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The effect of investment material type on the contamination zone and mechanical properties of commercially pure titanium castings
The effect of investment material type on the contamination zone and mechanical properties of commercially pure titanium castings Dimitris Eliopoulos, DDS, MSc,a Spiros Zinelis, PhD,b and Triantafillos Papadopoulos, PhDc Dental School, University of Athens, Athens, Greece Statement of problem. Different types of investment materials affect the formation of a surface contamination zone within commercially pure titanium (cpTi) castings. This contamination zone may possibly alter the mechanical properties of cast titanium, which may be problematic for castings used in the fabrication of removable and fixed prostheses. Purpose. The purpose of this study was to evaluate the effect of different types of investments on the extent of contamination zone and the modulus of elasticity, yield strength, elongation, and hardness of cpTi castings. Material and methods. Forty wax patterns were fabricated according to ISO 9693 for tensile testing. The patterns were divided into 2 groups of 20 patterns each, invested, and cast in pairs using cpTi. The first group (P) was invested with a phosphate-bonded silica-based investment material (Ticoat S1L), and the second group (M), with a magnesia-alumina investment material (Rematitan Ultra). Investment materials were examined by x-ray diffraction analysis (XRD). One specimen from each group was sectioned and prepared for metallographic observation. The extent of the contamination zone was determined by scanning electron microscopy, using back-scattering electron imaging and energy dispersive spectroscopy analysis, as well as microhardness testing. The tensile strength of the specimens was determined in a universal testing machine. From the derived tensile curves, the modulus of elasticity, yield strength, and percentage elongation were calculated and statistically evaluated among the groups using the Student t test (a=.05). Three fractured specimens from each group were examined by scanning electron microscopy to determine the mode of fracture. Results. XRD analysis showed that silica and magnesia were the dominant phases of Ticoat S1L and Rematitan Ultra, respectively. The contamination zone was found to extend 50 to 80 mm for the P specimens and 15 to 20 mm for the M specimens. No significance difference was found for the modulus of elasticity (P=85 6 11 GPa, M=79 6 13 GPa), whereas significant differences were found for the yield strength (P=462 6 48 MPa, M=321 6 54 MPa; P,.001) and percentage elongation (P=12 6 2, M=21 6 7; P=.002) between the groups tested. The fracture mode was brittle externally and ductile internally for both groups. Conclusions. According to the results of this study, the extent of the contamination zone as well as the yield strength and percentage elongation of the cpTi castings were significantly affected by the type of the investment material. (J Prosthet Dent 2005;94:539-48.)
CLINICAL IMPLICATIONS The use of a magnesia-based investment material reduces the extent of surface contamination and produces significantly less cpTi casting brittleness.
T
itanium (Ti) and its alloys are used in dentistry for the fabrication of fixed or removable partial dentures (FPDs or RPDs) due to their excellent biocompatibility, high corrosion resistance, low weight, and mechanical properties similar to Type IV dental gold alloys.1,2 However, the mechanical properties of titanium castings may be affected by the casting procedures.3,4 Titanium has an extremely high melting point and an inherent reactivity with the elements of atmospheric air (oxygen and nitrogen) and the components of several investment materials, especially at high temperatures.5,6 The
a
Postgraduate student, Department of Biomaterials. Lecturer, Metallurgical Engineer, Department of Biomaterials. c Associate Professor, Department of Biomaterials. b
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reactions between molten metal and some of the elements of the phosphate-bonded and other silica-based investment materials (oxygen, phosphorus, and silicon) result in the development of a surface contamination zone referred to as the ‘‘a-case,’’ consisting of compounds of titanium, primarily with silicone (Si), phosphorus (P), and oxygen (O).7,8 This zone is porous and brittle6 and is considered unfavorable because of reduced ductility, fatigue, and corrosion resistance.9,10 The microhardness of the a-case is up to 3 times greater than that of bulk titanium.9 It is also reported that this zone may have a surface depth of up to 200 mm and result in low bond strength between cast titanium and dental porcelain.11-13 The contamination zone cannot be completely removed from dental castings due to dimensional THE JOURNAL OF PROSTHETIC DENTISTRY 539
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Fig. 1. Yield strength of dental casting alloys.16-18
constraints, and, therefore, part of this zone usually remains, compromising the mechanical properties of the casting and the durability of the metal-ceramic bonding.12 To minimize the extent of the a-case, new investment materials consisting of oxides with a higher chemical affinity to oxygen than titanium, such as MgO and Al2O3, were introduced. However, a thin contamination zone of approximately 20 to 40 mm1,14,15 forms on Ti surfaces even with these newly developed magnesia investments. It is possible that this hard and brittle reaction layer may have an effect on the mechanical properties of the overall dental casting.12 The modulus of elasticity, yield strength, tensile strength, percentage elongation, and microhardness16,17 are among the most important mechanical properties of dental castings. The modulus of elasticity (E) is a measure of the stiffness of a material, demonstrating also the material’s resistance during elastic deformation.16 Dental alloys with higher E allow for a decrease in cross-sectional thickness of dental castings, and are preferable for the fabrication of RPD frameworks, whereas alloys with a lower E produce RPD clasps that can be easily deformed. The E is reported to be approximately 190 to 220 GPa for Co-Cr alloys, 180 to 200 GPa for Ni-Cr, and 85 to 105 GPa for cpTi.16,17 The low E for cpTi can be compensated for by increasing the thickness of the prosthesis. The yield strength refers to the point at which the material begins to undergo permanent deformation.16 If occlusal forces exceed that amount, the metal substrate will deform and the prosthesis may fail. Yield strength values are estimated to be approximately 500 to 700 MPa for base metal alloys and 320 to 415 MPa for cpTi17,18 (Fig. 1). The acceptable yield strength threshold value according to ISO standards 969319 and 687120,21 for alloys used for the construction of RPDs and metal-ceramic 540
FPDs is 500 MPa and 250 MPa, respectively. Thus, cpTi does not meet the minimal acceptable values for yield limit when used for RPDs. For this reason, if Ti is used for RPD frameworks, it is preferable to use a Ti alloy, such as Ti6Al4V or Ti6Al7Nb, with yield limits over the minimal acceptable values.22 Tensile strength indicates the ability of a metal structure to endure the applied forces.16 With RPDs, for example, tensile strength is an indication of the clasp durability to withstand the loads during prosthesis placement and removal. Base metal alloys have tensile strength values of approximately 640 to 820 MPa, whereas cpTi exhibits values of about 370 to 550 MPa.16,18 The ductility and brittleness of alloys are relevant to the percentage elongation. The percentage elongation is up to 1.5% to 2% for base metal alloys and 13% to 20% for cpTi.17,18 Specific elongation values and adequate yield strength values are both necessary for alloys used for FPDs and RPDs. The final grinding and polishing of the metal substrate are directly related to the microhardness of the material. The higher the microhardness values, the more difficult the grinding and polishing procedures are to accomplish. As previously mentioned, mechanical properties must be precisely controlled in the production of clinically adequate metal castings. The aim of this study was to evaluate the effect of the type of the investment material on the thickness of the contamination zone and, consequently, the mechanical properties, including E, yield strength, elongation, and hardness of cpTi castings.
MATERIAL AND METHODS Powder constituents of a phosphate-bonded silicabased investment material (Ticoat S1L; Manfredi, VOLUME 94 NUMBER 6
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Fig. 2. XRD spectrum of Ticoat S1L investment material.
Fig. 3. XRD spectrum of Rematitan Ultra investment material.
Torino, Italy) and a magnesia-alumina investment material (Rematitan Ultra; Dentaurum, Ispringen, Germany) were analyzed using x-ray diffraction (XRD) (D-500; Siemens, Stuttgart, Germany) for phase determination. Forty dumbbell-shaped (total length 42 mm, gauge length 18 mm, and 3 mm in diameter) wax patterns, divided into 2 groups of 20 patterns each, were invested and cast in pairs according to ISO 9693 for tensile testing.19 Commercially pure titanium (cpTi) grade II (Morita Mfg Corp, Kyoto, Japan) was used for the production of the castings. The first group of specimens (P) were invested with the phosphate-bonded investment DECEMBER 2005
material, whereas the specimens of the second group (M) were invested with the magnesia-alumina investment. All casting procedures followed manufacturers’ instructions. Group P specimens were cast in an induction melting/centrifugal-casting machine (EasyTi; Manfredi) with 5.5 atm of argon pressure, whereas group M specimens were cast in an arc-melting/gas pressure machine (Dor-A-Matic; Schutz, Rosbach, Germany) with 3.5 to 4 atm of argon pressure. The mold temperatures of the 2 groups followed manufacturer’s recommendations. Casting of group P specimens was performed at 250°C, and of group M, at 650°C. 541
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Fig. 4. BEI micrograph (original magnification 3800) of external area of specimen invested with phosphate-bonded investment (Ticoat S1L). Darker area (white arrow) consistently appears approximately 50 mm from surface within matrix of large lighter areas (patterned arrow), indicating that contamination zone consists of elements with lower atomic weight than Ti. Black arrows point to acicular oxygen-rich structures.
Investment was removed from the specimen surfaces using airborne-particle abrasion (110-mm Al2O3 and 3-bar pressure). All specimens were examined using a dental radiographic unit (Gendex 756 DC; Dentsply, Milano, Italy) for the evaluation of internal porosity. One specimen from each group was cross-sectioned and prepared for metallographic observation and microhardness testing. These specimens were embedded in epoxy resin (Durofix-2; Struers, Copenhagen, Denmark) and ground to a smooth surface using SiC paper (Struers) up to 320 grit. Final polishing was completed using a 9-mm diamond paste (DP paste; Struers) and a solution of 70-mL colloidal silica (OP-S suspension solution; Struers) and 30 mL H2O2. Specimens were ultrasonically cleaned (Ultramatic 150; Gunter Jaschke, Freiburg, Germany) in a distilled water bath for 5 minutes and acid-etched with a solution of 3 parts HNO3 and 1 part HF. The cross-sections of the embedded specimens were carbon coated and observed using a scanning electron microscope (SEM) (Quanta 200; FEI, Hillsboro, Ore) with secondary electron imaging (SEI), back-scattered electron imaging (BEI), and spot analysis for elemental determination. Spectra were obtained using an Si (Li) energy-dispersive spectroscopic (EDS) detector (Sapphire; EDAX, Mahwah, NJ) with a super-ultrathin window (Be), under 25-kV accelerating voltage, 100mA beam current, 100-second acquisition time, and 542
30% to 40% dead time. Quantitative analysis of the percent weight concentration of the probed elements was performed by nonstandard analysis and atomic number, absorbance, fluorescence (ZAF) correction methods. The Vickers microhardness test was performed at 10 mm, 30 mm, 50 mm, 100 mm, 150 mm, and 200 mm from the outermost surface, applying a 100-g load for 15 seconds (HMV-200; Shimadzu, Kyoto, Japan). This procedure was repeated 8 times for each specimen. Castings were subjected to tensile testing according to the relevant ISOs.19-21,23 The specimens were loaded to fracture in a universal testing machine (Tensometer 10; Monsanto, Akron, Ohio) operating at a crosshead speed of 1.5 mm/min. The E, yield strength, and percentage elongation for each specimen were calculated from the derived tensile curves. Differences in E, yield strength, and percentage elongation between the 2 groups were statistically evaluated by the Student t test (a=.05). Three fractured specimens from each group were investigated under SEM to characterize the fracture mode.
RESULTS Diagrams from the XRD analysis for the 2 investment materials are shown in Figures 2 and 3. Large amounts of SiO2 were detected in Ticoat S1L, whereas large VOLUME 94 NUMBER 6
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Fig. 5. Representative EDS spectra from spot analysis performed 5 mm from surface of phosphate-bonded (Ticoat S1L) invested specimen at lighter (A) and darker (B) areas of Figure 4 (white arrows).
amounts of MgO with Al2O3 were detected in Rematitan Ultra. Radiographic evaluation revealed severe porosity in 2 of the cast specimens of group M, which were discarded. In group P, BEI revealed a contamination zone of approximately 50 to 80 mm in depth from the outermost surface of the specimen (Fig. 4). EDS spot analysis at a distance 5 mm from the surface showed darker areas consisting of titanium compounds with Si, P, Al, Ca, and O, and lighter areas with traces of Si, Al, and O (Fig. 5). In group M specimens, BEI revealed that the length of the a-case was not greater than 15 to 20 mm. Spot analysis traced Al, Ca, Si, and O at 3-mm length (Fig. 6, A), but none of the above elements were found to extend beyond 6 mm from the outermost surface (Fig. 6, B). The results of the microhardness test (Fig. 7) showed (for both groups) the presence of an external, harder zone, whereas a more abrupt decrease of hardness was found for group M specimens compared to group P. DECEMBER 2005
Representative tensile curves for both groups are presented in Figure 8. The values of E, yield strength, and percentage elongation of the 2 groups are presented in Table I. The statistical evaluation of the tensile fracture values indicated that there were no significant differences between the E of the 2 groups tested (P=.28), whereas significant differences were found for the yield strength (P,.001) and percentage elongation (P=.002). The mean yield strength of the specimens of group P was higher than that of group M specimens, whereas the magnesia group exhibited higher percentage elongation values. The examination of the fracture surfaces indicated a common fracture mode for both groups. Topographically, the fracture mode was brittle at the periphery and ductile in the bulk of the material. For the specimens of group P, brittle fracture was detected in greater depths (150 to 180 mm) than in specimens of group M (30 to 50 mm) (Fig. 9). 543
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Fig. 6. Representative EDS spectra of specimen surfaces cast in magnesia-invested material (Rematitan Ultra) at 3 mm (A) and 6 mm (B) from outermost surface.
Fig. 7. Microhardness diagrams (VHN), in correlation with distance from outermost surface for both groups tested.
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Fig. 8. Representative tensile test curves for phosphate-bonded and magnesia-invested specimens.
Fig. 9. SEI micrograph showing brittle fracture (acicular grain structures are indicated by horizontal arrows) at external area (vertical arrow) of specimen of group P (original magnification 3260).
Table I. Mean, SD, and P values for comparison of modulus of elasticity, yield strength, and percentage elongation for groups P and M
Modulus of elasticity (GPa) Yield strength (MPa) Elongation (%)
Group P
Group M
P
85 6 11 462 6 48 12 6 21
79 6 13 321 6 54 21 6 71
.28 ,.001* .002*
*
Significant difference at P,.05.
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Table II. Chemical composition, yield limit, and percentage elongation as described by ISO standards, with results of present study for comparison purposes23,24 Yield limit (MPa) Alloy
High gold (I)* High gold (I)* High gold (III)* High gold (IV)* Medium gold (I)* Medium gold (II)* Medium gold (III)* Medium gold (IV)* Base alloys (RPDs) Base alloys metal-ceramic (FPDs) Group My Group Py
Chemical composition
Au (.65%) Pt Pd Ir Ru Au (.65%) Pt Pd Ir Ru Au (.65%) Pt Pd Ir Ru Au (.65%) Pt Pd Ir Ru Au (.25%) Pt Pd Ir Ru Au (.25%) Pt Pd Ir Ru Au (.25%) Pt Pd Ir Ru Au (.25%) Pt Pd Ir Ru Ni 1 Co 1 Cr .85% Ni 1 Co 1 Cr .85% cpTi Grade II cpTi Grade II
Rh Rh Rh Rh Rh Rh Rh Rh
(.75%) (.75%) (.75%) (.75%) (,75%) (,75%) (,75%) (,75%)
Percentage elongation (%)
ISO standard no.
Minimum
Maximum
Minimum
1562 1562 1562 1562 8891 8891 8891 8891 6871 9693
80 180 240 300 80 180 240 300 .500 .250 321 462
180 240
18 12 12 10 18 12 12 10 .3 .2 21 12
180 240
*
Values reported after softening thermal treatment procedures. Mean values.
y
DISCUSSION In this study, the high reactivity of Ti with the phosphate-bonded silica-based investment material was manipulated by using either a new, silica-free, investment material or by lowering the investment temperature during the time when molten Ti met the walls of the mold. In previous studies, it was reported that casting Ti into phosphate-bonded investments resulted in a surface contamination layer extending up to 200 mm from the surface of the casting.5,6 From the results of the present study, it is evident that the lower mold temperature (250°C) minimized the contamination zone (up to 80 mm), whereas the use of a silica-free investment material (Rematitan Ultra) confined its extension to 20 mm. In a similar study,12 the extent of the reaction zone was also examined. It was reported that this zone was greater than 50 mm for the castings invested directly in a phosphate-bonded investment and less than 35 mm when an intermediate zirconite coating was used. The microhardness test was used to verify the results from the SEM observation. Indeed, for group P, hardness was gradually reduced from the outermost surface towards the bulk of the specimen, stabilizing at about 90 mm from the outermost surface of the casting. The same situation exists with the microhardness test for group M specimens, whereas the contamination zone was stabilized at 25 mm from the surface of the casting. The difference between the microhardness curves and the contamination zone in SEM images for both groups indicates a microstructural change some microns deeper than the deepest limits of the dark zone. The brittle type of fracture at the periphery of the specimens in both groups is explained by the hard, porous structure of the contamination zone. In contrast,
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the noncontaminated internal Ti areas fractured in a ductile mode. Brittle fracture occurred 150 to 180 mm from the surface for group P specimens and 30 to 50 mm for group M. For group P, the extent of the brittle fracture was longer than the extent of the detected contamination zone. It is possible that brittle fracture starts at the periphery of the contamination zone and continues into the acicular grains toward the bulk of the material. Watanabe et al4 reported the same tensile fracture mode for cpTi castings invested with a phosphate-bonded investment material under different pressures during casting: brittle in the outer region and ductile in the interior. Elongated axial dimples are described by other investigators5 to occur in the area of the ductile mode of fracture, in the interior region of the specimens—an observation that was also detected in both groups in the present study (Fig.9). The mechanical values such as yield strength and percentage elongation are also in agreement with the results found in a previous study.4 The results of the present study clearly demonstrate the effect of the investment type in the thickness of the contamination zone in Ti castings. Phosphatebonded investment material produced contamination approximately 4 times thicker than the magnesia-based investment material. The correlation of the thickness of the contamination zone with the mechanical properties of the castings showed that yield strength was significantly higher and percentage elongation was significantly lower with the phosphate-bonded investment material. Results were probably affected by the extent of the hard and brittle a-case, which participates in a higher proportion in the quantitative formation of the final mechanical values for group P specimens, relative to group M. The E demonstrated no significant VOLUME 94 NUMBER 6
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differences among the groups. An explanation for this may be that E is in direct correlation with interatomic structural forces of the overall cross-sectional area of the specimens and was not considerably affected by the existence of the contamination zone. According to ISO standard 9693,19 both experimental groups are within the recommended yield strength (.250 MPa) and percentage elongation (.3%) demands for FPD metal-ceramic prostheses, as demonstrated in Table II. Although, there is currently no available ISO referring to mechanical requirements of Ti castings for the production of RPDs, both groups demonstrated inferior yield strength compared to ISO specifications (.500MPa) for the production of RPDs with Ni-Cr and Co-Cr base casting alloys.20,21 However both groups demonstrated substantial percentage elongation after fracture (.3%).20,21 The higher extent of the contamination layer makes its removal difficult and time-consuming, and may compromise the accuracy of the prosthesis. Relative to this, cpTi cast in magnesia investment materials showed a minimized contamination layer that can be more easily removed. Based on ISO requirements,19-21 cpTi cast in magnesia investment materials has adequate yield strength and percentage elongation values, suitable for metal-ceramic FPD restorations. In all situations it is apparent that the contamination layer should be completely removed from the surface of the castings, and from this standpoint, magnesia-based investments provide a more reliable choice for the production of Ti casting.
CONCLUSIONS This study demonstrated that the type of investment affects the extent of the contamination zone and, consequently, the mechanical properties of the cast cpTi. The following conclusions were drawn: 1. The extent of the contamination zone was almost 4 times greater when phosphate-bonded investment was used, when compared with magnesia-based investment material. 2. The mechanical properties of cpTi castings in either phosphate-bonded silica-based or magnesia-based investment materials do not fulfil the ISO requirements for FPD fabrication. 3. Specimens produced with the phosphate-bonded investment showed a more brittle behaviour, derived from the correlation of the values of yield strength and the percentage of elongation in combination with the fracture mode.
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REFERENCES 1. Titanium applications in dentistry. J Am Dent Assoc 2003;134:347-9. 2. King AW, Chai J, Lautenschlager E, Gilbert J. The mechanical properties of milled and cast titanium for ceramic veneering. Int J Prostodont 1994;7: 532-7. 3. Zinelis S. Effect of pressure of helium, argon, kyrpton and xenon on the porosity, microstructure and mechanical properties of commercially pure titanium castings. J Prosthet Dent 2000;84:575-82. 4. Watanabe I, Watkins J, Nakajima H, Atsuta M, Okabe T. Effect of pressure difference on the quality of titanium casting. J Dent Res 1997; 76:773-9. 5. Papadopoulos Tr, Zinelis S, Vardavoulias M. A metallurgical study of the contamination zone at the surface of dental Ti castings, due to the phosphate-bonded investment material: the protection efficacy of a ceramic coating. J Mater Sci 1999;34:3639-46. 6. Koike M, Cai Z, Fujii H, Brezner M, Okabe T. Corrosion behavior of cast titanium with reduced surface reaction layer made by a face-coating method. Biomaterials 2003;24:4541-9. 7. Takahashi J, Kimura H, Lautenschlager E, Chern Lin JH, Moser JB, Greener EH. Casting pure titanium into commercial phosphate-bonded SiO2 investment molds. J Dent Res 1990;69:1800-5. 8. Miyakawa O, Watanabe K, Okawa S, Nakano S, Kobayashi M, Shiokawa N. Layered structure of cast titanium surface. Dent Mater J 1989;8: 175-85. 9. Cai Z, Nakajima H, Woldu M, Berglund A, Bergman M, Okabe T. In vitro corrosion resistance of titanium made using different fabrication methods. Biomaterials 1999;20:183-90. 10. Cai Z, Bunce N, Nunn ME, Okabe T. Porcelain adherence to dental cast CP titanium: effects of surface modifications. Biomaterials 2001;22: 979-86. 11. Bourcier R, Koss D, Smelser R, Richmond O. The influence of porosity on the deformation and fracture of alloys. Acta Metallurgica 1986;34: 2443-53. 12. Luo XP, Guo TW, Ou YG, Liu Q. Titanium casting into phosphate bonded investment with zirconite. Dent Mater 2002;18:512-5. 13. Ferenczi AM, Demri B, Moritz M, Muster D. Casted titanium for dental applications: an XPS and SEM study. Biomaterials 1998;19: 1513-5. 14. Atwood RC, Lee PD, Curtis RV. Modeling the surface contamination of dental titanium castings. Dent Mater 2005;21:178-86. 15. Syverud M, Okabe T, Hero H. Casting of Ti-6Al-4V alloy compared with pure Ti in an Ar-arc casting machine. Eur J Oral Sci 1995;103: 327-30. 16. Craig R, Powers J. Restorative dental materials. 11th ed. St. Louis: Elsevier; 2001. p. 479-514. 17. Anusavice KJ, Phillips RW. Phillips’ science of dental materials. 11th ed. St. Louis: Elsevier; 2003. p. 73-102. 18. Van Noort R. Introduction to dental materials. 2nd ed. St Louis: Mosby; 2002. p. 227-30. 19. International Organization for Standardization. Dental materials— Metal-ceramic dental restorative systems. ISO 9693:1999. Available at: http://www.iso.ch/iso/en/prods-services/ISOstore/store.html. Accessed July 28, 2005. 20. International Organization for Standardization. Dental materials—Dental base metal casting alloys. Part 1: cobalt-based alloys. ISO 6871-1: 1994. Available at: http://www.iso.ch/iso/en/prods-services/ISOstore/store.html. Accessed July 28, 2005. 21. International Organization for Standardization. Dental materials—Dental base metal casting alloys. Part 2: nickel-based alloys. ISO 6871-2:1994. Available at: http://www.iso.ch/iso/en/prods-services/ISOstore/store.html. Accessed July 28, 2005. 22. Boyer R, Lampman S. Titanium alloys. Materials Park (OH): ASM Intl; 1994. p. 517-27, 693. 23. International Organization for Standardization. Dental materials— Dental casting alloys with noble metal content of at least 25% but less than 75%. ISO 8891:1998. Available at: http://www.iso.ch/iso/en/ prods-services/ISOstore/store.html. Accessed July 28, 2005. 24. International Organization for Standardization. Dental materials— Dental casting gold alloys. ISO 1562:1993. Available at: http:// www.iso.ch/iso/en/prods-services/ISOstore/store.html. Accessed July 28, 2005.
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Reprint requests to: DR TRIANTAFILLOS PAPADOPOULOS DEPARTMENT OF BIOMATERIALS, DENTAL SCHOOL UNIVERSITY OF ATHENS THIVON 2, 115 27 ATHENS GREECE FAX: +3102107240633 E-MAIL: [email protected]
Noteworthy Abstracts of the Current Literature
ELIOPOULOS, ZINELIS, AND PAPADOPOULOS
0022-3913/$30.00 Copyright Ó 2005 by The Editorial Council of The Journal of Prosthetic Dentistry.
doi:10.1016/j.prosdent.2005.09.017
Microleakage of various cementing agents for full cast crowns Piwowarczyk A, Lauer H-C, Sorensen JA. Dent Mater 2005; 21:445-53.
Objectives: To evaluate microleakage and marginal gaps in full cast crown restorations bonded with six different types of cementing agents. Methods: Sixty non-carious human premolars and molars were prepared in a standardized manner for full cast crown restorations. The mesial and distal margins were located in dentin, while the vestibular and palatal/ lingual margins were located in enamel. Crowns were made from a high-gold alloy using a standardized technique. The specimens were randomized to six groups of cementing agents: one zinc-phosphate cement (Harvard cement), one conventional glass–ionomer cement (Fuji I), one resin-modified glass–ionomer cement (Fuji Plus), two standard resin cements (RelyX ARC, Panavia F), and one self-adhesive universal resin cement (RelyX Unicem). After 4 weeks of storage in distilled water at 37°C, the specimens were subjected to 5000 thermocycles ranging from 5 to 55°C. Then, they were placed in a silver nitrate solution, embedded in resin blocks, and vertically cut in buccolingual and mesiodistal direction. Subsequently, the objects were evaluated for microleakage and marginal gap using a high-resolution digital microscope camera. Results: A number of inter-group differences were statistically significant. RelyX Unicem showed the smallest degree of microleakage both in enamel and in dentin. Panavia F und RelyX Unicem were associated with significantly larger marginal gaps than all other cementing agents. No association was observed between microleakage and marginal gap other than a weak direct correlation when using Harvard cement on enamel. Significance: The cementing agents investigated revealed different sealing abilities. These differences were not associated with specific types of materials.—Reprinted with permission of The Academy of Dental Materials.
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CLINICAL IMPLICATIONS The use of a magnesia-based investment material reduces the extent of surface contamination and produces significantly less cpTi casting brittleness.
T
itanium (Ti) and its alloys are used in dentistry for the fabrication of fixed or removable partial dentures (FPDs or RPDs) due to their excellent biocompatibility, high corrosion resistance, low weight, and mechanical properties similar to Type IV dental gold alloys.1,2 However, the mechanical properties of titanium castings may be affected by the casting procedures.3,4 Titanium has an extremely high melting point and an inherent reactivity with the elements of atmospheric air (oxygen and nitrogen) and the components of several investment materials, especially at high temperatures.5,6 The
a
Postgraduate student, Department of Biomaterials. Lecturer, Metallurgical Engineer, Department of Biomaterials. c Associate Professor, Department of Biomaterials. b
DECEMBER 2005
reactions between molten metal and some of the elements of the phosphate-bonded and other silica-based investment materials (oxygen, phosphorus, and silicon) result in the development of a surface contamination zone referred to as the ‘‘a-case,’’ consisting of compounds of titanium, primarily with silicone (Si), phosphorus (P), and oxygen (O).7,8 This zone is porous and brittle6 and is considered unfavorable because of reduced ductility, fatigue, and corrosion resistance.9,10 The microhardness of the a-case is up to 3 times greater than that of bulk titanium.9 It is also reported that this zone may have a surface depth of up to 200 mm and result in low bond strength between cast titanium and dental porcelain.11-13 The contamination zone cannot be completely removed from dental castings due to dimensional THE JOURNAL OF PROSTHETIC DENTISTRY 539
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Fig. 1. Yield strength of dental casting alloys.16-18
constraints, and, therefore, part of this zone usually remains, compromising the mechanical properties of the casting and the durability of the metal-ceramic bonding.12 To minimize the extent of the a-case, new investment materials consisting of oxides with a higher chemical affinity to oxygen than titanium, such as MgO and Al2O3, were introduced. However, a thin contamination zone of approximately 20 to 40 mm1,14,15 forms on Ti surfaces even with these newly developed magnesia investments. It is possible that this hard and brittle reaction layer may have an effect on the mechanical properties of the overall dental casting.12 The modulus of elasticity, yield strength, tensile strength, percentage elongation, and microhardness16,17 are among the most important mechanical properties of dental castings. The modulus of elasticity (E) is a measure of the stiffness of a material, demonstrating also the material’s resistance during elastic deformation.16 Dental alloys with higher E allow for a decrease in cross-sectional thickness of dental castings, and are preferable for the fabrication of RPD frameworks, whereas alloys with a lower E produce RPD clasps that can be easily deformed. The E is reported to be approximately 190 to 220 GPa for Co-Cr alloys, 180 to 200 GPa for Ni-Cr, and 85 to 105 GPa for cpTi.16,17 The low E for cpTi can be compensated for by increasing the thickness of the prosthesis. The yield strength refers to the point at which the material begins to undergo permanent deformation.16 If occlusal forces exceed that amount, the metal substrate will deform and the prosthesis may fail. Yield strength values are estimated to be approximately 500 to 700 MPa for base metal alloys and 320 to 415 MPa for cpTi17,18 (Fig. 1). The acceptable yield strength threshold value according to ISO standards 969319 and 687120,21 for alloys used for the construction of RPDs and metal-ceramic 540
FPDs is 500 MPa and 250 MPa, respectively. Thus, cpTi does not meet the minimal acceptable values for yield limit when used for RPDs. For this reason, if Ti is used for RPD frameworks, it is preferable to use a Ti alloy, such as Ti6Al4V or Ti6Al7Nb, with yield limits over the minimal acceptable values.22 Tensile strength indicates the ability of a metal structure to endure the applied forces.16 With RPDs, for example, tensile strength is an indication of the clasp durability to withstand the loads during prosthesis placement and removal. Base metal alloys have tensile strength values of approximately 640 to 820 MPa, whereas cpTi exhibits values of about 370 to 550 MPa.16,18 The ductility and brittleness of alloys are relevant to the percentage elongation. The percentage elongation is up to 1.5% to 2% for base metal alloys and 13% to 20% for cpTi.17,18 Specific elongation values and adequate yield strength values are both necessary for alloys used for FPDs and RPDs. The final grinding and polishing of the metal substrate are directly related to the microhardness of the material. The higher the microhardness values, the more difficult the grinding and polishing procedures are to accomplish. As previously mentioned, mechanical properties must be precisely controlled in the production of clinically adequate metal castings. The aim of this study was to evaluate the effect of the type of the investment material on the thickness of the contamination zone and, consequently, the mechanical properties, including E, yield strength, elongation, and hardness of cpTi castings.
MATERIAL AND METHODS Powder constituents of a phosphate-bonded silicabased investment material (Ticoat S1L; Manfredi, VOLUME 94 NUMBER 6
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Fig. 2. XRD spectrum of Ticoat S1L investment material.
Fig. 3. XRD spectrum of Rematitan Ultra investment material.
Torino, Italy) and a magnesia-alumina investment material (Rematitan Ultra; Dentaurum, Ispringen, Germany) were analyzed using x-ray diffraction (XRD) (D-500; Siemens, Stuttgart, Germany) for phase determination. Forty dumbbell-shaped (total length 42 mm, gauge length 18 mm, and 3 mm in diameter) wax patterns, divided into 2 groups of 20 patterns each, were invested and cast in pairs according to ISO 9693 for tensile testing.19 Commercially pure titanium (cpTi) grade II (Morita Mfg Corp, Kyoto, Japan) was used for the production of the castings. The first group of specimens (P) were invested with the phosphate-bonded investment DECEMBER 2005
material, whereas the specimens of the second group (M) were invested with the magnesia-alumina investment. All casting procedures followed manufacturers’ instructions. Group P specimens were cast in an induction melting/centrifugal-casting machine (EasyTi; Manfredi) with 5.5 atm of argon pressure, whereas group M specimens were cast in an arc-melting/gas pressure machine (Dor-A-Matic; Schutz, Rosbach, Germany) with 3.5 to 4 atm of argon pressure. The mold temperatures of the 2 groups followed manufacturer’s recommendations. Casting of group P specimens was performed at 250°C, and of group M, at 650°C. 541
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Fig. 4. BEI micrograph (original magnification 3800) of external area of specimen invested with phosphate-bonded investment (Ticoat S1L). Darker area (white arrow) consistently appears approximately 50 mm from surface within matrix of large lighter areas (patterned arrow), indicating that contamination zone consists of elements with lower atomic weight than Ti. Black arrows point to acicular oxygen-rich structures.
Investment was removed from the specimen surfaces using airborne-particle abrasion (110-mm Al2O3 and 3-bar pressure). All specimens were examined using a dental radiographic unit (Gendex 756 DC; Dentsply, Milano, Italy) for the evaluation of internal porosity. One specimen from each group was cross-sectioned and prepared for metallographic observation and microhardness testing. These specimens were embedded in epoxy resin (Durofix-2; Struers, Copenhagen, Denmark) and ground to a smooth surface using SiC paper (Struers) up to 320 grit. Final polishing was completed using a 9-mm diamond paste (DP paste; Struers) and a solution of 70-mL colloidal silica (OP-S suspension solution; Struers) and 30 mL H2O2. Specimens were ultrasonically cleaned (Ultramatic 150; Gunter Jaschke, Freiburg, Germany) in a distilled water bath for 5 minutes and acid-etched with a solution of 3 parts HNO3 and 1 part HF. The cross-sections of the embedded specimens were carbon coated and observed using a scanning electron microscope (SEM) (Quanta 200; FEI, Hillsboro, Ore) with secondary electron imaging (SEI), back-scattered electron imaging (BEI), and spot analysis for elemental determination. Spectra were obtained using an Si (Li) energy-dispersive spectroscopic (EDS) detector (Sapphire; EDAX, Mahwah, NJ) with a super-ultrathin window (Be), under 25-kV accelerating voltage, 100mA beam current, 100-second acquisition time, and 542
30% to 40% dead time. Quantitative analysis of the percent weight concentration of the probed elements was performed by nonstandard analysis and atomic number, absorbance, fluorescence (ZAF) correction methods. The Vickers microhardness test was performed at 10 mm, 30 mm, 50 mm, 100 mm, 150 mm, and 200 mm from the outermost surface, applying a 100-g load for 15 seconds (HMV-200; Shimadzu, Kyoto, Japan). This procedure was repeated 8 times for each specimen. Castings were subjected to tensile testing according to the relevant ISOs.19-21,23 The specimens were loaded to fracture in a universal testing machine (Tensometer 10; Monsanto, Akron, Ohio) operating at a crosshead speed of 1.5 mm/min. The E, yield strength, and percentage elongation for each specimen were calculated from the derived tensile curves. Differences in E, yield strength, and percentage elongation between the 2 groups were statistically evaluated by the Student t test (a=.05). Three fractured specimens from each group were investigated under SEM to characterize the fracture mode.
RESULTS Diagrams from the XRD analysis for the 2 investment materials are shown in Figures 2 and 3. Large amounts of SiO2 were detected in Ticoat S1L, whereas large VOLUME 94 NUMBER 6
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Fig. 5. Representative EDS spectra from spot analysis performed 5 mm from surface of phosphate-bonded (Ticoat S1L) invested specimen at lighter (A) and darker (B) areas of Figure 4 (white arrows).
amounts of MgO with Al2O3 were detected in Rematitan Ultra. Radiographic evaluation revealed severe porosity in 2 of the cast specimens of group M, which were discarded. In group P, BEI revealed a contamination zone of approximately 50 to 80 mm in depth from the outermost surface of the specimen (Fig. 4). EDS spot analysis at a distance 5 mm from the surface showed darker areas consisting of titanium compounds with Si, P, Al, Ca, and O, and lighter areas with traces of Si, Al, and O (Fig. 5). In group M specimens, BEI revealed that the length of the a-case was not greater than 15 to 20 mm. Spot analysis traced Al, Ca, Si, and O at 3-mm length (Fig. 6, A), but none of the above elements were found to extend beyond 6 mm from the outermost surface (Fig. 6, B). The results of the microhardness test (Fig. 7) showed (for both groups) the presence of an external, harder zone, whereas a more abrupt decrease of hardness was found for group M specimens compared to group P. DECEMBER 2005
Representative tensile curves for both groups are presented in Figure 8. The values of E, yield strength, and percentage elongation of the 2 groups are presented in Table I. The statistical evaluation of the tensile fracture values indicated that there were no significant differences between the E of the 2 groups tested (P=.28), whereas significant differences were found for the yield strength (P,.001) and percentage elongation (P=.002). The mean yield strength of the specimens of group P was higher than that of group M specimens, whereas the magnesia group exhibited higher percentage elongation values. The examination of the fracture surfaces indicated a common fracture mode for both groups. Topographically, the fracture mode was brittle at the periphery and ductile in the bulk of the material. For the specimens of group P, brittle fracture was detected in greater depths (150 to 180 mm) than in specimens of group M (30 to 50 mm) (Fig. 9). 543
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Fig. 6. Representative EDS spectra of specimen surfaces cast in magnesia-invested material (Rematitan Ultra) at 3 mm (A) and 6 mm (B) from outermost surface.
Fig. 7. Microhardness diagrams (VHN), in correlation with distance from outermost surface for both groups tested.
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Fig. 8. Representative tensile test curves for phosphate-bonded and magnesia-invested specimens.
Fig. 9. SEI micrograph showing brittle fracture (acicular grain structures are indicated by horizontal arrows) at external area (vertical arrow) of specimen of group P (original magnification 3260).
Table I. Mean, SD, and P values for comparison of modulus of elasticity, yield strength, and percentage elongation for groups P and M
Modulus of elasticity (GPa) Yield strength (MPa) Elongation (%)
Group P
Group M
P
85 6 11 462 6 48 12 6 21
79 6 13 321 6 54 21 6 71
.28 ,.001* .002*
*
Significant difference at P,.05.
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Table II. Chemical composition, yield limit, and percentage elongation as described by ISO standards, with results of present study for comparison purposes23,24 Yield limit (MPa) Alloy
High gold (I)* High gold (I)* High gold (III)* High gold (IV)* Medium gold (I)* Medium gold (II)* Medium gold (III)* Medium gold (IV)* Base alloys (RPDs) Base alloys metal-ceramic (FPDs) Group My Group Py
Chemical composition
Au (.65%) Pt Pd Ir Ru Au (.65%) Pt Pd Ir Ru Au (.65%) Pt Pd Ir Ru Au (.65%) Pt Pd Ir Ru Au (.25%) Pt Pd Ir Ru Au (.25%) Pt Pd Ir Ru Au (.25%) Pt Pd Ir Ru Au (.25%) Pt Pd Ir Ru Ni 1 Co 1 Cr .85% Ni 1 Co 1 Cr .85% cpTi Grade II cpTi Grade II
Rh Rh Rh Rh Rh Rh Rh Rh
(.75%) (.75%) (.75%) (.75%) (,75%) (,75%) (,75%) (,75%)
Percentage elongation (%)
ISO standard no.
Minimum
Maximum
Minimum
1562 1562 1562 1562 8891 8891 8891 8891 6871 9693
80 180 240 300 80 180 240 300 .500 .250 321 462
180 240
18 12 12 10 18 12 12 10 .3 .2 21 12
180 240
*
Values reported after softening thermal treatment procedures. Mean values.
y
DISCUSSION In this study, the high reactivity of Ti with the phosphate-bonded silica-based investment material was manipulated by using either a new, silica-free, investment material or by lowering the investment temperature during the time when molten Ti met the walls of the mold. In previous studies, it was reported that casting Ti into phosphate-bonded investments resulted in a surface contamination layer extending up to 200 mm from the surface of the casting.5,6 From the results of the present study, it is evident that the lower mold temperature (250°C) minimized the contamination zone (up to 80 mm), whereas the use of a silica-free investment material (Rematitan Ultra) confined its extension to 20 mm. In a similar study,12 the extent of the reaction zone was also examined. It was reported that this zone was greater than 50 mm for the castings invested directly in a phosphate-bonded investment and less than 35 mm when an intermediate zirconite coating was used. The microhardness test was used to verify the results from the SEM observation. Indeed, for group P, hardness was gradually reduced from the outermost surface towards the bulk of the specimen, stabilizing at about 90 mm from the outermost surface of the casting. The same situation exists with the microhardness test for group M specimens, whereas the contamination zone was stabilized at 25 mm from the surface of the casting. The difference between the microhardness curves and the contamination zone in SEM images for both groups indicates a microstructural change some microns deeper than the deepest limits of the dark zone. The brittle type of fracture at the periphery of the specimens in both groups is explained by the hard, porous structure of the contamination zone. In contrast,
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the noncontaminated internal Ti areas fractured in a ductile mode. Brittle fracture occurred 150 to 180 mm from the surface for group P specimens and 30 to 50 mm for group M. For group P, the extent of the brittle fracture was longer than the extent of the detected contamination zone. It is possible that brittle fracture starts at the periphery of the contamination zone and continues into the acicular grains toward the bulk of the material. Watanabe et al4 reported the same tensile fracture mode for cpTi castings invested with a phosphate-bonded investment material under different pressures during casting: brittle in the outer region and ductile in the interior. Elongated axial dimples are described by other investigators5 to occur in the area of the ductile mode of fracture, in the interior region of the specimens—an observation that was also detected in both groups in the present study (Fig.9). The mechanical values such as yield strength and percentage elongation are also in agreement with the results found in a previous study.4 The results of the present study clearly demonstrate the effect of the investment type in the thickness of the contamination zone in Ti castings. Phosphatebonded investment material produced contamination approximately 4 times thicker than the magnesia-based investment material. The correlation of the thickness of the contamination zone with the mechanical properties of the castings showed that yield strength was significantly higher and percentage elongation was significantly lower with the phosphate-bonded investment material. Results were probably affected by the extent of the hard and brittle a-case, which participates in a higher proportion in the quantitative formation of the final mechanical values for group P specimens, relative to group M. The E demonstrated no significant VOLUME 94 NUMBER 6
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differences among the groups. An explanation for this may be that E is in direct correlation with interatomic structural forces of the overall cross-sectional area of the specimens and was not considerably affected by the existence of the contamination zone. According to ISO standard 9693,19 both experimental groups are within the recommended yield strength (.250 MPa) and percentage elongation (.3%) demands for FPD metal-ceramic prostheses, as demonstrated in Table II. Although, there is currently no available ISO referring to mechanical requirements of Ti castings for the production of RPDs, both groups demonstrated inferior yield strength compared to ISO specifications (.500MPa) for the production of RPDs with Ni-Cr and Co-Cr base casting alloys.20,21 However both groups demonstrated substantial percentage elongation after fracture (.3%).20,21 The higher extent of the contamination layer makes its removal difficult and time-consuming, and may compromise the accuracy of the prosthesis. Relative to this, cpTi cast in magnesia investment materials showed a minimized contamination layer that can be more easily removed. Based on ISO requirements,19-21 cpTi cast in magnesia investment materials has adequate yield strength and percentage elongation values, suitable for metal-ceramic FPD restorations. In all situations it is apparent that the contamination layer should be completely removed from the surface of the castings, and from this standpoint, magnesia-based investments provide a more reliable choice for the production of Ti casting.
CONCLUSIONS This study demonstrated that the type of investment affects the extent of the contamination zone and, consequently, the mechanical properties of the cast cpTi. The following conclusions were drawn: 1. The extent of the contamination zone was almost 4 times greater when phosphate-bonded investment was used, when compared with magnesia-based investment material. 2. The mechanical properties of cpTi castings in either phosphate-bonded silica-based or magnesia-based investment materials do not fulfil the ISO requirements for FPD fabrication. 3. Specimens produced with the phosphate-bonded investment showed a more brittle behaviour, derived from the correlation of the values of yield strength and the percentage of elongation in combination with the fracture mode.
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REFERENCES 1. Titanium applications in dentistry. J Am Dent Assoc 2003;134:347-9. 2. King AW, Chai J, Lautenschlager E, Gilbert J. The mechanical properties of milled and cast titanium for ceramic veneering. Int J Prostodont 1994;7: 532-7. 3. Zinelis S. Effect of pressure of helium, argon, kyrpton and xenon on the porosity, microstructure and mechanical properties of commercially pure titanium castings. J Prosthet Dent 2000;84:575-82. 4. Watanabe I, Watkins J, Nakajima H, Atsuta M, Okabe T. Effect of pressure difference on the quality of titanium casting. J Dent Res 1997; 76:773-9. 5. Papadopoulos Tr, Zinelis S, Vardavoulias M. A metallurgical study of the contamination zone at the surface of dental Ti castings, due to the phosphate-bonded investment material: the protection efficacy of a ceramic coating. J Mater Sci 1999;34:3639-46. 6. Koike M, Cai Z, Fujii H, Brezner M, Okabe T. Corrosion behavior of cast titanium with reduced surface reaction layer made by a face-coating method. Biomaterials 2003;24:4541-9. 7. Takahashi J, Kimura H, Lautenschlager E, Chern Lin JH, Moser JB, Greener EH. Casting pure titanium into commercial phosphate-bonded SiO2 investment molds. J Dent Res 1990;69:1800-5. 8. Miyakawa O, Watanabe K, Okawa S, Nakano S, Kobayashi M, Shiokawa N. Layered structure of cast titanium surface. Dent Mater J 1989;8: 175-85. 9. Cai Z, Nakajima H, Woldu M, Berglund A, Bergman M, Okabe T. In vitro corrosion resistance of titanium made using different fabrication methods. Biomaterials 1999;20:183-90. 10. Cai Z, Bunce N, Nunn ME, Okabe T. Porcelain adherence to dental cast CP titanium: effects of surface modifications. Biomaterials 2001;22: 979-86. 11. Bourcier R, Koss D, Smelser R, Richmond O. The influence of porosity on the deformation and fracture of alloys. Acta Metallurgica 1986;34: 2443-53. 12. Luo XP, Guo TW, Ou YG, Liu Q. Titanium casting into phosphate bonded investment with zirconite. Dent Mater 2002;18:512-5. 13. Ferenczi AM, Demri B, Moritz M, Muster D. Casted titanium for dental applications: an XPS and SEM study. Biomaterials 1998;19: 1513-5. 14. Atwood RC, Lee PD, Curtis RV. Modeling the surface contamination of dental titanium castings. Dent Mater 2005;21:178-86. 15. Syverud M, Okabe T, Hero H. Casting of Ti-6Al-4V alloy compared with pure Ti in an Ar-arc casting machine. Eur J Oral Sci 1995;103: 327-30. 16. Craig R, Powers J. Restorative dental materials. 11th ed. St. Louis: Elsevier; 2001. p. 479-514. 17. Anusavice KJ, Phillips RW. Phillips’ science of dental materials. 11th ed. St. Louis: Elsevier; 2003. p. 73-102. 18. Van Noort R. Introduction to dental materials. 2nd ed. St Louis: Mosby; 2002. p. 227-30. 19. International Organization for Standardization. Dental materials— Metal-ceramic dental restorative systems. ISO 9693:1999. Available at: http://www.iso.ch/iso/en/prods-services/ISOstore/store.html. Accessed July 28, 2005. 20. International Organization for Standardization. Dental materials—Dental base metal casting alloys. Part 1: cobalt-based alloys. ISO 6871-1: 1994. Available at: http://www.iso.ch/iso/en/prods-services/ISOstore/store.html. Accessed July 28, 2005. 21. International Organization for Standardization. Dental materials—Dental base metal casting alloys. Part 2: nickel-based alloys. ISO 6871-2:1994. Available at: http://www.iso.ch/iso/en/prods-services/ISOstore/store.html. Accessed July 28, 2005. 22. Boyer R, Lampman S. Titanium alloys. Materials Park (OH): ASM Intl; 1994. p. 517-27, 693. 23. International Organization for Standardization. Dental materials— Dental casting alloys with noble metal content of at least 25% but less than 75%. ISO 8891:1998. Available at: http://www.iso.ch/iso/en/ prods-services/ISOstore/store.html. Accessed July 28, 2005. 24. International Organization for Standardization. Dental materials— Dental casting gold alloys. ISO 1562:1993. Available at: http:// www.iso.ch/iso/en/prods-services/ISOstore/store.html. Accessed July 28, 2005.
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Reprint requests to: DR TRIANTAFILLOS PAPADOPOULOS DEPARTMENT OF BIOMATERIALS, DENTAL SCHOOL UNIVERSITY OF ATHENS THIVON 2, 115 27 ATHENS GREECE FAX: +3102107240633 E-MAIL: [email protected]
Noteworthy Abstracts of the Current Literature
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0022-3913/$30.00 Copyright Ó 2005 by The Editorial Council of The Journal of Prosthetic Dentistry.
doi:10.1016/j.prosdent.2005.09.017
Microleakage of various cementing agents for full cast crowns Piwowarczyk A, Lauer H-C, Sorensen JA. Dent Mater 2005; 21:445-53.
Objectives: To evaluate microleakage and marginal gaps in full cast crown restorations bonded with six different types of cementing agents. Methods: Sixty non-carious human premolars and molars were prepared in a standardized manner for full cast crown restorations. The mesial and distal margins were located in dentin, while the vestibular and palatal/ lingual margins were located in enamel. Crowns were made from a high-gold alloy using a standardized technique. The specimens were randomized to six groups of cementing agents: one zinc-phosphate cement (Harvard cement), one conventional glass–ionomer cement (Fuji I), one resin-modified glass–ionomer cement (Fuji Plus), two standard resin cements (RelyX ARC, Panavia F), and one self-adhesive universal resin cement (RelyX Unicem). After 4 weeks of storage in distilled water at 37°C, the specimens were subjected to 5000 thermocycles ranging from 5 to 55°C. Then, they were placed in a silver nitrate solution, embedded in resin blocks, and vertically cut in buccolingual and mesiodistal direction. Subsequently, the objects were evaluated for microleakage and marginal gap using a high-resolution digital microscope camera. Results: A number of inter-group differences were statistically significant. RelyX Unicem showed the smallest degree of microleakage both in enamel and in dentin. Panavia F und RelyX Unicem were associated with significantly larger marginal gaps than all other cementing agents. No association was observed between microleakage and marginal gap other than a weak direct correlation when using Harvard cement on enamel. Significance: The cementing agents investigated revealed different sealing abilities. These differences were not associated with specific types of materials.—Reprinted with permission of The Academy of Dental Materials.
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