Abstract
Objectives
The aim of the study was to characterize the chemical composition, microstructure and roughness of two commercially available zirconia dental implants (WhiteSky and Zit-Z).
Methods
The chemical composition of the cervical collar and threaded root parts of the implants ( n = 2) were studied by XPS and HV-EDX. LV-SEM was used for morphological assessment, Raman microanalysis for microstructural characterization and optical profilometry for surface roughness measurements. XRD, HV-EDX and Raman microanalysis of bulk regions (longitudinal sections) were used as reference.
Results
XPS showed the presence of C, O, Zr and Y (collar) plus Al (root) at implant surfaces. More C (10–26 at%) and a lower Al/Zr ratio were found in WhiteSky (1.05 vs 1.26 in Zit-Z). Zr, Y and Al were detected in single, fully oxidized states. The same elements, plus Hf, were identified by HV-EDX at bulk and surface regions, with a Al/Zr ratio higher in WhiteSky (0.17 vs 0.09 in Zit-Z). Na, K and Cl contaminants were traced at implant root parts by both methods. XRD analysis of cross-sectioned specimens revealed the presence of monoclinic and tetragonal zirconia along with cubic yttria phases. Raman microanalysis showed that the monoclinic zirconia volume fraction was higher at root surfaces than the collar. No monoclinic phase was found at bulk regions. Significantly higher Sa and Sq values were recorded in WhiteSky than Zit-Z, whereas Zit-Z showed higher Rt value.
Significance
The differences found between the implants in the extent of carbon contamination, residual alumina content, tetragonal to monoclinic ZrO 2 phase transformation and 3D-roughness parameters may contribute to a substantial differentiation in the cellular and tissue response.
1
Introduction
Tetragonal stabilized ZrO 2 , with the addition of 3 mol% Y 2 O 3 (3Y-TZP) has long been considered as a strong, tough, wear-resistant and osseoconductive ceramic suitable for stress-bearing implantable applications . The less inflammatory response and better stabilization of soft-tissues in contact with zirconia , the lower plaque retention capacity and higher affinity to ostoeblasts along with the more aesthetic tooth-like colour have made 3Y-TZP a viable alternative to titanium implants.
Already, several zirconia implants have been introduced with proven efficacy in animal studies . However, long-term human trials to establish their clinical success are still missing .
From the material standpoint, concerns have been expressed on the low temperature degradation of the tetragonal to monoclinic ZrO 2 phase, that has been associated with in-service failures of orthopaedic implants . Treatments to avoid or reverse such transformations have been advocated during manufacturing or sterilization . Moreover, it has been shown that the osseointegration capacity of machined ZrO 2 surfaces is substantially increased after modification by Al 2 O 3 sandblasting . Although such techniques have already been adopted in some products, there is lack of information on the surface chemistry, structure and morphology of currently commercially available ZrO 2 dental implants that would facilitate understanding of the implant–bone interactions and the bone augmentation mechanisms involved.
The aim of the present study was to investigate the surface chemistry, morphology and structure of two commercially available zirconia implants. The testing hypothesis was that significant differences exist in these properties between the two implants.
2
Materials and methods
The products tested were WhiteSky (Ø: 3.5 mm, l: 12 mm, lot 59235, Bredent Medical, Senden, Germany) and Zit-Z (Ø: 3.5 mm, l: 13 mm, lot 49744, Ziterion, Uffenheim, Germany). Five specimens from each product were subjected to the following testing procedures:
2.1
X-ray photoelectron spectroscopy (XPS)
The samples as received ( n = 2) were placed in the ultra-high vacuum chamber of an X-ray photoelectron spectrometer (Scienta 300 ESCA system, NCESS, Cheshire, UK). Two regions, one at the cervical collar and the other at the threaded root portion, were located on each implant and analyzed under the following conditions: Al Ka (1486.7 eV) monochromated rotating anode, 14 kV accelerating voltage, 0.2 A current emission, 2.8 kW maximum power, 10 −9 mbar pressure, ∼90° electron take-off angle, 0.30 eV energy resolution and 6 mm × 0.5 mm sampling area. An electron flood gun operated at 4 eV was used for charge compensation. Energy calibration was performed based on Ag3d 5/2 peak standard. Survey scans (150 eV pass energy) were taken from each region to identify the elements present on the surface. Then, high resolution narrow scans (150 eV pass energy) were recorded over the predominant peaks and the elemental binding states were determined. All spectra were aligned on the binding energy scale to the C 1s peak arising from adventitious hydrocarbons (–CH 2 –, 285 eV BE). The core level data were analyzed using the CASA XPS software package. A Shirley background was subtracted from the data and 80:20 Gaussian:Lorenzian peaks fitted to give binding energy positions. The fitted peak areas in conjunction with relative intensity factors allowed the elemental ratios at the surface to be quantified. The depth of analysis was estimated as to 3 nm.
2.2
Low-vacuum scanning electron microscopy and high vacuum X-ray energy dispersive microanalysis (LV-SEM/HV-EDX)
The same specimens analyzed by XPS were subjected to LV-SEM imaging. Secondary electron images were acquired employing a large field detector (LFD) attached to a SEM unit (Quanta 200, FEI, Hillsboro, OR, USA) operated at 30 kV accelerating voltage, 90 μA beam current, 1 Torr pressure (without electron conductive coating to avoid masking off microstructural features) at 40×, 300×, 600× and 2400× magnifications.
The elemental composition of cervical collar, threaded root and bulk regions (the latter prepared for XRD analysis as described below) of each implant were determined by HV-EDX analysis (this technique provides better accuracy in quantitative determinations than LV-EDX). All the specimens were coated with a thin layer of conductive carbon in a sputter-coating unit (SCD 004 Sputter-Coater with OCD 30 attachment, Bal-Tec, Vaduz, Liechtenstein). EDX analysis was performed using a liquid N 2 -cooled Si(Li) detector with super ultra-thin Be window (Sapphire SUTW+ CDU, EDAX Int, Mahwah, NJ, USA) attached to the SEM unit under the following conditions: 30 kV energy range, 10 −6 Torr pressure, 110 μA beam current, 128 eV resolution, 250 s acquisition time, 210 μm × 210 μm sampling window and 28–34% detector dead time. The depth of analysis was estimated as to 1 μm. The quantitative analysis was performed in non-standard mode using ZAF and coating corrections employing Genesis v. 5.2 software (EDAX, Int). Elemental mapping of regions of interest was based on compositional backscattered electron images, obtained with a solid-state detector (SSD) at the same conditions as above, but at 15 kV accelerating voltage.
2.3
Raman microanalysis
The cervical collar, threaded root parts bulk regions (prepared for XRD analysis as described below) of the implants ( n = 2) were analyzed by Raman microscopy to identify and map the distribution of the tetragonal (145 and 262 cm −1 Raman shift) and monoclinic (180 and 190 cm −1 Raman shift) ZrO 2 phases at the surface region . A Raman microscope was used (LabRAM Aramis, Horiba Jobin-Yvon, Villeneuve d’Ascq, France) operated under the following conditions: Ar laser (532 nm), 10 mW power at sample, 50× LWD objective, 1800 grit/mm grating, 1000 μm confocal hole (defocused mode), 100 μm slit, 10 s acquisition. Three regions were randomly located at each implant surface location (collar/root) with the optical system of the microscope (10× optical objective) and analyzed. For mapping of the monoclinic ZrO 2 phase, 20 μm × 35 μm areas were selected and scanned at 5 μm steps and 5 s acquisition time.
From the net peak height intensities of the monoclinic and tetragonal ZrO 2 phases, the percentage volume of the monoclinic phase ( V m %) was calculated according to the equation: V m % = { I 180 + I 190 /0.97( I 145 + I 262 ) + I 180 + I 190 } × 100, where I the net peak height intensities at the corresponding Raman shifts .
2.4
X-ray diffraction (XRD)
In order to examine the presence of the tetragonal and monoclinic ZrO 2 phases in the entire implant, one specimen from each implant was embedded in epoxy resin and sectioned at a longitudinal direction using a microtome under continuous water cooling. The specimens were ground to a smooth surface using SiC paper up to 1200 grit size, polished with a 3 μm diamond paste and ultrasonically cleaned in a distilled water bath for 3 min. The sections were studied in an X-ray diffractometer (D8 Focus, Bruker AXS, Karlsruhe, Germany) under the following conditions: CuKa anode, 40 kV accelerating voltage, 40 mA beam current, 20–90° 2 range, 0.02° step, 2 s exposure per step. The identification of phases was based on the ICSD database .
2.5
Optical profilometry
The 3D-surface roughness parameters of the threaded parts of the implants ( n = 2) were evaluated by optical profilometry. The area between two successive implant threads was analyzed by an optical profiler (NT 1100, Veeco, S. Barbara, CA, USA) equipped with Michelson/Mirau interferometric objectives at 10–100× magnifications. Quantification of the 3D-surface roughness parameters Sa (arithmetic mean deviation), Sq (root mean square deviation), and Rt (maximum peak to valley height) was performed by Veeco-Vision software at 40× magnification (160 μm × 120 μm).
2.6
Statistical analysis
The results of elemental at% obtained from XPS and EDX were subjected to a two-way ANOVA (implant type and region as independent variables, and at% per element as dependent variable), whereas a two-way ANOVA was used for V m % comparisons (implant type and region as independent variables). A Tukey’s test was used for pairwise multiple comparisons. Finally, the differences in the roughness parameters between the implants were evaluated by a t -test. In all comparisons a 95% confidence level was used ( α : 0.05). Statistical analysis was performed by SigmaStat software (Jandel, St. Raphael, CA, USA).
2
Materials and methods
The products tested were WhiteSky (Ø: 3.5 mm, l: 12 mm, lot 59235, Bredent Medical, Senden, Germany) and Zit-Z (Ø: 3.5 mm, l: 13 mm, lot 49744, Ziterion, Uffenheim, Germany). Five specimens from each product were subjected to the following testing procedures:
2.1
X-ray photoelectron spectroscopy (XPS)
The samples as received ( n = 2) were placed in the ultra-high vacuum chamber of an X-ray photoelectron spectrometer (Scienta 300 ESCA system, NCESS, Cheshire, UK). Two regions, one at the cervical collar and the other at the threaded root portion, were located on each implant and analyzed under the following conditions: Al Ka (1486.7 eV) monochromated rotating anode, 14 kV accelerating voltage, 0.2 A current emission, 2.8 kW maximum power, 10 −9 mbar pressure, ∼90° electron take-off angle, 0.30 eV energy resolution and 6 mm × 0.5 mm sampling area. An electron flood gun operated at 4 eV was used for charge compensation. Energy calibration was performed based on Ag3d 5/2 peak standard. Survey scans (150 eV pass energy) were taken from each region to identify the elements present on the surface. Then, high resolution narrow scans (150 eV pass energy) were recorded over the predominant peaks and the elemental binding states were determined. All spectra were aligned on the binding energy scale to the C 1s peak arising from adventitious hydrocarbons (–CH 2 –, 285 eV BE). The core level data were analyzed using the CASA XPS software package. A Shirley background was subtracted from the data and 80:20 Gaussian:Lorenzian peaks fitted to give binding energy positions. The fitted peak areas in conjunction with relative intensity factors allowed the elemental ratios at the surface to be quantified. The depth of analysis was estimated as to 3 nm.
2.2
Low-vacuum scanning electron microscopy and high vacuum X-ray energy dispersive microanalysis (LV-SEM/HV-EDX)
The same specimens analyzed by XPS were subjected to LV-SEM imaging. Secondary electron images were acquired employing a large field detector (LFD) attached to a SEM unit (Quanta 200, FEI, Hillsboro, OR, USA) operated at 30 kV accelerating voltage, 90 μA beam current, 1 Torr pressure (without electron conductive coating to avoid masking off microstructural features) at 40×, 300×, 600× and 2400× magnifications.
The elemental composition of cervical collar, threaded root and bulk regions (the latter prepared for XRD analysis as described below) of each implant were determined by HV-EDX analysis (this technique provides better accuracy in quantitative determinations than LV-EDX). All the specimens were coated with a thin layer of conductive carbon in a sputter-coating unit (SCD 004 Sputter-Coater with OCD 30 attachment, Bal-Tec, Vaduz, Liechtenstein). EDX analysis was performed using a liquid N 2 -cooled Si(Li) detector with super ultra-thin Be window (Sapphire SUTW+ CDU, EDAX Int, Mahwah, NJ, USA) attached to the SEM unit under the following conditions: 30 kV energy range, 10 −6 Torr pressure, 110 μA beam current, 128 eV resolution, 250 s acquisition time, 210 μm × 210 μm sampling window and 28–34% detector dead time. The depth of analysis was estimated as to 1 μm. The quantitative analysis was performed in non-standard mode using ZAF and coating corrections employing Genesis v. 5.2 software (EDAX, Int). Elemental mapping of regions of interest was based on compositional backscattered electron images, obtained with a solid-state detector (SSD) at the same conditions as above, but at 15 kV accelerating voltage.
2.3
Raman microanalysis
The cervical collar, threaded root parts bulk regions (prepared for XRD analysis as described below) of the implants ( n = 2) were analyzed by Raman microscopy to identify and map the distribution of the tetragonal (145 and 262 cm −1 Raman shift) and monoclinic (180 and 190 cm −1 Raman shift) ZrO 2 phases at the surface region . A Raman microscope was used (LabRAM Aramis, Horiba Jobin-Yvon, Villeneuve d’Ascq, France) operated under the following conditions: Ar laser (532 nm), 10 mW power at sample, 50× LWD objective, 1800 grit/mm grating, 1000 μm confocal hole (defocused mode), 100 μm slit, 10 s acquisition. Three regions were randomly located at each implant surface location (collar/root) with the optical system of the microscope (10× optical objective) and analyzed. For mapping of the monoclinic ZrO 2 phase, 20 μm × 35 μm areas were selected and scanned at 5 μm steps and 5 s acquisition time.
From the net peak height intensities of the monoclinic and tetragonal ZrO 2 phases, the percentage volume of the monoclinic phase ( V m %) was calculated according to the equation: V m % = { I 180 + I 190 /0.97( I 145 + I 262 ) + I 180 + I 190 } × 100, where I the net peak height intensities at the corresponding Raman shifts .
2.4
X-ray diffraction (XRD)
In order to examine the presence of the tetragonal and monoclinic ZrO 2 phases in the entire implant, one specimen from each implant was embedded in epoxy resin and sectioned at a longitudinal direction using a microtome under continuous water cooling. The specimens were ground to a smooth surface using SiC paper up to 1200 grit size, polished with a 3 μm diamond paste and ultrasonically cleaned in a distilled water bath for 3 min. The sections were studied in an X-ray diffractometer (D8 Focus, Bruker AXS, Karlsruhe, Germany) under the following conditions: CuKa anode, 40 kV accelerating voltage, 40 mA beam current, 20–90° 2 range, 0.02° step, 2 s exposure per step. The identification of phases was based on the ICSD database .
2.5
Optical profilometry
The 3D-surface roughness parameters of the threaded parts of the implants ( n = 2) were evaluated by optical profilometry. The area between two successive implant threads was analyzed by an optical profiler (NT 1100, Veeco, S. Barbara, CA, USA) equipped with Michelson/Mirau interferometric objectives at 10–100× magnifications. Quantification of the 3D-surface roughness parameters Sa (arithmetic mean deviation), Sq (root mean square deviation), and Rt (maximum peak to valley height) was performed by Veeco-Vision software at 40× magnification (160 μm × 120 μm).
2.6
Statistical analysis
The results of elemental at% obtained from XPS and EDX were subjected to a two-way ANOVA (implant type and region as independent variables, and at% per element as dependent variable), whereas a two-way ANOVA was used for V m % comparisons (implant type and region as independent variables). A Tukey’s test was used for pairwise multiple comparisons. Finally, the differences in the roughness parameters between the implants were evaluated by a t -test. In all comparisons a 95% confidence level was used ( α : 0.05). Statistical analysis was performed by SigmaStat software (Jandel, St. Raphael, CA, USA).
3
Results
3.1
XPS
Fig. 1 (a and b) shows representative survey XPS spectra of the implants taken from cervical collar and threaded root regions. The elements identified were C, O, Al, Zr and Y. The results of the XPS elemental atomic percentage (at%) are listed in Table 1 . The implant surfaces have similar qualitative composition. Al was identified only at threaded root regions. WhiteSky demonstrated more C contamination and less O content at the cervical collar. The highest Zr content was found at Zit-Z collar. The XPS Y/Zr atomic ratios, as derived from the data listed in Table 1 , ranged from 0.05 to 0.07 ( p > 0.05), with the exception of Zit-Z collar, where the highest value of 0.15 was recorded ( p < 0.05). The Al/Zr atomic ratios for WhiteSky and Zit-Z were 1.05 and 1.27 respectively ( p < 0.05). The Zr 3d and Y 3d peaks showed only one doublet arising from 3d 5/2 and 3d 3/2 spin orbit splitting, indicating that Zr and Y exist in one oxidation state in the implants ( Fig. 1 c and d). The binding energy of the Zr 3d 5/2 peak at 182.4 eV corresponds to ZrO 2 , whereas that of theY 3d 5/2 peak at 156.8 eV to Y 2 O 3 . For Al, the binding energy of the Al 2p peak at 74.68 eV corresponds to Al 2 O 3 . Traces of Na were also identified (Na 1s at 1023 eV and Na Auger at 499 eV) in both implants.
