Press Release

A combination of scientific improvements and an ageing population have led to incredible growth in the number of THAs' completed each year. In 2003, some 650,000 hip implant procedures were completed on a global basis.

Wear reduction in hip implants
Aseptic loosening resulting from wear debris has been highlighted as a major factor in THA failure [1]. Consequently the limitation of wear in hip implant components is critical to their long-term survival. In implants with UHMWPE cups and metal / ceramic heads, polymeric debris can lead to osteolysis and the associated proximal femoral bone loss. This is a real problem when one considers that polymeric wear loss can be in the region of 56mm3 per year for UHMWPE (2.8mm3 per year for cross-linked UHMWPE). [2]

The alternative material combinations in THA are metal on metal or ceramic on ceramic, which can offer reduced wear characteristics. However they come with associated risks, which would include:

metal on metal
- Adverse biological effects to increased metal ion levels in the body.

ceramic on ceramic
- Chipping of ceramic cup liner and the risk of fracture.

Additionally the manufacturing challenges of obtaining improved conforming surfaces and consistently correct clearances impose more stringent constraints on manufacture.

Importance of surface texture
A number of researches have indicated that wear rate of the polymer acetabular cup is greatly affected by the surface texture characteristics of the femoral head [3]. This is because polymer (cup) implants typically operate in either a boundary or mixed lubrication regime. However the ability to develop thick film lubrication in metal on metal or ceramic on ceramic implants is also dependent on surface texture quality. Lubrication regimes are dictated by the lambda ratio (λ), which is defined as [4,5]:

hcen
λ = where Rq* = (Rq12 + Rq22)0.5
Rq*

Where hcen= theoretical fluid film thickness and Rq = the root mean square roughness

With the aim of improving surface texture characterisation knowledge, this article compares the use of two instruments (contact & non-contact) in the measurement of hip implant components. It looks at the instrument factors affecting nanometric accuracy and the parameters available, which can be used to improve functional performance knowledge.

Methodology of study
Four 36mm CoCr femoral head implants were measured using both contact and non contact systems. The analysis reviewed parameters that could be used to improve surface characterisation knowledge.

Contact Vs Non Contact (PGI Vs CCI)
One of the initial considerations in selecting an instrument for surface characterisation is whether to use a contact or a non-contact system [6]. Typically contact profiling systems are used in (ball) bearing applications as they:
- Measure the mechanical interface
- Can deal with contamination typically found in ball bearing manufacturing arenas.

The key benefits to contact and non-contact systems are summarised in table 1.

Table 1 Benefits of contact and non-contact probes

Principle of a contact profiling system

The Form Talysurf Phase Grating Interferometric (PGI) system is an example of a contact profiling system. Contact profiling systems operate by tracing a stylus across a surface and data logging each point in the X (horizontal) and Z (vertical) direction. X data logging is usually achieved by means of a grating or a time based motor. Z deviation is monitored on the PGI by means of a laser interferometric method (fig.1).

A laser is directed down a beam splitter onto a convex diffractive grating. This splits the beam into two elements, which are reflected back along different paths onto detectors. Movement of the stylus beam - as it moves across the surface - creates a phase difference between the reflected beams which is measured.

Table 1 Benefits of contact and non-contact probes

The PGI offers both good vertical range and resolution which makes it ideally suited to measuring form, radius and surface texture all of which are important in THA components. It should be noted that for stable and accurate measurement of form and radius on hip implant components, the stylus trace should cover an angle of at least 60 degrees.

Before considering the parameters available in 2D analysis it is worth briefly reflecting on the factors that affect accuracy in contact profiling systems.

The factors include:

1. Stylus tip size and geometry
2. Gauge linearity
3. Gauge frequency response
4. Vertical resolution of gauge
5. Data logging in the Z direction
6. System noise

2D parameters available to describe the surface

There are a wide range of 2D parameters available for describing surfaces [7]. However it helps to consider the parameters in three categories, which are:
Amplitude parameters which are determined solely by peak or valley heights, or both, irrespective of horizontal spacing, e.g. Ra, Rz, Rt, Rq etc.
Spacing parameters, which are determined solely by spacing of irregularities along the surface, e.g. Sm, Rsm, HSC, Pc etc.
Hybrid parameters which are determined by amplitude and spacing in combination, e.g. Rda, Rdq, Rla, Rlq etc.

A standard parameter used in 2D analysis of hip implants is Ra (average roughness), which is the arithmetic average of the absolute departure of the profile from the reference line throughout the sampling length. In mathematical terms:

Due to the nature of its calculation, Ra is a reasonably useful tool for process control however it is less useful as a predictor of functional performance. The reason for this can be explained by reviewing fig.2, which shows a number of different profiles. It is quite feasible for each of these profiles to have the same Ra figure, however it easy to see that they will have quite different tribological characteristics.

There are alternative 2D parameters available which can offer more insight into the bearing characteristics of hip implant components. These would include Rq and Rsk.

Rq is the root mean square of the distance of the filtered or unfiltered profile from its mean line. In mathematical terms:
Because this parameter squares amplitudes, it is more sensitive to peaks and valleys.

Another parameter is Rsk (Skew) which is a measure of the symmetry of a profile about a mean line. It can distinguish between asymmetrical profiles with the same Ra or Rq. Negative Skew indicates a predominance of valleys, while positive Skew will be seen on "peaky" surfaces. Examples of profiles with Rsk negative and Rsk positive are shown in figure 3.

Principle of non-contact interferometry

The Talysurf CCI (Coherent Correlation Interferometer) is an example of a non-contact areal (3D) measurement system. The system works on a Mirau interferometric principle shown in figure 5.

An upper beam splitter directs light from a white light source towards the objective lens. The lower beam splitter splits the light into two separate beams, one is reflected onto a reference surface, the other onto the surface. The two beams recombine to create an interference pattern which is monitored by a CCD (Charged Coupled Device). The interference pattern is processed and a digital image of the surface comprising (X,Y,Z) data points over the measurement
area is obtained.

As with contact profiling systems, there are a number of factors that affect the accuracy of areal interferometric systems.

The factors include:

1. Vertical resolution of gauge
2. Amount of returned light
3. Data points in X,Y (pixel array on CCD)
4. System noise

3D parameters available to describe the surface

There are a wide range of 3D parameters available for describing surfaces which can in general be broken into four categories, which are amplitude, spatial, hybrid and functional [8]. The first three are similar in definition to 2D parameters except they are calculated over an area rather than a profile and as such they use an "S" rather than an "R" prefix. The Functional parameters are used to characterise fluid retention properties, and are therefore concerned with volume: they are designated as "V" parameters.

It is possible to look at hip implant components using 3D equivalents of the 2D parameters we have discussed, i.e. Sa, Sq and Ssk. As these parameters are over an area they are double integrals (summations) in the x and y direction. For example in mathematical terms Ssk would be defined as:

As with the 2D analysis Sa is a limited parameter for gaining functional information from surfaces. However parameters like, Ssk and analysis related to volume properties of the surface can be used to gain a greater insight into functional performance.

Summary and further research

Table 2 shows a number of 2D and 3D parameters calculations on unworn CoCr femoral heads, manufactured by different processes. The 2D calculations were completed on a 24mm trace taken through the apex of each femoral head. In general the surface finish Ra ranges from 11nm (no.3) to 3nm (no.4). At first glance all surface finishes (Ra) would appear to be acceptable however reviewing some other parameters provides a greater insight. For example the Rsk ranges from very negative (no.2) to slightly positive (no.1). In light of our earlier discussion it is easy to understand that these surfaces will have quite different tribological characteristics i.e. peak or pit dominated.

The 3D calculations were completed on a 0.9mm2 area taken from the apex of each femoral head. The Sa and Sq parameters for head no.1 are within acceptable limits, however the values for the other three heads are very high. Closer inspection of these three heads using 3D visualisation tools highlights a severe problem with pitting on these heads, which is invariably due to a poor process. It is interesting to note how a 3D tool can identify issues with bearing surfaces that can be masked by a 2D profiling system.

This article has provided a brief insight into 2D and 3D surface characterisation on hip implants. It has highlighted that there are more parameters available than the average mean (Ra / Sa) for understanding and quantifying the functional performance of hip implants.

Further study should be considered into detailing the best parameters to be used and the acceptable limits on these parameters. There should also be some consideration into defining surface texture direction (lay) on hip implant components as this could have a large influence on the ability to generate a thick lubrication film.

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References
[1] D.W. Murray, N Rushton, "Macrophages stimulate resorption when they phagocytose particles", J Bone Surg 205H (1990)73-79
[2] Heisel H. Et al.: J Bone Joint Surg-Am 2003; 85-A: 1366-79
[3] D.J.R. Cooper and J Fisher , Clin Matls 14 (1993) 295
[4] Smith, S.L., Dowson, D. and Goldsmith, A.A.J., (2001), 'The Lubrication of Metal-on-Metal Hip Joints: A slide Down the Stribeck Curve', Proceedings of the Institute of Mechanical Engineers, Part 'J' Engineering Tribology, Volume 215, No. J5, 483-484
[5] Z.Jin, D. Dowson, J. Fisher. 'Analysis of fluid film lubrication in artificial hip joint replacements with surfaces of high elastic modulus'. Proceedings of the Institute of Mechanical Engineers, Vol 211, Part H, 247-256, 1997
[6] L. Blunt and X Jiang "Three dimensional measurement of the surface topography of ceramic and metallic orthopedic joint prostheses" J. Matl's Sci; Matl's in Medicine 11(2000) 235-246.
[7] D. Whitehouse, "Surfaces and their Measurement", Hermes Penton Science, London.
[8] L. Blunt and X. Jiang, "Assessment Surface Topography", Kogan Page Science, London.

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