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  • 1. Laboratory Strength of Glass Ionomer and Zinc Phosphate Cements Andree Piwowarczyk, Dr med dent,1 Peter Ottl, Dr med dent,1 and Hans-Christoph Lauer, Dr med dent2 Purpose: The present in vitro study examined 3 mechanical properties, namely compressive, flexural, and diametral tensile strength, of various commercially available cements and core materials as a function of time after mixing. Materials and Methods: The examined materials were 2 cermet cements (Ketac Silver [ESPE, Seefeld, Germany] and Chelon Silver [ESPE]), 1 metal-reinforced glass ionomer cement (Miracle Mix [GC Dental Industrial Corp, Tokyo, Japan]), 2 conventional glass ionomer cements (Ketac Bond [ESPE] and Ketac Cem [ESPE]), 1 standard cure zinc phosphate cement (Harvard Cement [Richter and Hoffmann, Berlin, Germany]), and 1 zinc phosphate cement with the addition of 30% silver amalgam alloy powder (Harvard Cement 70% with Dispersalloy 30% [Richter and Hoffmann/ Johnson and Johnson, East Windsor, NJ]). Properties were measured using a universal testing machine at 15 minutes, 1 hour, and 24 hours after first mixing. Results: Compressive strengths varied widely between the 3 times of measurement from 5.8 ؎ 6.6 MPa for Ketac Cem to 144.3 ؎ 10.2 MPa for Ketac Silver. Twenty-four hours after mixing, the Bonferroni test showed significant (p < .01) differences between Ketac Silver and all other materials tested. Diametral tensile strengths ranged widely from 4.4 ؎ 0.9 MPa for Ketac Cem to 11.5 ؎ 2.2 MPa for Chelon Silver. At 15 minutes, 1 hour, and 24 hours after first mixing, the analysis of variance did not show any significant differences between Ketac Silver, Chelon Silver, and Miracle Mix. The 3-point flexural strength of Ketac Silver showed, at 15 minutes with 13.5 ؎ 3.9 MPa and at 24 hours with 27.2 ؎ 7.4 MPa, the highest values. Conclusions: Setting time influences the mechanical properties of the materials tested in this study. Ketac Silver, a glass ionomer cement reinforced with sintered glass-silver particles, showed the highest mechanical properties of the examined materials. J Prosthodont 2001;10:140-147. Copyright © 2001 by The American College of Prosthodontists. INDEX WORDS: core restoration, mechanical properties, compressive strength, flexural strength, diametral tensile strength FIXED PROSTHODONTIC restorations often require the placement of foundation restora- tions to establish retentive and resistance forms to the teeth used as prosthesis retainers. These resto- rations will ideally possess sufficient mechanical strength, retention to the hard dental tissues, and a high degree of biocompatibility.1 Glass ionomer cements2,3 have been used as core build-up materials. They provide mechanical and chemical adhesion to the hard dental tissues that is on the order of one fifth to one third of that ob- tained with composite resins using the acid-etch technique.2 The reported results differ on the basis of brand of material4 and conditioning techniques.5 Metal-reinforced glass ionomer cements, known as cermet cements, are a mixture of glass and metal powder6 sintered to high density. This material sets to form a cement via an acid-base reaction.7 By sintering metal particles into the glass phase, it is possible to improve the material’s abrasion,8,9 and to yield a modest increase in diametral tensile strength.10 The objective of this study was to determine and compare the mechanical properties of various ce- ments used as core build-up materials relative to time after first mixing. The parameters examined were compressive strength, flexural strength, and diametral tensile strength at times of 15 minutes, 1 hour, and 24 hours. From Department of Prosthetic Dentistry, School of Dentistry, Johann Wolfgang Goethe University, Frankfurt, Germany. 1 Assistant Professor. 2 Professor and Director. Accepted June 25, 2001. Correspondence to: Dr. A. Piwowarczyk, Department of Pros- thetic Dentistry, School of Dentistry, Johann Wolfgang Goethe Uni- versity, Theodor-Stern-Kai 7, D-60590 Frankfurt, Germany. E-mail: piwowarczyk@t-online.de Copyright © 2001 by The American College of Prosthodontists 1059-941X/01/1003-0003$35.00/0 doi:10.1053/jpro.2001.27338 140 Journal of Prosthodontics, Vol 10, No 3 (September), 2001: pp 140-147
  • 2. Materials and Methods This study evaluated 2 cermet cements (Ketac Silver [ESPE, Seefeld, Germany] and Chelon Silver [ESPE]), 1 metal-reinforced glass ionomer cement (Miracle Mix [GC Dental Industrial Corp, Tokyo, Japan]), 2 conven- tional glass ionomer cements (Ketac Bond [ESPE] and Ketac Cem [ESPE]), 1 standard cure zinc phosphate cement (Harvard Cement [Richter and Hoffmann, Ber- lin, Germany]), and 1 zinc phosphate cement with the addition of 30% silver amalgam alloy powder (Harvard Cement 70% with Dispersalloy 30% [Richter and Hoff- mann/Johnson and Johnson, East Windsor, NJ]) under compressive, tensile, and flexural forces (Table 1). Powder was weighed using an analytical balance (Ϯ1 mg) following the manufacturer’s recommendation, while the liquid volume suggested by the manufacturer was established using a measuring pipette with an accu- racy of Ϯ0.03 mL. One material, Ketac Silver, was sup- plied in premeasured capsules. For this material, the manufacturer’s recommendation of a trituration time of 10 seconds was used for the Capmix (ESPE) amalgam- ator. To measure the compressive and tensile strengths, specimens were prepared in cylindrical Teflon molds (Figs 1 and 2) to the dimensions of 6 and 8 mm diameter and 12 and 4 mm height, respectively, as dictated by the British Standards Institute Specification 6039.11 The specification BS 6039 refers to the manufacturer’s rec- ommendations concerning the mixing process along with the need to fill the molds within 2 minutes of the initia- tion of mixing. Specimens used to determine flexural strength were produced in brass molds 25 ϫ 2 ϫ 2 mm in size that had been insulated with polytetrafluoroethylene spray (DIN 13922/EN 24049).12 The molds were placed in an incu- bator (Heraeus Kulzer, Hanau, Germany) at 37.0 Ϯ 1.0°C and at 95% to 100% relative humidity 3 minutes after mixing. Specimens were removed from the mold within 60 minutes of testing. Specimens for the 24-hour measurements were stored in the incubator submerged in distilled water until examined. The dimensions of the specimens were checked with a micrometer (Mauser, Figure 1. Precision mold and specimen for determining the compressive strength (6 ϫ 12 mm). Table 1. Core Materials and Manufacturers Material Classification Manufacturer Mixing Process Powder/Liquid Ratio Ketac Silver Cermet cement Espe, Seefeld, Germany Mechanical Chelon Silver Cermet cement Espe, Seefeld, Germany Manual 3.8:1 Miracle Mix Metal-reinforced glass ionomer cement GC Dental Industrial Corporation, Tokyo, Japan Manual 5.0:1 Ketac Bond Conventional glass ionomer cement Espe, Seefeld, Germany Manual 3.4:1 Ketac Cem Conventional glass ionomer cement Espe, Seefeld, Germany Manual 3.4:1 Harvard Cement Zinc phosphate cement Richter & Hoffmann, Berlin, Germany Manual 2.5:1 Harvard Cement 70%/ Dispersalloy 30% Zinc phosphate cement with admixture amalgam alloy powder Richter & Hoffmann, Berlin, Germany/Johnson & Johnson, East Windsor, NJ Manual 2.5:1 Figure 2. Precision mold and specimen for determining the diametral tensile strength (4 ϫ 8 mm). 141September 2001, Volume 10, Number 3
  • 3. Nu¨rnberg, Germany; precision: 0.01 mm). Testing was performed at 23.0 Ϯ 1.0°C and 50% Ϯ 5% relative hu- midity. A Zwick 1435 Universal testing machine (Zwick, Ulm, Germany) was used at a crosshead speed of 1 mm/min to test compressive and diametral tensile strengths (Fig 3). Specimens were loaded until fracture at a constant crosshead speed of 1 mm/min (BS 6039).11 Measurements for the 3-point flexural strength test (Fig 4) were performed at a constant crosshead speed of 1 mm/min (DIN 13922/EN 24049). Flexural, compressive, and diametral tensile strengths were assessed at 15 min- utes, 1 hour, and 24 hours after first mixing. Ten mea- surements each were made for each material and for each of the 3 parameters examined. Statistical data analysis was performed by one-way analysis of variance with regard to the time factor and by 2-sided t tests with regard to the compressive, flexural, and diametral tensile strength variables. The level of significance was fixed at p Յ .05 for significant differ- ences. The Bonferroni test was used for post-hoc analysis of variance comparing multiple means. Results Compressive Strength At the earliest testing period of 15 minutes after mixing, mean compressive strength values ranged from 35.8 Ϯ 6.6 MPa for Ketac Cem to 74.9 Ϯ 7.2 MPa for Harvard zinc phosphate cement with ad- mixture of silver amalgam alloy powder (Fig 5). Compressive tests at 1 hour showed increased com- pressive strength for all materials. Metal-reinforced glass ionomer cements Ketac Silver and Chelon Silver increased by 40.1% and 47.8%, respectively. The lowest value was found for the glass ionomer cement Ketac Cem with 64.2 Ϯ 7.3 MPa after 1 hour. At 24 hours, the compressive strength values showed further increases to 144.3 Ϯ 10.2 MPa by Ketac Silver, followed by Chelon Silver and Miracle Mix with values of 119.2 Ϯ 17.8 MPa and 111.4 Ϯ 13.2 MPa, Harvard cement with 95.7 Ϯ 19.1 MPa, and the admixture of Harvard cement with amal- gam alloy powder with 91.5 Ϯ 15.0 MPa. The Bonferonni test identified statistically significant differences among groups concerning the times of measurement (Table 2). Diametral Tensile Strength Diametral tensile strength means and standard deviations are displayed in Fig 6. At 15 minutes, the highest value of 7.8 Ϯ 1.0 MPa was obtained with the combination of Harvard cement and amalgam alloy powder; conventional glass ionomer cements (Ketac Bond, Ketac Cem) were measured at 5.3 Ϯ 0.7 MPa and 4.4 Ϯ 0.9 MPa. At 1 hour, results for the tested materials ranged from 6.3 Ϯ 1.0 MPa (Ketac Cem) to 8.5 Ϯ 1.9 MPa (Miracle Mix). At 24 hours, Chelon Silver at 11.5 Ϯ 2.2 MPa, Ketac Silver at 11.2 Ϯ 1.6 MPa, and Miracle Mix at 10.7 Ϯ 1.5 MPa showed the highest values. Between 1 and 24 hours after first mixing, Chelon Silver, Ketac Silver, Miracle Mix, and Ketac Cem showed a sig- nificant (p Յ .01) increase in diametral tensile strength. In addition to the 3 times of measure- Figure 3. Testing procedure for determining the diame- tral tensile strength. Figure 4. Testing procedure for determining the flex- ural strength. 142 Laboratory Strength of Cements ● Piwowarczyk, Ottl, and Lauer
  • 4. ment, the analysis of variance again identified sig- nificant differences among the materials (Table 3). Three-point Flexural Strength Figure 7 illustrates the 3-point flexural strength means and standard deviations. At 15 minutes after first mixing, Harvard cement with amalgam alloy powder at 16.2 Ϯ 1.4 MPa, Harvard cement at 14.5 Ϯ 1.7 MPa, and Ketac Silver at 13.5 Ϯ 3.9 MPa yielded the highest values. At 15 minutes, Ketac Cem showed the value of 4.7 Ϯ 1.7 MPa. During the first hour, the flexural strength value for Mir- acle Mix increased by 87.3% to 17.7 Ϯ 4.3 MPa. The corresponding values for Chelon Silver and Ketac Silver were 15.9 Ϯ 5.0 MPa and 15.0 Ϯ 4.6 MPa. Twenty-four hours after first mixing, Ketac Silver, at 27.2 Ϯ 7.4 MPa, yielded the highest values of all the cements examined, while also showing the greatest increase in strength (81.0%) over the 1-hour value. The other materials showed results ranging from 9.6 Ϯ 4.6 MPa for Ketac Bond to 24.8 Ϯ 8.0 MPa for Chelon Silver (Fig 7). The analysis of variance showed that Ketac Silver and Chelon Silver differed significantly from Miracle Mix (p Յ .01), Ketac Cem (p Յ .01), Ketac Bond (p Յ .01), Harvard cement (p Յ .01), and Harvard cement with amalgam alloy powder (p Յ .05). As a result of the Bonferonni test at p Յ .05, the vertical lines in Table 4 bracket statistically equivalent groups. Discussion In designing this study, the decision was made to make evaluations on standardized molds. This al- lowed the testing to conform to the British Stan- Figure 5. Compressive strength (BS 6039) of various core restoration materials (0.25, 1, and 24 hours after first mixing; n ϭ 10 measure- ments each per material and time). KS, Ketac Silver; CS, Chelon Silver; MM, Miracle Mix; KB, Ketac Bond; KC, Ketac Cem; HZ, Harvard ce- ment; HA, Harvard cement with amalgam alloy powder. Table 2. Compressive Strength (BS 6039) of Various Core Restoration Materials (0.25, 1, and 24 Hours After First Mixing; n ϭ 10 Measurements Each per Material and Time) Material 0.25 h 1 h 24 h x (MPa) x (MPa) x (MPa) Ketac Silver 66.0 Ϯ 3.5 92.5 Ϯ 9.5 144.3 Ϯ 10.2 Chelon Silver 55.1 Ϯ 2.6 81.5 Ϯ 5.9 119.2 Ϯ 17.8 Miracle Mix 40.7 Ϯ 4.1 77.6 Ϯ 6.1 111.4 Ϯ 13.2 Ketac Bond 37.7 Ϯ 4.0 66.2 Ϯ 6.7 84.1 Ϯ 18.3 Ketac Cem 35.8 Ϯ 6.6 64.2 Ϯ 7.3 80.1 Ϯ 201 Harvard cement 72.9 Ϯ 5.9 84.0 Ϯ 13.3 95.7 Ϯ 19.1 Harvard cement 70%/ amalgam alloy powder (Dispersalloy) 30% 74.9 Ϯ 7.2 86.9 Ϯ 11.4 91.5 Ϯ 15.0 Note. The vertical bars in respective lines to the right of the points of measurement indicate groups that do not significantly differ as measured by the Bonferroni test (p Յ .05). Abbreviations: x, arithmetic mean; s, standard deviation. 143September 2001, Volume 10, Number 3
  • 5. dards Institute Specification 6039.11 The alternative use of extracted teeth brought with it the variability associated with differences in dimensions, degree of calcification, water content, and the potential for pre-existing hard tissue defects.13-15 Although the latter approach may allow simpler interpretation of clinical expectations, it eliminates standardization and complicates meaningful comparisons. Considerable differences in compressive, flex- ural, and diametral tensile strength were seen among the tested materials. All 3 of the recognized tests showed a similar ranking order for the mate- rials (Figs 5, 6, and 7). Under the conditions of this study, Ketac Silver presented the highest values of compressive and flexural strengths at 24 hours of all the cements examined. This material showed statistically significant differences (p Յ .05) in test- ing compressive strength at all times of measure- ment. Between 1 and 24 hours, Ketac Silver showed a significant (p Յ .01) increase in strength within all the 3 testing parameters. Previous reports showed that the setting reaction of glass ionomer cements is complex, requiring 24 hours for maturation.7,16,17 This is in contrast to the zinc phosphate materials, Harvard cement, and Harvard cement with added amalgam alloy, which showed less increase in strength over time. The data in this report failed to show a consis- tent relationship between the method of mixing (manual or mechanical) and strength properties. Strict adherence to the prescribed mixing ratio is Figure 6. Diametral tensile strength (BS 6039) of various core restoration materials (0.25, 1, and 24 hours after first mixing; n ϭ 10 measure- ments each per material and time). KS, Ketac Silver; CS, Chelon Silver; MM, Miracle Mix; KB, Ketac Bond; KC, Ketac Cem; HZ, Harvard ce- ment; HA, Harvard cement with amalgam alloy powder. Table 3. Diametral Tensile Strength (BS 6039) of Various Core Restoration Materials (0.25, 1, and 24 Hours After First Mixing; n ϭ 10 Measurements Each per Material and Time) Material 0.25 h 1 h 24 h x (MPa) s (MPa) x (MPa) s (MPa) x (MPa) s (MPa) Ketac Silver 6.5 1.5 8.1 1.5 11.2 1.6 Chelon Silver 7.2 1.6 8.2 1.4 11.5 2.2 Miracle Mix 7.2 1.0 8.5 1.9 10.7 1.5 Ketac Bond 5.3 0.7 6.9 1.2 7.4 1.2 Ketac Cem 4.4 0.9 6.3 1.0 7.5 1.3 Harvard cement 6.9 0.9 7.4 1.1 7.9 1.7 Harvard cement 70%/ amalgam alloy powder (Dispersalloy) 30% 7.8 1.0 8.3 1.1 9.2 1.8 Note. The vertical bars in respective lines to the right of the points of measurement indicate groups that do not significantly differ as measured by the Bonferroni test (p Յ .05). Abbreviations: x, arithmetic mean; s, standard deviation. 144 Laboratory Strength of Cements ● Piwowarczyk, Ottl, and Lauer
  • 6. said to be especially important for the successful use of glass ionomer cements. Negative influences on strength, solubility, and surface structure as a result of having too little powder have been cited in the literature. Conversely, with too high of a pow- der-to-liquid ratio, a reduction in setting time and bond strength has been described.18,19 The manu- ally mixed product Chelon Silver, chemically iden- tical to Ketac Silver, has a lower compressive strength at 15 minutes (p Յ .01), 1 hour (p Յ .05), and 24 hours (p Յ .01), but Chelon Silver and Ketac Silver showed no significant differences in flexural and diametral tensile strengths over these time periods (p Յ .05). The present study showed significantly (p Յ .05) lower values for the glass ionomer cements Ketac Cem and Ketac Bond concerning compressive, flex- ural, and diametral tensile strengths at all times of measurement, compared with the cermet cements, Ketac Silver and Chelon Silver. The data reported by Chung10 showed higher diametral tensile strength values for metal-reinforced materials, Ketac Silver and Miracle Mix, compared with the glass ionomer cement, Fuji II (GC Dental Indus- trial Corp). Cermet cements were intended to pos- sess higher flexural strength than glass ionomer cements.20 Walls and coworkers21 showed an in- crease in compressive strength and compressive fatigue limit for cermet-type materials compared with the strength of a conventional glass ionomer cement from the same manufacturer. In contrast to the above-mentioned studies, Cho et al22 docu- mented that silver-reinforced glass ionomer cement Ketac Silver did not improve the compressive and diametral tensile strength in comparison with the conventional glass ionomer cement, Ketac Fil Figure 7. Three-point flex- ural strength (DIN 13922/EN 24049) of various core resto- ration materials (0.25, 1, and 24 hours after first mixing; n ϭ 10 measurements each per material and time). KS, Ketac Silver; CS, Chelon Sil- ver; MM, Miracle Mix; KB, Ketac Bond; KC, Ketac Cem; HZ, Harvard cement; HA, Harvard cement with amalgam alloy powder. Table 4. Three-point Flexural Strength (DIN 13922/EN 24049) of Various Core Restoration Materials (0.25, 1, and 24 Hours After First Mixing; n ϭ 10 Measurements Each per Material and Time) Material 0.25 h 1 h 24 h x (MPa) s (MPa) x (MPa) s (MPa) x (MPa) s (MPa) Ketac Silver 13.5 3.9 15.0 4.6 27.2 7.4 Chelon Silver 11.8 4.1 15.9 5.0 24.8 8.0 Miracle Mix 9.5 1.1 17.7 4.3 18.5 1.9 Ketac Bond 8.4 3.2 7.3 4.4 9.6 4.6 Ketac Cem 4.7 1.7 8.2 3.4 11.6 5.6 Harvard cement 14.5 1.7 17.7 2.9 20.3 1.6 Harvard cement 70%/ amalgam alloy powder (Dispersalloy) 30% 16.2 1.4 18.8 1.3 22.0 1.6 Note. The vertical bars in respective lines to the right of the points of measurement indicate groups that do not significantly differ as measured by the Bonferroni test (p Յ .05). Abbreviations: x, arithmetic mean; s, standard deviation. 145September 2001, Volume 10, Number 3
  • 7. (ESPE). The work of Williams et al23 did not show any significant reduction in compressive and diame- tral tensile strengths for the glass ionomer ce- ments, Chelon Fil (ESPE) and Fuji II (GC Dental Industrial Corp), compared with reinforced mate- rials, Chelon Silver and Miracle Mix. Peutzfeldt24 reported a similar flexural strength of Ketac Silver and Miracle Mix to 5 conventional glass ionomer cements. In the present study, Miracle Mix, a metal- reinforced, nonsintered glass ionomer cement, showed a marked increase in compressive, tensile, and flexural strengths during the first hour after first mixing. At 24 hours, Miracle Mix was inferior to the cermet cements Ketac Silver (p Յ .01) and Chelon Silver (p Յ .05) in flexural strength. After 7 days, Peutzfeldt24 found, in the 3-point flexural strength tests with specimens of 10 ϫ 2 ϫ 2 mm, a flexural strength of 13 Ϯ 2 MPa for Miracle Mix and 29 Ϯ 13 MPa for Ketac Silver. Nakajima et al25 showed no significant differences in 3-point flexural strength tests on Miracle Mix glass ionomer ce- ments (23.0 Ϯ 3.1 MPa) and Ketac Silver (22.6 Ϯ 10.4 MPa) when tested at 24 hours after storage in 100% humidity at 37°C. When measuring com- pressive strengths (American Dental Association [ADA] specification #66), values of 132.1 Ϯ 13.7 MPa and 121.9 Ϯ 12.7 MPa for Miracle Mix and Ketac Silver, respectively, were shown. Cohen et al26 examined the diametral tensile strength and compressive strength (ADA specifica- tion #27) of titanium-reinforced composite resins, Ti-Core (Essential Dental Systems, Hackensack, NJ) and Flexi-Flow Cem (Essential Dental Sys- tems), compared with commercially available core build-up materials and cements. All specimens had been stored at 100% humidity for 24 hours. The glass ionomer cements Ketac Silver, Miracle Mix, and Ketac Cem yielded 60.1% to 80.8% lower values than titanium-reinforced composite resins. Of the 3 glass ionomer cements tested, Ketac Silver achieved the highest values with regard to diame- tral tensile strength and compressive strength at 12.5 Ϯ 2.2 MPa and 115.1 Ϯ 16.0 MPa, respectively. The present study showed that after 24 hours, zinc phosphate cement, with and without the ad- mixture of silver amalgam alloy powder, is stronger than conventional glass ionomer cements and weaker than cermet cements under compressive, tensile, and flexural forces. The cement strength is almost linearly dependent on the powder:liquid ra- tio.27 For zinc phosphate cement, Abraham28 deter- mined an increasing compressive strength and de- clining solubility by increasing the powder-to-liquid ratio. On a graphic curve, this results in a rise to a maximum value and subsequently a decline in re- lation to the compressive strength. Smith29 con- cluded that the compressive and tensile strengths of properly mixed zinc phosphate cement are ade- quate to resist masticatory stress. The reported results concerning the mechanical properties of cements are broad and variable. In particular, measurements of strength are depen- dent on the method of specimen preparation, the operator, and labor variability.30 In addition to me- chanical properties, other factors such as pulp com- patibility,31-33 allergic potential,34-36 and expansion caused by water absorption37-39 should be consid- ered when choosing a material for a foundation restoration. To confirm the results of laboratory studies, longitudinal clinical trials should be con- ducted to establish the most important predictors of clinical success for core materials. Conclusions 1. Compressive strength, flexural strength, and diametral tensile strength varied among the tested materials. 2. Under the conditions of this study, 24 hours after first mixing, cermet cements were stronger than metal-reinforced glass ionomer cement, conven- tional glass ionomer cements, and zinc phos- phate cement without and with the addition of silver amalgam alloy powder. 3. The compressive strength of Chelon Silver was significantly lower than the chemically identical capsule product Ketac Silver at the 3 times of measurement. Acknowledgment The authors would like to thank the Institute for Medical Information Processing, Biometry, and Epidemiology (di- rector: Professor K. U¨berla) of the University of Munich for its support in performing the statistical analysis of the data. References 1. Paul SJ, Scha¨rer P: Plastische Aufbauten in der Kronen- und Bru¨ckenprothetik. [Plastic build-ups in crown and bridge prosthodontics.] Quintessenz 1996;47:1519-1531 2. Hickel R, Kunzelmann K-H: Glasionomer- und Kompomer- fu¨llung [Glass ionomer and compomer restorations], in Hei- 146 Laboratory Strength of Cements ● Piwowarczyk, Ottl, and Lauer
  • 8. demann D (ed): Kariologie und Fu¨llungstherapie. Praxis der Zahnheilkunde (ed 4). Mu¨nchen, Germany, Urban & Schwarzenberg, 1999, pp 154-176 3. Hickel R: Moderne Fu¨llungswerkstoffe. [Modern restorative materials.] Dtsch Zahna¨rztl Z 1997;52:572-585 4. Lin A, McIntyre NS, Davidson RD: Studies on the adhesion of glass-ionomer cements to dentin. J Dent Res 1992;71: 1836-1841 5. Powis DR, Follerås T, Merson SA, et al: Improved adhesion of a glass ionomer cement to dentin and enamel. J Dent Res 1982;61:1416-1422 6. McLean JW, Gasser O: Glass-cermet cements. Quintes- sence Int 1985;16:333-343 7. Wilson AD, McLean JW: Glass-ionomer cement. Chicago, IL, Quintessence Publishing, 1988 8. McKinney JE, Antonucci JM, Rupp NW: Wear and micro- hardness of a silver-sintered glass-ionomer cement. J Dent Res 1988;67:831-835 9. Moore BK, Swartz ML, Phillips RW: Abrasion resistance of metal reinforced glass ionomer cements. J Dent Res 1985; 64:371 (abstr 1766) 10. Chung KH: The properties of metal-reinforced glass iono- mer materials. J Oral Rehabil 1993;20:79-87 11. British Standards Institution: Specification for dental glass ionomer cements. BS 6039, 1981 12. DIN Deutsches Institut fu¨r Normung e.V, DIN-Taschen- buch 267: Zahnheilkunde-Werkstoffe: Normen, Gesetze, Richtlinien. [Dental Materials: Standards, Laws, Direc- tives.] Berlin, Germany, Beuth, 1997, pp 105-116 13. Kantor ME, Pines MS: A comparative study of restorative techniques for pulpless teeth. J Prosthet Dent 1977;38:405- 412 14. Mc Donald AV, King PA, Setchell DJ: An in vitro study to compare impact fracture resistance of intact root-treated teeth. Int Endod J 1990;23:304-312 15. Trope M, Maltz DO, Tronstad L: Resistance to fracture of restored endodontically treated teeth. Endod Dent Trauma- tol 1985;1:108-111 16. Smith DC: Composition and characteristics of glass ionomer cements. J Am Dent Assoc 1990;120:20-22 17. Nicholson JW: Chemistry of glass-ionomer cements: A re- view. Biomaterials 1998;19:485-494 18. Kullmann W: Glasionomer-Zemente—Physikalisch-tech- nische Eigenschaften in Abha¨ngigkeit von der Verarbeitung. [Glass ionomer cements—Physical-technical characteristics as a function of the processing.] Dtsch Zahna¨rztl Z 1986;41: 751-754 19. Mount GJ, Makinson OF: Glass-ionomer restorative ce- ments: Clinical implications of the setting reaction. Oper Dent 1982;7:134-141 20. McLean JW: Cermet cements. J Am Dent Assoc 1990;120: 43-47 21. Walls AWG, Adamson J, McCabe JF, et al: The properties of a glass polyalkenoate (ionomer) cement incorporating sin- tered metallic particles. Dent Mater 1987;3:113-116 22. Cho GC, Kaneko LM, Donovan TE, et al: Diametral and compressive strength of dental core materials. J Prosthet Dent 1999;82:272-276 23. Williams JA, Billington RW, Pearson GJ: The compar- ative strengths of commercial glass-ionomer cements with and without metal additions. Br Dent J 1992;172: 279-282 24. Peutzfeldt A: Compomers and glass ionomers: Bond strength to dentin and mechanical properties. Am J Dent 1996;9:259-263 25. Nakajima H, Watkins JH, Arita K, et al: Mechanical prop- erties of glass ionomers under static and dynamic loading. Dent Mater 1996;12:30-37 26. Cohen BI, Deutsch AS, Condos S, et al: Compressive and diametral tensile strength of titanium-reinforced compos- ites. J Esthet Dent 1992;4:50-55 27. Bruce WL, Stevens L: Strength properties of three zinc phosphate cements mixed to two different consistencies. Aust Dent J 1989;34:132-135 28. Abraham B: Untersuchung von vier Zinkphosphatzementen auf Druckfestigkeit und Lo¨slichkeit nach der FDI-Spezifika- tion Nr. 6 bei verschiedenem Pulver-Flu¨ssigkeits-Verha¨ltnis. [Study of four zinc phosphate cements for compression strength and solubility according to the FDI specification No. 6 with different powder-liquid ratio.] Thesis, Berlin, 1973 29. Smith DC: Dental cements. Current status and future pros- pects. Dent Clin North Am 1983;27:763-792 30. McCabe JF, Watts DC, Wilson HJ, et al: An investigation of test house variability in the mechanical testing of material and statistical treatment of results. J Dent Res 1990;18: 90-97 31. Hume WR, Mount GJ: In vitro studies on the potential for pulpal cytotoxicity of glass-ionomer cements. J Dent Res 1988;67:915-918 32. Schmalz G, Thonemann B, Riedel M, et al: Biological and clinical investigations of a glass ionomer base material. Dent Mater 1994;10:304-313 33. Schmalz G: The biocompatibility of non-amalgam dental filling materials. Eur J Oral Sci 1998;106:696-706 34. Geurtsen W: Substances released from dental resin compos- ites and glass ionomer cements. Eur J Oral Sci 1998;106: 687-695 35. Arenholt-Bindslev D: Composites and Compomers. 1st In- ternational ESPE Dental Symposium, Mu¨nchen, Germany, September 28, 1998 36. Arenholt-Bindslev D: Environmental aspects of dental filling materials. Eur J Oral Sci 1998;106:713-720 37. Cooley RL, Robbins JW, Barnwell S: Dimensional stability of glass ionomer used as a core material. J Prosthet Dent 1990;64:651-653 38. Small IC, Watson TF, Chadwick AV, et al: Water sorption in resin-modified glass-ionomer cements: An in vitro comparison with other materials. Biomaterials 1998;19:545-550 39. Cattani-Lorente MA, Dupuis V, Moya F, et al: Comparative study of the physical properties of a polyacid-modified com- posite resin and a resin-modified glass ionomer cement. Dent Mater 1999;15:21-32 147September 2001, Volume 10, Number 3