F r a c t u r e r e s i s t a n c e of p i n - r e t a i n e d a m a l g a m , c o m p o s i t e resin, and a l l o y - r e i n f o r c e d g l a s s i o n o m e r core m a t e r i a l s E l i z a b e t h C. K a o , D M D a
The Ohio State University, College of Dentistry, Columbus, Ohio This study i n v e s t i g a t e d the influence of pins on the fracture r e s i s t a n c e of three core materials. Two or four s t a i n l e s s steel pins w e r e incorporated in either amalgam, composite resin, or alloy:reinforced g l a s s ionomer specimens. Half of the pins w e r e s u r f a c e - t r e a t e d with mercury, P a n a v i a EX resin, or hydrochloric acid before t h e y w e r e incorporated in the r e s p e c t i v e materials. The pins w e r e oriented in a direction r e l a t i v e to the t e n s i l e s t r e s s / a x i s of the specimen: parallel/perpendicular (PL/PR), perPendicular/parallel (PR/PL), or perpendicular/perpendicular (PR/PR). ANOVA tested significant differences in d i a m e t r a l tensile Strength among materials, in number of pins, in pin orientations, in surface treatments, and in other interactions. Incorporation of pins w e a k e n e d a m a l g a m the most, followed by composite resin. Pins did not w e a k e n a m a l g a m - r e i n f o r c e d glass ionomer. Pin orientation improved the fracture r e s i s t a n c e of some s p e c i m e n s by two times that of the controls. Orientation of pins parallel to the t e n s i l e s t r e s s w a s most favorable. As the n u m b e r of pins increased, the fracture r e s i s t a n c e of a m a l g a m significantly decreased. Acid treatment of the pin surface e n h a n c e d the bond with composite resin. Both t r e a t m e n t s resulted in significant i m p r o v e m e n t in fracture resistance. (J PROSTHET DENT 1991;66:463-71.)
F o r extensively broken down teeth and endodontically treated teeth, the most popular core buildup materials are amalgam and composite resin. Early efforts to increase tensile strength of dental amalgam by reinforcement was made with a silver alloy plate. 1 Later, Markley 2' 3 introduced stainless steel pins for retention of amalgam and suggested t h a t pins reinforce amalgam as steel rods reinforce concrete. Since then, various parameters such as the number of pins within the amalgam matrix, the surface design of pins, pin orientation, shape of pin ends, and surface treatment of pins have been Studied. 41° The compresSive strength of a restorative material has been found not to be altered by the use of pins, 47 but the tensile strength is affected J, 6, 7 Both amalgam and composite resin possess a coefficient of thermal expansion that is two to three times higher than that of tooth structures. 11Both materials lack the ability to chemically bond to dentin. The use of these materials may result in increased microleakage and subsequently may lead to recurrent caries, solubility of post-luting cement, and corrosion of pins. In light of the marginal leakage problems encountered in composite resins, a hydrophilic system, glass ionomer
This project was supported by The Ohio State University, College of Dentistry research fund. aAssistant Professor, Department of Restorative and Prosthetic Dentistry. 10/1/29116
THE J O U R N A L OF PROSTHETIC DENTISTRY
cement, has been developed by Wilson, Kent et alJ 2' 13According to McLean, 14 the cement forms an adhesive bond to enamel, dentin, and treated platinum and gold. Other studies with polycarboxylate cement, a polyacrylic acidbased material similar to glass ionomer cement, have shown that an adhesive bond exists between stainless steel and this cement.15-1s Additionally, glass contains a large amount of fluoride (about 20% ).19 This fluoride can leach out from the set cement matrix by ion exchange, producing a potential anticariogenic effect. Studies have supported the material's biocompatibility with tooth structures and pulp.20.21 Since glass ionomer cements have low tensile strength, improvements in the flexural strength of these cements have been made by the incorporation of stainless steel powders, Is silver/tin alloy fibers or flakes, 22"23 and by high temperature sintering of silver and glass powders. 2426 Wilson 27 has shown that sintered cermet glass/metal composition could improve the mechanical properties of the glass ionomer, especially the increases in ductility and energy to fracture. The compressive strength, 2s abrasion resistance, 29 and flexural strength 8° of glass ionomer cement has also been found to be significantly increased with the addition of silver alloy powder. These alloy-reinforced glass ionomer cements are used by dentists for posterior restorations, post and cores, and pin-core buildups. The objectives of this study were: (1) to determine the effect of pin type, number of pins, surface treatment of pins, and their orientation on the fracture resistance of three core materials: amalgam, composite resin, and an al-
463
KAO
Occlusal Load
~lilil
lilil Tensile Stress
A Compressive Load
Tensile Stress,
B
t/
PR / PL
PR / PR
PL / PR
Fig. 1. A, Pin placement in tooth and their orientations relative to occlusal load and tensile stress. B, Orientations are designated in relation to tensile stress applied and axis of the specimen.
T a b l e I. Sample distribution of experimental groups for each core material Pin orientation PL/PR
Number of pins Surface treatment* Sample size for each core materialt
2 Y 10
N 10
PR/PL
4 Y 10
2 Y 10
N 10
N 10
PR/PR
4 Y 10
N 10
2 Y 10
4
N 10
Y 10
N 10
*Y, Pins with surface treatment; N, no surface t r e a t m e n t on pins. t T e n samples each of each core material with no pin incorporation acted as controls.
loy-reinforced glass ionomer and (2) to determine the modes of fracture when the pin-core materials were subjected to diametral tensile stress.
METHODS AND MATERIAL Three core restorative materials--high-copper admixed amalgam (Valiant phD, L. D. Caulk Co., Milford, Del.), composite resin (P-30, 3M Company, St. Paul, Minn.), and a silver amalgam reinforced glass ionomer (Ketac Silver,
464
Espe Premiere, Norristown, Pa.) were used. One hundred thirty 6 X 12 mm cylindrical specimens of each material were made in Teflon split molds. The experimental design is presented in Table I. In the 12 experimental groups of 10 each, half of the specimens would receive two pins (Minim pins, Whaledent International, New York, N.Y.), each 0.6 mm in diameter, and half would receive four pins. Ten specimens of each material without pins served as controls for fracture resistance. The pins were oriented in one of the
OCTOBER 1991
VOLUME 66 NUMBER 4
PIN INFLUENCE ON FRACTURE RESISTANCE
Fig. 2. A, Scores were made on internal surfaces of metal mold so that equal amounts of materials could be placed between pins arranged in P L / P R and P R / P R orientations. B, Plastic templates with predrilled pin positions are placed in mold so equal amounts of material can be condensed around pins arranged in the P R / P L orientation.
three orientations simulating clinical placement. The three pin orientations were: (1) parallel to the tensile force/perpendicular to the axis of the specimen (PL/PR), (2) perpendicular to the tensile force/parallel to the axis of the specimen (PR/PL), and (3) perpendicular to the tensile force/perpendicular to the axis of the specimen (PR/PR), respectively (Fig. 1). For each material, the pins were further divided into two groups: surface-treated and nonsurface-treated. For amalgam, half of the stainless steel pins were surface-treated by spinning in liquid mercury for 5 minutes before they were incorporated into the specimens. For composite resin, half of the pins were coated with a thin layer of modified phosphate ester Bis-GMA resin (Panavia EX, Kuraray Co., Osaka, Japan). The coated pins were immediately covered with composite resin because Panavia EX adhesive does not cure in the presence of oxygen. For reinforced glass ionomer, half of the pins were treated with a 15% aqueous solution of hydrochloric acid for 5 minutes and rinsed with deionized water and anhydrous methanol before they were
THE JOURNAL OF PROSTHETIC DENTISTRY
embedded in the specimen. The other half of the pins did not receive any surface treatment. Amalgam pellets were prepared by trituration in an amalgamator (Vari-Mix, L. D. Caulk Co.) for 15 seconds. Increments of amalgam were hand condensed into the split mold by the same operator using condensers varying from 1 to 3 mm in diameter. Composite resin was syringed directly into the mold and light-cured in six increments of 2 mm each for a total of 180 seconds with a visible curing light (Elipar light, Espe Premiere). Precapsulated KetacFil glass ionomer (Espe Premiere) was triturated in an amalgamator for 9 seconds and syringed directly into the mold with an applicator provided by the manufacturer. All samples were overfilled and excesses were expressed by compressing against two glass slides, held tightly by a Cclamp. Scores were made on the split mold to guide the positioning of pins in the P L / P R or P R / P R directions so that an equal amount of materials would be embedded between pins. Pins oriented in the PR/PL direction were
465
KAO t 350 1200 1050
PR/PL > PL/PR; for glass ionomers reductions are PR/PR = PR/ PL > PL/PR.
Data were analyzed by multifactor analysis of variance. For a significant F value, Tukey's Studentized test was performed to compare the mean fracture force among groups. The 95 % confidence level was selected for statistical significance.
THE JOURNAL OF PROSTHETIC DENTISTRY
RESULTS The mean fracture forces of pin-incorporated amalgam, composite resin, and silver-reinforced glass ionomer specimens are presented in Figs. 3 through 5. Their relative fracture resistance to the controls are presented in Fig. 6.
467
KAO
i i
Fig. 7. Pins arranged in all orientations were separated completely from amalgam substrate upon fracture. Serrations of pins were relatively free of alloy, with exception of fresh mercury-pretreated pins.
Multifactor analysis of variance revealed significant differences among materials, number of pins, pin orientation, and surface treatment of pins (Table II). For amalgam specimens under all conditions, incorporation of pins resulted in significantly lower fracture forces than for the controls. Pins oriented in the P L / P R direction provided a significantly higher fracture force than those in the PR/PL or P R / P R orientations. The fracture resistance of amalgam decreased as the number of pins increased from two to four, with the exception of specimens with pins oriented in the P L / P R direction. Surface treatment of pins with liquid mercury did not significantly alter the fracture resistance of amalgam. Pin incorporation resulted in a significant decrease in fracture resistance of composite resin, with the exception of the four-pin group that was surface-treated and arranged in the PL/PR direction. Pins treated with Panavia EX adhesive significantly enhanced the fracture resistance of composite resin. Pins in silver-reinforced glass ionomer specimens did not reduce the fracture resistance of the materials. However, pins oriented in the P L / P R direction improved the fracture resistance of the materials significantly. Four pins oriented in the PL/PR directions improved the fracture resistance of the material by almost twofold over that of the controls. The fracture strength of silver-reinforced glass ionomer specimens was improved when acid-pretreated pins were incorporated. The presence of pins within the specimens significantly influenced the gross fracture pattern. All control specimens had a clean-cut fracture along the midaxis of the specimen, perpendicular to the tensile force. As pins were incorporated, fracture lines of amalgam were observed to pass
468
through the sites of the pins. In general, pins were separated completely from the surrounding set amalgam upon fracture. Some amalgam alloys were observed to adhere to the mercury-wetted pins (Fig. 7). This alloy did not separate from the pins upon washing with alcohol or drying with air. Pins in composite resin created fracture lines through the pin sites as well as at some distance from them. Pins treated with Panavia EX adhesive remained bonded to composite resin even in thin fragments. Untreated pins became totally detached from surrounding composite resin, and manifested a shiny appearance (Fig. 8). Pin-incorporated silver-reinforced glass ionomer specimens presented a multifragmented fracture pattern. Upon fracture, glass ionomer specimens were observed to adhere to the pins (Fig. 9). No differences were found in the fracture pattern for specimens with incorporated surfacetreated pins compared with those with nontreated pins.
DISCUSSION This study indicated that amalgam, composite resin, and alloy-reinforced glass ionomer core materials were affected differently in the diametral tensile testing mode when pins were incorporated. Although the pins were not partially embedded in teeth as in the clinical condition, interaction between pins and the core materials should remain the same. For amalgam, the results are generally in agreement with the findings of other studiesJ 7 Pins were mechanically retained within the alloy by their threaded surface irregularities. Since there were no chemical bonds between pins and amalgam, the pins acted as voids in the restorative material. With the exception of pins arranged in the P L / P R orientation, as the number of pins increased, the fracture resistance of the material decreased. Surface treatment of pins might have improved the adherence between surfacetreated pins and amalgam, as suggested by Bapna and Lugassy. 1° However, the apparent fracture resistance of amalgam was not improved. For composite resin, pin incorporation also weakened the material, with the exception of the surface-treated four-pin model oriented in the P L / P R direction. Surface treatment of pins with Panavia EX adhesive significantly improved the overall fracture resistance of composite resin. The results support those findings by other investigators 3133 that Panavia adhesive forms a chemical bond with base metal alloys. Stainless steel pins did not result in a reduction of fracture resistance of the glass ionomer materials. In some instances, pins increased the fracture resistance of the core material. Formation of chemical and mechanical bonds between pins and their glass ionomer substrates have contributed to such reinforcement. Hotz et al. 34 and Sarkar et al. 35 reported that a chemical bond could be formed between the oxide films of certain metals and the carboxylate groups of polyacrylic acid. Similarly, the nickel,
O C T O B E R 1991
V O L U M E 66
NUMBER 4
PIN INFLUENCE ON FRACTURE RESISTANCE
F i g . 8. A, W i t h o u t surface treatment, stainless steel pins were separated from composite substrate upon fracture with a shiny appearance. B, P a n a v i a adhesive surface-treated pins were bonded to composite resin substrate even in thin 1 m m fragments.
Table
II.
Analysis of variance for fracture force (lb) of experimental groups relative to controls
S o u r c e of significant variation
df*
Mean square
F value
Pt
Model Core material (M) Number of pins (N) MxN Pin orientation (O) M×O NxO MxNxO Surface treatment (T) MxT Error
35 2 1 2 2 4 2 4 1 2 144
423,687.28 11,881,781.68 32,009.31 89,034.09 1,870,121.00 279,504.38 118,938.43 256,926.15 87,450.50 187,702.61 2,509.01
168.87 2,367.83 12.76 17.74 372.68 27.85 23.70 25.60 34.85 37.41
The Ohio State University, College of Dentistry, Columbus, Ohio This study i n v e s t i g a t e d the influence of pins on the fracture r e s i s t a n c e of three core materials. Two or four s t a i n l e s s steel pins w e r e incorporated in either amalgam, composite resin, or alloy:reinforced g l a s s ionomer specimens. Half of the pins w e r e s u r f a c e - t r e a t e d with mercury, P a n a v i a EX resin, or hydrochloric acid before t h e y w e r e incorporated in the r e s p e c t i v e materials. The pins w e r e oriented in a direction r e l a t i v e to the t e n s i l e s t r e s s / a x i s of the specimen: parallel/perpendicular (PL/PR), perPendicular/parallel (PR/PL), or perpendicular/perpendicular (PR/PR). ANOVA tested significant differences in d i a m e t r a l tensile Strength among materials, in number of pins, in pin orientations, in surface treatments, and in other interactions. Incorporation of pins w e a k e n e d a m a l g a m the most, followed by composite resin. Pins did not w e a k e n a m a l g a m - r e i n f o r c e d glass ionomer. Pin orientation improved the fracture r e s i s t a n c e of some s p e c i m e n s by two times that of the controls. Orientation of pins parallel to the t e n s i l e s t r e s s w a s most favorable. As the n u m b e r of pins increased, the fracture r e s i s t a n c e of a m a l g a m significantly decreased. Acid treatment of the pin surface e n h a n c e d the bond with composite resin. Both t r e a t m e n t s resulted in significant i m p r o v e m e n t in fracture resistance. (J PROSTHET DENT 1991;66:463-71.)
F o r extensively broken down teeth and endodontically treated teeth, the most popular core buildup materials are amalgam and composite resin. Early efforts to increase tensile strength of dental amalgam by reinforcement was made with a silver alloy plate. 1 Later, Markley 2' 3 introduced stainless steel pins for retention of amalgam and suggested t h a t pins reinforce amalgam as steel rods reinforce concrete. Since then, various parameters such as the number of pins within the amalgam matrix, the surface design of pins, pin orientation, shape of pin ends, and surface treatment of pins have been Studied. 41° The compresSive strength of a restorative material has been found not to be altered by the use of pins, 47 but the tensile strength is affected J, 6, 7 Both amalgam and composite resin possess a coefficient of thermal expansion that is two to three times higher than that of tooth structures. 11Both materials lack the ability to chemically bond to dentin. The use of these materials may result in increased microleakage and subsequently may lead to recurrent caries, solubility of post-luting cement, and corrosion of pins. In light of the marginal leakage problems encountered in composite resins, a hydrophilic system, glass ionomer
This project was supported by The Ohio State University, College of Dentistry research fund. aAssistant Professor, Department of Restorative and Prosthetic Dentistry. 10/1/29116
THE J O U R N A L OF PROSTHETIC DENTISTRY
cement, has been developed by Wilson, Kent et alJ 2' 13According to McLean, 14 the cement forms an adhesive bond to enamel, dentin, and treated platinum and gold. Other studies with polycarboxylate cement, a polyacrylic acidbased material similar to glass ionomer cement, have shown that an adhesive bond exists between stainless steel and this cement.15-1s Additionally, glass contains a large amount of fluoride (about 20% ).19 This fluoride can leach out from the set cement matrix by ion exchange, producing a potential anticariogenic effect. Studies have supported the material's biocompatibility with tooth structures and pulp.20.21 Since glass ionomer cements have low tensile strength, improvements in the flexural strength of these cements have been made by the incorporation of stainless steel powders, Is silver/tin alloy fibers or flakes, 22"23 and by high temperature sintering of silver and glass powders. 2426 Wilson 27 has shown that sintered cermet glass/metal composition could improve the mechanical properties of the glass ionomer, especially the increases in ductility and energy to fracture. The compressive strength, 2s abrasion resistance, 29 and flexural strength 8° of glass ionomer cement has also been found to be significantly increased with the addition of silver alloy powder. These alloy-reinforced glass ionomer cements are used by dentists for posterior restorations, post and cores, and pin-core buildups. The objectives of this study were: (1) to determine the effect of pin type, number of pins, surface treatment of pins, and their orientation on the fracture resistance of three core materials: amalgam, composite resin, and an al-
463
KAO
Occlusal Load
~lilil
lilil Tensile Stress
A Compressive Load
Tensile Stress,
B
t/
PR / PL
PR / PR
PL / PR
Fig. 1. A, Pin placement in tooth and their orientations relative to occlusal load and tensile stress. B, Orientations are designated in relation to tensile stress applied and axis of the specimen.
T a b l e I. Sample distribution of experimental groups for each core material Pin orientation PL/PR
Number of pins Surface treatment* Sample size for each core materialt
2 Y 10
N 10
PR/PL
4 Y 10
2 Y 10
N 10
N 10
PR/PR
4 Y 10
N 10
2 Y 10
4
N 10
Y 10
N 10
*Y, Pins with surface treatment; N, no surface t r e a t m e n t on pins. t T e n samples each of each core material with no pin incorporation acted as controls.
loy-reinforced glass ionomer and (2) to determine the modes of fracture when the pin-core materials were subjected to diametral tensile stress.
METHODS AND MATERIAL Three core restorative materials--high-copper admixed amalgam (Valiant phD, L. D. Caulk Co., Milford, Del.), composite resin (P-30, 3M Company, St. Paul, Minn.), and a silver amalgam reinforced glass ionomer (Ketac Silver,
464
Espe Premiere, Norristown, Pa.) were used. One hundred thirty 6 X 12 mm cylindrical specimens of each material were made in Teflon split molds. The experimental design is presented in Table I. In the 12 experimental groups of 10 each, half of the specimens would receive two pins (Minim pins, Whaledent International, New York, N.Y.), each 0.6 mm in diameter, and half would receive four pins. Ten specimens of each material without pins served as controls for fracture resistance. The pins were oriented in one of the
OCTOBER 1991
VOLUME 66 NUMBER 4
PIN INFLUENCE ON FRACTURE RESISTANCE
Fig. 2. A, Scores were made on internal surfaces of metal mold so that equal amounts of materials could be placed between pins arranged in P L / P R and P R / P R orientations. B, Plastic templates with predrilled pin positions are placed in mold so equal amounts of material can be condensed around pins arranged in the P R / P L orientation.
three orientations simulating clinical placement. The three pin orientations were: (1) parallel to the tensile force/perpendicular to the axis of the specimen (PL/PR), (2) perpendicular to the tensile force/parallel to the axis of the specimen (PR/PL), and (3) perpendicular to the tensile force/perpendicular to the axis of the specimen (PR/PR), respectively (Fig. 1). For each material, the pins were further divided into two groups: surface-treated and nonsurface-treated. For amalgam, half of the stainless steel pins were surface-treated by spinning in liquid mercury for 5 minutes before they were incorporated into the specimens. For composite resin, half of the pins were coated with a thin layer of modified phosphate ester Bis-GMA resin (Panavia EX, Kuraray Co., Osaka, Japan). The coated pins were immediately covered with composite resin because Panavia EX adhesive does not cure in the presence of oxygen. For reinforced glass ionomer, half of the pins were treated with a 15% aqueous solution of hydrochloric acid for 5 minutes and rinsed with deionized water and anhydrous methanol before they were
THE JOURNAL OF PROSTHETIC DENTISTRY
embedded in the specimen. The other half of the pins did not receive any surface treatment. Amalgam pellets were prepared by trituration in an amalgamator (Vari-Mix, L. D. Caulk Co.) for 15 seconds. Increments of amalgam were hand condensed into the split mold by the same operator using condensers varying from 1 to 3 mm in diameter. Composite resin was syringed directly into the mold and light-cured in six increments of 2 mm each for a total of 180 seconds with a visible curing light (Elipar light, Espe Premiere). Precapsulated KetacFil glass ionomer (Espe Premiere) was triturated in an amalgamator for 9 seconds and syringed directly into the mold with an applicator provided by the manufacturer. All samples were overfilled and excesses were expressed by compressing against two glass slides, held tightly by a Cclamp. Scores were made on the split mold to guide the positioning of pins in the P L / P R or P R / P R directions so that an equal amount of materials would be embedded between pins. Pins oriented in the PR/PL direction were
465
KAO t 350 1200 1050
PR/PL > PL/PR; for glass ionomers reductions are PR/PR = PR/ PL > PL/PR.
Data were analyzed by multifactor analysis of variance. For a significant F value, Tukey's Studentized test was performed to compare the mean fracture force among groups. The 95 % confidence level was selected for statistical significance.
THE JOURNAL OF PROSTHETIC DENTISTRY
RESULTS The mean fracture forces of pin-incorporated amalgam, composite resin, and silver-reinforced glass ionomer specimens are presented in Figs. 3 through 5. Their relative fracture resistance to the controls are presented in Fig. 6.
467
KAO
i i
Fig. 7. Pins arranged in all orientations were separated completely from amalgam substrate upon fracture. Serrations of pins were relatively free of alloy, with exception of fresh mercury-pretreated pins.
Multifactor analysis of variance revealed significant differences among materials, number of pins, pin orientation, and surface treatment of pins (Table II). For amalgam specimens under all conditions, incorporation of pins resulted in significantly lower fracture forces than for the controls. Pins oriented in the P L / P R direction provided a significantly higher fracture force than those in the PR/PL or P R / P R orientations. The fracture resistance of amalgam decreased as the number of pins increased from two to four, with the exception of specimens with pins oriented in the P L / P R direction. Surface treatment of pins with liquid mercury did not significantly alter the fracture resistance of amalgam. Pin incorporation resulted in a significant decrease in fracture resistance of composite resin, with the exception of the four-pin group that was surface-treated and arranged in the PL/PR direction. Pins treated with Panavia EX adhesive significantly enhanced the fracture resistance of composite resin. Pins in silver-reinforced glass ionomer specimens did not reduce the fracture resistance of the materials. However, pins oriented in the P L / P R direction improved the fracture resistance of the materials significantly. Four pins oriented in the PL/PR directions improved the fracture resistance of the material by almost twofold over that of the controls. The fracture strength of silver-reinforced glass ionomer specimens was improved when acid-pretreated pins were incorporated. The presence of pins within the specimens significantly influenced the gross fracture pattern. All control specimens had a clean-cut fracture along the midaxis of the specimen, perpendicular to the tensile force. As pins were incorporated, fracture lines of amalgam were observed to pass
468
through the sites of the pins. In general, pins were separated completely from the surrounding set amalgam upon fracture. Some amalgam alloys were observed to adhere to the mercury-wetted pins (Fig. 7). This alloy did not separate from the pins upon washing with alcohol or drying with air. Pins in composite resin created fracture lines through the pin sites as well as at some distance from them. Pins treated with Panavia EX adhesive remained bonded to composite resin even in thin fragments. Untreated pins became totally detached from surrounding composite resin, and manifested a shiny appearance (Fig. 8). Pin-incorporated silver-reinforced glass ionomer specimens presented a multifragmented fracture pattern. Upon fracture, glass ionomer specimens were observed to adhere to the pins (Fig. 9). No differences were found in the fracture pattern for specimens with incorporated surfacetreated pins compared with those with nontreated pins.
DISCUSSION This study indicated that amalgam, composite resin, and alloy-reinforced glass ionomer core materials were affected differently in the diametral tensile testing mode when pins were incorporated. Although the pins were not partially embedded in teeth as in the clinical condition, interaction between pins and the core materials should remain the same. For amalgam, the results are generally in agreement with the findings of other studiesJ 7 Pins were mechanically retained within the alloy by their threaded surface irregularities. Since there were no chemical bonds between pins and amalgam, the pins acted as voids in the restorative material. With the exception of pins arranged in the P L / P R orientation, as the number of pins increased, the fracture resistance of the material decreased. Surface treatment of pins might have improved the adherence between surfacetreated pins and amalgam, as suggested by Bapna and Lugassy. 1° However, the apparent fracture resistance of amalgam was not improved. For composite resin, pin incorporation also weakened the material, with the exception of the surface-treated four-pin model oriented in the P L / P R direction. Surface treatment of pins with Panavia EX adhesive significantly improved the overall fracture resistance of composite resin. The results support those findings by other investigators 3133 that Panavia adhesive forms a chemical bond with base metal alloys. Stainless steel pins did not result in a reduction of fracture resistance of the glass ionomer materials. In some instances, pins increased the fracture resistance of the core material. Formation of chemical and mechanical bonds between pins and their glass ionomer substrates have contributed to such reinforcement. Hotz et al. 34 and Sarkar et al. 35 reported that a chemical bond could be formed between the oxide films of certain metals and the carboxylate groups of polyacrylic acid. Similarly, the nickel,
O C T O B E R 1991
V O L U M E 66
NUMBER 4
PIN INFLUENCE ON FRACTURE RESISTANCE
F i g . 8. A, W i t h o u t surface treatment, stainless steel pins were separated from composite substrate upon fracture with a shiny appearance. B, P a n a v i a adhesive surface-treated pins were bonded to composite resin substrate even in thin 1 m m fragments.
Table
II.
Analysis of variance for fracture force (lb) of experimental groups relative to controls
S o u r c e of significant variation
df*
Mean square
F value
Pt
Model Core material (M) Number of pins (N) MxN Pin orientation (O) M×O NxO MxNxO Surface treatment (T) MxT Error
35 2 1 2 2 4 2 4 1 2 144
423,687.28 11,881,781.68 32,009.31 89,034.09 1,870,121.00 279,504.38 118,938.43 256,926.15 87,450.50 187,702.61 2,509.01
168.87 2,367.83 12.76 17.74 372.68 27.85 23.70 25.60 34.85 37.41
