J. Dent. 1992;
20: 225-230
225
Dentine bonding agents-characteristic bond strength as a function of dentine depth J. F. McCabe and S. Rusby Dental School, University
of Newcastle
upon Tyne, UK
ABSTRACT The tensile bond strength to dentine of four dentine bonding agents has been measured at various depths within dentine and the results analysed using Weibull analysis. There were significant differences in the bond strength values among the four products particularly with near-surface dentine. Despite the differences in chemical composition among the materials the trend of decreasing bond strength with increasing depth of dentine was common to them all. This suggests that the underlying mechanism of adhesion may be common to all four products despite their differing chemical compositions. KEY WORDS:
Dentine bonding agents, Bond strength,
J. Dent. 1992; January 1992) Correspondence Prosthodontics,
20: 225-230
(Received
15 November
Dentine depth 199 1; reviewed
are
1991;
accepted 29
should be addressed to: Dr J. F. McCabe, Dental Materials Science Unit, Department Dental School, Framlington Place, Newcastle upon Tyne NE2 4BW. UK.
INTRODUCTION There
16 December
now
a variety
of
materials
available
for
achieving bonding between resin-based composite filling materials and dentine. The chemical composition of the products varies considerably and it is far from clear in many cases how the bonding is achieved. The removal or modification of the dentine smear layer appears to play a significant part in the mechanism through which the dentine and resin become linked. The compositions of some of the available products are reported in various texts (e.g. Setcos, 1988). Since the chemical composition of the materials varies markedly it would be expected that the mechanism of bonding would be equally variable. One means of achieving some insight into the mechanism of bonding is to study the relationship between bond strength and the depth of dentine. Causton (1984) for example found that the bond strength of Scotchbond. an adhesive based on a chlorinated phosphate ester of BisGMA, was significantly lower for deep dentine than for upper dentine. Likewise, Suzuki and Finger (1988) reported a significant reduction in bond strength for three materials when comparing upper dentine to deep dentine and related this to residual @ 1992 Butterworth-Heinemann 0300-57 12/92/040225-06
Ltd.
of
area of solid dentine-implying that a bond to the residual bulk dentine is responsible for adhesion. In contrast to Causton (1984) Tao and Pashley (1988) reported that the variation in bond strength with dentine depth for Scotchbond was only evident when the smear layer was removed by acidic pretreatment, which had the effect of significantly lowering the mean bond strength irrespective of dentine depth. Many of the most recently developed materials involve a proposed concept of achieving adhesion which is different from that of products studied previously. It was therefore decided to investigate the relationship between tensile bond strength and depth of dentine in the hope that this would give some insight into the mechanism of bonding.
MATERIALS
AND METHODS
Four commercially available dentine bonding agents were used in the study: Gluma (Bayer, Leverkusen, Germany, Tenure (Den-Mat Corp., Santa Maria, CA, USA), Scotchbond 2 (3M Dental Products, St Paul, MN, USA) and Tripton (ICI, Macclesfield, UK).
226
J. Dent.
1992;
20:
No. 4
1
.o0
0
8
0.8-
0
Stress
Tensile bond strength testing was carried out using a method similar to that described previously (Walls et al., 1985; Welbury et al., 1988). Caries-free permanent molars were used in the study. They were stored for up to 1 week after extraction in 10 per cent formal saline and were then rinsed and transferred into distilled water and stored for a further 4-8 weeks in a refrigerator before use. There is some controversy over the storage of teeth in formaldehyde-containing solutions prior to bond testing. Storage in this medium was therefore confined to a few days following extraction. The occlusal surface of each test tooth was ground away using 180 grit wet and dry paper and then the teeth were mounted in polyester resin with the occlusal surface being close to one flat end of the cylindrical block of resin. The end of the resin block was ground to remove resin until a complete section of occlusal dentine was exposed. The grinding procedure was carried out in a jig involving a vertical drill stand mounted on a horizontal surface grinder. This ensured that the ground surface was perpendicular to the axis of the resin cylinder block. 180 grit wet and dry carborundum paper was used for initial rapid removal whilst 800 grit paper was used for final finishing under water irrigation. The first bond strength determination was made after removing all the occlusal enamel and exposing a complete layer of dentine. Further tests were carried out with each tooth after removing a further 0.5 mm of dentine by grinding with 800 grit paper. Bonds were prepared using an alignment jig and Teflon (PTFE) insert. The insert established an area for bonding of 5 mm diameter at the centre of the dentine and ensured that the flat exposed surface of composite material was parallel to the initial ground surface of dentine. The insert was 2 mm thick so the adhesive and overlying composite tilled a space of 5 mm diameter by 2 mm high.
10 Stress
(MPa)
Fig. 7. Cumulative probability of failure against stress for composite resin bonded to dentine with Gluma. Different symbols reflect different depths within dentine as follows: 0, near surface dentine; A, 0.5 mm deep; *, 1 .O mm deep; Cl, 1.5 mm deep.
5
15
20
25
(MPa)
fig. 2. Cumulative probability of failure against stress for composite resin bonded to dentine with Tenure. For key see Fig. 7.
All the adhesives were applied according to the manufacturer’s recommended procedures and the cavity was finally filled with Occlusin composite (ICI, Macclestield, UK) which was covered with a matrix and cured with a Luxor unit (ICI, Macclesfield, UK) for 60 s. After 5 min the alignment jig was disassembled, the Teflon insert removed and the bonded specimens placed in an incubator for storage at 37°C and 100 per cent relative humidity for 7 days prior to bond strength testing. After 7 days of storage the specimens were removed from the storage incubator and a PMMA (Perspex; ICI) rod was bonded to the surface of the composite resin using cyanoacrylate cement. The ends of the short sections of acrylic rod were flattened using the same grinding jig as that used for the dentine surfaces to ensure continued alignment. A radial hole through the Perspex rod facilitated attachment to the test machine. The tensile bond strength between dentine and adhesive was measured using a cross-head speed of 1 mm mine1 on an Instron testing machine. Thirty teeth were nominally used to test each adhesive. The use of teeth was discontinued if, following grinding, any part of the pulp chamber became exposed. Hence, in each case 30 results were obtained for the outer dentine and fewer for the deeper layers. In addition, a few bonds were inadvertently disturbed or fractured prior to testing and these were discounted from the analysis. It was not possible to obtain results at greater than 1.5 mm depth due to insufficient numbers of teeth remaining at this stage.
RESULTS The bond strength results were analysed using Weibull analysis (McCabe and Carrick, 1986; McCabe and Walls, 1986). Figs I-4 show plots of probability of failure against
McCabe
and Rusby: Bond strength
of dentine
bonding
agents
227
1 .oA 0.8-
0
2
4
8
6
Stress
,$ ._ n x
0.6-
:
0.4-
0
0
O0
8’
12
10
AA
5
(MPa)
10 Stress
15
(MPa)
Fig.
3. Cumulative probability of failure against stress for composite resin bonded to dentine with Scotchbond 2. For key see Fig. 1.
Fig. 4. Cumulative probability of failure against stress for composite resin bonded to dentine with Tripton. For key see Fig. 7.
stress for each adhesive system and at each depth of dentine tested. In each case there is a trend for the bond strength to dentine to decrease as the depth of dentine increases. Table I summarizes the results giving the characteristic strength and Weibull modulus for each test group. Table I also gives the observed modes of failure for each material at all the depths of dentine studied. Failures were either apparently adhesive at the dentine surface or mixed adhesive/cohesive at the dentine surface and within the adhesive or composite. There were no completely cohesive failures and no cohesive failures within dentine.
means of overcoming this problem. In the Weibull analysis, the characteristic strength (also sometimes called the normalizing parameter) is the equivalent of the mean strength in a normal distribution whilst the Weibull modulus, m, gives an indication of reliability and can be considered the equivalent of the standard deviation in a normal distribution. The higher the value of m the more closely grouped are the observed values and therefore the more reliable the value of characteristic strength in determining the true strength of a material or bond. The values of m recorded in Table 1 are all considered to be very low, confirming that the bond strength produced with each material is very variable. As reported previously (McCabe and Carrick, 1986; McCabe and Walls, 1986; Meechan and McCabe, 1986), one way of comparing the results from such tests is to calculate the probability of failure at a given value of stress. Table I1 gives values of probability of failure at a stress of 1 MPa. Treating the results in this way enables the results to be interpreted in a manner which is potentially more meaningful. Hence, from Table II, at a stress of 1 MPa there is less than a 3 per
DISCUSSION Tensile bond strength testing invariably produces results having a wide scatter. Standard deviations of mean bond strength values are often equivalent to or greater than the mean values and in such cases the data clearly do not tit a Gaussian distribution. The use of Weibull statistics is one
Table 1. Tensile bond strength values: characteristic modulus (m) at various depths within dentine
0.5
Omm 00
Gluma
4.6 2.2 ( 1 OOA)
Tenure
Scotchbond
m
10.4 (37A: 2
1.7 63M)
5.0 2.5 (1 OOA)
mm
00
strength
1.0
m
3.4 1.3 (1 OOA)
3.2
1.2
(1OOA)
4.3 2.2 (1 OOA)
(%A
2.4
(%!A:
1 :ii,
: 2:;) 1.6
(1 OOA) 2.8
(1OOA)
Figures in parentheses indicate percentage of specimens failing in adhesive mode (M).
adhesive/cohesive
mm
00
m
2.8
1.1
(1OOA)
3.2 1::)
2.7 4:;)
1.5
CT0
5.1 (85A:
and Weibull
mm
m
Tripton (5%:
(oo (MPa))
1.8
(1OOA)
2.2
1.1
( 1 OOA) 2.3 1.8 ( 1 OOA) mode (A) or mixed
228
J. Dent. 1992;
20:
No. 4
Fig. 5. Surface of dentine after treatment with Gluma cleaner (conditioner).
Fig. 6. Surface of dentine after treatment conditioner.
Fig. 7. Surface of dentine after debonding following use of Tenure bonding agent.
Fig. 8. Surface of dentine following
cent probability of failure for each adhesive bonded to surface dentine, with Tripton having a probability of less than 1 per cent. For deep dentine (1.5 mm deep) the probability of failure is between 17 and 33 per cent. Perhaps the most surprising thing about the results is the consistency with which bond strength varies with dentine depth amongst the four adhesives. Despite the fact that there are significant differences in composition
between the materials and that their mode of action may be expected to vary, the pattern of changing bond strength with dentine depth suggests that theunderlyingmechanism is the same in all cases. Comparison of Table I with Table II shows that those adhesives with higher values of bond strength are more likely to exhibit some cohesive bond failure. Also, for Tenure and Tripton. the proportion of cohesive failure reduced as dentine depth increased. This trend mirrored the reduction in bond strength in both cases. Apart from differences in composition of the four adhesives another major difference between the products is the way in which the dentine is treated prior to bond formation. With Tenure and Gluma the dentine is treated with an acid (nitric) or a chelating agent (EDTA) and then rinsed. This has the effect of removing the dentine smear layer and exposing dentinal tubules. In the case of Gluma
Table II. Probability of failure at stress of 1 MPa at various depths within dentine
Gluma Tenure Scotchbond 2 Tripton
Omm
0.5 mm
l.Omm
1.5mm
0.03 0.02 0.02 < 0.01
0.19 0.09 0.04 0.07
0.23 0.12 0.22 0.06
0.29 0.17 0.33 0.20
with Tenure
conditioning
with
Scotchbond 2 primer (Scotchprep).
McCabe and Rusby: Bond strength of dentine bonding agents
the dentine surface is left smooth with the tubules wide open (Fig 5). Clearly, if mechanical interlocking into open tubules were the primary mechanism of bonding Gluma would appear to have an advantage. It is clear from Table I that the apparent ability of Gluma to provide a means of direct mechanical interlocking into open tubules has not produced a higher characteristic bond strength than those products in which this mechanism is not available. Of interest however is the fact that this product is the least affected by dentine depth and in the deepest dentine its characteristic bond strength is higher than two of the other products. For Tenure, treatment of the dentine with conditioner, followed by rinsing leaves a surface bereft of smear layer (Fig. 6). The outline of the tubules appears etched but the majority appear to remain occluded with the only possibility of mechanical attachment being in the fissures surrounding the blocked tubule openings. The debonded Tenure/Dentine surface (Fig. 7) shows some areas which are essentially unchanged from the structure seen in Fig. 6, but there is evidence of some cohesive failure within the bonding resin, some parts of which appear to be tenaciously attached to the underlying dentine confirming the observations reported in Table II. This product has the highest values of characteristic bond strength in superficial dentine but the value decreases markedly in deeper dentine to approach that for other products. For Scotchbond 2, the surface of dentine treated with the acidic primer (Scotchprep) was not of uniform structure. Typically, some areas showed evidence of an intact smear layer surrounding areas in which the smear layer had been disturbed and tubules opened (Fig 8). One possible explanation for the apparently undisturbed areas is that they represent regions in which the smear layer has been redeposited on the surface along with the adhesion promoter. This would certainly concur with the manufacturer’s view on the mode of action for this material. For Tripton, initial dentine treatment is with a mildly acidic surface active agent (polyhexanide). This produces a surface which is similar in appearance to that for Scotchbond 2 (Fig. 8). The results confirm that the mechanism of adhesion is almost certainly multifactorial in nature. Mechanical attachment through penetration of bonding resins into dentinal tubules would not appear to be a prerequisite for high bond strengths since those products where this mechanism was evident did not have the highest tensile bond strength values. A similar finding was made by Tao and Pashley (1988) when investigating light-cured Scotchbond, an adhesive based upon a phosphate ester of BisGMA. It is tempting to speculate that the variation of bond strength with depth of dentine implies an important role for the mineral content of the dentine in bond formation for each of the materials. This is possible although it is unlikely that any direct chemical bonding to calcium
229
occurs except possibly in Tripton which contains a phosphate adhesion promoter. The pattern of decreasing bond strength with decreasing thickness of dentine follows that previously reported for a phosphate ester adhesive (Scotchbond). as reported by Causton (1984) and Mitchem and Gronas (1986). Tao and Pashley (1988) reported a similar finding for this material but only after the smear layer had been removed by acidic pretreatment. The reasoning for this observation was that the bond strength may be a function of the degree of mineralization and this suggests a specific involvement of mineral in the bond formation. The fact that the same pattern of bond strength variation occurs even with products which are unlikely to show specific affinity for apatite minerals casts doubts on the proposed mechanism of adhesion. Suzuki and Finger (1988) proposed a relationship between bond strength and area of solid dentine for three different materials. The results of the present study may be seen to confirm these findings, given the relationship between residual dentine thickness and area of solid dentine as demonstrated by Suzuki and Finger (1988). This finding however does not, on its own, offer any insight into the mechanism of bonding. Bonding through the formation of a ‘hybrid layer’ involving a mixture of intertubular dentine, smear dentine and resin has been proposed as a possible mechanism of bonding for some materials (Nakabayashi et al., 1991). The results of this study and those of Suzuki and Finger (1988) are compatible with the formation of a hybrid layer attached primarily to intertubular dentine, although in this study there was no direct evidence of such a layer being formed. For those materials in which the smear layer is completely removed prior to bonding the hybrid layer must be formed between bulk intertubular dentine and resin and one could speculate that the exceptionally close adaptation of resin to dentine observed for Tenure (Fig. 7) can only be achieved by penetration of resin into a porous dentine surface-as proposed in the ‘hybrid-layer’ theory. Given the importance of the dentine smear layer in the bonding process it is important to consider whether the nature of such a layer may be affected by the method of its production. The use of abrasive papers, in this work, may not give a smear layer of the same structure as that produced by dental instruments. Tao et al. (1988) reported that similar bond strengths were obtained for one adhesive system when it was bonded to a smear layer produced by either abrasive papers (320 and 600 grit) or burs. This suggests that preparing dentine with abrasive papers prior to bond studies is acceptable. Since the prerequisite for good bonding in any system is effective wetting of the dentine surface by the adhesive, it is possible that the results obtained for each system can be explained by the fact that the resin is less capable of wetting deep dentine than near surface dentine. It may be concluded from this work that all four adhesives studied exhibit greater characteristic bond
230
J. Dent.
strength
to near surface
that acidic tubules
1992;
pretreatments
20:
dentine which
are not a prerequisite
No. 4
than result
to deep dentine in patently
to effective
and
opened
bonding.
References Causton B. E. (1984) Improved bonding of composite restorative to dentine. Br. Dent J. 156, 93-95. McCabe J. F. and Carrick T. E. (1986) A statistical approach to the mechanical testing of dental materials. Dent. Mater. 2, 139-142. McCabe J. F. and Walls A. W. G. (1986) The treatment of results for tensile bond strength testing. J. Dent. 14, 165-168. Meechan J. G. and McCabe J. F. (1986) A model for investigating the probability of fracture of dental local anaesthetic cartridges. Br. Dent. J. 160, 326-328. Mitchem J. C. and Gronas D. G. (1986) Effects of time after extraction and depth of dentin on resin dentin adhesives. J. Am. Dent. Assoc. 113,285-287.
Nakabayashi N.. Nakamura M. and Yasuda N. (1991) Hybrid layer as a dentine bonding mechanism. .I. Esthetic Dent 3, 133-138. Setcos J. C. (1988) Dentin bonding in perspective. Am. J. Dent. 1, (Special Issue), 173-175. Suzuki T. and Finger W. J. (1988) Dentin adhesives: site of dentin vs. bonding of composite resins. Dent. Mater. 4, 379-383. Tao L. and Pashley D. H. (1988) Shear bond strengths to dentin: effects of surface treatments, depth and position. Dent. Mater. 4, 317-378. Tao L.. Pashley D. H. and Boyd L. (1988) Effect of different types of smear layers on dentin and enamel shear bond strengths. Dent. Mater. 4, 208-216. Walls A. W. G., McCabe J. F. and Murray J. J. (1985) The bond strength of composite laminate veneers. J. Dent. Res. 64, 1261-1264. Welbury R. R., McCabe J. F.. Murray J. J. et al. (1988) Factors affecting the bond strength of composite resin to etched glass ionomer cement. J. Dent. Res. 16, 188-193.
Book Reviews Laboratory Techniques for the Branemark System. R. Taylor and G. Bergman. Pp. 79. 1990. New Malden, Quintessence. Softback, f 34.00. Dental implants: Are They For Me? T. Cl. Taylor. Pp. 66. 1990. New Malden, Quintessence, Softback, f 19.00. The third printing of Laboratory Technique for the Branemark System a paperback, ring-bound manual, is intended to update and expand the original Branemark concept, which was intended for completely edentulous patients, to include treatment for the partially dentete. It is written for the restorative dentist and laboratory technician. The manual begins with a thorough discussion of components for abutment connection and an excellent, well-illustrated description of the construction of surgical templates. The succeeding text is divided into five sections: construction of a mandibular fixed complete denture; construction of an implant-retained overdenture; construction of an implant-supported fixed partial denture; construction of a single tooth replacement; and finally, the angulated abutment. Each topic is superbly illustrated with crisp, clear colour and black-and-white photographs, supported by a concise text, describing the entire technical procedure from component selection to final insertion. The section on mandibular fixed complete dentures includes a technique for utilizing light-cured composite resin to veneer the metal framework. At the end of the book is a short section describing the sequence of treatment which assumes a team effort between a ‘dentist’ and ‘surgeon’. One could take exception to both the terms and responsibilities allocated to each of these disciplines. This section could be easily corrected, or readers can place it in their own perspective. The only other objection to this reviewer is the lack of consideration, or even mention, of the several valuable
after-market prosthetic devices available from other manufacturers. This omission makes this otherwise excellent book seem like a ‘company’ publication. This is a book that all who restore occlusal function using Branemark implants should possess. Dental Implants: Are They For Me? is written for the potential patient and intended to be displayed in the dentist’s reception area. It sets out to answer many pertinent questions such a patient may ask. The questions are in sections broadly dealing with what implants are, what types of patients are good candidates, how they are placed, prosthodontics, and home care. The section dealing with patient selection is well done and should serve as a nidus for stimulating questions to the dentist regarding potential treatment. The section on the types of implants succinctly differentiates between endosseous, subperiosteal and transosteal implants, and is followed by the presently obligatory discussion of ‘osseointegration’. Unless the dentist intends to place only two-stage endosteal root or plate-form implants, this section is inappropriate since it would turn patients away from acceptable alternatives (e.g. subperiosteal). The discussion on cost is pertinent, but omits periodontists from the possible treatment team. The segment regarding course of treatment explains only two-stage implantation, which is described as ‘typical’. This might eliminate use of the booklet by practitioners who provide other types of implant. Similar considerations apply to the surgical section and, to a lesser extent, that on prosthodontics which is more generic, although the solutions illustrated are only root forms. The section on implant maintenance is excellent and contains a separate patient pamphlet. The book as a whole is clearly enough written to be understood by the majority of patients. Dentists who practise other forms of implant besides ‘osseointegration’ might be advised, however, to look elsewhere. D. Kott
20: 225-230
225
Dentine bonding agents-characteristic bond strength as a function of dentine depth J. F. McCabe and S. Rusby Dental School, University
of Newcastle
upon Tyne, UK
ABSTRACT The tensile bond strength to dentine of four dentine bonding agents has been measured at various depths within dentine and the results analysed using Weibull analysis. There were significant differences in the bond strength values among the four products particularly with near-surface dentine. Despite the differences in chemical composition among the materials the trend of decreasing bond strength with increasing depth of dentine was common to them all. This suggests that the underlying mechanism of adhesion may be common to all four products despite their differing chemical compositions. KEY WORDS:
Dentine bonding agents, Bond strength,
J. Dent. 1992; January 1992) Correspondence Prosthodontics,
20: 225-230
(Received
15 November
Dentine depth 199 1; reviewed
are
1991;
accepted 29
should be addressed to: Dr J. F. McCabe, Dental Materials Science Unit, Department Dental School, Framlington Place, Newcastle upon Tyne NE2 4BW. UK.
INTRODUCTION There
16 December
now
a variety
of
materials
available
for
achieving bonding between resin-based composite filling materials and dentine. The chemical composition of the products varies considerably and it is far from clear in many cases how the bonding is achieved. The removal or modification of the dentine smear layer appears to play a significant part in the mechanism through which the dentine and resin become linked. The compositions of some of the available products are reported in various texts (e.g. Setcos, 1988). Since the chemical composition of the materials varies markedly it would be expected that the mechanism of bonding would be equally variable. One means of achieving some insight into the mechanism of bonding is to study the relationship between bond strength and the depth of dentine. Causton (1984) for example found that the bond strength of Scotchbond. an adhesive based on a chlorinated phosphate ester of BisGMA, was significantly lower for deep dentine than for upper dentine. Likewise, Suzuki and Finger (1988) reported a significant reduction in bond strength for three materials when comparing upper dentine to deep dentine and related this to residual @ 1992 Butterworth-Heinemann 0300-57 12/92/040225-06
Ltd.
of
area of solid dentine-implying that a bond to the residual bulk dentine is responsible for adhesion. In contrast to Causton (1984) Tao and Pashley (1988) reported that the variation in bond strength with dentine depth for Scotchbond was only evident when the smear layer was removed by acidic pretreatment, which had the effect of significantly lowering the mean bond strength irrespective of dentine depth. Many of the most recently developed materials involve a proposed concept of achieving adhesion which is different from that of products studied previously. It was therefore decided to investigate the relationship between tensile bond strength and depth of dentine in the hope that this would give some insight into the mechanism of bonding.
MATERIALS
AND METHODS
Four commercially available dentine bonding agents were used in the study: Gluma (Bayer, Leverkusen, Germany, Tenure (Den-Mat Corp., Santa Maria, CA, USA), Scotchbond 2 (3M Dental Products, St Paul, MN, USA) and Tripton (ICI, Macclesfield, UK).
226
J. Dent.
1992;
20:
No. 4
1
.o0
0
8
0.8-
0
Stress
Tensile bond strength testing was carried out using a method similar to that described previously (Walls et al., 1985; Welbury et al., 1988). Caries-free permanent molars were used in the study. They were stored for up to 1 week after extraction in 10 per cent formal saline and were then rinsed and transferred into distilled water and stored for a further 4-8 weeks in a refrigerator before use. There is some controversy over the storage of teeth in formaldehyde-containing solutions prior to bond testing. Storage in this medium was therefore confined to a few days following extraction. The occlusal surface of each test tooth was ground away using 180 grit wet and dry paper and then the teeth were mounted in polyester resin with the occlusal surface being close to one flat end of the cylindrical block of resin. The end of the resin block was ground to remove resin until a complete section of occlusal dentine was exposed. The grinding procedure was carried out in a jig involving a vertical drill stand mounted on a horizontal surface grinder. This ensured that the ground surface was perpendicular to the axis of the resin cylinder block. 180 grit wet and dry carborundum paper was used for initial rapid removal whilst 800 grit paper was used for final finishing under water irrigation. The first bond strength determination was made after removing all the occlusal enamel and exposing a complete layer of dentine. Further tests were carried out with each tooth after removing a further 0.5 mm of dentine by grinding with 800 grit paper. Bonds were prepared using an alignment jig and Teflon (PTFE) insert. The insert established an area for bonding of 5 mm diameter at the centre of the dentine and ensured that the flat exposed surface of composite material was parallel to the initial ground surface of dentine. The insert was 2 mm thick so the adhesive and overlying composite tilled a space of 5 mm diameter by 2 mm high.
10 Stress
(MPa)
Fig. 7. Cumulative probability of failure against stress for composite resin bonded to dentine with Gluma. Different symbols reflect different depths within dentine as follows: 0, near surface dentine; A, 0.5 mm deep; *, 1 .O mm deep; Cl, 1.5 mm deep.
5
15
20
25
(MPa)
fig. 2. Cumulative probability of failure against stress for composite resin bonded to dentine with Tenure. For key see Fig. 7.
All the adhesives were applied according to the manufacturer’s recommended procedures and the cavity was finally filled with Occlusin composite (ICI, Macclestield, UK) which was covered with a matrix and cured with a Luxor unit (ICI, Macclesfield, UK) for 60 s. After 5 min the alignment jig was disassembled, the Teflon insert removed and the bonded specimens placed in an incubator for storage at 37°C and 100 per cent relative humidity for 7 days prior to bond strength testing. After 7 days of storage the specimens were removed from the storage incubator and a PMMA (Perspex; ICI) rod was bonded to the surface of the composite resin using cyanoacrylate cement. The ends of the short sections of acrylic rod were flattened using the same grinding jig as that used for the dentine surfaces to ensure continued alignment. A radial hole through the Perspex rod facilitated attachment to the test machine. The tensile bond strength between dentine and adhesive was measured using a cross-head speed of 1 mm mine1 on an Instron testing machine. Thirty teeth were nominally used to test each adhesive. The use of teeth was discontinued if, following grinding, any part of the pulp chamber became exposed. Hence, in each case 30 results were obtained for the outer dentine and fewer for the deeper layers. In addition, a few bonds were inadvertently disturbed or fractured prior to testing and these were discounted from the analysis. It was not possible to obtain results at greater than 1.5 mm depth due to insufficient numbers of teeth remaining at this stage.
RESULTS The bond strength results were analysed using Weibull analysis (McCabe and Carrick, 1986; McCabe and Walls, 1986). Figs I-4 show plots of probability of failure against
McCabe
and Rusby: Bond strength
of dentine
bonding
agents
227
1 .oA 0.8-
0
2
4
8
6
Stress
,$ ._ n x
0.6-
:
0.4-
0
0
O0
8’
12
10
AA
5
(MPa)
10 Stress
15
(MPa)
Fig.
3. Cumulative probability of failure against stress for composite resin bonded to dentine with Scotchbond 2. For key see Fig. 1.
Fig. 4. Cumulative probability of failure against stress for composite resin bonded to dentine with Tripton. For key see Fig. 7.
stress for each adhesive system and at each depth of dentine tested. In each case there is a trend for the bond strength to dentine to decrease as the depth of dentine increases. Table I summarizes the results giving the characteristic strength and Weibull modulus for each test group. Table I also gives the observed modes of failure for each material at all the depths of dentine studied. Failures were either apparently adhesive at the dentine surface or mixed adhesive/cohesive at the dentine surface and within the adhesive or composite. There were no completely cohesive failures and no cohesive failures within dentine.
means of overcoming this problem. In the Weibull analysis, the characteristic strength (also sometimes called the normalizing parameter) is the equivalent of the mean strength in a normal distribution whilst the Weibull modulus, m, gives an indication of reliability and can be considered the equivalent of the standard deviation in a normal distribution. The higher the value of m the more closely grouped are the observed values and therefore the more reliable the value of characteristic strength in determining the true strength of a material or bond. The values of m recorded in Table 1 are all considered to be very low, confirming that the bond strength produced with each material is very variable. As reported previously (McCabe and Carrick, 1986; McCabe and Walls, 1986; Meechan and McCabe, 1986), one way of comparing the results from such tests is to calculate the probability of failure at a given value of stress. Table I1 gives values of probability of failure at a stress of 1 MPa. Treating the results in this way enables the results to be interpreted in a manner which is potentially more meaningful. Hence, from Table II, at a stress of 1 MPa there is less than a 3 per
DISCUSSION Tensile bond strength testing invariably produces results having a wide scatter. Standard deviations of mean bond strength values are often equivalent to or greater than the mean values and in such cases the data clearly do not tit a Gaussian distribution. The use of Weibull statistics is one
Table 1. Tensile bond strength values: characteristic modulus (m) at various depths within dentine
0.5
Omm 00
Gluma
4.6 2.2 ( 1 OOA)
Tenure
Scotchbond
m
10.4 (37A: 2
1.7 63M)
5.0 2.5 (1 OOA)
mm
00
strength
1.0
m
3.4 1.3 (1 OOA)
3.2
1.2
(1OOA)
4.3 2.2 (1 OOA)
(%A
2.4
(%!A:
1 :ii,
: 2:;) 1.6
(1 OOA) 2.8
(1OOA)
Figures in parentheses indicate percentage of specimens failing in adhesive mode (M).
adhesive/cohesive
mm
00
m
2.8
1.1
(1OOA)
3.2 1::)
2.7 4:;)
1.5
CT0
5.1 (85A:
and Weibull
mm
m
Tripton (5%:
(oo (MPa))
1.8
(1OOA)
2.2
1.1
( 1 OOA) 2.3 1.8 ( 1 OOA) mode (A) or mixed
228
J. Dent. 1992;
20:
No. 4
Fig. 5. Surface of dentine after treatment with Gluma cleaner (conditioner).
Fig. 6. Surface of dentine after treatment conditioner.
Fig. 7. Surface of dentine after debonding following use of Tenure bonding agent.
Fig. 8. Surface of dentine following
cent probability of failure for each adhesive bonded to surface dentine, with Tripton having a probability of less than 1 per cent. For deep dentine (1.5 mm deep) the probability of failure is between 17 and 33 per cent. Perhaps the most surprising thing about the results is the consistency with which bond strength varies with dentine depth amongst the four adhesives. Despite the fact that there are significant differences in composition
between the materials and that their mode of action may be expected to vary, the pattern of changing bond strength with dentine depth suggests that theunderlyingmechanism is the same in all cases. Comparison of Table I with Table II shows that those adhesives with higher values of bond strength are more likely to exhibit some cohesive bond failure. Also, for Tenure and Tripton. the proportion of cohesive failure reduced as dentine depth increased. This trend mirrored the reduction in bond strength in both cases. Apart from differences in composition of the four adhesives another major difference between the products is the way in which the dentine is treated prior to bond formation. With Tenure and Gluma the dentine is treated with an acid (nitric) or a chelating agent (EDTA) and then rinsed. This has the effect of removing the dentine smear layer and exposing dentinal tubules. In the case of Gluma
Table II. Probability of failure at stress of 1 MPa at various depths within dentine
Gluma Tenure Scotchbond 2 Tripton
Omm
0.5 mm
l.Omm
1.5mm
0.03 0.02 0.02 < 0.01
0.19 0.09 0.04 0.07
0.23 0.12 0.22 0.06
0.29 0.17 0.33 0.20
with Tenure
conditioning
with
Scotchbond 2 primer (Scotchprep).
McCabe and Rusby: Bond strength of dentine bonding agents
the dentine surface is left smooth with the tubules wide open (Fig 5). Clearly, if mechanical interlocking into open tubules were the primary mechanism of bonding Gluma would appear to have an advantage. It is clear from Table I that the apparent ability of Gluma to provide a means of direct mechanical interlocking into open tubules has not produced a higher characteristic bond strength than those products in which this mechanism is not available. Of interest however is the fact that this product is the least affected by dentine depth and in the deepest dentine its characteristic bond strength is higher than two of the other products. For Tenure, treatment of the dentine with conditioner, followed by rinsing leaves a surface bereft of smear layer (Fig. 6). The outline of the tubules appears etched but the majority appear to remain occluded with the only possibility of mechanical attachment being in the fissures surrounding the blocked tubule openings. The debonded Tenure/Dentine surface (Fig. 7) shows some areas which are essentially unchanged from the structure seen in Fig. 6, but there is evidence of some cohesive failure within the bonding resin, some parts of which appear to be tenaciously attached to the underlying dentine confirming the observations reported in Table II. This product has the highest values of characteristic bond strength in superficial dentine but the value decreases markedly in deeper dentine to approach that for other products. For Scotchbond 2, the surface of dentine treated with the acidic primer (Scotchprep) was not of uniform structure. Typically, some areas showed evidence of an intact smear layer surrounding areas in which the smear layer had been disturbed and tubules opened (Fig 8). One possible explanation for the apparently undisturbed areas is that they represent regions in which the smear layer has been redeposited on the surface along with the adhesion promoter. This would certainly concur with the manufacturer’s view on the mode of action for this material. For Tripton, initial dentine treatment is with a mildly acidic surface active agent (polyhexanide). This produces a surface which is similar in appearance to that for Scotchbond 2 (Fig. 8). The results confirm that the mechanism of adhesion is almost certainly multifactorial in nature. Mechanical attachment through penetration of bonding resins into dentinal tubules would not appear to be a prerequisite for high bond strengths since those products where this mechanism was evident did not have the highest tensile bond strength values. A similar finding was made by Tao and Pashley (1988) when investigating light-cured Scotchbond, an adhesive based upon a phosphate ester of BisGMA. It is tempting to speculate that the variation of bond strength with depth of dentine implies an important role for the mineral content of the dentine in bond formation for each of the materials. This is possible although it is unlikely that any direct chemical bonding to calcium
229
occurs except possibly in Tripton which contains a phosphate adhesion promoter. The pattern of decreasing bond strength with decreasing thickness of dentine follows that previously reported for a phosphate ester adhesive (Scotchbond). as reported by Causton (1984) and Mitchem and Gronas (1986). Tao and Pashley (1988) reported a similar finding for this material but only after the smear layer had been removed by acidic pretreatment. The reasoning for this observation was that the bond strength may be a function of the degree of mineralization and this suggests a specific involvement of mineral in the bond formation. The fact that the same pattern of bond strength variation occurs even with products which are unlikely to show specific affinity for apatite minerals casts doubts on the proposed mechanism of adhesion. Suzuki and Finger (1988) proposed a relationship between bond strength and area of solid dentine for three different materials. The results of the present study may be seen to confirm these findings, given the relationship between residual dentine thickness and area of solid dentine as demonstrated by Suzuki and Finger (1988). This finding however does not, on its own, offer any insight into the mechanism of bonding. Bonding through the formation of a ‘hybrid layer’ involving a mixture of intertubular dentine, smear dentine and resin has been proposed as a possible mechanism of bonding for some materials (Nakabayashi et al., 1991). The results of this study and those of Suzuki and Finger (1988) are compatible with the formation of a hybrid layer attached primarily to intertubular dentine, although in this study there was no direct evidence of such a layer being formed. For those materials in which the smear layer is completely removed prior to bonding the hybrid layer must be formed between bulk intertubular dentine and resin and one could speculate that the exceptionally close adaptation of resin to dentine observed for Tenure (Fig. 7) can only be achieved by penetration of resin into a porous dentine surface-as proposed in the ‘hybrid-layer’ theory. Given the importance of the dentine smear layer in the bonding process it is important to consider whether the nature of such a layer may be affected by the method of its production. The use of abrasive papers, in this work, may not give a smear layer of the same structure as that produced by dental instruments. Tao et al. (1988) reported that similar bond strengths were obtained for one adhesive system when it was bonded to a smear layer produced by either abrasive papers (320 and 600 grit) or burs. This suggests that preparing dentine with abrasive papers prior to bond studies is acceptable. Since the prerequisite for good bonding in any system is effective wetting of the dentine surface by the adhesive, it is possible that the results obtained for each system can be explained by the fact that the resin is less capable of wetting deep dentine than near surface dentine. It may be concluded from this work that all four adhesives studied exhibit greater characteristic bond
230
J. Dent.
strength
to near surface
that acidic tubules
1992;
pretreatments
20:
dentine which
are not a prerequisite
No. 4
than result
to deep dentine in patently
to effective
and
opened
bonding.
References Causton B. E. (1984) Improved bonding of composite restorative to dentine. Br. Dent J. 156, 93-95. McCabe J. F. and Carrick T. E. (1986) A statistical approach to the mechanical testing of dental materials. Dent. Mater. 2, 139-142. McCabe J. F. and Walls A. W. G. (1986) The treatment of results for tensile bond strength testing. J. Dent. 14, 165-168. Meechan J. G. and McCabe J. F. (1986) A model for investigating the probability of fracture of dental local anaesthetic cartridges. Br. Dent. J. 160, 326-328. Mitchem J. C. and Gronas D. G. (1986) Effects of time after extraction and depth of dentin on resin dentin adhesives. J. Am. Dent. Assoc. 113,285-287.
Nakabayashi N.. Nakamura M. and Yasuda N. (1991) Hybrid layer as a dentine bonding mechanism. .I. Esthetic Dent 3, 133-138. Setcos J. C. (1988) Dentin bonding in perspective. Am. J. Dent. 1, (Special Issue), 173-175. Suzuki T. and Finger W. J. (1988) Dentin adhesives: site of dentin vs. bonding of composite resins. Dent. Mater. 4, 379-383. Tao L. and Pashley D. H. (1988) Shear bond strengths to dentin: effects of surface treatments, depth and position. Dent. Mater. 4, 317-378. Tao L.. Pashley D. H. and Boyd L. (1988) Effect of different types of smear layers on dentin and enamel shear bond strengths. Dent. Mater. 4, 208-216. Walls A. W. G., McCabe J. F. and Murray J. J. (1985) The bond strength of composite laminate veneers. J. Dent. Res. 64, 1261-1264. Welbury R. R., McCabe J. F.. Murray J. J. et al. (1988) Factors affecting the bond strength of composite resin to etched glass ionomer cement. J. Dent. Res. 16, 188-193.
Book Reviews Laboratory Techniques for the Branemark System. R. Taylor and G. Bergman. Pp. 79. 1990. New Malden, Quintessence. Softback, f 34.00. Dental implants: Are They For Me? T. Cl. Taylor. Pp. 66. 1990. New Malden, Quintessence, Softback, f 19.00. The third printing of Laboratory Technique for the Branemark System a paperback, ring-bound manual, is intended to update and expand the original Branemark concept, which was intended for completely edentulous patients, to include treatment for the partially dentete. It is written for the restorative dentist and laboratory technician. The manual begins with a thorough discussion of components for abutment connection and an excellent, well-illustrated description of the construction of surgical templates. The succeeding text is divided into five sections: construction of a mandibular fixed complete denture; construction of an implant-retained overdenture; construction of an implant-supported fixed partial denture; construction of a single tooth replacement; and finally, the angulated abutment. Each topic is superbly illustrated with crisp, clear colour and black-and-white photographs, supported by a concise text, describing the entire technical procedure from component selection to final insertion. The section on mandibular fixed complete dentures includes a technique for utilizing light-cured composite resin to veneer the metal framework. At the end of the book is a short section describing the sequence of treatment which assumes a team effort between a ‘dentist’ and ‘surgeon’. One could take exception to both the terms and responsibilities allocated to each of these disciplines. This section could be easily corrected, or readers can place it in their own perspective. The only other objection to this reviewer is the lack of consideration, or even mention, of the several valuable
after-market prosthetic devices available from other manufacturers. This omission makes this otherwise excellent book seem like a ‘company’ publication. This is a book that all who restore occlusal function using Branemark implants should possess. Dental Implants: Are They For Me? is written for the potential patient and intended to be displayed in the dentist’s reception area. It sets out to answer many pertinent questions such a patient may ask. The questions are in sections broadly dealing with what implants are, what types of patients are good candidates, how they are placed, prosthodontics, and home care. The section dealing with patient selection is well done and should serve as a nidus for stimulating questions to the dentist regarding potential treatment. The section on the types of implants succinctly differentiates between endosseous, subperiosteal and transosteal implants, and is followed by the presently obligatory discussion of ‘osseointegration’. Unless the dentist intends to place only two-stage endosteal root or plate-form implants, this section is inappropriate since it would turn patients away from acceptable alternatives (e.g. subperiosteal). The discussion on cost is pertinent, but omits periodontists from the possible treatment team. The segment regarding course of treatment explains only two-stage implantation, which is described as ‘typical’. This might eliminate use of the booklet by practitioners who provide other types of implant. Similar considerations apply to the surgical section and, to a lesser extent, that on prosthodontics which is more generic, although the solutions illustrated are only root forms. The section on implant maintenance is excellent and contains a separate patient pamphlet. The book as a whole is clearly enough written to be understood by the majority of patients. Dentists who practise other forms of implant besides ‘osseointegration’ might be advised, however, to look elsewhere. D. Kott
