- Email: [email protected]
Flexural strength of provisional restorative materials
Flexural
strength
of provisional
restorative
materials
Y. I. Osman, BChD, MChD,a and C. P. Owen, BDS, MScDent, MChDb Faculty of Dentistry, University of the Western Cape, Cape Town, South Africa A provisional restorative material must be strong enough to resist fracture during function. This study tested flve autopolymerizing provisional resin materials under conditions that related the stresses acting on them to those acting on a axed partial denture. The highest values for fracture resistance were displayed by Snap poly(ethy1 methacrylate) material. However, two of the 11 samples of this material displayed markedly lower values for fracture resistance. This finding warrants further investigation, because inconsistency has clinical implications. In decreasing order, the fracture resistance of the other materials was as follows: the poly(methy1 methacrylate) materials, Caulk temporary bridge resin and G-C Unifast temporary resin; the composite material, Protemp; and the epimine material, Scutan. (J PROSTHET DENT 1993;70:94-6.)
A
provisional restoration must fulfill several functions, not least of which is that it must be strong enough to
resist fracture.lW5A number of studies have examined the mechanical properties of resins found to be acceptable for use as provisional restorations, but there has been little consistency in the methods used.6-g Braden et a1.7in 1971 tested a newly introduced ethylene-imine (epimine) material and found that the impact strength, tensile strength, and extension to fracture were all decidedly inferior to a poly(methy1 methacrylate). In a later study a resin based on a poly(ethy1 methacrylate) polymer powder and n-butyl methacrylate monomer system was compared with a conventional acrylic resin, an epimine, and a poly(ethy1 methacrylate) resin of composition similar to that being tested, but which used isobutyl instead of n-butyl methacrylate monomer.” Rectangular specimens 60 x 5 x 1.5 mm were used. Flexural strength
aHead, Department of Conservative Dentistry. bProfeasor and Head, Department of Prosthetic Dentistry. Copyright @ 1993 by The Editorial Council of THE JOURNAL OF PROSTHETIC DENTISTRY. 0022-3913/93/$1.00 + .lO. 10/l/45956
Table
94
I.
was tested by clamping one end of each specimen and applying varying loads to the other. The new resin and the epimine showed higher values under these test conditions, but the new resin and the poly(ethy1 methacrylate) had lower Young’s modulus values. Increased flexibility can be an advantage in a provisional resin material, but the flexural strength test used may not be directly applicable clinically.
Fracture toughness has been tested by placement of specimens in tension in a servohydraulic loading apparatus.sThe specimens were in the form of a 14.4 x 15 x 4 mm slotted block in which a precrack was formed. An epimine and two poly(ethy1 methacrylate) resins demonstrated the greatest fracture toughness. A poly(ethy1 methacrylate) had the lowest fracture toughness, and a composite material was intermediate in toughness. In the clinical situation, a fixed partial denture is subjected to a variety of forces when under load. These forces have been shown by photoelastic stress analysis studies to be compressive at the points of application of load, and tensile and shear at the points of resistance to that load.iO This study compared the flexural strength of a variety of provisional resins with a destruct test that would simulate
a clinical
situation.
Materials used Trade name
Resin type
Caulk Temporary Bridge Besin (“Caulk’s”)
Poly(methy1 methacrylate)
G-C Unifast temporary resin (“Unifast”) Protemp
Poly(methy1 methacrylate)
Scutan Snap
Epimine Poly(ethy1 methacrylate)
Composite
Manufacturer
L. D. Caulk Co. Dentaply International Milford, Delaware G-C Dental Industrial Corporation Tokyo, Japan ESPE Fabrik GMbH Seefeld, Germany ESPE Fabrik GMbH Parke11Bio-Materials Division Farmingdale, h’. Y.
Batch No. 940378
090661 N112 Do17 35151
VOLUME70
NUMBER1
OSMAN
AND
THE
OWEN
JOURNAL
OF PROSTHETIC
DENTISTRY
SNAP
LOAD
I
CAULK’S UNIFAST PROTEMP
Fig. 1. Diagrammatic representation of the destruct test used. Force applied is resolved as a combination of compressive CC),tensile (T), and shear (S) stresses.
SCUTAN MATERIAL
AND
METHOD
The materials used are listed in Table I. Standard specimens of each material were produced from a brass mold designed on a split-cast principle. Tinfoil was used as the separating medium. Each mold yielded two identical specimens, the dimensions of which were 3 x 5 x 90 mm. A standardized procedure was used for constructing the specimens. All of the materials used came from the same batch for each manufacturer. The materials were mixed with weighed quantities of powder and a predetermined volume of liquid pipetted into the powder. The materials were mixed under clinical conditions according to the manufacturers’ instructions. They were syringed into the mold and allowed to bench cure for 20 minutes under a constant pressure of 500 gm. The specimens were stored at room temperature for 24 hours and then incubated in normal saline at 37’ C for at least 24 hours. Each specimen was assigned a number from a computergenerated list of random numbers. The specimens were then subjected to fracture tests on a tensile testing machine. The operator performing the fracture tests did not know which numbers were assigned to which material. The specimens were placed such that a central length of 10 mm of the specimen was loaded (Fig. 1). The machine used was the J.J.Tensile testing machine, type T5001 with a crosshead speed of 5 mm per minute. The machine was calibrated to a load cell sensitivity of 1, with a paper crosshead ratio of 1:l. It had a load cell rating of five kilonewtons (5 kN). Thus a pen movement of 250 mm on the graph paper was representative of a force of 5 kN applied to the specimen. When the specimens were fractured, the ends were observed under a laboratory magnifying glass. The presence of any air bubbles within the fracture face of the specimen excluded the specimen from the experiment. One way analysis of variance (ANOVA) was used to test for a significant difference between the materials. A RyanEinot-Gabriel-Welsch (REGWQ) multiple range test was used to test for differences between the groups.‘l
JULY
1993
2. Materials not significantly are linked by the bars.
Fig.
Table
different at p < 0.05
Force (in kilonewtons) required to fracture
II.
specimens
Mean SD
Snap
Caulk’s
Unifast
Protemp
Scutan
0.47 0.54 0.53 0.54 0.13 0.56 0.55 0.57 0.53 0.15 0.54 0.46 0.16
0.23 0.23 0.23 0.25 0.23 0.24 0.23 0.21 0.23 0.26 0.25 0.23 0.01
0.27 0.24 0.25 0.25 0.22 0.23 0.24 0.23 0.22 0.22 0.23 0.23 0.02
0.13 0.20 0.19 0.15 0.20 0.19 0.21 0.18 0.15 0.15 0.16 0.17 0.03
0.13 0.12 0.15 0.14 0.17 0.17 0.14 0.18 0.17 0.12 0.12 0.14 0.02
RESULTS The force required to fracture the specimens is shown in Table II. The highest figure was obtained by Snap poly(ethyl methacrylate) material. ANOVA revealed an f value of 31.4, which is significant at p < 0.001. The REGWQ multiple range test revealed that the difference between the Snap material and all other materials tested was significant at p < 0.05. At this level, the test also revealed that there was no significant difference between Caulk’s, Unifast, and Protemp materials and no difference between Protemp and Scutan materials (Fig. 2). DISCUSSION The test used in this study is an attempt to simulate the clinical situation, where a combination of compressive, tensile, and shear stresses have been shown to act as indicated in Fig. l.‘O Under the test conditions used, the poly95
(ethyl methacrylate) material (Snapj displayed the greatest. value for resistance to fracture, followed by the poly(methyl methacrylate) materials, the composite, and lastly the epimine resin. These results differ from those reported by Gegauff and Pryor,s but can be explained by the difference in test method used; the study cited placed the test specimens in tension only. Despite the higher figures obtained for the poly(ethy1 methacrylate) material, this material displayed a larger standard deviation than the other materials. This was because of the effect of two of the specimens, which displayed markedly lower values. If these two results are removed, the standard deviation obtained becomes 0.03 and the mean increases to 0.54 kN. The source of this variation is not known and requires further investigation as to the consistency of the behavior of this material, because of the evident clinical implications. The flexural strength of a provisional resin is only one of a number of factors to be taken into account in selecting suitable materials for clinical use. This study has shown that, under the test conditions used, the poly(ethy1 methacrylate) materials would be expected to provide a greater flexural strength when used for provisional fixed partial dentures.
CONCLUSIONS A method to test the flexural strength of five provisional resin materials that provided a simulation of the clinical condition of a fixed partial denture revealed the following: 1. The highest values for fracture resistance were displayed by the Snap poly(methy1 methacrylate) material. This material also displayed a large standard deviation because of the effect of two of 11 specimens, which displayed markedly lower values. This finding requires further investigation. 2. In decreasing order, the fracture resistance of the other materials was as follows: the poly(methy1 methacry-
96
late) materials Caulk Temporary Bridge tieslrl and ( i-l: Unifast Temporary resin, the Protemp composire mat r~%~l. and the Scutan epimine material. We acknowledge with grateful thanks the kind Iwlp antl ativ~cc, of Prof. C. Jooste of the Faculty of Dentistry. t’niversit?; crf Strllenbosch. We are also indebted to that Faculty l’or their kind I,v~mission f’or the use of their tensilr testing machine. Dr. (’ I.ill:!bard of the Medical Research Council carried or11t-he st;li.isl II,;II analysis. REFERENCES 1. Krug RS. Temporary resin crowns and bridges. Iknr (‘lin North Am 1975;19:313-20. 2. Kastenbeum F. Lahoraton, processed prowsiona~ prosr.heses. N\ ,I Dent 1982;52:39-44. 3. McCabe JF. Temporary crown and bridge rehnw. In: >lcCubr
VOLUME
70
NUMRER
I
strength
of provisional
restorative
materials
Y. I. Osman, BChD, MChD,a and C. P. Owen, BDS, MScDent, MChDb Faculty of Dentistry, University of the Western Cape, Cape Town, South Africa A provisional restorative material must be strong enough to resist fracture during function. This study tested flve autopolymerizing provisional resin materials under conditions that related the stresses acting on them to those acting on a axed partial denture. The highest values for fracture resistance were displayed by Snap poly(ethy1 methacrylate) material. However, two of the 11 samples of this material displayed markedly lower values for fracture resistance. This finding warrants further investigation, because inconsistency has clinical implications. In decreasing order, the fracture resistance of the other materials was as follows: the poly(methy1 methacrylate) materials, Caulk temporary bridge resin and G-C Unifast temporary resin; the composite material, Protemp; and the epimine material, Scutan. (J PROSTHET DENT 1993;70:94-6.)
A
provisional restoration must fulfill several functions, not least of which is that it must be strong enough to
resist fracture.lW5A number of studies have examined the mechanical properties of resins found to be acceptable for use as provisional restorations, but there has been little consistency in the methods used.6-g Braden et a1.7in 1971 tested a newly introduced ethylene-imine (epimine) material and found that the impact strength, tensile strength, and extension to fracture were all decidedly inferior to a poly(methy1 methacrylate). In a later study a resin based on a poly(ethy1 methacrylate) polymer powder and n-butyl methacrylate monomer system was compared with a conventional acrylic resin, an epimine, and a poly(ethy1 methacrylate) resin of composition similar to that being tested, but which used isobutyl instead of n-butyl methacrylate monomer.” Rectangular specimens 60 x 5 x 1.5 mm were used. Flexural strength
aHead, Department of Conservative Dentistry. bProfeasor and Head, Department of Prosthetic Dentistry. Copyright @ 1993 by The Editorial Council of THE JOURNAL OF PROSTHETIC DENTISTRY. 0022-3913/93/$1.00 + .lO. 10/l/45956
Table
94
I.
was tested by clamping one end of each specimen and applying varying loads to the other. The new resin and the epimine showed higher values under these test conditions, but the new resin and the poly(ethy1 methacrylate) had lower Young’s modulus values. Increased flexibility can be an advantage in a provisional resin material, but the flexural strength test used may not be directly applicable clinically.
Fracture toughness has been tested by placement of specimens in tension in a servohydraulic loading apparatus.sThe specimens were in the form of a 14.4 x 15 x 4 mm slotted block in which a precrack was formed. An epimine and two poly(ethy1 methacrylate) resins demonstrated the greatest fracture toughness. A poly(ethy1 methacrylate) had the lowest fracture toughness, and a composite material was intermediate in toughness. In the clinical situation, a fixed partial denture is subjected to a variety of forces when under load. These forces have been shown by photoelastic stress analysis studies to be compressive at the points of application of load, and tensile and shear at the points of resistance to that load.iO This study compared the flexural strength of a variety of provisional resins with a destruct test that would simulate
a clinical
situation.
Materials used Trade name
Resin type
Caulk Temporary Bridge Besin (“Caulk’s”)
Poly(methy1 methacrylate)
G-C Unifast temporary resin (“Unifast”) Protemp
Poly(methy1 methacrylate)
Scutan Snap
Epimine Poly(ethy1 methacrylate)
Composite
Manufacturer
L. D. Caulk Co. Dentaply International Milford, Delaware G-C Dental Industrial Corporation Tokyo, Japan ESPE Fabrik GMbH Seefeld, Germany ESPE Fabrik GMbH Parke11Bio-Materials Division Farmingdale, h’. Y.
Batch No. 940378
090661 N112 Do17 35151
VOLUME70
NUMBER1
OSMAN
AND
THE
OWEN
JOURNAL
OF PROSTHETIC
DENTISTRY
SNAP
LOAD
I
CAULK’S UNIFAST PROTEMP
Fig. 1. Diagrammatic representation of the destruct test used. Force applied is resolved as a combination of compressive CC),tensile (T), and shear (S) stresses.
SCUTAN MATERIAL
AND
METHOD
The materials used are listed in Table I. Standard specimens of each material were produced from a brass mold designed on a split-cast principle. Tinfoil was used as the separating medium. Each mold yielded two identical specimens, the dimensions of which were 3 x 5 x 90 mm. A standardized procedure was used for constructing the specimens. All of the materials used came from the same batch for each manufacturer. The materials were mixed with weighed quantities of powder and a predetermined volume of liquid pipetted into the powder. The materials were mixed under clinical conditions according to the manufacturers’ instructions. They were syringed into the mold and allowed to bench cure for 20 minutes under a constant pressure of 500 gm. The specimens were stored at room temperature for 24 hours and then incubated in normal saline at 37’ C for at least 24 hours. Each specimen was assigned a number from a computergenerated list of random numbers. The specimens were then subjected to fracture tests on a tensile testing machine. The operator performing the fracture tests did not know which numbers were assigned to which material. The specimens were placed such that a central length of 10 mm of the specimen was loaded (Fig. 1). The machine used was the J.J.Tensile testing machine, type T5001 with a crosshead speed of 5 mm per minute. The machine was calibrated to a load cell sensitivity of 1, with a paper crosshead ratio of 1:l. It had a load cell rating of five kilonewtons (5 kN). Thus a pen movement of 250 mm on the graph paper was representative of a force of 5 kN applied to the specimen. When the specimens were fractured, the ends were observed under a laboratory magnifying glass. The presence of any air bubbles within the fracture face of the specimen excluded the specimen from the experiment. One way analysis of variance (ANOVA) was used to test for a significant difference between the materials. A RyanEinot-Gabriel-Welsch (REGWQ) multiple range test was used to test for differences between the groups.‘l
JULY
1993
2. Materials not significantly are linked by the bars.
Fig.
Table
different at p < 0.05
Force (in kilonewtons) required to fracture
II.
specimens
Mean SD
Snap
Caulk’s
Unifast
Protemp
Scutan
0.47 0.54 0.53 0.54 0.13 0.56 0.55 0.57 0.53 0.15 0.54 0.46 0.16
0.23 0.23 0.23 0.25 0.23 0.24 0.23 0.21 0.23 0.26 0.25 0.23 0.01
0.27 0.24 0.25 0.25 0.22 0.23 0.24 0.23 0.22 0.22 0.23 0.23 0.02
0.13 0.20 0.19 0.15 0.20 0.19 0.21 0.18 0.15 0.15 0.16 0.17 0.03
0.13 0.12 0.15 0.14 0.17 0.17 0.14 0.18 0.17 0.12 0.12 0.14 0.02
RESULTS The force required to fracture the specimens is shown in Table II. The highest figure was obtained by Snap poly(ethyl methacrylate) material. ANOVA revealed an f value of 31.4, which is significant at p < 0.001. The REGWQ multiple range test revealed that the difference between the Snap material and all other materials tested was significant at p < 0.05. At this level, the test also revealed that there was no significant difference between Caulk’s, Unifast, and Protemp materials and no difference between Protemp and Scutan materials (Fig. 2). DISCUSSION The test used in this study is an attempt to simulate the clinical situation, where a combination of compressive, tensile, and shear stresses have been shown to act as indicated in Fig. l.‘O Under the test conditions used, the poly95
(ethyl methacrylate) material (Snapj displayed the greatest. value for resistance to fracture, followed by the poly(methyl methacrylate) materials, the composite, and lastly the epimine resin. These results differ from those reported by Gegauff and Pryor,s but can be explained by the difference in test method used; the study cited placed the test specimens in tension only. Despite the higher figures obtained for the poly(ethy1 methacrylate) material, this material displayed a larger standard deviation than the other materials. This was because of the effect of two of the specimens, which displayed markedly lower values. If these two results are removed, the standard deviation obtained becomes 0.03 and the mean increases to 0.54 kN. The source of this variation is not known and requires further investigation as to the consistency of the behavior of this material, because of the evident clinical implications. The flexural strength of a provisional resin is only one of a number of factors to be taken into account in selecting suitable materials for clinical use. This study has shown that, under the test conditions used, the poly(ethy1 methacrylate) materials would be expected to provide a greater flexural strength when used for provisional fixed partial dentures.
CONCLUSIONS A method to test the flexural strength of five provisional resin materials that provided a simulation of the clinical condition of a fixed partial denture revealed the following: 1. The highest values for fracture resistance were displayed by the Snap poly(methy1 methacrylate) material. This material also displayed a large standard deviation because of the effect of two of 11 specimens, which displayed markedly lower values. This finding requires further investigation. 2. In decreasing order, the fracture resistance of the other materials was as follows: the poly(methy1 methacry-
96
late) materials Caulk Temporary Bridge tieslrl and ( i-l: Unifast Temporary resin, the Protemp composire mat r~%~l. and the Scutan epimine material. We acknowledge with grateful thanks the kind Iwlp antl ativ~cc, of Prof. C. Jooste of the Faculty of Dentistry. t’niversit?; crf Strllenbosch. We are also indebted to that Faculty l’or their kind I,v~mission f’or the use of their tensilr testing machine. Dr. (’ I.ill:!bard of the Medical Research Council carried or11t-he st;li.isl II,;II analysis. REFERENCES 1. Krug RS. Temporary resin crowns and bridges. Iknr (‘lin North Am 1975;19:313-20. 2. Kastenbeum F. Lahoraton, processed prowsiona~ prosr.heses. N\ ,I Dent 1982;52:39-44. 3. McCabe JF. Temporary crown and bridge rehnw. In: >lcCubr
VOLUME
70
NUMRER
I