Material selection can be a challenge for hip implant manufacturers. A major factor in long-term success for implant procedures is limiting the gradual and progressive shedding of material from the implant surfaces. This wear and the resulting presence of particulates in the body can lead to bone resorption, granuloma formation, loosening of the joint, and ultimately, implant failure.

This failure mechanism is driving improvements in biometric materials that provide several options for hip replacement surgery. Today’s top-performing 3-D optical microscopes provide quantitative evaluation of materials prior to implantation to determine the wear characteristics of surfaces in a variety of preparations (uncoated, coated, polished, etc.). This evaluation enables the manufacture of implants with improved wear, comfort, flexibility, and prolonged life.

Technology Driven Solution

Three-dimensional microscopy uses white light interferometry (WLI) and is one of the most accurate, repeatable, and versatile methods of precision surface metrology. Systems based on this technology successfully measure materials in research and production line environments in subnanometer vertical resolution for a wide range of industries, including medical, automotive, aerospace, electronics, solar, MEMS, data storage, and general manufacturing and precision machining inspection. A 3-D microscope offers a noncontact advantage over contact stylus systems that have mechanical filtering due to tip radius, which also can damage a surface during measurement.

In WLI microscopy, light from a source is focused on a sample while at the same time reflected from a reference mirror in the instrument (see Figure 1). The light reflected from the sample is precisely compared to the light reflected from the reference mirror to obtain local height information as a function of lateral position on the sample. The use of high-brightness LEDs as source illumination and proper filtering allows users to tailor the measurement conditions to match the application requirements for a vast array of sample surfaces. The presence of the helium-neon laser delivers a self-calibrating reference for the movement of the scanner in order to account for changes in the position due to environmental variations.

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Figure 1. Basic design of a 3-D microscope with a self-calibrating helium-neon laser.

Applications for this technology are being increasingly developed outside the traditional surface measurement realm, specifically for areas such as film thickness metrology, environmental response (corrosion) studies, and actuated response studies. Most recently, the technology is enabling users to examine the behavior of materials over a range of timescales as they are worn via cutting or grinding operations, or where material wear is produced due to constant contact with other components. This particular wear mechanism is prominent in implants-specifically hip implants-where there is necessary contact by device design and function.

Plastic-on-metal, ceramic, and metal-on-metal implants have drawn negative attention for their tendency to produce friction-created debris, which in turn causes inflammation of the tissues surrounding the implants. This debris can lead to increased residual presence in the body of the polyethylene or metal components. The wear and resulting inflammation can lead to osteolysis (bone destruction), pseudotumors, or in limited cases, loud frictional squeaking due to ceramic-on-ceramic stripe-wear patterns. Careful characterization of the wear and wear rate for the materials involved is critical to improving the long-term performance and stability of these products. This type of information is readily attained using 3-D optical microscopy.

Figure 2. 1 mg of material removed, which correlates to ~7.5×108 µm3/mg volume of material and matching the nominal mass density of PEEK.

Wear Study Example

The following example is an alternative method of performing wear studies based on 3-D microscopic metrology that not only gives the volume of material removed, but also can provide other critical parameters to characterize the wear scar. This information can be extremely useful during process development, material property analysis, and material selection. It is invaluable to process development engineers, materials science engineers, and product development engineers, because the final performance of a material in an application can be correlated to the wear measured in such a manner. Using a 1-in. diameter virgin PEEK sphere to simulate a hip joint, the top of the surface was mapped using a 3-D microscope. The measurements of the surface consist of several fields of view measured at 5x magnification and combined to produce an accurate and wide-angle view of the top portion of the surface presented for inspection. A photograph of the PEEK sphere and the corresponding 3-D microscopic image of the portion to be inspected are shown in Images 1 and 2.

After initial characterization of the PEEK sphere, a wear mark was made to simulate wear and volume removal. After this small volume of material was removed, the surface was remapped in the same manner as it was done previously. Images of the worn surface and the leveled representation of the wear mark are shown in Image 3. By subtracting the original pristine surface from the second, worn surface, the material mass displaced and the volume loss can be precisely calculated. This methodology is accurate due to the high vertical resolution and high lateral resolution achieved by WLI-based 3-D optical microscopy.

Image 1. A 1-in. diameter PEEK sphere.	Image 2. A 3-D surface map of the PEEK sphere before wear.	Image 3. A remapping of the worn PEEK sphere surface.	Image 4. The wear with sphere terms removed.

The data presented in Image 4 are used to compute the volume displacement. The depth and area over which the material was removed is used to make a computation of the material density, which is highly correlated with gravimetric machines. The repeatability of the surface metrology technique is more than an order of magnitude higher than the gravimetric method of measurement, due to the higher uncertainty in measurement of such small mass. This aids a materials researcher who may be trying to correlate mass loss using a material properties model in development for the particular material under analysis. The analysis and volume computation for the initial wear mark and computation can be seen in Figure 2.

To verify the repeatability of the technique, a repeated wear mark was introduced to remove approximately 4 mg of additional material. This material was removed from the same surface scar location and the surface was remapped. The final volume removed was then computed based on analysis of the final depth and width of the resulting wear scar (see Figure 3).

Figure 3. The 3D profile reveals a new wear mark, with 4 mg of material removed.	The image shows the new volume loss calculation.

For this second stage of the material wear and volume removal experiment, slightly less than 5 milligrams of total material was eventually removed. This mass removal corresponds to approximately ~3.5×10⁹ µm³/mg of volume lost due to wear on the PEEK sphere surface. Because some of the final volume of material removed was taken from the sphere slightly outside the measurement field of view, the final measurement of volume lost was slightly less than the theoretical loss calculation (resulting in slight mismatch to nominal mass density). For an even more accurate representation of this removal, a large initial map could be made to accommodate the additional removal volume. Even with this mismatch, the associated error in mass determination is on the order of ten times smaller than gravimetric means.

Noncontact 3-D Microscope Metrology

The speed and versatility of 3-D microscopy brings accuracy and data-quality advantages to the user. The filtering that takes place with a mechanical stylus measurement does not occur with noncontact imaging, where the sampling of the image is set by a combination of the magnification of the observing lenses and the charge-coupled device camera spacing. Lateral resolutions at the diffraction limit are achievable in this manner, enabling higher sampling than is possible with a stylus of 2, 5, or 10 µm (some typical stylus tip dimensions). Important 3-D surface parameters, such as roughness (Sa), peak/valley (St), and radius of curvature (RoC), can easily be obtained from the same data used for the volume calculations above. These parameters are computed automatically for the entire field of view (FOV) of the optical inspection area. The data, if desired, can also be examined in 2-D, thus offering the operator hundreds of equivalent stylus traces for just a single FOV. The speed with which this information can be collected provides an advantage in production settings.

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Figure 4. Listing of various 3-D areal surface parameters measured via 3-D microscopy. Click Figure for larger image.

Additionally, 3-D parameters of interest (especially when computed in accordance with ISO 21578 standards, which are currently scheduled for adoption across a wide range of industries) provide an accurate and consistent quantification of important surface texture characteristics in a well-known, industry-accepted manner. A few of these parameters, representing average 3-D roughness, slope, skewness, kurtosis, height, and peak/valley information are listed in Figure 4.

Such 3-D parameters allow materials researchers to correlate properties that are highly quantifiable to product performance in a fast, accurate, and repeatable manner. These measurements also exhibit high reproducibility, a hallmark of good metrology for process and quality control.

Conclusion

3-D microscopes provide versatile and rapid noncontact means of performing surface finish and tribology measurements in several areas, including the engineering lab and the production floor. Fast, accurate, and repeatable measurements based on the capabilities of this technology are ideal for the medical industry. The versatility of 3-D microscopy enables end users to measure materials in a wide range of applications. The specific study presented in this article demonstrates the ability of 3-D microscopy to deliver a highly accurate wear metrology solution tailored to the medical implant industry. The results showed an accurate computation of wear volume on a hip implant (a PEEK simulated hip ball) obtained using 3-D microscopy while also giving form and surface finish parameters as value-added information. These accurate results and the additional 3-D surface texture parameters offer implant materials development and production teams a significant value over traditional gravimetric approaches to materials characterization.

Roger Posusta is senior marketing applications engineer at Bruker Corp.’s Nano Surfaces division (Tucson, AZ). Matt Novak, PhD, is marketing applications manager at the company.