Shear r e s i s t a n c e of composite resin to e n a m e l using color-modifying resins and variously applied unfilled bonding resins MS, D D S , a a n d S u e a n n L o n g e n e c k e r , The Ohio State University, College of Dentistry, Columbus, Ohio R o l a n d P. P a g n i a n o ,
RDH, MEd b
The effects o f bond l a y e r t h i c k n e s s of c o l o r - m o d i f y i n g r e s i n s and v a r i o u s l y applied unfilled r e s i n s on c o m p o s i t e r e s i n / e n a m e l s h e a r bond s t r e n g t h s w e r e compared. Composite resin columns w e r e bonded to etched e n a m e l s u r f a c e s in vitro w i t h either applied unfilled r e s i n s or w i t h filled c o l o r - m o d i f y i n g resins. The bond l a y e r t h i c k n e s s and the force r e q u i r e d to s h e a r the 17 s p e c i m e n s of each of six groups w e r e m e a s u r e d . ANOVA w i t h Tukey's test w a s p e r f o r m e d to d e t e r m i n e i f significant differences at the a = 0.05 l e v e l e x i s t e d for the m e a n bond l a y e r t h i c k n e s s e s as w e l l as the m e a n forces r e q u i r e d to debond the s p e c i m e n s of six groups. Colorm o d i f y i n g r e s i n s r e s u l t e d in a 5 to 327 t i m e s g r e a t e r bond t h i c k n e s s than the unfilled resins. Only one o f the unfilled r e s i n groups s h o w e d a s t a t i s t i c a l l y significant difference in s h e a r r e s i s t a n c e c o m p a r e d w i t h a c o l o r - m o d i f y i n g resin group. The m e t h o d of application and r e s u l t i n g t h i c k n e s s o f unfilled r e s i n a p p e a r e d to h a v e little effect on c o m p o s i t e r e s i n / e n a m e l s h e a r bond strengths. (J PROSTHET DENT 1991;66:445-50.)
T h e best retention of composite resins is obtained when they are used in conjunction with an etched enamel surface followed by a thorough rinse and application of a bonding agent. 1 Studies have been reported concerning bond strength of unfilled resins with composites and direct composite laminate veneers. 24 Rider et al. 5 investigated the relationship between thickness and strength of unfilled bonding resins and reported that the enamel/composite system did not show a decrease in shear bond strength with increasing bond layer thickness. 5 Since the advent of composite laminate veneering techniques, light-cured opaquing and color-modifying resins containing inorganic filler particles and intrinsic coloring have been used in place of unfilled bonding resins to enhance masking of discolored teeth. 6' 7 In this respect, Davis and Waters s' 9 found that fracture toughness increased with increased filler content. However, Hagen and Robbins 1° showed no significant difference in bond strengths with the addition of up to 20 % resin tint to the composite luting material. This study compared the effects of the bond layer thickness of color-modifying resins and variously applied unfilled resins on composite resin/enamel shear bond strengths. MATERIAL
One hundred two extracted human permanent maxillary central or lateral incisors were cleaned and were placed in
aAssociate Professor, Section of Restorative and Prosthetic Dentistry. bAssistant Professor, Section of Restorative and Prosthetic Dentistry. 10/1/29080
THE JOURNAL OF PROSTHETIC DENTISTRY
distilled water, where they were stored for a minimum of 4 weeks prior to use. The teeth were then partially embedded in epoxy resin (Leco Corporation, St. Joseph, Mich.) with only the labial surface of the tooth exposed. To facilitate the placement of the tooth in its individual embedment, the tooth was first attached loosely to the bottom of the disk-forming mold by interposing a small piece of soft compound between the tooth and the mold bottom. The labial enamel surface was then positioned parallel to the mold surface before pouring the resin into the mold. After removal from the mold, the labial enamel surface was planar reduced with a medium grit carbide disk to a depth of 0.5 mm. This enamel reduction provided (1) a surface parallel to the mold surface; (2) a fiat surface for bonding; and (3) simulated a typical reduction for a laminate veneer preparation. The 102 teeth were then randomly divided into six equal groups of 17 specimens and were stored in 100% humidity until ready for use. The prepared enamel surfaces were cleaned with pumice, then rinsed and dried before etching with 37 % phosphoric acid gel for 45 seconds. The surface was then rinsed with a gentle stream of water for 45 seconds and dried with clean air prior to placement and curing of the specific bonding resin. The method of application of the bonding resins tested in this study either followed the manufacturer's recommendations (groups 1, 4, 5, and 6) or deviated from these directions to adapt to those situations often encountered in clinical practice (groups 2 and 3). Treatments were assigned to the six groups according to percentage of inorganic filler and/or the method of application. The methods of application recommended by the manufacturers (groups 1 and 4) may be considered control groups for unfilled resins used in this study.
PAGNIANO AND LONGENECKER
Table I. ANOVA of dependent variables for comparison of unfilled and color-modifying resins (groups 1 to 6): thickness and force
Degrees of freedom
S u m of squares
Table IL ANOVA of dependent variables for comparison of unfilled resins (groups 1 to 4): thickness and force
D e g r e e s of freedom
Sum of squares
Group 1 (TU) Specimens in this group had a single, cured layer of Caulk Universal Bond resin (0 % inorganic filler) (L. D. Caulk Co., Milford, Del.) applied thinly with a brush after excess resin was removed by five lapping strokes of the brush against the wall of a dappen dish, as recommended by the manufacturer.
Group 2 (U) Specimens in this group had a single, cured layer of Caulk Universal Bond resin (0% inorganic filler) applied thinly by five lapping strokes of the brush on the enamel surface.
Group 3 (2U) Specimens in this group had two cured layers of Caulk Universal Bond resin (0% inorganic filler). The first layer was applied with a brush, blown thinly with clean forced air, and polymerized prior to duplication of the procedure for the second layer.
Group 4 (HB) Specimens in this group had a single cured layer of Vivadent Heliobond resin (0% inorganic filler) (Vivadent, Tonawanda, N.Y.) applied thinly (five lapping strokes of a brush on the enamel surface), as recommended by the manufacturer.
Group 5 (E) Specimens in this group had Caulk Prisma Enhancer L resin (15 % inorganic filler by weight plus intrinsic coloring) in a single cured layer moderately applied with a brush wetted with the resin. To maintain the color-modifying effectiveness of the material, no effort was made to thin the layer. G r o u p 6 (T) Specimens in this group had Caulk Prisma Tint SB resin (5 % filled by weight) in a single, cured layer moderately applied with a brush wetted with the resin. To maintain the
color-modifying effectiveness of the material, no attempt was made to thin the layer. Limitations of this study are seen at this point, since there is no practical method of measuring the thickness of the resin bond during placement to assure uniformity. Following the bond application and curing, a No. 5 gelatin capsule (Eli Lilly and Company, Indianapolis, Ind.) was filled to the three-quarter level with Caulk Prisma Microfine resin and cured to allow handling and to maintain the nearly circular configuration of the capsule. The inside diameter of an individual capsule was determined with a machinist's microscope to range from 4.43 to 4.47 ram, depending on the location of the measurement. Similar measurements were found with five additional capsules, and a mean diameter of 4.45 mm was used to calculate the 15.55 mm 2 area of the bond interface. The remainder of the capsule was then filled with Prisma Microfine resin (L. D. Caulk Co.) and was positioned with light pressure over and perpendicular to the air-inhibited layer of the polymerized bonding resin on the prepared enamel surface. A dark shade of composite resin was used to fill the capsule to aid in the layer identification after debonding procedures. After the flash of material outside the capsule was removed, the composite resin was cured for 80 seconds through the gelatin capsule by 20-second exposure to a Prisma-Lite light (L. D. Caulk) in each of four quadrants. The samples were then stored in 100% humidity for 24 hours until the gelatin capsules were removed and debonding procedures were begun. To standardize the debonding procedures, each specimen was placed into a stainless steel jig holder designed to firmly retain the embedded specimen after it was mounted on a testing machine. During the debonding procedures, each composite resin column, positioned perpendicular to the enamel surface, was positioned perpendicular to the edge of the shearing device mounted in line with the specimen holder. The shearing edge, with a crosshead speed of 0.02 inch per minute, was directed over the bond layer between the enamel and composite resin column and the force
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COMPOSITE RESIN/ENAMEL SHEAR RESISTANCE
300 ¸ A
Fig. 1. Mean thickness and standard error of mean for unfilled and color-modifying bonding resin groups (n = 17). (Means of same letter are not significantly different.)
required to fracture the specimen was recorded by means of an MTS Testing Machine equipped with a graphic recorder (MTS Systems Corp., Minneapolis, Minn.). Depending upon the fracture site, either the tooth, the composite resin column, or both were sectioned to determine the bonding layer thickness for that specimen. Bond layer thicknesses for those specimens that fractured entirely through the bond layer were measured in the area immediately adjacent to the fracture site. Seven specimens that were estimated to have fractured 25 % or more through the enamel and that fractured with little force were not used in the study because of the possible influence of weakened enamel on the results. The bond layer thicknesses for specimens in groups 2 through 6 were measured directly with a Leitz Machinist's microscope (Opto-Metric Tools, Inc., N.Y.), equipped with a Mitutoyo Digitizer, Model MUX 10 instrument (Mitutoyo Manufacturing Co., Tokyo, Japan), having a resolution of 1 ttm. Because of the thinness of the bond layers in group 1, a Leco M-400 measuring microscope (Leco Corp.) with a resolution of 0.5 ~m at 400 power magnification was also used for determinations of the thickness of these specimens. To achieve random thickness measurements of the bond layers, the length of each measurable layer was divided into eight increments and a reading was made at approximately each of the seven locations~ The mean bond thickness for each specimen was then determined from the seven random locations of each specimen. Group means were then derived from the 17 individual specimen means. The bonding resin types, the method of bonding layer
THE J O U R N A L OF PROSTHETIC D E N T I S T R Y
application, the bond thickness, and the shear force required to fracture the specimens were the only variables evaluated. An analysis of variance (ANOVA) with a Tukey Studentized range test was performed to determine if significant differences at an a = 0.05 level existed for the mean bond layer thicknesses as well as to determine the mean shear forces required to debond the specimens of the six groups. A Pearson correlation analysis was done to determine if a relationship existed between the mean thickness of the bonding layer and the force required to debond each of the 17 specimens within a group. RESULTS The predominant mode of fracture was adhesive fracture at the enamel/composite resin interface. All specimens showed the fracture site to be within the bond resin, either exclusively or including some involvement with the enamel and/or composite resin column. Bond layer thicknesses of seven specimens in group 1 (TU) were not discernible at 400 power magnification and were considered zero thickness. Other specimens within this group showed bond layer thicknesses from 1 to 3 t~m. Results of the ANOVA for the combined unfilled and color-modifying resin bond groups (1 to 6) are seen in Table I. ANOVA results for only the unfilled resin bond groups (1 through 4) are seen in Table II. The mean, standard error of the mean, and the Tukey grouping of the means for the thickness of the bond layers and the force required to shear the 17 specimens in each group can be seen in Figs. 1 through 4. An overall compar-
O 1.1. c
0 1 (TU)
Fig. 2. Mean force and standard error of mean for unfilled and color-modifying bonding resin groups (n = 17). (Means of same letter are not significantly different.)
Bonding Groups Fig. 3. Mean thickness and standard error of mean for unfilled bonding resin groups (n = 17). (Means of same letter are not significantly different.)
ison of the unfilled resin groups (1 through 4) and colormodifying resin groups (5 and 6) showed a significant difference between the bond layer thicknesses for these two categories (Fig. 1). A statistically significant difference
(p < 0.05) in the mean shear forces of group 3 (2U) and group 6 (T) was also evident, as seen in Fig. 2. No other significant differences between these categories were found. When only the unfilled resin bond layers (groups 1 through
OCTOBER 1991 VOLUME 66 NUMBER 4
COMPOSITE RESIN/ENAMEL SHEAR RESISTANCE
2O o 0
BondingGroups Fig. 4. Mean force and standard error of mean for unfilled bonding resin groups (n = 17). (Means of same letter are not significantly different.)
4) were compared, significant differences were evident for both layer thicknesses (Fig. 3) and forces required to shear the specimens (Fig. 4). Strengths of relationships (Pearson correlation coefficients) between the thickness of the bonding resin and the force required to shear the specimens of each group are given in Table III. The thickness of the bonding layers had both weak positive or negative relationships with the forces required to debond the specimens. DISCUSSION The effective application of color-modifying resins during direct laminate veneer restorations with composite resin would permit less composite resin thickness than would otherwise be required to mask tooth discoloration. Placement of a color-modifying resin, as seen in this study, resulted in from 5 to 327 times greater thickness of bond layer than would be necessary if only unfilled resin were used. Nevertheless, these increased thicknesses were found to provide debond resistance of the enamel/composite resin interface comparable to that of the unfilled resins. The means for the shear force required to debond the specimens of each of the six groups ranged from 22.7 to 26.9 kg. Though a statistically significant difference was evident between a color-modifier and an unfilled resin (group 3 [2U] and group 6 [T]), it is doubtful that this difference is clinically significant. The various methods of application as described for the unfilled bonding resins (groups 1 through 4) resulted in thicknesses ranging from 1 to 37 #m and in shear bond
THE J O U R N A L OF P R O S T H E T I C D E N T I S T R Y
Table III. Pearson correlation analysis between thickness and force of each group (n = 17) Bonding groups
i (TU) 2 (U) 3 (2U) 4 (HB) 5 (E) 6 (T)
0.26084 -0.28354 0.59361 0.05902 0.16923 -0.27059
0.3119 0.2701 0.0120 0.8220 0.5161 0.2935
strengths ranging from 23.2 to 26.9 kg. Group I(TU), with the bonding agent applied quite thinly according to the manufacturer's directions, had the smallest mean thickness (1 #m) and force (23.2 kg) necessary to shear the specimens. This group was the only one to show a statistically significant difference yet a comparable shear force to others in the unfilled resin category. It would be reasonable to assume from the results that even though statistically significant differences existed among the thicknesses and shear forces of the variously applied unfilled resins, any of the described methods of application would be clinically acceptable. The method of application of the bonding resins tested in this study either followed the manufacturer's recommendations (groups 1, 4, 5, and 6) or deviated from these directions to include those situations often encountered in clinical practice (groups 2 and 3)i A direct comparison of
PAGNIANO AND LONGENECKER
the thickness of the bond layers and the forces required to shear the color-modifying bond specimens should be avoided, since the effect of the inorganic filler particles must also be considered. The method of application and/or the resulting thickness of the bond layers of groups 1 through 4 may be directly compared, since all are unfilled resins. SUMMARY
This investigation was conducted to compare the effects of bond layer thicknesses of color-modifying resins and variously applied unfilled resins on composite resin/enamel shear bond strengths. The conclusions were: 1. Because of the inorganic filler content of color-modifying resins (tints and enhancers) as well as the need for them to be esthetically effective, they must be applied in thicker layers than unfilled enamel bonding resins. 2. Because of the more lucent nature of tints, they must be applied in thicker layers than the opaque enhancers co be effective color modifiers. 3. The use of the inorganically filled color-modifying resins, within the parameters of thickness as described in this study, show comparable shear resistance of composite resin to etched enamel, both with themselves and with the groups of unfilled resins. 4. Unfilled bonding resins will vary in thickness depending upon the method of application.
5. The method of application and resulting thickness of unfilled bonding resins as described in this study appeared to have little effect upon the shear resistance of composite resin to etched enamel. REFERENCES 1. Farah JW. Dougherty EW. Unfilled, filled, and microfilled composite resins. Oper Dent 1981;6:95-9. 2. Ortiz RF. Phillips RW. Sw~artz MS, Osborne JW. Effect of composite resin bond agent on microleakage and bond strength. J PROSTHETDENT 1979;41:51-7. 3. Khairy AE, Simonson RJ. Bonding of laminate veneer materials ~o enamel [Abstract]. J Dent Res 1987:66:209, 4, Boyer DB, Chalkley Y, Chan KC. Correlation between s~rength of bonding to enamel and mechanical properties of dental composites. J Biomed Mater Res 1982:16:775-83. 5. Rider M, Kenny B, Tanner AN. The effect 0f enamel bonding agents on in vitro composite strengths. J Dent Res 1977;5:295-302. 6. Caulk Prismetics Technique Manual. Milford. Del: L D Caulk Co. 1985. 7. Zinck JH, McInnis-Ledoux P. Effectiveness of opaquer and color-m0difying materials [Abstract]. J Dent Res 1986;65:256. 8. Davis DM, Waters NE. An investigation into the fracture behavior of particulate-filled bis-GMA resin. J Dent Res 1987;66:1128-33. 9. Davis DM, Waters NE. Fractography of a bis-GMA resin. J Dent Res 1989;68:1194-8, 10. Hagen BD. Robbins JW. Effect of resin tint on composite/enamel shear bond strengths [Abstract], J Dent Res 1988;67:378.
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