General appearance

The polished Ti discs had a smooth surface with a mirror finish but no distinct tint or colour. This indicates that the polishing and cleaning procedures do not alter the surface visibly. Lausmaa et al stated that naturally formed titanium oxides are too thin to give rise to any interference colours (Lausmaa at

a/. 1985). Ungersbock at a!, demonstrated that the surface appearance and tint of CP Ti samples varied according to preparation methods. Samples that were anodized to a fine finish displayed a gold colour as compared to samples anodized then roughened by blasting which were darker in colour. Ti plates that were hand ground, blasted with AI2O3 or electropolished did not display any interference colours. The authors used reflecting light microscopy to gain information on the thickness of the oxide layers, which will result in the Ti appearing with different colours. The gold / yellow tint observed on smooth Ti

metallic Ti delimited by metal grain boundaries of the oxide layer (Ungerbock et al. 1994). These surface discolorations are a result of light reflected from varying layers of the surface (namely the oxide layer and the underlying Ti substrate), which can either be in-phase or out-of-phase resulting in constructive or destructive interference respectively and hence the surface tint. Lausmaa at a! offered two possible explanations to observed blue and brown discolouration on Ti surfaces following autoclaving. These were thought to be either light absorption due to contamination layers deposition during autoclaving or light interference following oxide growth >

0.01

pm

(>100 A)

(Lausmaa at ai.

1985). Keller attributed the dark blue discolouration on autoclaved Ti surfaces to thickening of the oxide layer in the region of 0.025 - 0.07 pm (250 - 700

A)

(Keller at ai. 1990). As the samples produced in the current work did not display such colour changes and based on the XPS findings, the thickness of the oxide layer could be estimated at <100 A.

4 2.3.2 SEM

It appears that although care was practiced during handling of the polished CP Ti samples, scratches were still apparent on the surface as shown by SEM. This is due to the softness of Ti and poor abrasion resistance. The surface is characterised by pits and grooves, with similar features displayed on different areas of the same disc. Although these surface features are not uniformly distributed across the surface the orientation of grooving is generally in the direction of polishing although some are not and appear to be scratches on the surface due to handling. This is in agreement with several reports on similarly prepared surfaces. Ameen at al have studied the surface of CP Ti prepared to either a rough finish to simulate the surface of the fixture, or a

smooth finish similar to that of the abutment. They demonstrated the presence of pitting and pores on all surfaces (although they were of the opinion that these were related to the hydrocarbon overlayer) (Ameen et al. 1993).

Similar features are also reported for other Ti grades. Lincks at al.

reported on the surface characteristics of Grade 2 CP Ti polished to 1200 grit using AI2O3 paper, where small pits of up to 2 pm diameter and randomly oriented scratches were seen on the surface and thought to arise from the polishing procedure (Lincks at al. 1998). Grade 4 CP Ti samples polished using 600 grit SiC with 1 pm finish displayed striations and pits using SEM which the author believed to have resulted from the mechanical polishing process (Placko

at al. 2000). Similar findings were reported for clinically prepared Ti surfaces.

CP Ti samples prepared using the Brànemark procedure of electropolishing and cleaning illustrated grooving and/or steps of different width and height as well as pore-like structures when viewed using SEM (Baro at al. 1986). A group of researchers reported that even with electropolishing, CP Ti surfaces had small pores and structural inhomogeneities (Ungerbock at al. 1994). The topography of implant fixtures was studied by Esposito at al. who reported the presence of grooves and ridges along machining direction (Esposito at al. 1999). This was also reported by Mouhyi at al. who describe the grooved appearance of two fixtures to be a result of machining, adding that the surface illustrated pits at higher magnification (Mouhyi at al. 1998). In a more recent study of dental fixtures, the surface was found to be characterized by grooves and ridges of widths less than 10 pm and pits (induced by machining) that were oriented in the turning direction (Sul atal. 2002).

4.2.3 3 WLI

It is now established that surface roughness will influence the biological response around the implant, as porosities in the region of >10 pm will provide mechanical interlocking allowing in-growth of tissues. Surface topography on a smaller scale (10 nm - 10 pm) may also influence the interface biology affecting cellular and biomolecular interactions (Baro et al. 1986, Lincks et al. 1998). The thickness, structure and cellular composition of the soft tissue layers covering an implant will depend on several factors including the roughness of the implant surface.

The Rq roughness values measured by WLI in this work are not significantly different from those reported for CP Ti surfaces prepared in a similar manner. As these samples were polished metallographically, surface variations are to be expected, however these were not found to significantly affect the roughness values of the surface as a minimum of three areas were randomly measured on each disc and were found to be very similar as indicated by the low standard deviation. The roughness (Rq) value on mechanically

polished CP Ti surfaces was reported to be 0.08 pm (Chauvy et al. 1998) while

Ra values on CP Ti surfaces polished mechanically to 1200 grit were reported to be 0.07 pm (Yoshinari et al. 2000). These values are broadly in agreement with the Rq = 0.06 (± 0.02) pm and Ra = 0.04 (± 0.01) pm measured in the current work.

It should be noted however that the roughness values obtained using different measurement techniques can only be compared within the same spatial frequency domain as they are influenced by the measured area, lateral resolution of the equipment used and differences in machining parameters

(Lausmaa 1996, Brunette et al. 2001). To emphasize the role of lateral resolution and measured areas a comparison was offered by Lausmaa on the reported roughness values of laser measurement over areas of 100 x 100 pm^ which resulted in Ra values of 0.5 pm as compared to AFM measurement over areas of 1 x 1 pm^ which gave Rq values of 0.2 pm when same group of researchers used the same Ti substrate to gauge the bone response to Ti surfaces (Larsson et ai. 1994b). The following are reports on the roughness values measured on Ti surfaces prepared using different polishing or grit blasting regimes to highlight the role of mechanical preparation on altering the surface roughness, although these do not offer a direct comparison to the roughness values presented in the current work for the reasons stated above.

Baro reported that the surface roughness of CP Ti prepared by clinical methods and measured using scanning tunnelling microscopy was 0.1 pm, a value expected for machined samples in which large variations in surface topography are commonly observed on a single sample (Baro et ai. 1986). Ungersbock et al. used a profilometer to measure the Rq values on CP Ti surfaces. Those anodized to a fine finish were reported to be at 0.41 pm as compared to values of 0.3 pm for hand ground surfaces and values as high as 1.9 pm for Ti prepared by AI2O3 blasting (Ungerbock et al. 1994). Lincks et al.

measure the surface roughness on CP Ti (Grade 2) prepared to a smooth finish of 1200 grit to have a value Ra of 0.22 - 0.23 pm (Lincks et al. 1998). Roughness values were reported on Ti grade 2 CP Ti surfaces polished to 600 grit using SiC paper. The Ra and Rq values as measured using AFM were 0.14 pm and 0.18 pm respectively (Kilpadi et al. 1998b). Larsson et al. reported Rq

values on CP Ti surfaces of 0.2 pm over an area of 1 x 1 pm^ (Larsson et al.

1994a).

In a recent report, Sul at al. measured the average surface roughness of CP Ti dental implants to be 0.83 pm and reported an increase in surface roughness following anodic treatment (Sul at al. 2002), which again highlights the effect of surface treatment and preparation procedures of the substrate on the macrostructure, microstructure and ultrastructure of the surface as reported by Ellingsen (Ellingsen 1998).

Most of these values are much higher than values reported in the current work which is understandable seeing that the surface preparation of samples used in this research was to a 2400 grit followed by polishing in a suspension of 0.1 pm particle size (Chapter 3).

4.2.S.4 XRD

The most prominent peaks in the X-ray diffractogram indicate the structure of the Ti to be hexagonal. XRD is primarily a technique for the analysis of the bulk structure of a material. Even in the grazing incidence geometry used in the current research, the sampling depth is of the order of 5 - 10 pm. If the surface oxide is largely crystalline and sufficiently thick, it should be possible to see reflections in the diffractograms.

Ti02 can exist in three crystalline forms namely anatase, rutile and less commonly reported, brookite. Table 4.5 lists the 20 angles and the corresponding planes for the strongest rutile and anatase planes as well as hexagonal Ti.

Hexagonal 20 Intensity Rutile 20 Intensity Anatase 20 Intensity

In document Ion Implantation As A Route To Enhancing Osseointegration On Modified Titanium Surfaces (Page 130-136)