Journal of Oral Rehabilitation, 1979, Volume 6, pages 61-66
A clinical study of marginal integrity and tarnish behaviour of three Cu-rich amalgam systems
T. H. LARSON, D. SABOTT, R. COOLEY and ^. H. GREENER Northwestern University, Chicago
Summary
Three high Cu-y2 free amalgams and one conventional amalgam were used in a clinical study to evaluate degree of marginal integrity and surface discoloration occurring for time periods of up to two years. Two hundred and seventy restorations were placed in Class I and II cavities in sixty-two paedodontic patients. Amalgams were prepared from conventional alloy, Cu admixed, Dispersalloy and three parts 50Ag20 Sn-30Cu mixed with one part conventional alloy. Restorations were evaluated by three groups of clinicians. Ridit means obtained from all three groups of evaluations were in good agreement. The lower ridits indicate better marginal integrity and less tarnish. Marginal integrity of amalgams prepared from Cu-admix did not differ appreciably (0-41) from the ridit associated with initial placement (0-39). Conventional amalgams had the highest ridit (0-65) for marginal integrity whereas the ternary/conventional and Dispersalloy restorations were intermediate (0-55 and 0-52). The ridit for tarnish and surface discoloration was decidedly higher for the Cu-admix (0-73) than for the other systems investigated which had ridits of the order of 0-50 compared to an initial ridit for tarnish of 0-28. Introduction
The in vitro saline polarization of yt free amalgams has not displayed the conversion of tin containing phases to tin oxychloride and tin oxide (Sarkar, 1973). It may be expected, however, that -/2 free amalgams would tend to have a higher concentration ratio of yi and would, therefore, be more susceptible to formation of sulphide tarnish films (Finkelstein, 1976). In addition, clinical evaluation of marginal integrity has shown representative y^ free amalgams to maintain a higher degree of marginal integrity over conventional amalgams for comparable time periods (Mahler, Terkla & Van Eysden, 1973; Sabott, 1975). The purpose of this investigation was to compare the clinical marginal stability and the tarnish characteristics of three dental alloys known to produce restorations free of 72 and a conventional dental alloy containing the 72 reaction product. Correspondence: Dr E.H. Greener, Department of Biological Materials, Northwestern University, Dental School, Ward Memorial Building, 311E Chicago Ave., Chicago, Illinois 60611, U.S.A.
0305-182X/79/0100-0061 S02.00 © 1979 Blackwell Scientific Publications 61
61
T. H. Larson et al.
:
.
;
Materials and methods
A clinical evaluation was initiated using the following amalgam alloys: (1) conventional amalgam alloy (Aristaloy) prepared according to the manufacturer's recommendations. (2) An admixed high copper amalgam, prepared by mixing copper amalgam and conventional amalgam in a ratio of 1:2 by volume, by the technique reported by Sabott. The resulting amalgam contained approximately 11-6% copper by weight (Sabott, 1975). (3) Commercial amalgam alloy (Dispersalloy), containing 1 part spherical particles of AgCu eutectic dispersed in two parts of Ag3Sn alloy powder. This amalgam contained about 6% copper (Marshall, Finkelstein & Greener, 1976). (4) Three parts of an as-cast 30% Cu, 52-5% Ag, 17-5% Sn ternary alloy, admixed with one part conventional alloy. This alloy may be regarded as an inverse dispersant phase alloy, i.e. a high copper matrix using conventional amalgam as a dispersant (Marshall et al., 1976). The copper content ofthe amalgam was 12%. Fifty-two paedodontic patients received a total of 242 Class I and II amalgam restorations using approximately equal numbers of the four alloys. All restorations were placed by two operators using a standardized technique, including use of the rubber dam. All patients attended the outpatient clinics of the Children's Memorial Hospital in Chicago, Illinois. Clinical indications were present to require placement of these restorations. The sample was collected by recording photographically the occlusal surface of each tooth restored. The magnification ratio obtained on the 35 mm colour transparency was approximately 1:1-3 (Sabott, 1975). The slides were projected for evaluation onto a screen to obtain an image at least fifty-two times life size (Sabott, 1975). A random sub-sample was taken of restorations made from each type of alloy at six months and one year, and a second sub-sample ofthe conventional alloy and the high copper admixed alloy at two years. These transparencies were randomly mixed and projected for evaluation by four dentists. During evaluation a category comparison slide remained in view with examples of four categories of marginal breakdown. Verbal descriptions of the categories were also used to aid the evaluators in category selection. The following descriptions were given for marginal integrity: (1) no marginal perimeter breakdown; (2) good marginal integrity in over 50% of marginal perimeter; (3) good marginal integrity in less than 50% of marginal perimeter; (4) relative 'gross' marginal chipping over the entire marginal perimeter (Sahott, 1975). The sample of restorations was compared to a category comparison slide for surface discoloration. The verbal description of the categories was as follows: (1) high surface lustre; (2) silver colour without high lustre; (3) dull gray colour; (4) dark gray colour. The mode of each restoration was the category assigned to it. The distribution obtained was used to perform a ridit transformation to obtain ridit numbers for each category. The ridit numbers were given to restorations in corresponding categories and ridit means for each type of alloy were calculated for each time period (Bross, 1958). A two-way analysis of variance was used to compare the alloys and the time periods. When the analysis of variance contained more than twa time periods, or more than two types of alloys, a pair-wise comparison was made, using the Mest, in
Clinical study of three Cu-rich amalgam systems
63
the form of least significant difference, to determine which pairs were significantly different from the sample mean. Results and Discussion
Figure 1 is a plot of the ridit means for margins of the four alloys tested at the time intervals they were evaluated. A higher ridit mean indicates a greater amount of marginal breakdown. The numbers on the right correspond to the ridit values assigned to the evaluation categories used in this study. From this graph it can be seen that all four types of amalgams had an increase in marginal breakdown with time. The order of severity of marginal breakdown remained the same throughout all evaluation periods. The conventional dental alloy maintained values in the category (2) to (4) range, as indicated by the appropriate ridits, while the three 72 free alloys stayed between categories (1) and (2), as indicated by the appropriate ridits. max
0 I yr
6 mo
2 yr
Fig. 1. Ridit means for margins of the four alloys tested at the times they were evaluated, P, at time of polish. The evaluation categories (1) through (4) listed on the right are placed at their ridit values. Table 1. Amalgam margins, two-factor analysis of variance (6 month, 1 year) (Admix, Tern, Disp., Conv.) Source of variation
Sum of squares
Time Alloy 2-Way interactions Alloy Time Error
d.f.
Mean square
/
Significance of/
0163 2-410
1 3
0-163 0-803
3-606 17-803
0-057 0-001
0-102 5-415
3 120
' 0-034 0-045
0-750
0-999
Paired comparisons, significant difference from the sample mean Tern. Tern. Disp. Tern.
1 yr-Admix 1 yr 0-185 6 mo-Conv. 6 mo 0-189 6 mo-Conv. 6 mo 0-194 1 yr-Conv. 1 yr 0-265
Admix 1 yr-Conv. 6 mo 0-291 Admix 6 mo-Conv. 6 mo 0-308 Disp. 1 yr-Conv. 1 yr 0-325 Admix 1 yr-Conv. 1 yr 0-450 LSD (a 0-01) (120 d.f.) = 0-177
64
T. H. Larson et al
'
V
•
v
In Table 1 the analysis of variance for the four alloys through six months and one year showed a significant diflerence in the type of alloy used. The / test confirms a diflerence in margins with time only to the 0-057 level, therefore, it seems that after six months parallel curves are established, i.e. that the early failures in marginal integrity set a trend that is significant without regard for time, even at six months. The/test gives a 0-001 level of significance with regard to type of alloy used. When the four alloys were compared pair-wise, it was found that at the 0-01 level most pairings between the conventional alloy and a high copper 72 free alloy were significantly diflerent from the sample mean. In addition, the ternary alloy and admix alloy, when compared at one year, were diflerent from the sample mean at the 0-01 level or better. In Table 2, when only the admix and conventional alloy were compared at six months, one year and two years, it was found that there was a significant diflerence between time periods at the 0-004 level. When the three time periods were compared by pairs using the least significant diflerence method, it was found that there was a diflerence from the sample mean between the conventional alloy at six months and one year.
Table 2. Amalgam margins, two-factor analysis of variance (6 month, 1 year, 2 year) (Admix, Conv.) Source of variation
Time Alloy 2-Way interactions Alloy Time Error
Sum of squares
d.f.
Mean square
/
Significance of/
0 403 3-838
2 1
0-202 3-838
6-001 114-220
0-004 0-001
0100 3-024
2 90
0-050 0-034
1-490
0-299
Paired comparisons, significant difference from the sample mean Conv. 6 mo-Conv. 1 yr 0-159
LSD (a 0-01) (90 d.f.) = 0 ] 5 6
Figure 2 compares the ridit means of surface discoloration for the four alloys throughout the evaluation periods. Higher ridit means represent a greater degree of surface discoloration. In this study, the conventional alloy, Dispersalloy, and ternary alloy remained between category 2 and 3 of surface discoloration and the admix alloy was plotted between category 3 and 4. The analysis of variance for discoloration on Table 3 compares the four alloys at the six month and one year time period. A significant diflerence between alloy type was found at the 0-001 level. When the alloys were compared pair-wise, a diflerence was found between the conventional alloy, ternary alloy and Dispersalloy when compared to the admix at the 0-01 level or better. The analysis of variance in Table 4 is calculated for the admix and conventional alloy through two years. This test showed a diflerence between alloy types at the 0-001 level, but no diflerence between time periods.
Clinical study of three Cu-rich amalgam systems max
^ ^
65
(4)
Fig. 2. Ridit means for tarnish of the four alloys tested at the time they were evaluated, P, at time of polish. The evaluation categories (1) through (4) listed on the right are placed at their ridit values.
Table 3. Amalgam discoloration, two-factor analysis of variance (6 month, 1 year) (Admix, Tern., Disp., Conv.) Source of variation
Time Alloy 2-Way interactions Alloy Time Error
Sum of squares
d.f.
Mean square
/
Significance of/
0-055 2-564
1 3
0-005 0-855
1-072 16-658
0-303 0-001
0-606 6-156
3 120
0-020 0-051
0-393
0-999
Paired comparisons, significant difference from the sample mean Admix 6mo-Conv. 6 mo 0-207 Admix 6 mo-Disp. 6 mo 0-276 Admix 1 yr-Conv. 1 yr 0-320
Admix 1 yr-Disp. Admix 6 mo-Tern. Admix 1 yr-Tern. LSD (a 0 01) (120
1 yr 6 mo 1 yr d.f.) =
0-323 0-329 0-423 O-188
Table 4. Amalgam discoloration, two-factor analysis of variance (6 month, 1 year, 2 year) (Admix, Conv.) Source of variation
Time Alloy 2-Way interactions Time Alloy Error
Significance of/
Sum of squares
df.
Mean square
0-307 1-343
2 1
0-153 1-343
4-045 35-408
0-020 0-001
0-086 3-413
2 90
0-043 0-038
1-129
0-328
66
T. H. Larson et al.
^
Conclusions
In this evaluation it was found that the high copper 72 free amalgam alloys maintained a greater degree of marginal integrity when tested against a conventional dental alloy containing the 72 reaction product. This improvement was found, regardless of which of the three methods was used to produce the high copper amalgam. The diflerence was noticed at the six month evaluation period and the order of marginal integrity did not change with the later evaluation times. The copper admix did show tarnish. The other two high copper alloys had a tarnish level less than the conventional amalgam, but were at a level similar to the conventional amalgam. The tarnish layer was easily removed from the copper admix by repolishing with pumice. The level of tarnish, in general, seemed highly patient dependent. The ridit transformation and analysis of variance seemed to be a good method for clinical evaluation of early changes in marginal perimeter as well as tarnish behaviour. Although the categories of surface discoloration could not be as well controlled or defined, good alloy discrimination was achieved. References (1958) How to use ridit analysis. Biometrics, March, 18. FiNKELSTEiN, G.F. (1976) Electrochemical corrosion of in vitro corrosion of dental amalgam. M.S. Thesis, Northwestern Univ., Chicago, Illinois. MAHLER, D.B., TERKLA, L . G . & VAN EYSDEN, J. (1973) Marginal Fracture of Amalgam Restorations. Journal of Dental Research, 52, 823. MARSHALL, G.W., FINKELSTEIN, G.F. & GREENER, E . H . (1975) Microstructural changes of dental amalgam by copper additions. Journal of Oral Rehabilitation, 3, 359. SABOTT, D . G . (1975) A one year clinical evaluation of an admixed high copper amalgam alloy. M.S. Thesis, Northwestern University, Chicago, Illinois. SARKAR, N . K . (1973) The electrochemical behaviour of dental amalgams and their component phases. PhD. Thesis, Northwestern University, Chicago, Illinois. BROSS, L D . J .
Manuscript accepted 7 August 1977
A clinical study of marginal integrity and tarnish behaviour of three Cu-rich amalgam systems
T. H. LARSON, D. SABOTT, R. COOLEY and ^. H. GREENER Northwestern University, Chicago
Summary
Three high Cu-y2 free amalgams and one conventional amalgam were used in a clinical study to evaluate degree of marginal integrity and surface discoloration occurring for time periods of up to two years. Two hundred and seventy restorations were placed in Class I and II cavities in sixty-two paedodontic patients. Amalgams were prepared from conventional alloy, Cu admixed, Dispersalloy and three parts 50Ag20 Sn-30Cu mixed with one part conventional alloy. Restorations were evaluated by three groups of clinicians. Ridit means obtained from all three groups of evaluations were in good agreement. The lower ridits indicate better marginal integrity and less tarnish. Marginal integrity of amalgams prepared from Cu-admix did not differ appreciably (0-41) from the ridit associated with initial placement (0-39). Conventional amalgams had the highest ridit (0-65) for marginal integrity whereas the ternary/conventional and Dispersalloy restorations were intermediate (0-55 and 0-52). The ridit for tarnish and surface discoloration was decidedly higher for the Cu-admix (0-73) than for the other systems investigated which had ridits of the order of 0-50 compared to an initial ridit for tarnish of 0-28. Introduction
The in vitro saline polarization of yt free amalgams has not displayed the conversion of tin containing phases to tin oxychloride and tin oxide (Sarkar, 1973). It may be expected, however, that -/2 free amalgams would tend to have a higher concentration ratio of yi and would, therefore, be more susceptible to formation of sulphide tarnish films (Finkelstein, 1976). In addition, clinical evaluation of marginal integrity has shown representative y^ free amalgams to maintain a higher degree of marginal integrity over conventional amalgams for comparable time periods (Mahler, Terkla & Van Eysden, 1973; Sabott, 1975). The purpose of this investigation was to compare the clinical marginal stability and the tarnish characteristics of three dental alloys known to produce restorations free of 72 and a conventional dental alloy containing the 72 reaction product. Correspondence: Dr E.H. Greener, Department of Biological Materials, Northwestern University, Dental School, Ward Memorial Building, 311E Chicago Ave., Chicago, Illinois 60611, U.S.A.
0305-182X/79/0100-0061 S02.00 © 1979 Blackwell Scientific Publications 61
61
T. H. Larson et al.
:
.
;
Materials and methods
A clinical evaluation was initiated using the following amalgam alloys: (1) conventional amalgam alloy (Aristaloy) prepared according to the manufacturer's recommendations. (2) An admixed high copper amalgam, prepared by mixing copper amalgam and conventional amalgam in a ratio of 1:2 by volume, by the technique reported by Sabott. The resulting amalgam contained approximately 11-6% copper by weight (Sabott, 1975). (3) Commercial amalgam alloy (Dispersalloy), containing 1 part spherical particles of AgCu eutectic dispersed in two parts of Ag3Sn alloy powder. This amalgam contained about 6% copper (Marshall, Finkelstein & Greener, 1976). (4) Three parts of an as-cast 30% Cu, 52-5% Ag, 17-5% Sn ternary alloy, admixed with one part conventional alloy. This alloy may be regarded as an inverse dispersant phase alloy, i.e. a high copper matrix using conventional amalgam as a dispersant (Marshall et al., 1976). The copper content ofthe amalgam was 12%. Fifty-two paedodontic patients received a total of 242 Class I and II amalgam restorations using approximately equal numbers of the four alloys. All restorations were placed by two operators using a standardized technique, including use of the rubber dam. All patients attended the outpatient clinics of the Children's Memorial Hospital in Chicago, Illinois. Clinical indications were present to require placement of these restorations. The sample was collected by recording photographically the occlusal surface of each tooth restored. The magnification ratio obtained on the 35 mm colour transparency was approximately 1:1-3 (Sabott, 1975). The slides were projected for evaluation onto a screen to obtain an image at least fifty-two times life size (Sabott, 1975). A random sub-sample was taken of restorations made from each type of alloy at six months and one year, and a second sub-sample ofthe conventional alloy and the high copper admixed alloy at two years. These transparencies were randomly mixed and projected for evaluation by four dentists. During evaluation a category comparison slide remained in view with examples of four categories of marginal breakdown. Verbal descriptions of the categories were also used to aid the evaluators in category selection. The following descriptions were given for marginal integrity: (1) no marginal perimeter breakdown; (2) good marginal integrity in over 50% of marginal perimeter; (3) good marginal integrity in less than 50% of marginal perimeter; (4) relative 'gross' marginal chipping over the entire marginal perimeter (Sahott, 1975). The sample of restorations was compared to a category comparison slide for surface discoloration. The verbal description of the categories was as follows: (1) high surface lustre; (2) silver colour without high lustre; (3) dull gray colour; (4) dark gray colour. The mode of each restoration was the category assigned to it. The distribution obtained was used to perform a ridit transformation to obtain ridit numbers for each category. The ridit numbers were given to restorations in corresponding categories and ridit means for each type of alloy were calculated for each time period (Bross, 1958). A two-way analysis of variance was used to compare the alloys and the time periods. When the analysis of variance contained more than twa time periods, or more than two types of alloys, a pair-wise comparison was made, using the Mest, in
Clinical study of three Cu-rich amalgam systems
63
the form of least significant difference, to determine which pairs were significantly different from the sample mean. Results and Discussion
Figure 1 is a plot of the ridit means for margins of the four alloys tested at the time intervals they were evaluated. A higher ridit mean indicates a greater amount of marginal breakdown. The numbers on the right correspond to the ridit values assigned to the evaluation categories used in this study. From this graph it can be seen that all four types of amalgams had an increase in marginal breakdown with time. The order of severity of marginal breakdown remained the same throughout all evaluation periods. The conventional dental alloy maintained values in the category (2) to (4) range, as indicated by the appropriate ridits, while the three 72 free alloys stayed between categories (1) and (2), as indicated by the appropriate ridits. max
0 I yr
6 mo
2 yr
Fig. 1. Ridit means for margins of the four alloys tested at the times they were evaluated, P, at time of polish. The evaluation categories (1) through (4) listed on the right are placed at their ridit values. Table 1. Amalgam margins, two-factor analysis of variance (6 month, 1 year) (Admix, Tern, Disp., Conv.) Source of variation
Sum of squares
Time Alloy 2-Way interactions Alloy Time Error
d.f.
Mean square
/
Significance of/
0163 2-410
1 3
0-163 0-803
3-606 17-803
0-057 0-001
0-102 5-415
3 120
' 0-034 0-045
0-750
0-999
Paired comparisons, significant difference from the sample mean Tern. Tern. Disp. Tern.
1 yr-Admix 1 yr 0-185 6 mo-Conv. 6 mo 0-189 6 mo-Conv. 6 mo 0-194 1 yr-Conv. 1 yr 0-265
Admix 1 yr-Conv. 6 mo 0-291 Admix 6 mo-Conv. 6 mo 0-308 Disp. 1 yr-Conv. 1 yr 0-325 Admix 1 yr-Conv. 1 yr 0-450 LSD (a 0-01) (120 d.f.) = 0-177
64
T. H. Larson et al
'
V
•
v
In Table 1 the analysis of variance for the four alloys through six months and one year showed a significant diflerence in the type of alloy used. The / test confirms a diflerence in margins with time only to the 0-057 level, therefore, it seems that after six months parallel curves are established, i.e. that the early failures in marginal integrity set a trend that is significant without regard for time, even at six months. The/test gives a 0-001 level of significance with regard to type of alloy used. When the four alloys were compared pair-wise, it was found that at the 0-01 level most pairings between the conventional alloy and a high copper 72 free alloy were significantly diflerent from the sample mean. In addition, the ternary alloy and admix alloy, when compared at one year, were diflerent from the sample mean at the 0-01 level or better. In Table 2, when only the admix and conventional alloy were compared at six months, one year and two years, it was found that there was a significant diflerence between time periods at the 0-004 level. When the three time periods were compared by pairs using the least significant diflerence method, it was found that there was a diflerence from the sample mean between the conventional alloy at six months and one year.
Table 2. Amalgam margins, two-factor analysis of variance (6 month, 1 year, 2 year) (Admix, Conv.) Source of variation
Time Alloy 2-Way interactions Alloy Time Error
Sum of squares
d.f.
Mean square
/
Significance of/
0 403 3-838
2 1
0-202 3-838
6-001 114-220
0-004 0-001
0100 3-024
2 90
0-050 0-034
1-490
0-299
Paired comparisons, significant difference from the sample mean Conv. 6 mo-Conv. 1 yr 0-159
LSD (a 0-01) (90 d.f.) = 0 ] 5 6
Figure 2 compares the ridit means of surface discoloration for the four alloys throughout the evaluation periods. Higher ridit means represent a greater degree of surface discoloration. In this study, the conventional alloy, Dispersalloy, and ternary alloy remained between category 2 and 3 of surface discoloration and the admix alloy was plotted between category 3 and 4. The analysis of variance for discoloration on Table 3 compares the four alloys at the six month and one year time period. A significant diflerence between alloy type was found at the 0-001 level. When the alloys were compared pair-wise, a diflerence was found between the conventional alloy, ternary alloy and Dispersalloy when compared to the admix at the 0-01 level or better. The analysis of variance in Table 4 is calculated for the admix and conventional alloy through two years. This test showed a diflerence between alloy types at the 0-001 level, but no diflerence between time periods.
Clinical study of three Cu-rich amalgam systems max
^ ^
65
(4)
Fig. 2. Ridit means for tarnish of the four alloys tested at the time they were evaluated, P, at time of polish. The evaluation categories (1) through (4) listed on the right are placed at their ridit values.
Table 3. Amalgam discoloration, two-factor analysis of variance (6 month, 1 year) (Admix, Tern., Disp., Conv.) Source of variation
Time Alloy 2-Way interactions Alloy Time Error
Sum of squares
d.f.
Mean square
/
Significance of/
0-055 2-564
1 3
0-005 0-855
1-072 16-658
0-303 0-001
0-606 6-156
3 120
0-020 0-051
0-393
0-999
Paired comparisons, significant difference from the sample mean Admix 6mo-Conv. 6 mo 0-207 Admix 6 mo-Disp. 6 mo 0-276 Admix 1 yr-Conv. 1 yr 0-320
Admix 1 yr-Disp. Admix 6 mo-Tern. Admix 1 yr-Tern. LSD (a 0 01) (120
1 yr 6 mo 1 yr d.f.) =
0-323 0-329 0-423 O-188
Table 4. Amalgam discoloration, two-factor analysis of variance (6 month, 1 year, 2 year) (Admix, Conv.) Source of variation
Time Alloy 2-Way interactions Time Alloy Error
Significance of/
Sum of squares
df.
Mean square
0-307 1-343
2 1
0-153 1-343
4-045 35-408
0-020 0-001
0-086 3-413
2 90
0-043 0-038
1-129
0-328
66
T. H. Larson et al.
^
Conclusions
In this evaluation it was found that the high copper 72 free amalgam alloys maintained a greater degree of marginal integrity when tested against a conventional dental alloy containing the 72 reaction product. This improvement was found, regardless of which of the three methods was used to produce the high copper amalgam. The diflerence was noticed at the six month evaluation period and the order of marginal integrity did not change with the later evaluation times. The copper admix did show tarnish. The other two high copper alloys had a tarnish level less than the conventional amalgam, but were at a level similar to the conventional amalgam. The tarnish layer was easily removed from the copper admix by repolishing with pumice. The level of tarnish, in general, seemed highly patient dependent. The ridit transformation and analysis of variance seemed to be a good method for clinical evaluation of early changes in marginal perimeter as well as tarnish behaviour. Although the categories of surface discoloration could not be as well controlled or defined, good alloy discrimination was achieved. References (1958) How to use ridit analysis. Biometrics, March, 18. FiNKELSTEiN, G.F. (1976) Electrochemical corrosion of in vitro corrosion of dental amalgam. M.S. Thesis, Northwestern Univ., Chicago, Illinois. MAHLER, D.B., TERKLA, L . G . & VAN EYSDEN, J. (1973) Marginal Fracture of Amalgam Restorations. Journal of Dental Research, 52, 823. MARSHALL, G.W., FINKELSTEIN, G.F. & GREENER, E . H . (1975) Microstructural changes of dental amalgam by copper additions. Journal of Oral Rehabilitation, 3, 359. SABOTT, D . G . (1975) A one year clinical evaluation of an admixed high copper amalgam alloy. M.S. Thesis, Northwestern University, Chicago, Illinois. SARKAR, N . K . (1973) The electrochemical behaviour of dental amalgams and their component phases. PhD. Thesis, Northwestern University, Chicago, Illinois. BROSS, L D . J .
Manuscript accepted 7 August 1977
