Physico-chemical characterization of the surface of 9 dental implants 
with 3 different surface treatments

RODRÍGUEZ-RIUS D, GARCÍA-SABAN F.J. PHYSICO-CHEMICAL CHARACTERIZATION OF THE SURFACE OF 9 DENTAL IMPLANTS WITH 3 DIFFERENT SURFACE TREATMENTS. MED ORAL PATOL ORAL CIR BUCAL 2005;10:58-65.

 

ABSTRACT

There are many surface treatments applied to dental implants. The aim of the present investigation is to compare the physico-chemical characteristics of titanium dental implant surfaces with different surface treatments. 9 dental implants from the same batch were divided in 3 groups and received 3 different surface treatments: machined, acid etched and a new chemical surface treatment called Avantblast^®. Scanning electron microscopy and confocal microscopy were used to image the treated surfaces, and energy-dispersive spectrometry and X-ray photoelectron spectrometry to provide a chemical characterization of the surfaces.
Results:
The acid etched and chemical etched surfaces had an increased roughness over the machined one. Surface chemical composition had differences between processes, as the surface with the new treatment presented a reduced level of impurities and increased thickness of the titanium oxide layer
Conclusions:
Surface roughness of titanium dental implants and thickness of the titanium oxide layer can be increased with a suitable surface treatment.

Keywords: Dental implants, surface treatment, roughness, titanium oxide

 

INTRODUCTION

In the last years different surface treatments have been developed in order to achieve an increase of surface roughness on titanium dental implants, such as TiOblast^®(1), SLA^®(2), Osseotite^®(3) or TiUnite^®(4), for example. Application of surface treatments to increase the dental implant surface roughness (5-8), proves that implant osseointegration at immediate and early stages is improved with a micrometric surface roughness. This improvement is possibly related, as indicated by some studies (8-11), to the existence on the implant surface of a microroughness that promotes cellular adhesion, increased cell differentiation as well as improved osteoblast expression. This effect produces an improved osseointegration and more bone formation.

Other surface modifications recently developed aim to increase the thickness and crystallinity of the titanium oxide surface layer, as some studies suggest the existence of a relationship between an increase of titanium oxide thickness and/or crystallinity with protein adsorption to the surface (12, 13), increased differentiation and osteoblastic growth (14, 15) and better osseointegration of the treated implants (16, 17).

A new surface treatment, Avantblast^® (Impladent, Sentmenat, Spain), has been developed. It combines the improvement of osteoblast response achieved with an increment of surface roughness with the advantages of an increased thickness and crystallinity of the titanium oxide layer. The increment of roughness is attained with an homogenization of surface stresses and chemical etching of the surface, while the increase in thickness and crystallinity of the oxide layer is due to a thermal treatment (14, 18, 19). In the present work three different surface treatments, machined, acid etched and Avantblast^®, have been studied. The analysis included morphology, chemical surface composition and roughness produced by the different treatments, as well as the thickness of the titanium oxide layer.

MATERIALS AND METHODS

9 Defcon^TM TSA^® dental implants (Impladent, Sentmenat, Spain), machined in grade 2 titanium, were selected from the same batch. They were washed and divided in 3 groups of 3 implants each.

The first group of 3 implants (“machined”) did not receive any surface treatment. The next group (“acid etched”) was acid etched with a aqueous solution of hydrofluoric acid, followed by passivation with an aqueous solution of hydrofluoric and nitric acids, as described in ASTM F86 standard. The third group (“Avantblast”) was treated in two steps. An homogenization of surface stresses was followed by chemical etching of the surface with an aqueous solution of hydrofluoric and sulfuric acids. Afterwards, the implant received a thermal treatment.

The prepared samples were individually packaged and gamma-ray sterilized with a minimum dose of 25kGy from a Cobalt-60 source (Aragogamma S.A., Granollers, Spain). Afterwards the samples were analyzed without any other preparation in the region of the implant designed to contact bone.

Surface finishing of the treatments was examined with a scanning electron microscope Jeol JSM-840 (Serveis Científico-Tècnics, Universitat de Barcelona). The beam acceleration voltage was 15kV. Magnification ranged from 14-10.000 x.

Surface chemical composition was studied with two complementary techniques in a flat zone at the end of the implant (1 implant per group). It was analyzed by energy-dispersive spectrometry (EDS) with a Link-Inca equipment attached to a Leica Electroscan 360 SEM (Serveis Científico-Tècnics, Universitat de Barcelona), as well as with an X-ray photoelectron spectrometer (XPS) Physical Electronics 5500 (Serveis Científico-Tècnics, Universitat de Barcelona). The XPS detects the chemical composition of the outer nanometers of the surface. The equipment operated with a monochromatic AlKα radiation in a ultra-high vacuum of 0.6·10^-6 Pa. Electron detection angle was 90º for all measurements. The instrument was calibrated by measuring the reference peak Ag3d5/2 (367.8eV) and 0.44eV of mean peak amplitude with aluminium as excitation source. An argon ion cannon was used for in situ surface ion milling as well as to measure the thickness of the titanium oxide layer, with a mean milling speed of 6nm/min.

Roughness was measured, both surface and profile, on three places of the screw and the flat zone of the implant end (1 implant per group) with a white light confocal microscope Sensofar^® PLµ (Sensofar, Terrassa, Spain). This equipment consist of a Nikon L150 microscope with a SLWD20x objective and a reconstruction and analysis software. The equipment has a vertical reproducibility under 20nm and a lateral resolution of 0.91µm.

For each zone were reported, as surface roughness parameters, the mean roughness (S_a), the root mean square (RMS) roughness (S_q), the maximum peak value (S_p), the surface void volume (S_v) and the maximum peak-valley distance (S_t). The profile parameters were recorded after applying a gaussian filter and a cut-off filter of 800 µm to the profiles measured following the DIN 4768 standard. The reported parameters were the average roughness (R_a), the RMS roughness (R_q), the maximum profile peak height (R_p), the maximum profile valley depth (R_v), the maximum height of the profile (R_t) and the mean spacing of profile peaks (S_m). An analysis of variance (ANOVA) was applied as well as the Tukey post hoc comparative test to the parameter S_a in order to study statistically the roughness differences between sample groups.

RESULTS

The analysis of the three groups of implants shows the presence of important differences of surface properties due to the applied surface treatments.

Figure 1 illustrates the surface morphology of the individual surfaces. At low magnification, all implants show a well defined geometry. At higher magnifications (2000x) differences in topography become apparent. The implants from the “machined” group present a surface with parallel valleys, likely the result of machining (figure 1a). The surface of the implants from the group “acid etched”, much rougher than the “machined” implants, had a surface with blocks due to the preferential attack of the hydrofluoric acid (figure 1b), while the “Avantblast” group of implants presented a surface morphology with cavities of sharp edges and a marked porosity evenly distributed, due to the action of the chemical reactive (figure 1c).

Measurements of the chemical composition with EDS did not provided a discrimination between the studied surface treatments, as it just detected titanium and oxygen in all the groups of samples. However, the XPS analysis detected differences between surface treatments (table 1). The results show a reduced presence of carbon and other impurities on the “Avantblast” batch when compared to other surface treatments (20, 21).

Titanium oxide layer thickness was found to be three times higher in the “Avantblast” group implants than in the other two implant groups (table 1).

Implant roughness parameters are shown in tables 2 and 3 (mean and standard deviation of the four measurements per batch). Implant batches “acid etched” and “Avantblast” had statistically rougher surface (p-value: 0.000) than the “machined” one. In this batch, the roughness is caused by the waviness inherent to the machining operation (figure 2a).

The chemical attack in the implant batch “acid etched” increased roughness up to values over 1 mm (figure 2b). An important component of the acid etched surface roughness are prominent peaks and valleys (with high S_p and R_p values), that notably increase the mean roughness.

The implant batch “Avantblast” (figure 2c) had roughness values statistically higher than the two other surfaces (Tukey comparative test, p-value < 0.05). Implant roughness in this batch is also due to a chemical etching, although more homogeneous than the “acid etched” batch, as shown by comparison of the parameters maximum peak and maximum profile peak height (S_p and R_p).

DISCUSSION

Some studies (5, 22) have shown that a dental implant with a low roughness value, like the measured on the “machined” implant batch, promotes the formation of fibrous tissue around the implant, reduces bone-implant contact and has a lower removal torque value than implants with a rougher surface. However, mean roughness values of 1 mm or above, like the values measured for “acid etched” and “Avantblast” batches, improves bone bonding to the implant surface (5, 10).

The Avantblast^® surface fulfill the dental implant roughness requirements for a good anchoring of bone cells to the surface (5-11, 23), as well as providing an homogeneous roughness to the treated surface. Besides, the morphology is close to a theoretical optimum for implant retention, as deducted from biomechanical principles, consisting of a surface with rounded cavities (24).

XPS chemical analysis, as a more sensitive technique, has allowed the detection of chemical elements on the implant surface not detected by EDS. The presence of organic contamination (carbon) on every studied surface is unavoidable, as atmospheric hydrocarbons are readily adsorbed on exposed titanium surfaces. However, a high presence of carbon could signal the presence of lubricant or another fabrication contaminants. The presence of elements not related to the surface treatment, like sodium, chlorine or silicon, is also an indication of the presence of remaining impurities after the cleaning process.

The increased thickness of the titanium oxide layer in the Avantblast^® surface treatment is achieved with a thermal treatment that favors the oxygen diffusion into titanium. The increased thickness reduces the ion release into the medium, as the titanium oxide layer acts as a barrier for the ion diffusion (25), and reduces ion release exponentially (26, 27).

The increase of titanium oxide thickness and/or crystallinity has also been related with an increase of protein adsorption to surface (12, 15), affecting the cell response. For example, chondrocytes are sensitive to the crystallinity of TiO2 and its response improves with titanium oxide thickness and/or crystallinity (15). In vivo response to an increment of the titanium oxide thickness is also positive, as doubling the thickness by means of thermal treatment doubled the bone-implant bonding strength (14).

It is expected that the merging of both implant surface modifications in the Avantblast^® treatment has as a consequence an increased osseointegration and bone formation in contact with the surface of the implants when compared with other surfaces.

CONCLUSIONS

Surface roughness of titanium dental implants can be increased by means of acid etching surface treatments. The surface treatment Avantblast^®, in addition to producing a surface roughness of micrometer range, triple the thickness of the titanium oxide layer, and reduces the presence of impurities on the surface.

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