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Compressive strength comparison and crystal morphology of dental cements
Compressive strength comparison and crystal morphology of dental cements Drummond JL, Lenke JW, Randolph R G . Compressive strength comparison and crystal morphology of dental cements. Dent Mater 1988: 4: 3 8 4 0 . Abstract - Three dental cements were aged as cylindrical specimens in distilled water for periods up to 2 years. Zinc phosphate cement showed no change in compressive strength over the aging time, while glass ionomer and polycarboxylate cements increased in compressive strength. Energy dispersive spectroscopy of the crystalline surface of the zinc phosphate cement samples indicated a wide range of crystalline structures and compositions.
The use of cements as luting agents is w e l l established, with their effectiveness based on strength (compressive, tensile, and shear) and solubility. The compressive strengths for the three cement systems studied here are zinc phosphate cements 80-110 MPa (1, 2, 5, 8), polycarboxylate cements 55-85 MPa (2, 5, 6, 7, 8), and glass ionomer cements 90-140 (3, 4, 8, 9). Previous studies have determined only hopeite and/or zinc phosphate as the crystals formed on the exterior of zinc phosphate cements after setting (10-11). The purpose of this study was to evaluate the compressive strength of dental cements after aging in distilled water for periods up to 758 days, and to analyze the oxide composition of the various crystals formed on the exterior surface during aging.
J. L. Drummond 1, J. W. Lenke 2, R. G. Randolph 3 1Department of Operative Dentistry, College of Dentistry, University of Illinois at Chicago, Chicago, Illinois, USA, 2American Dental Association Health Foundation, American Dental Association, Chicago, Illinois, USA and 3Department of Fixed Partial Prosthodontics, College of Dentistry, University of Illinois at Chicago, Chicago, Illinois, USA
Key words: dental cements, aging, crystalline compositions. James L. Drummond, D.D.S., PhD, University of Illinois at Chicago, College of Dentistry, Department of Operative Dentistry, Room 335 B, 801 S. Paulina, Chicago, Illinois 60612, USA.
Received February 6; accepted May 5, 1987.
ml of distilled water in sealed polyethylene containers at 37~ Aging periods were from 122 to 758 days. At the end of the aging period, the samples were removed from the distilled water, stored in a humidor for 24 h, and then tested in compression using a universal testing machine 4 at a loading rate of 5 mm/min. Statistical analysis was conducted using an one-way analysis of variance with a Student-Neuman-Keuls (SNK) multiple comparison test of the controls and the aged specimens. Scanning electron micrographs (SEM) were taken of the internal and external surfaces~before and after aging to evaluate the effect of the aging media on these surfaces.
The crystals and matrix of the zinc phosphate cement were analyzed by energy dispersive spectroscopys (EDS) according to oxide percentage. The cement samples were coated with carbon and SEM pictures taken of the analyzed areas.
4Instron Model 1125, Instron Corporation, Canton, Massachusetts, U.S.A.
5Princeton-Gamma-Tech, Jersey, U.S.A.
Results and discussion The compressive strength data are presented in Table 1. The zinc phosphate cement showed a slight decrease in strength over the aging time, but this decrease was not statistically significant. The polycarboxylate cement showed an increase in strength over time, with a significant difference Princeton, New
Table 1. Mean compressive strength of dental cements after aging in distilled water. Material and methods
Group
N
The cements were formed in stainless steel molds to produce cylindrical samples 12 mm in height and 6 mm in diameter. The cements were mixed according to manufacturers' instructions. The cylindrical samples were stored for 24 h in a humidor before testing or aging. The three cements used were zinc phosphate 1, polycarboxylate2, and glass ionomer 3. The aging solution was 500
Zinc phosphate cement Controls 758 days 122 days 402 days
17 15 14 15
Polycarboxylate cements 472 days 124 days 283 days Controls
15 15 15 32
Glass ionomer cements 369 days 259 days 124 days Controls 488 days
15 15 14 25 11
1Fleck's, Mizzy, Inc., Clifton Forge, Virginia, U.S.A. 2Durelon, Premier Dental Products, Philadelphia, Pennsylvania, U.S.A. 3Chemglass, L. D. Caulk, Milford, Delaware, U.S.A.
Mean compressive strength (MPa) _+ SD
~ [
T J
T 1
56.45_+23.25 53.66_+25.81 50.86_+21.73 37.10+14.88
~ t
64.47+11.04 59.69_+7.22 59.43_+ 8.62 53.33+ 8.69
T 1
101.96+14.35 99.37_+31.27 92.50_+15.85 86.19+25.52 77.26+20.56
Ranking is from highest to lowest. Values joined by a common line do not differ significantly.
Crystal morphology of dental cements Table 2. Energy dispersive spectroscopy analysis by oxide percentage. Specimen
MgO
A1203
P205
CaO
ZnO
Na20
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
13.75 8.61 12.25 0.00 13.02 11.40 6.04 4.44 16.67 3.16 0.00
0.83 12.74 6.77 0.61 0.96 1.72 2.31 11.24 4.82 3.34 0.01
33.97 41.79 30.92 5.14 31.72 30.82 27.26 25.06 38.21 8.68 8.26
0.04 0.00 0.06 0.00 0.01 0.04 0.00 5.15 0.13 0.19 0.00
51.41 35.86 49.68 92.25 54.29 56.01 64.40 54.11 40.17 84.62 91.72
0.00 0.65 0.00 0.00 0.00 0.00 0.00 0.00 12.32 0.00 0.00
Control Control-matrix 122 days-matrix 122 days 122 days 402 days 402 days 402 days-matrix 758 days 758 days 758 days
Table 3. Natural occurring crystals of zinc phosphate and atomic weight ratios of ZnO to P205. Crystal
Composition
Hopeite Parahopeite Hibbenite Spencerite Tarbuttite
Zn3(PO4)2 9 4H20 Z n 3 ( P O 4 ) 2 9 4H20
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
Zn7(PO4) 4 - 7H20
Zn4(PO4)2 (OH)2 - 3H20 Zn2 (PO4) OH
Control Control-matrix 122 days-matrix 122 days 122 days 402 days 402 days 402 days-matrix 758 days 758 days 758 days
found between the control and the samples aged 472 days. The glass ionomer cement also showed an increase in
Ratio ZnO/P205 1.7 1.7 2.0 2.3 2.3 1.5 0.9 1.6 18.3 1.7 1.8 2.4 2.2 9.7 1.1 11.1
strength over time, except for the longest aging time of 488 days, which was the lowest value and significantly dif-
39
ferent from the other data. Crisp et al. found a similar increase in strength at the end of one year and attributed this gain to an increase in the number of ionic crosslinks (3). The unanswered question is whether the increase in cross linking is negated over time as the cement is subject to attack by the aging media. The compressive strength values of the cements are within the range of values reported in the literature. The effect of exposure to distilled water on the external surface of the zinc phosphate cement resulted in several crystal morphologies and compositions. The EDS analysis (Table 2) is according to oxide percentage. Table 3 lists the naturally occurring hydrated crystals of zinc phosphate and the respective zinc oxide to phosphorous oxide by weight ratios of the natural crystals and the analyzed specimens (12). The analyzed matrix specimens 2, 3, and 8 are characterized by a higher concentration of aluminum, which is added in the liquid to allow the formation of complexes with phosphoric acid AI (H3PO4) (13). The reported crystal formation on the exterior of zinc phosphate cement after initial set is hopeite, which has a ratio of 1.72 for 4ZnO/P205 which is the same for parahopeite. Specimens 1, 5, and 6 fall into this category. Spencerite and Tarbuttite with a ratio of 2.29 are close to specimen 7. The very high ratios of ZnO/P205 are most likely a combination of ZnO hydrated crystals, specimens 4, 9, and 11. The substitution of Mg in zinc phosphate to form MgZnP205 may account for specimen 10. Mg is added in the liquid to delay the development of crystals of hopeite (8). The different crystal morphologies are evident in Figures 1 and 2 for the zinc phosphate cement samples aged 758 days. The variation in crystal composition and morphology indicates that, although hopeite is the first crystal formed after mixing, as the cement is aged the crystal composition and morphology change.
Summary
Fig. l. Crystal analyzed by energy dispersive spectroscopy on exterior of zinc phosphate cement aged 758 days (specimen 11) in distilled water.
The compressive strength of zinc phosphate cement decreased with aging in distilled water while that of polycarboxylate and glass ionomer cement increased. EDS analysis of the exterior surface of zinc phosphate cement indicated a wide range of crystal mot-
40
Drummond
et al.
Fig. 2. Crystal analyzed by energy dispersive spectroscopy on exterior of zinc phosphate ce5 ment aged 758 days (specimen 10) in distilled water.
phology a n d c o m p o s i t i o n d e p e n d i n g o n t h e length of aging.
Acknowledgements - The authors are grate-
ful to Dr Maharukh Kravich for assistance in the cement sample preparation. This research was supported in part by a grant from the Campus Research Board, University of Illinois at Chicago. 1. Swartz ML, Phillips RW, Norman RD, Oldham DF. Strength, hardness and
abrasion characteristics of dental cements. J A m Dent Assoc 1963:67 (9): 367-374. 2. Osborne JW, Swartz MJ, Goodacre CJ, Phillips RW, Gale EN. A method for assessing the clinical solubility and disintegration of luting cements. J Prosthet Dent 1978:40 (4): 413-417. 3. Crisp S, Lewis BG, Wilson AD. Characterization of glass-ionomer cements. I. Long-term hardness and compressive strength. J Dent (Bristol) 1976:4 (4): 162-166.
4. Causton BE. The physio-mechanical consequences of exposing glass ionomer cements to water during setting. Biomaterials 1981: 2: 112-115. 5. Branco R, Hegdahl T. Physical properties of some zinc phosphate and polycarboxylate cements. Acta Odontol Scand 1983: 41: 349-353. 6. Mortimer KV, Tranter TC. A preliminary laboratory evaluation of polycarboxylate cements. Br Dent J 1969: 10: 365-370. 7. Powers JM, Johnson ZG, Craig RG. Physical and mechanical properties of zinc polyacrylate dental cements. J A m Dent Assoc 1974: 88: 380-383. 8. Smith DC. Dental cements - current status and future prospects. Dent Clin N A m 1983:6 (3): 763-792. 9. McComb D, Sirisko R, Brown J. Comparison of physical properties of commercial glass ionomer luting cements. J Can Dent Assoc 1984: 9: 699-701. 10. Kikuchi H, Hirose R, Ide R, et al. Surface analysis of cements after having been immersed in water-surface state and elementary analysis of zinc phosphate cement and carboxylate cement. J Nihon Univ Sch Dent 1983:25 (4): 277283. 11. Cartz L, Servais G, Rossi E Surface structure of zinc phosphate dental cements. J Dent Res 1972:51 (6): 16681671. 12. Palache C, Berman H, Frondell C. Dana's System o f Mineralogy, 7th edn, Volume 11, John Wiley and Sons, Inc., New York, 1951: 1-1124. 13. Smith DC, Norman RD, Swartz ML. Dental cements: current status and future prospects. In: Restorative Dental Materials. A n Overview. Vol. 1, Eds. J Reese and TA Valgea, London, Quintessence, 1985: 33-74.
The use of cements as luting agents is w e l l established, with their effectiveness based on strength (compressive, tensile, and shear) and solubility. The compressive strengths for the three cement systems studied here are zinc phosphate cements 80-110 MPa (1, 2, 5, 8), polycarboxylate cements 55-85 MPa (2, 5, 6, 7, 8), and glass ionomer cements 90-140 (3, 4, 8, 9). Previous studies have determined only hopeite and/or zinc phosphate as the crystals formed on the exterior of zinc phosphate cements after setting (10-11). The purpose of this study was to evaluate the compressive strength of dental cements after aging in distilled water for periods up to 758 days, and to analyze the oxide composition of the various crystals formed on the exterior surface during aging.
J. L. Drummond 1, J. W. Lenke 2, R. G. Randolph 3 1Department of Operative Dentistry, College of Dentistry, University of Illinois at Chicago, Chicago, Illinois, USA, 2American Dental Association Health Foundation, American Dental Association, Chicago, Illinois, USA and 3Department of Fixed Partial Prosthodontics, College of Dentistry, University of Illinois at Chicago, Chicago, Illinois, USA
Key words: dental cements, aging, crystalline compositions. James L. Drummond, D.D.S., PhD, University of Illinois at Chicago, College of Dentistry, Department of Operative Dentistry, Room 335 B, 801 S. Paulina, Chicago, Illinois 60612, USA.
Received February 6; accepted May 5, 1987.
ml of distilled water in sealed polyethylene containers at 37~ Aging periods were from 122 to 758 days. At the end of the aging period, the samples were removed from the distilled water, stored in a humidor for 24 h, and then tested in compression using a universal testing machine 4 at a loading rate of 5 mm/min. Statistical analysis was conducted using an one-way analysis of variance with a Student-Neuman-Keuls (SNK) multiple comparison test of the controls and the aged specimens. Scanning electron micrographs (SEM) were taken of the internal and external surfaces~before and after aging to evaluate the effect of the aging media on these surfaces.
The crystals and matrix of the zinc phosphate cement were analyzed by energy dispersive spectroscopys (EDS) according to oxide percentage. The cement samples were coated with carbon and SEM pictures taken of the analyzed areas.
4Instron Model 1125, Instron Corporation, Canton, Massachusetts, U.S.A.
5Princeton-Gamma-Tech, Jersey, U.S.A.
Results and discussion The compressive strength data are presented in Table 1. The zinc phosphate cement showed a slight decrease in strength over the aging time, but this decrease was not statistically significant. The polycarboxylate cement showed an increase in strength over time, with a significant difference Princeton, New
Table 1. Mean compressive strength of dental cements after aging in distilled water. Material and methods
Group
N
The cements were formed in stainless steel molds to produce cylindrical samples 12 mm in height and 6 mm in diameter. The cements were mixed according to manufacturers' instructions. The cylindrical samples were stored for 24 h in a humidor before testing or aging. The three cements used were zinc phosphate 1, polycarboxylate2, and glass ionomer 3. The aging solution was 500
Zinc phosphate cement Controls 758 days 122 days 402 days
17 15 14 15
Polycarboxylate cements 472 days 124 days 283 days Controls
15 15 15 32
Glass ionomer cements 369 days 259 days 124 days Controls 488 days
15 15 14 25 11
1Fleck's, Mizzy, Inc., Clifton Forge, Virginia, U.S.A. 2Durelon, Premier Dental Products, Philadelphia, Pennsylvania, U.S.A. 3Chemglass, L. D. Caulk, Milford, Delaware, U.S.A.
Mean compressive strength (MPa) _+ SD
~ [
T J
T 1
56.45_+23.25 53.66_+25.81 50.86_+21.73 37.10+14.88
~ t
64.47+11.04 59.69_+7.22 59.43_+ 8.62 53.33+ 8.69
T 1
101.96+14.35 99.37_+31.27 92.50_+15.85 86.19+25.52 77.26+20.56
Ranking is from highest to lowest. Values joined by a common line do not differ significantly.
Crystal morphology of dental cements Table 2. Energy dispersive spectroscopy analysis by oxide percentage. Specimen
MgO
A1203
P205
CaO
ZnO
Na20
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
13.75 8.61 12.25 0.00 13.02 11.40 6.04 4.44 16.67 3.16 0.00
0.83 12.74 6.77 0.61 0.96 1.72 2.31 11.24 4.82 3.34 0.01
33.97 41.79 30.92 5.14 31.72 30.82 27.26 25.06 38.21 8.68 8.26
0.04 0.00 0.06 0.00 0.01 0.04 0.00 5.15 0.13 0.19 0.00
51.41 35.86 49.68 92.25 54.29 56.01 64.40 54.11 40.17 84.62 91.72
0.00 0.65 0.00 0.00 0.00 0.00 0.00 0.00 12.32 0.00 0.00
Control Control-matrix 122 days-matrix 122 days 122 days 402 days 402 days 402 days-matrix 758 days 758 days 758 days
Table 3. Natural occurring crystals of zinc phosphate and atomic weight ratios of ZnO to P205. Crystal
Composition
Hopeite Parahopeite Hibbenite Spencerite Tarbuttite
Zn3(PO4)2 9 4H20 Z n 3 ( P O 4 ) 2 9 4H20
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
Zn7(PO4) 4 - 7H20
Zn4(PO4)2 (OH)2 - 3H20 Zn2 (PO4) OH
Control Control-matrix 122 days-matrix 122 days 122 days 402 days 402 days 402 days-matrix 758 days 758 days 758 days
found between the control and the samples aged 472 days. The glass ionomer cement also showed an increase in
Ratio ZnO/P205 1.7 1.7 2.0 2.3 2.3 1.5 0.9 1.6 18.3 1.7 1.8 2.4 2.2 9.7 1.1 11.1
strength over time, except for the longest aging time of 488 days, which was the lowest value and significantly dif-
39
ferent from the other data. Crisp et al. found a similar increase in strength at the end of one year and attributed this gain to an increase in the number of ionic crosslinks (3). The unanswered question is whether the increase in cross linking is negated over time as the cement is subject to attack by the aging media. The compressive strength values of the cements are within the range of values reported in the literature. The effect of exposure to distilled water on the external surface of the zinc phosphate cement resulted in several crystal morphologies and compositions. The EDS analysis (Table 2) is according to oxide percentage. Table 3 lists the naturally occurring hydrated crystals of zinc phosphate and the respective zinc oxide to phosphorous oxide by weight ratios of the natural crystals and the analyzed specimens (12). The analyzed matrix specimens 2, 3, and 8 are characterized by a higher concentration of aluminum, which is added in the liquid to allow the formation of complexes with phosphoric acid AI (H3PO4) (13). The reported crystal formation on the exterior of zinc phosphate cement after initial set is hopeite, which has a ratio of 1.72 for 4ZnO/P205 which is the same for parahopeite. Specimens 1, 5, and 6 fall into this category. Spencerite and Tarbuttite with a ratio of 2.29 are close to specimen 7. The very high ratios of ZnO/P205 are most likely a combination of ZnO hydrated crystals, specimens 4, 9, and 11. The substitution of Mg in zinc phosphate to form MgZnP205 may account for specimen 10. Mg is added in the liquid to delay the development of crystals of hopeite (8). The different crystal morphologies are evident in Figures 1 and 2 for the zinc phosphate cement samples aged 758 days. The variation in crystal composition and morphology indicates that, although hopeite is the first crystal formed after mixing, as the cement is aged the crystal composition and morphology change.
Summary
Fig. l. Crystal analyzed by energy dispersive spectroscopy on exterior of zinc phosphate cement aged 758 days (specimen 11) in distilled water.
The compressive strength of zinc phosphate cement decreased with aging in distilled water while that of polycarboxylate and glass ionomer cement increased. EDS analysis of the exterior surface of zinc phosphate cement indicated a wide range of crystal mot-
40
Drummond
et al.
Fig. 2. Crystal analyzed by energy dispersive spectroscopy on exterior of zinc phosphate ce5 ment aged 758 days (specimen 10) in distilled water.
phology a n d c o m p o s i t i o n d e p e n d i n g o n t h e length of aging.
Acknowledgements - The authors are grate-
ful to Dr Maharukh Kravich for assistance in the cement sample preparation. This research was supported in part by a grant from the Campus Research Board, University of Illinois at Chicago. 1. Swartz ML, Phillips RW, Norman RD, Oldham DF. Strength, hardness and
abrasion characteristics of dental cements. J A m Dent Assoc 1963:67 (9): 367-374. 2. Osborne JW, Swartz MJ, Goodacre CJ, Phillips RW, Gale EN. A method for assessing the clinical solubility and disintegration of luting cements. J Prosthet Dent 1978:40 (4): 413-417. 3. Crisp S, Lewis BG, Wilson AD. Characterization of glass-ionomer cements. I. Long-term hardness and compressive strength. J Dent (Bristol) 1976:4 (4): 162-166.
4. Causton BE. The physio-mechanical consequences of exposing glass ionomer cements to water during setting. Biomaterials 1981: 2: 112-115. 5. Branco R, Hegdahl T. Physical properties of some zinc phosphate and polycarboxylate cements. Acta Odontol Scand 1983: 41: 349-353. 6. Mortimer KV, Tranter TC. A preliminary laboratory evaluation of polycarboxylate cements. Br Dent J 1969: 10: 365-370. 7. Powers JM, Johnson ZG, Craig RG. Physical and mechanical properties of zinc polyacrylate dental cements. J A m Dent Assoc 1974: 88: 380-383. 8. Smith DC. Dental cements - current status and future prospects. Dent Clin N A m 1983:6 (3): 763-792. 9. McComb D, Sirisko R, Brown J. Comparison of physical properties of commercial glass ionomer luting cements. J Can Dent Assoc 1984: 9: 699-701. 10. Kikuchi H, Hirose R, Ide R, et al. Surface analysis of cements after having been immersed in water-surface state and elementary analysis of zinc phosphate cement and carboxylate cement. J Nihon Univ Sch Dent 1983:25 (4): 277283. 11. Cartz L, Servais G, Rossi E Surface structure of zinc phosphate dental cements. J Dent Res 1972:51 (6): 16681671. 12. Palache C, Berman H, Frondell C. Dana's System o f Mineralogy, 7th edn, Volume 11, John Wiley and Sons, Inc., New York, 1951: 1-1124. 13. Smith DC, Norman RD, Swartz ML. Dental cements: current status and future prospects. In: Restorative Dental Materials. A n Overview. Vol. 1, Eds. J Reese and TA Valgea, London, Quintessence, 1985: 33-74.