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

1. modify the surface,
2. eliminate defects, and
3. 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

1. rounded the sharp-curved contour of the sandblasted surface;
2. made the topography of that rough surface much more regular;
3. removed the embedded particles; and
4. 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.

1. Albrektsson T, Branemark P-I, Hannsson HA, et al. Osseointegrated titanium implants: Requirements for ensuring a long-lasting direct bone anchoring in man. Acta Orthop Scand. 1981;52:155– 170.
2. Adell R, Lekholm U, Rockler B, et al. A 15-year study of osseointegrated implants in the treatment of the edentulous jaws. Int J Oral Surg. 1981;10:387– 416.
3. Lazzara RJ, Porter SS, Testori T, et al. A prospective multicenter study evaluating loading of Osseotite implants two months after placement: One-year results. J Esthet Dent. 1998;10:280–289.
4. Buser D, Belser UC, Lang NP. The original one-stage dental implant system and its clinical application. Periodontol 2000. 1998;17:106–118.
5. Brunski JB,. Forces on dental implants and interfacial stress transfer. In: Laney WR, Tolman DF, eds. Proceedings of the Second International Congress on Tissue Integration in Oral, Orthopedic, and Maxillofacial Reconstruction. Chicago: Quintessence ; 1990:108–124.
6. Deporter DA, Friedlant B, Watson PA, et al. A clinical and radiographic assessment of a porous-surfaced, titanium alloy dental implant system in dogs. J Dent Res. 1986;65:1071–1077.
7. Piattelli A, Paolantonio M, Corigliano M, et al. Immediate loading of titanium plasma-sprayed screw-shaped implants in man: A clinical and histological report of two cases. J Periodontol. 1997; 68:591–597.
8. Tarnow DP, Emtiaz S, Classi A. Immediate loading of threaded implants at stage 1 surgery in edentulous arches: Ten consecutive case reports with 1- to 5-year data. Int J Oral Maxillofac Implants. 1997;12:319–324.
9. Balshi TJ, Wolfinger GJ. Immediate loading of Branemark implants in edentulous mandibles: A preliminary report. Implant Dent. 1997;6:83–88.
10. Wilke HJ, Claes L, and Steinemann S. The influence of various titanium surfaces on the interface shear strength between implants and bone. In: Heimke G, Soltesz U, Lee ALC, eds. Clinical Implant Materials. Advances in Biomaterials, Vol 9. Amsterdam: Elsevier Science Publishers; 1990:309–315.
11. Martin JY, Schwartz Z, Hummert TM, et al. Effect of titanium surface roughness on proliferation, differentiation, and protein synthesis of human osteoblast-like cells (MG63). J Biomed Mater Res. 1995;29:389–401.
12. Watzek G. Preprosthetic and periimplant surgery–general principles. In: Watzek G, ed. Endosseous Implants: Scientific and Clinical Aspects. Chicago: Quintessence; 1996:183–209.
13. Yue S, Pilliar RM, Weatherly GC. The fatigue strength of porous-coated Ti-6%AL-4%V implant alloy. J Biomed Mater Res. 1984;18:1043–1058.
14. Ducheyne P, Willems G, Martens M, et al. In vivo metal-ion release from porous titanium-fiber material. J Biomed Mater Res. 1984;18:293–308.
15. Thomas KA, Kay JF, Cook SD, et al. The effect of surface macrotexture and hydroxylapatite coating on the mechanical strengths and histological profiles of titanium implant materials. J Biomed Mater Res. 1987;21:1395–1414.
16. Li D-H, Liu B-L, Zou J-C, et al. The histological study of the effects of modified sandblasting surface treatment on the osseointegration of titanium dental implant. Pract J Stomatol. 1999;15:431–433.
17. Li D-H, Liu B-L, Zou J-C, et al. Improvement of osseointegration of titanium dental implants by a modified sandblasting surface treatment: An in vivo interfacial biomechanics study. Implant Dent. 1999;8:289–294.
18. Healy KE, Ducheyne P. Hydration and preferential molecular adsorption on titanium in vitro. Biomaterials. 1992;13: 553–561.
19. Weinggart D, Steinemann S, Schilli W, et al. Titanium deposition in regional lymph nodes after insertion of titanium screw implants in maxillofacial region. Int J Oral Maxillofac Surg. 1994;23: 450–452.
20. Gotfredsen K, Wennerberg A, Johansson C, et al. Anchorage of TiO2- blasted, HA-coated and machined implants: An experimental study with rabbits. J Biomed Mater Res. 1995;29: 1223–1231.
21. Han CH, Johansson CB, Wennerberg A, et al. Quantitative and qualitative investigations of surface enlarged titanium and titanium alloy implants. Clin Oral Implants Res. 1998;9:1–10.
22. Wong M, Eulenberger J, Schenk R, et al. Effects of surface topology on the osseointegration of implant materials in trabecular bone. J Biomed Mater Res. 1995;29:1567–1575.