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Shear bond strengths to dentin: effects of surface treatments, depth and position
Shear bond strengths to dentin: effects of surface treatments,
L. Tao, D. H. Pashley Department of Oral Biology/Physiology, Medical College of Georgia, Augusta, Georgia, USA
depth and position Tao L, Pashley DH. Shear bond strengths to dentin: effects of surface treatments, depth and position. Dent Mater 1988: 4: 371-378. Abstract - Shear bond strengths of Scotchbond/Silux were tested at different dentin depths and positions before and after different pretreatments, i.e. no treatment, sonication, 0.2% EDTA, 6% citric acid and 37% phosphoric acid. Smear layers were created using either sandpaper or low-speed burs. This study showed that there were significant differences in bond strength on different depths of dentin after etching the surface but not if the smear layer was left undisturbed.
One of the dilemmas in dentin bonding procedures is that if materials are placed on smear layers rather than the underlying dentin, the bonding occurs to a layer of cutting debris that make up the smear layer. If the smear layer is removed so that direct bonding to dentin can take place, one runs the risk of also removing smear plugs and causing a large increase in dentin permeability (1). That would not be a problem if dentin bonding were uniform, complete and strong. However, if dentin bonding is not perfect following treatments that remove smear layers, the resulting high dentin permeability may lead to increased dentin sensitivity and pulpal irritation. The composition of smear layers presumably reflects the composition of the dentin from which they are created. Dentin just beneath the dentinoenamel junction (DE J) has few dentinal tubules and has been reported to give high bond strengths with Scotchbond (2). Dentin near the pulp has far more tubules, each of which contains and is lined by organic material. Such deep dentin has been reported to give low bond strengths with Scotchbond and to benefit from the use of a mineralizing solution (2). As the flat crown segments used in most dentin bonding studies include both superficial dentin in the center and deep dentin over the pulp horns, there may be regional differences in bond strength between these regions. The composition of smear layers may
also reflect the instruments that are used to shape dentin. There is always concern that smear layers created in the laboratory with silicon-carbide sandpaper may be different from those created with dental burs operated in handpieces. The purpose of this study was to compare the bond strengths of Scotchbond/Silux to dentin smear layers created with 320-grit SiC sandpaper with those created with #37 inverted-cone burs operated in a low-speed handpiece. Bonds made to superficial, middle and deep dentin before and after surface treatments designed to remove variable amounts of smear layer were compared, as were those made in the center of crown segments versus those made to dentin over the pulp horns.
Material and methods
Unerupted, noncarious human third molars stored in 4~ isotonic saline (containing 0.2% sodium azide as a preservative) were cemented on plastic stubs using Epoxy cement (Cole Parmer Instrument, Chicago, IL, USA) and prepared utilizing a lowspeed diamond saw (Buehler, Isomet, 11-1180 low-speed saw, Evanston, IL, USA) with water coolant. Two sections perpendicular to the tooth axis were made. The first section was made just under the DEJ, to obtain a flat superficial dentin surface. The second section, parallel to the first, was made ad-
Key words: smear layer; Tubulicid; citric acid; phosphoric acid; superficial and deep dentin; dentin bonding. Dr. David H. Pashley, Oral Biology/Physiology, School of Dentistry, Medical College of Georgia, Augusta, GA 30912, USA.
ReceivedOctober 7; accepted December 23, 1987.
jacent to the cementoenamel junction to remove the roots. Pulp tissue was removed with cotton forceps. The resulting crown segments were glued with Histoacry blue (B. Melsungen, FRG) to 2 x 2 x 0.7 cm pieces of Plexiglas. Smear layers were created on dentin using: 1) 320-grit silicon carbide sandpaper (SIC, Carbimet Paper Discs, Buehler); 2) #37 tungsten-carbide inverted-cone burs in a low-speed handpiece (4500 rpm, Star, Titan II) operated without air-water spray. The 320grit sandpaper-created smear layers were produced utilizing a modified disc-sanding machine (Buehler Ecomet III grinder) operated at 108 rpm (0.2 mA) for 5 s under a 500-g force with continuous flowing water as a lubricant. Each smear layer was produced with fresh sandpaper. Smear layers created with a #37 inverted bur were created using a handpiece affixed on to a modified microscope-stage. The fixed crown segments were moved manually at 15 cm/min horizontally, without vertical force, to produce a very flat, uniform smear layer surface. Every crown segment was prepared such that bonding could be sequentially performed on superficial, middle and deep dentin. At each level, bonding was done in the center of the crown segment and in the periphery over one of the pulp horns. Five different surface treatments were applied to the smear layers: 1) they were treated with water for 1 min (i.e. no treatment); 2) they were sonicated for 1 h; 3) they were treated with
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Table 1. Shear bond strengths (MNm 2) on dentin with different treatments [~+SSD(n)] Treatment None Son• 1h 0.2% EDTA, 1 min 6% citric acid, i rain 37% H3PO4, 15s
320-grit-SiC
Low-speed bur
5.7_+2.1(96) (0.7-13.3,37%)* 4.2• (0.7-8.6,48%) 3.4• (0.6-7.9,47%) 1.7_+1.7(64) (0-6.7,100%) 1.0_+1.7(34) (0-4.5,170%)
5.0• (0.7-10.8,34%) 5.3• (0.7-8.6,40%) 3.7• (0.7-7.4,46%) 2.0• (0-6.1,110%) 0.6_+1.3(34) (0-6.1,217%)
* Range, coefficient of variation in %
Tubulicid (Blue-Label, 0.2% EDTA pH 7.1, Dental Therapeutics, Nacka, Sweden) by scrubbing the dentin surface for 1 min with a cotton ball soaked with the liquid; 4) they were acidetched for 1 min with 6% (w/v) citric adic; or 5) they were acid-etched for 15 s with 37% (w/v) phosphoric acid. The son• was done by placing the specimens into 20-ml polystyrene micro-beakers (Fisher Scientific Cat. No. 2-544-37, Philadephia, PA, USA) containing 2 ml of distilled water. The beaker was then suspended in a 1-pint-capacity ultrasonic cleaner (Cole Parmer Instrument, Model 8849-00) filled with distilled water. This indirect son• method was used to permit collection of the smear layer debris from the tooth, the subject of another future report. After rinsing the treated surface with water for 15 s, they were air-dried for 5 s, and then a nylon cylinder (Small Parts RSN-4/2, Miami, FL, USA) 3 mm high with an ID of 3 mm was positioned on the surface, and held in place with Scotchtape. The bonding position was either on the center of the dentin surface or over one of the pulp horns. Light-cured Scotchbond (3M Dental Products, St. Paul, MN, USA) was
spread on the tooth surface inside of the nylon tube using a tiny brush, and then light-cured for 10 s (Visilux, 3M). Two layers of Universal Silux composite material (3M) were applied sequentially in the tube using a plastic condenser and light-cured for 30 s, respectively. After allowing the material to polymerize for 5 min, the tape was removed and the specimens were stored in distilled water at 37~ for 24 h. Shear force was applied as lead shot mechanically dropped into a container at a constant rate of 829 g/min. The container was connected to the nylon cylinder containing the composite by a thin wire cable (diameter 0.61 mm). When the bond broke, the container dropped several centimeters and activated a shut-off switch that terminated the flow of lead shot to the container. The container was then removed and weighed to calculate the bond strength by dividing the weight by the surface area of the inside of the nylon cylinder (0.067 cm2). This bonding testing device was calibrated against an Instron machine and was found to give results that were not statistically different (p>0.5). Data were subjected to a one-way analysis of variance and Duncan's mul-
tiple range test at the 5% level to determine statistically significant differences. Scanning electron microscopy was used to examine the dentin surfaces receiving the various treatments to permit characterization of the modifications that were produced in the smear layers, the presence or absence of the smear layer after breaking the bonds, and the presence or absence of resin tags at the dentin-Scotchbond interface. Following debonding, the dentin and Silux cylinders were collected separately for SEM examination of their surfaces. The samples were dried in a 37~ oven for 24 h. After gold-coating the specimens were examined with a scanning electron JEOL Model JSM-35CF (JEOL, Peabody, M A , USA) microscope operated at 25 kV at magnifications varying from • 600 to • 18,000.
Results Control smear layers created with 320grit SiC sandpaper gave slightly higher shear bond stregths (p<0.05) than smear layers created with the lowspeed inverted-cone bur (Table l). Treatments of dentin smear layers with son• dilute EDTA, 6% citric acid, and 37% phosphoric acid, all led to significant reductions in bond strength. Son• lowered shear bond strengths developed on surfaces sanded with 320-grit sandpaper (p<0.25) but not those created with a low-speed #37 inverted-cone bur (Table 1). When these smear layers were scrubbed for 1 min with Tubulicid (0.2% EDTA), the bond strengths of both groups were significantly reduced (p<0.05) by about 25%. When these smear layers were acid-etched with 6% (w/v) critic acid for 1 min or 37% phosphoric acid for 15 s, there were even
Table 2. Shear bond strengths (MNm 2) at different dentin depths with different surface treatments [~ • SD(n)] Depth
Control
Son• lh
0.2% EDTA 1 rain
6% citric acid 1 min
37% H3PO4 15 s
Superficial
5.2+2.5(22) (0.7-11.8,48%)
3.5• 1.7(8) (2.1-5.6,49%)
3.7+ f.7(12) (0.7-6.2,46%)
3.3_+2.5(13) (0.7-8.8,76%)
3.4+ 1.9(7) (0-5.0,56%)
Middle
6.0+2.5(38) (2.2-13.3,38%)
3.0• (2.5-6.1,37%)
2.7+1.3(19) (0.6-4.6,48%)
1.9+1.6(17) (0.6-6.7,84%)
0.9+1.3(10) (0-3.7,144%)
Deep
5.4_+t.8(36) (2.9-9.9,33%)
4.7• (3.9-5.6,15%)
4.2+ 1.8(13) (1.2-6.3,43%)
1.2+2.2(19) (0-6.8,183%)
0.1+0.2(10) (0-0.5,200%)
Dentin surface originally had smear layer created by 320-grit SiC sandpaper. *Range, coefficient of variation in %.
Bonding after removal of smear layers Table 3. Shear bond strength (MNm-2) development on different dentin positions and with different surface treatments [~ _+ SD(n)] Position
Sonication 1h
0.2% EDTA 1 min
6% citric acid 1 min
37% H3PO4 15 s
Pulp
4.2_+1.0(6) (3.2-5.6,24%)
3.3_+1.6(27) 2.1_+2.2(31) 0.8+1.5(14) (0.6-7.9,48%) (0-8.8,105%) (0-5.0,188%)
Center
3.0+1.6(4) (1.3-4.9,53%)
3.8_+1.6(17) 1.8_+2.1(18) 1.6_+1.9(14) (0.7-5.5,42%) (0-6.7,117%) (0-5.4,119%)
p
p<0.1
p>0.2
p>0.5
Dentin surface originally had smear layer created by 320-grit SiC sandpaper. *Range, coefficient of variation in %.
greater reductions in bond strength (40-60%). The results of these treatments at different dentin depths are shown in Table 2. Smear layers that received no treatment gave bond strengths of between 5.2-6.0 MN m -2. There were no significant differences between superficial, middle and deep dentin bond strengths in the control group. Sonication of smear layers created with 320-grit sandpaper led to a decrease in Scotchbond/ Silux bond strengths at all levels compared with controls. The fall in bond strength was greatest in middle dentin and least in deep dentin. Treatment of dentin at all levels with dilute EDTA led to an uniform reduction in bond stregth at all levels, although they were not all statistically significant. However, after 6% citric acid or 37% phosphoric acid-etching of 320-grit SiC-created smear layers, the bond strengths on deep dentin were significantly lower than that on the superficial dentin (p<0.05). Acid-etching produced a preferential reduction in middle and deep dentin bond strengths, with 37% phosphoric acid producing greater reduction than those produced by 6% citric acid. When these data were analyzed according to position in the crown, sonication produced a greater reduction in bond strengths over central dentin (compare Tables 2 and 3) than over pulp horns, although due to the variance, the differences were not statistically significant. Treatment with 0.2% EDTA produced a uniform reduction in sandpaper-created smear layer bond strength at both positions. Citric acid treatment of 320-grit SiC-created smear layers caused a further, but uniform reduction in bond strength. A preferential sensitivity of smear layers over pulp horns to 37% phosphoric acid was seen (Table 3).
p>0.2
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Scanning electron microscopy showed that there were many smear layer particles on both dentin and composite sides after breaking bonds in the control group. No composite tags were found on either the dentin or composite surface. Only a few dentinal tubules were evident (Fig. 1A). Scanning electron microscopy of the failed bonds indicated a cohesive failure of the smear layer rather than an adhesive failure between the bonding material and the dentin, since the composite side of the failed bond was covered by smear layer debris (Fig. 1B). On the sonicated surfaces, there
Fig. 1. Scanning electron micrographs of the interface between Scotchbond/Silux and control dentin. A. Dentin side. B. Composite side (original • 6000).
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Tao & Pashley with such a thick layer of adhering smear layer debris that one could not discern the underlying Scotchbond (Fig. 3B). In the group that was acid-etched with 6% citric acid, most of the dentinal tubules were open, but some still had smear layer particles occluding their orifices (Fig. 4A). Bonding to the 6% citric acid-treated dentin produced few tags remaining in the dentin (Fig. 4A). The composite side of the failed bond was covered with short (2-5 ~tm) resin tags extending from the composite surface (Fig. 4B). When dentin surfaces covered with smear layers created with 320-grit SiCsandpaper were acid-etched with 37% phosphoric acid for 15 s, the smear layer was completely stripped away from the surface and all traces of peritubular dentin were removed, creating tubular orifices that were about 5 ~tm in diameter (Fig. 5A). After the bonds were broken during shear strength testing, the 2 sides of the failed bond were examined by SEM. The dentin side (Fig. 5A) revealed numerous open tubules. About 10% of the tubules were occupied by what appeared to be resin tags that were broken off close to the surface. The composite side (Fig. 5B) revealed numerous resin tags projecting out of the composite surface. Perhaps 20-30% of them were 30-100 ~tm long, which permitted them to clump together near the termination of their free ends during specimen processing. The remainder of the surface was covered with a combination of trace impressions of tubule orifices and very short (2-5 Itm) resin tags. Discussion
Fig. 2. Scanning electron micrographs of the interface between Scotchbond/Silux and dentin covered with a 320-grit-SiC-created smear layer followed by sonication for 1 h. A. Dentin side. B. Composite side (original magnification x 6000).
seemed to be a preferential loss of smear plugs from the smear layer, leaving considerable smear layer debris on the surface. When Scotchbond/Silux bonds made to the sonicated surface were examined by SEM, the dentin surface revealed a very thin residual smear layer with many broken resin plugs protruding from the dentin surface (Fig. 2A). The composite side showed broken resin tags 6-7 p.m long extending from the resin surface (Fig. 2B). When dentin surfaces covered with 320-grit SiC-created smear layer were
scrubbed with 0.2% EDTA, prior to bonding and the failed bonds examined by SEM, the superficial smear layer debris was removed (Fig. 3A), leaving many smear layer plugs remaining in the tubules. Most of the dentinal tubules remained plugged. When these EDTA-treated surfaces were then bonded with Scotchbond/Silux and the bonds subsequently broken, the dentin surfaces remained covered with a thin layer of smear layer (Fig. 3A). Examination of the composite side of the failed bond revealed that it was covered
Bonds made to smear layers created with 320-grit SiC sandpaper were statistically significantly (but only slightly) higher than those made to smear layers created with a #37-inverted-cone bur operated at low speed in a dental handpiece (no treatment, Table 1). This was an important observation because most bonding studies are done to tooth surfaces made flat with sandpaper. Clinically, dentin is prepared with dental burs rather than sandpaper. The observation that there is only a small difference in the magnitude of the bond strengths validates the use of sandpaper. When smear layers were created using 320-grit SiC, there were no statistically significant differences in the the
Bonding after removal of smear layers
Fig. 3. Scanning electron micrographs of the interface between Scotchbond/Silux and dentin covered with a 320-grit-SiC-created smear layer subsequently treated with 0.2% EDTA for 1 rain. A. Dentin side (original x 6000). B. Composite side (original x 2000).
bond strengths of Scotchbond/Silux to superficial, middle or deep dentin. This is in contrast to the results of Causton (2), Stanford et al. (3) and Mitchem & Gronas (4), who all found higher bond strengths in superficial versus deep dentin. Perhaps our standardized methods produce a different type of smear layer t h a n those produced by hand. When we removed the smear layers by acid-etching, then our results were consistent with the literature reports. That is, after etching with 6% citric acid or 37% phosphoric acid to remove the smear layers, the resulting bond
strengths developed on superficial dentin were significantly higher that those on middle or deep dentin, although etching of dentin generally lowered the bond strengths, as has been previously reported (5). Examination of these surfaces by SEM showed that 37% phosphoric acid removed nearly all of the smear layer and opened most of dental tubules. On deep dentin there were more tubules with larger diameters than on superficial dentin, as had been reported by other researchers (6, 7). Also, there were more tubules per unit surface area
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over pulpal horns than over central dentin, even though there were no significant differences in bond strengths between these positions. There was, however, a tendency for bonds to central dentin to be higher than those of dentin over pulpal horns. There were more long resin tags in both deep dentin and the composite side of bonds to deep dentin than on the superficial dentin, yet the strength of bonds to deep dentin was lower than those made on superficial dentin. This suggests that mechanical interlocks contribute little to the strength of dentin bonds, confirming previous investigations (8). In the present study, following acid-etching, there were no traces of remaining smear layer debris, so that the bond strengths we observed may be solely related to calcium concentrations on inter- and peritubular dentin, both of which were reduced by etching. Superficial dentin may have more calcium and fewer tubules than deep dentin, leading to higher bond strengths in the former compared with the latter. The bonds strengths on superficial and middle dentin covered with the different types of smear layers were very consistent. The results obtained in deep dentin were more variable. Treatment of smear layers with acids (phosphoric acid, citric acid) or EDTA not only removes smear layers but depletes any residual smear layer debris and the underlying dentin matrix of calcium. Thus, chemical attempts to remove smear layers apparently also remove calcium, an element thought to participate in the bonding of Scotchbond to dentin (2, 9). A n alternative approach to the removal of smear layers is the use of ultrasonic energy. .In the 320-grit-created smear-layer group, the bond strength was significantly higher in the no-treatment group than that in the sonication group (compare 5.6 MN m 2 with 4.1 MN m 2, Table 2). After sonication for 1 h, SEM examination showed that most of the smear layer had been removed. However, sonication may only remove loose smear layer debris on the surface, leaving enough residual smear layer debris for the development of relatively high bond strengths. Deep dentin covered with 320-grit-created smear layers was more tenacious and was not similarly removed by sonication. This was associated with higher bond strengths, which were not statistically different from nonsonicated controls. It should be noted that the indirect sonication
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Fig. 4. Scanning electron micrographs of the interface between Scotchbond/Silux and dentin with 320-grit-SiC-created smear layer treated with 6% citric acid for 1 rain. A. Dentin side. B. Composite side (original x 600).
method used in these experiments is much less effective than the direct sonication. That is, sonication of samples directly in a sonication bath removes smear layers more rapidly than the indirect method used in this study (unpublished observations). When smear layers were created using a low-speed bur, the mean bond strength was 5.0 MN m -2. When similar samples were sonicated for 1 h, there was no significant change in bond strength. Apparently, the forces holding the bur-created smear-layer parti-
cles together exceeded the amount of energy available using indirect sonication for 1 h. The smear-layer thickness created by the use of low-speed bur is larger (1.0 ~tm) than that of 320-grit SiC (0.4 ~tm, unpublished observations). Also, the former was produced at much higher speeds (4500 rpm) without water than the latter (108 rpm) with water. Therefore, the former method may have produced higher cutting temperatures, which might denature collagen within the particles of debris constituting the smear layer, making it more
cohesive. Thus, even after 1 h o f indirect sonication, most of the smear layer debris created by low-speed burs remained on the dentin surface (not shown). The resulting bond strengths reflected the cohesive strength of the smear layers, since both sides of the failed bonds were covered by smear layer debris (not shown). Br~innstr6m et al. (10, 11) have long advocated scrubbing smear layers with a dilute solution of EDTA containing a bactericidal surface-active agent (benzalkonium chloride) to kill any bacteria that might remain in the smear layer. He has suggested that the top half of smear layers is composed of loose debris that might be removed within 1 min by scrubbing with 0.2% EDTA (as opposed to 17% EDTA used for completely removing the smear layer in the Gluma bonding system). Br~innstr6m's goal was to remove most of the smear layer but none of the smear plugs in the tubule limina. This would decrease the thickness of the smear layer without increasing dentin permeability. In this study, after treating the dentin smear layer with 0.2% EDTA, the dentin bond strength decreased, but not to a level that was statistically significant. This was in agreement with the reports of Ishiok & Caputo (12) and Solomon & Beech (13). However, Stangel et al. (14) found that a l-rain treatment of Tubulicid increased the bond strength of a bonding system. Tubulicid (i.e. 0.2% EDTA) scrubbing of the smear layer removed some of the smear layer and may have also removed some calcium from the remaining smear layer. A t the same time, the collagen that was in the smear layer particles dissolved by the EDTA may have remained on the residual smear layer, thereby enriching it with loose organic material, resulting in lower bond strengths. Alternatively, the EDTA may have percolated around the particles constituting the smear layer, which may have lowered the cohesive forces holding the smear layer particles together. These changes may be below the resolving power of scanning electron microscopy. In the studies reported here, acidetching led to dramatically lower bond strengths of Scotchbond/Silux, even though resin tags were able to penetrate into dentinal tubules. Bonds made to intact smear layers, which would not permit resin penetration into tubules, were always higher, as other investigators have reported (15-17). This indicates that the mechanical retention af-
Bonding after removal o f smear layers
377
amount of smear layer left on the surface. T h e more the smear layer was r e m o v e d , the lower were the resulting bond strengths. Phosphoric acid-etching produced the lowest bond strength. Sonication or dilute E D T A treatment had little effect on either the S E M appearance of the smear layer or in bond strengths. When smear layers covered the dentin, there were no significant differences in b o n d strengths between superficial, middle and deep dentin; after removal of the smear layers by acidetching, the bond strength on superficial dentin was significantly higher than that on deep dentin. Acknowledgement - Supported, in part, by
grant DEO6427 from the National Institute of Dental Research.
References
Fig. 5.Scanning electron micrographs of the interface betwen Scotchbond/Silux and dentin with 320-grit-SiC sandpaper treated with 37% phosphoric acid for 15 s. A. Dentin side. B. Composite side (original x 600).
forded by resin tags in tubules is not as important as chemical or ionic bonding of smear layers. The effects of acid-etching on composite bond strengths in the literature range from decreases to increases in bond strength. Some researchers found no effect on bond strengths of citric acid plzetreatment of dentin (16, 18) while others reported that it prevented bonding (15, 19). In our results, 37% phosphoric acid-treated dentin produced the lowest bond strengths (Table 1). Other reports have also shown very low bond strengths after 37% phos26
Dental Materials 4:6, 1988
phoric acid pretreatment of dentin (13, 20). Without any treatment, the shear bond strengths developed on dentin prepared at different depths, covered by different types of smear layers were not significantly different. Also, S E M examination revealed that the bonding occurred on the smear layer rather than on the underlying dentin. T h e r e f o r e , the bond strength seems to be determined by the forces holding the smear layer together. Dentin bond strength with Scotchbond/Silux appeared to depend on the
1. Gwinnett AJ. Smear layer: morphological considerations. Oper Dent 1984: Supplement 3, 3-12. 2. Causton BE. Improved bonding of composite restorative to dentin, a study in vitro of the use of a commercial halogenated phosphate ester. Br Dent J 1984: 156(3): 93-5. 3. Stanford JW, Zaker S, Jose S. A comparison of the effectiveness of dentine bonding agents. Int Dent J 1985: 35: 139-44. 4. Mitchem JC, Gronas DG. Effects of time after extraction and depth of dentin on resin dentin adhesives. J A m Dent Assoc 1986: 113(2): 285-7. 5. Nolden R. Bonding of restorative materials to dentine: the present status in the Federal Republic of Germany. lnt Dent J 1985: 35: 166-72. 6. Ketterl W. Studie l~lber das Dentin der permanenten Z~ihne des Menschen. Stoma 1961: 14: 79-112. 7. Garberolio R, Brfinnstr6m M. Scanning clectron microscopic investigation of human dentinal tubules. Arch Oral Biol 1976: 21: 355-62. 8. Nakabayashi N. Biocompatibility and promotion of adhesion to tooth substrates. CRC Crit Rev Biocompatibility 1982: 6: 25-52. 9. Asmussen E. Clinical relevance of physical, chemical and bonding properties of composite resins. Oper Dent 1985: 10: 61-73. 10. Brfinnstr6m M, Glantz P-O. The effect of some cleaning solutions on the morphology of dentin prepared in different ways: an in vivo study. A S D C J Dent Child 1979: 66: 291-5. 11. Br~innstr6m M, Nordenvall KJ, Glantz P-O. The effect of EDTA-containing surface-active solutions on the morphology of prepared dentin: an in vivo study. J Dent Res 1980: 59(7): 1127-31. 12. Ishiok S, Caputo AA. Interaction be-
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tween the dentinal smear layer and composite bond strength. J Dent Res 1986: 65: (Spec Issue 35) 765, Abstr. 351. 13. Solomon A, Beech DR. Bonding of composites to dentin using primers. Dent Mater 1985: 1: 79-82. 14. Stangel I, Ostro E, Benko J. Effect of surface treatment on bond strength of dentin adhesive. J Dent Res 1986: 65: 174, Abstr. No. 41. 15. Erickson RL, Glasspoole EA. Effect of
acid rinsing on dentin smear layer. J Dent Res 1985: 64: 276, Abstr. No. 914. 16. Retief DH, Gross JD, Bradley EL, Denys FR. Tensile bond strengths of dentin bonding agents to dentin. Dent Mater 1986: 2: 72-7. 17. Hinoura K, Moore BK, Philips RW. Influence of dentin surface treatments on the bond strengths of dentin lining cements. Oper Dent 1986: 11: 147-54. 18. Chan DCN, Reinhardt JW, Schulein TM. Bond strengths of restorative ma-
terials to dentin. Gen Dent 1985: 33(3): 236-8. 19. Serobro L, Ben-Aman A, Liverman R, Rotfeld I. In vitro study of the effect of acids on the bond strength of composite bonding agents using bovine dentin. J Dent Res 1986: 65(4): 574, Abstr. No. 16. 20. Hill GL, Jenson ME, Zidan O. Shearbond strengths of a new dentinal bonding agent. J Dent Res 1983: #469, 62: 221, Abstr. No. 469.
L. Tao, D. H. Pashley Department of Oral Biology/Physiology, Medical College of Georgia, Augusta, Georgia, USA
depth and position Tao L, Pashley DH. Shear bond strengths to dentin: effects of surface treatments, depth and position. Dent Mater 1988: 4: 371-378. Abstract - Shear bond strengths of Scotchbond/Silux were tested at different dentin depths and positions before and after different pretreatments, i.e. no treatment, sonication, 0.2% EDTA, 6% citric acid and 37% phosphoric acid. Smear layers were created using either sandpaper or low-speed burs. This study showed that there were significant differences in bond strength on different depths of dentin after etching the surface but not if the smear layer was left undisturbed.
One of the dilemmas in dentin bonding procedures is that if materials are placed on smear layers rather than the underlying dentin, the bonding occurs to a layer of cutting debris that make up the smear layer. If the smear layer is removed so that direct bonding to dentin can take place, one runs the risk of also removing smear plugs and causing a large increase in dentin permeability (1). That would not be a problem if dentin bonding were uniform, complete and strong. However, if dentin bonding is not perfect following treatments that remove smear layers, the resulting high dentin permeability may lead to increased dentin sensitivity and pulpal irritation. The composition of smear layers presumably reflects the composition of the dentin from which they are created. Dentin just beneath the dentinoenamel junction (DE J) has few dentinal tubules and has been reported to give high bond strengths with Scotchbond (2). Dentin near the pulp has far more tubules, each of which contains and is lined by organic material. Such deep dentin has been reported to give low bond strengths with Scotchbond and to benefit from the use of a mineralizing solution (2). As the flat crown segments used in most dentin bonding studies include both superficial dentin in the center and deep dentin over the pulp horns, there may be regional differences in bond strength between these regions. The composition of smear layers may
also reflect the instruments that are used to shape dentin. There is always concern that smear layers created in the laboratory with silicon-carbide sandpaper may be different from those created with dental burs operated in handpieces. The purpose of this study was to compare the bond strengths of Scotchbond/Silux to dentin smear layers created with 320-grit SiC sandpaper with those created with #37 inverted-cone burs operated in a low-speed handpiece. Bonds made to superficial, middle and deep dentin before and after surface treatments designed to remove variable amounts of smear layer were compared, as were those made in the center of crown segments versus those made to dentin over the pulp horns.
Material and methods
Unerupted, noncarious human third molars stored in 4~ isotonic saline (containing 0.2% sodium azide as a preservative) were cemented on plastic stubs using Epoxy cement (Cole Parmer Instrument, Chicago, IL, USA) and prepared utilizing a lowspeed diamond saw (Buehler, Isomet, 11-1180 low-speed saw, Evanston, IL, USA) with water coolant. Two sections perpendicular to the tooth axis were made. The first section was made just under the DEJ, to obtain a flat superficial dentin surface. The second section, parallel to the first, was made ad-
Key words: smear layer; Tubulicid; citric acid; phosphoric acid; superficial and deep dentin; dentin bonding. Dr. David H. Pashley, Oral Biology/Physiology, School of Dentistry, Medical College of Georgia, Augusta, GA 30912, USA.
ReceivedOctober 7; accepted December 23, 1987.
jacent to the cementoenamel junction to remove the roots. Pulp tissue was removed with cotton forceps. The resulting crown segments were glued with Histoacry blue (B. Melsungen, FRG) to 2 x 2 x 0.7 cm pieces of Plexiglas. Smear layers were created on dentin using: 1) 320-grit silicon carbide sandpaper (SIC, Carbimet Paper Discs, Buehler); 2) #37 tungsten-carbide inverted-cone burs in a low-speed handpiece (4500 rpm, Star, Titan II) operated without air-water spray. The 320grit sandpaper-created smear layers were produced utilizing a modified disc-sanding machine (Buehler Ecomet III grinder) operated at 108 rpm (0.2 mA) for 5 s under a 500-g force with continuous flowing water as a lubricant. Each smear layer was produced with fresh sandpaper. Smear layers created with a #37 inverted bur were created using a handpiece affixed on to a modified microscope-stage. The fixed crown segments were moved manually at 15 cm/min horizontally, without vertical force, to produce a very flat, uniform smear layer surface. Every crown segment was prepared such that bonding could be sequentially performed on superficial, middle and deep dentin. At each level, bonding was done in the center of the crown segment and in the periphery over one of the pulp horns. Five different surface treatments were applied to the smear layers: 1) they were treated with water for 1 min (i.e. no treatment); 2) they were sonicated for 1 h; 3) they were treated with
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Table 1. Shear bond strengths (MNm 2) on dentin with different treatments [~+SSD(n)] Treatment None Son• 1h 0.2% EDTA, 1 min 6% citric acid, i rain 37% H3PO4, 15s
320-grit-SiC
Low-speed bur
5.7_+2.1(96) (0.7-13.3,37%)* 4.2• (0.7-8.6,48%) 3.4• (0.6-7.9,47%) 1.7_+1.7(64) (0-6.7,100%) 1.0_+1.7(34) (0-4.5,170%)
5.0• (0.7-10.8,34%) 5.3• (0.7-8.6,40%) 3.7• (0.7-7.4,46%) 2.0• (0-6.1,110%) 0.6_+1.3(34) (0-6.1,217%)
* Range, coefficient of variation in %
Tubulicid (Blue-Label, 0.2% EDTA pH 7.1, Dental Therapeutics, Nacka, Sweden) by scrubbing the dentin surface for 1 min with a cotton ball soaked with the liquid; 4) they were acidetched for 1 min with 6% (w/v) citric adic; or 5) they were acid-etched for 15 s with 37% (w/v) phosphoric acid. The son• was done by placing the specimens into 20-ml polystyrene micro-beakers (Fisher Scientific Cat. No. 2-544-37, Philadephia, PA, USA) containing 2 ml of distilled water. The beaker was then suspended in a 1-pint-capacity ultrasonic cleaner (Cole Parmer Instrument, Model 8849-00) filled with distilled water. This indirect son• method was used to permit collection of the smear layer debris from the tooth, the subject of another future report. After rinsing the treated surface with water for 15 s, they were air-dried for 5 s, and then a nylon cylinder (Small Parts RSN-4/2, Miami, FL, USA) 3 mm high with an ID of 3 mm was positioned on the surface, and held in place with Scotchtape. The bonding position was either on the center of the dentin surface or over one of the pulp horns. Light-cured Scotchbond (3M Dental Products, St. Paul, MN, USA) was
spread on the tooth surface inside of the nylon tube using a tiny brush, and then light-cured for 10 s (Visilux, 3M). Two layers of Universal Silux composite material (3M) were applied sequentially in the tube using a plastic condenser and light-cured for 30 s, respectively. After allowing the material to polymerize for 5 min, the tape was removed and the specimens were stored in distilled water at 37~ for 24 h. Shear force was applied as lead shot mechanically dropped into a container at a constant rate of 829 g/min. The container was connected to the nylon cylinder containing the composite by a thin wire cable (diameter 0.61 mm). When the bond broke, the container dropped several centimeters and activated a shut-off switch that terminated the flow of lead shot to the container. The container was then removed and weighed to calculate the bond strength by dividing the weight by the surface area of the inside of the nylon cylinder (0.067 cm2). This bonding testing device was calibrated against an Instron machine and was found to give results that were not statistically different (p>0.5). Data were subjected to a one-way analysis of variance and Duncan's mul-
tiple range test at the 5% level to determine statistically significant differences. Scanning electron microscopy was used to examine the dentin surfaces receiving the various treatments to permit characterization of the modifications that were produced in the smear layers, the presence or absence of the smear layer after breaking the bonds, and the presence or absence of resin tags at the dentin-Scotchbond interface. Following debonding, the dentin and Silux cylinders were collected separately for SEM examination of their surfaces. The samples were dried in a 37~ oven for 24 h. After gold-coating the specimens were examined with a scanning electron JEOL Model JSM-35CF (JEOL, Peabody, M A , USA) microscope operated at 25 kV at magnifications varying from • 600 to • 18,000.
Results Control smear layers created with 320grit SiC sandpaper gave slightly higher shear bond stregths (p<0.05) than smear layers created with the lowspeed inverted-cone bur (Table l). Treatments of dentin smear layers with son• dilute EDTA, 6% citric acid, and 37% phosphoric acid, all led to significant reductions in bond strength. Son• lowered shear bond strengths developed on surfaces sanded with 320-grit sandpaper (p<0.25) but not those created with a low-speed #37 inverted-cone bur (Table 1). When these smear layers were scrubbed for 1 min with Tubulicid (0.2% EDTA), the bond strengths of both groups were significantly reduced (p<0.05) by about 25%. When these smear layers were acid-etched with 6% (w/v) critic acid for 1 min or 37% phosphoric acid for 15 s, there were even
Table 2. Shear bond strengths (MNm 2) at different dentin depths with different surface treatments [~ • SD(n)] Depth
Control
Son• lh
0.2% EDTA 1 rain
6% citric acid 1 min
37% H3PO4 15 s
Superficial
5.2+2.5(22) (0.7-11.8,48%)
3.5• 1.7(8) (2.1-5.6,49%)
3.7+ f.7(12) (0.7-6.2,46%)
3.3_+2.5(13) (0.7-8.8,76%)
3.4+ 1.9(7) (0-5.0,56%)
Middle
6.0+2.5(38) (2.2-13.3,38%)
3.0• (2.5-6.1,37%)
2.7+1.3(19) (0.6-4.6,48%)
1.9+1.6(17) (0.6-6.7,84%)
0.9+1.3(10) (0-3.7,144%)
Deep
5.4_+t.8(36) (2.9-9.9,33%)
4.7• (3.9-5.6,15%)
4.2+ 1.8(13) (1.2-6.3,43%)
1.2+2.2(19) (0-6.8,183%)
0.1+0.2(10) (0-0.5,200%)
Dentin surface originally had smear layer created by 320-grit SiC sandpaper. *Range, coefficient of variation in %.
Bonding after removal of smear layers Table 3. Shear bond strength (MNm-2) development on different dentin positions and with different surface treatments [~ _+ SD(n)] Position
Sonication 1h
0.2% EDTA 1 min
6% citric acid 1 min
37% H3PO4 15 s
Pulp
4.2_+1.0(6) (3.2-5.6,24%)
3.3_+1.6(27) 2.1_+2.2(31) 0.8+1.5(14) (0.6-7.9,48%) (0-8.8,105%) (0-5.0,188%)
Center
3.0+1.6(4) (1.3-4.9,53%)
3.8_+1.6(17) 1.8_+2.1(18) 1.6_+1.9(14) (0.7-5.5,42%) (0-6.7,117%) (0-5.4,119%)
p
p<0.1
p>0.2
p>0.5
Dentin surface originally had smear layer created by 320-grit SiC sandpaper. *Range, coefficient of variation in %.
greater reductions in bond strength (40-60%). The results of these treatments at different dentin depths are shown in Table 2. Smear layers that received no treatment gave bond strengths of between 5.2-6.0 MN m -2. There were no significant differences between superficial, middle and deep dentin bond strengths in the control group. Sonication of smear layers created with 320-grit sandpaper led to a decrease in Scotchbond/ Silux bond strengths at all levels compared with controls. The fall in bond strength was greatest in middle dentin and least in deep dentin. Treatment of dentin at all levels with dilute EDTA led to an uniform reduction in bond stregth at all levels, although they were not all statistically significant. However, after 6% citric acid or 37% phosphoric acid-etching of 320-grit SiC-created smear layers, the bond strengths on deep dentin were significantly lower than that on the superficial dentin (p<0.05). Acid-etching produced a preferential reduction in middle and deep dentin bond strengths, with 37% phosphoric acid producing greater reduction than those produced by 6% citric acid. When these data were analyzed according to position in the crown, sonication produced a greater reduction in bond strengths over central dentin (compare Tables 2 and 3) than over pulp horns, although due to the variance, the differences were not statistically significant. Treatment with 0.2% EDTA produced a uniform reduction in sandpaper-created smear layer bond strength at both positions. Citric acid treatment of 320-grit SiC-created smear layers caused a further, but uniform reduction in bond strength. A preferential sensitivity of smear layers over pulp horns to 37% phosphoric acid was seen (Table 3).
p>0.2
373
Scanning electron microscopy showed that there were many smear layer particles on both dentin and composite sides after breaking bonds in the control group. No composite tags were found on either the dentin or composite surface. Only a few dentinal tubules were evident (Fig. 1A). Scanning electron microscopy of the failed bonds indicated a cohesive failure of the smear layer rather than an adhesive failure between the bonding material and the dentin, since the composite side of the failed bond was covered by smear layer debris (Fig. 1B). On the sonicated surfaces, there
Fig. 1. Scanning electron micrographs of the interface between Scotchbond/Silux and control dentin. A. Dentin side. B. Composite side (original • 6000).
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Tao & Pashley with such a thick layer of adhering smear layer debris that one could not discern the underlying Scotchbond (Fig. 3B). In the group that was acid-etched with 6% citric acid, most of the dentinal tubules were open, but some still had smear layer particles occluding their orifices (Fig. 4A). Bonding to the 6% citric acid-treated dentin produced few tags remaining in the dentin (Fig. 4A). The composite side of the failed bond was covered with short (2-5 ~tm) resin tags extending from the composite surface (Fig. 4B). When dentin surfaces covered with smear layers created with 320-grit SiCsandpaper were acid-etched with 37% phosphoric acid for 15 s, the smear layer was completely stripped away from the surface and all traces of peritubular dentin were removed, creating tubular orifices that were about 5 ~tm in diameter (Fig. 5A). After the bonds were broken during shear strength testing, the 2 sides of the failed bond were examined by SEM. The dentin side (Fig. 5A) revealed numerous open tubules. About 10% of the tubules were occupied by what appeared to be resin tags that were broken off close to the surface. The composite side (Fig. 5B) revealed numerous resin tags projecting out of the composite surface. Perhaps 20-30% of them were 30-100 ~tm long, which permitted them to clump together near the termination of their free ends during specimen processing. The remainder of the surface was covered with a combination of trace impressions of tubule orifices and very short (2-5 Itm) resin tags. Discussion
Fig. 2. Scanning electron micrographs of the interface between Scotchbond/Silux and dentin covered with a 320-grit-SiC-created smear layer followed by sonication for 1 h. A. Dentin side. B. Composite side (original magnification x 6000).
seemed to be a preferential loss of smear plugs from the smear layer, leaving considerable smear layer debris on the surface. When Scotchbond/Silux bonds made to the sonicated surface were examined by SEM, the dentin surface revealed a very thin residual smear layer with many broken resin plugs protruding from the dentin surface (Fig. 2A). The composite side showed broken resin tags 6-7 p.m long extending from the resin surface (Fig. 2B). When dentin surfaces covered with 320-grit SiC-created smear layer were
scrubbed with 0.2% EDTA, prior to bonding and the failed bonds examined by SEM, the superficial smear layer debris was removed (Fig. 3A), leaving many smear layer plugs remaining in the tubules. Most of the dentinal tubules remained plugged. When these EDTA-treated surfaces were then bonded with Scotchbond/Silux and the bonds subsequently broken, the dentin surfaces remained covered with a thin layer of smear layer (Fig. 3A). Examination of the composite side of the failed bond revealed that it was covered
Bonds made to smear layers created with 320-grit SiC sandpaper were statistically significantly (but only slightly) higher than those made to smear layers created with a #37-inverted-cone bur operated at low speed in a dental handpiece (no treatment, Table 1). This was an important observation because most bonding studies are done to tooth surfaces made flat with sandpaper. Clinically, dentin is prepared with dental burs rather than sandpaper. The observation that there is only a small difference in the magnitude of the bond strengths validates the use of sandpaper. When smear layers were created using 320-grit SiC, there were no statistically significant differences in the the
Bonding after removal of smear layers
Fig. 3. Scanning electron micrographs of the interface between Scotchbond/Silux and dentin covered with a 320-grit-SiC-created smear layer subsequently treated with 0.2% EDTA for 1 rain. A. Dentin side (original x 6000). B. Composite side (original x 2000).
bond strengths of Scotchbond/Silux to superficial, middle or deep dentin. This is in contrast to the results of Causton (2), Stanford et al. (3) and Mitchem & Gronas (4), who all found higher bond strengths in superficial versus deep dentin. Perhaps our standardized methods produce a different type of smear layer t h a n those produced by hand. When we removed the smear layers by acid-etching, then our results were consistent with the literature reports. That is, after etching with 6% citric acid or 37% phosphoric acid to remove the smear layers, the resulting bond
strengths developed on superficial dentin were significantly higher that those on middle or deep dentin, although etching of dentin generally lowered the bond strengths, as has been previously reported (5). Examination of these surfaces by SEM showed that 37% phosphoric acid removed nearly all of the smear layer and opened most of dental tubules. On deep dentin there were more tubules with larger diameters than on superficial dentin, as had been reported by other researchers (6, 7). Also, there were more tubules per unit surface area
375
over pulpal horns than over central dentin, even though there were no significant differences in bond strengths between these positions. There was, however, a tendency for bonds to central dentin to be higher than those of dentin over pulpal horns. There were more long resin tags in both deep dentin and the composite side of bonds to deep dentin than on the superficial dentin, yet the strength of bonds to deep dentin was lower than those made on superficial dentin. This suggests that mechanical interlocks contribute little to the strength of dentin bonds, confirming previous investigations (8). In the present study, following acid-etching, there were no traces of remaining smear layer debris, so that the bond strengths we observed may be solely related to calcium concentrations on inter- and peritubular dentin, both of which were reduced by etching. Superficial dentin may have more calcium and fewer tubules than deep dentin, leading to higher bond strengths in the former compared with the latter. The bonds strengths on superficial and middle dentin covered with the different types of smear layers were very consistent. The results obtained in deep dentin were more variable. Treatment of smear layers with acids (phosphoric acid, citric acid) or EDTA not only removes smear layers but depletes any residual smear layer debris and the underlying dentin matrix of calcium. Thus, chemical attempts to remove smear layers apparently also remove calcium, an element thought to participate in the bonding of Scotchbond to dentin (2, 9). A n alternative approach to the removal of smear layers is the use of ultrasonic energy. .In the 320-grit-created smear-layer group, the bond strength was significantly higher in the no-treatment group than that in the sonication group (compare 5.6 MN m 2 with 4.1 MN m 2, Table 2). After sonication for 1 h, SEM examination showed that most of the smear layer had been removed. However, sonication may only remove loose smear layer debris on the surface, leaving enough residual smear layer debris for the development of relatively high bond strengths. Deep dentin covered with 320-grit-created smear layers was more tenacious and was not similarly removed by sonication. This was associated with higher bond strengths, which were not statistically different from nonsonicated controls. It should be noted that the indirect sonication
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Fig. 4. Scanning electron micrographs of the interface between Scotchbond/Silux and dentin with 320-grit-SiC-created smear layer treated with 6% citric acid for 1 rain. A. Dentin side. B. Composite side (original x 600).
method used in these experiments is much less effective than the direct sonication. That is, sonication of samples directly in a sonication bath removes smear layers more rapidly than the indirect method used in this study (unpublished observations). When smear layers were created using a low-speed bur, the mean bond strength was 5.0 MN m -2. When similar samples were sonicated for 1 h, there was no significant change in bond strength. Apparently, the forces holding the bur-created smear-layer parti-
cles together exceeded the amount of energy available using indirect sonication for 1 h. The smear-layer thickness created by the use of low-speed bur is larger (1.0 ~tm) than that of 320-grit SiC (0.4 ~tm, unpublished observations). Also, the former was produced at much higher speeds (4500 rpm) without water than the latter (108 rpm) with water. Therefore, the former method may have produced higher cutting temperatures, which might denature collagen within the particles of debris constituting the smear layer, making it more
cohesive. Thus, even after 1 h o f indirect sonication, most of the smear layer debris created by low-speed burs remained on the dentin surface (not shown). The resulting bond strengths reflected the cohesive strength of the smear layers, since both sides of the failed bonds were covered by smear layer debris (not shown). Br~innstr6m et al. (10, 11) have long advocated scrubbing smear layers with a dilute solution of EDTA containing a bactericidal surface-active agent (benzalkonium chloride) to kill any bacteria that might remain in the smear layer. He has suggested that the top half of smear layers is composed of loose debris that might be removed within 1 min by scrubbing with 0.2% EDTA (as opposed to 17% EDTA used for completely removing the smear layer in the Gluma bonding system). Br~innstr6m's goal was to remove most of the smear layer but none of the smear plugs in the tubule limina. This would decrease the thickness of the smear layer without increasing dentin permeability. In this study, after treating the dentin smear layer with 0.2% EDTA, the dentin bond strength decreased, but not to a level that was statistically significant. This was in agreement with the reports of Ishiok & Caputo (12) and Solomon & Beech (13). However, Stangel et al. (14) found that a l-rain treatment of Tubulicid increased the bond strength of a bonding system. Tubulicid (i.e. 0.2% EDTA) scrubbing of the smear layer removed some of the smear layer and may have also removed some calcium from the remaining smear layer. A t the same time, the collagen that was in the smear layer particles dissolved by the EDTA may have remained on the residual smear layer, thereby enriching it with loose organic material, resulting in lower bond strengths. Alternatively, the EDTA may have percolated around the particles constituting the smear layer, which may have lowered the cohesive forces holding the smear layer particles together. These changes may be below the resolving power of scanning electron microscopy. In the studies reported here, acidetching led to dramatically lower bond strengths of Scotchbond/Silux, even though resin tags were able to penetrate into dentinal tubules. Bonds made to intact smear layers, which would not permit resin penetration into tubules, were always higher, as other investigators have reported (15-17). This indicates that the mechanical retention af-
Bonding after removal o f smear layers
377
amount of smear layer left on the surface. T h e more the smear layer was r e m o v e d , the lower were the resulting bond strengths. Phosphoric acid-etching produced the lowest bond strength. Sonication or dilute E D T A treatment had little effect on either the S E M appearance of the smear layer or in bond strengths. When smear layers covered the dentin, there were no significant differences in b o n d strengths between superficial, middle and deep dentin; after removal of the smear layers by acidetching, the bond strength on superficial dentin was significantly higher than that on deep dentin. Acknowledgement - Supported, in part, by
grant DEO6427 from the National Institute of Dental Research.
References
Fig. 5.Scanning electron micrographs of the interface betwen Scotchbond/Silux and dentin with 320-grit-SiC sandpaper treated with 37% phosphoric acid for 15 s. A. Dentin side. B. Composite side (original x 600).
forded by resin tags in tubules is not as important as chemical or ionic bonding of smear layers. The effects of acid-etching on composite bond strengths in the literature range from decreases to increases in bond strength. Some researchers found no effect on bond strengths of citric acid plzetreatment of dentin (16, 18) while others reported that it prevented bonding (15, 19). In our results, 37% phosphoric acid-treated dentin produced the lowest bond strengths (Table 1). Other reports have also shown very low bond strengths after 37% phos26
Dental Materials 4:6, 1988
phoric acid pretreatment of dentin (13, 20). Without any treatment, the shear bond strengths developed on dentin prepared at different depths, covered by different types of smear layers were not significantly different. Also, S E M examination revealed that the bonding occurred on the smear layer rather than on the underlying dentin. T h e r e f o r e , the bond strength seems to be determined by the forces holding the smear layer together. Dentin bond strength with Scotchbond/Silux appeared to depend on the
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acid rinsing on dentin smear layer. J Dent Res 1985: 64: 276, Abstr. No. 914. 16. Retief DH, Gross JD, Bradley EL, Denys FR. Tensile bond strengths of dentin bonding agents to dentin. Dent Mater 1986: 2: 72-7. 17. Hinoura K, Moore BK, Philips RW. Influence of dentin surface treatments on the bond strengths of dentin lining cements. Oper Dent 1986: 11: 147-54. 18. Chan DCN, Reinhardt JW, Schulein TM. Bond strengths of restorative ma-
terials to dentin. Gen Dent 1985: 33(3): 236-8. 19. Serobro L, Ben-Aman A, Liverman R, Rotfeld I. In vitro study of the effect of acids on the bond strength of composite bonding agents using bovine dentin. J Dent Res 1986: 65(4): 574, Abstr. No. 16. 20. Hill GL, Jenson ME, Zidan O. Shearbond strengths of a new dentinal bonding agent. J Dent Res 1983: #469, 62: 221, Abstr. No. 469.