurements were made on each disk with a 300 kg indentor load. The Knoop hardness number {KHN) was calculated as the mean of the means for the five disks of each material. Multiple internal reflection infrared spectroscopy (MIR) was conducted on all five composite resins after polymerization by their prescribed method. A specimen of HXR cured with the Dentacolor XS oven was also examined. The use of the MIR technique for analysis of the surface chemistry of a composite resin has been described in detail elsewhere.lO The selective absorption of radiation in the 5 pm of surface material exhibits absorption bands that may be used to determine the presence of carbon-carbon double bonds, an indicator of unpolymerized resin. Two samples of each composite resin were prepared in a four-sided mold with an internal size of 10 X 40 X 1 mm. Glass slabs and Mylar strips (DuPont Co., Wilmington, Del.) were used for the top and bottom of the mold to produce smooth surfaces and to prevent inhibition of surface polymerization by oxygen during curing. Materials were tested on an SX-60 Fourier transform infrared spectrometer (Nicolet Instrument Corp., Madison, Wise.) in a reflectance mode. Two samples were secured in close contact to each side of a thallium-bromide-iodide crystal approximately the sample size. This combination was analyzed by midrange infrared light (5000 to 400 cm-l). The spectra were analyzed by peaks specific to the chemical groups present on the surface of the samples. Residual carbon-carbon bonds of nonpolymerized resin were identified by a peak at 1638 cm-l. The means and standard deviations of the samples in the tests for flexural strength, repair flexural strength, solubility, sorption, and KHN were calculated. The data were analyzed by analysis of variance (ANOVA) and were sub-
SEPTEMBER
1992
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RESINS
Table V. Sorption and solubility means and standard deviations of composite resins after T-day immersion (mg/cm’) Product
C
I Ia_I
1
1629
1643
1657
VG HM DC HXR
1671
Solubility
Sorption
0.01 (0.01) 0.02 (0.02)
0.3 (0.01) 0.2 (0.01)
0.05
(0.01)
0.2
0.05 (0.01) 0.08 (0.02)
(0.01)
0.2 (0.02) 0.2 (0.03)
Vertical lines, No significant difference between products, p 5 0.05. Abbreviations as in body of Table I.
VI. Mean Knoop hardness number of test materials (kg/mm2)
Table
Composite
VG
bl
‘4 1629
l&3
l&s7 WAVENUMBERS
lb71
2. Postpolymerization MIR infrared analysis spectra of: a, Visio-Gem (dotted line); Dentacolor (open circles); and Concept (solid line), the usual method of cure. b, Herculite (solid line), usual method of cure; (open circles), strobe light cure; and Heliomolar (dotted line), usual method of cure. Residual C-C bonds identified at 1638 cm-l. Fig.
jetted to the SchefIe contrast.14 Coefficients were calculated for correlations between physical properties. RESULTS The mean transverse flexural strengths and the mean transverse repair flexural strengths of each material are reported in Table III. Statistical analysis of the mean flexural strengths demonstrated significant differences: HXR and C were stronger than Heliomolar (HM; Vivadent, Tonawanda, N.Y.), VG and DC (p i 0.01). Repair of VG and DC showed the largest mean repair values. Three of the five repaired C samples fractured upon removal from the mold, and contributed to the lowest mean of any of the materials. The HXR repair with HXR was less strong than the HM repaired with HXR. There was no statistical difference among any of the means. The type of repair bond failures are listed in Table IV. Possible fractures would be cohesive in the primary resin or in the repair resin, adhesive at the repair interface or mixed, with portions of either of the materials remaining in an otherwise adhesive failure. None of the fractures occurred entirely cohesively. All of the mixed failures involved small fragments of the broken resin; the failure interface was mainly adhesive. All of the C failures were adhesive; the others all had both adhesive and mixed fractures.
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HM DC HXR C
KHN
(S.D.)
14 (1) 30 (10) 31 (3) 48 (5)
52 (1) I
Vertical line, No significant difference, p i 0.05. KHN, Knoop hardness number; other abbreviations as in body of Table I.
Comparison of the radiopacity of the materials is shown in Fig. 1. Radiopacity was found to be acceptable in the 2 mm samples of HXR, HM, and C, as compared with the same thickness of aluminum. VG and DC were less radiopaque than the 2 mm aluminum IS0 standard. The mean solubility and water sorption values are reported in Table V. The solubility of HXR, HM, and DC were not significantly different from one another, but all demonstrated significantly more solubility than C and VG. C and VG were not significantly different in solubility. C demonstrated significantly more sorption than the other materials, which were not different from one another. In the depth of cure test two of the indirect materials, C and VG, were cured the full 6 mm depth of the mold; DC cured to a mean depth of 1.9 mm in the mold. The two direct materials polymerized to a depth slightly more than 4 mm, significantly less than C or VG, but more than twice the IS0 minimum standard of 2 mm. The opacity of HXR, HM, C and VG was judged acceptable with all samples. DC was judged to be too translucent to be acceptable. Table VI reports the mean Knoop hardness for the materials tested. VG was significantly less hard than C, HXR, and DC at the 99 % confidence level, and less hard than HM at the 95 % level. C was statistically harder than all other materials except HXR; HXR was harder than the other three materials (p i 0.05). Analysis of the surfaces of the five composite resins by 409
GREGORY
MIR spectroscopy confirmed the presence of unpolymerized materinl at the surface after cure by the presence of a C-C double bond peak found at 1638 cm-i wave numbers. (Fig. 2). The sample of HXR cured with the Dentacolor strobe light oven demonstrated a lower level of unconverted carbon bonds than the sample cured with the intraoral light. DISCUSSION The materials examined in this study are different in several aspects, which will contribute to differences in physical properties. Of the two direct placement composite resins, HXR is a hybrid, HM, a microfill, with manufacturers’ reported load by weight of 78% and 79%, respectively. The three indirect materials are all microfills. VG’s load is reported as 38 % , while the load of C and DC are reported to be 73 % and 72 % . C is marketed as a posterior composite resin; VG are DC are crown and bridge veneer materials. Methods of cure are different: C is oven polymerized at 250° F and 6 psi for 10 minutes; VG has a IO-second visible light-cure followed by 15 minutes’ light exposure in a chamber under vacuum (5 to 6 mbar); DC is cured under a xenon light for 90 seconds. The mean flexural strengths of the direct materials were 90 MN/m2 (HM) and 115 MN/m2 (HXR), similar to those reported by McLean. I5 The mean flexural strength of C, 152, was significantly higher (p = 0.01) than that of all others except HXR. The heat and pressure cure method for C may be more conversion-effective, improving flexural strength. Flexural strength is an important factor with respect to resistance to restoration deformation or rupture by occlusal loads, as well as maintenance of the marginal seal.16 This property is less important for facing veneers. Lightcured composite resins are brittle in thin sections and require bulk for occlusal stress areas.12Conscientious cavity design will contribute to successful material use, and indirect composite resin restorations should not necessarily be based on preparations designed for metal castings. Studies of composite resin repairs have demonstrated that about 50 % of diametral tensile strengths can be generated without physical retention.17 Repairs rely upon residual unconverted carbon bonds for chemical bonding plus mechanical retention. The 600 grit finish to the primary bar in this study limited the repair bond to chemical bonding. Less conversion of or increases in resin matrix in the primary specimen should provide increased linkages for a chemical bonded repair. Lack of any cohesive failure of repairs indicts the repair interface. C, which did not repair well, had high flexural strength and KHN and a low solubility, all of which suggest a high degree of conversion. A -0.549 correlation coefficient of flexural strength and repair flexural strength, though not significant, reflects the inverse relationship of these two properties. Materials with higher repair flexural strengths (although not significantly different) generally had lower flexural strengths. Of 410
ET AL
the mixed failures, three left portions of the primary resin on the repair bar while seven left repair resin on the primary bar. These results substantially agree with the findings of another investigation of repair failures.‘s Radiopacity of materials is essential to radiographic differentiation of tooth and restoration, particularly in the posterior dentition. The IS0 depth of cure test does not apply well to the indirect materials because of their varied polymerization methods, but its minimum requirement may be used as a standard for comparisons. Statistical significance was demonstrated between the mean solubility values of C and VG as compared with HXR, HM, and DC (p = 0.05). A correlation coefficient of -0.2 between flexural strength and solubility suggests that factors contributing to loss of uncured resin, that is, low degree of cure and high percentage of resin present, contribute to lower flexural strength. The higher mean sorption value of C than of the other four products was significant (p 5 O.Ol), perhaps because of its unique polymerization process. A correlation coefficient of 0.76 between flexural strength and sorption was significant. While one might expect that low sorption, partially dependent upon low adsorption and low solubility, would produce an opposite result, the short time of this test may be inaccurate with respect to their long-term effects. Sorption has been found to be a poor predictor of resin conversion and did not vary with different conversion levels.lg Other studies20,21have demonstrated solubility and sorption levels of microfilled resins higher than those of hybrids and have related them to lower filler content and smaller size particles. Agreater degree of cure of microfilled resins should decrease leaching of matrix components. The value of hardness tests of composite resins is compromised by the biphasic nature of the material. The test stylus may fall on an area not representative of the matrix/ filler proportions. The hardness of the filler itself may adversely affect the results. KHNs of 600 kg/mm2 for quartz filler particles and 400 for barium glass have been reported.22Mean KHN value has been found to be a good test of the degree of polymerization for a specific resin, but not for comparing polymerization of different resins.llaig The higher KHNs of C and HXR may be the result of the type of cure and particle size, respectively. The low KHN of VG may relate to its low filler load (38 % ), exposing more matrix to the testing device. MIR analysis cannot be used to compare the degree of resin conversion of different products because formulations are so diverse, but it can measure the effects of polymerization techniques on individual materials. As indicated by the wave presence at 1638 cm-l in the spectra, none of the techniques eliminated residual unpolymerized resin. The greater degree of cure of HXR using a strobe light cure (a smaller wave peak as measured from the adjacent trough) compared with the intraoral light is consistent with other research on the effect of heat and light on the degree of cure.1° Any technique to improve the cure of indirect or
SEPTEMBER
1992
VOLUME
68
NUMBER
3
REPAIR
BOND
STRENGTHS
direct materials tics.
OF COMPOSITE
RESINS
may improve their functional
characteris-
CQNCLUSIONS 1. The laboratory use of pressure and heat or light and vacuum for indirect polymerization of microfilled composite resin significantly increased flexural strength and KHN, and decreased solubility compared with polymerization by light for intraoral application. 2. The laboratory use of pressure and heat for polymerization of a micro611 composite resin significantly increased flexural strength and KHN, and decreased solubility compared with laboratory polymerization of two microfill resins by strobe light and vacuum or by strobe light alone. 3. Materials demonstrating higher flexural strength of composite resin had lower repair bond strengths. 4. Of the directly cured resins, the hybrid composite resin demonstrated higher flexural strength, KHN, and lower solubility than the microfill. 5. Solubility and sorption did not directly correlate in composite resins for either direct or indirect use. 6. Laboratory methods of resin cure did not eliminate residual unreacted carbon-carbon bonds. Strobe light application to a directly cured hybrid resin achieved a greater degree of carbon bond conversion than cure with the intraoral light. REFERENCES 1. Bowen RL. Dental filling material comprising vinyl silane treated fused silica and a binder consisting of the reaction product of Bis phenol and glycidyl acrylate. 1962 United States Patent No. 3,066,112. 2. Buonocore MG. A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J Dent Res 1955;34:849-53. 3. Phillips RW, Avery DR, Mehra R, Schwartz MI, McCune RS. Observations on a composite resin for class II restorations: two-year report. J PROSTHET DENT 1972;28:164-9. 4. Klaff M, Ward G. Composite technique for restoration of malformed teeth. Dent Surv 1973;49:34-7.
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volumes
available
5. Ameye C, Lambrechts R, Vanherle G. Conventional and microfilled composite resins. Part I. Color stability and marginal adaptation. J PROSTHET DENT 1981;46:623-30. 6. Leinfelder KF. Composite resins in posterior teeth. Dent Clin North Am 1981;25:357-64. 7. Leinfelder KF, Sleder TB, Sockwell CL, Wall JT. Clinical evaluation of composite resins as anterior and posterior restorative materials. J PROSTHET DENT 1975;33:407-16. 8. Loeys K, Lambrechts P, Vanherle G, Davidson CL. Material development and clinical performance of composite resins. J PROSTHET DENT 1982;48:664-72. 9. Craig RG. Restorative dental materials. St. Louis: CV Mosby Co, 1985:137. 10. Van Kerckhoven H, Lambrechts P, Van Beylen M, Davidson CL, Vanherle G. Unreacted methacrylate groups on the surfaces of composite resins. J Dent Res 1982;61:791-5. 11. Ferracane JL. Correlation between hardness and degree of conversion during the setting reaction of unfilled dental restorative resins. Dent Mater 1985;1:11-4. 12. Raffeto RF, Greenberg JR. Practical applications of composite resins in the modern dental laboratory. Trends Techniques 1985;5:15-7. 13. Greenberg JR, Rafetto RF. Laboratory light-cured composite resins: a clinical study. Part I. Compend Contin Educ 1985;6:402-2. 14. Scheffe H. The analysis of variance. New York: John Wiley & Sons, 1959:67. 15. McLean JW. Cermet cements. Am Dent Assoc J 1990;120:43-7. of cavities and 16. Jorgensen KD, Matono R, Shimokobe H. Deformation resin fillings in loaded teeth. Stand J Dent Res 1976;84:46-50. 17. Boyer DB, Chan DC, Torney DL. The strength of multi-layer and repaired composite resin. J PROSTHET DENT 1978;39:63-7. 18. Gregory WA, Pounder B, Bakus E. Bond strengths of chemically dissimilar repaired composite resins. J PROSTHET DENT (In press) 19. Rueggeberg FA, Craig RG. Correlation of parameters used to estimate monomer conversion in a light-cured composite. J Dent Res 1988;67:932-7. 20. van Fraunhofer JA, Hammer DW. Microleakage of composite resin restorations. J PROSTHET DENT 1984;51:209-13. 21. van Fraunhofer JA. The physical and mechanical properties of anterior and posterior composite restorative materials. Dent Mater 1989;5:365-8. 22. Leinfelder KF. Current developments in posterior composite resins. Adv. Dent Res 1988;2:111-21.
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