J. Dent. 1992; 20: 245-249
245
Bonding characteristics of a phosphonated anaerobic adhesive to amalgam D. C. Watts, H. Devlin and J. E. Fletcher Department
of Restorative
Dentistry,
University
of Manchester
Dental /-/ospita/,
UK
ABSTRACT The shear bond strength of a phosphonated anaerobic resin dental adhesive (Panavia-Ex) to amalgam was determined and clinically important parameters affecting the bond strength: amalgam type, surface finish and corrosion, were investigated. It is possible to bond to high copper set amalgam using this adhesive with a clinically useful shear bond strength. Three copper-enriched, single composition alloys were studied. One was a spherical alloy and two were lathe cut. The bond strength to one of the lathe-cut alloys was significantly lower than to the other two alloys. A significantly higher bond strength to aged lathe-cut amalgam than to fresh amalgam was found. However, Weibull analysis indicated that these latter changes were less consistent and indicated patchy, localized changes in the amalgam surface. The bond strength to the spherical alloy was significantly reduced by polishing with a finishing bur or Prophy-Jet. Therefore when bonding to a large amalgam restoration with such a phosphonated anaerobic adhesive. no polishing of the amalgam should be undertaken. KEY WORDS:
Amalgam,
J. Dent. 1992; 20: November 199 1)
Bonding, Adhesion
245-249
(Received
26
July
1991;
reviewed
18 September
1991;
accepted
7
Correspondence should be addressed too: Dr D. C. Watts, Department of Restorative Dentistry, University of Manchester Dental Hospital, Higher Cambridge Street, Manchester Ml 5 6FH. UK.
INTRODUCTION Recently, some studies (Staninec and Holt, 1988; Staninec, 1989) have reported significant bond strengths between freshly mixed amalgam applied to enamel coated with the anaerobic adhesive, Panavia-Ex (Kuraray Co. Ltd. Kuarashiki, Japan). This product has not been directly promoted as an amalgam-tooth bonding agent, but may provide reduced microleakage (Torii et al.. 1988) and resistance to fracture of amalgam restored teeth (Eakle et al.. 1990). Bonding amalgam to the cavity margin and a more conservative removal of tooth substance may prevent weakening of the restored tooth and aid retention of intracoronal restorations. The bond strength of Panavia-Px to fully set amalgam has not been shown to be sufficient to aid the retention of extracoronal restorations. Rueggerberg et al. (1989) reported tensile bond strengths of less than 10 MPa after 48 h. when bonding to various amalgams with PanaviaEx. They concluded that the coverage of amalgam surfaces by the metal framework of etched metal resinbonded (Maryland) retainers should be avoided. However, a 1992 Butterworth-Heinemann 0300-5712/92/040245-05
Ltd
the amalgam surfaces used in their tests were milled with silicon carbide and therefore were smooth and corrosion free. Maryland bridges are more likely to be required to be attached to already existing, rough, tarnished, amalgam restorations with additional possibilities for retention (Rawlinson, 1987). The aim of the present study was to determine the effect of some clinically important parameters on the adhesion of a commercially available prosthodontic adhesive to amalgam. These are: (a) the type of amalgam alloy used, (h) the surface finish of the amalgam, and (c) the age of the amalgam, i.e. the time since insertion of the triturated amalgam.
MATERIALS
AND METHODS
Copper-enriched, single composition amalgam (non-y,) alloys were used in this investigation because of their clinically superior properties: less creep, superior marginal integrity and higher early strength. The amalgam alloys used were Sybraloy (Kerr, Romulus, MN, USA), a spherical alloy, and two lathe-cut alloys, ANA 2000
246
J. Dent. 1992;
20: No. 4
Table 1. Descriptive statistics of amalgam/Panavia
Group
Alloy
bond strengths
Specimens (n0.l
Sybraloy ANA 2000 Matticap Plus 43 Aged ANA 2000
46 39 36 45
8.02 7.33 4.72 16.36
3.68 3.63 1.77 8.81
Sybraloy Prophy-Jet Aged Matticap Plus 43 Sybraloy Finishing Bur
42 32 36
11.02 4.72 5.14
4.99 2.30 2.93
(Nordiska Dental AB. Helsingborg, Sweden) and Matticap Plus 43 (Johnson Matthey Medical, Birmingham, UK). Panavia-Ex, a composite resin adhesive, was used and consists of initiators. filler, aromatic and aliphatic methacrylates, phosphate monomer, activators and stabilizers. Amalgam specimens were bonded by the adhesive to brass rods, the surfaces of which had been sandblasted. This ensured that on de-bonding the weakest test interface was consistently the amalgam/Panavia-Ex interface and the brass/Panavia-Ex interface remained intact.
Determination of shear bond strength of adhered specimens The shear strength of bonded specimens was measured by placing each specimen in a ‘shear’ loading assembly which permitted neither bending nor rotational forces. The specimens were loaded in a calibrated Universal testing machine (Model 50 TS, RDP Howden Ltd. Learnington Spa, UK), using a crosshead speed of 5 mm minG. The peak shear fracture force was determined and this was re-expressed as a stress value by taking into account the cross-sectional area of the bond. The type of failure at the adhesive/amalgam interface (cohesive, adhesive, or combined adhesive/cohesive) was determined by examination of the surface with a stereo-zoom microscope. Selected specimens were also examined by electron microscopy. A total of seven groups of specimens was prepared for investigation of variables affecting bond strengths (Table I).
manufacturer’s instructions. Five millimetre diameter brass rods were placed centrally on top of each amalgam surface, perpendicular to the Perspex disc. Excess resin was removed and the specimens protected from atmospheric oxygen with a polyethylene glycol gel. After 6 min the gel was removed by rinsing with water. Each specimen was stored in distilled and deionized water at 37 “C for a further 7 days prior to testing.
Variation of shear strength with surface of amalgam: groups I, 6 and 7
of shear strength with alloys: groups 1-3
different
The three different amalgams were triturated according to manufacturers’ instructions and each condensed manually into cavities 5 mm diameter and 2 mm deep, prepared centrally in 25 mm diameter clear poly (methylmethacrylate) (Perspex) discs. The set amalgam was carved level with the surface of the Perspex. Forty-six specimens of Sybraloy, 39 of ANA 2000 and 36 of Matticap Plus 43 were used. After 7 days storage at 37°C in water, the fully set specimens were thoroughly dried and coated with Panavia-Ex, which was mixed according to the
finish
Only Sybraloy amalgam was used to investigate this factor. After 7 days, 36 specimens were lightly polished with a pear-shaped finishing bur to a high lustre. A further 32 were polished with an air abrasive prophylaxis device (Prophy-Jet, Dentsply International Inc.. York, PA. USA) for 15 s. Each specimen was bonded to a brass rod with Panavia-Ex, placed in water at 37°C for 7 days, and the shear strength measured.
Variation amalgam:
of shear groups
strength
with
age of
2-5
Forty-five specimens of ANA 2000 and 42 of Matticap Plus 43 were prepared and stored at 37 “C in a humidifier at 98 per cent humidity for 37 days. Brass rods were bonded to each amalgam surface with Panavia-Ex as previously described. All bonded specimens were stored for a further 7 days in water at 37 “C prior to measurement of the shear strength.
Statistical Variation amalgam
Bond strength (MPa) Mean s.d.
analysis
Statistical analysis was performed on the bond strength data using analysis of variance and the Student-NewmanKeuls (SNK) test to analyse significant differences between means. This was followed by Weibull analysis (McCabe and Walls, 1986).
RESULTS The bond strength data for the seven groups are presented in Table I, in terms of descriptive statistics, and pertinent statistically significant differences are noted in Table II. Cumulative failure probability plots are given in Figs f-4
Watts et al.: Bonding to amalgam
Table II. Pertinent differences between groups (as in Table I) that were statistically significant (P < 0.05)
Group 1 2 3 4
7
2
3
Group 4
S s
s
5
6
7
s
s
S S
and the derived Weibull parameters are listed in Table III. The modes of adhesive failure are noted in Table IV and electron micrographs of the surfaces of Sybraloy amalgam are illustrated in Figs 5-7. The salient features of these results may be considered in terms of the major variables.
Differences
in amalgam
247
alloy: groups l-3
Panavia-Ex exhibited comparable bond strengths to both Sybraloy and ANA 2000, which substantially exceeded those to Matticap Plus amalgam (Tables I, II). Nevertheless. similar trends were also observed in the stress levels for 10 per cent failure probability (Table HZ), However, a slightly higher Weibull ‘reliability’ modulus was found with the bonds to Matticap Plus (Table III), together with the greatest incidence (8 per cent) of pure adhesive failures (Table IV).
Effects of surface finish groups 1, 6 and 7
(Sybraloy):
Finishing the surface of Sybraloy amalgam specimens with either a Prophy-Jet or a finishing bur led to a
0
5
10 Bond
15 ‘allure
20
25
stress CMP.3)
fig. 7. Failure probability versus stress level for bond failure: effect of amalgam type. Mt. Matticap Plus; An, ANA 2000; Sy, Sybraloy.
Fig. 2. Failure probability versus stress level for bond failure: effect of surface finish of Sybraloy. pj, Prophy Jet; fb, finishing bur; fc, flat carved.
fig. 3. Failure probability versus stress level for bond failure to ANA 2000. Effect of different ageing periods: 7 and 37
Fig. 4. Failure probability versus stress level for bond failure to Matticap Plus. Effect of different ageing periods: 7 and 37 days.
24%
J. Dent.
1992;
20:
No. 4
Table 111.Weibull statistical parameters for amalgam bonding by Panavia-Ex
Group
Alloy
Weibull modulus (m)
1 2 3 4 5 6 7
Sybraloy ANA 2000 Matticap Plus 43 Aged ANA 2000 Aged Matticap Plus 43 Sybraloy Prophy-Jet Sybraloy Finishing Bur
2.72 2.38 3.43 1.65 1.93 2.58 2.34
Tab/e IV. Mode of amalgam/adhesive
Critical stress, S, (MPa)
Stress level (MPa) for 70% failure probability
9.54 8.87 5.68 19.46 13.36 5.99 6.37
4.18 3.46 2.87 5.09 4.15 2.56 2.43
bond failure
Group
Alloy
Cohesive 1%)
Adhesive I%)
Mixed: cohesive/ adhesive (%I
1 2 3 4 5 6 7
Sybraloy ANA 2000 Matticap Plus 43 Aged ANA 2000 Aged Matticap Plus 43 Sybraloy Prophy-Jet Sybraloy Finishing Bur
67.4 23.1 38.9 20.0 9.5 34.4 2.8
0.0 2.6 8.3 11.1 38.1 18.7 19.4
32.6 74.3 52.8 68.9 52.4 46.9 77.8
substantial and highly significant reduction in the mean bond strengths and the stress levels for 10 per cent failure probability, compared to the flat carved specimens (Tables Z-111). However, there was little associated change in the Weibull modulus(m), which ranged from 2.72 down to 2.34. A substantial jump in pure adhesive failures, from 0 to 18-19 per cent, was seen when the mode of surface finishing was changed from the flat-carved protocol. Electron micrographs (Figs 5-7) illustrate the relatively rough surface of the flat-carved Sybraloy, with extensive possibilities of tag-formation by the adhesive resin, and the smooth and unretentive surface produced by use of a finishing bur.
Effects of amalgam
ageing:
groups 2-5
Ageing of the specimens prior to bonding led to a two-fold increase in the mean bond strengths with both ANA 2000 and Matticap Plus 43 (Tables I, II). However, the increases in the stress levels for 10 per cent failure probability (Table Ill) were less pronounced. This is clarified by Table 111 and Figs 3 and 4 which show reductions in the Weibull modulus for bonds to both 37-day aged alloys, compared with the 7-day bond strengths. With regard to bond-failure modes, there were increases in pure adhesive failure: from 2.6 to 11.1 per cent for ANA 2000 and from 8.3 to 38.1 per cent for Matticap Plus 43. These changes in Matticap Plus were at the‘expense’ of reduction in the incidence of pure cohesive failures, from 38.9 to 9.5 per cent.
DISCUSSION So-called ‘shear’ bond testing is one convenient mode of testing adhesive interfaces that is well established in biomaterials research. However, the actual mode of disruption of the interface is virtually never a pure shear deformation. Nevertheless, the resultant data are fairly reproducible between groups working to the same protocol and provide a reasonable insight into the strengths of bonded structures. The application of Weibull analysis has recently become a valuable adjunct to adhesive testing in that the modulus (m) parameter gives a measure of the reliability of the Characteristic or Critical Strength parameter (S,) (Table III). This is particularly so when groups consisting of 30 or more specimens are analysed, enabling the determination of m with greater accuracy. Care should be taken, however, not to overinterpret the Weibull parameters unless rigorous statistical tests are conducted to assess the goodness of tit of the Weibull function to the data in question. The present results on bonding to dental amalgams with Panavia-Ex may be rationalized in terms of the reproducibility or uniformity of the amalgam surface as an adherend following ‘environmental’ changes. Alternative finishing treatments-including application of finishing burs or Prophy-Jet-may be considered as ‘environmental’ treatments which uniformly alter the surface (Lubow and Cooley. 1986; Cooley ct al.. 1989).
Watts et al.: Bonding to amalgam
Fig. 5. (x 270).
Electron
micrograph
of
flat
carved
Sybraloy
Fig. 6. Electron rough
surface
micrograph
of flat carved
Sybraloy
249
showing
(X 1350).
microscopic roughening of metallic restorations by means of corrosion processes is generally unwelcome. there is some beneficial enhancement of bonding that may well result. The relative decrease in critical bond strength-reliability suggests that surface changes are patchy and this factor merits further investigation. Meanwhile, the bond strengths are of a reasonable magnitude, and at least match the range of values exhibited by dentine-bonding agents. This provides a basis for judicious application of this adhesive agent for bonding to aged amalgam. Alternative techniques which involve replacing existing amalgams with composite resin prior to cementing resin-bonded bridges have some disadvantages, especially where the resulting gingival floor extends below the amelocemental junction. Fig. 7. Electron finishing bur, (x 270).
micrograph showing
of Sybraloy after application of a a featureless smooth surface
References Cooley R. L.. McCourt J. and Train T. (1989) Bond strength
Notably. therefore, the Weibull modulus is maintained at a reasonably constant level (2.3-2.6) by such changes. Nevertheless, although the critical bond strength reliability was maintained despite rapid. operator-induced surface change, there were substantial decreases in mean bond strength. By contrast. the effects of storage for 4-5 weeks. albeit in a uniform environment. produce less consistent changes in the critical bond strength. manifested by substantial decreases in the Weibull modulus. These must be attributed to changes in the amalgam surfaces during this period, probably involving surface-localized oxidation/ corrosion changes or at least surface reactions of some type. Associated with this slow chemically induced surface change (oxidation?). which might reflect slow changes in clinical surface, there were substantial increases in mean bond strength. This evidence points to micromechanical tag-formation as a basic component of effective surface bonding to amalgam by Panavia-Ex, supplemented in suitable aged amalgams by chemical bonding mediated by surface oxidation layers. Although surface oxide formation and
of resin to amalgam as affected by surface finish. Quintessence Inr. 20, 231-239. Eakle W. S. et al. (1990) Effect of bonded amalgams on fracture resistance of teeth. J. Dent. Res. 69, (spec. issue). 287. Lubow R. M. and Cooley R. L. (1986) Effect of air-powder abrasive instrument on restorative materials. J. Prosther. Dent. 55, 462-465. McCabe J. F. and Walls A. W. G. (1986) The treatment of results for tensile bond strength testing. J. Dent. 14, 165-168. Martin N. and Jedynakiewicz N. (1991) Amalgam-enamel adhesive bond strengths. J. Dent. Res. (in press). Rawlinson A. (1987) Maryland bridgework using restored posterior abutment teeth. Rest Dent. 3, 68-74. Rueggerberg F. A, Caughman W. F., Gao F. et al. (1989) Bond strength of Panavia Ex to dental amalgam. Int. J. Prosthodont. 2, 371-375. Staninec M. (1989) Retention of amalgam restorations: undercuts versus bonding. Quintessence Int 20, 347-351. Staninec M. and Holt M. (1988) Bonding of amalgam to tooth structure: tensile adhesion and microleakage tests. J. Prosthet. Dent. 59, 397402. Torii Y. er al. (1988) Inhibition of caries around amalgam restorations by amalgam bonding. J. Dent. Res. 67, (spec. issue). 308 (abstr.).
245
Bonding characteristics of a phosphonated anaerobic adhesive to amalgam D. C. Watts, H. Devlin and J. E. Fletcher Department
of Restorative
Dentistry,
University
of Manchester
Dental /-/ospita/,
UK
ABSTRACT The shear bond strength of a phosphonated anaerobic resin dental adhesive (Panavia-Ex) to amalgam was determined and clinically important parameters affecting the bond strength: amalgam type, surface finish and corrosion, were investigated. It is possible to bond to high copper set amalgam using this adhesive with a clinically useful shear bond strength. Three copper-enriched, single composition alloys were studied. One was a spherical alloy and two were lathe cut. The bond strength to one of the lathe-cut alloys was significantly lower than to the other two alloys. A significantly higher bond strength to aged lathe-cut amalgam than to fresh amalgam was found. However, Weibull analysis indicated that these latter changes were less consistent and indicated patchy, localized changes in the amalgam surface. The bond strength to the spherical alloy was significantly reduced by polishing with a finishing bur or Prophy-Jet. Therefore when bonding to a large amalgam restoration with such a phosphonated anaerobic adhesive. no polishing of the amalgam should be undertaken. KEY WORDS:
Amalgam,
J. Dent. 1992; 20: November 199 1)
Bonding, Adhesion
245-249
(Received
26
July
1991;
reviewed
18 September
1991;
accepted
7
Correspondence should be addressed too: Dr D. C. Watts, Department of Restorative Dentistry, University of Manchester Dental Hospital, Higher Cambridge Street, Manchester Ml 5 6FH. UK.
INTRODUCTION Recently, some studies (Staninec and Holt, 1988; Staninec, 1989) have reported significant bond strengths between freshly mixed amalgam applied to enamel coated with the anaerobic adhesive, Panavia-Ex (Kuraray Co. Ltd. Kuarashiki, Japan). This product has not been directly promoted as an amalgam-tooth bonding agent, but may provide reduced microleakage (Torii et al.. 1988) and resistance to fracture of amalgam restored teeth (Eakle et al.. 1990). Bonding amalgam to the cavity margin and a more conservative removal of tooth substance may prevent weakening of the restored tooth and aid retention of intracoronal restorations. The bond strength of Panavia-Px to fully set amalgam has not been shown to be sufficient to aid the retention of extracoronal restorations. Rueggerberg et al. (1989) reported tensile bond strengths of less than 10 MPa after 48 h. when bonding to various amalgams with PanaviaEx. They concluded that the coverage of amalgam surfaces by the metal framework of etched metal resinbonded (Maryland) retainers should be avoided. However, a 1992 Butterworth-Heinemann 0300-5712/92/040245-05
Ltd
the amalgam surfaces used in their tests were milled with silicon carbide and therefore were smooth and corrosion free. Maryland bridges are more likely to be required to be attached to already existing, rough, tarnished, amalgam restorations with additional possibilities for retention (Rawlinson, 1987). The aim of the present study was to determine the effect of some clinically important parameters on the adhesion of a commercially available prosthodontic adhesive to amalgam. These are: (a) the type of amalgam alloy used, (h) the surface finish of the amalgam, and (c) the age of the amalgam, i.e. the time since insertion of the triturated amalgam.
MATERIALS
AND METHODS
Copper-enriched, single composition amalgam (non-y,) alloys were used in this investigation because of their clinically superior properties: less creep, superior marginal integrity and higher early strength. The amalgam alloys used were Sybraloy (Kerr, Romulus, MN, USA), a spherical alloy, and two lathe-cut alloys, ANA 2000
246
J. Dent. 1992;
20: No. 4
Table 1. Descriptive statistics of amalgam/Panavia
Group
Alloy
bond strengths
Specimens (n0.l
Sybraloy ANA 2000 Matticap Plus 43 Aged ANA 2000
46 39 36 45
8.02 7.33 4.72 16.36
3.68 3.63 1.77 8.81
Sybraloy Prophy-Jet Aged Matticap Plus 43 Sybraloy Finishing Bur
42 32 36
11.02 4.72 5.14
4.99 2.30 2.93
(Nordiska Dental AB. Helsingborg, Sweden) and Matticap Plus 43 (Johnson Matthey Medical, Birmingham, UK). Panavia-Ex, a composite resin adhesive, was used and consists of initiators. filler, aromatic and aliphatic methacrylates, phosphate monomer, activators and stabilizers. Amalgam specimens were bonded by the adhesive to brass rods, the surfaces of which had been sandblasted. This ensured that on de-bonding the weakest test interface was consistently the amalgam/Panavia-Ex interface and the brass/Panavia-Ex interface remained intact.
Determination of shear bond strength of adhered specimens The shear strength of bonded specimens was measured by placing each specimen in a ‘shear’ loading assembly which permitted neither bending nor rotational forces. The specimens were loaded in a calibrated Universal testing machine (Model 50 TS, RDP Howden Ltd. Learnington Spa, UK), using a crosshead speed of 5 mm minG. The peak shear fracture force was determined and this was re-expressed as a stress value by taking into account the cross-sectional area of the bond. The type of failure at the adhesive/amalgam interface (cohesive, adhesive, or combined adhesive/cohesive) was determined by examination of the surface with a stereo-zoom microscope. Selected specimens were also examined by electron microscopy. A total of seven groups of specimens was prepared for investigation of variables affecting bond strengths (Table I).
manufacturer’s instructions. Five millimetre diameter brass rods were placed centrally on top of each amalgam surface, perpendicular to the Perspex disc. Excess resin was removed and the specimens protected from atmospheric oxygen with a polyethylene glycol gel. After 6 min the gel was removed by rinsing with water. Each specimen was stored in distilled and deionized water at 37 “C for a further 7 days prior to testing.
Variation of shear strength with surface of amalgam: groups I, 6 and 7
of shear strength with alloys: groups 1-3
different
The three different amalgams were triturated according to manufacturers’ instructions and each condensed manually into cavities 5 mm diameter and 2 mm deep, prepared centrally in 25 mm diameter clear poly (methylmethacrylate) (Perspex) discs. The set amalgam was carved level with the surface of the Perspex. Forty-six specimens of Sybraloy, 39 of ANA 2000 and 36 of Matticap Plus 43 were used. After 7 days storage at 37°C in water, the fully set specimens were thoroughly dried and coated with Panavia-Ex, which was mixed according to the
finish
Only Sybraloy amalgam was used to investigate this factor. After 7 days, 36 specimens were lightly polished with a pear-shaped finishing bur to a high lustre. A further 32 were polished with an air abrasive prophylaxis device (Prophy-Jet, Dentsply International Inc.. York, PA. USA) for 15 s. Each specimen was bonded to a brass rod with Panavia-Ex, placed in water at 37°C for 7 days, and the shear strength measured.
Variation amalgam:
of shear groups
strength
with
age of
2-5
Forty-five specimens of ANA 2000 and 42 of Matticap Plus 43 were prepared and stored at 37 “C in a humidifier at 98 per cent humidity for 37 days. Brass rods were bonded to each amalgam surface with Panavia-Ex as previously described. All bonded specimens were stored for a further 7 days in water at 37 “C prior to measurement of the shear strength.
Statistical Variation amalgam
Bond strength (MPa) Mean s.d.
analysis
Statistical analysis was performed on the bond strength data using analysis of variance and the Student-NewmanKeuls (SNK) test to analyse significant differences between means. This was followed by Weibull analysis (McCabe and Walls, 1986).
RESULTS The bond strength data for the seven groups are presented in Table I, in terms of descriptive statistics, and pertinent statistically significant differences are noted in Table II. Cumulative failure probability plots are given in Figs f-4
Watts et al.: Bonding to amalgam
Table II. Pertinent differences between groups (as in Table I) that were statistically significant (P < 0.05)
Group 1 2 3 4
7
2
3
Group 4
S s
s
5
6
7
s
s
S S
and the derived Weibull parameters are listed in Table III. The modes of adhesive failure are noted in Table IV and electron micrographs of the surfaces of Sybraloy amalgam are illustrated in Figs 5-7. The salient features of these results may be considered in terms of the major variables.
Differences
in amalgam
247
alloy: groups l-3
Panavia-Ex exhibited comparable bond strengths to both Sybraloy and ANA 2000, which substantially exceeded those to Matticap Plus amalgam (Tables I, II). Nevertheless. similar trends were also observed in the stress levels for 10 per cent failure probability (Table HZ), However, a slightly higher Weibull ‘reliability’ modulus was found with the bonds to Matticap Plus (Table III), together with the greatest incidence (8 per cent) of pure adhesive failures (Table IV).
Effects of surface finish groups 1, 6 and 7
(Sybraloy):
Finishing the surface of Sybraloy amalgam specimens with either a Prophy-Jet or a finishing bur led to a
0
5
10 Bond
15 ‘allure
20
25
stress CMP.3)
fig. 7. Failure probability versus stress level for bond failure: effect of amalgam type. Mt. Matticap Plus; An, ANA 2000; Sy, Sybraloy.
Fig. 2. Failure probability versus stress level for bond failure: effect of surface finish of Sybraloy. pj, Prophy Jet; fb, finishing bur; fc, flat carved.
fig. 3. Failure probability versus stress level for bond failure to ANA 2000. Effect of different ageing periods: 7 and 37
Fig. 4. Failure probability versus stress level for bond failure to Matticap Plus. Effect of different ageing periods: 7 and 37 days.
24%
J. Dent.
1992;
20:
No. 4
Table 111.Weibull statistical parameters for amalgam bonding by Panavia-Ex
Group
Alloy
Weibull modulus (m)
1 2 3 4 5 6 7
Sybraloy ANA 2000 Matticap Plus 43 Aged ANA 2000 Aged Matticap Plus 43 Sybraloy Prophy-Jet Sybraloy Finishing Bur
2.72 2.38 3.43 1.65 1.93 2.58 2.34
Tab/e IV. Mode of amalgam/adhesive
Critical stress, S, (MPa)
Stress level (MPa) for 70% failure probability
9.54 8.87 5.68 19.46 13.36 5.99 6.37
4.18 3.46 2.87 5.09 4.15 2.56 2.43
bond failure
Group
Alloy
Cohesive 1%)
Adhesive I%)
Mixed: cohesive/ adhesive (%I
1 2 3 4 5 6 7
Sybraloy ANA 2000 Matticap Plus 43 Aged ANA 2000 Aged Matticap Plus 43 Sybraloy Prophy-Jet Sybraloy Finishing Bur
67.4 23.1 38.9 20.0 9.5 34.4 2.8
0.0 2.6 8.3 11.1 38.1 18.7 19.4
32.6 74.3 52.8 68.9 52.4 46.9 77.8
substantial and highly significant reduction in the mean bond strengths and the stress levels for 10 per cent failure probability, compared to the flat carved specimens (Tables Z-111). However, there was little associated change in the Weibull modulus(m), which ranged from 2.72 down to 2.34. A substantial jump in pure adhesive failures, from 0 to 18-19 per cent, was seen when the mode of surface finishing was changed from the flat-carved protocol. Electron micrographs (Figs 5-7) illustrate the relatively rough surface of the flat-carved Sybraloy, with extensive possibilities of tag-formation by the adhesive resin, and the smooth and unretentive surface produced by use of a finishing bur.
Effects of amalgam
ageing:
groups 2-5
Ageing of the specimens prior to bonding led to a two-fold increase in the mean bond strengths with both ANA 2000 and Matticap Plus 43 (Tables I, II). However, the increases in the stress levels for 10 per cent failure probability (Table Ill) were less pronounced. This is clarified by Table 111 and Figs 3 and 4 which show reductions in the Weibull modulus for bonds to both 37-day aged alloys, compared with the 7-day bond strengths. With regard to bond-failure modes, there were increases in pure adhesive failure: from 2.6 to 11.1 per cent for ANA 2000 and from 8.3 to 38.1 per cent for Matticap Plus 43. These changes in Matticap Plus were at the‘expense’ of reduction in the incidence of pure cohesive failures, from 38.9 to 9.5 per cent.
DISCUSSION So-called ‘shear’ bond testing is one convenient mode of testing adhesive interfaces that is well established in biomaterials research. However, the actual mode of disruption of the interface is virtually never a pure shear deformation. Nevertheless, the resultant data are fairly reproducible between groups working to the same protocol and provide a reasonable insight into the strengths of bonded structures. The application of Weibull analysis has recently become a valuable adjunct to adhesive testing in that the modulus (m) parameter gives a measure of the reliability of the Characteristic or Critical Strength parameter (S,) (Table III). This is particularly so when groups consisting of 30 or more specimens are analysed, enabling the determination of m with greater accuracy. Care should be taken, however, not to overinterpret the Weibull parameters unless rigorous statistical tests are conducted to assess the goodness of tit of the Weibull function to the data in question. The present results on bonding to dental amalgams with Panavia-Ex may be rationalized in terms of the reproducibility or uniformity of the amalgam surface as an adherend following ‘environmental’ changes. Alternative finishing treatments-including application of finishing burs or Prophy-Jet-may be considered as ‘environmental’ treatments which uniformly alter the surface (Lubow and Cooley. 1986; Cooley ct al.. 1989).
Watts et al.: Bonding to amalgam
Fig. 5. (x 270).
Electron
micrograph
of
flat
carved
Sybraloy
Fig. 6. Electron rough
surface
micrograph
of flat carved
Sybraloy
249
showing
(X 1350).
microscopic roughening of metallic restorations by means of corrosion processes is generally unwelcome. there is some beneficial enhancement of bonding that may well result. The relative decrease in critical bond strength-reliability suggests that surface changes are patchy and this factor merits further investigation. Meanwhile, the bond strengths are of a reasonable magnitude, and at least match the range of values exhibited by dentine-bonding agents. This provides a basis for judicious application of this adhesive agent for bonding to aged amalgam. Alternative techniques which involve replacing existing amalgams with composite resin prior to cementing resin-bonded bridges have some disadvantages, especially where the resulting gingival floor extends below the amelocemental junction. Fig. 7. Electron finishing bur, (x 270).
micrograph showing
of Sybraloy after application of a a featureless smooth surface
References Cooley R. L.. McCourt J. and Train T. (1989) Bond strength
Notably. therefore, the Weibull modulus is maintained at a reasonably constant level (2.3-2.6) by such changes. Nevertheless, although the critical bond strength reliability was maintained despite rapid. operator-induced surface change, there were substantial decreases in mean bond strength. By contrast. the effects of storage for 4-5 weeks. albeit in a uniform environment. produce less consistent changes in the critical bond strength. manifested by substantial decreases in the Weibull modulus. These must be attributed to changes in the amalgam surfaces during this period, probably involving surface-localized oxidation/ corrosion changes or at least surface reactions of some type. Associated with this slow chemically induced surface change (oxidation?). which might reflect slow changes in clinical surface, there were substantial increases in mean bond strength. This evidence points to micromechanical tag-formation as a basic component of effective surface bonding to amalgam by Panavia-Ex, supplemented in suitable aged amalgams by chemical bonding mediated by surface oxidation layers. Although surface oxide formation and
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