Introduction - Materials and methods.
Dehua Li, MD, DDS, PhD,
Associate Professor, Dept. of Oral and Maxillofacial Surgery, Qindu Stomatological College, Xi’an 710032, P.R. China.
Baolin Liu, MD, DDS,
Professor and Chairman, Craniofacial Implant Center, Qindu Stomatological College, Xi’an 710032, P.R. China.
Yong Han, PhD,
Associate Professor, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, P.R. China.
Kewei Xu, PhD,
Professor and Dean, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, P.R. China.
To seek ideal osseointegration is a major focus of study in contemporary oral implantology.1 The osseointegration currently achieved is far from ideal. It takes a long time for dental implants to osseointegrate ordinarily from 3–6 months2. Histologically, only capsule-like bone attachment forms at the interface, rather than the real bone-fiber-perpendicularly connecting integration (D.-H. Li, unpublished data). This is reflected clinically with lengthy therapeutics courses for most current dental implant systems (irrespective of submerged or nonsubmerged) and the detrimental periimplant alveolar bone loss resulting from the unfavorable distribution of stress at the bone interface. Although there are immediately loaded nonsubmerged implants for shortening the time of implant therapy, they are still at the preliminary stage of development.
Surface modification of dental implants is essential for seeking the ideal osseointegration. Regarding the implant-bone interface, it was demonstrated that a rough surface is more beneficial than a smooth surface. Several methods, such as titanium plasma spraying, sintering, hydroxylapatite coating, electrophoretic deposition, and ion-beam or radiofrequency sputtering, have been adopted with the aim of optimizing the roughness and topography of the implant surface. Nearly all of these methods are coating techniques. To enhance the interfacial biomechanical properties, these methods also present some new problems. Of these problems, the major one is the existence of an additional interface between the coating and the substrate of the implant, which could further complicate the maintenance of osseointegration, especially with repetitive function.
Presently, as an alternative, the noncoating, surface-roughening method has drawn some attention. In light of the status of coating techniques, a modified sandblasting noncoating surface modification was developed. In vivo experimental studies initially demonstrated that it can improve osseointegration of dental implants and enhance the interfacial shear strength. This paper studies the effects of this newly developed modification on the topographic and chemical properties of the implant surface and is intended to verify its feasibility and reliability in treating titanium dental implants.
MATERIALS AND METHODS.
Surface Treatments of Titanium and Preparation of Samples.
A total of 27 titanium discs (10-mm diameter and 1-mm thickness) were cut from grade 2 commercially pure titanium plates (ASTM F67, unalloyed titanium for surgical implant applications), which were supplied by the Northwest Nonferrous Metal Institute (Xi’an, P.R. China). Samples were equally divided into three groups: the smooth surface (S) group, the sandblasted surface (SB) group, and the modified sandblasted surface (SBA) group. S group disks were polished through a series of silicon carbide papers to 800 grit, SB group disks were sandblasted by 0.15- to 0.21-mm corundum (Al2O3) at 0.45 MPa, and SBA group disks were etched by oxalic acid after the sandblast treatment. Before use, all samples were degreased by cleaning in trichloroethylene and ethanol using ultrasonics for 10 minutes, respectively, and passivation by immersion in 40% nitric acid for 30 minutes. They were then rinsed with distilled water three times for 5 minutes each time. After air-drying, they were used for the further tests, each of which was done on a separate disc.
Observation of Surface Topography.
Topographic variations of samples (two samples each group) were observed using a S-520 scanning electron microscope (SEM; S-520, Hitachi Co., Japan) with an energy dispersive electron probe x-ray analyzer (TN-5500, Thermo Noran, Middleton, WI) for the element analysis of surface.
Detection of Heteroelement Pollution of Titanium Surface.
Heteroelement pollution of the treated samples (two samples each group) was detected by phase composition analysis with x-ray diffraction (XRD; D/Max-3c, Rigaku Co., Japan), using monochromated CuKa radiation at 40 kV and 20 mA. Data were collected from 20 to 100 degrees, with a step size of 0.02 degrees and a count time of 0.15 second. This examination was used solely to detect the element pollution of Al2O3 on the surfaces.
Assay of Titanium Anti-Corrosiveness.
Anti-corrosiveness of titanium was examined by the in vitro titanium ion-releasing test, which was performed as follows. Samples of various groups, sterilized by autoclaving, were soaked in the sterile simulated body fluid 18 at 37°C, 2 mL per sample, and five samples per group. After soaking for 3 months, the samples were removed, and the concentration of titanium ion in the simulated body fluid was assayed using a standard curve of visible spectrometry (Shanghai 722, the 3rd of Shanghai Analytical Instrument Manufacturer, Shanghai, P.R. China) at 385 nm. A standard curve of Ti-ion concentrations, ranging from 0 to 2 mg/mL, was measured using the same testing method as described below; TiO2 was used to produce the standard solutions, which were done by dissolution of analytically pure TiO2 powder (Beijing Chemical Industry Company, Beijing, P.R. China) in 1:1 HCl.
The principle for this method is that the mixture of Ti ion and diantipyrylmethane in acidic conditions (0.25–2.00 mol/L of H2SO4) causes formation of a yellow complex compound. The measurements were performed three times for each sample, and the final value was the average. In detail, 1.5 mL per sample was transferred to a 25-mL flask, 2 mL of 1:1 H2SO4, and 2 mL of 50% ascorbic acid was added; the solution was incubated for 2 minutes; then 10 mL of 2% diantipyrylmethane was added; and finally the sample was diluted to 25 mL with distilled water. The resultant solution was incubated in a boiling-water bath for 2 minutes then cooled to room temperature. The absorbance of each sample was measured using Shanghai-722 Visible Spectrometry at 385 nm, and the corresponding Ti-ion concentration was calculated from that standard curve.
Results - Discussion - References.
RESULTS.SEM Observation of Surface Topography.
Dramatic differences were found among the three surfaces: groups S, SB, and SBA. Figure 1 shows that the mechanically polished surface (S group) appeared smooth with some small parallel scratches due to grounding. However, the SB surface was rather rough with an irregular and sharp-edged contour and many Al2O3 particles embedded in it, which were identified by energy dispersive electron probe x-ray analysis. Compared with the surface of group SB, the topography of the SBA rough surface was much more regular, and its contour was rounded. Furthermore, numerous secondary micropores, 2.0 mm in diameter, were created and inhabited the rough macrotexture. Embedded Al2O3 particles could not be detected. XRD Analysis of Samples.
The XRD patterns of various groups are shown in Figure 6. The polished samples were composed of pure titanium, corresponding to JCPPS 50682. The sandblasted samples consisted of biphasic structures of Ti and Al2O3. The relative amount of the latter was 11.8% . Meanwhile, there was no phase of Al2O3 in the group SBA samples. Instead, a slight amount of TiH2 was detected other than the Ti phase. Test of Ti-Ion Release.
Figure 10 shows that there was a statistical difference in titanium ion concentrations among these three surface groups. The highest was the concentration of group SB, 0.65 6 0.014 mg/mL, significantly higher than the titanium ion concentrations of the two other groups (P , .01). The lowest titanium ion concentration was in group SBA, 0.14 6 0.028 mg/mL, even lower than the smooth group (P , .01), which was 0.23 6 0.020 mg/mL. DISCUSSION.
As a conventional surfaceroughening technique, sandblasting is done using a high-speed sputtering of corundum (Al2O3) grits at a high pressure. Compared with coating methods, it has some advantages in modifying dental implant surfaces. It is a simple process that has no additional interface between the implant and the surrounding bone. There is no additional damage to the physiochemical properties of implants that might be caused by the high temperature of the coating process. Nevertheless, the defects of a sandblasted surface severely compromise those potential advantages. The rough surface contour is irregular and sharp. The sharp edges and tips could adversely affect osteogenesis at the implant-bone interface. Second, the titanium ion-releasing rate of an implant is increased by sandblasting treatment because of the enlarged surface area and the severely uneven distribution of surface tension. In this study, the titanium ion-releasing in vitro rate of sandblasted titanium surface is about two times higher than that of the smooth surface group. The increased titanium ion concentration around dental implants has been demonstrated to be detrimental to osseointegration. Third, the embedded particles caused by sandblasting bring some additional elements to the treated surface and pollute it. Although a small number of embedded particles generally do not affect the purpose in industrial processing, in biomaterials, the heteroelement pollution could possibly destroy the biocompatibility of implants. To avoid the adverse effects of element pollution on biocompatibility, some researchers specifically selected TiO2 as a blasting grit. TiO2 decrease the element pollution, but it does not improve the topographic and corrosive properties of the SB surface. Therefore, the prerequisite for the implantological application of sandblasting is to
- modify the surface,
- eliminate defects, and
- promote advantages.
With these aims, acid etching was used to modify the sandblasting technique. Oxalic acid was used for this purpose; and an oxalic acid– attacking method was developed. This study shows that the oxalic acid processing attack can eliminate the defects of single sandblast treatment and endow the sandblasted surface with some new advantageous properties: specific treatment of oxalic acid attack
- rounded the sharp-curved contour of the sandblasted surface;
- made the topography of that rough surface much more regular;
- removed the embedded particles; and
- created numerous secondary micropores on the basis of the rough macrotexture.
The existence of the numerous secondary micropores is considered to be important for improving the interfacial biomechanics of implants. It has been shown that the secondary micropores of the modified sandblasted surface were filled with the calcified bone matrix and that there was no observable gap between them. Except for the surface appearance changes, the anticorrosiveness of titanium was significantly increased by the oxalic acid attack, even beyond the level of the smooth surface. The Al2O3 pollution was removed. There was no Al2O3 phase detected on the modified sandblasted surface by XRD, and no Al2O3 particles were observed by SEM. This suggests that the specific oxalic acid attack can remove the embedded particles from the sandblasted titanium surface.REFERENCES.
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