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Silver pins: Their influence on the strength and adaptation of amalgam
Operative dentistry
Silver
pins:
strength
Their
and
influence
adaptation
on the of am&am
Joseph P. Moffa, D.D.S., M.S.D.,* Robert E. Going, D.D.S., MS.,** and Lawrence Geftleman, D.M.D., M.S.D.*** IJnited States Public Health Service Hospital, San Francisco, Calif.
D
Previous studies have shown that threaded stainless steel pins placed in dental amalgam for reinforcement do not increase the strength of the amalgam.lm4 Stainless steel pins are not adhesive to amalgam but depend upon the adaptation of the amalgam into the pin’s surface irregularities for retention. In addition, adequate condensation around the pin is essential to achieve optimal adaptation and strength of the amalgam. Recent innovations in pins and instruments related to pin techniques have been said to achieve bonding of the pin to the amalgam matrix and to improve condensation. The purpose of this study was to assess the influence of certain proprietary threaded pins made of silver? and stainless steel pins plated with silver* upon the ultimate compressive strength of amalgam. Strength values were compared using conventional hand-condensing instruments and a special hand-condensing instrument.5 Adaptation at the amalgam-pin interface and the nature of the compressive fracture were also studied using metallographic procedures. MATERIALS
AND
METHODS
Cylindrical specimens were prepared for evaluation by condensing dental amalgam into a 6 by 12 mm. steel die and by storing the completed specimens at Portions of this article Association
Meeting,
Atlantic
were presented City, N. J.
at the scientific
session of the American
*Dental Research Coordinator, Federal Health Programs Service, Health Mental Health Administration, Department of Health, Education and Welfare, Public Health Service. **Associate Professor and College of Dentistry, Gainesville, ***Assistant Professor, cine, Boston, Mass. +Silver
Chairman, Fla.
Prosthetic
Dentistry,
pin wire, E. A. Beck Company,
$Silver-threaded
wire
$Special Condenser
with
stainless
Division
of Biomaterials,
Harvard
Anaheim,
University
Services and United States
University School
Dental
of Florida,
of Dental
Medi-
Calif.
steel core, E. A. Beck Company,
No. 12 with holes, E. A. Beck Company,
Anaheim,
Anaheim,
Calif,
Calif.
491
492
Moffa,
J. Prosthet. Dent. November, 1972
G oing, and Gettleman
room temperature for seven days prior to testing. All specimens were prepared and tested according to a random scheme, The orientation of four pins within each specimen was maintained parallel to the long axis of the die as described in a previous artic1e.l The pins were cut from bulk lengths threaded wire 0.64 mm. (0.025 inch) in diameter. Pins made of silver (Ag) and stainless steel plated with silver (AgSS) were cut to 14.5 mm. lengths. Before testing, the 4.5 mm. lengths of pins protruding from the base of all test specimens were removed. This left pins 10 mm. long incorporated within the 6 by 12 mm. amalgam specimens. Test specimens were made with New True Dentalloy” tablets at a 1:l mercuryalloy ratio. Trituration was completed in Bakelite capsules inserted for 18 seconds in a Wig-L-Bug? amalgamator. Both serrated and smooth-faced hand pluggers varying from 1 to 4 mm. in diameter were used as conventional hand condensers. smoothFor hand condensing of amalgam around the pins, special double-ended faced pluggers, 1.5 and 2.5 mm. in diameter with a hole 0.68 mm. (0.027 inch) in diameter running longitudinally through the nib, were used (Fig. 1). Condensing thrusts of 3 to 6 pounds, standardized on an Instron load cell,3 were directed vertically for both condensing methods. Compressive strength determinations were made for ten replications of each factorial combination of variables, or a total of 50 test specimens. Variables included silver pins (AG) , stainless steel pins plated with silver (AgSS) , and conventional and special condensation instruments. Specimens containing no pins condensed with conventional hand instruments served as controls. Additional controls without pins but condensed with the special instrument (Fig. 1) would have been meaningless since the center of the nib has a hole. Strength determinations were made on a mechanical testing instrumentt at a loading rate of 0.05 cm. per minute. Test specimens were loaded parallel with their long axis until failure occurred. Metallographic evaluations were performed on 10 representative specimens before and after fracture. Uncrushed specimens containing stainless steel pins plated with silver (AgSS) and stainless steel pins5 (SS) were examined, as well as cylinders containing silver (Ag) and gold-plated steel pins11 (A&S). Both crushed and uncrushed cylinders were imbedded in polymethyl methacrylate resin to prevent crumbling of the amalgam fragments. These specimens were then sliced with a diamond disc near the center of the crushed cylinder and remounted in methacrylatefilled metallurgic rings. Wet grinding was done with 240, 320, 400, and 600 grit silicon carbide paper followed by polishing with 6 pm diamond paste on a metallographic polisher. Etching was accomplished wth KCN and KI following the method of Allan, Asgar, and Peyton.” *S. S. White Dental Mfg. Company, King-of-Prussia, *Crescent Dental Mfg. Company, Chicago, Ill. SInstron Corp., Canton, Mass. SThreaded wire, K & R Dental [ITMS
regular,
Whaledent,
Products
Inc., Brooklyn,
Company, N. Y.
Pa.
Blue Island, Ill.
volume 28 Number 5
Silver
pins
493
Fig. 1. A special amalgam condenser which was theoretically designed to improve the condensation of amalgam surrounding a pin-retaining device. Note the hole in the nib for thP pk.
Table I. Effect of various of amalgam
types of pins and condensers upon the compressive
strength .-.-
Type Conventional Type
of pin*
Silver Silver-plated stainless steel *Control tstandard
of condemer Special
I
p.s.i.
S.D.f
54,000 52,200
1,000 1,200
p.s.i. .513,700 35,700
!
S.D.t 1:lOO 1,400 --.
with no pins = 54,600 ( 1,200). deviation.
RESULTS The mean compressive strength for the seven-day-old amalgam specimens. which were condensed with both conventional and special condensers and contained threaded silver pins (Ag), threaded silver-plated stainless steel pins (A&). and no pins (controls), are presented in Table I. The data from this factorial experiment with a single control group were analyzed by an analysis of variance technique (Table II) as described by Winer.” The 99 per cent level of confidenct was selected to determine statistical significance. The statistical analysis indicated that there was no significant difference between the compressive strength of the control specimen with no pins and the strength of those specimens which contained either the silver pins of the silver-plated stainless steel pins. Similarly, there was no significant difference between strengths of specimens which were condensed by means of the conventional instruments. However. those specimens which contained the silver pins were found to be significantl) weaker than specimens containing the silver-plated stainless steel pins (Table II )
494
Going,
Moffa,
J. Prosthet. Dent. November, 1972
and Gettleman
Fig. 2. Surface patterns
Table
II. Factorial
Source
analysis of variance
of variation
Control vs. all others Type of pin Type of condenser Pin and condenser Within cell *Not
of pins.
with a single control group
ss
df
163,192 18,491,920 103,938 3,392,480 66.410.553
1 1 1 1 45
1
MS
F
P
163,192 18,491,920 103,938 3,392,480 l-475,790
0.1105 12.5301 0.0704 2.2987
0.259* 0.999t 0.208* 0.864*
significant.
TSignificant.
Microscopic examination of the pins revealed that they differ considerably in their circumferential contour (Fig. 2). All pins were threaded to some extent, with less thread depth in the stainless steel and silver pins. Much sharper threads with greater depth were found in the gold-plated stainless steel pins (AuSS) . Examination of cross sections of imbedded pins in amalgam revealed that the adaptation of the amalgam to the silver-plated, gold-plated, and unplated stainless steel pins appeared incomplete and contained numerous voids (Figs. 3 and 4). Also, the conventional and special condensation procedures produced no significant difference in the occurrence of these voids. Conversely, the adaptation of the amalgam matrix to the silver pin appeared to be excellent in that the interface was distinguishable only by a slight difference in color. The presence or absence of pins within the specimens significantly influenced the gross type of fracture pattern (Fig. 5) . The specimen without pins produced the typical cone-shaped fracture patterns at both ends of the specimen. However,
Volume 28 Number 5
views of metallographic Fig. 3. High- and low-power and silver-plated gold. -plated stainless steel (Au%) amal Igam.
Silver pins
495
sections illustrate the interface betwt XII stainless steel (AgSS) pins and dec ita1
Fig. 4. High- and low-power views of metallographic sections illustrate silver (Ag) and stainless steel (SS) pins and dental amalgam.
the interface
between
496
Moffa,
Going,
.I, Prosthet. Dent. November, 1972
and Gettleman
Fig. 5. Specimens which reflect the different the presence or absence of stainless steel pins.
modes of compressive
fracture
as influenced
by
amalgam specimens which contain stainless steel pins demonstrated an aberration of this fracture pattern. The longitudinal compressive stresses caused the pins to bend outward, fracturing the outer surface of amalgam and usually producing a fracture cone at only one end of the specimen. The most significant microscopic observation was the striking difference in the nature of amalgam fracture in those specimens which contained the stainless steel and silver pins (Fig. 6). The lines of fracture in the specimens containing stainless steel pins (either plated or unplated) passed through the site of the pins. This failure at the pin-amalgam interface virtually left the pins remaining in empty cavities. In marked contrast, the silver pins remained enclosed within the amalgam matrix. The fracture passed at some distance from the pins and was restricted to the matrix. DISCUSSION Nonparallel pin-retaining devices usually are made of stainless steel, and numerous studies have shown that they do not increase the strength of amalgam.‘-* These stainless steel pins are not adhesive to dental amalgam but rely upon their threaded surface irregularities to achieve retention within the alloy by a mechanical interlocking phenomenon. It has been suggested that this failure to achieve an adhesive bond is also partially responsible for the failure to improve the strength properties of amalgam. Recent attempts have been made to correct this deficiency by the de-
Silver pins
Fig. 6. High-
and low-power views of metallographic sections illustrate the differencr site of amalgam fracture between silver (Ag) and stainless steel (SS) pins.
497
in thr
velopment of a true adhesive bond between the pin and the amalgam matrix either by silver-plating the stainless steel pins or by using silver pins. This study has demonstrated that silver-plated stainless steel or silver pins do not increase the ultimate compressive strength of dental amalgam. It was apparent from viewing the metallographic sections of the interface between the silver-plated stainless steel pins and amalgam that they appeared to be identical to the non-plated pins, and no evidence of a bond was present. This may be attributed to complete dissolution of the silver plate within the amalgam matrix or, as Duperon and KoslofP found: a poor bond between the stainless steel core wire and the electro-deposited metal. Gross observation of the specimens also revealed that the pins bend under the compressive loading, and they act as stress concentrators, contributing significantly to crack propagation throughout the alloy. Although the bond between the silver pin and amalgam appeared to be excellent. it is apparent that the strength properties of silver do not enhance the over-all compressive strength of the silver amalgam. The results of this study are in agreement with those of Duperon and Kos10f-I~ and Bapna and Lugassy8 who also found that silver-plating of platinum-gold-palladium pins and gold-plating of stainless pins did not significantly increase the compressive and tensile strength properties of amalgam.
498
Moffa,
Going,
and Gettleman
J. Prosthet. Dent. November, 1972
Although the presence of voids within the amalgam is a very real problem, the special condensing instrument used in this study was not found to be effective in decreasing their presence. Further, this instrument did not improve the adaptation of the amalgam to the special deformations in the pins. This type of instrument has an additional disadvantage in that it compels the dentist to use short, straight pins. Frequently, pins must be bent in order to keep them confined within the anatomic contour of the clinical crown of the tooth. The fact that pins do not reinforce the restorative material should not diminish their value in clinical practice but merely re-emphasizes their limitations. Pins should be used primarily to retain nonadhesive restorative materials to excessively damaged tooth structure that could not otherwise be expected to support or retain the restoration. A previous study with stainless steel pins by this laboratory has shown that, due to surface threading, a 2 mm. pin length provides optimum retention to dental amalgam.s Although it is not possible to demonstrate bonding of the silver-plated stainless steel pin to the alloy, the surface thread patterns may be expected to provide adequate retention to the alloy, and we cannot offer any contraindication to their clinical use. Solid-silver pins are soft and easily deformed. The failure of the pins to resist elongation and flexure is an important consideration and could present serious retentive problems when such pins are employed to retain restorative materials to tooth structure. In view of these potential disadvantages, we seriously question the rationale for the clinical use of solid-silver pin-retaining devices. CONCLUSIONS
Based upon a determination of ultimate compressive strength and microscopic evaluation of metallographic sections, the following points were established : 1. Silver pins and stainless steel pins plated with silver did not significantly increase the ultimate compressive strength of amalgam cylinders over control specimens without pins, 2. Silver-plating of stainless steel pins did not improve the adaptation of the amalgam to the pin surface. 3. The use of a special instrument for condensing amalgam around pins did not increase the compressive strength or adaptation of amalgam over specimens condensed by a conventional hand instrument. 4. Stainless steel pins act as areas of stress concentration causing fracture of the amalgam surrounding the pins. We would like to thank Doctors Gerald L. Courtade and John J. Timmermans for permission to use illustrations from Chapter 2, Rationale for the Use of Pins Based on Research by Joseph P. Moffa, which appeared in their text, Pins in Restorative Dentistry, St. Louis, 1971, The C. V. Mosby Company. We also wish to thank the E. A. Beck Company, K & R Dental Products Company, and the Whaledent Corp. for their cooperation in furnishing materials used in this study.
References 1. Going, R. E., Moffa, J. P., Nostrant, G. W., and Johnson, B. E.: The Strength Amalgam as Influenced by Pins, J. Am. Dent. Assoc. 77: 1331-1334, 1968.
of Dental
Silver
pins
499
2. Going, R. E., and Nostrant, G. W.: Early Strength of Dental Amalgam as Influenced by Pins, J. Dent. Res. 48: 489, 1969. 3. Welk, D. A., and Dilts, W. E.: Influence of Pins on the Compressive and Transverse Strength of Dental Amalgam and Retention of Pins in Amalgam, J. Am. Dent. Assoc. 78: 101-104, 1969. Int. Assoc 4. Cecconi, B. T., and Asgar, L.: Pins in Amalgam: A Study of Reinforcement, Dent. Res., Abst. No. 120, 1968. 5. Allan, F. Cl., Asgar, K., and Peyton, F. A.: Microstructure of Dental Amalgam, J. Dent. Res. 44: 1002-1012, 1965. 6. Winer, B. J.: Statistical Principles in Experimental Design, New York, 1962, McGraw-Hill Book Company, Inc., pp. 263-267. 7. Duperon, D. F., and Kosloff, Z.: The Effects of Three Types of Pins on Some Mechanical Properties of Amalgam, Int. Assoc. Dent. Res., Abst. No. 574, 1971. 8. Bapna, M. S., and Lugassy, A. A.: Influence of Gold Plating of Stainless Steel Pins on the Tensile Strength of Dental Amalgam, J. Dent. Res. 50: 846-849, 1971. 9. Moffa, J. P., Razzano, M. R., and Doyle, M. G.: Pins-A Comparison of Their Retentive Properties, J. Am. Dent. Assoc. 78: 529-535, 1969. DR. MOFFA 15~~ & LAKE ST. SAN FRANCISCO, CALIF. 94118 DR. GOING UNIVERSITY OF FLORIDA COLLEGE OF DENTISTRY GAINESVILLE, FLA. 32601 DR. GETTLEMAN HARVARD UNIVERSITY SCHOOL OF DENTAL MEDICINE 188 LONGWOOD AVE. BOSTON, MASS. 02115
Silver
pins:
strength
Their
and
influence
adaptation
on the of am&am
Joseph P. Moffa, D.D.S., M.S.D.,* Robert E. Going, D.D.S., MS.,** and Lawrence Geftleman, D.M.D., M.S.D.*** IJnited States Public Health Service Hospital, San Francisco, Calif.
D
Previous studies have shown that threaded stainless steel pins placed in dental amalgam for reinforcement do not increase the strength of the amalgam.lm4 Stainless steel pins are not adhesive to amalgam but depend upon the adaptation of the amalgam into the pin’s surface irregularities for retention. In addition, adequate condensation around the pin is essential to achieve optimal adaptation and strength of the amalgam. Recent innovations in pins and instruments related to pin techniques have been said to achieve bonding of the pin to the amalgam matrix and to improve condensation. The purpose of this study was to assess the influence of certain proprietary threaded pins made of silver? and stainless steel pins plated with silver* upon the ultimate compressive strength of amalgam. Strength values were compared using conventional hand-condensing instruments and a special hand-condensing instrument.5 Adaptation at the amalgam-pin interface and the nature of the compressive fracture were also studied using metallographic procedures. MATERIALS
AND
METHODS
Cylindrical specimens were prepared for evaluation by condensing dental amalgam into a 6 by 12 mm. steel die and by storing the completed specimens at Portions of this article Association
Meeting,
Atlantic
were presented City, N. J.
at the scientific
session of the American
*Dental Research Coordinator, Federal Health Programs Service, Health Mental Health Administration, Department of Health, Education and Welfare, Public Health Service. **Associate Professor and College of Dentistry, Gainesville, ***Assistant Professor, cine, Boston, Mass. +Silver
Chairman, Fla.
Prosthetic
Dentistry,
pin wire, E. A. Beck Company,
$Silver-threaded
wire
$Special Condenser
with
stainless
Division
of Biomaterials,
Harvard
Anaheim,
University
Services and United States
University School
Dental
of Florida,
of Dental
Medi-
Calif.
steel core, E. A. Beck Company,
No. 12 with holes, E. A. Beck Company,
Anaheim,
Anaheim,
Calif,
Calif.
491
492
Moffa,
J. Prosthet. Dent. November, 1972
G oing, and Gettleman
room temperature for seven days prior to testing. All specimens were prepared and tested according to a random scheme, The orientation of four pins within each specimen was maintained parallel to the long axis of the die as described in a previous artic1e.l The pins were cut from bulk lengths threaded wire 0.64 mm. (0.025 inch) in diameter. Pins made of silver (Ag) and stainless steel plated with silver (AgSS) were cut to 14.5 mm. lengths. Before testing, the 4.5 mm. lengths of pins protruding from the base of all test specimens were removed. This left pins 10 mm. long incorporated within the 6 by 12 mm. amalgam specimens. Test specimens were made with New True Dentalloy” tablets at a 1:l mercuryalloy ratio. Trituration was completed in Bakelite capsules inserted for 18 seconds in a Wig-L-Bug? amalgamator. Both serrated and smooth-faced hand pluggers varying from 1 to 4 mm. in diameter were used as conventional hand condensers. smoothFor hand condensing of amalgam around the pins, special double-ended faced pluggers, 1.5 and 2.5 mm. in diameter with a hole 0.68 mm. (0.027 inch) in diameter running longitudinally through the nib, were used (Fig. 1). Condensing thrusts of 3 to 6 pounds, standardized on an Instron load cell,3 were directed vertically for both condensing methods. Compressive strength determinations were made for ten replications of each factorial combination of variables, or a total of 50 test specimens. Variables included silver pins (AG) , stainless steel pins plated with silver (AgSS) , and conventional and special condensation instruments. Specimens containing no pins condensed with conventional hand instruments served as controls. Additional controls without pins but condensed with the special instrument (Fig. 1) would have been meaningless since the center of the nib has a hole. Strength determinations were made on a mechanical testing instrumentt at a loading rate of 0.05 cm. per minute. Test specimens were loaded parallel with their long axis until failure occurred. Metallographic evaluations were performed on 10 representative specimens before and after fracture. Uncrushed specimens containing stainless steel pins plated with silver (AgSS) and stainless steel pins5 (SS) were examined, as well as cylinders containing silver (Ag) and gold-plated steel pins11 (A&S). Both crushed and uncrushed cylinders were imbedded in polymethyl methacrylate resin to prevent crumbling of the amalgam fragments. These specimens were then sliced with a diamond disc near the center of the crushed cylinder and remounted in methacrylatefilled metallurgic rings. Wet grinding was done with 240, 320, 400, and 600 grit silicon carbide paper followed by polishing with 6 pm diamond paste on a metallographic polisher. Etching was accomplished wth KCN and KI following the method of Allan, Asgar, and Peyton.” *S. S. White Dental Mfg. Company, King-of-Prussia, *Crescent Dental Mfg. Company, Chicago, Ill. SInstron Corp., Canton, Mass. SThreaded wire, K & R Dental [ITMS
regular,
Whaledent,
Products
Inc., Brooklyn,
Company, N. Y.
Pa.
Blue Island, Ill.
volume 28 Number 5
Silver
pins
493
Fig. 1. A special amalgam condenser which was theoretically designed to improve the condensation of amalgam surrounding a pin-retaining device. Note the hole in the nib for thP pk.
Table I. Effect of various of amalgam
types of pins and condensers upon the compressive
strength .-.-
Type Conventional Type
of pin*
Silver Silver-plated stainless steel *Control tstandard
of condemer Special
I
p.s.i.
S.D.f
54,000 52,200
1,000 1,200
p.s.i. .513,700 35,700
!
S.D.t 1:lOO 1,400 --.
with no pins = 54,600 ( 1,200). deviation.
RESULTS The mean compressive strength for the seven-day-old amalgam specimens. which were condensed with both conventional and special condensers and contained threaded silver pins (Ag), threaded silver-plated stainless steel pins (A&). and no pins (controls), are presented in Table I. The data from this factorial experiment with a single control group were analyzed by an analysis of variance technique (Table II) as described by Winer.” The 99 per cent level of confidenct was selected to determine statistical significance. The statistical analysis indicated that there was no significant difference between the compressive strength of the control specimen with no pins and the strength of those specimens which contained either the silver pins of the silver-plated stainless steel pins. Similarly, there was no significant difference between strengths of specimens which were condensed by means of the conventional instruments. However. those specimens which contained the silver pins were found to be significantl) weaker than specimens containing the silver-plated stainless steel pins (Table II )
494
Going,
Moffa,
J. Prosthet. Dent. November, 1972
and Gettleman
Fig. 2. Surface patterns
Table
II. Factorial
Source
analysis of variance
of variation
Control vs. all others Type of pin Type of condenser Pin and condenser Within cell *Not
of pins.
with a single control group
ss
df
163,192 18,491,920 103,938 3,392,480 66.410.553
1 1 1 1 45
1
MS
F
P
163,192 18,491,920 103,938 3,392,480 l-475,790
0.1105 12.5301 0.0704 2.2987
0.259* 0.999t 0.208* 0.864*
significant.
TSignificant.
Microscopic examination of the pins revealed that they differ considerably in their circumferential contour (Fig. 2). All pins were threaded to some extent, with less thread depth in the stainless steel and silver pins. Much sharper threads with greater depth were found in the gold-plated stainless steel pins (AuSS) . Examination of cross sections of imbedded pins in amalgam revealed that the adaptation of the amalgam to the silver-plated, gold-plated, and unplated stainless steel pins appeared incomplete and contained numerous voids (Figs. 3 and 4). Also, the conventional and special condensation procedures produced no significant difference in the occurrence of these voids. Conversely, the adaptation of the amalgam matrix to the silver pin appeared to be excellent in that the interface was distinguishable only by a slight difference in color. The presence or absence of pins within the specimens significantly influenced the gross type of fracture pattern (Fig. 5) . The specimen without pins produced the typical cone-shaped fracture patterns at both ends of the specimen. However,
Volume 28 Number 5
views of metallographic Fig. 3. High- and low-power and silver-plated gold. -plated stainless steel (Au%) amal Igam.
Silver pins
495
sections illustrate the interface betwt XII stainless steel (AgSS) pins and dec ita1
Fig. 4. High- and low-power views of metallographic sections illustrate silver (Ag) and stainless steel (SS) pins and dental amalgam.
the interface
between
496
Moffa,
Going,
.I, Prosthet. Dent. November, 1972
and Gettleman
Fig. 5. Specimens which reflect the different the presence or absence of stainless steel pins.
modes of compressive
fracture
as influenced
by
amalgam specimens which contain stainless steel pins demonstrated an aberration of this fracture pattern. The longitudinal compressive stresses caused the pins to bend outward, fracturing the outer surface of amalgam and usually producing a fracture cone at only one end of the specimen. The most significant microscopic observation was the striking difference in the nature of amalgam fracture in those specimens which contained the stainless steel and silver pins (Fig. 6). The lines of fracture in the specimens containing stainless steel pins (either plated or unplated) passed through the site of the pins. This failure at the pin-amalgam interface virtually left the pins remaining in empty cavities. In marked contrast, the silver pins remained enclosed within the amalgam matrix. The fracture passed at some distance from the pins and was restricted to the matrix. DISCUSSION Nonparallel pin-retaining devices usually are made of stainless steel, and numerous studies have shown that they do not increase the strength of amalgam.‘-* These stainless steel pins are not adhesive to dental amalgam but rely upon their threaded surface irregularities to achieve retention within the alloy by a mechanical interlocking phenomenon. It has been suggested that this failure to achieve an adhesive bond is also partially responsible for the failure to improve the strength properties of amalgam. Recent attempts have been made to correct this deficiency by the de-
Silver pins
Fig. 6. High-
and low-power views of metallographic sections illustrate the differencr site of amalgam fracture between silver (Ag) and stainless steel (SS) pins.
497
in thr
velopment of a true adhesive bond between the pin and the amalgam matrix either by silver-plating the stainless steel pins or by using silver pins. This study has demonstrated that silver-plated stainless steel or silver pins do not increase the ultimate compressive strength of dental amalgam. It was apparent from viewing the metallographic sections of the interface between the silver-plated stainless steel pins and amalgam that they appeared to be identical to the non-plated pins, and no evidence of a bond was present. This may be attributed to complete dissolution of the silver plate within the amalgam matrix or, as Duperon and KoslofP found: a poor bond between the stainless steel core wire and the electro-deposited metal. Gross observation of the specimens also revealed that the pins bend under the compressive loading, and they act as stress concentrators, contributing significantly to crack propagation throughout the alloy. Although the bond between the silver pin and amalgam appeared to be excellent. it is apparent that the strength properties of silver do not enhance the over-all compressive strength of the silver amalgam. The results of this study are in agreement with those of Duperon and Kos10f-I~ and Bapna and Lugassy8 who also found that silver-plating of platinum-gold-palladium pins and gold-plating of stainless pins did not significantly increase the compressive and tensile strength properties of amalgam.
498
Moffa,
Going,
and Gettleman
J. Prosthet. Dent. November, 1972
Although the presence of voids within the amalgam is a very real problem, the special condensing instrument used in this study was not found to be effective in decreasing their presence. Further, this instrument did not improve the adaptation of the amalgam to the special deformations in the pins. This type of instrument has an additional disadvantage in that it compels the dentist to use short, straight pins. Frequently, pins must be bent in order to keep them confined within the anatomic contour of the clinical crown of the tooth. The fact that pins do not reinforce the restorative material should not diminish their value in clinical practice but merely re-emphasizes their limitations. Pins should be used primarily to retain nonadhesive restorative materials to excessively damaged tooth structure that could not otherwise be expected to support or retain the restoration. A previous study with stainless steel pins by this laboratory has shown that, due to surface threading, a 2 mm. pin length provides optimum retention to dental amalgam.s Although it is not possible to demonstrate bonding of the silver-plated stainless steel pin to the alloy, the surface thread patterns may be expected to provide adequate retention to the alloy, and we cannot offer any contraindication to their clinical use. Solid-silver pins are soft and easily deformed. The failure of the pins to resist elongation and flexure is an important consideration and could present serious retentive problems when such pins are employed to retain restorative materials to tooth structure. In view of these potential disadvantages, we seriously question the rationale for the clinical use of solid-silver pin-retaining devices. CONCLUSIONS
Based upon a determination of ultimate compressive strength and microscopic evaluation of metallographic sections, the following points were established : 1. Silver pins and stainless steel pins plated with silver did not significantly increase the ultimate compressive strength of amalgam cylinders over control specimens without pins, 2. Silver-plating of stainless steel pins did not improve the adaptation of the amalgam to the pin surface. 3. The use of a special instrument for condensing amalgam around pins did not increase the compressive strength or adaptation of amalgam over specimens condensed by a conventional hand instrument. 4. Stainless steel pins act as areas of stress concentration causing fracture of the amalgam surrounding the pins. We would like to thank Doctors Gerald L. Courtade and John J. Timmermans for permission to use illustrations from Chapter 2, Rationale for the Use of Pins Based on Research by Joseph P. Moffa, which appeared in their text, Pins in Restorative Dentistry, St. Louis, 1971, The C. V. Mosby Company. We also wish to thank the E. A. Beck Company, K & R Dental Products Company, and the Whaledent Corp. for their cooperation in furnishing materials used in this study.
References 1. Going, R. E., Moffa, J. P., Nostrant, G. W., and Johnson, B. E.: The Strength Amalgam as Influenced by Pins, J. Am. Dent. Assoc. 77: 1331-1334, 1968.
of Dental
Silver
pins
499
2. Going, R. E., and Nostrant, G. W.: Early Strength of Dental Amalgam as Influenced by Pins, J. Dent. Res. 48: 489, 1969. 3. Welk, D. A., and Dilts, W. E.: Influence of Pins on the Compressive and Transverse Strength of Dental Amalgam and Retention of Pins in Amalgam, J. Am. Dent. Assoc. 78: 101-104, 1969. Int. Assoc 4. Cecconi, B. T., and Asgar, L.: Pins in Amalgam: A Study of Reinforcement, Dent. Res., Abst. No. 120, 1968. 5. Allan, F. Cl., Asgar, K., and Peyton, F. A.: Microstructure of Dental Amalgam, J. Dent. Res. 44: 1002-1012, 1965. 6. Winer, B. J.: Statistical Principles in Experimental Design, New York, 1962, McGraw-Hill Book Company, Inc., pp. 263-267. 7. Duperon, D. F., and Kosloff, Z.: The Effects of Three Types of Pins on Some Mechanical Properties of Amalgam, Int. Assoc. Dent. Res., Abst. No. 574, 1971. 8. Bapna, M. S., and Lugassy, A. A.: Influence of Gold Plating of Stainless Steel Pins on the Tensile Strength of Dental Amalgam, J. Dent. Res. 50: 846-849, 1971. 9. Moffa, J. P., Razzano, M. R., and Doyle, M. G.: Pins-A Comparison of Their Retentive Properties, J. Am. Dent. Assoc. 78: 529-535, 1969. DR. MOFFA 15~~ & LAKE ST. SAN FRANCISCO, CALIF. 94118 DR. GOING UNIVERSITY OF FLORIDA COLLEGE OF DENTISTRY GAINESVILLE, FLA. 32601 DR. GETTLEMAN HARVARD UNIVERSITY SCHOOL OF DENTAL MEDICINE 188 LONGWOOD AVE. BOSTON, MASS. 02115