Scand. J. Dent. Res. 1976: 84: 430-433 (Key words: dental amalgam)
Marginal fracture of dental amalgams L. FORSTEN AND M.-L. KALLIO Institute of Dentistry, University of Turku, Turku, Finland ABSTRACT — The purpose of this study was to determine the marginal fracture of different amalgams, using a semiclinical method which facilitates standardization and minimizes the observation time, in 10 patients receiving complete dentures. Cavities were cut occlusally in acrylic premolars and molars. The margin was then beveled to 45° with a cone-shaped diamond. Two dispersion-strengthened amalgams and one conventional amalgam were used in each mouth. The cavities of each quadrant were filled with the same amalgam. Thus, conventional amalgam fillings were always placed in opposing quadrants to those filled with a dispersion-strengthened amalgam. The degree of marginal fracture was evaluated from the prints (magnification X 5-7) by five dentists separately and blindly. After 6 months' service, severe marginal fracturing was frequently seen.
Deterioration of amalgam fillings with time is to a large degree due to galvanic corrosion. Efforts have been made to reduce this by decreasing the tin-mercury phase called gamma II. As a result, a dispersion-strengthened alloy with less tin was developed (INNES & YOUDELIS 1963). When clinically used, this amalgam showed less miarginal fracture than conventional amalgams (MAHLER et al. 1970, 1973). The degree of fracture was compared with the mechanical properties of the amalgarjis and a direct correlation between a low degree of fracture and a low creep value was demonstrated, without, however, considering the role of corrosion. Due to the absorption of mercury liberated by corrosion, the margin of amalgam fillings tend to bend away from the enamel (JORGENSEN 1965). An amalgam edge without tooth support fractures easi-
ly. It seems important to know whether this correlation between the creep value and the clinical behavior of the margins still exists when the effect of corrosion is excluded. The purpose of this study was to evaluate the degree of marginal fracture of different types of amalgams possessing different creep values when the effect of corrosion is reduced to a minimum. Furthermore, the aim was to design a semiclinical method which facilitates standardization and quickens marginal breakdown. Material and methods Ten patients receiving maxillary and mandibular complete dentures with acrylic teeth were selected for the study. After selective grinding, occlusal cavities of 2—2.5 mm in depth were cut into the premolars and molars using ultrahighspeed and a carbide fissure bur ( 0 = 1
431
FRACTURE OF AMALGAM
Fig. 1. Cross-section of acrylic tooth, showing beveled cavity and cone-shaped diamond. mm). After this, the margins of the cavities were beveled to approximately a 45° angle with a coneshaped diamond (Fig. 1). The preparation was controlled with a stereomicroscope ( X 40). After rinsing and drying, the cavity walls were covered with a thin layer of Copalite® (Dr. R. C. Cooley, USA) cavity varnish to prevent the thin edges of amalgam from being pulled off during carving. The amalgam was mixed in a Silamat® (Ivoclar AG, Liechtenstein) mixing device for 5 sec. One quadrant (3-4 cavities) was filled with each mix, beginning from the first premolar. Each filling was condensed using a hand pressure of 0.5 kg on a circular condenser ( 0 = 1 mm) applying 10 thrusts on each of three increments and 20 thrusts with a larger condensor ( 0 = 1.5 mm) on the fourth and last increment, which overfilled the cavity. The time
needed for filling the cavities in each quadrant was 3—5 min. The restorations were then carved using a modified Maehlum carving instrument. The amalgams were allowed to harden for 24 h before polishing with pear-shaped finishing burs and rubber-carborundum points using waterspray cooling. The amalgam alloys investigated in this study are presented together with their creep values in Table 1. All three alloys were used in every pair of dentures so that the conventional amalgam fillings were always placed opposite the dispersion-strengthened amalgam fillings. This resulted in four different combinations (Fig. 2). A total of 74 fillings of the conventional amalgam and 37 of each dispersion amalgam were made. Impressions were taken of the dentures with the fillings using a rubber base material (Impregum® Espe, Western Germany) and models were cast in plaster and then photographed. This was done immediately after finishing and after 6 months of service. The degree of marginal fracture was evaluated from the prints (magnification X 5-7) by five dentists separately and blindly. The evaluators graded the fracture of the restorations of each quadrant on a scale from 1 to 5. As a reference, four photographs were selected with different grades of marginal breakdown (Fig. 3). Grade 1 was
B
C
A
B
C
Fig. 2. Four combinations of placing three different amalgams in maxillary and mandibular complete dentures.
Table 1 Amalgam alloys studied Brand name
Description
Manufacturer
Greepvalue(%)*
A
Amalcap F.G.®
conventional
Vivadent AG Liechtenstein
4.77
B
Dispersalloy®
dispersion strengthened
Johnson & Johnson USA
0.31
G
Amalcap non-gamma®
dispersion strengthened
Vivadent AG Liechtenstein
0.30
Code
*S. Espevik, Scandinavian Institute of Dental Materials.
FORSTEN AND KALLIO
432
GRADE
Fig. 3. Glassification of marginal fracture of the amalgam restorations. A, minimal fracture. B, some fracture. C, moderate fracture. D, gross fracture.
given to those restorations that had less fracture than in Fig. 3 A. Grade 2 was given to those fillings that showed more fracture than in Fig. 3 A, but less than in Fig. 3 B, and so on. The mean values of the grades given by each evaluator of each amalgam brand were calculated and the differences were studied with Student's t-test. Results When the marginal fracture was studied after 6 months, all evaluators gave slightly lower mean grades to the dispersionstrengthened amalgams than to the corresponding fillings of the conventional amalgam (Table 2). There was only one exception where one dispersion amalgam got the same grade as the conventional amalgam. When the t-test was applied to the results, no significant differences could be shown at the 95 % confidence level, nor was there any significant difference in the marginal breakdown between the two dispersion-strengthened amalgams.
Table 2 Grade values of marginal fracture for two dispersion-strengthened amalgams and of conventional amalgam fillings in opposing quadrants (number of full dentures used = 10) Eval- Amalgam Amalgam Amalgam Amalgam uator A Mean B Mean A Mean G Mean I
3.2
II
2.8 3.3 3.0 3.3
in IV V
2.9 2.2 3.1 2.5 3.1
3.5
3.3
2.8 3.6 3.3 3.7
2.4 3.6 3.0 3.4
Results of Student's t-test Mean grades Amalgam B compared with amalgam A Amalgam G compared with amalgam A Amalgam B compared with amalgam G
t*
2.8
3.1
1.7
3.1
3.4
1.0
2.8
3.1
1.7
*t value at the 95 % confidence level = 2.0.
FRAGTURE OF AMALGAM Table 3 Mean
grade of marginal fracture for fillings from all four quadrants of each case
Case
Mean grade
1
2.5
2
4.0 3.8 3.4 2.1 3.0 3.8 1.7 4.5 2.2
3
4 5 6
7 8
9 10
When the average grade of fracture was calculated for all restorations of each pair of dentures, great differences were found between the different cases (Table 3).The case with the smallest marginal breakdown got an average grade of 1.7 whereas the case with the greatest breakdown got a grade of 4.5.
433
differences in biting forces and use of dentures. Since there were only minimal differences in marginal fracture between the individual restorations of each quadrant, the section was considered as a whole without evaluating each filling separately. All five evaluators rated the average marginal fracture as being smaller with the dispersion-strengthened amalgams than with the conventional amalgam. This trend was therefore quite clear. The differences were, however, very small and of no statistical significance in this study, where the short observation time did not allow corrosion to any greater extent. The large differences found in earlier clinical studies (MAHLER et al. 1970, 1973) may thus have been due more to the effect of corrosion than to a difference in creep values. Consequently, it may not be justifiable to predict the clinical behavior of the margins of an amalgam based on the creep value of the brand of the amalgam only.
References Discussion
The semiclinical method for studying the marginal fracture appears to have many advantages. It was easy to standardize the size and shape of the cavity and filling. It was possible to make the margin of the restoration sharp and thin in order to speed up the marginal breakdown. The acrylate, being more resilient than enamel, gives less support to the filling, which might have quickened the marginal fracturing as well. Thus, gross marginal fracturing was seen in many cases within half a year. It may, however, be noted that the degree of breakdown varied markedly between the different cases, obviously due to
& YOUDELIS, W . V.: Dispersion strengthened amalgams. / . Ca?i. Dent. Assoc. 1963: 29: 587-593.
INNES, D . B . K .
JORGENSEN, K . D . : The mechanism of marginal
fracture of amalgam fillings. Acta Odontol. Scand. 1965: 23: 347-389. MAHLER, D . B., TERKLA, L . G., EYSDEN, J. v. & REISBICK, M . H . : Marginal fracture vs.
mechanical properties of amalgam. / . Dent. Res. 1970: 49: 1452-1457. M.\HLER, D. B., TERKLA, L . G. & EYSDEN, J. v.:
Marginal fracture of amalgam restorations. /. Dent. Res. 1973: 52: 823-827. Address: L. Forsten Lemminkaisenkatu 2 SF-20520 Turku 52 Finland
Marginal fracture of dental amalgams L. FORSTEN AND M.-L. KALLIO Institute of Dentistry, University of Turku, Turku, Finland ABSTRACT — The purpose of this study was to determine the marginal fracture of different amalgams, using a semiclinical method which facilitates standardization and minimizes the observation time, in 10 patients receiving complete dentures. Cavities were cut occlusally in acrylic premolars and molars. The margin was then beveled to 45° with a cone-shaped diamond. Two dispersion-strengthened amalgams and one conventional amalgam were used in each mouth. The cavities of each quadrant were filled with the same amalgam. Thus, conventional amalgam fillings were always placed in opposing quadrants to those filled with a dispersion-strengthened amalgam. The degree of marginal fracture was evaluated from the prints (magnification X 5-7) by five dentists separately and blindly. After 6 months' service, severe marginal fracturing was frequently seen.
Deterioration of amalgam fillings with time is to a large degree due to galvanic corrosion. Efforts have been made to reduce this by decreasing the tin-mercury phase called gamma II. As a result, a dispersion-strengthened alloy with less tin was developed (INNES & YOUDELIS 1963). When clinically used, this amalgam showed less miarginal fracture than conventional amalgams (MAHLER et al. 1970, 1973). The degree of fracture was compared with the mechanical properties of the amalgarjis and a direct correlation between a low degree of fracture and a low creep value was demonstrated, without, however, considering the role of corrosion. Due to the absorption of mercury liberated by corrosion, the margin of amalgam fillings tend to bend away from the enamel (JORGENSEN 1965). An amalgam edge without tooth support fractures easi-
ly. It seems important to know whether this correlation between the creep value and the clinical behavior of the margins still exists when the effect of corrosion is excluded. The purpose of this study was to evaluate the degree of marginal fracture of different types of amalgams possessing different creep values when the effect of corrosion is reduced to a minimum. Furthermore, the aim was to design a semiclinical method which facilitates standardization and quickens marginal breakdown. Material and methods Ten patients receiving maxillary and mandibular complete dentures with acrylic teeth were selected for the study. After selective grinding, occlusal cavities of 2—2.5 mm in depth were cut into the premolars and molars using ultrahighspeed and a carbide fissure bur ( 0 = 1
431
FRACTURE OF AMALGAM
Fig. 1. Cross-section of acrylic tooth, showing beveled cavity and cone-shaped diamond. mm). After this, the margins of the cavities were beveled to approximately a 45° angle with a coneshaped diamond (Fig. 1). The preparation was controlled with a stereomicroscope ( X 40). After rinsing and drying, the cavity walls were covered with a thin layer of Copalite® (Dr. R. C. Cooley, USA) cavity varnish to prevent the thin edges of amalgam from being pulled off during carving. The amalgam was mixed in a Silamat® (Ivoclar AG, Liechtenstein) mixing device for 5 sec. One quadrant (3-4 cavities) was filled with each mix, beginning from the first premolar. Each filling was condensed using a hand pressure of 0.5 kg on a circular condenser ( 0 = 1 mm) applying 10 thrusts on each of three increments and 20 thrusts with a larger condensor ( 0 = 1.5 mm) on the fourth and last increment, which overfilled the cavity. The time
needed for filling the cavities in each quadrant was 3—5 min. The restorations were then carved using a modified Maehlum carving instrument. The amalgams were allowed to harden for 24 h before polishing with pear-shaped finishing burs and rubber-carborundum points using waterspray cooling. The amalgam alloys investigated in this study are presented together with their creep values in Table 1. All three alloys were used in every pair of dentures so that the conventional amalgam fillings were always placed opposite the dispersion-strengthened amalgam fillings. This resulted in four different combinations (Fig. 2). A total of 74 fillings of the conventional amalgam and 37 of each dispersion amalgam were made. Impressions were taken of the dentures with the fillings using a rubber base material (Impregum® Espe, Western Germany) and models were cast in plaster and then photographed. This was done immediately after finishing and after 6 months of service. The degree of marginal fracture was evaluated from the prints (magnification X 5-7) by five dentists separately and blindly. The evaluators graded the fracture of the restorations of each quadrant on a scale from 1 to 5. As a reference, four photographs were selected with different grades of marginal breakdown (Fig. 3). Grade 1 was
B
C
A
B
C
Fig. 2. Four combinations of placing three different amalgams in maxillary and mandibular complete dentures.
Table 1 Amalgam alloys studied Brand name
Description
Manufacturer
Greepvalue(%)*
A
Amalcap F.G.®
conventional
Vivadent AG Liechtenstein
4.77
B
Dispersalloy®
dispersion strengthened
Johnson & Johnson USA
0.31
G
Amalcap non-gamma®
dispersion strengthened
Vivadent AG Liechtenstein
0.30
Code
*S. Espevik, Scandinavian Institute of Dental Materials.
FORSTEN AND KALLIO
432
GRADE
Fig. 3. Glassification of marginal fracture of the amalgam restorations. A, minimal fracture. B, some fracture. C, moderate fracture. D, gross fracture.
given to those restorations that had less fracture than in Fig. 3 A. Grade 2 was given to those fillings that showed more fracture than in Fig. 3 A, but less than in Fig. 3 B, and so on. The mean values of the grades given by each evaluator of each amalgam brand were calculated and the differences were studied with Student's t-test. Results When the marginal fracture was studied after 6 months, all evaluators gave slightly lower mean grades to the dispersionstrengthened amalgams than to the corresponding fillings of the conventional amalgam (Table 2). There was only one exception where one dispersion amalgam got the same grade as the conventional amalgam. When the t-test was applied to the results, no significant differences could be shown at the 95 % confidence level, nor was there any significant difference in the marginal breakdown between the two dispersion-strengthened amalgams.
Table 2 Grade values of marginal fracture for two dispersion-strengthened amalgams and of conventional amalgam fillings in opposing quadrants (number of full dentures used = 10) Eval- Amalgam Amalgam Amalgam Amalgam uator A Mean B Mean A Mean G Mean I
3.2
II
2.8 3.3 3.0 3.3
in IV V
2.9 2.2 3.1 2.5 3.1
3.5
3.3
2.8 3.6 3.3 3.7
2.4 3.6 3.0 3.4
Results of Student's t-test Mean grades Amalgam B compared with amalgam A Amalgam G compared with amalgam A Amalgam B compared with amalgam G
t*
2.8
3.1
1.7
3.1
3.4
1.0
2.8
3.1
1.7
*t value at the 95 % confidence level = 2.0.
FRAGTURE OF AMALGAM Table 3 Mean
grade of marginal fracture for fillings from all four quadrants of each case
Case
Mean grade
1
2.5
2
4.0 3.8 3.4 2.1 3.0 3.8 1.7 4.5 2.2
3
4 5 6
7 8
9 10
When the average grade of fracture was calculated for all restorations of each pair of dentures, great differences were found between the different cases (Table 3).The case with the smallest marginal breakdown got an average grade of 1.7 whereas the case with the greatest breakdown got a grade of 4.5.
433
differences in biting forces and use of dentures. Since there were only minimal differences in marginal fracture between the individual restorations of each quadrant, the section was considered as a whole without evaluating each filling separately. All five evaluators rated the average marginal fracture as being smaller with the dispersion-strengthened amalgams than with the conventional amalgam. This trend was therefore quite clear. The differences were, however, very small and of no statistical significance in this study, where the short observation time did not allow corrosion to any greater extent. The large differences found in earlier clinical studies (MAHLER et al. 1970, 1973) may thus have been due more to the effect of corrosion than to a difference in creep values. Consequently, it may not be justifiable to predict the clinical behavior of the margins of an amalgam based on the creep value of the brand of the amalgam only.
References Discussion
The semiclinical method for studying the marginal fracture appears to have many advantages. It was easy to standardize the size and shape of the cavity and filling. It was possible to make the margin of the restoration sharp and thin in order to speed up the marginal breakdown. The acrylate, being more resilient than enamel, gives less support to the filling, which might have quickened the marginal fracturing as well. Thus, gross marginal fracturing was seen in many cases within half a year. It may, however, be noted that the degree of breakdown varied markedly between the different cases, obviously due to
& YOUDELIS, W . V.: Dispersion strengthened amalgams. / . Ca?i. Dent. Assoc. 1963: 29: 587-593.
INNES, D . B . K .
JORGENSEN, K . D . : The mechanism of marginal
fracture of amalgam fillings. Acta Odontol. Scand. 1965: 23: 347-389. MAHLER, D . B., TERKLA, L . G., EYSDEN, J. v. & REISBICK, M . H . : Marginal fracture vs.
mechanical properties of amalgam. / . Dent. Res. 1970: 49: 1452-1457. M.\HLER, D. B., TERKLA, L . G. & EYSDEN, J. v.:
Marginal fracture of amalgam restorations. /. Dent. Res. 1973: 52: 823-827. Address: L. Forsten Lemminkaisenkatu 2 SF-20520 Turku 52 Finland
