Variables influencing the repair strength of dental composites KARL-JOHAN M, SODERHOLM AND MARION J, ROBERTS Department of Dental Biomaterials, College of Dentistry, University of Florida, Gainesville, Florida,
USA
Soderholm K-JM, Roberts MJ: Variables influencing the repair strength of dental composites, Scand J Dent Res 1991; 99: 173-80, Abstract - Differences in the Qexural strength of composites (P-30) repaired with four different adhesive techniques were determined and compared to the flexural strength of unrepaired specimens. Besides investigating the effects of differences in repair methods, both the effect of composite age at repair time and the elfect of water storage on Qexural strength were evaluated. No significant differences were found between specimens which had been stored for different times (1 or 60 days) before they were repaired. All specimens, including the unrepaired ones, became weaker with time in water. After both 90 and 360 days at 37°C in water, the strength of the repaired specimens ranged between 25 and 50% of the strength of the unrepaired specimens. The weakest group was the one which had been repaired after acid etching, water rinsing, and air drying only. Key words: acid etching; bonding; composites; silanes, Karl-Johan Soderholm, Department of Dental Biomaterials, College of Dentistry, Box J-446, Gainesville, Florida 32610, USA, Accepted for publication 22 July 1990,
Dental composite materials shrink when they polymerize, and wear and discolor when used (1, 2), These shortcomings sometimes require that composite restorations be replaced. Even though entire replacements are often justified, it is sometimes advantageous if discolored and worn restorations are veneered rather than completely removed and remade. The more conservative veneering procedure reduces the risk for both pulpal omplications and inconvenience to the paiient during tooth preparation (3), In addigion, a reliable bond to tooth structure still :equires that acid etched enamel be present,
Unfortunately, complete remakes of bonded restorations require previously etched and resin impregnated enamel to be removed and etched in order to optimize the enamel bond, Repeated remakes will increase the risk for dentin exposure which will result in decreased bonding ability. The above con,siderations suggest that it is beneficial if a layer of the old composite is left adjacent to the tooth-composite interface and that new material is bonded to this layer. Such an approach, however, can result in weaker restorations (3-6) and increase the risk that caries is being left undetected under the old
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restoration (2, 7). Regarding this risk, the probability of leaving caries undetected should decrease as a result of continued decrease in caries frequency. By reducing this risk, the justification for refurbishing worn and discolored restorations without complete restoration removal should be strengthened. Before different repair techniques can be recommended, we must know the repair strength of different adhesive systems and strength needed under clinical conditions. A previous study showed that the flexural strength of repaired composites ranged from 20 to 85% of the strength of unrepaired specimens (6). This variation was caused by differences in treatment protocol of the cut surfaces before new material was added (6). The strength reduction of repaired composites was explained as a result of ineomplete bond formation between the cut composite surface and the added material. The ineomplete bond formation was a result of silane removal from cut and exposed filler surfaces and lack of reactive bond sites available in the matrix of the cut composite. By acid etching and then silane-toluene treating the cut composite surface, it was possible to restore 85% of the strength of homogeneously cured composites. If there was no pretreatment of the cut surface at all, the restored strength was only 20% of the strength of unrepaired specimens. No silane-toluene solution as described (6) is commereially available at this time. Therefore, there is a need to identify commercial adhesive products that can be used to optimize the repair strength of composites. The objective of this study is to determine how some commercial adhesive products and repair conditions sueh as age of composite and effect of time in water influence the repair strength. Material and methods Sixty rectangular composite material bars, 3.5 mm wide, 3.5 mm deep, and 35.0 mm long, were made
by inserting approximately 2 g of P-30 (Batch No. 7 RUIV, Dental Products/3M, St Paul, MI) into a 3.5-mm-thick split aluminum mold. The two open surfaces of the mold were covered with a celluloid sheet and a glass slab on each side. Finger pressure was applied to the two glass slabs to . assure constant specimen thickness and flat specimen sides. Immediately after insertion, the specimens were light-cured on the top surface at five evenly spaced locations using a Translux (Kultzer, Irvine, CA) light unit. The light exposure time at each location was 40 s. After the top surface had been cured, the mold was turned upside down, and the lower surface was cured similarly as the top surface. After curing was completed, the specimen was removed from the split mold and transferred lo an oven in which the specimen was stored dry for either 24 h or 60 days al 37°C. After 24 h at 37°C, 30 of the 60 specimens were drawn and divided into five groups, each containing six specimens. The remaining 30 specimens were divided into five similar groups after another 59 days in the oven. The first of the five groups. Group A, consisted of unrepaired specimens and served as a control group. The other specimens were cut into two pieces with a diamond saw (Buehler Isomet low speed saw, Buehler Ltd., f L, USA) cooled with tap water. One of the cut pieces was always cut to a length of 17.5 mm. Immediately after the specimens had been cut, they were carefully rinsed in room temperature tap water and dried for 1 h at room temperature. After a preceding acid etching (60 s) (Scotchbond Etching Gel, batch 6 AR, Dental Products/ 3M, St Paul, MI), water rinsing (30 s), and air drying (30 s), the cut surfaces of the 17.5-mmlong specimens were treated with one of the following four methods: Specimens belonging to Group B were coaled with Scotchprime (Batch No. 6 AK, Dental Products/3M, St Paul, MI) and air-dried for 10 s, coated with Scotchbond (Batch No. 7 AVf and 7 C, Dental Products/3M, St Paul, MI) and air-dried for 5 s, light-cured for 10 s, inserted into the aluminum mold, rebuilt with P30 to the initial length of 35.0 mm, and finally light cured. Specimens belonging to Group C were coated with Scotchbond and air-dried for 5 s, lightcured for 10 s, inserted into the aluminum mold, rebuilt with P-30 to the initial length of 35.0 mm, and finally light cured. Regarding the specimens belonging to Group D, these specimens were treated with toluene fbr 30 s (Gertified A.C.S. toluene.
REPAIR STRENGTH OF COMPOSITES
175
UNREPAIRED
REPAIRED
FLEXURAL STRENGTH (MPa) Fig. 1. Lengths of bars represent mean flexural strengths of different groups, and attached lines are their standard deviations. Filled bars represent speciments being stored dry for 1 day before being water immersed, while unfilled bars are those specimens that were stored dry for 60 days before water immersion.
707131, Fischer Scientific Company, PA, USA), air-dried for 30 s, coated with Scotchprime and air-dried for 10 s, coated with Scotchbond and airdried for 5 s, light-cured for 10 s, inserted into the aluminum mold, rebuilt with P-30 to the initial length of 35,0 mm and finally light cured. Group E did not receive any particular surface treatment other than the preceding acid etch before these specimens were repaired. After the specimens belonging to Groups B, C, D, and E had been reconstructed and removed from the aluminum mold, these specimens and the Group A specimens were stored for another 24 h period at 37°C before excess material in the form of flash was removed with sandpaper (400 grit). Following this process, the diflerent groups were divided into two subgroups, each consisting of three specimens. Each of these subgroups was transferred to polyethylene bottles containing 100 niL distilled water. These subgroups were stored at 37°C for either 3 or 12 months. Following these storage periods, the flexure strengths of the specimens were determined using :i four-point bending test method (6), Immediately before the specimens were tested, their widths and depths were measured with a micrometer. The siecimens were then transferred to an oven (In,st on Environmental Chamber-Model 3111, Ins-
tron Corporation, Canton, MA, USA) kept at 37°C and attached to the testing machine. The four-point bending test was performed inside this oven. To avoid dehydration before and during the testing procedure, the specimens were loosely wrapped in wet paper towels except those surfaces contacting the four-point bending support (Instron Flexure Fixtures, A 336-9b and A 336-6b, Instron Corporation, Canton, MA, USA), The specimens were positioned so the repaired midsection of the specimens were in the center of the span. The distance between the lower span supports was 21,0 mm, while the distance between the upper load lines was 7,0 mm. The composite bars were loaded with a load rate of 0,2 mm/min using an Instron Universal Testing Instrument (Instron Universal Testing Instrument, Model 1125, Instron Corporation, Canton, MA, USA), with which the load was continuously registered until fracture. The flexural strength was determined from the relationship (8); FS = 6 P a / ( w l r ) where; FS = flexure strength; P = load at fracture/ 2; a = distance from lower span support to the closest upper load line; w = width; h = height. The strength values were compared by using
SODERHOLM AND ROBERTS
176
analysis of variance (ANOVA) testing at the 95% significance level. Besides the specimens being tested in flexure strength, cut specimens treated similarly to those in Groups B, C, and D were coated with a 10-nmthick gold-palladium layer and investigated with a scanning electron microscope (SEM) (Jeol C35, Jeol Inc., Tokyo, Japan).
been stored 1 day before repair was 11.7 MPa (SD = 5.85 MPa), while the mean strength for those that had been repaired after 60 days was 13.6 MPa (SD = 6.95 MPa). Although the mean value was higher for the specimens that had been prestored for 60 days (Fig. 1), this difference was not significant (P>0.05).
Results TIME BEFORE TRANSFER TO WATER The pooled mean values (90 and 360 days) for the unrepaired control groups prestored for 1 day dry before water immersion and those prestored dry for 60 days before water immersion are shown in Fig. 1. The mean value of the former group was 33.8 MPa (SD= 10.73 MPa), while the mean value of the latter group was 31.8 MPa (SD = 6.48 MPa). This difference was not significant {P>0.05). When the repaired specimens (Groups B, C, D, and E) were pooled and compared regarding time before water transfer, the mean strength of the specimens that had
TIME IN WATER The mean value of the unrepaired specimens prestored for 1 or 60 days in air before being kept in water for 3 months was 37.0 MPa (SD = 8.05 MPa). After 12 months in water the mean strength of the unrepaired group was 28.6 MPa (SD=7.24 MPa) (Fig. 2). This difference was not significant {P> 0.05). When the repaired specimens (Groups B, G, D, and E) were pooled and compared similarly as Group A, the mean strength of the specimens stored for 3 months was 15.4 MPa (SD = 5.85 MPa), while the mean strength of the specimens stored for 12 months was 10.0 MPa (SD = 6.95 MPa).
0 FLEXURAL STRENGTH (MPa) Fig. 2. Bar graph showing how storage time in water influenced flexural strength of the difTerent groups Filled bars represent specimens being stored for 90 days in water, and unfilled bars are those specimen stored for 360 days in water before testing.
REPAIR STRENGTH OF COMPOSITES This difference was not significant {P> 0.05). By comparing the individual groups it was found that only Group D showed a significant decrease (/'
USA
Soderholm K-JM, Roberts MJ: Variables influencing the repair strength of dental composites, Scand J Dent Res 1991; 99: 173-80, Abstract - Differences in the Qexural strength of composites (P-30) repaired with four different adhesive techniques were determined and compared to the flexural strength of unrepaired specimens. Besides investigating the effects of differences in repair methods, both the effect of composite age at repair time and the elfect of water storage on Qexural strength were evaluated. No significant differences were found between specimens which had been stored for different times (1 or 60 days) before they were repaired. All specimens, including the unrepaired ones, became weaker with time in water. After both 90 and 360 days at 37°C in water, the strength of the repaired specimens ranged between 25 and 50% of the strength of the unrepaired specimens. The weakest group was the one which had been repaired after acid etching, water rinsing, and air drying only. Key words: acid etching; bonding; composites; silanes, Karl-Johan Soderholm, Department of Dental Biomaterials, College of Dentistry, Box J-446, Gainesville, Florida 32610, USA, Accepted for publication 22 July 1990,
Dental composite materials shrink when they polymerize, and wear and discolor when used (1, 2), These shortcomings sometimes require that composite restorations be replaced. Even though entire replacements are often justified, it is sometimes advantageous if discolored and worn restorations are veneered rather than completely removed and remade. The more conservative veneering procedure reduces the risk for both pulpal omplications and inconvenience to the paiient during tooth preparation (3), In addigion, a reliable bond to tooth structure still :equires that acid etched enamel be present,
Unfortunately, complete remakes of bonded restorations require previously etched and resin impregnated enamel to be removed and etched in order to optimize the enamel bond, Repeated remakes will increase the risk for dentin exposure which will result in decreased bonding ability. The above con,siderations suggest that it is beneficial if a layer of the old composite is left adjacent to the tooth-composite interface and that new material is bonded to this layer. Such an approach, however, can result in weaker restorations (3-6) and increase the risk that caries is being left undetected under the old
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restoration (2, 7). Regarding this risk, the probability of leaving caries undetected should decrease as a result of continued decrease in caries frequency. By reducing this risk, the justification for refurbishing worn and discolored restorations without complete restoration removal should be strengthened. Before different repair techniques can be recommended, we must know the repair strength of different adhesive systems and strength needed under clinical conditions. A previous study showed that the flexural strength of repaired composites ranged from 20 to 85% of the strength of unrepaired specimens (6). This variation was caused by differences in treatment protocol of the cut surfaces before new material was added (6). The strength reduction of repaired composites was explained as a result of ineomplete bond formation between the cut composite surface and the added material. The ineomplete bond formation was a result of silane removal from cut and exposed filler surfaces and lack of reactive bond sites available in the matrix of the cut composite. By acid etching and then silane-toluene treating the cut composite surface, it was possible to restore 85% of the strength of homogeneously cured composites. If there was no pretreatment of the cut surface at all, the restored strength was only 20% of the strength of unrepaired specimens. No silane-toluene solution as described (6) is commereially available at this time. Therefore, there is a need to identify commercial adhesive products that can be used to optimize the repair strength of composites. The objective of this study is to determine how some commercial adhesive products and repair conditions sueh as age of composite and effect of time in water influence the repair strength. Material and methods Sixty rectangular composite material bars, 3.5 mm wide, 3.5 mm deep, and 35.0 mm long, were made
by inserting approximately 2 g of P-30 (Batch No. 7 RUIV, Dental Products/3M, St Paul, MI) into a 3.5-mm-thick split aluminum mold. The two open surfaces of the mold were covered with a celluloid sheet and a glass slab on each side. Finger pressure was applied to the two glass slabs to . assure constant specimen thickness and flat specimen sides. Immediately after insertion, the specimens were light-cured on the top surface at five evenly spaced locations using a Translux (Kultzer, Irvine, CA) light unit. The light exposure time at each location was 40 s. After the top surface had been cured, the mold was turned upside down, and the lower surface was cured similarly as the top surface. After curing was completed, the specimen was removed from the split mold and transferred lo an oven in which the specimen was stored dry for either 24 h or 60 days al 37°C. After 24 h at 37°C, 30 of the 60 specimens were drawn and divided into five groups, each containing six specimens. The remaining 30 specimens were divided into five similar groups after another 59 days in the oven. The first of the five groups. Group A, consisted of unrepaired specimens and served as a control group. The other specimens were cut into two pieces with a diamond saw (Buehler Isomet low speed saw, Buehler Ltd., f L, USA) cooled with tap water. One of the cut pieces was always cut to a length of 17.5 mm. Immediately after the specimens had been cut, they were carefully rinsed in room temperature tap water and dried for 1 h at room temperature. After a preceding acid etching (60 s) (Scotchbond Etching Gel, batch 6 AR, Dental Products/ 3M, St Paul, MI), water rinsing (30 s), and air drying (30 s), the cut surfaces of the 17.5-mmlong specimens were treated with one of the following four methods: Specimens belonging to Group B were coaled with Scotchprime (Batch No. 6 AK, Dental Products/3M, St Paul, MI) and air-dried for 10 s, coated with Scotchbond (Batch No. 7 AVf and 7 C, Dental Products/3M, St Paul, MI) and air-dried for 5 s, light-cured for 10 s, inserted into the aluminum mold, rebuilt with P30 to the initial length of 35.0 mm, and finally light cured. Specimens belonging to Group C were coated with Scotchbond and air-dried for 5 s, lightcured for 10 s, inserted into the aluminum mold, rebuilt with P-30 to the initial length of 35.0 mm, and finally light cured. Regarding the specimens belonging to Group D, these specimens were treated with toluene fbr 30 s (Gertified A.C.S. toluene.
REPAIR STRENGTH OF COMPOSITES
175
UNREPAIRED
REPAIRED
FLEXURAL STRENGTH (MPa) Fig. 1. Lengths of bars represent mean flexural strengths of different groups, and attached lines are their standard deviations. Filled bars represent speciments being stored dry for 1 day before being water immersed, while unfilled bars are those specimens that were stored dry for 60 days before water immersion.
707131, Fischer Scientific Company, PA, USA), air-dried for 30 s, coated with Scotchprime and air-dried for 10 s, coated with Scotchbond and airdried for 5 s, light-cured for 10 s, inserted into the aluminum mold, rebuilt with P-30 to the initial length of 35,0 mm and finally light cured. Group E did not receive any particular surface treatment other than the preceding acid etch before these specimens were repaired. After the specimens belonging to Groups B, C, D, and E had been reconstructed and removed from the aluminum mold, these specimens and the Group A specimens were stored for another 24 h period at 37°C before excess material in the form of flash was removed with sandpaper (400 grit). Following this process, the diflerent groups were divided into two subgroups, each consisting of three specimens. Each of these subgroups was transferred to polyethylene bottles containing 100 niL distilled water. These subgroups were stored at 37°C for either 3 or 12 months. Following these storage periods, the flexure strengths of the specimens were determined using :i four-point bending test method (6), Immediately before the specimens were tested, their widths and depths were measured with a micrometer. The siecimens were then transferred to an oven (In,st on Environmental Chamber-Model 3111, Ins-
tron Corporation, Canton, MA, USA) kept at 37°C and attached to the testing machine. The four-point bending test was performed inside this oven. To avoid dehydration before and during the testing procedure, the specimens were loosely wrapped in wet paper towels except those surfaces contacting the four-point bending support (Instron Flexure Fixtures, A 336-9b and A 336-6b, Instron Corporation, Canton, MA, USA), The specimens were positioned so the repaired midsection of the specimens were in the center of the span. The distance between the lower span supports was 21,0 mm, while the distance between the upper load lines was 7,0 mm. The composite bars were loaded with a load rate of 0,2 mm/min using an Instron Universal Testing Instrument (Instron Universal Testing Instrument, Model 1125, Instron Corporation, Canton, MA, USA), with which the load was continuously registered until fracture. The flexural strength was determined from the relationship (8); FS = 6 P a / ( w l r ) where; FS = flexure strength; P = load at fracture/ 2; a = distance from lower span support to the closest upper load line; w = width; h = height. The strength values were compared by using
SODERHOLM AND ROBERTS
176
analysis of variance (ANOVA) testing at the 95% significance level. Besides the specimens being tested in flexure strength, cut specimens treated similarly to those in Groups B, C, and D were coated with a 10-nmthick gold-palladium layer and investigated with a scanning electron microscope (SEM) (Jeol C35, Jeol Inc., Tokyo, Japan).
been stored 1 day before repair was 11.7 MPa (SD = 5.85 MPa), while the mean strength for those that had been repaired after 60 days was 13.6 MPa (SD = 6.95 MPa). Although the mean value was higher for the specimens that had been prestored for 60 days (Fig. 1), this difference was not significant (P>0.05).
Results TIME BEFORE TRANSFER TO WATER The pooled mean values (90 and 360 days) for the unrepaired control groups prestored for 1 day dry before water immersion and those prestored dry for 60 days before water immersion are shown in Fig. 1. The mean value of the former group was 33.8 MPa (SD= 10.73 MPa), while the mean value of the latter group was 31.8 MPa (SD = 6.48 MPa). This difference was not significant {P>0.05). When the repaired specimens (Groups B, C, D, and E) were pooled and compared regarding time before water transfer, the mean strength of the specimens that had
TIME IN WATER The mean value of the unrepaired specimens prestored for 1 or 60 days in air before being kept in water for 3 months was 37.0 MPa (SD = 8.05 MPa). After 12 months in water the mean strength of the unrepaired group was 28.6 MPa (SD=7.24 MPa) (Fig. 2). This difference was not significant {P> 0.05). When the repaired specimens (Groups B, G, D, and E) were pooled and compared similarly as Group A, the mean strength of the specimens stored for 3 months was 15.4 MPa (SD = 5.85 MPa), while the mean strength of the specimens stored for 12 months was 10.0 MPa (SD = 6.95 MPa).
0 FLEXURAL STRENGTH (MPa) Fig. 2. Bar graph showing how storage time in water influenced flexural strength of the difTerent groups Filled bars represent specimens being stored for 90 days in water, and unfilled bars are those specimen stored for 360 days in water before testing.
REPAIR STRENGTH OF COMPOSITES This difference was not significant {P> 0.05). By comparing the individual groups it was found that only Group D showed a significant decrease (/'
