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Bond strength of a light-cured and two auto-cured glass ionomer liners
J. Dent. 1990; 18: 271-275
271
Bond strength of a light-cured and two auto-cured glass ionomer liners J. R. Holtan, G. P. Nystrom, P. S. Olin, J. Rudney and W. H. Douglas Biomaterials
Program, School of Dentistry,
University
of Minnesota,
Minneapolis,
Minnesota,
USA
ABSTRACT Ninety-nine extracted human molar teeth were used in this study comparing the shear bond strengths on dentine of one light-cured and two auto-cured polyalkenoate (glass ionomer) cements. Bond strength can be influenced by differences in tooth structure. A balanced-incomplete block design (Hull and Nie, 1981) was used to reduce variation attributable to such differences. Cements were applied to paired dentine surfaces in combinations such that 66 tooth sides were treated with each material. A light-cured dentinal adhesive and composite resin restorative material were then placed and shear bond strength testing was conducted exactly 24 h after the completion of each specimen. Mean forces (MPa) for the three materials were compared using an appropriate analysis of variance model (balanced-incomplete-blocks) The shear bond strengths (MPa) of the light-cured liner (Espe, Seefeld/Oberbay, FRG) was 4.71 f 1.16. Vitrabond showed the greatest variance of all three materials tested, however this material’s average bond strength was greater than the maximum achieved for the other materials. Student-Newman-Keuls comparison of means showed that all cements differed significantly from each other (a = O.O5).It is concluded that the light-cured glass ionomer liner exhibited significantly better shear bond strength performance than the two auto-cured glass ionomers tested. KEY WORDS: J. Dent 1990)
1990:
Polyalkenoate (glass ionomer) cement liners, Bond strengths 18: 271-275
(Received 11 December 1989;
reviewed 26 January 1990;
accepted 10 July
Correspondence should be addressed to: Dr G. P. Nystrom, Biomaterials Program, School of Dentistry, 515 Delaware Street South East, University of Minnesota, Minneapolis, MN 55455, USA.
INTRODUCTION Polyalkenoate (glass ionomer) cements have been suggested as liners for composite resin restorations. This application is advocated because of these materials’ ability to bond both to dentine and composite resin materials (McLean and Wilson, 1977; Powis et al., 1982; McLean et al., 1985) as well as their reported improvement in microleakage performance as compared to composite resin restorative materials (Hembree and Andrews, 1978; Welsh and Hembree, 1985) the release of fluoride over time (Forsten, 1977; Swartz et al.. 1984) and their biocompatibility with dental tissues (Tobias et al., 1978; Heys et al., 1987). High bond strengths of glass ionomer cements to dentine and enamel are critical if they are to be effective in this regard. Handling properties of these materials, however, make them especially technique sensitive because of their susceptibility to moisture contamination or dehydration during the early stages of the setting reaction. Contamina0 1990 Butterworth-Heinemann 0300-5712/90/050271-05
Ltd.
tion by saliva of freshly prepared dentine surface results in a marked decrease in the bond strength (Prodger and Symmonds, 1977). Light-cured forms of glass ionomer cements are now available which may improve some of the adverse handling characteristics. A reduction in setting time because of photocuring within the cavity preparation may assist in decreasing moisture contamination and dehydration difficulties associated with the early stage of the setting reaction. The purpose of this study was to evaluate and compare the shear bond strength performance of a light-cured glass ionomer cement liner with two commonly used chemically cured glass ionomer liners. MATERIALS
AND METHODS
Study design This study compared bond strengths of three glass ionomer cements in a random sample of 99 caries-free
272
J. Dent. 1990; 18: No. 5
Table 1. Glass ionomer liners investigated Material
Source
Vitrabond
3M Dental Products Division, St Paul, MN, USA GC Dental Industrial Corp., Tokyo, Japan Espe, Seefeld/Oberbay, FRG
GC Lining Cement Ketac-Bond
Table II. Details of comparison of materials Paired tooth sides Materials compared Vitrabond GC Lining Cement Vitrabond Ketac-Bond GC Lining Cement Ketac-Bond
(no.1 vs.
33
vs.
::
vs.
:z 33
66 tooth sides were treated with each material. extracted human third molars. Bond strength can be influenced by differences in tooth structure. This was a major consideration in study design. One way of controling tooth structure effects would have been to test all three materials within each of the sample teeth. However, this would have required that teeth be divided in thirds, which would have made it difficult to obtain uniform surfaces for comparison. The alternative was to bisect teeth and compare opposing faces, however, this approach would permit only two of the three materials to be compared in any given tooth. The problem was addressed through use of a ‘balanced-incomplete-block’ study design. Balancedincomplete-block designs are employed when the number of treatments to be compared (three cements) within a ‘block’ (tooth) is greater than the number of treatments that can be physically applied to a block (only two halves of a tooth are available). In this approach, each block receives a subset of treatments. All combinations of treatments must be used, and each combination must be applied to equal numbers of blocks. When these conditions are met, data can be analysed in a manner similar to a complete randomized block design in which each block receives every treatment (Cochran and Cox, 1957; Hull and Nie, 1981). As each of the 99 sample teeth were bisected, materials were randomly assigned to each half with the stipulation that opposing halves receive different materials. This resulted in 66 halves being treated with each material, and with each of the three possible combinations of materials (A with B, B with C, and A with C) represented in 33 teeth. The glass ionomer materials investigated are detailed in Table I. Details of comparison of materials are presented in Table II.
Dentine
specimen
preparation
Teeth for this study were obtained from the School of Dentistry’s Oral Surgery clinic and several oral surgery
Class Light-cured Auto-cured Auto-cured
offices in the community. Teeth were stored briefly in distilled water at room temperature and were prepared shortly after they were obtained. None were stored longer than 30 days prior to preparation. After mechanical debridement of the teeth the surfaces were cleaned with a pumice-water slurry. Each tooth was reduced under constant water spray on mesial and distal surfaces to expose a flat surface of dentine. The tooth was then sectioned buccolingually. The paired dentinal surfaces were centred on flat Teflon disks which were placed in the ends of aluminium cylinders. The cylinders were inverted and filled with mounting medium (orthodontic resin) in two increments. Following setting of the resin, each specimen was repositioned and secured in the aluminium cylinder with the resin-encased dentinal surface protruding slightly beyond the face of the cylinder. This assembly was mounted in a jig attached to a modified lathe with an end-cutting, water-lubricated 7.5 cm diameter 600 grit diamond wheel. The specimens, with constant water irrigation, were brought orthogonally against and across the diamond wheel to obtain a flat dentine test surface (Fig. 1). This surface was then drawn 25 times across, and parallel to a dry, flat-mounted sheet of 600 grit aluminium oxide wet-or-dry sandpaper (3M, St Paul, MN, USA). A
Fig. 7. Assembly mounted in a jig attached to a modified lathe with an end-cutting, water-lubricated 7.5 cm diameter diamond wheel. The specimens were brought orthogonally against and across the diamond wheel to obtain a flat dentine test surface.
Holtan et a/.: Bond strength
Composite
resin
Alumini urn cylinder
,Teflon ‘Glass
ring ionomer
‘Mounting
resin
Fig. 2. Resin-encased dentine specimen secured in an aluminium mounting cylinder in direct apposition to a doughnut-shaped split Teflon disk bored with a 5 mm diameter hole positioned flush with the end of the cylinder. Placement of the glass ionomer liner on the dentine is followed by application of the dentinal adhesive and the composite resin restorative material.
fresh section of sandpaper was used for each specimen. Each pair of specimens was wrapped in gauze moistened with water and stored very briefly in a closed container at room temperature until placement of the glass ionomer and restorative materials. The resin-encased dentine specimen (Fig. 2) was again placed and secured in an aluminium mounting cylinder in direct apposition to a split Teflon disk bored with a 5 mm diameter hole positioned flush with the end of the cylinder. A ‘cavity’ preparation 5 mm in diameter and 3 mm in depth was thus formed by the dentinal surface and the cylindrical internal surface of the disk.
Restorative
procedures
The glass ionomer liners were applied to the entire dentinal surface of the ‘cavity preparations’ according to manufacturer’s instructions:
4 m
lnstron mounting post
Mounting resin
Fig. 3. Specimen applied.
mounted
in a jig and shear forces
liners
273
- Vitrabond was light cured for 30 s. - GC Lining Cement was placed on the dentinal surface and allowed to set for 4 min. - Ketac-Bond: Ketac-Conditioner was applied for 10 s to remove the smear layer, rinsed with water for 30 s and dried with air. Ketac-Bond was then placed on dentine and allowed to cure (set) for 4 min. After completion of the setting time the exposed surfaces of the auto-cured glass ionomer cements were etched, according to manufacturer’s instructions, with a phosphoric acid gel (Scotchbond Etching Gel, 3M) for 30 s, rinsed with water for 30 s, and dried with oil-free compressed air. The Vitrabond was not etched prior to placement of the dentine bonding adhesive. From this point, all of the restorative procedures were identical for all materials at all sites. A light-cured dentinal adhesive system (Scotchbond 2 with a primer, Scotchprep, 3M) was used which includes the immediate use of the primer. Scotchprep was constantly agitated within the confines of the preparation for 60 s and then dried for 15 s under moderate air volume (but not washed). The resin bonding agent (Scotchbond 2) was then applied, moderately air thinned, and cured under visible light for 20 s. Silux Plus resin (3M) was placed over the resin bonding agent in two increments and light cured for 60 s according to manufacturer’s instructions, the second of which was packed even with the outer surface of the Teflon split disk. The split disks were separated and the specimens carefully cleared of flash to maintain the circular 5 mm diameter interface of dentine to the ‘cavity’preparation. The sample pairs, now complete with the glass ionomers being evaluated and a composite stub, were removed from the aluminium cylinders, wrapped in water-moistened gauze, and stored as pairs in a closed container at room temperature prior to bond strength testing exactly 24 h later. Specimens were mounted in a jig and shear forces applied by the flat blade of a moveable slide (Fig. 3). The jig was mounted in an Instron testing machine (Model 4204, Instron Corporation, Canton, MA USA) which was configured with a cross-head speed of 0.5 mm/min. Peak break points (kg) were recorded for the paired specimens. Statistical
Moveable slide *
of glass ionomer
analysis
The study was similar to a paired design in that the objective was to remove variation between teeth and compare materials between tooth sides. Examples from Hull and Nie (198 1) were used to set up Statistical Package for the Social Sciences (SPSS) program MANOVA for an analysis of variance (ANOVA) of a balanced-incompleteblock design. Variation attributable to differences between teeth first was removed from the total sum of squares. Treatment effects then were tested over the residual error variance. This provided an overall test for differences between material means. Comparisons between specific materials then were carried out with the StudentNewman-Keuls test (SNK). The ANOVA residual mean
274
J. Dent. 1990;
18: No. 5
Table 111.Shear bond strength values
No.
Vitrabond/GC 6.306 13.607 9.737 4.658 11.42 4.926 3.847 7.27 6.71 1 6.886 8.349 7.635 8.169 7.435 12.828 12.198 14.505 1 1.674 5.852 11.225 3.1 1 8.793 8.653 12.468 3.666 4.363 5.642 6.776 7.75 9.867 11.41 10.576 9.228
: 3 4 5 6 7 98 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Liner
Shear bond strength (MPa) Vitrabond /Ketac-Bond
4.182 3.244 4.028 4.893 3.867 2.935 2.54 1.977 3.626 3.27 3.304 3.512 2.46 4.511 2.493 1.977 2.647 3.304 1.749 2.42 2.708 1.89 3.593 3.177 2.051 3.079 2.285 3.237 1.381 2.1 18 4.698 5.098 4.899
4.437 8.094 7.29 1 1.689 6.476 7.07 9.452 8.289 11.155 10.181 1 1.789 10.05 1 6.46 1 5.542 5.283 10.216 3.626 6.226 12.633 9.727 5.947 4.484 6.386 6.271 6.871 3.733 4.672 4.53 1 5.233 9.637 10.236 11.11 8.089
square was used as the SNKerror term, which showed that SNK results also were adjusted for variation between teeth (Wirier, 197 1).
RESULTS Shear bond strength values (kg/cmz) were calculated from peak break points as the load to produce failure (kg) divided by the area (cmz, which was assumed to be a constant) of the bonding agent. These values were then converted to MPa (Table ZZZ).The balanced-incompleteblock analysis of variance showed an overall difference materials that was statistically significant among (a = 0.05) after variation due to differences among teeth Table IV. Analysis of variance Source of variation
d.f.
Mean square
Teeth Cements Residual
98 2 97
422 33359 359
F
92.89
P
< 0.00001
Balanced-incomplete-block analysis of variance of shear bond strength of the light-cured liner and two auto-cured glass ionomers. An overall difference among materials was statistically significant !t;otit5) after variation due to differences among teeth was
4.196 3.559 4.088 4.557 2.5 3.445 4.337 6.142 4.222 4.531 3.452 3.854 3.699 4.94 3.881 3.485 4.578 3.646 4.778 3.425 4.175 5.228 4.725 4.59 1 4.323 3.606 5.383 3.84 5.872 4.725 6.46 1 4.953 4.035
GC Liner / Ketac-Bond 4.731 4.866 4.283 3.364 3.096 4.765 4.597 3.344 4.068 6.536 4.088 3.686 3.478 5.507 3.572 3.425 5.418 3.445 5.482 5.812 4.839 5.173 4.799 4.782 4.108 5.872 4.866 3.928 3.954 4.645 2.493 2.929 5.418
5.447 6.446 6.311 6.59 1 4.852 4.711 5.582 7.045 4.645 4.718 5.058 6.641 4.504 4.048 6.376 4.149 6.122 5.348 5.233 3.478 4.128 5.437 1.77 6.107 7.425 3.043 5.368 5.183 5.597 3.385 2.366 4.202 6.251
was removed (Table IV). Mean shear bond strength values for each of the three materials is presented in Table V. Vitrabond showed greater bond strength (but also greater variance) than either GC Lining Cement or Ketac-Bond, while Ketac-Bond in turn displayed greater bond strength than GC Lining Cement. Student-Newman-Keuls comparison of means for individual materials showed that all cements differed significantly from each other (a = 0.05).
DISCUSSION The bond strength of the light-cured liner in the present report was significantly stronger than either of the autocured ionomers. This improvement in bond strength is due to an increase in the cohesive strength of the lightcured liner. It is widely known that the bond failure of glass ionomers is mainly cohesive, therefore immediate improvement in bond strength could be obtained by improving the cohesive strength of the material. This appears to have been achieved in the case of the lightcured liner, where the improved strength is due to the development of a new molecule, an unsaturated hydrophilic polymer which offers curing both by chemical means (polyalkenoic acid) and by the more popular
Holtan et a/.: Bond strength
of glass ionomer
liners
275
Table V. Mean shear bond strength of materials Material
n*
Mean
Vitrabond GC Lining Cement Ketac-Bond
66 66 66
8.03 3.76 4.71
+ s.d. f 2.86 + 1.17 k 1.16
Minimum
Maximum
Range
3.21 1.38 1.77
14.51 6.54 7.42
1 1.40 5.16 5.65
Each cement was significantly different from every other by the Student-Newman-Keuls (a = 0.05). *n based on the number of tooth sides treated with each material.
The light-cured liner is photo-curing (vinyl insaturation). comprised of one main molecular species, and in this respect can be distinguished from many other light-curing liners in this category, which may be mixtures of a resin and an ionomer liquid (Creo. 1988). The benefits of light curing, familiar to photo-cured composites, are also available in the light-curing ionomers. However, it is likely that the contraction of the light-curing ionomers may be greater than that of the auto-cured materials. This contraction is mostly complete before the resin and the composite material is added onto the ionomer, and therefore its effects may not be as great as originally anticipated. The light-cured liner showed greater variation in bond strength than either auto-cured cement. However, the average bond strength for the light-cured liner was greater than the maximum achieved for the other materials. Moreover, the averages for the auto-cured materials were only slightly greater than the minimum bond strength of the light-cured liner. The performances of the auto-cured materials were much closer to each other in comparison. Ketac-Bond, however, was still significantly stronger than GC Lining Cement. As noted, the problem with the autocured materials is in the slowness of set, sensitivity to clinical conditions, and above all, the decrease in cohesive strength which led to a decrease in the bond strength. The results of this study would indicate a significant advantage for photo-cured glass ionomer materials. The chief remaining advantage of the auto-cured glass ionomers, as already noted, lies in the fact that the contraction may be less. The reason for this may be the vinylization of the light-cured glass ionomer cement which tends to impart a composite-like linkage. An important consideration is that even the auto-cured materials contract (Kemp-Scholte and Davidson, 1988), but the contraction is extremely slow and may not challenge the bond that has formed. It remains to be seen whether the differences in contraction between photo-and auto-cured materials have any impact on their durability or longevity. Given the significant advantages of photocured materials, the continued use of auto-cured materials should be questioned. Research should continue comparing bond strength performance and other qualities of light-curing glass ionomer materials which are becoming available to the dental profession. In conclusion, the light-cured liner (Vitrabond) had significantly higher shear bond strength performance than the auto-cured glass ionomers tested. Although the
test
light-cured liner performed much better than either of the auto-cured glass ionomer cements tested, there was still a difference between the auto-cured materials themselves, Ketac-Bond having significantly better shear bond strength than the GC Lining Cement. Vitrabond showed the greatest variance of all three materials tested, however its average bond strength was greater than the maximum achieved for the other materials. Long-term significance of shrinkage of light-cured and auto-cured glass ionomer cements has yet to be determined.
References Cochran W. G. and Cox G. M. (1957) Experimental Design, 2nd edn. New York, Wiley, p. 443. Creo A. (1988) Technical Bulletin on Vitrabond. St Paul, MN, 3M Dental Products Division. Forsten L. (1977) Fluoride release from a glass ionomer cement. Stand. J. Dent. Res. 85, 503-504. Hembree J. H. Jr and Andrews J. T. (1978) Microleakage of several class V anterior restorative materials: a laboratory study. J. Am. Dent. Assoc. 97, 179-183. Heys R. J., Fitzgerald M., Heys D. R. et al. (1987) An evaluation of a glass ionomer luting agent: pulpal histologic response. J. Am. Dent. Assoc. 14, 607-611. Hull C. H. and Nie N. H. (1981) SPSS Update 7-9. In: New Procedures and Facilities for Release. New York, McGrawHill, pp. 18-19. Kemp-Scholte C. M. and Davidson C. L. (1988) Marginal sealing of curing contraction gaps in class V composite resin restorations. J. Dent. Res. 67, 841-845. McLean J. W. and Wilson A. D. (1977) The clinical development of the glass-ionomer cements. I. Formulations and properties. Aust. Dent. J. 22, 31-36. McLean J. W., Powis D. R., Prosser H. J. et al. (1985) The use of glass ionomer cements in bonding composite resins to dentine. Br. Dent. J. 158,410-414. Powis D. R., Foller& T., Merson S. A. et al. (1982) Improved adhesion of a glass ionomer cement to dentin and enamel. J. Dent. Res. 61, 1416-1422. Prodger T. E. and Symmonds M. (1977) ASPA adhesion study. Br. Dent. J. 143, 266-270. Swartz M. L., Phillips R. W. and Clark H. E. (1984) Longterm F release from glass ionomer cements. J. Dent. Res. 63, 158-160. Tobias R. S., Brown R. M., Plant C. G. et al. (1978) Pulpal response to a glass-ionomer cement. Br. Dent. J. 144, 345-350. Welsh E. L. and Hembree J. H. Jr (1985) Microleakage at the gingival wall with four class V anterior restorative materials. J. Prosthet. Dent. 54, 370-372. Winer B. J. (1971) Statistical Principles in Experimental Design, 2nd edn. New York, McGraw-Hill, p. 270.
271
Bond strength of a light-cured and two auto-cured glass ionomer liners J. R. Holtan, G. P. Nystrom, P. S. Olin, J. Rudney and W. H. Douglas Biomaterials
Program, School of Dentistry,
University
of Minnesota,
Minneapolis,
Minnesota,
USA
ABSTRACT Ninety-nine extracted human molar teeth were used in this study comparing the shear bond strengths on dentine of one light-cured and two auto-cured polyalkenoate (glass ionomer) cements. Bond strength can be influenced by differences in tooth structure. A balanced-incomplete block design (Hull and Nie, 1981) was used to reduce variation attributable to such differences. Cements were applied to paired dentine surfaces in combinations such that 66 tooth sides were treated with each material. A light-cured dentinal adhesive and composite resin restorative material were then placed and shear bond strength testing was conducted exactly 24 h after the completion of each specimen. Mean forces (MPa) for the three materials were compared using an appropriate analysis of variance model (balanced-incomplete-blocks) The shear bond strengths (MPa) of the light-cured liner (Espe, Seefeld/Oberbay, FRG) was 4.71 f 1.16. Vitrabond showed the greatest variance of all three materials tested, however this material’s average bond strength was greater than the maximum achieved for the other materials. Student-Newman-Keuls comparison of means showed that all cements differed significantly from each other (a = O.O5).It is concluded that the light-cured glass ionomer liner exhibited significantly better shear bond strength performance than the two auto-cured glass ionomers tested. KEY WORDS: J. Dent 1990)
1990:
Polyalkenoate (glass ionomer) cement liners, Bond strengths 18: 271-275
(Received 11 December 1989;
reviewed 26 January 1990;
accepted 10 July
Correspondence should be addressed to: Dr G. P. Nystrom, Biomaterials Program, School of Dentistry, 515 Delaware Street South East, University of Minnesota, Minneapolis, MN 55455, USA.
INTRODUCTION Polyalkenoate (glass ionomer) cements have been suggested as liners for composite resin restorations. This application is advocated because of these materials’ ability to bond both to dentine and composite resin materials (McLean and Wilson, 1977; Powis et al., 1982; McLean et al., 1985) as well as their reported improvement in microleakage performance as compared to composite resin restorative materials (Hembree and Andrews, 1978; Welsh and Hembree, 1985) the release of fluoride over time (Forsten, 1977; Swartz et al.. 1984) and their biocompatibility with dental tissues (Tobias et al., 1978; Heys et al., 1987). High bond strengths of glass ionomer cements to dentine and enamel are critical if they are to be effective in this regard. Handling properties of these materials, however, make them especially technique sensitive because of their susceptibility to moisture contamination or dehydration during the early stages of the setting reaction. Contamina0 1990 Butterworth-Heinemann 0300-5712/90/050271-05
Ltd.
tion by saliva of freshly prepared dentine surface results in a marked decrease in the bond strength (Prodger and Symmonds, 1977). Light-cured forms of glass ionomer cements are now available which may improve some of the adverse handling characteristics. A reduction in setting time because of photocuring within the cavity preparation may assist in decreasing moisture contamination and dehydration difficulties associated with the early stage of the setting reaction. The purpose of this study was to evaluate and compare the shear bond strength performance of a light-cured glass ionomer cement liner with two commonly used chemically cured glass ionomer liners. MATERIALS
AND METHODS
Study design This study compared bond strengths of three glass ionomer cements in a random sample of 99 caries-free
272
J. Dent. 1990; 18: No. 5
Table 1. Glass ionomer liners investigated Material
Source
Vitrabond
3M Dental Products Division, St Paul, MN, USA GC Dental Industrial Corp., Tokyo, Japan Espe, Seefeld/Oberbay, FRG
GC Lining Cement Ketac-Bond
Table II. Details of comparison of materials Paired tooth sides Materials compared Vitrabond GC Lining Cement Vitrabond Ketac-Bond GC Lining Cement Ketac-Bond
(no.1 vs.
33
vs.
::
vs.
:z 33
66 tooth sides were treated with each material. extracted human third molars. Bond strength can be influenced by differences in tooth structure. This was a major consideration in study design. One way of controling tooth structure effects would have been to test all three materials within each of the sample teeth. However, this would have required that teeth be divided in thirds, which would have made it difficult to obtain uniform surfaces for comparison. The alternative was to bisect teeth and compare opposing faces, however, this approach would permit only two of the three materials to be compared in any given tooth. The problem was addressed through use of a ‘balanced-incomplete-block’ study design. Balancedincomplete-block designs are employed when the number of treatments to be compared (three cements) within a ‘block’ (tooth) is greater than the number of treatments that can be physically applied to a block (only two halves of a tooth are available). In this approach, each block receives a subset of treatments. All combinations of treatments must be used, and each combination must be applied to equal numbers of blocks. When these conditions are met, data can be analysed in a manner similar to a complete randomized block design in which each block receives every treatment (Cochran and Cox, 1957; Hull and Nie, 1981). As each of the 99 sample teeth were bisected, materials were randomly assigned to each half with the stipulation that opposing halves receive different materials. This resulted in 66 halves being treated with each material, and with each of the three possible combinations of materials (A with B, B with C, and A with C) represented in 33 teeth. The glass ionomer materials investigated are detailed in Table I. Details of comparison of materials are presented in Table II.
Dentine
specimen
preparation
Teeth for this study were obtained from the School of Dentistry’s Oral Surgery clinic and several oral surgery
Class Light-cured Auto-cured Auto-cured
offices in the community. Teeth were stored briefly in distilled water at room temperature and were prepared shortly after they were obtained. None were stored longer than 30 days prior to preparation. After mechanical debridement of the teeth the surfaces were cleaned with a pumice-water slurry. Each tooth was reduced under constant water spray on mesial and distal surfaces to expose a flat surface of dentine. The tooth was then sectioned buccolingually. The paired dentinal surfaces were centred on flat Teflon disks which were placed in the ends of aluminium cylinders. The cylinders were inverted and filled with mounting medium (orthodontic resin) in two increments. Following setting of the resin, each specimen was repositioned and secured in the aluminium cylinder with the resin-encased dentinal surface protruding slightly beyond the face of the cylinder. This assembly was mounted in a jig attached to a modified lathe with an end-cutting, water-lubricated 7.5 cm diameter 600 grit diamond wheel. The specimens, with constant water irrigation, were brought orthogonally against and across the diamond wheel to obtain a flat dentine test surface (Fig. 1). This surface was then drawn 25 times across, and parallel to a dry, flat-mounted sheet of 600 grit aluminium oxide wet-or-dry sandpaper (3M, St Paul, MN, USA). A
Fig. 7. Assembly mounted in a jig attached to a modified lathe with an end-cutting, water-lubricated 7.5 cm diameter diamond wheel. The specimens were brought orthogonally against and across the diamond wheel to obtain a flat dentine test surface.
Holtan et a/.: Bond strength
Composite
resin
Alumini urn cylinder
,Teflon ‘Glass
ring ionomer
‘Mounting
resin
Fig. 2. Resin-encased dentine specimen secured in an aluminium mounting cylinder in direct apposition to a doughnut-shaped split Teflon disk bored with a 5 mm diameter hole positioned flush with the end of the cylinder. Placement of the glass ionomer liner on the dentine is followed by application of the dentinal adhesive and the composite resin restorative material.
fresh section of sandpaper was used for each specimen. Each pair of specimens was wrapped in gauze moistened with water and stored very briefly in a closed container at room temperature until placement of the glass ionomer and restorative materials. The resin-encased dentine specimen (Fig. 2) was again placed and secured in an aluminium mounting cylinder in direct apposition to a split Teflon disk bored with a 5 mm diameter hole positioned flush with the end of the cylinder. A ‘cavity’ preparation 5 mm in diameter and 3 mm in depth was thus formed by the dentinal surface and the cylindrical internal surface of the disk.
Restorative
procedures
The glass ionomer liners were applied to the entire dentinal surface of the ‘cavity preparations’ according to manufacturer’s instructions:
4 m
lnstron mounting post
Mounting resin
Fig. 3. Specimen applied.
mounted
in a jig and shear forces
liners
273
- Vitrabond was light cured for 30 s. - GC Lining Cement was placed on the dentinal surface and allowed to set for 4 min. - Ketac-Bond: Ketac-Conditioner was applied for 10 s to remove the smear layer, rinsed with water for 30 s and dried with air. Ketac-Bond was then placed on dentine and allowed to cure (set) for 4 min. After completion of the setting time the exposed surfaces of the auto-cured glass ionomer cements were etched, according to manufacturer’s instructions, with a phosphoric acid gel (Scotchbond Etching Gel, 3M) for 30 s, rinsed with water for 30 s, and dried with oil-free compressed air. The Vitrabond was not etched prior to placement of the dentine bonding adhesive. From this point, all of the restorative procedures were identical for all materials at all sites. A light-cured dentinal adhesive system (Scotchbond 2 with a primer, Scotchprep, 3M) was used which includes the immediate use of the primer. Scotchprep was constantly agitated within the confines of the preparation for 60 s and then dried for 15 s under moderate air volume (but not washed). The resin bonding agent (Scotchbond 2) was then applied, moderately air thinned, and cured under visible light for 20 s. Silux Plus resin (3M) was placed over the resin bonding agent in two increments and light cured for 60 s according to manufacturer’s instructions, the second of which was packed even with the outer surface of the Teflon split disk. The split disks were separated and the specimens carefully cleared of flash to maintain the circular 5 mm diameter interface of dentine to the ‘cavity’preparation. The sample pairs, now complete with the glass ionomers being evaluated and a composite stub, were removed from the aluminium cylinders, wrapped in water-moistened gauze, and stored as pairs in a closed container at room temperature prior to bond strength testing exactly 24 h later. Specimens were mounted in a jig and shear forces applied by the flat blade of a moveable slide (Fig. 3). The jig was mounted in an Instron testing machine (Model 4204, Instron Corporation, Canton, MA USA) which was configured with a cross-head speed of 0.5 mm/min. Peak break points (kg) were recorded for the paired specimens. Statistical
Moveable slide *
of glass ionomer
analysis
The study was similar to a paired design in that the objective was to remove variation between teeth and compare materials between tooth sides. Examples from Hull and Nie (198 1) were used to set up Statistical Package for the Social Sciences (SPSS) program MANOVA for an analysis of variance (ANOVA) of a balanced-incompleteblock design. Variation attributable to differences between teeth first was removed from the total sum of squares. Treatment effects then were tested over the residual error variance. This provided an overall test for differences between material means. Comparisons between specific materials then were carried out with the StudentNewman-Keuls test (SNK). The ANOVA residual mean
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J. Dent. 1990;
18: No. 5
Table 111.Shear bond strength values
No.
Vitrabond/GC 6.306 13.607 9.737 4.658 11.42 4.926 3.847 7.27 6.71 1 6.886 8.349 7.635 8.169 7.435 12.828 12.198 14.505 1 1.674 5.852 11.225 3.1 1 8.793 8.653 12.468 3.666 4.363 5.642 6.776 7.75 9.867 11.41 10.576 9.228
: 3 4 5 6 7 98 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Liner
Shear bond strength (MPa) Vitrabond /Ketac-Bond
4.182 3.244 4.028 4.893 3.867 2.935 2.54 1.977 3.626 3.27 3.304 3.512 2.46 4.511 2.493 1.977 2.647 3.304 1.749 2.42 2.708 1.89 3.593 3.177 2.051 3.079 2.285 3.237 1.381 2.1 18 4.698 5.098 4.899
4.437 8.094 7.29 1 1.689 6.476 7.07 9.452 8.289 11.155 10.181 1 1.789 10.05 1 6.46 1 5.542 5.283 10.216 3.626 6.226 12.633 9.727 5.947 4.484 6.386 6.271 6.871 3.733 4.672 4.53 1 5.233 9.637 10.236 11.11 8.089
square was used as the SNKerror term, which showed that SNK results also were adjusted for variation between teeth (Wirier, 197 1).
RESULTS Shear bond strength values (kg/cmz) were calculated from peak break points as the load to produce failure (kg) divided by the area (cmz, which was assumed to be a constant) of the bonding agent. These values were then converted to MPa (Table ZZZ).The balanced-incompleteblock analysis of variance showed an overall difference materials that was statistically significant among (a = 0.05) after variation due to differences among teeth Table IV. Analysis of variance Source of variation
d.f.
Mean square
Teeth Cements Residual
98 2 97
422 33359 359
F
92.89
P
< 0.00001
Balanced-incomplete-block analysis of variance of shear bond strength of the light-cured liner and two auto-cured glass ionomers. An overall difference among materials was statistically significant !t;otit5) after variation due to differences among teeth was
4.196 3.559 4.088 4.557 2.5 3.445 4.337 6.142 4.222 4.531 3.452 3.854 3.699 4.94 3.881 3.485 4.578 3.646 4.778 3.425 4.175 5.228 4.725 4.59 1 4.323 3.606 5.383 3.84 5.872 4.725 6.46 1 4.953 4.035
GC Liner / Ketac-Bond 4.731 4.866 4.283 3.364 3.096 4.765 4.597 3.344 4.068 6.536 4.088 3.686 3.478 5.507 3.572 3.425 5.418 3.445 5.482 5.812 4.839 5.173 4.799 4.782 4.108 5.872 4.866 3.928 3.954 4.645 2.493 2.929 5.418
5.447 6.446 6.311 6.59 1 4.852 4.711 5.582 7.045 4.645 4.718 5.058 6.641 4.504 4.048 6.376 4.149 6.122 5.348 5.233 3.478 4.128 5.437 1.77 6.107 7.425 3.043 5.368 5.183 5.597 3.385 2.366 4.202 6.251
was removed (Table IV). Mean shear bond strength values for each of the three materials is presented in Table V. Vitrabond showed greater bond strength (but also greater variance) than either GC Lining Cement or Ketac-Bond, while Ketac-Bond in turn displayed greater bond strength than GC Lining Cement. Student-Newman-Keuls comparison of means for individual materials showed that all cements differed significantly from each other (a = 0.05).
DISCUSSION The bond strength of the light-cured liner in the present report was significantly stronger than either of the autocured ionomers. This improvement in bond strength is due to an increase in the cohesive strength of the lightcured liner. It is widely known that the bond failure of glass ionomers is mainly cohesive, therefore immediate improvement in bond strength could be obtained by improving the cohesive strength of the material. This appears to have been achieved in the case of the lightcured liner, where the improved strength is due to the development of a new molecule, an unsaturated hydrophilic polymer which offers curing both by chemical means (polyalkenoic acid) and by the more popular
Holtan et a/.: Bond strength
of glass ionomer
liners
275
Table V. Mean shear bond strength of materials Material
n*
Mean
Vitrabond GC Lining Cement Ketac-Bond
66 66 66
8.03 3.76 4.71
+ s.d. f 2.86 + 1.17 k 1.16
Minimum
Maximum
Range
3.21 1.38 1.77
14.51 6.54 7.42
1 1.40 5.16 5.65
Each cement was significantly different from every other by the Student-Newman-Keuls (a = 0.05). *n based on the number of tooth sides treated with each material.
The light-cured liner is photo-curing (vinyl insaturation). comprised of one main molecular species, and in this respect can be distinguished from many other light-curing liners in this category, which may be mixtures of a resin and an ionomer liquid (Creo. 1988). The benefits of light curing, familiar to photo-cured composites, are also available in the light-curing ionomers. However, it is likely that the contraction of the light-curing ionomers may be greater than that of the auto-cured materials. This contraction is mostly complete before the resin and the composite material is added onto the ionomer, and therefore its effects may not be as great as originally anticipated. The light-cured liner showed greater variation in bond strength than either auto-cured cement. However, the average bond strength for the light-cured liner was greater than the maximum achieved for the other materials. Moreover, the averages for the auto-cured materials were only slightly greater than the minimum bond strength of the light-cured liner. The performances of the auto-cured materials were much closer to each other in comparison. Ketac-Bond, however, was still significantly stronger than GC Lining Cement. As noted, the problem with the autocured materials is in the slowness of set, sensitivity to clinical conditions, and above all, the decrease in cohesive strength which led to a decrease in the bond strength. The results of this study would indicate a significant advantage for photo-cured glass ionomer materials. The chief remaining advantage of the auto-cured glass ionomers, as already noted, lies in the fact that the contraction may be less. The reason for this may be the vinylization of the light-cured glass ionomer cement which tends to impart a composite-like linkage. An important consideration is that even the auto-cured materials contract (Kemp-Scholte and Davidson, 1988), but the contraction is extremely slow and may not challenge the bond that has formed. It remains to be seen whether the differences in contraction between photo-and auto-cured materials have any impact on their durability or longevity. Given the significant advantages of photocured materials, the continued use of auto-cured materials should be questioned. Research should continue comparing bond strength performance and other qualities of light-curing glass ionomer materials which are becoming available to the dental profession. In conclusion, the light-cured liner (Vitrabond) had significantly higher shear bond strength performance than the auto-cured glass ionomers tested. Although the
test
light-cured liner performed much better than either of the auto-cured glass ionomer cements tested, there was still a difference between the auto-cured materials themselves, Ketac-Bond having significantly better shear bond strength than the GC Lining Cement. Vitrabond showed the greatest variance of all three materials tested, however its average bond strength was greater than the maximum achieved for the other materials. Long-term significance of shrinkage of light-cured and auto-cured glass ionomer cements has yet to be determined.
References Cochran W. G. and Cox G. M. (1957) Experimental Design, 2nd edn. New York, Wiley, p. 443. Creo A. (1988) Technical Bulletin on Vitrabond. St Paul, MN, 3M Dental Products Division. Forsten L. (1977) Fluoride release from a glass ionomer cement. Stand. J. Dent. Res. 85, 503-504. Hembree J. H. Jr and Andrews J. T. (1978) Microleakage of several class V anterior restorative materials: a laboratory study. J. Am. Dent. Assoc. 97, 179-183. Heys R. J., Fitzgerald M., Heys D. R. et al. (1987) An evaluation of a glass ionomer luting agent: pulpal histologic response. J. Am. Dent. Assoc. 14, 607-611. Hull C. H. and Nie N. H. (1981) SPSS Update 7-9. In: New Procedures and Facilities for Release. New York, McGrawHill, pp. 18-19. Kemp-Scholte C. M. and Davidson C. L. (1988) Marginal sealing of curing contraction gaps in class V composite resin restorations. J. Dent. Res. 67, 841-845. McLean J. W. and Wilson A. D. (1977) The clinical development of the glass-ionomer cements. I. Formulations and properties. Aust. Dent. J. 22, 31-36. McLean J. W., Powis D. R., Prosser H. J. et al. (1985) The use of glass ionomer cements in bonding composite resins to dentine. Br. Dent. J. 158,410-414. Powis D. R., Foller& T., Merson S. A. et al. (1982) Improved adhesion of a glass ionomer cement to dentin and enamel. J. Dent. Res. 61, 1416-1422. Prodger T. E. and Symmonds M. (1977) ASPA adhesion study. Br. Dent. J. 143, 266-270. Swartz M. L., Phillips R. W. and Clark H. E. (1984) Longterm F release from glass ionomer cements. J. Dent. Res. 63, 158-160. Tobias R. S., Brown R. M., Plant C. G. et al. (1978) Pulpal response to a glass-ionomer cement. Br. Dent. J. 144, 345-350. Welsh E. L. and Hembree J. H. Jr (1985) Microleakage at the gingival wall with four class V anterior restorative materials. J. Prosthet. Dent. 54, 370-372. Winer B. J. (1971) Statistical Principles in Experimental Design, 2nd edn. New York, McGraw-Hill, p. 270.