Journal of Dental Research http://jdr.sagepub.com/
The Effect of Local Interfacial Geometry on the Measurement of the Tensile Bond Strength to Dentin R. Van Noort, G.E. Cardew, I.C. Howard and S. Noroozi J DENT RES 1991 70: 889 DOI: 10.1177/00220345910700050501 The online version of this article can be found at: http://jdr.sagepub.com/content/70/5/889
Published by: http://www.sagepublications.com
On behalf of: International and American Associations for Dental Research
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>> Version of Record - May 1, 1991 What is This?
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The Effect of Local Interfacial Geometry on the Measurement of the Tensile Bond Strength to Dentin R. VAN NOORT, G.E. CARDEW', I.C. HOWARD', and S. NOROOZI Department of Restorative Dentistry and 1Department of Mechanical and Process Engineering, University of Sheffield, Western Bank, Sheffield S10 2SZ, United Kingdom
The local detail of the geometry of the adhesive interface can have a significant effect on the measurement of dentin bond strengths and may be a contributory factor in the discrepancies among data in the published literature. The potential effect on the dentin bond strength due to modifications of the local stress distribution at the adhesive/dentin interface has been assessed. Tensile bond strength measurements for specimens with and without an adhesive flash were carried out and compared with the stress distribution at the adhesive interface determined by finite element stress analysis. The results showed that when the adhesive was constrained to the interface only, the tensile bond strength was 3.10 MPa, which increased to 6.90 MPa when a flash of adhesive was present. For a realistic measurement of dentin bond strength, the adhesive should be constrained to the interface only. Extension of the adhesive beyond the interface will result in an artificially high value for the dentin bond strength. A standardized method for the measurement of dentin bond strength is urgently needed, but must take these as well as all other known factors into account if results from different testing centers are to be directly comparable.
local to the interface will have a significant effect on the measurement of bond strength to dentin. Differences in local interfacial geometry can arise when different methods of applying the adhesive to the dentin surface are adopted. The manufacturer's literature for bond strength measurement data for a new dentin bonding agent (3M Dental Products, Bracknell, UK) recommends that the adhesive be applied uniformly over the whole dentin surface prior to the placement of the PTFE mould for bonding the resin composite cylinder. This technique is commonly used and will effectively result in the formation of a flash, the thickness of which will depend on how liberally or how many coats of the adhesive are applied. Alternatively, the adhesive may be constrained to the adhesive interface only. In this study, the potential effect on the dentin bond strength due to modifications of the local stress distribution at the adhesive/dentin interface was more fully explored. Tensile bond strength measurements for specimens with and without an adhesive flash were carried out and compared with the stress distribution at the adhesive interface determined when finiteelement stress analysis was used.
J Dent Res 70(5):889-893, May, 1991
Materials and methods.
Introduction. The variation in the values of bond strengths of dentin-bonding agents reported by different testing centers has been variously attributed to differences in measurement techniques, dentin surface preparation, and the age of the dentin itself (Mitchem and Gronas, 1986; Causton, 1987). In a previous study, van Noort et al. (1989) showed that the techniques commonly used for the measurement of dentin bond strength could give only a nominal value for the bond strength and not the true stress at fracture because of the non-uniform nature of the stress distribution at the interface between the adhesive and the dentin. Additionally, it was shown that the interfacial stress distribution was affected by the mode of load application, materials properties such as elastic modulus, and the size of the resin composite cylinder used. In order for the data from different centers to be directly comparable by a standardized method of testing, all the parameters affecting the measurement of the bond strength need to be identified. In this context, van Noort et al. (1989) also noted that the presence of a small fillet or flash can have a profound effect on the detail of the local stress distribution at the edge of the interface between the adhesive and the dentin, where there is an inherent stress concentration due to the geometry of the system. As a consequence of these observations, it is speculated that the detail of the geometry Received for publication August 8, 1990 Accepted for publication January 16, 1991 This research was supported by the Trent Regional Health Authority Locally Organized Research Fund.
The materials used for the experimental component of the work were a relatively new dentin bonding agent (Scotchbond2, 3M Dental Products, Bracknell, UK) and a complementary posterior resin composite (P-50, 3M Dental Products, Bracknell, UK). The dentin-bonding agent consists of a dentin-surface primer of an aqueous solution of hydroxyethyl methacrylate (HEMA) and maleic acid and a dentin adhesive of HEMA, Bis-GMA, a photo-initiator, and viscosity controllers. The dentin primer is hydrophilic, readily wets the dentin surface, and is believed to solubilize the dentin smear layer. The adhesive, by virtue of the presence of the HEMA, penetrates this solubilized layer and, after polymerization, is locked into it, both sealing the surface and making it more hydrophobic and able to accept the resin composite. The resin composite then bonds to the methacrylate groups of the HEMA and the Bis-GMA by free radical polymerization. Sample preparation and measurement of bond strength. The crowns of 24 caries-free extracted human molar teeth, previously stored in tap water, were sectioned to expose the dentin surface. The teeth were embedded in a clear polyester resin (Buehler UK Ltd, Coventry, UK), and the surface was ground flat with 150-, then 320-, and (finally) 600-grit sandpaper under continuous water spray on a flat-bed grinder. The surfaces were thoroughly washed for 60 s with copious water and blown dry for a further 60 s. The dentin primer was applied liberally with a brush for 60 s and continuously agitated to allow efficient reaction with the smear layer. The surface was then dried with oil-free compressed air for 60 s. At no stage was the primer washed off the surface. In order for samples with and without a flash of adhesive to be obtained, the teeth were split randomly into two equal groups of 12 specimens each.
Downloaded from jdr.sagepub.com at University of Leeds on November 2, 2014 For personal use only. No other uses without permission.
889
890
VAN NOORT et al.
J Dent Res May 1991
2 mm
50 gm Adhesive
pin
Embedded tooth
+-
6 mm
i
Fig. 2-Mesh designs for group B samples.
Fig. 1-Test assembly for the measurement of tensile bond strengths.
Group A. -For this group, the adhesive was applied uniformly and liberally over the whole surface of the dried primed dentin, according to the manufacturer's instructions. No attempt was made to remove excess adhesive, although care was taken that the adhesive did not pool. The uniform layer of adhesive was cured for 20 s with a Luxor (ICI Dental Products, Macclesfield, UK) visible-light-curing unit. This process resulted in a uniform coating of the adhesive with a thickness of approximately 50 jlm, as estimated later from SEM micrographs of replicas of the fracture surface. A resin composite cylinder was subsequently built up using a PTFE mould that was designed to ensure that the resin composite cylinder was central to the block containing the dentin and perpendicular to the surface. The resin composite was packed and cured (with the visible-light-curing unit) in increments of 2 mm, with each increment cured for 60 s. The shape of the resin composite cylinder was that of half a dumbbell, with a diameter of 4 mm at the adhesive interface, broadening out to 6 mm. The total length of the cylinder was 6 mm. A metal pin was bonded to the resin composite cylinder with a resin-bonded bridge cement
(Kerr UK Ltd, Peterborough, UK), and the full assembly is shown schematically in Fig. 1. Group B. -This group was treated differently from group A in that the PTFE mould was clamped in position on the dentin surface after the surface was treated with the dentin primer only. The adhesive was then applied down the central hole in the PTFE mould and cured for 20 s. Care was taken that the adhesive did not penetrate between the PTFE mould and the dentin surface. This procedure meant that the adhesive was applied only to the interfacial area and not over the whole surface of the dentin, as for group A above. Therefore, this group did not contain a flash of adhesive. Further preparation was the same as for group A. All the samples were given an additional cure of 60 s after removal of the PTFE mould and then stored for 24 h in distilled water at 370C prior to being tested. The tensile bond strength measurements were performed on a tensile testing machine (Lloyd Instruments Plc, Southampton, UK) at a cross-head speed of 2 mm/min. The tensile bond strength was calculated by dividing the force at failure by the cross-sectional area of the resin composite cylinder. The results were analyzed for statistical significance by one-way analysis of variance. Impressions were taken of selected fracture surfaces by use of an addition-cured silicone impression material. These were replicated in resin, gold-coated, and examined under a scanning electron microscope (Philips P-500). Measurement of interfacial stress distribution. -Two-dimensional axisymmetric models with eight-noded isoparametric elements were prepared of the two geometries
Downloaded from jdr.sagepub.com at University of Leeds on November 2, 2014 For personal use only. No other uses without permission.
Vol. 70 No. 5
891
GEOMETRIC FACTORS AND DENTIN BOND STRENGTH ayy (MPa) 1 =2.22 2=4.65 3=7.07 4=9.50 5=1 1.92 6=1 4.35 7=1 6.77 8=1 9.20 9=21.62 1 0=24.05 1 1 =26.47 12=28.90 13=31.32 14=33.74
Fig. 4-The pattern of stress distribution in the region of the edge between the adhesive and the dentin for Group B. Fig. 3-The pattern of stress distribution in the region of the edge between the adhesive and the dentin for Group A.
and the dentin, with no discernible adhesive remaining on the dentin surface.
representing groups A and B. The mesh design for the condition with the constrained adhesive interface (group B) is shown in Fig. 2 and consisted of 628 elements with 3800 degrees of freedom. The thickness of the adhesive layer was 50 Aim. The results were produced with use of the University of Sheffield's own finite element program, TOMECH. The elastic moduli of the dentin, the adhesive, and the resin composite were 15 GPa, 4 GPa, and 20 GPa, respectively. A distributed load was applied to the top surface of the resin composite cylinder, which produced an interfacial nominal tensile stress of 10 MPa.
Results. The data presented in the Table show that the tensile bond strength to dentin was significantly affected by the method of application of the adhesive. In this particular experimental arrangement, a two-fold increase in the tensile bond strength resulted when the adhesive was applied to the whole of the dentin surface prior to the bonding of the resin composite cylinder, compared with the condition where the adhesive was constrained to the same area on the dentin as the resin composite cylinder. In the samples where the adhesive was constrained, the tensile bond strength was 3.10 MPa, which increased to 6.90 MPa when the adhesive was spread over the whole of exposed dentin. The local detail of the stress distribution for tensile stresses acting perpendicular to the adhesive interface is shown in Figs. 3 and 4 for Group A and Group B geometries, respectively. Two examples of the SEM appearance of the edge of the fracture surface of samples in Group A are shown in Figs. 5 and 6. These clearly show the adhesive layer extending beyond the interface. For Group B, the fracture appearance was that of a clean separation between the adhesive
Discussion. The reasons for the large discrepancy in the tensile bond strength data for the two approaches can be found in the effects on the local stress distribution in the region near the interface arising from the different methods of application of the adhesive. Both computer models (Figs. 3 and 4) show that there is a large stress concentration around the rim of the resin composite cylinder where it meets the dentin surface, but the exact locations of these stress concentrations are different. The local stress pattern for Group A (Fig. 3) shows that a stress concentration is situated around the circumference of the resin composite cylinder at the adhesive/resin composite interface. The SEM micrograph of Fig. 5 shows that where the fracture comes to the surface is coincident with the position of this stress concentration. This suggests that fracture may have initiated from some point around the circumference at the adhesive/resin composite interface where there was a flaw of a critical size that subsequently traveled along the adhesive/dentin interface. Consequently, the tensile bond strength deterTABLE TENSILE BOND STRENGTHS OF SCOTCHBOND-2 AND P-50 TO DENTIN FOR TWO DIFFERENT TEST GEOMETRIES Standard Mean MPa Deviation Range 2.76-9.49 2.05 6.90 Group A
(with flash) 3.10 Group B (without flash) t-value = 4.692; p
The Effect of Local Interfacial Geometry on the Measurement of the Tensile Bond Strength to Dentin R. Van Noort, G.E. Cardew, I.C. Howard and S. Noroozi J DENT RES 1991 70: 889 DOI: 10.1177/00220345910700050501 The online version of this article can be found at: http://jdr.sagepub.com/content/70/5/889
Published by: http://www.sagepublications.com
On behalf of: International and American Associations for Dental Research
Additional services and information for Journal of Dental Research can be found at: Email Alerts: http://jdr.sagepub.com/cgi/alerts Subscriptions: http://jdr.sagepub.com/subscriptions Reprints: http://www.sagepub.com/journalsReprints.nav Permissions: http://www.sagepub.com/journalsPermissions.nav Citations: http://jdr.sagepub.com/content/70/5/889.refs.html
>> Version of Record - May 1, 1991 What is This?
Downloaded from jdr.sagepub.com at University of Leeds on November 2, 2014 For personal use only. No other uses without permission.
The Effect of Local Interfacial Geometry on the Measurement of the Tensile Bond Strength to Dentin R. VAN NOORT, G.E. CARDEW', I.C. HOWARD', and S. NOROOZI Department of Restorative Dentistry and 1Department of Mechanical and Process Engineering, University of Sheffield, Western Bank, Sheffield S10 2SZ, United Kingdom
The local detail of the geometry of the adhesive interface can have a significant effect on the measurement of dentin bond strengths and may be a contributory factor in the discrepancies among data in the published literature. The potential effect on the dentin bond strength due to modifications of the local stress distribution at the adhesive/dentin interface has been assessed. Tensile bond strength measurements for specimens with and without an adhesive flash were carried out and compared with the stress distribution at the adhesive interface determined by finite element stress analysis. The results showed that when the adhesive was constrained to the interface only, the tensile bond strength was 3.10 MPa, which increased to 6.90 MPa when a flash of adhesive was present. For a realistic measurement of dentin bond strength, the adhesive should be constrained to the interface only. Extension of the adhesive beyond the interface will result in an artificially high value for the dentin bond strength. A standardized method for the measurement of dentin bond strength is urgently needed, but must take these as well as all other known factors into account if results from different testing centers are to be directly comparable.
local to the interface will have a significant effect on the measurement of bond strength to dentin. Differences in local interfacial geometry can arise when different methods of applying the adhesive to the dentin surface are adopted. The manufacturer's literature for bond strength measurement data for a new dentin bonding agent (3M Dental Products, Bracknell, UK) recommends that the adhesive be applied uniformly over the whole dentin surface prior to the placement of the PTFE mould for bonding the resin composite cylinder. This technique is commonly used and will effectively result in the formation of a flash, the thickness of which will depend on how liberally or how many coats of the adhesive are applied. Alternatively, the adhesive may be constrained to the adhesive interface only. In this study, the potential effect on the dentin bond strength due to modifications of the local stress distribution at the adhesive/dentin interface was more fully explored. Tensile bond strength measurements for specimens with and without an adhesive flash were carried out and compared with the stress distribution at the adhesive interface determined when finiteelement stress analysis was used.
J Dent Res 70(5):889-893, May, 1991
Materials and methods.
Introduction. The variation in the values of bond strengths of dentin-bonding agents reported by different testing centers has been variously attributed to differences in measurement techniques, dentin surface preparation, and the age of the dentin itself (Mitchem and Gronas, 1986; Causton, 1987). In a previous study, van Noort et al. (1989) showed that the techniques commonly used for the measurement of dentin bond strength could give only a nominal value for the bond strength and not the true stress at fracture because of the non-uniform nature of the stress distribution at the interface between the adhesive and the dentin. Additionally, it was shown that the interfacial stress distribution was affected by the mode of load application, materials properties such as elastic modulus, and the size of the resin composite cylinder used. In order for the data from different centers to be directly comparable by a standardized method of testing, all the parameters affecting the measurement of the bond strength need to be identified. In this context, van Noort et al. (1989) also noted that the presence of a small fillet or flash can have a profound effect on the detail of the local stress distribution at the edge of the interface between the adhesive and the dentin, where there is an inherent stress concentration due to the geometry of the system. As a consequence of these observations, it is speculated that the detail of the geometry Received for publication August 8, 1990 Accepted for publication January 16, 1991 This research was supported by the Trent Regional Health Authority Locally Organized Research Fund.
The materials used for the experimental component of the work were a relatively new dentin bonding agent (Scotchbond2, 3M Dental Products, Bracknell, UK) and a complementary posterior resin composite (P-50, 3M Dental Products, Bracknell, UK). The dentin-bonding agent consists of a dentin-surface primer of an aqueous solution of hydroxyethyl methacrylate (HEMA) and maleic acid and a dentin adhesive of HEMA, Bis-GMA, a photo-initiator, and viscosity controllers. The dentin primer is hydrophilic, readily wets the dentin surface, and is believed to solubilize the dentin smear layer. The adhesive, by virtue of the presence of the HEMA, penetrates this solubilized layer and, after polymerization, is locked into it, both sealing the surface and making it more hydrophobic and able to accept the resin composite. The resin composite then bonds to the methacrylate groups of the HEMA and the Bis-GMA by free radical polymerization. Sample preparation and measurement of bond strength. The crowns of 24 caries-free extracted human molar teeth, previously stored in tap water, were sectioned to expose the dentin surface. The teeth were embedded in a clear polyester resin (Buehler UK Ltd, Coventry, UK), and the surface was ground flat with 150-, then 320-, and (finally) 600-grit sandpaper under continuous water spray on a flat-bed grinder. The surfaces were thoroughly washed for 60 s with copious water and blown dry for a further 60 s. The dentin primer was applied liberally with a brush for 60 s and continuously agitated to allow efficient reaction with the smear layer. The surface was then dried with oil-free compressed air for 60 s. At no stage was the primer washed off the surface. In order for samples with and without a flash of adhesive to be obtained, the teeth were split randomly into two equal groups of 12 specimens each.
Downloaded from jdr.sagepub.com at University of Leeds on November 2, 2014 For personal use only. No other uses without permission.
889
890
VAN NOORT et al.
J Dent Res May 1991
2 mm
50 gm Adhesive
pin
Embedded tooth
+-
6 mm
i
Fig. 2-Mesh designs for group B samples.
Fig. 1-Test assembly for the measurement of tensile bond strengths.
Group A. -For this group, the adhesive was applied uniformly and liberally over the whole surface of the dried primed dentin, according to the manufacturer's instructions. No attempt was made to remove excess adhesive, although care was taken that the adhesive did not pool. The uniform layer of adhesive was cured for 20 s with a Luxor (ICI Dental Products, Macclesfield, UK) visible-light-curing unit. This process resulted in a uniform coating of the adhesive with a thickness of approximately 50 jlm, as estimated later from SEM micrographs of replicas of the fracture surface. A resin composite cylinder was subsequently built up using a PTFE mould that was designed to ensure that the resin composite cylinder was central to the block containing the dentin and perpendicular to the surface. The resin composite was packed and cured (with the visible-light-curing unit) in increments of 2 mm, with each increment cured for 60 s. The shape of the resin composite cylinder was that of half a dumbbell, with a diameter of 4 mm at the adhesive interface, broadening out to 6 mm. The total length of the cylinder was 6 mm. A metal pin was bonded to the resin composite cylinder with a resin-bonded bridge cement
(Kerr UK Ltd, Peterborough, UK), and the full assembly is shown schematically in Fig. 1. Group B. -This group was treated differently from group A in that the PTFE mould was clamped in position on the dentin surface after the surface was treated with the dentin primer only. The adhesive was then applied down the central hole in the PTFE mould and cured for 20 s. Care was taken that the adhesive did not penetrate between the PTFE mould and the dentin surface. This procedure meant that the adhesive was applied only to the interfacial area and not over the whole surface of the dentin, as for group A above. Therefore, this group did not contain a flash of adhesive. Further preparation was the same as for group A. All the samples were given an additional cure of 60 s after removal of the PTFE mould and then stored for 24 h in distilled water at 370C prior to being tested. The tensile bond strength measurements were performed on a tensile testing machine (Lloyd Instruments Plc, Southampton, UK) at a cross-head speed of 2 mm/min. The tensile bond strength was calculated by dividing the force at failure by the cross-sectional area of the resin composite cylinder. The results were analyzed for statistical significance by one-way analysis of variance. Impressions were taken of selected fracture surfaces by use of an addition-cured silicone impression material. These were replicated in resin, gold-coated, and examined under a scanning electron microscope (Philips P-500). Measurement of interfacial stress distribution. -Two-dimensional axisymmetric models with eight-noded isoparametric elements were prepared of the two geometries
Downloaded from jdr.sagepub.com at University of Leeds on November 2, 2014 For personal use only. No other uses without permission.
Vol. 70 No. 5
891
GEOMETRIC FACTORS AND DENTIN BOND STRENGTH ayy (MPa) 1 =2.22 2=4.65 3=7.07 4=9.50 5=1 1.92 6=1 4.35 7=1 6.77 8=1 9.20 9=21.62 1 0=24.05 1 1 =26.47 12=28.90 13=31.32 14=33.74
Fig. 4-The pattern of stress distribution in the region of the edge between the adhesive and the dentin for Group B. Fig. 3-The pattern of stress distribution in the region of the edge between the adhesive and the dentin for Group A.
and the dentin, with no discernible adhesive remaining on the dentin surface.
representing groups A and B. The mesh design for the condition with the constrained adhesive interface (group B) is shown in Fig. 2 and consisted of 628 elements with 3800 degrees of freedom. The thickness of the adhesive layer was 50 Aim. The results were produced with use of the University of Sheffield's own finite element program, TOMECH. The elastic moduli of the dentin, the adhesive, and the resin composite were 15 GPa, 4 GPa, and 20 GPa, respectively. A distributed load was applied to the top surface of the resin composite cylinder, which produced an interfacial nominal tensile stress of 10 MPa.
Results. The data presented in the Table show that the tensile bond strength to dentin was significantly affected by the method of application of the adhesive. In this particular experimental arrangement, a two-fold increase in the tensile bond strength resulted when the adhesive was applied to the whole of the dentin surface prior to the bonding of the resin composite cylinder, compared with the condition where the adhesive was constrained to the same area on the dentin as the resin composite cylinder. In the samples where the adhesive was constrained, the tensile bond strength was 3.10 MPa, which increased to 6.90 MPa when the adhesive was spread over the whole of exposed dentin. The local detail of the stress distribution for tensile stresses acting perpendicular to the adhesive interface is shown in Figs. 3 and 4 for Group A and Group B geometries, respectively. Two examples of the SEM appearance of the edge of the fracture surface of samples in Group A are shown in Figs. 5 and 6. These clearly show the adhesive layer extending beyond the interface. For Group B, the fracture appearance was that of a clean separation between the adhesive
Discussion. The reasons for the large discrepancy in the tensile bond strength data for the two approaches can be found in the effects on the local stress distribution in the region near the interface arising from the different methods of application of the adhesive. Both computer models (Figs. 3 and 4) show that there is a large stress concentration around the rim of the resin composite cylinder where it meets the dentin surface, but the exact locations of these stress concentrations are different. The local stress pattern for Group A (Fig. 3) shows that a stress concentration is situated around the circumference of the resin composite cylinder at the adhesive/resin composite interface. The SEM micrograph of Fig. 5 shows that where the fracture comes to the surface is coincident with the position of this stress concentration. This suggests that fracture may have initiated from some point around the circumference at the adhesive/resin composite interface where there was a flaw of a critical size that subsequently traveled along the adhesive/dentin interface. Consequently, the tensile bond strength deterTABLE TENSILE BOND STRENGTHS OF SCOTCHBOND-2 AND P-50 TO DENTIN FOR TWO DIFFERENT TEST GEOMETRIES Standard Mean MPa Deviation Range 2.76-9.49 2.05 6.90 Group A
(with flash) 3.10 Group B (without flash) t-value = 4.692; p
