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Characterizing three-dimensional topography of engineering and biomaterial surfaces by confocal laser scanning and stylus techniques
Copyright
PII: s1350-4533(95)00005-7
ELSEVIER
Med. Eng. Phys. Vol. 18, No. 7, pp. 548-556. 1996 0 1996 Elsevier Science Ltd for IPEMB. All rights reserved kinted in Great Britain 1350-4533/96 $15.00 + 0.00
Characterizing three-dimensional topography of engineering and biomaterial surfaces by confocal laser scanning and stylus techniques A. Wennerberg *, R. Ohlssont,
B.-G. Rosknt
and B. Andersson*
“Biomaterials Group, Department of Biomaterials/Handicap Research, Institute for Surgical Sciences, Giiteborg University, Medicinaregatan 8, S-413 90 Giiteborg, Sweden; and tchalmers Surface Geometry Group, Department of Production Engineering, Chalmers University of Technology, S-412 96 Gbteborg, Sweden Received
12 May
1995,
revised
13 November
1995, accepted
18 December
1995
ABSTRACT Threedimensional
measurements of surface topography were pe$%rrned using a confocal laser scanner and a contact stylus instrument. Three surfaces known to be dificult to evaluate were chosen to be measured on the same area with the two instruments. The measurements from the optical and the contact stylus projilometer were compared with each other and with measurements obtained from highresolution atomic force microscopy, which served as a reference instrument. Six implants manufactured f?om commonly used biomaterials were also measured on the same part of the implant, but not on the same area, with the optical and the contact profilome~er in order to simulate the measurements that would be perfo77nd when diff erent laboratories measure similarly treated surfaces. The numerical and visual differaces achiezed when measun’ng the same area with the two instruments investigated were compared. In general, we found an underestimation of the suface features with the contact stylus measurement and an overestimation with the confocal scanner. The stylus readings are mainly influenced by the radius of the stylus tip, the pressure of the stylus tip on the surface, and the hardness of the material. The optical projlometer has a tendency for creating spikes when surfaces with deep slopes are measured. For relatively soft metallic biomaterials, we found that using the optical shaped surface
instrument implants
is the most appropriate
method
for surface
Keywords: 3D surface biomaterial surfaces
topography,
stylus profilometer,
Med. Eng. Phys., 1996, Vol. 18, 548-556,
characterization,
particularly
The surface properties of an implant are widely known to influence the local tissue response’- . To be able to control the surface preparation and maintain quality there is an increasing demand for surface characterization. Several methods for surface analysis are now in frequent use. The surface composition is commonly investigated with auger electron spectroscopy @ES), X-ray photoemission spectroscopy (XPS) , and/or secondary ion mass spectroscopy (SIMS)4-6. To evaluate the thickness and composition of the oxide film, AES and ion etching are common methods7-g, and for surface morphology, cellular as well as material, scannin electron microscopy (SEM) is often usedlo-’ 8. For surface roughness characterization, several systems have been used in evaluating biomaterials1z-14. The most commonly used method is a two-dimensional (2D) profile measurement to: Ann
Wennerberg,
DDS.
confocal
laser
scanner,
when screw slopes within the
engineering
and
October
INTRODUCTION
Correspondence
roughness
are analysed, whereas the stylus is pefared when larger areas with substantial structure are to be evaluated. Copyright 0 1996 Elsevier Science Ltd for IPEMB.
with a contact instrument. However, 2D measurements are not always enough to describe a surface, and increasing numbers of devices for 3D measurements are now commercially available. All measuring systems, 2D as well as 3D, have limitations in terms of wavelengths and resolution, and it is difficult to select the most appropriate instrument. In the biomaterials field several authors have published values for surface roughnesses of experimental as well as commercially available implants, measured with different kinds of instruments’5-1s. This raises the question of whether it is possible to compare results from one instrument with those obtained from another. The purpose of the present study is to compare two, in principle quite different, devices adapted to 3D surface roughness measurements in the micrometre scale, when measuring different kinds of machined surfaces. In part 1 of this investigation, three different surfaces were measured with the two instruments on the same areas. This
procedure enables comparison of measuring techniques with respect to different surface types. Furthermore, an AFM instrument was used as a reference system. In Part 2 we compared measurements on non-relocated (different) areas for the purpose of investigating the differences that ma.y occur when different investigators measure simllar surfaces. MATERTALS Surface
AND
measuring
METHODS equipment
A mechanical contact measuring instrument, PerthometerTM C5D with a PRK drive unit (Feinpriif Perthen GmbH, GGttingen, Germany) and an measuring device, TopScan 3DTM optical (Heidelberg Instruments, Heidelberg, Germany), were used in this study. Contact method. For this method a stylus tip is drawn along a surface with constant speed and pressure. The Perthometer instrument has a possible measuring range of 60 x 300 mm, Z-ranges of f2.5-625 pm and a measuring force of 5 mN. The measuring range used was, in part 1: 2 x 2 mm in the X and Y directions, +25 pm in Z in part 2: 0.250 x 0.250 mm in the X and Y directions, f25 pm in Z. The stylus has a diamond with a radius of 5 pm. The resolutions used in the X and Y directions were 5 pm, the resolution used in % was 0.01 pm. This equipment is a traditional 2D instrument complemented with a Y steppingtable to enable SD measurements, as described by Bengtsson I”. Non-contacting method. The TopScan 3D instrument uses the principle of laser confocal scanning microscopy2”. A He-Ne laser beam works as an optical stylus, the diameter of the laser beam being about 1 pm. The possible measuring ranges are 0.25-2 mm in the X and Y directions, 25108 pm in Z. The measuring range used was, in part 1: 2 x 2 mm in the X and Y directions, 54 pm in 8 in part 2: 0.250 x 0.250 mm in the X and Y directions, 108 pm in Z. The resolution used in the X and Y directions was 5 pm, 0.01 pm in Z. The magnification of the objective was x20. The laser spot is scanned across the fixed specimen. To be able to distinguish between the true surface features and features missed or created by the above measuring systems, we used an AFM, the Rasterscope 4000TM (Danish Micro Engineering A/S, KDbenhavn, Denmark). The AFM provides very high-resolution images compared to the other two instruments used in this study. These properties make the AFM system suitable for use as a reference method to the stylus and the confocal laser scanning methods. The stylus tip is of silicon, the diameter is lo-20 nm and the measuring force is in the range of O-3.421 pN. The possible measuring ranges in X and Y are 6-125 pm, 1.3-6.2 pm in Z. The measuring range used was 0.125 x 0.125 mm in the X and Y directions, 6.2 pm in Z. The resolutions used in Xand Ywere 0.20 pm, 1.5 pm in Z. When measuring biomater-
ial surfaces the AFM approach has clear limitations in its small measurement range, particularly in the 2 direction. This limitation does not prohibit using AFM as a reference for specially selected areas. Evaluating deviations between the measuring systems used Part 1: Relocated areas on surfaces with different properties Three quite differently machined surfaces were prepared with three Vickers hardness numbers. These indentions were made to make sure that the same area was measured with the two measuring instruments (I;igure 1). This method for an exact relocation was first introduced by Ohlsson and Rosen in 1993 *l . B y making the indentions we were also provided with the value of the material hardness for each of the three samples. Surface3 investigated. 1. A plateau-honed cylinder liner. This is an important surface for the automotive industry. The surface interacts with the internal combustion engine piston ring, with oil and surface coatings as the interfacing media. The working environment is harsh, owing to high pressures, temperatures and varying relative speed, causing lubrication regimes from mild hydrodynamic to severe boundary ones. The surface consists of two independent roughness structures, fine plateaux and coarser intersecting valleys, the latter working as oil reservoirs. The complex surface structure with a superimposed cylindrical shape constitutes a difficult-to-measure surface and, therefore, is suitable for a comparative study. 2. A turned c.p. (commercially pure) titanium disc, blasted with 25 pm-sized particles of TiO,.
Figure 1 Photograph of the TiO,-blasted c.p. titanium disr. showing the indentions used for rrlocation of the measurements in Part 1. hrro~~ indicate the bar-det- 01’ rhr area mcasur4 h\ tht, @us instrum(‘,1t
549
Characterizing
30 tofography:
A, Wennerberg
et al.
Blasting is a frequently used method to enlarge the surface structure and has been subjected to several experimental studies where the bonetissue responses to implants of different surface roughness have been evaluated22-25. The blasting particles impinge the surface at random positions and orientations. A blasted surface represents a random structure in amplitude as well as in spatial direction26. 3. A hardened and polished steel roller-a roller from a bearing with a smooth and anisotropic structure. The roller is made of hardened, ground and polished bearing steel. The intention of this polishing is to produce a surface with an extremely low average roughness value. The process is performed by multidirectional movements. This polished surface represents an extremely hard and smooth surface where evidence from the machining process prior to the polishing procedure still is present. Part 2: Measurements biomaterial implants
on unrelocated
In this second part of the present study, we measured the surface structure of six implants, four screw-shaped and two cylindrical ones, manufactured from commonly used biomaterials. Each of the six samples was measured with a stylus and with the optical equipment on different areas. The purpose was to explain the differences or similarities by relating them to the differences found in Part 1, thereby seeking explanations as to why numerical values sometimes differ remarkably when measurements are performed with different equipment and by different investigators. Samples used in this part.
screw, blasted with 25 Frn-sized 1. A c.p. titanium particles of TiO,. This specimen represents an isotropic surface with a moderately enlarged surface roughness. For this implant the hardness was 153 HV. 2. A c.p. titanium screw blasted with 250 pm-sized particles of Al,O,. Blasting with this particle size results in a highly enlarged surface structure. Material hardness = 150 I-IV. 3. A screw-shaped implant manufactured of bone cement, polymerized methyl methacrylate (PMMA) . Material hardness = 21 HV. This material has a broad field of applications, and from a quantitative aspect it is possibly the most used biomaterial. This material is described in detail by Morberg*‘. 4. A turned c.p. titanium screw. Material hardness = 145 HV. 5. A titanium alloy, Ti6A14V, cylinder. Material hardness = 350 HV. C.p. titanium as well as titanium alloys are extensively used for fabrication of different oral and orthopaedic implants. A thorough description of these materials can be found in Johansson28. 6. A hydroxyapatite (HA)-coated cylinder. Material hardness = 71 HV. The chemical formula for this material is Ca,,(PO,),(OH),. It
550
is known to have a great variability in the quality of the HA-coating adhesive strength against the implant, due among other things, to the crystallinity and the purity of the HA*‘, which may influence the tendency of stylus to remove some portional of the coating during the measurement, thereby damaging the surface. To be able to measure the screws with the stylus, we had to measure these samples on the top or the bottom of the implants, i.e on a flat surface and not on the threaded part. Control
of table
error
and noise
induction
A flat glass was measured with the stylus and the optical instrument for the purpose of controlling the size and, even more importantly, the shape of the table error. It is also important to control the induced noise before surface roughness evaluation. The noise, either electrical or vibration induced, may never be of such magnitude that it will interfere with the actual signal from the surface topograPhY. Evaluation
of collected
data
To enable a correct comparison between the two instruments, the raw data was imported into a commercially available evaluation system, UB soft V.2.6 (UBM Messtechnik GmbH, Ettlingen, Germany). In order to filter out the form and the cylindricity double curvature from the raw data before roughness parameter evaluation, a polynomial 3D surface of varying degree in X and Y was used. The polynomial fitting in this case acts as a high-pass filter enabling form deviations to be filtered out. The polynomial filtering technique has the advantage over the normally used Gaussian high-pass filter (DIN 4777)30 that the length and width of the measured area can be maintained throughout the parametric evaluation, whereas the Gaussian filter reduces the width and the length by one cut-off length, respectively. This represents a considerable loss of measured data. The disadvantage with a polynomial filter is that the transferring function of wavelengths is not defined and, therefore, visual inspection of the polynomial fitting had to be done in the software used. This is in contrast to the Gaussian filter, which has a well-defined transferring curve for different wavelengths3’, providing stricter numerical control of the filter performance. Because it was important to compare as big an area as possible we selected the polynomial filter for the present investigation. The roughness parameters were calculated in software implemented in accordance with Stout et aL3’. Three-dimensional
parameters
used
parameters (i.e. height-descriptive). S, (R, for 2D) is the arithmetic mean of the absolute values of the surface departures from the mean plane within the sampling area. The parameter is measured in pm and is a general and commonly used parameter. Amplitude
.S, (R for 2D) is the root mean square value of the su I-Face departures within the sampling area (measured in pm). This parameter is more sensitive to extreme values than is the S, parameter owing to the squaring operation. S4 has statistical significance as the standard deviation of the height distribution. S, (R, for 2D) is the average value in pm of the absolute heights of the five highest peaks and the absolute value of the five deepest valleys within the sampling area. This parameter is sensitive to the changes of pronounced topography features. S,, (&, for 2D), skewness, is the measure of the symmetry of surface deviations about the mean plane. A negatively skewed surface has more valleys than peaks. For a Gaussian distribution the skewness is zero. Sk, (& for 2D), k ur t osis, is the measure of the sharpness of the surface height distribution. A perfect Gaussian distribution has a kurtosis of 3, a lower value indicating relatively few high peaks and low valleys. If the surface has many high peaks as well as low valleys the kurtosis will be larger than three.
produces more serious artefacts because it is nonrepetitive and of an irregular character, and therefore more difficult to subtract (fi,pre 3).
Functional parameters (i.e. the particular description of surface characteristics that are important for a specific functional application). The parameters used describing bearing properties are from DIN standard 4776”’ and are also presented in Figure 2. S, (& for 2D), core roughness depth. This part of the profile forms a dense bearing core. S,,, (R,, for 2D), reduced peak height. Calculated as the height of the peak area. S,,, (&k for 2D), reduced valley depth. Calculated as the height of the valley area. S,, (M,, for 2D), peak material ratio. Material ratio at the top of the roughness core. S,, (MfiL for 2D), valley material ratio. Material ratio at the bottom of the roughness core.
Turned disc blasted with 25 pm-sized particles of TiOZ. The material hardness for this specimen was 153 HV. the softest material investigated in this part of the present study. The numerical characterization demonstrated quite a good agreement for the averaging parameters, but not as good as for the plateau-honed cylinder liner ( Table I). For this comparatively soft material we could detect a clear influence of the stylus tip (f~igure I). The AFM measurement outside the stylus-measured area showed higher values than inside the measured area, demonstrating that the stylus had smoothed the surface structure (Figure 6). However, an overestimation was shown for the optical instrument using AFM as a reference method. The true values are most likely somewhere in between (Table 2).
RESULTS Table error influence The measurement of the flat glass demonstrated a repeatable and large error for the optical laser scanner. Because of its shape it is possible to filter it out from the measured surface, and we regard this as an acceptable table. The table for the stylus
Noise The noise produced by vibrations and/or electrical noise was of the same magnitude for the stylus and the laser scanner (Figure 4). Part 1: Relocated areas on surfaces with different properties Plateau-honed cylinder liner. This surface had a material hardness of 441 HV. The surface roughness parameters achieved from the two devices demonstrated very good agreement ( Table I). Comparison with the AFM measurement on a small area of a cylinder plateau showed that a more featureless surface structure was obtained with the stylus, while more features were measured by the laser scanner. For this surface the laser scanner produced an image closer to the appearance of the AFM measurements (Table 2, Figure 5).
Hardened a?ld polished ,roller. The material hardness was 760 HV. The values for the different surface roughness parameters exhibited for this specimen a greater variance than the other evaluated surfaces in Part 1 ( Tables 1, 2). Higher values were obtained with the optical scanner, and the AFM measurement supported the image from the stylus measurement. The higher values from the
551
Characterizing
3D topography:
A. Wmmrberg
et al.
Laser
Stylus
Figure 3 Measurements on a flat glass plate, left, laser scanner on the right
Point
Figure 4 Single profile showing noise was of similar magnitude
the electrical
showing
dens.
the shape and size of the table error
: 500
Sa % SZ Ssk Sku Sk SPk Svk Srl Sr2
liner
PE
TE
0.71 0.95 10.60 -1.81 11.43 2.16 0.73 1.59 0.04 0.84
0.74 0.95 8.68 -0.89 4.75 2.37 0.65 1.43 0.06 0.88
Blasted
and vibration
Ti disc
PE
noise
0.55 0.70 5.56 0.27 3.51 1.60 0.82 0.44 0.11 0.91
Roller
TS
PE
0.75 0.98 14.24 0.41 11.85 2.30 1.21 0.83 0.09 0.90
0.13 0.17 1.25 -0.13 2.98 0.32 0.09 0.15 0.08 0.90
TS 0.28 0.36 4.38 0.08 4.94 0.79 0.32 0.51 0.08 0.89
Table 2 Summary of parameters, obtained by stylus (PE) and optical device (TS), using the AFM as a numerical reference on small (approximately 50 x 50 pm) non-relocated areas on the three reference surfaces Cylinder
Sa
liner
Blasted
Ti disc
Roller
PE
TS
AFM
PE
TS
AFM
PE
TS
AFM
0.16 0.21 1.04
0.29 0.35 1.47
0.24 0.31 1.84
0.36 0.46 1.79
0.76 1.05 4.70
0.41 0.49 2.19
0.11 0.13 0.44
0.19 0.29 1.40
0.11 0.14 0.87
optical scanner were probably due to difficulties in detecting the focus because of high slopes in the surface which resulted in induced spikes. However, these spikes could be removed by a lowpass Gaussian filter (Figure 7)) after which the laser scanner values were similar to the stylus results.
552
instruments.
Stylus
on the
p/mm
Table 1 Summary of parameters for relocated areas (approximately 1 x 1 mm) on the reference surfaces, obtained by stylus (PE) and optical device (TS) Cylinder
of the two investigated
from
the stylus on the left, laser scanner
Part 2: Measurements biomaterial implants
on the right.
The
demonstrated
on non-relocated
In general, the stylus equipment showed lower values than the optical instrument, for the surface roughness parameters and the results in Part 2 confirm the findings in Part 1, i.e. an underestimation of the surface features made by the stylus and an overestimation made by the laser scanner. However, the differences between the two measuring instruments were much more pronounced in Part 2 than in Part 1. This was mainly caused by the non-relocated and smaller measured area investigated in Part 2 ( Table 3). A visual comparison between the topographical images from the stylus, the AFM and the laser scanner measurement together with a SEM image as another visual possibility is demonstrated in Figure 8. These pictures are from a measuring area at the bottom aspect of a TiO*-blasted screw (Figure 9). DISCUSSION The present investigation demonstrated very good agreement between the two investigated instruments for the plateau-honed surface, and good agreement for the blasted surface, while the roller bearing measurement disagreed to a much higher extent when the averaging parameters (S,, S ) were compared. The extreme parameter s, showed a more pronounced difference for all investigated surfaces. The functional parameters (S,, S k, S,,, ST,, S,,) showed very good agreement for t R e plateau-honed surface but not for the roller bearing. The reason for this disagreement is that the induced spikes from the optical instrument convert the appearance of this flat surface to an isotropic surface like the blasted one. Blasted
Figure 6 AFM measurrment showing the influence on a TiO,-blasted sutixe, outside (left) and inside ured at-ra. The stylus had smoothened the surface cl-olled by rhc calculated average roughness value
Laser measurement
of’ the stylus tip (right) Ihe mea\structurr as COW
of a roller
Low pass filtered surface (cut-off 15pm)
Figure 7 Measurement of’ the roller. demonstrating spikes created by the optical equipment. These spikes could be removed with a low-pass filter and .m image closer to the stylus instrument cotlId tx achirvcd
surfaces are not suitable for this parameter set, owing to the lack of an S-shaped material ratio curve”“. The size of the measured areas will influence the values of the surface roughness parameters, as demonstrated by a comparison between the values in Tables 2 and 3. This points to the importance of a clear specification of the measured areas if different investigations are to be compared. We have also shown that for some surfaces it is preferable to use a contact and for others a non-contacting method. The measured biomaterials exhibited much more pronounced differences with respect to the surface roughness parameters than did the three investigated surfaces where the same area was evaluated. However, the basic findings in Part 1 of this investigation agree with the findings in Part 2; in general the stylus presented a smoother surface than did the laser scanner. One explanation for this is the size of the stylus tip which actually works as a lowpass filter, i.e. the tip is not able to reach the bottom of narrow valleys so that some of the information about the roughness is lost. Furthermore, the pressure acting on the surface asperities causes deformation of the weaker ones. The laser scanner in general gives too high values for the surface roughness, owing to induced spikes, which leads to an overestimation of the surface roughness’i4. These findings are in agreement with a study by Blunt et aZ.“, where the authors measured the same piece of a plateau-honed cylinder liner, as we did in the present study, with four contact stylus instruments and two optical instruments. The contact instruments demonstrated an underestimation and the optical instruments an overestimation of the true surface when using an AFM instrument for reference. The contact instruments were considered to best reproduce the true surface in the study by Blunt et ~1.““. This is in
553
Sa % SZ Ssk Sku Sk SPk Svk Srl Sr2
Table
3
0.47 0.59 3.80 0.24 3.07 1.49 0.54 0.47 0.10 0.91
-
_.
0.75 0.97 8.84 -0.23 3.81 2.24 0.94 1.23 0.09 0.89
(PE)
-
1.04 1.43 11.55 0.51 6.00 2.93 2.07 1.98 0.09 0.86
PE
TS
by stylus
PE
obtained
C.p. titanium Blasted 250
of parameters
C.p. titanium Blasted 25
Summary
-.
TS 2.97 4.15 38.55 -0.64 6.53 8.26 4.42 7.56 0.10 0.87
and optical
device
0.32 0.42 3.23 -0.32 4.37 0.93 0.48 0.52 0.10 0.89
- ..-
-.
0.84 1.07 9.51 -0.21 3.74 2.76 1.11 1.12 0.08 0.90
-”
screws.
_
In general,
2.00 2.48 12.85 -0.34 2.69 6.30 1.95 2.61 0.08 0.86
PE
TS
biomaterial
PE
measuring PMMA
when
Cp. titanium Untreated
(TS)
TS 2.39 3.03 24.01 -0.32 3.57 7.69 3.02 3.12 0.05 0.87
the optical
shows higher
PE 0.86 1.07 7.05 -0.16 2.86 2.78 0.93 0.90 0.09 0.90
TiGA14V
device
TS 1.17 1.52 14.42 -0.55 4.02 3.66 1.36 2.15 0.07 0.87
values
3.68 4.52 23.96 0.26 2.55 11.75 4.76 3.01 0.13 0.93
TS
..” I.. ._. -” - -
PE
Hydroxyapatite HA
for these surfaces
3.19 3.92 25.49 -0.06 2.93 11.30 3.55 3.93 0.07 0.92
?
b) Laser
a) Stylus
.&--7
f i”/ .r.< -c-__ ‘ .e--
c--I 20 wn 005mm,400 'P'mm
d) SEM
c) AFM
Figure
8
Topographical
Figure 9 Photograph particles of TiO,. The and has a pitch-height twed area
..+A: \,..A /‘, c ! 30’ , iiiii’ - -_.-Y
images
on the X0,-blasted
SCI-ew pr-educed
of a c.p. titanium screw blasted with 25 wm screw is 7 mm in length, 3.7 mm in diameter. of 0.6 mm. The arrow indicates the meas-
ha the st~lilus. the AFklI.
the laser scanner
and a SE:M imagr
contrast to the present study where we found a very good numerical agreement between the optical and the contact profilometer and, when using the AFM as a reference, we found the optical scanner to reproduce best the surface topography for this specimen (Tabk 2, Figure 5). The laser scanner was also better for measuring the cylinder liner surface compared with the other optical instruments investigated by Blunt et aZ.““. One reason for this may be the confocal principle allowing a higher numerical aperture when using standard objectives, i.e. more light to be collected, which is important for tilted or porous surfaces. For the blasted, relatively soft, surface the numerical values had about the same validity for the two investigated instruments. However, owing to the stylus influence on the surface, it will not have the same topography after the measurement as before and, thereby, the possibility exists that the measurement itself may influence the outcome of the biological response. For a hard-polished surface like the roller bearing, the traces remaining after the grinding operation cause high slopes in the surface, resulting in induced spikes; this is one likely explanation for the quite high surface roughness values achieved with the laser scanner. For such a surface it is important to know the errors and to use a low-pass filter before evaluation if this optical method is to be used. The stylus missed some of the valleys but was far closer to the true surface, and was a better method for this surface.
555
Characterizing
30 topography:
A.
Wane&erg et al.
CONCLUSIONS The two instruments, a stylus and a laser scanning device, are both acceptable for use in the evaluation of surface topographies. Even so, it is important to know the advantages and the disadvantages of each of the two approaches. Typical soft metal biomaterials are best evaluated with the laser approach, whereas hard industrial materials with high slopes within the surface are best evaluated with the stylus instrument. For threaded screw implants, the laser method is the most advantageous approach since the screw geometry will prevent proper stylus reading. ACKNOWLEDGEMENTS This study has been supported by grants from the Swedish Board for Technical Evaluation (NUTEK) , Sylvan’s Foundation, the Swedish Dental Society, the Anna Ahrenberg Foundation, the Greta and Einar Asker Foundation, the Wilhelm and Martina Lundgren Science Foundation, the Swedish Medical Research Council and the Hjalmar Svensson Research Foundation. REFERENCES 1. Albrektsson T, B&remark P-I, Hansson H-A, Lindstrijm J. Osseointegrated implants. Requirements for ensuring a long-lasting, direct bone-to-implant anchorage in man. Acta Orthop Stand 1981; 52: 155. 2. Ratner BD. Biomaterial surfaces. J Biomed Mater Res A#1 Biomater 1987; 21: 59-90. 3. Smith DC. Dental implants: materials and design considerations. Int J Prosthodont 1993; 6: 106-17. 4. Ask M, Lausmaa J, Kasemo B. Preparation and surface of oxide films on spectroscopic characterization Ti6A14V. Appl Surface Sci 1989; 35: 283-301. 5. Keller JC, Stanford CM, Wightman JP, Draughn A, Zaharias R. Characterizations of titanium implant surfaces III. J Biomed
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28: 939-46.
6. Meyle J, Giiltig K, Hiitteman W, von Recum A, Elssner G, Wolburg H, Nisch W. Oberflachenmikromorphologie und Zellreaktion. Z Zahnantl Implantol 1994; 10: 51-64. 7. Klauber C, Lenz LJ, Henry PJ. Oxide thickness and surface contamination of six endosseous dental implants determined by electron spectroscopy for chemical analysis: a preliminary report. Int J Oral Maxillofac Implants 1990; 5: 264-71. 8. Olefjord I, Hansson S. Surface analysis of four dental implant systems. Int J Oral Maxillofac Implants 1993; 8: 3240. 9. Lausmaa J, Kasemo B, Mattsson H. Surface spectroscopic characterization of titanium implant materials. A#1 Surface Sci 1990; 44: 133-46. 10. Helsingen AL, Lyberg T. Comparative surface analysis and clinical performance studies of B&remark implants and related clones. Znt J Oral Moxillof~ Implants 1994,9: 422-30. 11. Mtiller-Mai CM, Voigt C, Gross U. Incorporation and degradation of hydroxyapatite implants of different surface roughness and surface structure in bone. Scanning Microscopy 1990; 4: 613-24. 12. Quirynen M, Marechal M, Busscher HJ, Weerkamp AH, Darius PL, Van Steenberghe D. The influence of surface free energy and surface roughness on early plaque formation. J Clin Periodontol 1990; 17: 138-44. 13. Baguet J, Sommer F, Due TM. Imaging surfaces of hydrophilic contact lenses with the atomic force microscope. Biomattials 1993; 14: 279-84.
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14. Bianco PD, Ducheyne P, Bonnell D. Scanning tunnelling microscopy and tunnelling spectroscopy of titanium before and after in vitro immersion. J Mater Sci Muter Med 1992; 3: 28-32. 15. Bowers KT, Keller JC, Randolph BA, Wick DG, Michaels CM. Optimization of surface micromorphology for enhanced osteoblast responses in vitro. Int J Oral Maxillofuc Implants 1992; 7: 302-10. 16. Wennerberg A, Albrektsson T, Andersson B. 13 commercially available oral implants. Int J Oral Maxillofac Zmplants 1993; 8: 622-633. 17. Schmidt JA, von Recum AF. Surface characterization of microtextured silicone. Biomaterials 1992; 13: 675-81. 18. Nentwig GH, Reichel M. Vergleichende untersuchungen zur mikromorphologie und gesamtoberflache enossaler implantate. Z Zahnarztl Zmplantol 1994; 10: 150-4. 19. Bengtsson A. On threedimensional measurement of surface roughness. Thesis, Dept of Production Engineering, Chalmers University of Technology, Giiteborg, 1991. 20. Wilson T. Confocal microscopy. In Confocal Microscopy, Wilson T, ed. Academic Press, London, 1990. 21. Ohlsson R, Rosen B-G. On replication and 3D stylus profilometry techniques for measurement of plateau-honed cylinder liner surfaces. In Proceedings of tk ASPE 1993 Annual Meeting Hocken RJ, ed. Seattle, 7-12 November 1993. 22. Buser D, Schenk RK, Steinemann S, Fiorellini JP, Fox CH, Stich H. Influence of surface characteristics on bone integration of titanium implants. A histomorphometric study in miniature. J Biomed Mater Res 1991; 25: 889-902. 23. Wennerberg A, Albrektsson T, Andersson B, Krol JJ. A histomorphometric and removal torque study of screwshaped titanium implants with three different surface topographies. Clin Oral Implant Res 1995; 6: 2430. 24. Wennerberg A, Albrektsson T, Johansson CB, Andersson B. An experimental study of turned and grit blasted screw-shaped implants with special emphasis on effects of blasting material and surface topography. Biomateriuls 1996; 17: 15-22. 25. Wennerberg A, Albrektsson T, Andersson B. An animal study of c.p. titanium screws with different surface topographies. J Mater Sci: Mater Med 1995; 6: 302-309. 26. Stout KJ, David EJ, Sullivan PJ. Atlas of Machined Surfnces. Chapman and Hall, London, 1990. 27. Morberg P. On bone tissue reactions to acrylic’ cement. Thesis, University of G&eborg, Goteborg, 1991. 28. Johansson CB. On tissue reactions to metal implants. Thesis, University of GGteborg, Giiteborg, 1991. 29. Soballe K Hydroxyapatite ceramic coating for bone implant fixation. Mechanical and histological studies in dogs. Acta Orthop Stand 1993; Suppl 255, 64. 30. DIN Meterology of surfaces, profile filters for electrical contact stylus instruments, phase corrected filters. German Standard DIN 4777, 1990. 31. Stout KJ, Sullivan PJ, Dong WP, Luo NL. A proposal of parameters for charaterizing three-dimensional surface topography. For the EC project on ‘Development of methods for the characterization of surface topography in three dimensions’. EC Contract No 3374/1/O/170/90/2 1992. Centre for Metrology, Birmingham, UK 32, DIN. Measurement of surface roughness, parameters Rk, Rpk, Rvk, Mrl, Mr2 for the description of the material portion in the roughness, German Standard, DIN 4776, 1990. 33. Whitehouse DJ. Handbook of Surface Metrology. Institute of Physics, Bristol, 1994. 34. Tricot C, Ferland P, Baran G. Fractal analysis of worn surfaces. Wear 1993; 172: 127-33. 35. Blunt L, Ohlsson R, Rosen B-G. A comprehensive comparative study of 3D surface topography measuring instruments. In Proceedings of the 6th Nordic Symposium on Tribology, NOFDTRIB 94, Vol. 2, Hedenqvist P, Hogmark S, Jacobson S, eds. NORTRIB Uppsala, Sweden 1994; 359-67.
PII: s1350-4533(95)00005-7
ELSEVIER
Med. Eng. Phys. Vol. 18, No. 7, pp. 548-556. 1996 0 1996 Elsevier Science Ltd for IPEMB. All rights reserved kinted in Great Britain 1350-4533/96 $15.00 + 0.00
Characterizing three-dimensional topography of engineering and biomaterial surfaces by confocal laser scanning and stylus techniques A. Wennerberg *, R. Ohlssont,
B.-G. Rosknt
and B. Andersson*
“Biomaterials Group, Department of Biomaterials/Handicap Research, Institute for Surgical Sciences, Giiteborg University, Medicinaregatan 8, S-413 90 Giiteborg, Sweden; and tchalmers Surface Geometry Group, Department of Production Engineering, Chalmers University of Technology, S-412 96 Gbteborg, Sweden Received
12 May
1995,
revised
13 November
1995, accepted
18 December
1995
ABSTRACT Threedimensional
measurements of surface topography were pe$%rrned using a confocal laser scanner and a contact stylus instrument. Three surfaces known to be dificult to evaluate were chosen to be measured on the same area with the two instruments. The measurements from the optical and the contact stylus projilometer were compared with each other and with measurements obtained from highresolution atomic force microscopy, which served as a reference instrument. Six implants manufactured f?om commonly used biomaterials were also measured on the same part of the implant, but not on the same area, with the optical and the contact profilome~er in order to simulate the measurements that would be perfo77nd when diff erent laboratories measure similarly treated surfaces. The numerical and visual differaces achiezed when measun’ng the same area with the two instruments investigated were compared. In general, we found an underestimation of the suface features with the contact stylus measurement and an overestimation with the confocal scanner. The stylus readings are mainly influenced by the radius of the stylus tip, the pressure of the stylus tip on the surface, and the hardness of the material. The optical projlometer has a tendency for creating spikes when surfaces with deep slopes are measured. For relatively soft metallic biomaterials, we found that using the optical shaped surface
instrument implants
is the most appropriate
method
for surface
Keywords: 3D surface biomaterial surfaces
topography,
stylus profilometer,
Med. Eng. Phys., 1996, Vol. 18, 548-556,
characterization,
particularly
The surface properties of an implant are widely known to influence the local tissue response’- . To be able to control the surface preparation and maintain quality there is an increasing demand for surface characterization. Several methods for surface analysis are now in frequent use. The surface composition is commonly investigated with auger electron spectroscopy @ES), X-ray photoemission spectroscopy (XPS) , and/or secondary ion mass spectroscopy (SIMS)4-6. To evaluate the thickness and composition of the oxide film, AES and ion etching are common methods7-g, and for surface morphology, cellular as well as material, scannin electron microscopy (SEM) is often usedlo-’ 8. For surface roughness characterization, several systems have been used in evaluating biomaterials1z-14. The most commonly used method is a two-dimensional (2D) profile measurement to: Ann
Wennerberg,
DDS.
confocal
laser
scanner,
when screw slopes within the
engineering
and
October
INTRODUCTION
Correspondence
roughness
are analysed, whereas the stylus is pefared when larger areas with substantial structure are to be evaluated. Copyright 0 1996 Elsevier Science Ltd for IPEMB.
with a contact instrument. However, 2D measurements are not always enough to describe a surface, and increasing numbers of devices for 3D measurements are now commercially available. All measuring systems, 2D as well as 3D, have limitations in terms of wavelengths and resolution, and it is difficult to select the most appropriate instrument. In the biomaterials field several authors have published values for surface roughnesses of experimental as well as commercially available implants, measured with different kinds of instruments’5-1s. This raises the question of whether it is possible to compare results from one instrument with those obtained from another. The purpose of the present study is to compare two, in principle quite different, devices adapted to 3D surface roughness measurements in the micrometre scale, when measuring different kinds of machined surfaces. In part 1 of this investigation, three different surfaces were measured with the two instruments on the same areas. This
procedure enables comparison of measuring techniques with respect to different surface types. Furthermore, an AFM instrument was used as a reference system. In Part 2 we compared measurements on non-relocated (different) areas for the purpose of investigating the differences that ma.y occur when different investigators measure simllar surfaces. MATERTALS Surface
AND
measuring
METHODS equipment
A mechanical contact measuring instrument, PerthometerTM C5D with a PRK drive unit (Feinpriif Perthen GmbH, GGttingen, Germany) and an measuring device, TopScan 3DTM optical (Heidelberg Instruments, Heidelberg, Germany), were used in this study. Contact method. For this method a stylus tip is drawn along a surface with constant speed and pressure. The Perthometer instrument has a possible measuring range of 60 x 300 mm, Z-ranges of f2.5-625 pm and a measuring force of 5 mN. The measuring range used was, in part 1: 2 x 2 mm in the X and Y directions, +25 pm in Z in part 2: 0.250 x 0.250 mm in the X and Y directions, f25 pm in Z. The stylus has a diamond with a radius of 5 pm. The resolutions used in the X and Y directions were 5 pm, the resolution used in % was 0.01 pm. This equipment is a traditional 2D instrument complemented with a Y steppingtable to enable SD measurements, as described by Bengtsson I”. Non-contacting method. The TopScan 3D instrument uses the principle of laser confocal scanning microscopy2”. A He-Ne laser beam works as an optical stylus, the diameter of the laser beam being about 1 pm. The possible measuring ranges are 0.25-2 mm in the X and Y directions, 25108 pm in Z. The measuring range used was, in part 1: 2 x 2 mm in the X and Y directions, 54 pm in 8 in part 2: 0.250 x 0.250 mm in the X and Y directions, 108 pm in Z. The resolution used in the X and Y directions was 5 pm, 0.01 pm in Z. The magnification of the objective was x20. The laser spot is scanned across the fixed specimen. To be able to distinguish between the true surface features and features missed or created by the above measuring systems, we used an AFM, the Rasterscope 4000TM (Danish Micro Engineering A/S, KDbenhavn, Denmark). The AFM provides very high-resolution images compared to the other two instruments used in this study. These properties make the AFM system suitable for use as a reference method to the stylus and the confocal laser scanning methods. The stylus tip is of silicon, the diameter is lo-20 nm and the measuring force is in the range of O-3.421 pN. The possible measuring ranges in X and Y are 6-125 pm, 1.3-6.2 pm in Z. The measuring range used was 0.125 x 0.125 mm in the X and Y directions, 6.2 pm in Z. The resolutions used in Xand Ywere 0.20 pm, 1.5 pm in Z. When measuring biomater-
ial surfaces the AFM approach has clear limitations in its small measurement range, particularly in the 2 direction. This limitation does not prohibit using AFM as a reference for specially selected areas. Evaluating deviations between the measuring systems used Part 1: Relocated areas on surfaces with different properties Three quite differently machined surfaces were prepared with three Vickers hardness numbers. These indentions were made to make sure that the same area was measured with the two measuring instruments (I;igure 1). This method for an exact relocation was first introduced by Ohlsson and Rosen in 1993 *l . B y making the indentions we were also provided with the value of the material hardness for each of the three samples. Surface3 investigated. 1. A plateau-honed cylinder liner. This is an important surface for the automotive industry. The surface interacts with the internal combustion engine piston ring, with oil and surface coatings as the interfacing media. The working environment is harsh, owing to high pressures, temperatures and varying relative speed, causing lubrication regimes from mild hydrodynamic to severe boundary ones. The surface consists of two independent roughness structures, fine plateaux and coarser intersecting valleys, the latter working as oil reservoirs. The complex surface structure with a superimposed cylindrical shape constitutes a difficult-to-measure surface and, therefore, is suitable for a comparative study. 2. A turned c.p. (commercially pure) titanium disc, blasted with 25 pm-sized particles of TiO,.
Figure 1 Photograph of the TiO,-blasted c.p. titanium disr. showing the indentions used for rrlocation of the measurements in Part 1. hrro~~ indicate the bar-det- 01’ rhr area mcasur4 h\ tht, @us instrum(‘,1t
549
Characterizing
30 tofography:
A, Wennerberg
et al.
Blasting is a frequently used method to enlarge the surface structure and has been subjected to several experimental studies where the bonetissue responses to implants of different surface roughness have been evaluated22-25. The blasting particles impinge the surface at random positions and orientations. A blasted surface represents a random structure in amplitude as well as in spatial direction26. 3. A hardened and polished steel roller-a roller from a bearing with a smooth and anisotropic structure. The roller is made of hardened, ground and polished bearing steel. The intention of this polishing is to produce a surface with an extremely low average roughness value. The process is performed by multidirectional movements. This polished surface represents an extremely hard and smooth surface where evidence from the machining process prior to the polishing procedure still is present. Part 2: Measurements biomaterial implants
on unrelocated
In this second part of the present study, we measured the surface structure of six implants, four screw-shaped and two cylindrical ones, manufactured from commonly used biomaterials. Each of the six samples was measured with a stylus and with the optical equipment on different areas. The purpose was to explain the differences or similarities by relating them to the differences found in Part 1, thereby seeking explanations as to why numerical values sometimes differ remarkably when measurements are performed with different equipment and by different investigators. Samples used in this part.
screw, blasted with 25 Frn-sized 1. A c.p. titanium particles of TiO,. This specimen represents an isotropic surface with a moderately enlarged surface roughness. For this implant the hardness was 153 HV. 2. A c.p. titanium screw blasted with 250 pm-sized particles of Al,O,. Blasting with this particle size results in a highly enlarged surface structure. Material hardness = 150 I-IV. 3. A screw-shaped implant manufactured of bone cement, polymerized methyl methacrylate (PMMA) . Material hardness = 21 HV. This material has a broad field of applications, and from a quantitative aspect it is possibly the most used biomaterial. This material is described in detail by Morberg*‘. 4. A turned c.p. titanium screw. Material hardness = 145 HV. 5. A titanium alloy, Ti6A14V, cylinder. Material hardness = 350 HV. C.p. titanium as well as titanium alloys are extensively used for fabrication of different oral and orthopaedic implants. A thorough description of these materials can be found in Johansson28. 6. A hydroxyapatite (HA)-coated cylinder. Material hardness = 71 HV. The chemical formula for this material is Ca,,(PO,),(OH),. It
550
is known to have a great variability in the quality of the HA-coating adhesive strength against the implant, due among other things, to the crystallinity and the purity of the HA*‘, which may influence the tendency of stylus to remove some portional of the coating during the measurement, thereby damaging the surface. To be able to measure the screws with the stylus, we had to measure these samples on the top or the bottom of the implants, i.e on a flat surface and not on the threaded part. Control
of table
error
and noise
induction
A flat glass was measured with the stylus and the optical instrument for the purpose of controlling the size and, even more importantly, the shape of the table error. It is also important to control the induced noise before surface roughness evaluation. The noise, either electrical or vibration induced, may never be of such magnitude that it will interfere with the actual signal from the surface topograPhY. Evaluation
of collected
data
To enable a correct comparison between the two instruments, the raw data was imported into a commercially available evaluation system, UB soft V.2.6 (UBM Messtechnik GmbH, Ettlingen, Germany). In order to filter out the form and the cylindricity double curvature from the raw data before roughness parameter evaluation, a polynomial 3D surface of varying degree in X and Y was used. The polynomial fitting in this case acts as a high-pass filter enabling form deviations to be filtered out. The polynomial filtering technique has the advantage over the normally used Gaussian high-pass filter (DIN 4777)30 that the length and width of the measured area can be maintained throughout the parametric evaluation, whereas the Gaussian filter reduces the width and the length by one cut-off length, respectively. This represents a considerable loss of measured data. The disadvantage with a polynomial filter is that the transferring function of wavelengths is not defined and, therefore, visual inspection of the polynomial fitting had to be done in the software used. This is in contrast to the Gaussian filter, which has a well-defined transferring curve for different wavelengths3’, providing stricter numerical control of the filter performance. Because it was important to compare as big an area as possible we selected the polynomial filter for the present investigation. The roughness parameters were calculated in software implemented in accordance with Stout et aL3’. Three-dimensional
parameters
used
parameters (i.e. height-descriptive). S, (R, for 2D) is the arithmetic mean of the absolute values of the surface departures from the mean plane within the sampling area. The parameter is measured in pm and is a general and commonly used parameter. Amplitude
.S, (R for 2D) is the root mean square value of the su I-Face departures within the sampling area (measured in pm). This parameter is more sensitive to extreme values than is the S, parameter owing to the squaring operation. S4 has statistical significance as the standard deviation of the height distribution. S, (R, for 2D) is the average value in pm of the absolute heights of the five highest peaks and the absolute value of the five deepest valleys within the sampling area. This parameter is sensitive to the changes of pronounced topography features. S,, (&, for 2D), skewness, is the measure of the symmetry of surface deviations about the mean plane. A negatively skewed surface has more valleys than peaks. For a Gaussian distribution the skewness is zero. Sk, (& for 2D), k ur t osis, is the measure of the sharpness of the surface height distribution. A perfect Gaussian distribution has a kurtosis of 3, a lower value indicating relatively few high peaks and low valleys. If the surface has many high peaks as well as low valleys the kurtosis will be larger than three.
produces more serious artefacts because it is nonrepetitive and of an irregular character, and therefore more difficult to subtract (fi,pre 3).
Functional parameters (i.e. the particular description of surface characteristics that are important for a specific functional application). The parameters used describing bearing properties are from DIN standard 4776”’ and are also presented in Figure 2. S, (& for 2D), core roughness depth. This part of the profile forms a dense bearing core. S,,, (R,, for 2D), reduced peak height. Calculated as the height of the peak area. S,,, (&k for 2D), reduced valley depth. Calculated as the height of the valley area. S,, (M,, for 2D), peak material ratio. Material ratio at the top of the roughness core. S,, (MfiL for 2D), valley material ratio. Material ratio at the bottom of the roughness core.
Turned disc blasted with 25 pm-sized particles of TiOZ. The material hardness for this specimen was 153 HV. the softest material investigated in this part of the present study. The numerical characterization demonstrated quite a good agreement for the averaging parameters, but not as good as for the plateau-honed cylinder liner ( Table I). For this comparatively soft material we could detect a clear influence of the stylus tip (f~igure I). The AFM measurement outside the stylus-measured area showed higher values than inside the measured area, demonstrating that the stylus had smoothed the surface structure (Figure 6). However, an overestimation was shown for the optical instrument using AFM as a reference method. The true values are most likely somewhere in between (Table 2).
RESULTS Table error influence The measurement of the flat glass demonstrated a repeatable and large error for the optical laser scanner. Because of its shape it is possible to filter it out from the measured surface, and we regard this as an acceptable table. The table for the stylus
Noise The noise produced by vibrations and/or electrical noise was of the same magnitude for the stylus and the laser scanner (Figure 4). Part 1: Relocated areas on surfaces with different properties Plateau-honed cylinder liner. This surface had a material hardness of 441 HV. The surface roughness parameters achieved from the two devices demonstrated very good agreement ( Table I). Comparison with the AFM measurement on a small area of a cylinder plateau showed that a more featureless surface structure was obtained with the stylus, while more features were measured by the laser scanner. For this surface the laser scanner produced an image closer to the appearance of the AFM measurements (Table 2, Figure 5).
Hardened a?ld polished ,roller. The material hardness was 760 HV. The values for the different surface roughness parameters exhibited for this specimen a greater variance than the other evaluated surfaces in Part 1 ( Tables 1, 2). Higher values were obtained with the optical scanner, and the AFM measurement supported the image from the stylus measurement. The higher values from the
551
Characterizing
3D topography:
A. Wmmrberg
et al.
Laser
Stylus
Figure 3 Measurements on a flat glass plate, left, laser scanner on the right
Point
Figure 4 Single profile showing noise was of similar magnitude
the electrical
showing
dens.
the shape and size of the table error
: 500
Sa % SZ Ssk Sku Sk SPk Svk Srl Sr2
liner
PE
TE
0.71 0.95 10.60 -1.81 11.43 2.16 0.73 1.59 0.04 0.84
0.74 0.95 8.68 -0.89 4.75 2.37 0.65 1.43 0.06 0.88
Blasted
and vibration
Ti disc
PE
noise
0.55 0.70 5.56 0.27 3.51 1.60 0.82 0.44 0.11 0.91
Roller
TS
PE
0.75 0.98 14.24 0.41 11.85 2.30 1.21 0.83 0.09 0.90
0.13 0.17 1.25 -0.13 2.98 0.32 0.09 0.15 0.08 0.90
TS 0.28 0.36 4.38 0.08 4.94 0.79 0.32 0.51 0.08 0.89
Table 2 Summary of parameters, obtained by stylus (PE) and optical device (TS), using the AFM as a numerical reference on small (approximately 50 x 50 pm) non-relocated areas on the three reference surfaces Cylinder
Sa
liner
Blasted
Ti disc
Roller
PE
TS
AFM
PE
TS
AFM
PE
TS
AFM
0.16 0.21 1.04
0.29 0.35 1.47
0.24 0.31 1.84
0.36 0.46 1.79
0.76 1.05 4.70
0.41 0.49 2.19
0.11 0.13 0.44
0.19 0.29 1.40
0.11 0.14 0.87
optical scanner were probably due to difficulties in detecting the focus because of high slopes in the surface which resulted in induced spikes. However, these spikes could be removed by a lowpass Gaussian filter (Figure 7)) after which the laser scanner values were similar to the stylus results.
552
instruments.
Stylus
on the
p/mm
Table 1 Summary of parameters for relocated areas (approximately 1 x 1 mm) on the reference surfaces, obtained by stylus (PE) and optical device (TS) Cylinder
of the two investigated
from
the stylus on the left, laser scanner
Part 2: Measurements biomaterial implants
on the right.
The
demonstrated
on non-relocated
In general, the stylus equipment showed lower values than the optical instrument, for the surface roughness parameters and the results in Part 2 confirm the findings in Part 1, i.e. an underestimation of the surface features made by the stylus and an overestimation made by the laser scanner. However, the differences between the two measuring instruments were much more pronounced in Part 2 than in Part 1. This was mainly caused by the non-relocated and smaller measured area investigated in Part 2 ( Table 3). A visual comparison between the topographical images from the stylus, the AFM and the laser scanner measurement together with a SEM image as another visual possibility is demonstrated in Figure 8. These pictures are from a measuring area at the bottom aspect of a TiO*-blasted screw (Figure 9). DISCUSSION The present investigation demonstrated very good agreement between the two investigated instruments for the plateau-honed surface, and good agreement for the blasted surface, while the roller bearing measurement disagreed to a much higher extent when the averaging parameters (S,, S ) were compared. The extreme parameter s, showed a more pronounced difference for all investigated surfaces. The functional parameters (S,, S k, S,,, ST,, S,,) showed very good agreement for t R e plateau-honed surface but not for the roller bearing. The reason for this disagreement is that the induced spikes from the optical instrument convert the appearance of this flat surface to an isotropic surface like the blasted one. Blasted
Figure 6 AFM measurrment showing the influence on a TiO,-blasted sutixe, outside (left) and inside ured at-ra. The stylus had smoothened the surface cl-olled by rhc calculated average roughness value
Laser measurement
of’ the stylus tip (right) Ihe mea\structurr as COW
of a roller
Low pass filtered surface (cut-off 15pm)
Figure 7 Measurement of’ the roller. demonstrating spikes created by the optical equipment. These spikes could be removed with a low-pass filter and .m image closer to the stylus instrument cotlId tx achirvcd
surfaces are not suitable for this parameter set, owing to the lack of an S-shaped material ratio curve”“. The size of the measured areas will influence the values of the surface roughness parameters, as demonstrated by a comparison between the values in Tables 2 and 3. This points to the importance of a clear specification of the measured areas if different investigations are to be compared. We have also shown that for some surfaces it is preferable to use a contact and for others a non-contacting method. The measured biomaterials exhibited much more pronounced differences with respect to the surface roughness parameters than did the three investigated surfaces where the same area was evaluated. However, the basic findings in Part 1 of this investigation agree with the findings in Part 2; in general the stylus presented a smoother surface than did the laser scanner. One explanation for this is the size of the stylus tip which actually works as a lowpass filter, i.e. the tip is not able to reach the bottom of narrow valleys so that some of the information about the roughness is lost. Furthermore, the pressure acting on the surface asperities causes deformation of the weaker ones. The laser scanner in general gives too high values for the surface roughness, owing to induced spikes, which leads to an overestimation of the surface roughness’i4. These findings are in agreement with a study by Blunt et aZ.“, where the authors measured the same piece of a plateau-honed cylinder liner, as we did in the present study, with four contact stylus instruments and two optical instruments. The contact instruments demonstrated an underestimation and the optical instruments an overestimation of the true surface when using an AFM instrument for reference. The contact instruments were considered to best reproduce the true surface in the study by Blunt et ~1.““. This is in
553
Sa % SZ Ssk Sku Sk SPk Svk Srl Sr2
Table
3
0.47 0.59 3.80 0.24 3.07 1.49 0.54 0.47 0.10 0.91
-
_.
0.75 0.97 8.84 -0.23 3.81 2.24 0.94 1.23 0.09 0.89
(PE)
-
1.04 1.43 11.55 0.51 6.00 2.93 2.07 1.98 0.09 0.86
PE
TS
by stylus
PE
obtained
C.p. titanium Blasted 250
of parameters
C.p. titanium Blasted 25
Summary
-.
TS 2.97 4.15 38.55 -0.64 6.53 8.26 4.42 7.56 0.10 0.87
and optical
device
0.32 0.42 3.23 -0.32 4.37 0.93 0.48 0.52 0.10 0.89
- ..-
-.
0.84 1.07 9.51 -0.21 3.74 2.76 1.11 1.12 0.08 0.90
-”
screws.
_
In general,
2.00 2.48 12.85 -0.34 2.69 6.30 1.95 2.61 0.08 0.86
PE
TS
biomaterial
PE
measuring PMMA
when
Cp. titanium Untreated
(TS)
TS 2.39 3.03 24.01 -0.32 3.57 7.69 3.02 3.12 0.05 0.87
the optical
shows higher
PE 0.86 1.07 7.05 -0.16 2.86 2.78 0.93 0.90 0.09 0.90
TiGA14V
device
TS 1.17 1.52 14.42 -0.55 4.02 3.66 1.36 2.15 0.07 0.87
values
3.68 4.52 23.96 0.26 2.55 11.75 4.76 3.01 0.13 0.93
TS
..” I.. ._. -” - -
PE
Hydroxyapatite HA
for these surfaces
3.19 3.92 25.49 -0.06 2.93 11.30 3.55 3.93 0.07 0.92
?
b) Laser
a) Stylus
.&--7
f i”/ .r.< -c-__ ‘ .e--
c--I 20 wn 005mm,400 'P'mm
d) SEM
c) AFM
Figure
8
Topographical
Figure 9 Photograph particles of TiO,. The and has a pitch-height twed area
..+A: \,..A /‘, c ! 30’ , iiiii’ - -_.-Y
images
on the X0,-blasted
SCI-ew pr-educed
of a c.p. titanium screw blasted with 25 wm screw is 7 mm in length, 3.7 mm in diameter. of 0.6 mm. The arrow indicates the meas-
ha the st~lilus. the AFklI.
the laser scanner
and a SE:M imagr
contrast to the present study where we found a very good numerical agreement between the optical and the contact profilometer and, when using the AFM as a reference, we found the optical scanner to reproduce best the surface topography for this specimen (Tabk 2, Figure 5). The laser scanner was also better for measuring the cylinder liner surface compared with the other optical instruments investigated by Blunt et aZ.““. One reason for this may be the confocal principle allowing a higher numerical aperture when using standard objectives, i.e. more light to be collected, which is important for tilted or porous surfaces. For the blasted, relatively soft, surface the numerical values had about the same validity for the two investigated instruments. However, owing to the stylus influence on the surface, it will not have the same topography after the measurement as before and, thereby, the possibility exists that the measurement itself may influence the outcome of the biological response. For a hard-polished surface like the roller bearing, the traces remaining after the grinding operation cause high slopes in the surface, resulting in induced spikes; this is one likely explanation for the quite high surface roughness values achieved with the laser scanner. For such a surface it is important to know the errors and to use a low-pass filter before evaluation if this optical method is to be used. The stylus missed some of the valleys but was far closer to the true surface, and was a better method for this surface.
555
Characterizing
30 topography:
A.
Wane&erg et al.
CONCLUSIONS The two instruments, a stylus and a laser scanning device, are both acceptable for use in the evaluation of surface topographies. Even so, it is important to know the advantages and the disadvantages of each of the two approaches. Typical soft metal biomaterials are best evaluated with the laser approach, whereas hard industrial materials with high slopes within the surface are best evaluated with the stylus instrument. For threaded screw implants, the laser method is the most advantageous approach since the screw geometry will prevent proper stylus reading. ACKNOWLEDGEMENTS This study has been supported by grants from the Swedish Board for Technical Evaluation (NUTEK) , Sylvan’s Foundation, the Swedish Dental Society, the Anna Ahrenberg Foundation, the Greta and Einar Asker Foundation, the Wilhelm and Martina Lundgren Science Foundation, the Swedish Medical Research Council and the Hjalmar Svensson Research Foundation. REFERENCES 1. Albrektsson T, B&remark P-I, Hansson H-A, Lindstrijm J. Osseointegrated implants. Requirements for ensuring a long-lasting, direct bone-to-implant anchorage in man. Acta Orthop Stand 1981; 52: 155. 2. Ratner BD. Biomaterial surfaces. J Biomed Mater Res A#1 Biomater 1987; 21: 59-90. 3. Smith DC. Dental implants: materials and design considerations. Int J Prosthodont 1993; 6: 106-17. 4. Ask M, Lausmaa J, Kasemo B. Preparation and surface of oxide films on spectroscopic characterization Ti6A14V. Appl Surface Sci 1989; 35: 283-301. 5. Keller JC, Stanford CM, Wightman JP, Draughn A, Zaharias R. Characterizations of titanium implant surfaces III. J Biomed
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