Titanium Dental Implant Surface Micromorphology Optimization

Branemark et al1 started the new era of modern implant dentistry in 1969 when they published their first findings about titanium dental implants. Since then, the shapes and surfaces of titanium implants have changed. Excellent titanium biocompatibility ensures good tissue integration.2^,3 Baier et al4 discussed what features play the most significant role in early acceptance and immobilization of the implant in the tissue bed. Texture, charge, and chemistry of the surface as well as cleanliness were considered to be the most important requirements for the implant material.5 Predecki et al6 observed rapid bone growth and good mechanical adhesion with an implant that had an irregular surface. Bowers et al7 confirmed these findings in a histologic study. Many researchers have been working during the last decade on the development of new surface textures in attempts to improve primary implant stability and bone healing.8,^–19 

Titanium implant surfaces have been modified by additive methods (eg, titanium plasma spraying) to increase the surface area and provide a more complex surface macrotopography. Subtractive methods (eg, blasting, acid etching) have also been used to increase the surface area and to alter its microtopography or texture.20 Buser et al21 analyzed the percentage of direct bone-implant contact for different surfaces: sandblasted, hydroxyapatite coated, titanium plasma sprayed, and acid etched (and different combinations of these processes). The highest percentage of bone-implant contact was recorded at the sandblasted surface treated by acid etching (hydrochloric and sulfuric acids). Acid etching of titanium is of particular interest because it creates a microtextured surface (fine rough surface with micropits of 1 to 3 μm and larger pits of approximately 6 to 10 μm) that appears to enhance early endosseous integration and stability of the implant.22 This may be related to a change in surface roughness and/or chemical composition.13 It has also been shown in rabbits that implants with a macrotextured surface (significant waviness with large elements of 10 to 30 μm and peaks of different size) provided better endosseous integration.20 A sandblasted and acid-etched surface (SAE) provides both the microroughness and waviness that seem to enhance bone contact with the implant surface. The titanium surface was first sandblasted with large particles, creating a grossly rough surface, followed by acid etching, which formed a microrough surface.23 

The purpose of this investigation was to create different implant surface textures using acid etching only, which would result in a surface similar to that gained by combining sandblasting with acid etching. The experimental surface was then compared with surfaces of commercially available screw-type implants.

Titanium grade 5 disks (8 mm in diameter and 2 mm in height) were machined in preparation for acid etching. All disks were etched using acids, either alone or in combination (Table 1). A series of etching processes was performed, with the duration of exposure and acid combination changed. Exposure times were as follows: 12 hours initial exposure followed by 6-hour increments until 72 hours of exposure were reached.

The titanium disks were etched with 4 different pure acids or a combination of these at 11 different exposure times at +20°C. Thus there were 44 experimental groups with 5 samples in each group, for a total of 220 disks.

All surfaces were examined with a scanning electron microscope (SEM) (JEOL JSM-5600, Tokyo, Japan) using ×27, ×200, and ×2000 magnification. Digital images were made for visual examination of the surfaces.

The machined implant surfaces were first characterized using the SEM. The surfaces were oriented in the direction of the machine grooves and the surface was rated on the degree of etching. Surface orientation was designated as unidirectional when the machining grooves were still present. When the machining grooves could not be distinguished, the surface was characterized as complex.

Another important indicator was regularity of etching. If the surface was etched unequally and had intact areas, it was characterized as an irregular surface. An equally etched surface was characterized as a regular or uniformly etched surface.

Digital photos were evaluated on the principle that darker spots represented pits and lighter ones represented peaks. The diameter but not depth of the pits was measured. Micropits of 1 to 3 μm and larger elements of approximately 6 to 10 μm formed the microtexture of the surface. The term “microtexture” was used to characterize the roughness of titanium surfaces, whereas “macrotexture” consisted of large elements of 10 to 30 μm and was characterized as waviness.

From these results, the etching method that achieved a surface most similar to an SAE surface was selected as the most acceptable. The profile of the selected surface was additionally evaluated visually with the SEM. Prepared titanium disks were fixed in acrylic resin. Two were cut and polished, and 2 were scored and fractured without polishing; a detailed examination of the surface profiles was then performed. Both profiles appeared to have significant roughness.

The second portion of the study investigated screw-shaped titanium implants. Twenty-eight screw-shaped implants with 4 threads each were manufactured from commercially available titanium grade 5 (3.5 mm in diameter and 6 mm in length). Three were chosen as controls (machined surface), and 25 implants were etched using the method created earlier on the titanium disks. Five series of surface-etching process were performed on 5 implants in each series. Exposition and temperature of the etching process were controlled for creation of new standardized implant surfaces.

Implants were ultrasonically cleaned prior to examination. Implants with the experimental surface were examined with the SEM using ×27, ×200, and ×2000 magnification, and digital images were made for visual evaluation according to the previously stated principles. The first 2 consecutive crests of threads, flanks, and roots of threads of each implant were analyzed.

A 3-dimensional optical interferometer (Micro-Xam, Phase-Shift/ADE, Tucson, AZ) was used to characterize the surface roughness of both control and test group implants. The surfaces of 3 implants in each group were analyzed topographically according to the method proposed by Wennerberg and Albrektsson.24 Three screws were selected from each group, and each screw was measured at 9 sites (3 times each on the thread crest, root, and flank). Each measured area was 200 × 200 μm. A Gaussian filter of 50 × 50 μm was used to distinguish between roughness and form or undulations in accordance of the requirements of the ISO standard (SS-ISO 11562:1996). To describe the surface roughness in numbers, the following parameters were used: average height deviation (Sa), developed interfacial area ratio (Sdr), fastest decay autocorrelation length (Sal), and density of summits (Sds). The surface of 5 different commercially produced screw-shaped implants23 and the implant with the experimental surface were comparatively analyzed by taking measurements at the same sites.

Statistical analyses were performed using the SPSS/PC+ version 10.0.1 program (SPSS Inc, Chicago, Ill). Means and standard deviations were calculated.

The titanium test disks were examined visually and described as follows.

The control group (machined surface) had regular unidirectional grooves with some irregular shallow roughness (Figure 1a).

Group 1 (etched with hydrochloric acid [HCl]) had a microtexture that was poor, without evidence of micropits (Figure 1b).

Group 2 (etched with HCl and sulfuric acid [H₂SO₄]) yielded a rather rough surface, but the microtexture was poor, with few micropits and smooth waviness (Figure 1c). The length of time that groups I and II were subjected to their respective acids did not change the surface texture.

Group 3 (etched with H₂SO₄/HCl and phosphoric acid [H₃PO₄]) yielded an interesting surface that showed distinct waviness without microtexture (Figure 1d).

Group 4 (etched with H₂SO₄ for 72 hours and HCl for 30 hours) showed significant surface roughness, with micropits of 1 to 10 μm and large valleys of 20 to 30 μm with peaks of different sizes (Figure 1e). The waviness and roughness of the surface were regular and without intact areas.

Evaluation of the surface profile in the cut and polished (Figure 2a) and scored and broken (Figure 2b) groups showed that the surface was rough with small depressions and prominences of 1 to 10 μm that were visible in profile. Wide trenches of 30 μm could be seen (Figure 2).

The group 4 acid-etching method (etched with H₂SO₄ for 72 hours and HCl for 30 hours) was selected for further investigation with screw-shaped titanium implants. Digital topographic images (Figure 3) were created so that the machined and acid-etched surfaces on screw-shaped titanium implants could be compared. The surfaces of the machined titanium implants were examined; these implants had mainly unidirectional machining grooves and ridges (Figure 3a). The acid-treated titanium implants showed roughness and waviness that were evenly spread over the entire surface (Figure 3b). Surface texture was characterized by regularly distributed peaks and valleys.

Electron microscopic scans of the machined surface of a control implant showed grooves, which were more pronounced on thread crests than at the roots or flanks of the threads (Figure 4a). The unidirectionality of deep grooves and ridges remained from the machining process. The implant surface treated with sulfuric and hydrochloric acids was found to have a very complex surface without any intact areas, but the roughness of the surface was more pronounced at the crests and flanks (Figure 4b and 4d) than in the root areas (Figure 4c).

Measurements with an optical interferometer established that Sa, Sdr, Sal, and Sds were significantly greater on acid-etched surfaces than on machined surfaces (Table 2). It is worth noting that the roughness of the acid-treated surface was significant, but, again, the roots were smoother than the crests or the flanks.

The surfaces of 5 commercially produced screw-shaped implants23 and the implant with the experimental surface were comparatively analyzed by taking measurements at the same sites (Table 3). The results showed that all commercially available implants had the smoothest surface at the flanks, whereas the flanks of the experimental implants were the roughest (Sa 2.24 ± 0.8 μm). It is worth noting that the experimental implant screws were generally rougher than other commercially available implants. However, the surface enlargement (Sdr) was rather similar to that of the SLA implant (sandblasted/acid-etched surface, Institut Straumann, Waldenburg, Switzerland).

Commercially available implants are present with several surface texture types. This study resulted in the development of a surface texture using acid etching technology. It has been shown that finely pitted (micropits of 1 to 3 μm and larger elements of approximately 6 to 10 μm) surfaces result in early enhancement of bone-implant integration.20 

Studies by Wennerberg et al12^,24,^–26 demonstrated that an optimal surface roughness (75-μm particles) made surfaces more resistant to torque and resulted in greater bone-to-metal contact than small (25-μm) or coarse (250-μm) particles. The optimal surface had an average height deviation of about 1.5 μm, resulting in a surface enlargement of 50%.

Implants with macrotextured surfaces (eg, plasma sprayed or hydroxyapatite coated) have shown enhanced bone-to-implant contact during the late osseointegration period.25 Some authors have reported erosion of the hydroxyapatite layer27 and peri-implant bone loss, resulting in a higher failure rate12^,28 for implants with hydroxyapatite-coated surfaces. On the other hand, Buser et al21 showed that implants with sandblasted and acid-etched surfaces had higher bone-to-implant contact percentages than implants with titanium plasma–sprayed surfaces. This confirms the presumption that a significantly roughened surface (titanium plasma–sprayed surface) does not by itself stimulate early bone and implant integration. The titanium surface was first sandblasted with large particles, creating a significantly rough surface that was then acid etched, forming a finely rough surface. This surface texture improved primary implant stability in bone of low density and improved the quality of the bone-to-implant interface.25 It should be emphasized that this titanium surface was gained using 2 methods of processing: sandblasting and acid etching. The probability of surface contamination and of microparticle dissemination into the surrounding tissues is extremely low.29 The study of Diniz et al30 showed that characterization of the titanium surface is essential in the evaluation of the material manufacturing process, because the presence of residual aluminum particles may have deleterious effects on the formation of the osseous peri-implant tissue. Furthermore, Mueller et al31 proved that metal-bone contact showed a tendency for more bone when bioceramics, rather than aluminum oxide, were used as blasting materials.

The present study used a single method—acid etching—to create a new titanium surface that included all the aforementioned surface texture features. The present study showed that precise selection of acids and of the sequence of the etching process played primary roles in preparation of the rough titanium surface. The surface was less rough if it was etched with HCl and then H₂SO₄. Very similar results were demonstrated when processing implants with HCl only or with H₂SO₄/HCl and then H₃PO₄. H₂SO₄ and HCl applied in sequence and time showed the best results. The topography of the newly created titanium surface was very much like that of a sandblasted and acid-etched surface. It combined the main properties of a roughened titanium surface: glossy microroughness and pronounced waviness. In general, the experimental surface was rougher that of than commercially available implants.

Although the implant surface created using specific acid-etching methods resembles a surface created by both sandblasting and acid etching, further research is necessary to study the biologic response to it.