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Transverse resistance of composite resin restorations on dentin
Transverse resistance of composite resin restorations on dentin R.H.J. Peerlings* P.J.J.M. Plasmans* M.M.A. VriJhoef** *Department of Occlusal Reconstruction Dental School University of Nijmegen Philips van Leydenlaan 25 6525 EX Nijmegen The Netherlands **Department of Dental Materials, Science and Technology University of Nijmegen The Netherlands Received July 15, 1987 Accepted May 5, 1988 *Address for correspondence (R.H.J.P.) Dent Mater 5:27-30, January, 1989
Abstract-The transverse resistance of a dentin-bonded composite resin restoration was tested. Dentin and enamel surfaces of human molars were pre-treated differently. The specimens were stored for various periods. The presence of enamel led to a transverse resistance significantly higher than when only dentin was present. When enamel was present, additional etching of dentin gave a lower resistance. This was possibly caused by the formation of a soluble monite precipitate. Etching of dentin without the presence of enamel gave a value significantly higher than the bond strength to unetched dentin.
ecently, an avalanche of resinous dentin-bonding agents has become available. These systems contain reactive groups. One type of group is claimed to bond to the mineral or organic component of dentin, while the other one associates with the resin matrix of restorative composite materials (Asmussen and Munksgaard, 1985; Beech, 1985; Gwinnett, 1985). Potential advantages of bonding to dentin have been recognized (Chan et al., 1985; Hinoura et al., 1986; McCullock and Smith, 1986). Among others, it would facilitate retentions of composite resin restorations in teeth, or regions therein, where little or no enamel is present. Dentin-bonding agents can be classified either according to their chemical characteristics or by the surface treatment of the dentin substrate. The desired structure or treatment of the dentin which is to be bonded depends on the type of dentin bonding agent used. The dentin-bonding material (Ca:zex Clearfil Newbond; Cavex Holland, Haarlem, The Netherlands) used in this experiment belongs, according to its chemical composition, to the category of resins with a phosphorus ester group (Munksgaard et al., 1984). When this material is being used, acid-etching-i.e., removal of the smear layer-is recommended so that resin tags will be created in the widened dentin tubules (Fusayama et al., 1979; Hansen, 1984; van Noort and Northeast, 1986). On the other hand, for an optimal bond, the preservation of the smear layer is promoted by several investigators, especially when using Scotch bond (Hansen, 1984; Eick and Welch, 1986; Stanford et al., 1985). This dentinbonding system belongs to the same chemical composition group as the dentin-bonding material mentioned above, but apparently requires a dif-
R
ferent surface treatment of the dentin. A point of concern with the application of dentin-bonding agents is whether the acid-etch technique should be considered to be noxious to the pulp, as reported by some investigators (Gwinnett, 1985; Stanford et al., 1985; Komatsu and Finger, 1986). In the case when the dentinbonding material tested in this experiment is used, the acid-etch technique is said not to lead to pulp damage because of the short etching time and a thorough washing treatment subsequently (Fusayama et al., 1979; Torstenson et al., 1982; Lee et al., 1973; Macko et al., 1978). Also, the better adhesion of this dentinbonding agent may result in less microleakage, which will, in the long term, prevent pulp damage. Questions arise about the required strength of the bond to dentin, especially in the situation of an extensive composite restoration. The initial strength of the dentin bond has to be greater than the forces exerted on the bond by the polymerization shrinkage (Beech, 1985; Gwinnett, 1985; Stanford et al., 1985). The resin tags created with the dentin-bonding material system used in this study do not seem to separate upon polymerization shrinkage (Fusayama, 1983). The shrinkage may be compensated for by hygroscopic expansion in a later stage (Huang and SSderholm, 1987). Attaining values for bond strengths to dentin comparable with those to etched enamel is the present goal of research in this field. Bond strengths to dentin in the range of 0.5-12 MPa have been reported (Chan et al., 1985; Huang and SSderholm, 1987; Hosado and Fusayama, 1984). Factors which may influence the variation of the in vitro bond strength to dentin are tooth type, region of dentin, the surface exposed (Beech, 1985; Stanford et al.,
Dental Materials/Janua~! 1989 27
1985), and storage time of the teeth (Fusayama et al., 1979; Causton and Johnson, 1979; Kimura et al., 1985; Smith and Ruse, 1986; Chan et al., 1985). In the present study, the transverse resistance of a dentin bonding system is evaluated, in order for the potentials of extensive composite restorations to be explored. The results obtained after different pretreatments of the dentin and enamel surfaces are compared. Also, the influences of aging time and the presence of enamel and/or dentin on transverse resistance are taken into consideration. MATERIALS AND METHODS
Sixty recently extracted sound human molar teeth were used. These were stored at 37°C in water except during those periods required for completion of the experimental proc e d u r e s . The s p e c i m e n s w e r e embedded in cold-curing acrylic resin in a uniform cylindrical mould up to the cemento-enamel junction. The coronal portions of the molar specimens were reduced with a conventional model trimmer, with water, for exposure of dentin up to 2 mm from the cemento-enamel junction, perpendicular to the long axis of the mould. The exposed dentin and enamel surfaces were finished with 400-grit waterproof carborundum paper. The p r e p a r e d specimens were randomly assigned to four groups of 15 m o l a r s each. The r e m a i n i n g enamel on the lateral surfaces of the molars in two of the four groups was removed with a diamond stone (Horico 243/016, Hopf, Ringlet & Co. GmbH & Cie., Berlin, West Germany). The design of the experiment is given in Table 1. Each group of 15 molars was then randomly subTABLE 1 TREATMENT COMBINATIONS OF THE RESPECTIVE EXPERIMENTAL GROUPS Group
Surface
Acid Etching
I
dentin enamel dentin enamel dentin dentin
+ + + + -
II III IV
+ acid etching. - no acid etching.
TABLE 2 SURFACE IN cm2 Group
Dentin
Enamel
Total
I II III IV
0.480 0.420 0.420 0.380
0.310 0.390 -
0.790 0.810 0.420 0.380
divided into t h r e e experimental groups of five molars each. Tests were conducted for two days, nine days, and 64 days. A standardized (12 x 12 cm) blackand-white photograph was taken of the occlusal surface of each specimen. The enlarging factor was calculated by comparison of the diameter of the mould on the photograph with the real diameter (1.6 cm). The areas of dentin and enamel (groups I and II only) were found by weighing of the appropriate areas of the photographs. The mean surface areas for both enamel and dentin are given in Table 2. A c o m p o s i t e m a t e r i a l (Cavex Clearfil Ray, Cavex Holland, Haarlem, The Netherlands) and its corresponding dentin-bonding material were used. For polymerization, a visible-light source (HL 500 Cavex, Cavex Holland) was available. The composite resin used was applied according to the manufacturer's instructions as follows: All specimens were dried. The entire surface of each specimen in experimental groups I and III, and only the enamel surfaces of those of group II, was etched for 40 sec with a 37% phosphoric acid, rinsed thoroughly for 20 sec, and dried for 15 sec. A Toffiemire retainer with metal matrix was placed around the specimens, the dentinbonding material was applied, and a 3-mm-thick composite layer placed. The light was applied for 40 sec. Tofflemire and matrix were removed, and the specimens were placed back into the water. They were stored for two days, nine days, or 64 days. For the transverse resistance test,
! ;
/
;
/
Fig. 1. A schematic representationof the transverse testing mode.
the specimens were placed in a mechanical testing device (Instron Corp., Canton, MA, U.SA.). Each specimen was loaded perpendicular to its long axis (Fig. 1). The load was placed at the w i d e s t side of the specimen. The transverse force was applied with a crosshead speed of 0.5 mm/min until failure occurred. The dislodgement and load were registered continuously. The level of resistance was determined by the point in the time/load graph where the line had a downward deflection. RESULTS
Table 3 depicts the results for the four treatment groups and three test times. Furthermore, the mean values p e r type of treatment are given. A significant difference was established between the mean transverse resistance of groups containing enamel (I + II) and that of those without (II! + IV) (p < 0.001). No significant difference was found between groups I and II (p = 0.09). On the other hand, a significant difference was detected between the mean resistance of groups III and IV (p = 0.001). The data clearly show that the presence of enamel results in a much higher transverse resistance compared with the situation with only dentin being present. If enamel is present, the additional acidetching of dentin gives a slightly lower transverse resistance. However, without any enamel being present, acid-etching of dentin gives significantly higher results.
TABLE 3 TRANSVERSE RESISTANCE ± STANDARD DEVIATION IN N
Group I II III IV
2 Days 578 673 297 196
± 197 ± 73 ± 125 ± 66
28 PEERLINGS et aL/RESISTANCE OF A DENTIN BONDED COMPOSITE
9 Days
64 Days
458 ± 177 542_+ 45 272 ± 130 92 ± 35
543__ 81 619 ± 119 385 ± 84 287 ± 92
Mean 521 611 322 192
± 157 _+ 94 ± 117 ± 104
A significant overall effect of time (p = 0.003) is found. For groups I and II, no significant storage-time effect (p = 0.06) was established, contrary to the significant storagetime effect for groups III and IV (p = 0.008). The nine-day resistance for group IV is significantly lower than that of those at the other storage times.
DISCUSSION The results of groups I and II make it clear that acid-etching in a situation with both dentin and enamel being present results in lower transverse resistance levels after both enamel and dentin are etched, compared with etching enamel only (Table 3). Photographs of specimens of corresponding t r e a t m e n t groups (Figs. 2-5) reveal white stains on the enamel (Figs. 4 and 5). Chow and Brown (1973) reported on the possibility of the formation of a soluble monite precipitate. Thus, it is feasible that the precipitate observed reduced the bond to enamel. Apparently, a rinsing time of 20 sec, as prescribed by the manufacturer, is insufficient for a complete removal of the precipitate in vitro. Consequently, the elimination of the precipitate from a preparation with difficult accessible line angles rather than a flat surface, as in this experiment, will require an even longer rinsing time. Therefore, in a general practice situation, a total etch system (including both enamel and dentin) m a y pose risks r a t h e r than advantages. Acid-etching of dentin in a situation without enamel, thus removing the smear layer, turns out to give a significantly higher transverse resistance. This is consistent with earlier reports (Fusayama et al., 1979; Hansen, 1984; van Noort and Northeast, 1986). Combination of the results, as given in Tables 2 and 3, gives a mean shear bond strength of 7.6 MPa for group III and 5 MPa for group IV. These values are in the same range as those of other in vitro tests in the shear mode (Chan et al., 1985; Huang and SSderholm, 1987; Stangel, 1987). It is also consistent with the findings that the shear bond strength of dentin-bonding systems to dentin is lower than that of those to enamel (Asmussen and Munks-
Fig. 2. The dentin-enamel junction after acid-etching of enamel for 40 sec and rinsing with water for 20 sec (magnification 100 x ). The smear layer on the dentin surface is partially removed near the enamel.
Fig. 3. A close-up (magnification 500 x ) of the designated area of the dentin-enamel junction of Fig. 2.
gaard, 1985; Stanford et al., 1985). The finding of no d e c r e a s e in transverse resistance in time is in agreement with the findings of other studies (Fusayama et al., 1979; Smith and Ruse, 1986; Chan et al., 1985). The minimum resistance observed after nine days for group IV might be explained in either of two ways. After loss of the pulp, changes take place in the tooth tissue (Causton and Johnson, 1979). Furthermore, hydrolytic degradation of the tooth-resin interface has been reported (Huang and SSderholm, 1987; Smith and Ruse, 1986). Another explanation
could be that hygroscopic expansion relieves interfacial stresses in time, and thus compensates for the deterioration of the bond to dentin (Huang and SSderholm, 1987). The resistance level found in this experiment is of the same order of magnitude as that of retention systems for extensive amalgam restorations. T h e s e w e r e said to be sufficient (Plasmans et al., 1987). Because extensive amalgam restorations are successful from a mechanical point of view, the transverse resistance of the dentin-bonding systern as used in this experiment has
Dental Materials~January 1989
29
Fig, 4. The dentin-enamel Junction after acid-etching of both enamel and dentin for 40 san and rinsing with wofor for 20 sec (magnHication 100 × ), The smear layer on the dentin surface is totally removed, Tim presence of the precipHate can be clearly seen.
Fig. $. A close-up (magnification 500 x ) of the designated area of the dentin-enamel junction of Fig. 4.
reached a level which is thought to be sufficient under oral conditions in the short term. However, clinical research is necessary to give the ultim a t e a n s w e r as to its l o n g - t e r m acceptability.
REFERENCES ASMUSSEN, E. and MUNKSGAARD, E. (1985): Bonding of Restorative Resins to Dentin Promoted by Aqueous Mixtures of Aldehydes and Active Monomers, Int Dent J 35: 160-165.
3{}
BEECH, D. (1985): Bonding of Restorative Resins to Dentin. In: Posterior Composite Resin Dental Restorative Materials, G. Vanherle and D. Smith, Eds., The Netherlands: P. Szulc Publishing Co., pp. 231-241. CAUSTOS, B. and JOHSSO~, N. (1979): Changes in the Dentin of Human Teeth Following Extraction and Their Implication for in vitro Studies of Adhesion to Tooth Substance, Arch Oral Biol 24: 229-232. CHAN, D.; REINHARDT, G.; and BOYER, D. (1985): Composite Resin Compatibility and Bond Longevity of a Dentin Bonding Agent, J Dent Res 64: 1402-1404. CRAN, D.; REINHARD'r,J.; and SCHULEIN,T. (1985):
P E E R L I N G S et a U R E S I S T A N C E O F A D E N T I N B O N D E D C O M P O S I T E
Bond Strengths of Restorative Materials to Dentin, Gen Dent 33: 236-238. CHow, L. and BROW~, W. (1973): Phosphoric Acid Conditioning of Teeth for Pit and Fissure Sealants, J Dent Res 52: 1158. EICK, J. and WELCH, F. (1986): Dentin Adhesives: Do They Protect the Dentin from Acid Etching?, Quint Int 17: 53.3-544. FUSAYAMA,T. (1983): Cavity l~reparation for a New Adhesive Restorative Resin, Quint lnt 14: 397409. FUSAYAMA,T.; NAKAMURA,M.; KUROSAKI,N.; and SWAKER, M. (1979): Non-pressure Adhesion of a New Adhesive Restorative Resin, J Dent Res 58: 1364-1370. GWINNETT, A. (1985): Dentin Bonding: Present Status, N Y State Dent J 51: 635-638. HANSEN, E. (1984): Effect of Scotchbond Dependent on Cavity Cleaning, Cavity Diameter and Cavosurface Angle, Scand J Dent Res 92: 141147. HINOURA, R.; MOORE, B.K.; and PHILLIPS, R. (1986): Influence of Dentin Surface Treatments on the Bond Strengths of Dentin-lining Cements, Oper Dent 11: 147-154. HOSADO, H. and FUSAYAMA, T. (1984): A Tooth Substance Saving Restorative Technique, Int De~£ J 34: 1-12. HUANG, G. and SODERHOLM,K. (1987): An in vitro Investigation of the Shear Bond Strength of Bondlite to Dentin, J Dent Res 66: 292, Abst. No. 1481. KIMURA, S.; SHIMIZU, T.; and FUJI, B. (1985): Influence of Dentin on Bonding of Composite Resin. Part I. Effect of Fresh Dentin and Storing Conditions, Dent Mater 4: 68-80. KOMATSU,M. and FINGER, W. (1986): Dentin Bonding Agents: Correlation of Early Bond Strength with Margin Gaps, Dent Mater 2: 257-262. LEE, H.; ORLOWSK~V,J.; SCHEIDT, G.; and LEE, J. (1973): Effects of Acid Etchants on Dentin, J De~t Res 52: 12"28-1233. MACHO, D.; RUTBERG, M.; and LANGELAND, K. (1978): Pulpal Response to the Application of Phosphoric Acid to Dentin, Oral Surg Oral Med Oral Pathol 45: 930-945. McCULLOCK, A. and SMITH, B. (1986): In vitro Studies of Cusp Reinforcement with Adhesive Restorative Material, Br Dent J 161: 450-452. MUNKSGAARD, E.; HANSEN, E.; and ASMUSSEN, E. (1984): Effect of Five Adhesives on Adaptation of Resin in Dentin Cavities, Scand J Dent Res 92: 544--548. PLASMANS, P.; KUSTERS, S.; DE JONGE, B.; VAN 'T HOF, M.; and VRIJHOEF, M. (1987): In vitro Resistance of Extensive Amalgam Restorations Using Various Retention Methods, J Prosthet Dent 57: 16-20. SMITH, D. and RUSE, N. (1986): In vitro Evaluation of Adhesion to Dentin of Resin Bonding Systems, J Dent Res 65 (Spec Iss): 239, Abst. No. 624 (AADR). STANFORD, J.; SABRI, Z.; and JOSE, S. (1985): A Comparison of the Effectiveness of Dentin Bonding Agents, Int Dent J 35: 139-144. STANGEL, I. (1987): The Bonding of Composite to Dentin Mediated by GLUMA, J Dent Res 66: 292, Abst. No. 1486. TORSTENSON, B.; NORDENVALL, K.; and BRANNSTROM, M. (1982): Pulpal Reaction and Microorganisms Under Clearfil Composite Resin in Deep Cavities with Acid Etched Dentin, Swed Dent J 6: 167-176. VAN NOORT, R. and NORTHEAST, S. (1986): The Potential Clinical Consequences of the New Dentinbonding Resins, Br Dent J 161: 437-443.
Abstract-The transverse resistance of a dentin-bonded composite resin restoration was tested. Dentin and enamel surfaces of human molars were pre-treated differently. The specimens were stored for various periods. The presence of enamel led to a transverse resistance significantly higher than when only dentin was present. When enamel was present, additional etching of dentin gave a lower resistance. This was possibly caused by the formation of a soluble monite precipitate. Etching of dentin without the presence of enamel gave a value significantly higher than the bond strength to unetched dentin.
ecently, an avalanche of resinous dentin-bonding agents has become available. These systems contain reactive groups. One type of group is claimed to bond to the mineral or organic component of dentin, while the other one associates with the resin matrix of restorative composite materials (Asmussen and Munksgaard, 1985; Beech, 1985; Gwinnett, 1985). Potential advantages of bonding to dentin have been recognized (Chan et al., 1985; Hinoura et al., 1986; McCullock and Smith, 1986). Among others, it would facilitate retentions of composite resin restorations in teeth, or regions therein, where little or no enamel is present. Dentin-bonding agents can be classified either according to their chemical characteristics or by the surface treatment of the dentin substrate. The desired structure or treatment of the dentin which is to be bonded depends on the type of dentin bonding agent used. The dentin-bonding material (Ca:zex Clearfil Newbond; Cavex Holland, Haarlem, The Netherlands) used in this experiment belongs, according to its chemical composition, to the category of resins with a phosphorus ester group (Munksgaard et al., 1984). When this material is being used, acid-etching-i.e., removal of the smear layer-is recommended so that resin tags will be created in the widened dentin tubules (Fusayama et al., 1979; Hansen, 1984; van Noort and Northeast, 1986). On the other hand, for an optimal bond, the preservation of the smear layer is promoted by several investigators, especially when using Scotch bond (Hansen, 1984; Eick and Welch, 1986; Stanford et al., 1985). This dentinbonding system belongs to the same chemical composition group as the dentin-bonding material mentioned above, but apparently requires a dif-
R
ferent surface treatment of the dentin. A point of concern with the application of dentin-bonding agents is whether the acid-etch technique should be considered to be noxious to the pulp, as reported by some investigators (Gwinnett, 1985; Stanford et al., 1985; Komatsu and Finger, 1986). In the case when the dentinbonding material tested in this experiment is used, the acid-etch technique is said not to lead to pulp damage because of the short etching time and a thorough washing treatment subsequently (Fusayama et al., 1979; Torstenson et al., 1982; Lee et al., 1973; Macko et al., 1978). Also, the better adhesion of this dentinbonding agent may result in less microleakage, which will, in the long term, prevent pulp damage. Questions arise about the required strength of the bond to dentin, especially in the situation of an extensive composite restoration. The initial strength of the dentin bond has to be greater than the forces exerted on the bond by the polymerization shrinkage (Beech, 1985; Gwinnett, 1985; Stanford et al., 1985). The resin tags created with the dentin-bonding material system used in this study do not seem to separate upon polymerization shrinkage (Fusayama, 1983). The shrinkage may be compensated for by hygroscopic expansion in a later stage (Huang and SSderholm, 1987). Attaining values for bond strengths to dentin comparable with those to etched enamel is the present goal of research in this field. Bond strengths to dentin in the range of 0.5-12 MPa have been reported (Chan et al., 1985; Huang and SSderholm, 1987; Hosado and Fusayama, 1984). Factors which may influence the variation of the in vitro bond strength to dentin are tooth type, region of dentin, the surface exposed (Beech, 1985; Stanford et al.,
Dental Materials/Janua~! 1989 27
1985), and storage time of the teeth (Fusayama et al., 1979; Causton and Johnson, 1979; Kimura et al., 1985; Smith and Ruse, 1986; Chan et al., 1985). In the present study, the transverse resistance of a dentin bonding system is evaluated, in order for the potentials of extensive composite restorations to be explored. The results obtained after different pretreatments of the dentin and enamel surfaces are compared. Also, the influences of aging time and the presence of enamel and/or dentin on transverse resistance are taken into consideration. MATERIALS AND METHODS
Sixty recently extracted sound human molar teeth were used. These were stored at 37°C in water except during those periods required for completion of the experimental proc e d u r e s . The s p e c i m e n s w e r e embedded in cold-curing acrylic resin in a uniform cylindrical mould up to the cemento-enamel junction. The coronal portions of the molar specimens were reduced with a conventional model trimmer, with water, for exposure of dentin up to 2 mm from the cemento-enamel junction, perpendicular to the long axis of the mould. The exposed dentin and enamel surfaces were finished with 400-grit waterproof carborundum paper. The p r e p a r e d specimens were randomly assigned to four groups of 15 m o l a r s each. The r e m a i n i n g enamel on the lateral surfaces of the molars in two of the four groups was removed with a diamond stone (Horico 243/016, Hopf, Ringlet & Co. GmbH & Cie., Berlin, West Germany). The design of the experiment is given in Table 1. Each group of 15 molars was then randomly subTABLE 1 TREATMENT COMBINATIONS OF THE RESPECTIVE EXPERIMENTAL GROUPS Group
Surface
Acid Etching
I
dentin enamel dentin enamel dentin dentin
+ + + + -
II III IV
+ acid etching. - no acid etching.
TABLE 2 SURFACE IN cm2 Group
Dentin
Enamel
Total
I II III IV
0.480 0.420 0.420 0.380
0.310 0.390 -
0.790 0.810 0.420 0.380
divided into t h r e e experimental groups of five molars each. Tests were conducted for two days, nine days, and 64 days. A standardized (12 x 12 cm) blackand-white photograph was taken of the occlusal surface of each specimen. The enlarging factor was calculated by comparison of the diameter of the mould on the photograph with the real diameter (1.6 cm). The areas of dentin and enamel (groups I and II only) were found by weighing of the appropriate areas of the photographs. The mean surface areas for both enamel and dentin are given in Table 2. A c o m p o s i t e m a t e r i a l (Cavex Clearfil Ray, Cavex Holland, Haarlem, The Netherlands) and its corresponding dentin-bonding material were used. For polymerization, a visible-light source (HL 500 Cavex, Cavex Holland) was available. The composite resin used was applied according to the manufacturer's instructions as follows: All specimens were dried. The entire surface of each specimen in experimental groups I and III, and only the enamel surfaces of those of group II, was etched for 40 sec with a 37% phosphoric acid, rinsed thoroughly for 20 sec, and dried for 15 sec. A Toffiemire retainer with metal matrix was placed around the specimens, the dentinbonding material was applied, and a 3-mm-thick composite layer placed. The light was applied for 40 sec. Tofflemire and matrix were removed, and the specimens were placed back into the water. They were stored for two days, nine days, or 64 days. For the transverse resistance test,
! ;
/
;
/
Fig. 1. A schematic representationof the transverse testing mode.
the specimens were placed in a mechanical testing device (Instron Corp., Canton, MA, U.SA.). Each specimen was loaded perpendicular to its long axis (Fig. 1). The load was placed at the w i d e s t side of the specimen. The transverse force was applied with a crosshead speed of 0.5 mm/min until failure occurred. The dislodgement and load were registered continuously. The level of resistance was determined by the point in the time/load graph where the line had a downward deflection. RESULTS
Table 3 depicts the results for the four treatment groups and three test times. Furthermore, the mean values p e r type of treatment are given. A significant difference was established between the mean transverse resistance of groups containing enamel (I + II) and that of those without (II! + IV) (p < 0.001). No significant difference was found between groups I and II (p = 0.09). On the other hand, a significant difference was detected between the mean resistance of groups III and IV (p = 0.001). The data clearly show that the presence of enamel results in a much higher transverse resistance compared with the situation with only dentin being present. If enamel is present, the additional acidetching of dentin gives a slightly lower transverse resistance. However, without any enamel being present, acid-etching of dentin gives significantly higher results.
TABLE 3 TRANSVERSE RESISTANCE ± STANDARD DEVIATION IN N
Group I II III IV
2 Days 578 673 297 196
± 197 ± 73 ± 125 ± 66
28 PEERLINGS et aL/RESISTANCE OF A DENTIN BONDED COMPOSITE
9 Days
64 Days
458 ± 177 542_+ 45 272 ± 130 92 ± 35
543__ 81 619 ± 119 385 ± 84 287 ± 92
Mean 521 611 322 192
± 157 _+ 94 ± 117 ± 104
A significant overall effect of time (p = 0.003) is found. For groups I and II, no significant storage-time effect (p = 0.06) was established, contrary to the significant storagetime effect for groups III and IV (p = 0.008). The nine-day resistance for group IV is significantly lower than that of those at the other storage times.
DISCUSSION The results of groups I and II make it clear that acid-etching in a situation with both dentin and enamel being present results in lower transverse resistance levels after both enamel and dentin are etched, compared with etching enamel only (Table 3). Photographs of specimens of corresponding t r e a t m e n t groups (Figs. 2-5) reveal white stains on the enamel (Figs. 4 and 5). Chow and Brown (1973) reported on the possibility of the formation of a soluble monite precipitate. Thus, it is feasible that the precipitate observed reduced the bond to enamel. Apparently, a rinsing time of 20 sec, as prescribed by the manufacturer, is insufficient for a complete removal of the precipitate in vitro. Consequently, the elimination of the precipitate from a preparation with difficult accessible line angles rather than a flat surface, as in this experiment, will require an even longer rinsing time. Therefore, in a general practice situation, a total etch system (including both enamel and dentin) m a y pose risks r a t h e r than advantages. Acid-etching of dentin in a situation without enamel, thus removing the smear layer, turns out to give a significantly higher transverse resistance. This is consistent with earlier reports (Fusayama et al., 1979; Hansen, 1984; van Noort and Northeast, 1986). Combination of the results, as given in Tables 2 and 3, gives a mean shear bond strength of 7.6 MPa for group III and 5 MPa for group IV. These values are in the same range as those of other in vitro tests in the shear mode (Chan et al., 1985; Huang and SSderholm, 1987; Stangel, 1987). It is also consistent with the findings that the shear bond strength of dentin-bonding systems to dentin is lower than that of those to enamel (Asmussen and Munks-
Fig. 2. The dentin-enamel junction after acid-etching of enamel for 40 sec and rinsing with water for 20 sec (magnification 100 x ). The smear layer on the dentin surface is partially removed near the enamel.
Fig. 3. A close-up (magnification 500 x ) of the designated area of the dentin-enamel junction of Fig. 2.
gaard, 1985; Stanford et al., 1985). The finding of no d e c r e a s e in transverse resistance in time is in agreement with the findings of other studies (Fusayama et al., 1979; Smith and Ruse, 1986; Chan et al., 1985). The minimum resistance observed after nine days for group IV might be explained in either of two ways. After loss of the pulp, changes take place in the tooth tissue (Causton and Johnson, 1979). Furthermore, hydrolytic degradation of the tooth-resin interface has been reported (Huang and SSderholm, 1987; Smith and Ruse, 1986). Another explanation
could be that hygroscopic expansion relieves interfacial stresses in time, and thus compensates for the deterioration of the bond to dentin (Huang and SSderholm, 1987). The resistance level found in this experiment is of the same order of magnitude as that of retention systems for extensive amalgam restorations. T h e s e w e r e said to be sufficient (Plasmans et al., 1987). Because extensive amalgam restorations are successful from a mechanical point of view, the transverse resistance of the dentin-bonding systern as used in this experiment has
Dental Materials~January 1989
29
Fig, 4. The dentin-enamel Junction after acid-etching of both enamel and dentin for 40 san and rinsing with wofor for 20 sec (magnHication 100 × ), The smear layer on the dentin surface is totally removed, Tim presence of the precipHate can be clearly seen.
Fig. $. A close-up (magnification 500 x ) of the designated area of the dentin-enamel junction of Fig. 4.
reached a level which is thought to be sufficient under oral conditions in the short term. However, clinical research is necessary to give the ultim a t e a n s w e r as to its l o n g - t e r m acceptability.
REFERENCES ASMUSSEN, E. and MUNKSGAARD, E. (1985): Bonding of Restorative Resins to Dentin Promoted by Aqueous Mixtures of Aldehydes and Active Monomers, Int Dent J 35: 160-165.
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