Effect of Non-ionizing Radio Frequency Signals of Magnetic Resonance Imaging on Physical Properties of Dental Alloys and Metal-Ceramic Adhesion Abu Bakr El-Bediwia / Abeer El-Fallalb / Samah Sakerc / Mutlu Özcand
Purpose: To assess the influence of non-ionizing radio frequency signals of magnetic resonance imaging (MRI) on physical properties of dental alloys and metal-ceramic adhesion. Materials and Methods: A total of 120 disk-shaped wax patterns (10 mm x 10 mm x 1 mm) were cast in a base metal alloy (Ni-Cr alloy) and commercially pure titanium (Ti) following the manufacturing recommendation. After casting, air abrasion and ultrasonic cleaning, feldspathic ceramic was applied and fired according to manufacturer’s instructions using a standard mold. The specimens were subjected to 3000 thermocycles in distilled water between 5°C and 55°C, then veneered alloy specimens were randomly assigned to three groups according to MRI exposure time: a) 15 min of MRI exposure, b) 30 min of MRI exposure and c) no MRI exposure (control group). The specimens were subjected to shear loading until failure. A separate set of Ni-Cr and Ti specimens were prepared, and after exposure to MRI for 15 and 30 min, x-ray diffraction (XRD) analysis, surface roughness, and Vicker’s hardness were measured. Results: Both the alloy type (p < 0.005) and exposure duration (p < 0.005) had a significant effect on the bond results. While the control group presented the highest bond strength for Ni-Cr and Ti (36.9 ± 1.4 and 21.5 ± 1.6 MPa, respectively), 30 min MRI exposure significantly decreased the bond strength for both alloys (29.4 ± 1.5 and 12.8 ± 1.5 MPa, respectively) (p < 0.05). XRD analysis indicated formation of the crystalline phase as well as change in crystal size and position for Ni-Cr and Ti after MRI. Compared to the control group where alloys were not exposed to MRI (Ni-Cr: 0.40 μm; Ti: 0.17 μm), surface roughness increased (Ni-Cr: 0.54 μm; Ti: 1.1 μm). Vicker’s hardness of both alloys decreased after 30 min MRI (Ni-Cr: 329.5; Ti: 216.1) compared to the control group c (Ni-Cr: 356.1; Ti: 662.1), being more significant for Ti (p < 0.005). Conclusion: Ni-Cr alloy is recommended over Ti for the fabrication of metal-ceramic restorations for patients with a history of frequent exposure to MRI. Keywords: adhesion, base alloy, chipping, magnetic resonance imaging, metal-ceramic, roughness, titanium. J Adhes Dent 2014; 16: 407–413 doi: 10.3290/j.jad.a32664
a
Professor, Metal Physics Laboratory, Physics Department, Faculty of Science, Mansoura University, Egypt. Performed experiments, collected and analyzed data, wrote the manuscript, discussed the results, commented on the manuscript at all stages.
b
Professor, Biomaterials Department, Faculty of Dentistry, Mansoura University, Egypt. Performed experiments, collected and analyzed data, wrote the manuscript, discussed the results, commented on the manuscript at all stages.
c
Asistant Professor, Conservative Dentistry Department, Faculty of Dentistry, Mansoura University, Egypt. Performed experiments, collected and analyzed data, wrote the manuscript, discussed the results, commented on the manuscript at all stages.
d
Professor, Dental Materials Unit, Clinic for Fixed and Removable Prosthodontics and Dental Materials Science, Center for Dental and Oral Medicine, University of Zürich, Zurich, Switzerland. Experimental design, analyzed data, wrote and edited manuscript, discussed the results, commented on the manuscript at all stages.
Correspondence: Dr. Samah Saker, Mansoura University, Faculty of Dentistry, Conservative Dentistry Department, Mansoura, 35516 Egypt. Tel: +20-128745-7890. e-mail: [email protected]
Vol 16, No 5, 2014
Submitted for publication: 28.03.13; accepted for publication: 17.06.14
A
lthough the use of all-ceramic or polymeric materials is increasing in dental applications, metal-ceramic restorations are still being widely used in dental practice due to the long-term clinical success.19,29 The considerable increase in the price of gold starting in the 1970s has resulted in the development of alternative metallic systems for dental use. In comparison to noble alloys, Ni-Cr and Co-Cr base metal alloys melt at higher temperatures, require more critical handling during melting, and are more difficult to finish. Such limitations are minimized with recent technological developments, and the biomechanical properties of these alloys could be considered superior to noble alloys.1 In prosthetic applications so far, the success of metal-ceramic fixed dental prostheses (FDP) depends primarily on optimal adhesion of the veneering ceramic on the framework alloy.2,3,7,17,23,26 Metal-ceramic adhesion necessitates 407
El-Bediwi et al
chemical and thermal compatibility during both ceramic sintering and function. Three possible mechanisms, namely micromechanical retention, chemical bonding, and van der Waals forces, dictate metal-ceramic adhesion. Chemical bonding of the union is characterized by the direct transfer of electrons between oxygen in the vitreous part of the ceramic and the metal oxides.15,18 Depending on the test method employed, bond strength results of Co-Cr or Ni-Cr dental alloys and ceramics range between 35 and 95 MPa in the dental literature.6,10,12 Among all dental alloys, commercial pure titanium (Ti) has been the material of choice in several disciplines in dentistry due to its biocompatibility, resistance to corrosion, and mechanical properties similar to that of gold alloys. Despite these favorable characteristics, Ti casting for prosthetic purposes has not been viable for many years, since casting procedures led to the formation of an undesirable crust resulting in high reactivity and fragility. This coating is called “alpha-case” and is formed by incorporation of the elements from the investment that may impair the adhesion between Ti and ceramics, yielding bond strength results between 29 and 32 MPa.20 Despite all improvements in casting methods, clinical studies still report ceramic fractures associated with FDPs, predominantly between metals and ceramics.22 Magnetic resonance imaging (MRI) is widely used as an important diagnostic tool for the whole body, but especially for orthopedic and brain surgery. MRI has remarkable advantages, as it provides cross-sectional views and assists diagnostics of diseases in the human body with no invasion and no exposure to x-ray radiation.16,19,24 Unfortunately, the use of MRI diagnostics can be risky when metals are present as implants or prosthetic devices in the body, that is, metallic materials become magnetized in the intense magnetic field of the MRI device.4,11 Recently, MRI was reported to impair the surface and bulk properties as well as corrosion resistance of Co-Cr and Ni-Cr.13,14 The information on magnetic susceptibility of dental alloys and its effect on metal-ceramic adhesion mechanical properties are not readily available for commonly used metals in dentistry. The objectives of this study were thus to evaluate the effect of non-ionizing radio frequency signals of MRI for short and long durations on the physical properties of NiCr and commercially pure Ti and metal-ceramic adhesion.
MATERIALS AND METHODS Specimen Preparation Ni-Cr (N = 30) specimens (Durabond; Sylmar, CA, USA) and commercially pure titanium (Ti) alloys (N = 30) (ASTM, Grade II, Modern Techniques and Materials Engineering Center; Nasr City, Cairo, Egypt) of 10 mm x 10 mm x 1 mm size were obtained. Initially, a wax sprue was attached perpendicular to the specimen at one end of the template and connected to a central wax rod of 5 mm diameter (wax wire for casting sprues, Dentaurum; Pforzheim, Germany). The assembly was mounted in a silicone ring and poured with investment 408
material (Rematitan, Ultra, Dentaurum) that was mixed at a ratio of 100 g of powder to 14 ml of liquid. After the investment material set, the silicone ring and sprue former were separated from the investment mold. Metallic frameworks of Ti were cast in an electrical induction furnace (Rematitan Autocast, Dentaurum) under argon gas. Ni-Cr was cast following the manufacturer’s instructions. Elimination of sprues and separation of metallic strips were performed using carbide disks at low speed. After divesting, the metal specimens were airborneparticle abraded with 110-μm aluminum oxide (Korax, Bego; Bremen Germany) at a pressure of 2.5 bar from a distance of approximately 2 cm at an angle of 45 degrees for 10 s (Blastmate II, CFI 9441-113, Ney Dental; Yucaipa, CA, USA). The metal specimens were then ultrasonically cleaned (Vitasonic II, Vita Zahnfabrik; Bad Säckingen, Germany) in distilled water and isopropyl alcohol for 5 min each. Two layers of opaque ceramic (thickness: 0.1 mm each) (VMK900, Vita Zahnfabrik) were applied by homogenously mixing the powder of opaque ceramic and liquid in a container, and applied with a thin brush by the same operator onto the metallic surface using a standard mold. The thickness of the opaque layer was carefully measured using a digital caliper (StarrettR 727, Starrett; Itu, Brazil). The veneering ceramic (Shade 2M1) was then fired onto Ni-Cr and Ti alloys. A specially designed split polyethylene mold (diameter: 6 mm, thickness: 4 mm) was positioned in the center of each plate and the ceramic was applied onto the metal plate. Sintering of the veneering ceramics was accomplished in an oven (Vacumat, VITA Zahnfabrik). A second firing was performed to compensate for sintering shrinkage of the ceramics according to manufacturer’s recommendations. The veneered alloy specimens were then thermocycled in distilled water 3000x between 5°C and 55°C (dwell time: 30 s; transfer time between baths: 2 s). The specimens were randomly assigned to three groups: a) 15 min of MRI non-ionizing radio frequency (RF) signal exposure (1.5 T, Magnetom Vision, Siemens; Erlangen, Germany), b) 30 min of MRI exposure, and c) no MRI exposure (control group). The specimens were embedded in auto-polymerized acrylic resin (Acrostone; Alexandria, Egypt) using plastic molds (diameter: 25 mm, height 20 mm). They were mounted in the jig of a universal testing machine (Lloyd Model TT-B, Instron; Canton, MA, USA) and loading was applied to the metal-ceramic interface until failure occurred (crosshead speed: 0.5 mm/min). The shearing blade was a 30-degree mono-bevelled chisel positioned 0.1 mm away from the bonded interface. The maximum force to produce failure (MPa) was recorded by the corresponding software, dividing the fracture load (F) in Newtons by the surface area (A) in mm2. X-Ray Diffraction Analysis (XRD) In another set of Ni-Cr and Ti specimens (N = 60, n = 10 per group), surface characterization was performed on the flat surfaces of the specimens using an x-ray diffractometer (Shimadzu, Dx–30; Tokyo, Japan). The Journal of Adhesive Dentistry
El-Bediwi et al
Table 1 Two-way ANOVA results comparing mean shear bond strength of ceramics onto Ni-Cr and commercially pure Ti as a function of MRI exposure duration Sum of squares
df
Mean square
F value
p
Type of alloy
4247.051
1
MRI duration
675.52
2
337.765
132.029 0.000
Type x duration 26.133
2
13.067
5.108 0.009
Total
60
Table 2 Mean (SD) bond strength values in MPa of ceramic-alloy combinations (Ni-Cr, commercially pure Ti) before (control) and after MRI exposure Alloy type
Groups
Mean
SD
Ni-Cr
Control
36.9
1.4
15 min
33.4
1.9
30 min
29.4
1.5
Control
21.5
1.6
15 min
14.8
1.8
30 min
12.8
1.5
4247.051 1660.133 0.000
Ti
41979.3
Similar to the shear test, the alloy surfaces were initially exposed to either a) 15 min of MRI non-ionizing RF signal exposure (1.5 T, Magnetom Vision) or b) 30 min of MRI exposure. The control group was not exposed to MRI. Cu-Kα radiation (l = 1.54056 Å at 45 kV and 35 mA) and Ni-filter in the angular range of 2q from 0 to 100 degrees was applied in continuous mode with a scan speed of 5 degrees/min for surface characterization. Surface Roughness Test Surface roughness (Ra) of the control group and the MRI exposed groups was measured by using a portable profilometer (Surftest SJ-201 P, Mitutoyo; Tokyo, Japan). A diamond stylus with a radius of 5 μm took three measurements and the average was calculated. Vicker’s Hardness Test The microhardness of each group was calculated using a digital Vicker’s microhardness tester (Model FM-7, Future Tech; Tokyo, Japan), applying a load of 100 g for 5 s by means of a Vicker’s diamond tip. Statistical Analysis Statistical analysis was performed using SPSS 11.0 software for Windows (SPSS; Chicago, IL, USA). Bond strength data (MPa) were submitted to two-way ANOVA with the bond strength as the dependent variable and the alloy type and MRI duration as independent variables. For surface roughness and Vicker’s hardness, two-way ANOVA and Tukey’s tests were used. Due to significant differences between groups, multiple comparisons were made using Tukey’s tests. P-values less than 0.05 were considered to be statistically significant in all tests.
RESULTS Shear Bond Strength Both the alloy type (p < 0.005) and exposure duration (p < 0.005) had a significant effect on the bond results. Interaction terms were also significant Vol 16, No 5, 2014
(p = 0.009) (Table 1). While the control group presented the highest bond strength for Ni-Cr and Ti (36.9 ± 1.4 and 21.5 ± 1.6 MPa, respectively), 30 min MRI exposure significantly decreased the bond strength of both alloys (29.4 ± 1.5 and 12.8 ± 1.5 MPa, respectively) (Table 2). XRD Analysis According to XRD analysis, Ni-Cr alloy consisted of gamma solid solution, γ (Ni-Cr) and Ni cubic phase. After exposure to MRI for 15 and 30 min, the intensity of the peaks indicated the formation of crystalline phases. Broad bands denoted changes in crystal size and position (Fig 1). For Ti in the control group, hexagonal Ti phase could be observed. Similar to Ni-Cr, exposure to MRI for 15 and 30 min changed the intensity of Ti phase, indicating position change (Fig 2). Surface Roughness Compared to the control group (Ni-Cr: 0.40 μm; Ti: 0.17 μm), surface roughness increased for Ni-Cr (0.54 μm) and Ti (1.1 μm). Surface roughness was significantly higher for Ti in all conditions (p < 0.05) (Table 3). The duration of MRI exposure did not show a significant difference between 15 and 30 min (p > 0.05). Vicker’s Hardness Vicker’s hardness of both alloys decreased significantly after 30 min MRI (Ni-Cr: 329.5; Ti: 216.1) compared to the control group (Ni-Cr: 356.1; Ti: 662.1) (Table 4). The decreased Vicker’s hardness for Ti was more significant (p
Purpose: To assess the influence of non-ionizing radio frequency signals of magnetic resonance imaging (MRI) on physical properties of dental alloys and metal-ceramic adhesion. Materials and Methods: A total of 120 disk-shaped wax patterns (10 mm x 10 mm x 1 mm) were cast in a base metal alloy (Ni-Cr alloy) and commercially pure titanium (Ti) following the manufacturing recommendation. After casting, air abrasion and ultrasonic cleaning, feldspathic ceramic was applied and fired according to manufacturer’s instructions using a standard mold. The specimens were subjected to 3000 thermocycles in distilled water between 5°C and 55°C, then veneered alloy specimens were randomly assigned to three groups according to MRI exposure time: a) 15 min of MRI exposure, b) 30 min of MRI exposure and c) no MRI exposure (control group). The specimens were subjected to shear loading until failure. A separate set of Ni-Cr and Ti specimens were prepared, and after exposure to MRI for 15 and 30 min, x-ray diffraction (XRD) analysis, surface roughness, and Vicker’s hardness were measured. Results: Both the alloy type (p < 0.005) and exposure duration (p < 0.005) had a significant effect on the bond results. While the control group presented the highest bond strength for Ni-Cr and Ti (36.9 ± 1.4 and 21.5 ± 1.6 MPa, respectively), 30 min MRI exposure significantly decreased the bond strength for both alloys (29.4 ± 1.5 and 12.8 ± 1.5 MPa, respectively) (p < 0.05). XRD analysis indicated formation of the crystalline phase as well as change in crystal size and position for Ni-Cr and Ti after MRI. Compared to the control group where alloys were not exposed to MRI (Ni-Cr: 0.40 μm; Ti: 0.17 μm), surface roughness increased (Ni-Cr: 0.54 μm; Ti: 1.1 μm). Vicker’s hardness of both alloys decreased after 30 min MRI (Ni-Cr: 329.5; Ti: 216.1) compared to the control group c (Ni-Cr: 356.1; Ti: 662.1), being more significant for Ti (p < 0.005). Conclusion: Ni-Cr alloy is recommended over Ti for the fabrication of metal-ceramic restorations for patients with a history of frequent exposure to MRI. Keywords: adhesion, base alloy, chipping, magnetic resonance imaging, metal-ceramic, roughness, titanium. J Adhes Dent 2014; 16: 407–413 doi: 10.3290/j.jad.a32664
a
Professor, Metal Physics Laboratory, Physics Department, Faculty of Science, Mansoura University, Egypt. Performed experiments, collected and analyzed data, wrote the manuscript, discussed the results, commented on the manuscript at all stages.
b
Professor, Biomaterials Department, Faculty of Dentistry, Mansoura University, Egypt. Performed experiments, collected and analyzed data, wrote the manuscript, discussed the results, commented on the manuscript at all stages.
c
Asistant Professor, Conservative Dentistry Department, Faculty of Dentistry, Mansoura University, Egypt. Performed experiments, collected and analyzed data, wrote the manuscript, discussed the results, commented on the manuscript at all stages.
d
Professor, Dental Materials Unit, Clinic for Fixed and Removable Prosthodontics and Dental Materials Science, Center for Dental and Oral Medicine, University of Zürich, Zurich, Switzerland. Experimental design, analyzed data, wrote and edited manuscript, discussed the results, commented on the manuscript at all stages.
Correspondence: Dr. Samah Saker, Mansoura University, Faculty of Dentistry, Conservative Dentistry Department, Mansoura, 35516 Egypt. Tel: +20-128745-7890. e-mail: [email protected]
Vol 16, No 5, 2014
Submitted for publication: 28.03.13; accepted for publication: 17.06.14
A
lthough the use of all-ceramic or polymeric materials is increasing in dental applications, metal-ceramic restorations are still being widely used in dental practice due to the long-term clinical success.19,29 The considerable increase in the price of gold starting in the 1970s has resulted in the development of alternative metallic systems for dental use. In comparison to noble alloys, Ni-Cr and Co-Cr base metal alloys melt at higher temperatures, require more critical handling during melting, and are more difficult to finish. Such limitations are minimized with recent technological developments, and the biomechanical properties of these alloys could be considered superior to noble alloys.1 In prosthetic applications so far, the success of metal-ceramic fixed dental prostheses (FDP) depends primarily on optimal adhesion of the veneering ceramic on the framework alloy.2,3,7,17,23,26 Metal-ceramic adhesion necessitates 407
El-Bediwi et al
chemical and thermal compatibility during both ceramic sintering and function. Three possible mechanisms, namely micromechanical retention, chemical bonding, and van der Waals forces, dictate metal-ceramic adhesion. Chemical bonding of the union is characterized by the direct transfer of electrons between oxygen in the vitreous part of the ceramic and the metal oxides.15,18 Depending on the test method employed, bond strength results of Co-Cr or Ni-Cr dental alloys and ceramics range between 35 and 95 MPa in the dental literature.6,10,12 Among all dental alloys, commercial pure titanium (Ti) has been the material of choice in several disciplines in dentistry due to its biocompatibility, resistance to corrosion, and mechanical properties similar to that of gold alloys. Despite these favorable characteristics, Ti casting for prosthetic purposes has not been viable for many years, since casting procedures led to the formation of an undesirable crust resulting in high reactivity and fragility. This coating is called “alpha-case” and is formed by incorporation of the elements from the investment that may impair the adhesion between Ti and ceramics, yielding bond strength results between 29 and 32 MPa.20 Despite all improvements in casting methods, clinical studies still report ceramic fractures associated with FDPs, predominantly between metals and ceramics.22 Magnetic resonance imaging (MRI) is widely used as an important diagnostic tool for the whole body, but especially for orthopedic and brain surgery. MRI has remarkable advantages, as it provides cross-sectional views and assists diagnostics of diseases in the human body with no invasion and no exposure to x-ray radiation.16,19,24 Unfortunately, the use of MRI diagnostics can be risky when metals are present as implants or prosthetic devices in the body, that is, metallic materials become magnetized in the intense magnetic field of the MRI device.4,11 Recently, MRI was reported to impair the surface and bulk properties as well as corrosion resistance of Co-Cr and Ni-Cr.13,14 The information on magnetic susceptibility of dental alloys and its effect on metal-ceramic adhesion mechanical properties are not readily available for commonly used metals in dentistry. The objectives of this study were thus to evaluate the effect of non-ionizing radio frequency signals of MRI for short and long durations on the physical properties of NiCr and commercially pure Ti and metal-ceramic adhesion.
MATERIALS AND METHODS Specimen Preparation Ni-Cr (N = 30) specimens (Durabond; Sylmar, CA, USA) and commercially pure titanium (Ti) alloys (N = 30) (ASTM, Grade II, Modern Techniques and Materials Engineering Center; Nasr City, Cairo, Egypt) of 10 mm x 10 mm x 1 mm size were obtained. Initially, a wax sprue was attached perpendicular to the specimen at one end of the template and connected to a central wax rod of 5 mm diameter (wax wire for casting sprues, Dentaurum; Pforzheim, Germany). The assembly was mounted in a silicone ring and poured with investment 408
material (Rematitan, Ultra, Dentaurum) that was mixed at a ratio of 100 g of powder to 14 ml of liquid. After the investment material set, the silicone ring and sprue former were separated from the investment mold. Metallic frameworks of Ti were cast in an electrical induction furnace (Rematitan Autocast, Dentaurum) under argon gas. Ni-Cr was cast following the manufacturer’s instructions. Elimination of sprues and separation of metallic strips were performed using carbide disks at low speed. After divesting, the metal specimens were airborneparticle abraded with 110-μm aluminum oxide (Korax, Bego; Bremen Germany) at a pressure of 2.5 bar from a distance of approximately 2 cm at an angle of 45 degrees for 10 s (Blastmate II, CFI 9441-113, Ney Dental; Yucaipa, CA, USA). The metal specimens were then ultrasonically cleaned (Vitasonic II, Vita Zahnfabrik; Bad Säckingen, Germany) in distilled water and isopropyl alcohol for 5 min each. Two layers of opaque ceramic (thickness: 0.1 mm each) (VMK900, Vita Zahnfabrik) were applied by homogenously mixing the powder of opaque ceramic and liquid in a container, and applied with a thin brush by the same operator onto the metallic surface using a standard mold. The thickness of the opaque layer was carefully measured using a digital caliper (StarrettR 727, Starrett; Itu, Brazil). The veneering ceramic (Shade 2M1) was then fired onto Ni-Cr and Ti alloys. A specially designed split polyethylene mold (diameter: 6 mm, thickness: 4 mm) was positioned in the center of each plate and the ceramic was applied onto the metal plate. Sintering of the veneering ceramics was accomplished in an oven (Vacumat, VITA Zahnfabrik). A second firing was performed to compensate for sintering shrinkage of the ceramics according to manufacturer’s recommendations. The veneered alloy specimens were then thermocycled in distilled water 3000x between 5°C and 55°C (dwell time: 30 s; transfer time between baths: 2 s). The specimens were randomly assigned to three groups: a) 15 min of MRI non-ionizing radio frequency (RF) signal exposure (1.5 T, Magnetom Vision, Siemens; Erlangen, Germany), b) 30 min of MRI exposure, and c) no MRI exposure (control group). The specimens were embedded in auto-polymerized acrylic resin (Acrostone; Alexandria, Egypt) using plastic molds (diameter: 25 mm, height 20 mm). They were mounted in the jig of a universal testing machine (Lloyd Model TT-B, Instron; Canton, MA, USA) and loading was applied to the metal-ceramic interface until failure occurred (crosshead speed: 0.5 mm/min). The shearing blade was a 30-degree mono-bevelled chisel positioned 0.1 mm away from the bonded interface. The maximum force to produce failure (MPa) was recorded by the corresponding software, dividing the fracture load (F) in Newtons by the surface area (A) in mm2. X-Ray Diffraction Analysis (XRD) In another set of Ni-Cr and Ti specimens (N = 60, n = 10 per group), surface characterization was performed on the flat surfaces of the specimens using an x-ray diffractometer (Shimadzu, Dx–30; Tokyo, Japan). The Journal of Adhesive Dentistry
El-Bediwi et al
Table 1 Two-way ANOVA results comparing mean shear bond strength of ceramics onto Ni-Cr and commercially pure Ti as a function of MRI exposure duration Sum of squares
df
Mean square
F value
p
Type of alloy
4247.051
1
MRI duration
675.52
2
337.765
132.029 0.000
Type x duration 26.133
2
13.067
5.108 0.009
Total
60
Table 2 Mean (SD) bond strength values in MPa of ceramic-alloy combinations (Ni-Cr, commercially pure Ti) before (control) and after MRI exposure Alloy type
Groups
Mean
SD
Ni-Cr
Control
36.9
1.4
15 min
33.4
1.9
30 min
29.4
1.5
Control
21.5
1.6
15 min
14.8
1.8
30 min
12.8
1.5
4247.051 1660.133 0.000
Ti
41979.3
Similar to the shear test, the alloy surfaces were initially exposed to either a) 15 min of MRI non-ionizing RF signal exposure (1.5 T, Magnetom Vision) or b) 30 min of MRI exposure. The control group was not exposed to MRI. Cu-Kα radiation (l = 1.54056 Å at 45 kV and 35 mA) and Ni-filter in the angular range of 2q from 0 to 100 degrees was applied in continuous mode with a scan speed of 5 degrees/min for surface characterization. Surface Roughness Test Surface roughness (Ra) of the control group and the MRI exposed groups was measured by using a portable profilometer (Surftest SJ-201 P, Mitutoyo; Tokyo, Japan). A diamond stylus with a radius of 5 μm took three measurements and the average was calculated. Vicker’s Hardness Test The microhardness of each group was calculated using a digital Vicker’s microhardness tester (Model FM-7, Future Tech; Tokyo, Japan), applying a load of 100 g for 5 s by means of a Vicker’s diamond tip. Statistical Analysis Statistical analysis was performed using SPSS 11.0 software for Windows (SPSS; Chicago, IL, USA). Bond strength data (MPa) were submitted to two-way ANOVA with the bond strength as the dependent variable and the alloy type and MRI duration as independent variables. For surface roughness and Vicker’s hardness, two-way ANOVA and Tukey’s tests were used. Due to significant differences between groups, multiple comparisons were made using Tukey’s tests. P-values less than 0.05 were considered to be statistically significant in all tests.
RESULTS Shear Bond Strength Both the alloy type (p < 0.005) and exposure duration (p < 0.005) had a significant effect on the bond results. Interaction terms were also significant Vol 16, No 5, 2014
(p = 0.009) (Table 1). While the control group presented the highest bond strength for Ni-Cr and Ti (36.9 ± 1.4 and 21.5 ± 1.6 MPa, respectively), 30 min MRI exposure significantly decreased the bond strength of both alloys (29.4 ± 1.5 and 12.8 ± 1.5 MPa, respectively) (Table 2). XRD Analysis According to XRD analysis, Ni-Cr alloy consisted of gamma solid solution, γ (Ni-Cr) and Ni cubic phase. After exposure to MRI for 15 and 30 min, the intensity of the peaks indicated the formation of crystalline phases. Broad bands denoted changes in crystal size and position (Fig 1). For Ti in the control group, hexagonal Ti phase could be observed. Similar to Ni-Cr, exposure to MRI for 15 and 30 min changed the intensity of Ti phase, indicating position change (Fig 2). Surface Roughness Compared to the control group (Ni-Cr: 0.40 μm; Ti: 0.17 μm), surface roughness increased for Ni-Cr (0.54 μm) and Ti (1.1 μm). Surface roughness was significantly higher for Ti in all conditions (p < 0.05) (Table 3). The duration of MRI exposure did not show a significant difference between 15 and 30 min (p > 0.05). Vicker’s Hardness Vicker’s hardness of both alloys decreased significantly after 30 min MRI (Ni-Cr: 329.5; Ti: 216.1) compared to the control group (Ni-Cr: 356.1; Ti: 662.1) (Table 4). The decreased Vicker’s hardness for Ti was more significant (p
